Title of the invention: Steel member and its manufacturing method
Technical field
[0001]
　The present invention relates to a steel member and a method for producing the same.
　The present application claims priority based on Japanese Patent Application No. 2018-082625 filed in Japan on April 23, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　In the field of steel sheets for automobiles, the application of steel sheets having high tensile strength is expanding in order to achieve both fuel efficiency and collision safety against the background of recent stricter environmental regulations and collision safety standards. However, as the strength increases, the press formability of the steel sheet decreases, which makes it difficult to manufacture a product having a complicated shape. Specifically, the ductility of the steel sheet decreases as the strength increases, so that the highly processed portion is likely to break. In addition, residual stress after machining may cause springback and wall warpage, which may reduce dimensional accuracy. Therefore, it is not easy to press-mold a steel sheet having high strength, particularly tensile strength of 780 MPa or more, into a product having a complicated shape. Although it is easy to process a high-strength steel sheet by roll forming instead of press forming, the application destination is limited to parts having a uniform cross section in the longitudinal direction.
[0003]
　In recent years, for example, as disclosed in Patent Documents 1 to 3, a hot stamping technique has been adopted as a technique for press-molding a material that is difficult to form, such as a high-strength steel sheet. The hot stamping technique is a hot stamping technique in which a material to be molded is heated and then molded. In this technique, since the material is heated before molding, the steel material is soft and has good moldability at the time of molding. As a result, even a high-strength steel material can be accurately formed into a complicated shape. Further, in the hot stamping technique, since quenching is performed at the same time as molding by a press die, the steel material after molding has sufficient strength.
[0004]
　For example, according to Patent Document 1, it is possible to impart a tensile strength of 1400 MPa or more to a steel material after forming by a hot stamping technique. Further, Patent Document 2 discloses a press-molded product which is hot press-molded and has excellent toughness and a tensile strength of 1.8 GPa or more. Further, Patent Document 3 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and further having good toughness and ductility. Further, Patent Document 4 discloses a steel material having a tensile strength of 1.4 GPa or more and excellent ductility. Further, Patent Document 5 discloses a hot press-molded product having excellent ductility. Further, Patent Document 6 discloses a press-molded member having a tensile strength of 980 MPa or more and excellent ductility. Further, Patent Document 7 discloses a molded member having a tensile strength of 1000 MPa or more and excellent ductility.
Prior art literature
Patent documents
[0005]
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-102980
Patent Document 2: Japanese Patent Application Laid-Open No. 2012-180594
Patent Document 3: Japanese Patent Application Laid-Open No. 2012-1802
Patent Document 4: International Publication No. 2016/1634468
Patent Document 5: International Publication No. 2012/16963
Patent Document 6: International Publication No. 2011/111333
Patent Document 7: International Publication No. 2012/091328
Outline of the invention
Problems to be solved by the invention
[0006]
　The steel sheet for automobiles applied to the vehicle body is required not only to have the above-mentioned formability but also to have collision safety after molding. The collision safety of an automobile is evaluated by the crush strength and absorbed energy in the collision test of the entire vehicle body or steel members. In particular, since the crushing strength greatly depends on the material strength, the demand for ultra-high-strength steel sheets is increasing dramatically. However, in general, the fracture toughness and deformability of the automobile member decrease as the strength of the steel plate material increases, so that the automobile member breaks early at the time of collision crushing of the automobile member or breaks at a portion where the deformation is concentrated. , The crushing strength commensurate with the material strength is not exhibited, and the absorbed energy decreases. Therefore, in order to improve collision safety, it is important to improve not only the material strength but also the fracture toughness and deformability of the automobile member, that is, the toughness and ductility of the steel sheet material.
[0007]
　In the techniques described in Patent Documents 1 and 2, although tensile strength and toughness are described, ductility is not considered. Further, according to the techniques described in Patent Documents 3 and 4, it is possible to improve tensile strength, toughness and ductility. However, the methods described in Patent Documents 3 and 4 may not be sufficient in eliminating the fracture origin and controlling the highly ductile structure, and may not be able to further improve toughness and ductility. Further, in the techniques of Patent Documents 5, 6 and 7, although the tensile properties and ductility are described, the toughness is not considered.
[0008]
　The present invention has been made to solve the above problems, and an object of the present invention is to provide a steel member having high tensile strength and excellent ductility and a method for producing the same. It is an object of the present invention to more preferably provide a steel member having the above-mentioned various properties and excellent toughness, and a method for producing the same.
Means to solve problems
[0009]
　The gist of the present invention is the following steel member and a method for manufacturing the same.
　In many cases, the hot-formed steel member is not a flat plate but a molded body, but in the present invention, the steel member is referred to as a "steel member" including the case where it is a molded body. Further, a steel sheet that is a material before heat treatment of a steel member is also referred to as a "material steel sheet".
[0010]
[1] The steel member according to one aspect of the present invention has a chemical composition of% by mass,
C: 0.10 to 0.60%,
Si: 0.40 to 3.00%,
Mn: 0.30 to 0.30 to 3.00%,
P: 0.050% or less,
S: 0.0500% or less,
N: 0.010% or less,
Ti: 0.0010 to 0.1000%,
B: 0.0005 to 0.0100% ,
Cr: 0 to 1.00%,
Ni: 0 to 2.0%,
Cu: 0 to 1.0%,
Mo: 0 to 1.0%,
V: 0 to 1.0%,
Ca: 0 to 0.010%,
Al: 0 to 1.00%,
Nb: 0 to 0.100%,
Sn: 0 to 1.00%,
W: 0 to 1.00%,
REM: 0 to 0.30%,
The balance is Fe and impurities, and the
　metallographic structure is 60.0 to 85.0% for martensite, 10.0 to 30.0% for baynite, and 5.0 to 5.0 to retained austenite. 15.0% and 0-4.0% of the
　residual structure , and the length of the maximum minor axis of the retained austenite is 30 nm or more.
　The number density of carbides having a circle-equivalent diameter of 0.1 μm or more and an aspect ratio of 2.5 or less is 4.0 × 10 3 pieces / mm 2 or less.
[2] In the steel member according to the above [1], the chemical composition is
Cr: 0.01 to 1.00%,
Ni: 0.01 to 2.0%,
Cu: 0.01 in mass%. ~ 1.0%,
Mo: 0.01 ~ 1.0%,
V: 0.01 ~ 1.0%,
Ca: 0.001 ~ 0.010%,
Al: 0.01 ~ 1.00%,
Contains one or more of Nb: 0.010 to 0.100%,
Sn: 0.01 to 1.00%,
W: 0.01 to 1.00%, and
REM: 0.001 to 0.30%. You may.
[3] In the steel member according to the above [1] or [2], the value of the strain-induced transformation parameter k represented by the following formula (1) may be less than 18.0.
　k = (logf γ0- logf γ (0.02)) /0.02 ... Equation (1)
　However, the meaning of each symbol in the above equation (1) is as follows.
　f γ0 : Volume fraction of retained austenite present in the steel member before applying true strain
　f γ (0.02): In the steel member after applying true strain of 0.02 to the steel member and unloading Volume Fraction of Retained Austenite Existing in
[4] In the steel member according to any one of the above [1] to [3], even if the tensile strength is 1400 MPa or more and the total elongation is 10.0% or more. Good.
[5] In the steel member according to any one of the above [1] to [4], the local elongation may be 3.0% or more.
[6] In the steel member according to any one of the above [1] to [5], the impact value at −80 ° C. may be 25.0 J / cm 2 or more.
[7] In the steel member according to any one of the above [1] to [6], the cleanliness value of the steel specified in JIS G 0555: 2003 may be 0.100% or less.
[8] The method for producing a steel member according to another aspect of the present invention is the method for producing a steel member according to any one of the above [1] to [7], wherein the
　chemical composition is mass%. ,
C: 0.10 to 0.60%,
Si: 0.40 to 3.00%,
Mn: 0.30 to 3.00%,
P: 0.050% or less,
S: 0.0500% or less,
N: 0.010% or less,
Ti: 0.0010 to 0.1000%,
B: 0.0005 to 0.0100%,
Cr: 0 to 1.00%,
Ni: 0 to 2.0%,
Cu: 0 to 1.0%,
Mo : 0 to 1.0%,
V: 0 to 1.0%,
Ca: 0 to 0.010%,
Al: 0 to 1.00%,
Nb: 0 to 0.100%,
Sn: 0 to 1. It contains 00%,
W: 0 to 1.00%,
REM: 0 to 0.30%,
the balance is Fe and impurities, the circle equivalent diameter is 0.1 μm or more, and the aspect ratio is 2.5 or less. A material steel sheet having a 　carbon number density of 8.0 × 10 3 pieces / mm 2 or less and an average value of the equivalent circle diameter of (Nb, Ti) C of 5.0 μm or less can be obtained from
Ac 3 points to (Ac). A heating step of heating to a temperature range of 3 points + 200) ° C. at an average heating rate of 5 to 300 ° C./s, and a
　first method of cooling to the Ms point at a first average cooling rate equal to or higher than the upper critical cooling rate after the heating step. Cooling process and
　After the first cooling step, a second average of 5 ° C./s or more and less than 150 ° C./s and slower than the first average cooling rate from (Ms-30) to (Ms-70) ° C. After the second cooling step of cooling at a cooling rate, the
　reheating step of heating to a temperature range of Ms to (Ms + 200) ° C. at an average temperature rise rate of 5 ° C./s or more, and the
　reheating step. A third cooling step of cooling at a third average cooling rate of 5 ° C./s or higher is provided.
[9] In the method for manufacturing a steel member according to the above [8], between the heating step and the first cooling step, 5 in the temperature range of the Ac 3 points to (Ac 3 points + 200) ° C. A holding step of holding for up to 200 seconds may be provided.
[10] In the method for producing a steel member according to the above [8] or [9], 3 in the temperature range of Ms to (Ms + 200) ° C. between the reheating step and the third cooling step. A holding step of holding for up to 60 seconds may be provided.
[11] In the method for manufacturing a steel member according to any one of [8] to [10] above, the material steel sheet is hot-formed between the heating step and the first cooling step. May be good.
[12] In the method for manufacturing a steel member according to any one of [8] to [10] above, in the first cooling step, cooling is performed at the first cooling rate, and at the same time, the material steel sheet is heated. Inter-molding may be performed.
Effect of the invention
[0011]
　According to the above aspect of the present invention, it is possible to provide a steel member having high tensile strength and excellent ductility and a method for producing the same. According to a preferred embodiment of the present invention, it is possible to provide a steel member having the above-mentioned various properties and excellent toughness and a method for producing the same.
A brief description of the drawing
[0012]
FIG. 1 is a diagram showing a temperature history of each step in the method for manufacturing a steel member according to the present embodiment.
Mode for carrying out the invention
[0013]
　Hereinafter, the steel member and the manufacturing method thereof according to the embodiment of the present invention will be described in detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention.
[0014]
(A) Chemical Composition of
　Steel Member The reasons for limiting each element of the steel member according to the present embodiment are as follows. In the following description, "%" for the content means "mass%". The numerical limitation range described below includes the lower limit value and the upper limit value. Numerical values ​​indicating "super" and "less than" do not include the value in the numerical range. All% of the chemical composition indicate mass%.
[0015]
　C: 0.10 to 0.60%
　C is an element that enhances the hardenability of steel and improves the strength of the steel member after quenching. However, if the C content is less than 0.10%, it becomes difficult to secure sufficient strength in the hardened steel member. Therefore, the C content is set to 0.10% or more. The C content is preferably 0.15% or more, or 0.20% or more. On the other hand, if the C content exceeds 0.60%, the strength of the steel member after quenching becomes too high, and the toughness deteriorates significantly. Therefore, the C content is set to 0.60% or less. The C content is preferably 0.50% or less, or 0.45% or less.
[0016]
　Si: 0.40 to 3.00%
　Si is an element that enhances the hardenability of steel and improves the strength of steel members by solid solution strengthening. Furthermore, since Si hardly dissolves in carbides, it suppresses the precipitation of carbides during hot molding and promotes C concentration in untransformed austenite. As a result, the Ms point is remarkably lowered, and a large amount of solid solution-enhanced austenite can remain. In order to obtain this effect, it is necessary to contain 0.40% or more of Si. When the Si content is 0.40% or more, the residual carbide tends to decrease. As will be described later, if there are many carbides precipitated in the material steel sheet before heat treatment, they remain undissolved during heat treatment, and sufficient hardenability cannot be ensured, low-strength ferrite precipitates, and the strength of the steel member is insufficient. There is. Therefore, in this sense as well, the Si content is set to 0.40% or more. The Si content is preferably 0.50% or more, or 0.60% or more.
　However, when the Si content in the steel exceeds 3.00%, the heating temperature required for the austenite transformation becomes remarkably high during the heat treatment. This may cause an increase in the cost required for the heat treatment, or the ferrite may remain without being sufficiently austenitized, and the desired metal structure and strength may not be obtained. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, or 2.00% or less.
[0017]
　Mn: 0.30 to 3.00%
　Mn is a very effective element for enhancing the hardenability of the material steel sheet and stably ensuring the strength after quenching. Further, Mn is an element that lowers the Ac 3 point and promotes the lowering of the quenching treatment temperature. However, if the Mn content is less than 0.30%, the above effect cannot be sufficiently obtained. Therefore, the Mn content is set to 0.30% or more. The Mn content is preferably 0.40% or more. On the other hand, when the Mn content exceeds 3.00%, the above effect is saturated and further deteriorates the toughness of the hardened portion. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.80% or less, more preferably 2.50% or less.
[0018]
　P: 0.050% or less
　P is an element that deteriorates the toughness of the steel member after quenching. In particular, when the P content exceeds 0.050%, the toughness of the steel member is significantly deteriorated. Therefore, the P content is limited to 0.050% or less. The P content is preferably limited to 0.030% or less, 0.020% or less, or 0.005% or less. Although P is mixed as an impurity, it is not necessary to limit the lower limit thereof, and in order to obtain the toughness of the steel member, the content of P is preferably low. However, if the P content is excessively reduced, the manufacturing cost increases. From the viewpoint of manufacturing cost, the P content may be 0.001% or more.
[0019]
　S: 0.0500% or less
　S is an element that deteriorates the toughness of the steel member after quenching. In particular, when the S content exceeds 0.0500%, the toughness of the steel member is significantly deteriorated. Therefore, the S content is limited to 0.0500% or less. The S content is preferably limited to 0.0030% or less, 0.0020% or less, or 0.0015% or less. Although S is mixed as an impurity, it is not necessary to limit the lower limit thereof, and in order to obtain the toughness of the steel member, the content of S is preferably low. However, if the S content is excessively reduced, the manufacturing cost increases. From the viewpoint of manufacturing cost, the S content may be 0.0001% or more.
[0020]
　N: 0.010% or less
　N is an element that deteriorates the toughness of the steel member after quenching. In particular, when the N content exceeds 0.010%, coarse nitrides are formed in the steel, and the local deformability and toughness of the steel member are significantly deteriorated. Therefore, the N content is 0.010% or less. The lower limit of the N content is not particularly limited, but it is economically unfavorable to set the N content to less than 0.0002% because it causes an increase in steelmaking cost. Therefore, the N content is preferably 0.0002% or more, and more preferably 0.0008% or more.
[0021]
　Ti: 0.0010 to 0.1000%
　Ti suppresses recrystallization when the material steel sheet is heated to a temperature of 3 points or more and heat-treated, and also forms fine carbides to suppress grain growth. As a result, it is an element that has the effect of making austenite grains finer. Therefore, the inclusion of Ti has the effect of greatly improving the toughness of the steel member. Further, Ti preferentially binds to N in the steel to suppress the consumption of B due to the precipitation of BN, and promotes the effect of improving the hardenability by B described later. If the Ti content is less than 0.0010%, the above effect cannot be sufficiently obtained. Therefore, the Ti content is set to 0.0010% or more. The Ti content is preferably 0.0100% or more, or 0.0200% or more. On the other hand, when the Ti content exceeds 0.1000%, the precipitation amount of TiC increases and C is consumed, so that the strength of the steel member after quenching decreases. Therefore, the Ti content is set to 0.1000% or less. The Ti content is preferably 0.0800% or less, or 0.0600% or less.
[0022]
　B: 0.0005 to 0.0100%
　B is a very important element in the present embodiment because it has an effect of dramatically improving the hardenability of steel even in a small amount. Further, B segregates at the grain boundaries to strengthen the grain boundaries and increase the toughness of the steel member. Further, B suppresses the grain growth of austenite when the material steel sheet is heated. If the B content is less than 0.0005%, the above effects may not be sufficiently obtained. Therefore, the B content is set to 0.0005% or more. The B content is preferably 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, when the B content exceeds 0.0100%, a large amount of coarse compounds are precipitated, and the toughness of the steel member deteriorates. Therefore, the B content is 0.0100% or less. The B content is preferably 0.0080% or less, or 0.0060% or less.
[0023]
　In the chemical composition of the steel member according to the present embodiment, elements other than those described above, that is, the balance are Fe and impurities. Here, the "impurity" is a component mixed with raw materials such as ore and scrap, and various factors in the manufacturing process when the steel sheet is industrially manufactured, and adversely affects the steel member according to the present embodiment. Means something that is acceptable to the extent that it does not exist.
　The steel member according to the present embodiment is one selected from Cr, Ni, Cu, Mo, V, Ca, Al, Nb, Sn, W and REM shown below instead of a part of the remaining Fe. The above optional elements may be contained. However, since the steel member according to the present embodiment can solve the problem even if the optional element shown below is not contained, the lower limit of the content when the optional element is not contained is 0%.
[0024]
　Cr: 0 to 1.00%
　Cr may be contained because it is an element that enhances the hardenability of steel and makes it possible to stably secure the strength of the steel member after quenching. In order to surely obtain this effect, the Cr content is preferably 0.01% or more, and more preferably 0.05% or more. However, when the Cr content exceeds 1.00%, the above effect is saturated and unnecessarily causes an increase in cost. Further, since Cr has an action of stabilizing iron carbides, if the Cr content exceeds 1.00%, coarse iron carbides remain undissolved when the material steel sheet is heated, and the toughness of the steel member deteriorates. Therefore, when Cr is contained, the Cr content is set to 1.00% or less. The Cr content is preferably 0.80% or less.
[0025]
　Ni: 0 to 2.0%
　Ni may be contained because it is an element that enhances the hardenability of steel and makes it possible to stably secure the strength of the steel member after quenching. In order to surely obtain this effect, the Ni content is preferably 0.01% or more, and more preferably 0.1% or more. However, if the Ni content exceeds 2.0%, the above effects are saturated and cause an increase in cost. Therefore, the Ni content when Ni is contained is set to 2.0% or less.
[0026]
　Cu: 0 to 1.0%
　Cu may be contained because it is an element that enhances the hardenability of steel and makes it possible to stably secure the strength of the steel member after quenching. In addition, Cu improves the corrosion resistance of steel members in a corrosive environment. In order to surely obtain these effects, the Cu content is preferably 0.01%, more preferably 0.1% or more. However, if the Cu content exceeds 1.0%, the above effects are saturated and cause an increase in cost. Therefore, when Cu is contained, the Cu content is 1.0% or less.
[0027]
　Mo: 0 to 1.0%
　Mo may be contained because it is an element that enhances the hardenability of steel and makes it possible to stably secure the strength of the steel member after quenching. In order to surely obtain this effect, the Mo content is preferably 0.01% or more, and more preferably 0.1% or more. However, if the Mo content exceeds 1.0%, the above effects are saturated and cause an increase in cost. Further, since Mo has an action of stabilizing iron carbides, if the Mo content exceeds 1.00%, coarse iron carbides remain undissolved when the material steel sheet is heated, and the toughness of the steel member deteriorates. Therefore, when Mo is contained, the Mo content is 1.0% or less.
[0028]
　V: 0 to 1.0%
　V is an element that forms fine carbides and makes it possible to increase the toughness of the steel member by the fine granulation effect, and therefore may be contained. In order to surely obtain this effect, the V content is preferably 0.01% or more, and more preferably 0.1% or more. However, if the V content exceeds 1.0%, the above effects are saturated and cause an increase in cost. Therefore, when V is contained, the V content is 1.0% or less.
[0029]
　Ca: 0 to 0.010%
　Ca may be contained because it is an element having an effect of refining inclusions in the steel and improving the toughness and ductility of the steel member after quenching. In order to surely obtain this effect, the Ca content is preferably 0.001% or more, and more preferably 0.002% or more. However, when the Ca content exceeds 0.010%, the above effect is saturated and unnecessarily causes an increase in cost. Therefore, when Ca is contained, the Ca content is 0.010% or less. The Ca content is preferably 0.005% or less, more preferably 0.004% or less.
[0030]
　Al: 0 to 1.00%
　Al is generally used as a deoxidizer for steel and may be contained. In order to be sufficiently deoxidized by Al, the Al content is preferably 0.01% or more. However, if the Al content exceeds 1.00%, the above effects are saturated and cause an increase in cost. Therefore, the Al content when Al is contained is set to 1.00% or less.
[0031]
　Nb: 0 to 0.100%
　Nb is an element that forms fine carbides and makes it possible to increase the toughness of the steel member by the fine granulation effect, and therefore may be contained. In order to surely obtain this effect, the Nb content is preferably 0.010% or more. However, if the Nb content exceeds 0.100%, the above effects are saturated and cause an increase in cost. Therefore, when Nb is contained, the Nb content is 0.100% or less.
[0032]
　Sn: 0 to 1.00%
　Sn may be contained in order to improve the corrosion resistance of the steel member in a corrosive environment. In order to surely obtain this effect, the Sn content is preferably 0.01% or more. However, if the Sn content exceeds 1.00%, the grain boundary strength decreases and the toughness of the steel member deteriorates. Therefore, when Sn is contained, the Sn content is set to 1.00% or less.
[0033]
　W: 0 to 1.00%
　W is an element that enhances the hardenability of steel and makes it possible to stably secure the strength of the steel member after quenching, and thus may be contained. W also improves the corrosion resistance of the steel member in a corrosive environment. In order to surely obtain these effects, the W content is preferably 0.01% or more. However, if the W content exceeds 1.00%, the above effects are saturated and cause an increase in cost. Therefore, when W is contained, the W content is set to 1.00% or less.
[0034]
　REM: 0 to 0.30%
　REM is an element having the effect of refining inclusions in the steel and improving the toughness and ductility of the steel member after quenching, as in Ca, and may be contained. In order to surely obtain this effect, the REM content is preferably 0.001% or more, and more preferably 0.002% or more. However, when the REM content exceeds 0.30%, the effect is saturated and unnecessarily causes an increase in cost. Therefore, when REM is contained, the REM content is set to 0.30% or less. The REM content is preferably 0.20% or less.
[0035]
　Here, REM refers to a total of 17 elements composed of lanthanoids such as Sc, Y and La, Nd, and the content of the REM means the total content of these elements. REM is added to molten steel using, for example, a Fe—Si—REM alloy, which alloys include, for example, Ce, La, Nd, Pr.
[0036]
(B) Metallic structure of steel member The steel member according to the
　present embodiment has a volume fraction of martensite of 60.0 to 85.0%, bainite of 10.0 to 30.0%, and retained austenite of 5. It has a metallographic structure with 0 to 15.0% and a residual structure of 0 to 4.0%.
　The maximum minor axis length of retained austenite is 30 nm or more.
[0037]
　The martensite present in the steel member according to the present embodiment also includes automatic tempered martensite. Automatic tempering martensite is tempering martensite generated during cooling during quenching without heat treatment for tempering, and the generated martensite is tempered by the heat generated by the martensite transformation. It is what is generated. Tempered martensite can be distinguished from as-quenched martensite by the presence or absence of fine cementite precipitated inside the lath.
[0038]
　Martensite: 60.0 to 85.0%
　Martensite is a hard phase and is a structure necessary for increasing the strength of steel members. If the volume fraction of martensite is less than 60.0%, the tensile strength of the steel member cannot be sufficiently secured. Therefore, the volume fraction of martensite is set to 60.0% or more. Preferably, it is 65.0% or more. On the other hand, if the volume fraction of martensite exceeds 85.0%, other tissues such as bainite and retained austenite, which will be described later, cannot be sufficiently secured. Therefore, the volume fraction of martensite is set to 85.0% or less. Preferably, it is 80.0% or less.
[0039]
　Bainite: 10.0 to 30.0%
　Bainite is a structure with a higher hardness than retained austenite and a lower hardness than martensite. The presence of bainite reduces the hardness gap between retained austenite and martensite, prevents the formation of cracks at the boundary between retained austenite and martensite when stress is applied, and improves the toughness and ductility of steel members. Improve. Since the above effect cannot be obtained when the volume fraction of bainite is less than 10.0%, the volume fraction of bainite is set to 10.0% or more. The preferred volume fraction of bainite is 15.0% or more. Further, if the volume fraction of bainite exceeds 30.0%, the strength of the steel member decreases, so the volume fraction of bainite is set to 30.0% or less. The preferred volume fraction of bainite is 25.0% or less, more preferably 20.0% or less.
[0040]
　Residual austenite: 5.0 to 15.0%
　Residual austenite undergoes martensitic transformation (work-induced transformation) during plastic deformation to prevent constriction, promote work hardening, and improve ductility (TRIP effect). There is. Further, the transformation of retained austenite relaxes the stress concentration at the crack tip, which has the effect of improving not only the ductility but also the toughness of the steel member. In particular, when the volume fraction of residual plasticity is less than 5.0%, the ductility of the steel member is remarkably lowered, the risk of breakage of the steel member is increased, and the collision safety is lowered. Therefore, the volume fraction of retained austenite is set to 5.0% or more. It is preferably 6.0% or more, and more preferably 7.0% or more. On the other hand, if the volume fraction of retained austenite is excessive, the intensity may decrease. Therefore, the volume fraction of retained austenite is set to 15.0% or less. Preferably, it is 12.0% or less, or 10.0% or less.
[0041]
　The retained austenite present in the steel member according to the present embodiment exists between martensite laths, between bainite bainitic ferrites, or at the former austenite grain boundaries (former γ grain boundaries). The retained austenite is preferably present between the laths of the martensite or between the bainitic ferrites of the bainite. Since the retained austenite present at these positions is flat, it has the effect of promoting deformation near these positions and improving the ductility and toughness of the steel member.
[0042]
　Remaining structure: 0 to 4.0%
　Ferrite and pearlite may coexist as the remaining structure in the steel member according to the present embodiment. In this embodiment, the total volume fraction of martensite, bainite and retained austenite needs to be 96.0% or more. That is, in the present embodiment, the residual tissue other than martensite, bainite and retained austenite is limited to 4.0% or less in volume fraction. Since the residual tissue may be 0%, the volume fraction of the residual tissue is 0 to 4.0%.
[0043]
　Maximum minor axis of retained austenite: 30 nm or more In the
　present embodiment, the maximum minor axis of retained austenite is 30 nm or more. Retained austenite having a maximum minor axis of less than 30 nm is not stable in deformation, that is, martensitic transformation occurs in a low strain region at the initial stage of plastic deformation, and thus cannot sufficiently contribute to the improvement of ductility and collision safety of steel members. Therefore, the maximum minor axis of retained austenite is 30 nm or more. The upper limit of the maximum minor axis of retained austenite is not particularly limited, but it may be 600 nm or less, 100 nm or less, or 60 nm or less because the TRIP effect is not sufficiently exhibited if it is excessively stable in deformation.
[0044]
　The volume fractions of martensite, bainite and retained austenite, the location of retained austenite, and the method for measuring the maximum minor axis of retained austenite will be described.
　The volume fraction of retained austenite is measured using X-ray diffraction. First, a test piece is collected from a position 100 mm away from the end of the steel member. If the test piece cannot be collected from a position 100 mm away from the end due to the shape of the steel member, the test piece may be collected from the heat equalizing portion avoiding the end. This is because the end portion of the steel member is not sufficiently heat-treated and may not have the metal structure of the steel member according to the present embodiment.
　Chemical polishing is performed from the surface of the test piece to a depth of 1/4 of the plate thickness using hydrofluoric acid and hydrogen peroxide solution. The measurement conditions are a Co tube and a range of 45 ° to 105 ° at 2θ. The diffraction X-ray intensity of the face-centered cubic lattice (residual austenite) contained in the steel member is measured, and the volume fraction of retained austenite is calculated from the area ratio of the diffraction curve. As a result, the volume fraction of retained austenite is obtained. According to the X-ray diffraction method, the volume fraction of retained austenite in the steel member can be measured with high accuracy.
[0045]
　The volume fraction of martensite and the volume fraction of baynite are measured by a transmission electron microscope (TEM) and an electron beam diffractometer attached to the TEM. A measurement sample is cut out from a position 100 mm away from the end of the steel member and a position at a depth of 1/4 of the plate thickness, and used as a thin film sample for TEM observation. If the measurement sample cannot be collected from a position 100 mm away from the end due to the shape of the steel member, the measurement sample may be collected from the soaking portion avoiding the end. The range of TEM observation is 50 μm 2 or more in area, and the magnification is 10,000 to 50,000 times. Martensite and iron carbide (Fe in bainite 3 C) heading by the diffraction pattern, and observing the precipitate forms, to determine the martensite and bainite, measuring the area fraction and area fraction of bainite martensite .. If the precipitation form of the iron carbide is three-way precipitation, it is judged to be martensite, and if it is limited to one direction, it is judged to be bainite. The fractions of martensite and bainite measured by TEM are measured as area fractions, but since the steel member according to this embodiment has an isotropic metal structure, the volume fraction values ​​are directly integrated. Can be replaced with a rate. Iron carbides are observed to distinguish between martensite and bainite, but in this embodiment, iron carbides are not included in the volume fraction of the metal structure.
[0046]
　Whether or not ferrite or pearlite is present as the residual structure is confirmed with an optical microscope or a scanning electron microscope. If ferrite or pearlite is present, the area fractions of these are obtained, and the value is directly converted into the volume fraction to obtain the volume fraction of the remaining structure. However, in many cases, the residual structure of the steel member according to the present embodiment is hardly observed.
　For the volume fraction of the residual structure, a measurement sample is cut out from a cross section at a position 100 mm away from the end of the steel member, and used as a measurement sample for observing the residual structure. If the measurement sample cannot be collected from a position 100 mm away from the end due to the shape of the steel member, the measurement sample may be collected from the soaking portion avoiding the end. The observation range with an optical microscope or a scanning electron microscope is 40,000 μm 2 or more in area , the magnification is 500 to 1000 times, and the observation position is 1/4 of the plate thickness. The cut-out measurement sample is mechanically polished and then mirror-finished. Next, etching is performed with a Nital corrosive solution (a mixed solution of nitric acid and ethyl or methyl alcohol) to reveal ferrite and pearlite, and the presence of ferrite or pearlite is confirmed by observing this under a microscope. A structure in which ferrite and cementite are alternately arranged in layers is discriminated as pearlite, and a structure in which cementite is precipitated in particles is discriminated as bainite. The total volume fraction of the observed ferrite and pearlite is obtained, and the value is directly converted into the volume fraction to obtain the volume fraction of the residual structure.
[0047]
　In the present embodiment, the volume fractions of martensite and bainite, the residual austenite volume fraction, and the volume fraction of the residual tissue are measured by different measurement methods, so that the total of the above three volume fractions is 100. It may not be 0.0%. If the total of the three volume fractions does not reach 100.0%, the three volume fractions may be adjusted so that the total becomes 100.0%. For example, if the volume fraction of martensite and baynite, the volume fraction of retained austenite, and the volume fraction of the residual tissue are 101.0%, the total is measured to be 100.0%. The value obtained by multiplying the obtained volume fraction of each tissue by 100.0 / 101.0 may be used as the volume fraction of each tissue.
　If the volume fraction of martensite and bainite, the volume fraction of retained austenite, and the volume fraction of the residual tissue are less than 95.0% or more than 105.0%, the volume is again. Measure the fraction.
[0048]
　The presence position of retained austenite is confirmed by using TEM.
　In the martensite in the metal structure of the steel member according to the present embodiment, a plurality of packets are present in the old austenite grains, and inside each packet, a block having a parallel strip-like structure is present, and further, each block has a block. There is a set of laths, which are martensite crystals with almost the same crystal orientation. When the laths are confirmed by TEM, the selected area diffraction pattern is measured near the boundary between the laths, the electron diffraction pattern near the boundary between the laths is confirmed, and the electron beam diffraction pattern of the face-to-center cubic lattice is detected. Determine that retained austenite is present between the laths. Since the lath is a body-centered cubic lattice and the retained austenite is a face-centered cubic lattice, it can be easily identified by electron diffraction.
[0049]
　Further, the bainite in the metal structure of the steel member according to the present embodiment exists in a state in which a plurality of bainitic ferrite crystal grains are aggregated. The crystal grains of bainitic ferrite are confirmed by TEM, and the selected area diffraction pattern is measured near the grain boundaries of the bainitic ferrite crystal grains to obtain the electron diffraction pattern near the grain boundaries of the bainitic ferrite crystal grains. When the electron diffraction pattern of the face-to-center cubic lattice is detected, it is determined that retained austenite is present between the bainitics. Since bainitic ferrite is a body-centered cubic lattice and retained austenite is a face-centered cubic lattice, it can be easily identified by electron diffraction.
[0050]
　Further, the former austenite grain boundaries are present in the metal structure of the steel member according to the present embodiment. When the selected area diffraction pattern is measured near the former austenite grain boundary to confirm the electron beam diffraction pattern near the former austenite grain boundary and the electron beam diffraction pattern of the face-centered cubic lattice is detected, it remains in the former austenite grain boundary. Determine that austenite is present. Since martensite or bainite in the body-centered cubic lattice exists in the vicinity of the prior austenite grain boundaries, the retained austenite in the face-centered cubic lattice can be easily identified by electron diffraction.
[0051]
　The maximum minor axis of retained austenite is measured by the following method.
　First, the thin film sample is collected from a position 100 mm away from the end of the steel member (if the test piece cannot be collected from the position, a heat equalizing portion avoiding the end) and a position at a depth of 1/4 of the plate thickness. This thin film sample is magnified 50,000 times with a transmission electron microscope, 10 fields of view are randomly observed (1 field of view is 1.0 μm × 0.8 μm), and retained austenite is identified using an electron beam diffraction pattern. .. Of the retained austenites identified in each field of view, the minor axis of the "maximum retained austenite" is measured, three "minor diameters" are selected from the 10 visual fields in descending order, and the average value thereof is calculated. To obtain the "maximum minor axis of retained austenite". Here, the "maximum retained austenite" measures the cross-sectional area of ​​the retained austenite crystal grains identified in each visual field, determines the circle-equivalent diameter of the circle having the cross-sectional area, and indicates the maximum circle-equivalent diameter. Is defined as. In addition, the "minor diameter" of retained austenite is the distance between the parallel lines when assuming two parallel lines that are in contact with the contour of the crystal grains and sandwich the crystal grains with respect to the crystal grains of retained austenite identified in each visual field. It is defined as the shortest interval (minimum ferre diameter) of parallel lines when parallel lines are drawn so as to be the shortest distance.
[0052]
(C) Carbide A carbide having a
　circle-equivalent diameter of 0.1 μm or more and an aspect ratio of 2.5 or less: 4.0 × 10 3 pieces / mm 2 or less When a
　material steel sheet is heat-treated, it is generally present in the material steel sheet. Sufficient hardenability can be ensured by the re-solid solution of carbides. However, if coarse carbides are present in the material steel sheet and the carbides are not sufficiently re-solidified, sufficient hardenability cannot be ensured and low-strength ferrite is precipitated. Therefore, the smaller the amount of coarse carbides in the material steel sheet, the better the hardenability, and the higher the strength of the steel member after the heat treatment can be obtained.
　If a large amount of coarse carbide is present in the material steel sheet, not only the hardenability is deteriorated, but also a large amount of carbide remains in the steel member (residual carbide). Since a large amount of this residual carbide is deposited at the old γ grain boundaries, the old γ grain boundaries are embrittled. Further, if the amount of residual carbide is excessive, the residual carbide becomes a void starting point at the time of deformation, and the connection becomes easy, so that the ductility of the steel member, particularly the local elongation, is lowered, and as a result, the collision safety is deteriorated.
[0053]
　In particular, when the number density of carbides having a circle-equivalent diameter of 0.1 μm or more in a steel member exceeds 4.0 × 10 3 pieces / mm 2 , the toughness and ductility of the steel member deteriorate. Therefore, the number density of carbides having a circle-equivalent diameter of 0.1 μm or more in the steel member is 4.0 × 10 3 pieces / mm 2 or less. It is preferably 3.5 × 10 3 pieces / mm 2 or less.
[0054]
　Even in the material steel sheet before the heat treatment, it is preferable that there are few coarse carbides. In the present embodiment, the number density of carbides having a circle-equivalent diameter of 0.1 μm or more present in the material steel sheet is preferably 8.0 × 10 3 pieces / mm 2 or less.
[0055]
　The carbides in the steel member and the material steel sheet refer to granular ones, and specifically, those having an aspect ratio of 2.5 or less are targeted. The composition of the carbide is not particularly limited. Examples of carbides include iron-based carbides, Nb-based carbides and Ti-based carbides.
　Further, since the carbides having a size of less than 0.1 μm do not have a great influence on the ductility, particularly the local elongation, the size of the carbides subject to the number limitation is set to 0.1 μm or more in this embodiment.
[0056]
　The number density of carbides is determined by the following method.
　The test piece is cut out from a position 100 mm away from the end of the steel member (if the test piece cannot be collected from the position, a heat equalizing part avoiding the end) or a 1/4 part of the plate width of the material steel plate. After mirror-processing the observation surface of the test piece, it is corroded with a picral solution, magnified 10,000 times with a scanning electron microscope, and randomly 10 fields of view (1 field of view is 10 μm × 8 μm) at 1/4 of the plate thickness. ) Is observed. At this time, by counting all the number of carbides having a circle equivalent diameter of 0.1 μm or more and an aspect ratio of 2.5 or less and calculating the number density with respect to the total viewing area, the circle equivalent diameter is 0.1 μm or more and Obtain the number density of carbides with an aspect ratio of 2.5 or less.
[0057]
(D) Mechanical Properties of Steel Member The steel member according to
　this embodiment can obtain high ductility by the TRIP effect utilizing the work-induced transformation of retained austenite. However, if retained austenite is transformed with low strain, high ductility due to the TRIP effect cannot be expected. That is, in order to further increase ductility, it is preferable to control not only the amount and size of retained austenite but also its properties.
[0058]
　When the value of the strain-induced transformation parameter k represented by the following equation (1) becomes large, the retained austenite is transformed with low strain. Therefore, it is preferable that the value of the strain-induced transformation parameter k is less than 18.0.
[0059]
　k = (logf γ0- logf γ (0.02)) / 0.02 Equation (1)
　However, the meaning of each symbol in the above equation (1) is as follows.
　f γ0 : Volume fraction of retained austenite present in the steel member before applying true strain
　f γ (0.02): Steel member after applying true strain of 0.02 to the steel member and removing it. Volume fraction of retained austenite present
　in the log in the above equation (1) is a logarithm having a base of 10, that is, a common logarithm.
[0060]
　f [gamma] 0 , f gamma volume fraction of retained austenite present in the steel member for (0.02) is measured by X-ray diffraction method described above.
　It is considered that the amount of solid solution C in the retained austenite controls whether or not the retained austenite is easily transformed when strain is applied, and the range of the Mn content in the steel member according to the present embodiment. Then, there is a positive correlation between the volume fraction of retained austenite and the amount of solid solution C in retained austenite. Then, for example, when the amount of solid solution C in retained austenite is about 0.8%, the value of k is about 15 and shows excellent ductility, but when the amount of solid solution C in retained austenite is about 0.2%. If there is, the value of k is about 53, so that all the retained austenite is transformed with low strain, the ductility is lowered, and as a result, the collision safety is deteriorated.
[0061]
　The steel member according to this embodiment preferably has a tensile strength of 1400 MPa or more and a total elongation of 10.0% or more. Further, it is more preferable that the impact value at −80 ° C. is 25.0 J / cm 2 or more while having these characteristics . Fuel economy and collision safety by having high tensile strength of 1400 MPa or more, excellent ductility of 10.0% or more in total elongation, and excellent impact value of 25.0 J / cm 2 or more at -80 ° C. This is because it is possible to meet the demand for both.
[0062]
　In order to achieve excellent ductility and improve collision safety, it is effective to increase the total elongation. The total elongation is the sum of the uniform elongation (uniform elongation) until the constriction occurs and the local elongation until the fracture after that in the tensile test. In the present embodiment, from the viewpoint of further improving collision safety, it is preferable to increase not only uniform elongation but also local elongation. From the viewpoint of further improving collision safety, the local elongation is preferably 3.0% or more.
[0063]
　In the present embodiment, ASTM E8-69 (ANNUAL BOOK OF ASTM STANDARD, PART10, AMERICAN SOCIETY FOR TESTING AND MATERIALS) is used to measure mechanical properties including the strain-induced transformation parameters k, tensile strength, total elongation and local elongation. , P120-140), use the half-size plate-shaped test piece specified. Specifically, the tensile test was carried out in accordance with the regulations of ASTM E8-69, and was performed on a plate-shaped test piece having a thickness of 1.2 mm, a parallel portion length of 32 mm, and a parallel portion plate width of 6.25 mm. Then, a room temperature tensile test is performed at a strain rate of 3 mm / min, and the maximum strength (tensile strength) is measured. In addition, a 25 mm rule is placed in advance in the parallel portion of the tensile test, and the elongation rate (total elongation) is measured by matching the fractured samples. Then, the plastic strain (uniform elongation) at the maximum strength is subtracted from the total elongation to obtain the local elongation.
[0064]
　The Charpy impact test for measuring the impact value shall be carried out in accordance with the provisions of JIS Z 2242: 2005. The steel member is ground to a thickness of 1.2 mm, a test piece having a length of 55 mm and a width of 10 mm is cut out in parallel with the rolling direction, and three pieces are laminated to prepare a test piece having a V notch. The V notch has an angle of 45 °, a depth of 2 mm, and a notch bottom radius of 0.25 mm. Perform a Charpy impact test at a test temperature of -80 ° C to determine the impact value.
[0065]
(E) Mn segregation degree of steel member Mn segregation
　degree α: 1.6 or less At the
　center of the cross section of the steel member (1/2 of the plate thickness), Mn is concentrated due to central segregation. When Mn is concentrated in the center of the plate thickness, MnS is concentrated in the center of the plate thickness as inclusions, and hard martensite is likely to be formed, which causes a difference in hardness from the surroundings and deteriorates the toughness of the steel member. May be done. In particular, if the value of the Mn segregation degree α represented by the following formula (2) exceeds 1.6, the toughness of the steel member may deteriorate. Therefore, in order to further improve the toughness of the steel member, the value of the Mn segregation degree α of the steel member may be set to 1.6 or less. In order to further improve the toughness, the value of Mn segregation degree α may be set to 1.2 or less. The lower limit does not need to be specified, but the lower limit may be 1.0.
[0066]
　Mn segregation degree α = [maximum Mn concentration (mass%) at 1/2 part of plate thickness] / [average Mn concentration (mass%) at 1/4 part of plate thickness] ... Equation (2)
[0067]
　The Mn segregation degree α is mainly controlled by the chemical composition, particularly the impurity content, and the conditions of continuous casting, and the value of the Mn segregation degree α does not change significantly by heat treatment or hot forming. By setting the Mn segregation degree α value of the steel sheet to 1.6 or less, the Mn segregation degree α value of the steel member after the heat treatment can also be set to 1.6 or less, that is, the toughness of the steel member is further increased. It is possible to improve.
[0068]
　The maximum Mn concentration at 1/2 part of the plate thickness and the average Mn concentration at 1/4 part of the plate thickness are obtained by the following methods.
　The observation surface is parallel to the rolling direction from a position 100 mm away from the end of the steel member (if the test piece cannot be collected from that position, the heat equalizing part avoiding the end) or the plate width 1/2 of the material steel plate. And cut out the sample so that it is parallel to the plate thickness direction. Using an electron probe microanalyzer (EPMA), line analysis (1 μm) was performed at 10 locations at random in the rolling direction at 1/2 part of the sample plate thickness, and 3 measured values ​​were selected from the analysis results in descending order of Mn concentration. By calculating the average value, the maximum Mn concentration at 1/2 part of the plate thickness can be obtained. In addition, the average Mn concentration at 1/4 part of the plate thickness is also analyzed at 10 points in 1/4 part of the plate thickness of the sample using EPMA, and the average value is calculated to 1/4 part of the plate thickness. The average Mn concentration in the above can be obtained.
[0069]
(F) Cleanliness of steel member Cleanliness
　: 0.100% or less If a
　large amount of A-based inclusions, B-based inclusions and C-based inclusions described in JIS G 0555: 2003 is present in the steel member, the steel member Toughness may deteriorate. This is because crack propagation easily occurs as the amount of these inclusions increases. In particular, in the case of a steel member having a tensile strength of 1400 MPa or more, it is preferable to keep the abundance ratio of these inclusions low. If the cleanliness value of the steel specified in JIS G 0555: 2003 exceeds 0.100%, it may be difficult to secure sufficient toughness for practical use due to the large amount of inclusions. Therefore, the cleanliness value of the steel member is preferably 0.100% or less. In order to further improve the toughness of the steel member, it is more preferable that the cleanliness value is 0.060% or less. The value of the cleanliness of the steel is a calculation of the area percentage occupied by the above-mentioned A-based inclusions, B-based inclusions and C-based inclusions.
[0070]
　Since the cleanliness value does not change significantly due to heat treatment or hot forming, the cleanliness value of the steel member is also 0.100 by setting the cleanliness value of the material steel sheet to 0.100% or less. It can be less than%.
[0071]
　In the present embodiment, the cleanliness value of the material steel plate or steel member is obtained by the point calculation method described in Annex 1 of JIS G 0555: 2003. For example, the sample is cut out from a plate width of 1/4 of the material steel plate or a position 100 mm away from the end of the steel member (if the test piece cannot be collected from the position, the heat equalizing part avoiding the end). A quarter of the plate thickness of the observation surface is magnified 400 times with an optical microscope, A-based inclusions, B-based inclusions and C-based inclusions are observed, and their area percentages are calculated by a point calculation method. Observations are randomly performed in 10 fields of view (1 field of view is 200 μm × 200 μm), and the value with the highest cleanliness value (lowest cleanliness) among all fields of view is the cleanliness of the material steel plate or steel member. Use as a value.
[0072]
　Although the steel member according to the present embodiment has been described above, the shape of the steel member is not particularly limited. A flat plate may be used, but in particular, a hot-formed steel member is often a molded body, and in the present embodiment, the steel member is referred to as a “steel member” including the case where it is a molded body.
[0073]
　Next, a method for manufacturing a steel member according to the present embodiment will be described.
　The steel member according to the present embodiment has the above-mentioned chemical composition, and has a circle-equivalent diameter of 0.1 μm or more and an aspect ratio of 2.5 or less, and has a number density of carbides of 8.0 × 10 3 pieces / mm. It can be produced by subjecting a material steel sheet having a value of 2 or less and an average value of (Nb, Ti) C equivalent to a circle of 5.0 μm or less to a heat treatment described later.
[0074]
　The reason for limiting the precipitation form of carbides in the material steel sheet to be heat-treated as described above is as follows.
　As described above, it is necessary to reduce the precipitation of coarse carbides in the steel member in order to suppress the decrease in ductility of the steel member, but it is preferable that the material steel sheet before the heat treatment also has a small amount of coarse carbides. Therefore, in the present embodiment, the number density of carbides having a circle-equivalent diameter of 0.1 μm or more and an aspect ratio of 2.5 or less existing in the material steel sheet is 8.0 × 10 3 pieces / mm 2 or less. The number density of carbides in the material steel sheet may be measured by cutting out a test piece from a quarter part from the widthwise end portion of the material steel sheet and measuring it by the same method as that for a steel member.
[0075]
　Further, among various carbides, when coarse (Nb, Ti) C is contained in the material steel sheet, the ductility of the steel member after the heat treatment, particularly the local elongation is lowered, and as a result, the collision safety is deteriorated. In addition, (Nb, Ti) C refers to Nb-based carbide and Ti-based carbide.
　In particular, when the average value of the circle-equivalent diameters of (Nb, Ti) C present in the material steel sheet exceeds 5.0 μm, the ductility of the steel member after the heat treatment deteriorates. Therefore, the average value of the circle-equivalent diameters of (Nb, Ti) C present in the material steel sheet is 5.0 μm or less.
　The method for obtaining the average value of the circle-equivalent diameters of (Nb, Ti) C is as follows. A cross section is cut out from the plate width 1/4 part of the material steel plate, the observation surface of the sample is mirror-polished, and then magnified 3000 times with a scanning electron microscope, and 10 visual fields (1 visual field is 40 μm × 30 μm) are randomly arranged. Make an observation. For all the observed (Nb, Ti) C, calculate the area of ​​each (Nb, Ti) C, and let the diameter of the circle having the same area as this area be the equivalent circle diameter of each (Nb, Ti) C. .. By calculating the average value of those circle-equivalent diameters, the average value of the circle-equivalent diameters of (Nb, Ti) C is obtained.
[0076]
　Next, a method for manufacturing the material steel sheet will be described.
(H) Method for Manufacturing Material
　Steel Sheet There is no particular limitation on the manufacturing conditions of the material steel sheet, which is the steel sheet before heat treatment of the steel member according to the present embodiment. However, by using the production method shown below, it is possible to produce a material steel sheet in which the precipitation form of carbides is controlled as described above. In the following manufacturing method, for example, continuous casting, hot rolling, pickling, cold rolling and annealing treatment are performed.
[0077]
　After melting steel having the above chemical composition in a furnace, a slab is produced by casting. At this time, in order to suppress the concentrated precipitation of MnS, which is the starting point of delayed fracture, it is desirable to perform a central segregation reduction treatment for reducing the central segregation of Mn. Examples of the central segregation reduction treatment include a method of discharging molten steel in which Mn is concentrated in the unsolidified layer before the slab is completely solidified.
　Specifically, by performing treatments such as electromagnetic agitation and reduction of the unsolidified layer, molten steel in which Mn is concentrated before complete solidification can be discharged.
[0078]
　In order to reduce the cleanliness of the material steel sheet to 0.100% or less, when continuously casting molten steel, the superheat temperature of the molten steel (molten steel superheat temperature) is set to a temperature 5 ° C. or higher higher than the liquidus temperature of the steel. Moreover, it is desirable to suppress the cast amount of molten steel per unit time to 6 t / min or less.
[0079]
　If the molten steel superheat temperature is less than 5 ° C higher than the liquidus temperature during continuous casting, the viscosity of the molten steel becomes high, and inclusions are less likely to float in the continuous casting machine, resulting in inclusions in the slab. It increases and the cleanliness cannot be reduced sufficiently. Furthermore, when the casting amount of molten steel exceeds 6 t / min per unit time, the molten steel flows quickly in the mold, so that inclusions are easily trapped in the solidified shell, and inclusions in the slab increase to improve cleanliness. Is likely to get worse.
　On the other hand, by setting the molten steel superheat temperature to a temperature 5 ° C. or higher higher than the liquidus temperature and the molten steel casting amount per unit time to be 6 t / min or less, inclusions are less likely to be carried into the slab. As a result, the amount of inclusions at the stage of producing the slab can be effectively reduced, and the cleanliness of the material steel sheet of 0.100% or less can be easily achieved.
[0080]
　When the molten steel is continuously cast, the molten steel superheat temperature of the molten steel is preferably 8 ° C. or higher higher than the liquidus temperature, and the molten steel casting amount per unit time is preferably 5 t / min or less. By setting the molten steel superheat temperature to a temperature 8 ° C. or higher higher than the liquidus temperature and setting the molten steel casting amount per unit time to 5 t / min or less, the cleanliness of the material steel sheet can be set to 0.060% or less. It is preferable because it becomes easy.
[0081]
　The slab obtained by the above method may be subjected to soaking (heat equalizing) treatment, if necessary. By performing the soaking treatment, the segregated Mn can be diffused and the degree of Mn segregation can be reduced. When the soaking treatment is performed, the preferable heat equalizing temperature is 1150 to 1300 ° C., and the preferable heat equalizing time is 15 to 50 hours.
[0082]
　The slab obtained by the above method is hot-rolled.
　The slab is heated at 1200 ° C. or higher to melt the coarse (Nb, Ti) C and subjected to hot rolling. Further, from the viewpoint of more uniform formation of carbides, it is preferable that the hot rolling start temperature is 1000 to 1300 ° C. and the hot rolling completion temperature is 950 ° C. or higher.
[0083]
　The take-up temperature after hot rolling is preferably high from the viewpoint of workability, but if it is too high, the yield will decrease due to scale formation, so it is preferably 450 to 700 ° C. Further, when the winding temperature is lowered, the carbides are more likely to be finely dispersed and the coarsening of the carbides can be suppressed.
[0084]
　The morphology of the carbide can be controlled by adjusting the subsequent annealing conditions in addition to the conditions in hot rolling. In this case, it is desirable to raise the annealing temperature to a high temperature, dissolve the carbides once in the annealing step, and then transform them at a low temperature. Since the carbide is hard, its form does not change in cold rolling, and its existing form after hot rolling is maintained even after cold rolling.
[0085]
　The material steel sheet according to the present embodiment may be a hot-rolled steel sheet or a hot-rolled annealed steel sheet, a cold-rolled steel sheet or a cold-rolled annealed steel sheet, or a surface-treated steel sheet such as a plated steel sheet. The processing step may be appropriately selected according to the required level of plate thickness accuracy of the product and the like. The hot-rolled steel sheet that has been descaled is annealed as necessary to obtain a hot-rolled annealed steel sheet. The hot-rolled steel sheet or the hot-rolled annealed steel sheet is cold-rolled as necessary to obtain a cold-rolled steel sheet, and the cold-rolled steel sheet is annealed as necessary to obtain a cold-rolled annealed steel sheet. When the steel sheet to be subjected to cold rolling is hard, it is preferable to perform annealing before cold rolling to improve the workability of the steel sheet to be subjected to cold rolling.
[0086]
　Cold rolling may be carried out by a usual method. From the viewpoint of ensuring good flatness, the cumulative rolling reduction in cold rolling is preferably 30% or more. On the other hand, in order to prevent the load from becoming excessive, the cumulative rolling reduction in cold rolling is preferably 80% or less.
[0087]
　When a hot-rolled annealed steel sheet or a cold-rolled annealed steel sheet is manufactured as a material steel sheet, the hot-rolled steel sheet or the cold-rolled steel sheet is annealed. In annealing, for example, a hot-rolled steel sheet or a cold-rolled steel sheet is held in a temperature range of 550 to 950 ° C.
[0088]
　By setting the temperature held by annealing to 550 ° C. or higher, the difference in characteristics due to the difference in hot-rolling conditions is reduced regardless of whether the hot-rolled annealed steel sheet or the cold-rolled annealed steel sheet is manufactured, and after quenching. The characteristics of can be made more stable. Further, by setting the temperature held by annealing of the cold-rolled steel sheet to 550 ° C. or higher, the cold-rolled steel sheet is softened by recrystallization, so that workability can be improved. That is, a cold-rolled annealed steel sheet having good workability can be obtained. Therefore, regardless of whether the hot-rolled annealed steel sheet or the cold-rolled annealed steel sheet is manufactured, the temperature held by annealing is preferably 550 ° C. or higher.
[0089]
　On the other hand, if the temperature held by annealing exceeds 950 ° C., the structure may become coarse-grained. Coarse-grained tissue may reduce toughness after quenching. Further, even if the temperature held by annealing exceeds 950 ° C., the effect of raising the temperature cannot be obtained, the cost increases, and the productivity only decreases. Therefore, regardless of whether the hot-rolled annealed steel sheet or the cold-rolled annealed steel sheet is manufactured, the temperature held by annealing is preferably 950 ° C. or lower.
[0090]
　After annealing, it is preferable to cool to a temperature range of 550 ° C. or lower at an average cooling rate of 3 to 20 ° C./s. By setting the average cooling rate to 3 ° C./s or higher, the formation of coarse pearlite and coarse cementite can be suppressed, and the characteristics after quenching can be improved. Further, by setting the average cooling rate to 20 ° C./s or less, it becomes easy to suppress the occurrence of strength unevenness and stabilize the material of the hot-rolled annealed steel sheet or the cold-rolled annealed steel sheet.
　The average cooling rate at the time of annealing is a value obtained by dividing the temperature drop width of the steel sheet from the end of annealing to 550 ° C by the time required from the end of annealing to 550 ° C.
[0091]
　In the case of a plated steel sheet, the plating layer may be an electroplating layer, a hot-dip galvanizing layer, or an alloyed hot-dip galvanizing layer. Examples of the electroplating layer include an electrogalvanizing layer and an electric Zn—Ni alloy plating layer. Examples of the hot-dip plating layer include a hot-dip aluminum plating layer, a hot-dip Al-Si plating layer, a hot-dip Al-Si-Mg plating layer, a hot-dip zinc plating layer, and a hot-dip Zn-Mg plating layer. The alloyed hot-dip galvanized layer includes an alloyed hot-dip aluminum plating layer, an alloyed hot-dip Al-Si plating layer, an alloyed hot-dip Al-Si-Mg plating layer, an alloyed hot-dip zinc plating layer, and an alloyed hot-dip Zn-Mg plating layer. Etc. are exemplified. The plating layer may contain Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti and the like. The amount of adhesion of the plating layer is not particularly limited, and may be, for example, a general amount of adhesion. Similar to the material steel sheet, the steel member after the heat treatment may be provided with a plating layer or an alloyed plating layer.
[0092]
　In this embodiment, a steel sheet having a tensile strength of 1400 MPa or more cannot be used as a material steel sheet. This is because when such a steel sheet is used as a material steel sheet, the strength is high and cracks occur during the manufacture of the steel member.
[0093]
(I) Manufacturing Method
　of Steel Member Next, a manufacturing method of the steel member will be described.
　By heat-treating the above-mentioned material steel sheet through the temperature history as shown in FIG. 1, the volume fraction is 60.0 to 85.0% for martensite and 10.0 to 30.0 for bainite. % And retained austenite are 5.0 to 15.0%, the maximum minor axis length of the retained austenite is 30 nm or more, the equivalent circle diameter is 0.1 μm or more, and the aspect ratio is 2.5 or less. It is possible to obtain a steel member having a metal structure having a number density of 4.0 × 10 3 pieces / mm 2 or less, having high strength and excellent ductility.
[0094]
　The average heating rate described below is a value obtained by dividing the temperature rise width of the steel sheet from the start of heating to the end of heating by the time required from the start of heating to the end of heating.
　The first average cooling rate is the value obtained by dividing the temperature drop width of the steel sheet from the start of cooling (when taken out from the heating furnace) to the Ms point by the time required for cooling from the start of cooling to the Ms point. To do. The second average cooling rate is a value obtained by dividing the temperature drop width of the steel sheet from the Ms point to the end of cooling by the time from the Ms point to the end of cooling. The third average cooling rate is the temperature drop width of the steel sheet from the start of cooling (when taken out from the heating furnace) to the end of cooling after the reheating step is performed after the second cooling step, and the cooling end from the start of cooling. The value is divided by the time required until time.
[0095]
　"Heating step" The above material steel sheet is heated to a temperature range of
　Ac 3 points to (Ac 3 points + 200) ° C. at an average heating rate of 5 to 300 ° C./s (heating step). By this heating process, the structure of the material steel sheet is made into austenite single phase. As long as the average rate of temperature rise is within the above range, the material steel sheet at room temperature may be heated, or the material steel sheet cooled to 550 ° C. or lower by the cooling after annealing may be heated.
　If the average temperature rise rate in the heating step is less than 5 ° C / s, or if the temperature reached in the heating step exceeds (Ac 3 points + 200) ° C, the γ grains become coarse and the strength of the steel member after heat treatment deteriorates. There is a risk. In addition, austenite may not sufficiently remain in the first cooling step and the second cooling step, which will be described later, and the ductility and toughness of the steel member may deteriorate. On the other hand, when the average temperature rise rate exceeds 300 ° C./s in the heating step, the dissolution of carbides does not proceed sufficiently and the hardenability deteriorates, and ferrite and pearlite are precipitated in the first cooling step and the second cooling step described later. , The strength of the steel member deteriorates. If the ultimate temperature is less than 3 points of Ac , ferrite remains in the metal structure of the material steel sheet after the heating step, and austenite cannot be made into a single phase, and the strength of the steel member after heat treatment deteriorates. There is.
　In the present embodiment, deterioration of strength, ductility and toughness of the steel member can be prevented by carrying out the heating step satisfying the above conditions.
[0096]
　"First cooling step" From the temperature range of
　Ac 3 points to (Ac 3 points + 200) ° C. so that diffusion transformation does not occur, in other words, ferrite and pearlite do not precipitate on the material steel plate that has undergone the above heating step. Cooling to the Ms point (martensite transformation start point) at a first average cooling rate equal to or higher than the upper critical cooling rate (first cooling step).
　The upper critical cooling rate is the minimum cooling rate at which austenite is supercooled to produce martensite without precipitating ferrite or pearlite in the metal structure. When cooled below the upper critical cooling rate, ferrite is formed and the strength of the steel member is insufficient. Further, when cooled below the upper critical cooling rate, pearlite is generated and carbon is precipitated as carbides. Therefore, carbon can be concentrated in untransformed austenite in the second cooling step and the reheating step in the subsequent step. It cannot be done, and the ductility and toughness of the steel member are insufficient.
[0097]
　The Ac 3 point, Ms point and upper critical cooling rate are measured by the following methods.
　A test piece having a width of 30 mm and a length of 200 mm is cut out from the material steel sheet having the above-mentioned chemical components. This test piece is heated to 1000 ° C. at a heating rate of 10 ° C./sec in a nitrogen atmosphere, held at that temperature for 5 minutes, and then cooled to room temperature at various cooling rates. The cooling rate is set at intervals of 10 ° C./sec from 1 ° C./sec to 100 ° C./sec. Ac 3 points and Ms points are measured by measuring the thermal expansion change of the test piece during heating and cooling .
　Further, as the upper critical cooling rate, the lowest cooling rate at which the ferrite phase is not precipitated is defined as the upper critical cooling rate among the test pieces cooled at the above various cooling rates.
[0098]
"Second cooling step" After
　the first cooling step (cooling to the Ms point at the first average cooling rate higher than the upper critical cooling rate), the temperature range from (Ms-30) to (Ms-70 ° C) is 5 ° C./s. As described above, cooling is performed at a second average cooling rate that is less than 150 ° C./s and slower than the first average cooling rate (second cooling step).
[0099]
　In the second cooling step of cooling the temperature range below the Ms point, cooling is performed at a second average cooling rate of 5 ° C./s or more and less than 150 ° C./s, which is slower than the first average cooling rate. It is important that the stop temperature is in the temperature range of (Ms-30) to (Ms-70) ° C. By this second cooling step, retained austenite having a maximum minor axis of 30 nm or more, which greatly contributes to the improvement of ductility and toughness of steel members, is formed between martensite laths, between bainitic ferrites, or at the old γ grain boundaries. be able to. In addition, in the temperature range below the Ms point, the supersaturated solid solution carbon from a part of the generated martensite is diffused and concentrated in untransformed austenite by the second cooling step, and it is difficult to transform with respect to plastic deformation. A stable retained austenite with a value of less than 18 can be produced.
[0100]
　In the second cooling step, when the second average cooling rate is less than 5 ° C./s, carbon is excessively concentrated in the untransformed austenite around martensite generated just below the Ms point, and carbon is precipitated as carbide. As a result, carbon is not sufficiently diffused to the entire untransformed austenite, and retained austenite cannot be secured between martensite laths, between bainitic ferrites or at the old γ grain boundaries, and the amount is not sufficient. , Insufficient ductility and toughness of steel members.
　When the second average cooling rate is 150 ° C./s or more, the time for carbon to diffuse into untransformed austenite is not sufficient, and martensite is formed one after another next to each other. As a result, the width of retained austenite between martensite becomes small (the maximum minor axis of retained austenite is less than 30 nm), and the amount is insufficient, resulting in insufficient ductility and toughness of the steel member.
[0101]
　In the second cooling step, when the cooling stop temperature is less than (Ms-70) ° C., a large amount of martensite is generated and the amount of retained austenite is insufficient, and the maximum minor axis of the retained austenite becomes small, so that the steel member Insufficient ductility. The cooling stop temperature is preferably more than 250 ° C, more preferably 300 ° C or higher.
　When the cooling stop temperature exceeds (Ms-30) ° C., only a small amount of martensite is generated, so that the amount of C that concentrates from martensite to untransformed austenite is insufficient. As a result, in the reheating step, which is a subsequent step, similarly, the amount of C that concentrates from martensite to untransformed austenite is insufficient, so that stable retained austenite cannot be secured, and martensite is again in the third cooling process described later. Due to the site generation, the ductility and toughness of the steel member is insufficient.
[0102]
"Reheating step" and "third cooling step" After
　the second cooling step (cooling to the temperature range of (Ms-30) to (Ms-70) ° C. at the second average cooling rate), Ms to (Ms + 200) ° C. It is reheated to a temperature range at an average temperature rise rate of 5 ° C./s or more (reheating step), and then cooled at a third average cooling rate of 5 ° C./s or more (third cooling step).
[0103]
　The reheating step promotes the diffusion and concentration of carbon into untransformed austenite, which can increase the stability of retained austenite. When the temperature reached in the reheating step is less than the Ms point, carbon diffusion and concentration in untransformed austenite are not sufficient, the stability of retained austenite is lowered, and the ductility and toughness of the steel member are insufficient. If the ultimate temperature in the reheating step exceeds (Ms + 200) ° C., ferrite and pearlite are formed or bainite is excessively formed, so that the strength of the steel member is insufficient.
　In the reheating step, when the average heating rate from Ms to (Ms + 200) ° C. is less than 5 ° C./s, carbon is excessively concentrated in untransformed austenite, and the temperature range from Ms to (Ms + 200) ° C. Since the formation of bainite is suppressed and the volume fraction of bainite is reduced, the ductility and toughness of the steel member are insufficient.
[0104]
　In the third cooling step, when the third average cooling rate is less than 5 ° C./s, concentrated carbon is precipitated as carbide in the untransformed austenite, and the stability of the retained austenite becomes insufficient. Insufficient ductility and toughness.
[0105]
　As described above, by performing a heat treatment that satisfies the above conditions on the material steel sheet, it is possible to prevent the formation of ferrite and pearlite when cooling to the Ms point, and the retained austenite between the martensite trusses when cooling below the Ms point. It can be secured in the form of a maximum minor axis of 30 nm or more between the bainitic ferrite and the old γ grain boundary. Further, by reheating to the Ms point or higher after cooling, the diffusion of carbon from the previously produced martensite to the untransformed austenite is promoted, and the stability of the retained austenite is increased. This makes it possible to obtain a steel member having excellent strength and ductility.
[0106]
　A holding step may be performed between the heating step and the first cooling step of cooling to the Ms point. That is, after the heating step , the first cooling step may be performed after holding for 5 to 200 seconds in a temperature range of Ac 3 points to (Ac 3 points + 200) ° C.
　Specifically, from the viewpoint of improving the hardenability of steel by advancing austenite transformation and dissolving carbides after heating to a temperature range of Ac 3 points to (Ac 3 points + 200) ° C., the material steel sheet is Ac 3 It is preferable to keep the temperature range from point to (Ac 3 points + 200) ° C. for 5 s or more. Further, the holding time is preferably 200 s or less from the viewpoint of productivity.
[0107]
　Further, a holding step may be performed between the reheating step and the third cooling step. That is, after the reheating step, the third cooling step may be performed after holding the product in a temperature range of Ms to (Ms + 200) ° C. for 3 to 60 seconds. In the holding step, the steel sheet temperature may be varied in the temperature range of Ms to (Ms + 200) ° C., or the steel sheet temperature may be kept constant in the temperature range of Ms to (Ms + 200) ° C.
　Specifically, after reheating to the temperature range of Ms to (Ms + 200) ° C., the steel sheet is held in the temperature range of Ms to (Ms + 200) ° C. for 3 s or more from the viewpoint of diffusing carbon and increasing the stability of retained austenite. Is preferable. Further, this holding time is preferably 60 s or less from the viewpoint of productivity.
[0108]
　By performing the holding step between the reheating step and the third cooling step, the retained austenite can be further stabilized, the k value can be lowered, and the TRIP effect can be further enhanced. In the retention step, it is presumed that the release of carbon from martensite and the concentration of carbon in retained austenite are further promoted, and the retained austenite is more stabilized. When the temperature range of the holding step is less than the Ms point, the concentration of carbon in the retained austenite is not promoted.
[0109]
　The holding temperature in the holding steps before the first cooling step and before the third cooling step does not have to be constant, and may vary as long as it is within a predetermined temperature range.
[0110]
　Here, in the above series of heat treatments, after heating to the temperature range of Ac 3 points to (Ac 3 points + 200) ° C. (after the heating step) and before cooling to the Ms point (before the first cooling step), the hot stamping is performed. Such hot molding may be performed. Examples of hot forming include bending, drawing, overhanging, hole expansion, flange forming and the like. Further, if a means for cooling the material steel sheet is provided at the same time as or immediately after the forming, a forming method other than press forming, for example, roll forming may be performed. If the heat history described above is followed, hot forming may be repeated.
　Further, hot molding may be performed at the same time as the first cooling step. The hot forming may be performed simultaneously with the first cooling step, that is, the first cooling step of cooling at a cooling rate equal to or higher than the upper critical cooling rate may be performed, and at the same time, the material steel plate may be hot formed. In this case, since the forming is performed hot, the material steel sheet is in a soft state, so that it is possible to obtain a steel member with high dimensional accuracy, which is preferable.
[0111]
　The above series of heat treatments can be carried out by any method, and may be carried out by, for example, high-frequency heating quenching, energization heating, or furnace heating.
Example
[0112]
　Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0113]
　First, in manufacturing the heat-treated steel sheet member, the heat-treated steel sheet which is the material steel sheet was produced in the following manner.
[0114]
"Material
　Steel Sheet" Steels having the chemical components shown in Tables 1A and 1B were melted in a test converter and continuously cast by a continuous casting tester to prepare slabs having a width of 1000 mm and a thickness of 250 mm. At this time, in order to control the cleanliness of the material steel sheet, the superheat temperature of the molten steel and the amount of molten steel cast per unit time were adjusted.
[0115]
[Table 1A]

[0116]
[Table 1B]

[0117]
　The cooling rate of the slab was controlled by changing the amount of water in the secondary cooling spray zone. Further, the central segregation reduction treatment was carried out by using a roll in the final solidification portion to carry out light reduction with a gradient of 1 mm / m and discharging the concentrated molten steel in the final solidification portion. Some slabs were then soaked at 1250 ° C. for 24 hours.
[0118]
　The obtained slab was hot-rolled with a hot-rolling tester to obtain a hot-rolled steel sheet having a thickness of 3.0 mm. In the hot rolling step, descaling was performed after rough rolling, and finally finish rolling was performed. Then, the hot-rolled steel sheet was pickled in the laboratory. Further, cold rolling was performed with a cold rolling tester to obtain a cold-rolled steel sheet having a thickness of 1.4 mm, and a raw steel sheet was obtained.
[0119]
　The obtained material steel sheet was evaluated by the following methods for the number density of carbides, the average value of the diameters equivalent to circles of (Nb, Ti) C, the Mn segregation degree and the cleanliness.
　The Ac 3 points, Ms points and upper critical cooling rates shown in Tables 4A and 4B were determined by the following experiments.
[0120]
When
　determining the number density of carbides with a circle-equivalent diameter of 0.1 μm or more, a sample is cut out from a quarter of the plate width of the material steel plate, the observation surface is mirror-processed, and then the picral solution is applied. It was corroded by using it, magnified 10000 times with a scanning electron microscope, and 10 fields (1 field was 10 μm × 8 μm) and 1/4 part of the plate thickness were randomly observed. At this time, by counting all the number of carbides having a circle equivalent diameter of 0.1 μm or more and an aspect ratio of 2.5 or less and calculating the number density with respect to the total viewing area, the circle equivalent diameter is 0.1 μm or more and The number density of carbides having an aspect ratio of 2.5 or less was obtained.
[0121]
When calculating the mean value of circle-equivalent diameter of (Nb, Ti) C, a sample is cut out from the plate width 1/4 part of the material steel plate and its observation surface. Was mirror-processed and then magnified 3000 times with a scanning electron microscope, and 10 fields of view (1 field of view is 40 μm × 30 μm) and 1/4 part of the plate thickness were observed. By calculating the areas of all the observed (Nb, Ti) C, the diameter of the circle having the same area as this area is defined as the diameter equivalent to the circle of each (Nb, Ti) C, and the average value thereof is calculated. , (Nb, Ti) C, the average value of the equivalent circle diameter was obtained.
[0122]
The
　Mn segregation degree was measured by the following procedure. A sample is cut out from the plate width 1/2 part of the material steel plate so that the observation surface is parallel to the rolling direction, and the rolling direction and the plate thickness are taken at the plate thickness 1/2 part of the steel plate using an electron probe microanalyzer (EPMA). Line analysis (1 μm) was performed at 10 locations parallel to the direction. After selecting three measured values ​​in descending order from the analysis results, the average value was calculated to obtain the maximum Mn concentration at the center of the plate thickness. In addition, at the 1/4 depth position (1/4 part of the plate thickness) from the surface of the material steel plate, analysis was performed at 10 locations using EPMA in the same manner, the average value was calculated, and the plate thickness was calculated from the surface. The average Mn concentration at the 1/4 depth position was determined. Then, by dividing the maximum Mn concentration at the center of the plate thickness by the average Mn concentration at a depth of 1/4 of the plate thickness from the surface, the Mn segregation degree α ([[1/2 part of the plate thickness]) Maximum Mn concentration (mass%)] / [Average Mn concentration at 1/4 part of plate thickness (mass%)]) was determined.
[0123]
For
　cleanliness, a sample is cut out from 1/4 of the width of the material steel plate, and 1/4 of the thickness of the observation surface is magnified 400 times with an optical microscope for 10 fields of view (1 field of view is 200 μm × 200 μm). Was observed. Then, the area percentages of the A-based inclusions, the B-based inclusions and the C-based inclusions were calculated by the point calculation method described in Annex 1 of JIS G 0555: 2003. The value with the highest cleanliness value (lowest cleanliness) in multiple fields of view was taken as the cleanliness value of the material steel sheet.
[0124]

　The Ac 3 points and upper critical cooling rate of each steel type were measured by the following method.
　A strip test piece having a width of 30 mm and a length of 200 mm was cut out from the obtained material steel sheet, and the test piece was heated to 1000 ° C. at a heating rate of 10 ° C./sec in a nitrogen atmosphere and kept at that temperature for 5 minutes. After that, it was cooled to room temperature at various cooling rates. The cooling rate was set at intervals of 10 ° C./sec from 1 ° C./sec to 100 ° C./sec. By measuring the change in thermal expansion of the test piece during heating and cooling at that time, the Ac 3 points and the Ms points were measured.
　As the upper critical cooling rate, the lowest cooling rate at which the ferrite phase was not precipitated was defined as the upper critical cooling rate among the test pieces cooled at the above cooling rate.
[0125]
　As described above, since the average value of the circle-equivalent diameter of (Nb, Ti) C, the Mn segregation degree, and the cleanliness value do not change significantly by the heat treatment or the hot forming treatment performed later, the above-mentioned material. The average value of the circle-equivalent diameter of (Nb, Ti) C of the steel sheet, the Mn segregation degree α and the cleanliness are the average value of the circle-equivalent diameter of (Nb, Ti) C of the steel member, the Mn segregation degree α and the cleanliness. The value was set to.
[0126]
　Next, the obtained material steel plate was subjected to the heat treatments shown in the following [Example 1] to [Example 3] to prepare a steel member.
[0127]
[Example 1]
　Samples having a thickness of 1.4 mm, a width of 30 mm, and a length of 200 mm were collected from each of the above-mentioned material steel sheets. The sample was collected so that the longitudinal direction of the sample was parallel to the rolling direction.
　Next, the collected sample was heated to a temperature range of (Ac 3 points + 50) ° C. at an average temperature rise rate of 10 ° C./s and held for 120 seconds, and then up to the Ms point at a first average cooling rate equal to or higher than the upper critical cooling rate. After cooling, it is cooled to (Ms-50) ° C. at an average cooling rate (10 ° C./s) slower than the first average cooling rate, and then heated to (Ms + 75) ° C. at an average heating rate of 10 ° C./s. After that, a steel member was obtained by performing a heat treatment for cooling at an average cooling rate of 8 ° C./s.
　After that, a test piece is cut out from the soaking part of the obtained steel member, and tensile test, Charpy impact test, X-ray diffraction, optical microscope observation, and transmission electron microscope observation are performed by the following methods to obtain mechanical properties and metallographic structure. evaluated. The evaluation results are shown in Table 2A and Table 2B.
[0128]
The
　tensile test was carried out with a tensile tester manufactured by Instron in accordance with the regulations of ASTM standard E8-69. After grinding the sample of the steel member to a thickness of 1.2 mm, a half-size plate-shaped test piece (parallel portion length: 32 mm, parallel portion plate width: 6.25 mm) specified in ASTM standard E8-69 was collected. In the energizing heating device cooling device used in the heat treatment of this example, since the soaking portion obtained from a sample having a length of about 200 mm is limited, a half-size plate-shaped test piece of ASTM standard E8-69 should be adopted. And said.
　Then, a strain gauge (KFGS-5 manufactured by Kyowa Electric Co., Ltd., gauge length: 5 mm) was attached to each test piece, and a room temperature tensile test was performed at a strain rate of 3 mm / min to measure the maximum strength (tensile strength). In addition, a 25 mm ruled line was previously placed in the parallel portion of the tensile test, and the fractured samples were put together to measure the elongation rate (total elongation). Then, the local elongation was obtained by subtracting the plastic strain (uniform elongation) at the maximum strength from the total elongation.
　In this example, when the tensile strength is 1400 MPa or more, it is judged to be acceptable because it is excellent in strength, and when it is less than 1400 MPa, it is judged to be unacceptable because it is inferior in strength.
　Further, when the total elongation was 10.0% or more, it was determined to be acceptable as having excellent ductility, and when the total elongation was less than 10.0%, it was determined to be rejected as being inferior in ductility.
　Further, the product of the tensile strength and the total elongation (tensile strength TS × total elongation EL) is obtained, and when TS × EL is 14,000 MPa ·% or more, it is judged that the strength-ductility balance is excellent, and when it is less than 14,000 MPa ·%. It was judged to be inferior in strength-ductility balance. Further, when TS × EL was 16000 MPa ·% or more, it was evaluated that the strength-ductility balance was superior, and when it was 18,000 MPa ·% or more, it was evaluated that the strength-ductility balance was further excellent.
[0129]
The
　Charpy impact test was carried out in accordance with the provisions of JIS Z 2242: 2005. The steel member was ground to a thickness of 1.2 mm, a test piece having a length of 55 mm and a width of 10 mm was cut out, and three pieces were laminated to prepare a test piece having a V notch. The V notch had an angle of 45 °, a depth of 2 mm, and a notch bottom radius of 0.25 mm. A Charpy impact test was performed at a test temperature of −80 ° C., and the impact value was determined. In this example, a case having an impact value of 25.0 J / cm 2 or more was evaluated as having excellent toughness.
[0130]
In
　X-ray diffraction, first, a test piece is taken from the soaking portion of the steel member, and hydrofluoric acid and hydrogen peroxide solution are used to reach a depth of 1/4 part of the plate thickness from the surface. Chemically polished. The diffracted X-ray intensity of the face-centered cubic lattice (residual austenite) was measured by measuring the test piece after chemical polishing in the range of 45 ° to 105 ° at 2θ using a Co tube. The volume fraction of retained austenite (f γ0 ) was obtained by calculating the volume fraction of retained austenite from the area ratio of the obtained diffraction curve .
[0131]

　A sample of the steel member is processed into the same shape as the tensile test piece, a constant plastic strain (true strain: ε = 0.02) is applied, and the tensile test piece added or subtracted is used as described above. A test piece for X-ray diffraction was prepared, and the body integration rate (f γ (0.02)) of retained austenite was determined by the same method as the above-mentioned X-ray diffraction . From these, the strain-induced transformation parameter k represented by the following equation (i) was calculated and used as an index of high ductility due to the TRIP effect. The larger k is, the lower the strain is and the more the retained austenite is transformed. Therefore, prevention of constriction at high strain, that is, high ductility due to the TRIP effect cannot be expected.
[0132]
　k = (logf γ0- logf γ (0.02)) /0.02 ... (i)
　However, the meaning of each symbol in the above formula is as follows.
　f γ0 : Volume fraction of retained austenite present in the steel member before the true strain is applied
　f γ (0.02): The steel member after the true strain of 0.02 is applied to the steel member and added or removed. Volume fraction of retained austenite present in
[0133]
A
　cross section is cut out from the heat equalizing part of the steel member, the cross section is mirror-finished, and then corroded with a picral solution, and a 1/4 part of the plate thickness is magnified 10,000 times with a scanning electron microscope. Observation of 10 visual fields (1 visual field is 10 μm × 8 μm) was performed. At this time, the circle equivalent diameter is 0 by counting all the number of carbides having a circle equivalent diameter of 0.1 μm or more and an aspect ratio of 2.5 or less and calculating the number (number density) with respect to the total viewing area. A number density of carbides of 1 μm or more and an aspect ratio of 2.5 or less was obtained.
[0134]
A
　thin film sample was collected by thin film processing from the heat equalizing part of the steel member and the position of the plate thickness 1/4 depth. Next, a transmission electron microscope was used to magnify the image 50,000 times, and 10 visual fields were randomly observed (1 visual field is 1.0 μm × 0.8 μm). At this time, the retained austenite was identified using the electron diffraction pattern. The minor axis of the "maximum retained austenite" is measured in each field of view, three "minor diameters" are selected from the largest in the 10 visual fields, and the average value is calculated to calculate the "residual austenite" of the steel member. "Maximum minor axis" was obtained. Here, the "maximum retained austenite" measures the cross-sectional area of ​​the retained austenite crystal grains identified in each visual field, determines the circle-equivalent diameter of the circle having the cross-sectional area, and indicates the maximum circle-equivalent diameter. And said. In addition, the "minor diameter" of retained austenite is the distance between the parallel lines when assuming two parallel lines that are in contact with the contour of the crystal grains and sandwich the crystal grains with respect to the crystal grains of retained austenite identified in each visual field. The shortest interval (minimum ferre diameter) of parallel lines when parallel lines were drawn so as to be the shortest distance was used.
[0135]
The
　method for measuring the tissue fraction (volume fraction) of martensite and bainite and the presence position of retained austenite was as follows.
　The volume fractions of martensite and bainite were measured by the electron beam diffractometer attached to the TEM. A measurement sample was cut out from the heat equalizing part of the steel member and the position of the plate thickness 1/4 depth, and used as a thin film sample for TEM observation. The range of TEM observation was 400 μm 2 in area, and the magnification was 50,000 times. Martensite and iron carbides in bainite (Fe 3 to C), If the precipitation form of the iron carbide was three-way precipitation, it was judged to be martensite, and if it was one-way limited precipitation, it was judged to be bainite. The fractions of martensite and bainite measured by electron diffraction of TEM are measured as the area fractions, but since the steel member of this example has an isotropic metal structure, the value of the area fractions is used. It was replaced with the volume fraction as it was. Iron carbides were observed to distinguish between martensite and bainite, but iron carbides were not included in the volume fraction of the metallographic structure.
[0136]
　The volume fractions of ferrite and pearlite, which are the remaining structures, were measured by the following methods.
　A measurement sample was cut out from the soaking portion of the steel member and used as a measurement sample for observing the remaining structure. The observation range with a scanning electron microscope was 40,000 μm 2 in area , the magnification was 1000 times, and the measurement position was 1/4 of the plate thickness. The cut-out measurement sample was mechanically polished and then mirror-finished. Next, etching was performed with a Nital corrosive solution (a mixed solution of nitric acid and ethyl or methyl alcohol) to reveal ferrite and pearlite, and the presence of ferrite or pearlite was confirmed by observing this under a microscope. A structure in which ferrite and cementite were alternately arranged in layers was discriminated as pearlite, and a structure in which cementite was precipitated in particles was discriminated as bainite. The total volume fraction of the observed ferrite and pearlite was calculated, and the value was directly converted into the volume fraction to obtain the volume fraction of the residual structure.
[0137]
　The position of retained austenite was confirmed using an electron diffraction pattern obtained by TEM. In the martensite of the steel member, there are multiple packets in the old austenite grains, and inside each packet, there are blocks that are parallel strips, and in each block, martensite with almost the same crystal orientation. There is a set of laths that are crystals of. The laths were confirmed by TEM, and the selected area diffraction pattern was measured near the boundary between the laths to confirm the electron beam diffraction pattern near the boundary between the laths. When the electron diffraction pattern of the face-centered cubic lattice was detected, it was determined that retained austenite was present between the laths.
　In addition, the crystal grain structure of bainitic ferrite is confirmed by TEM, the selected area diffraction pattern is measured near the grain boundaries of the bainitic ferrite crystal grains, and the electron beam diffraction near the grain boundaries of the bainitic ferrite crystal grains is performed. I confirmed the pattern. When the electron diffraction pattern of the face-centered cubic lattice was detected, it was determined that retained austenite was present between the bainitic ferrites.
　Furthermore, the selected area diffraction pattern was measured near the old austenite grain boundaries to confirm the electron beam diffraction pattern near the old austenite grain boundaries. When the electron diffraction pattern of the face-centered cubic lattice was detected, it was determined that retained austenite was present at the former austenite grain boundaries.
[0138]
　As shown in Table 2A, Invention Examples B1 to B28 satisfying the scope of the present invention have good results in terms of both metallographic structure and mechanical properties. On the other hand, Comparative Examples b1 to b16, which did not satisfy the scope of the present invention in Table 2B, did not satisfy at least one of the metallographic structure and the mechanical properties.
　In all of Invention Examples B1 to B28 in Table 2A, the Mn segregation degree was 1.6 or less and the cleanliness was 0.100% or less. Further, in Invention Examples B1 to B28, retained austenite was present between martensite laths, between bainite bainitic ferrites, and at the former austenite grain boundaries.
[0139]
[Table 2A]

[0140]
[Table 2B]

[0141]

　Among the steel types shown in Table 1A, the steel No. When casting slabs having the chemical compositions of A26 and A27, the superheat temperature, casting rate (casting amount), and slab cooling rate were changed to change the Mn segregation degree and cleanliness of the slab. Then, the slab was subjected to hot rolling, pickling, and cold rolling in the same manner as described above, and then heat-treated under the same conditions as in Example 1 to produce a steel member.
　The evaluation results of the obtained steel members C1 to C10 are shown in Table 3. The evaluation method of each characteristic was carried out in the same manner as in Example 1.
[0142]
　Inventive Examples C1, C3 and C5 having a good Mn segregation degree of 1.6 or less and a cleanliness of 0.100% or less have impact values ​​and local elongation as compared with Invention Examples C2 and C4 manufactured from the same steel. Is even better. Further, Invention Examples C6, C8 and C10 having a good Mn segregation degree of 1.6 or less and a cleanliness of 0.100% or less have impact values ​​and local parts as compared with Invention Examples C7 and C9 manufactured from the same steel. The growth is even better.
　On the other hand, Invention Example C2 having a slightly higher Mn segregation degree has a slightly lower impact value and local elongation than Invention Examples C1, C3 and C5 manufactured from the same steel. Inventive Example C7 having a slightly higher Mn segregation degree has a slightly lower impact value and local elongation than Invention Examples C6, C8 and C10 manufactured from the same steel. Inventive Example C4, which has a slightly higher cleanliness, has a slightly lower impact value and local elongation than Invention Examples C1, C3, and C5 manufactured from the same steel. Inventive Example C9 having a slightly higher cleanliness has a slightly lower impact value and local elongation than C6, C8 and C10 manufactured from the same steel.
　In Invention Examples C1 to C10, retained austenite was present between martensite laths, between bainite bainitic ferrites, and at the former austenite grain boundaries.
[0143]
[Table 3]

[0144]

　Among the steel types shown in Table 1A, the steel No. The material steel sheets having the chemical compositions of A26 and A27 were subjected to the heat treatments shown in Tables 4A and 4B to produce steel members.
　The evaluation results of the metallographic structure and mechanical properties of the obtained steel member are shown in Tables 5A and 5B.
　Looking at Tables 4A to 5B, Invention Examples D1 to D28 satisfying the scope of the present invention have good results in terms of both metallographic structure and mechanical properties, but Comparative Examples d1 to d34 not satisfying the scope of the present invention have The result did not satisfy at least one of the metallographic structure and mechanical properties.
　In all of Invention Examples D1 to D28, the Mn segregation degree was 1.6 or less and the cleanliness was 0.100% or less, which were good. Further, in Invention Examples D1 to D28, retained austenite was present between martensite laths, between bainite bainitic ferrites, and at the former austenite grain boundaries.
[0145]
[Table 4A]

[0146]
[Table 4B]

[0147]
[Table 5A]

[0148]
[Table 5B]

Industrial applicability
[0149]
　According to the above aspect of the present invention, it is possible to obtain a steel member having a tensile strength of 1400 MPa or more and excellent ductility. The steel member according to the present invention is particularly suitable for use as a collision-resistant part of an automobile.
The scope of the claims
[Claim 1]
　The chemical composition is mass%,
C: 0.10 to 0.60%,
Si: 0.40 to 3.00%,
Mn: 0.30 to 3.00%,
P: 0.050% or less,
S : 0.0500% or less,
N: 0.010% or less,
Ti: 0.0010 to 0.1000%,
B: 0.0005 to 0.0100%,
Cr: 0 to 1.00%,
Ni: 0 to 2.0%,
Cu: 0 to 1.0%,
Mo: 0 to 1.0%,
V: 0 to 1.0%,
Ca: 0 to 0.010%,
Al: 0 to 1.00%,
Nb: 0 to 0.100%,
Sn: 0 to 1.00%,
W: 0 to 1.00%,
REM: 0 to 0.30%,
the balance is Fe and impurities, and the
　metal structure is , Martensite is 60.0 to 85.0%, baynite is 10.0 to 30.0%, retained austenite is 5.0 to 15.0%, and residual tissue is 0 to 4.0%. , and the
　maximum length of the minor axis of said residual austenite is at 30nm or more,
A steel member having a 　circle-equivalent diameter of 0.1 μm or more and an aspect ratio of 2.5 or less, and having a number density of carbides of 4.0 × 10 3 pieces / mm 2 or less
.
[Claim 2]
　The chemical composition is, in mass%,
Cr: 0.01 to 1.00%,
Ni: 0.01 to 2.0%,
Cu: 0.01 to 1.0%,
Mo: 0.01 to 1. 0%,
V: 0.01 to 1.0%,
Ca: 0.001 to 0.010%,
Al: 0.01 to 1.00%,
Nb: 0.010 to 0.100%,
Sn: 0 The steel member according to claim 1, which contains one or more of
0.01 to 1.00%, W: 0.01 to 1.00%, and
REM: 0.001 to 0.30%.
..
[Claim 3]
　The steel member according to claim 1 or 2, wherein the value of the strain-induced transformation parameter k represented by the following formula (1) is less than 18.0.
　k = (logf γ0- logf γ (0.02)) /0.02 ... Equation (1)
　However, the meaning of each symbol in the above equation (1) is as follows.
　f γ0 : Volume fraction of retained austenite present in the steel member before the true strain is applied
　f γ (0.02): The steel member after the true strain of 0.02 is applied to the steel member and unloaded. Volume fraction of retained austenite present in
[Claim 4]
　The steel member according to any one of claims 1 to 3, wherein the tensile strength is 1400 MPa or more and the total elongation is 10.0% or more.
[Claim 5]
　The steel member according to any one of claims 1 to 4, wherein the local elongation is 3.0% or more.
[Claim 6]
The steel member according to any one of claims 1 to 5, 　wherein the impact value at −80 ° C. is 25.0 J / cm 2 or more.
[Claim 7]
　The steel member according to any one of claims 1 to 6, wherein the cleanliness value of the steel specified in JIS G 0555: 2003 is 0.100% or less.
[Claim 8]
　The method for producing a steel member according to any one of claims 1 to 7, wherein the
　chemical composition is mass%,
C: 0.10 to 0.60%,
Si: 0.40 to 3.00. %,
Mn: 0.30 to 3.00%,
P: 0.050% or less,
S: 0.0500% or less,
N: 0.010% or less,
Ti: 0.0010 to 0.1000%,
B: 0.0005 to 0.0100%,
Cr: 0 to 1.00%,
Ni: 0 to 2.0%,
Cu: 0 to 1.0%,
Mo: 0 to 1.0%,
V: 0-1 .0%,
Ca: 0 to 0.010%,
Al: 0 to 1.00%,
Nb: 0 to 0.100%,
Sn: 0 to 1.00%,
W: 0 to 1.00%,
REM : 0 to 0.30%,
wherein the balance being Fe and impurities, and the number density of 8.0 × 10 carbide is equivalent circle diameter 0.1μm or more and an aspect ratio of 2.5 or less 3 pieces / Mm 2A material steel plate having the following, and an average value of the equivalent circle diameter of (Nb, Ti) C of 5.0 μm or less, is subjected to an average heating rate of 5 to 300 in a temperature range of
　Ac 3 points to (Ac 3 points + 200) ° C. After the heating step of heating at °
　C./s, the first cooling step of cooling to the Ms point at the first average cooling rate equal to or higher than the upper critical cooling rate, and
　after the first cooling step (Ms-30). A second cooling step of cooling to a temperature range of ~ (Ms-70) ° C. at a second average cooling rate of 5 ° C./s or more and less than 150 ° C./s and slower than the first average cooling rate, and the
　above. After the second cooling step, a reheating step of heating to a temperature range of Ms to (Ms + 200) ° C. at an average heating rate of 5 ° C./s or more, and a
　third average cooling rate of 5 ° C./s or more after the reheating step. A
method for manufacturing a steel member , which comprises a third cooling step of cooling with .
[Claim 9]
　8. The eighth aspect of the present invention is characterized in that, between the heating step and the first cooling step, a holding step of holding the Ac in the temperature range of 3 points to (Ac 3 points + 200) ° C. for 5 to 200 seconds is provided. The method for manufacturing a steel member according to the description.
[Claim 10]
　8. Manufacturing method of steel members.
[Claim 11]
　The method for manufacturing a steel member according to any one of claims 8 to 10, wherein hot forming is performed on the material steel sheet between the heating step and the first cooling step.
[Claim 12]
　The production of a steel member according to any one of claims 8 to 10, wherein in the first cooling step, cooling is performed at the first cooling rate and at the same time, the material steel sheet is hot-formed. Method.