Technical field
[0001]
　The present invention relates to a heat treatment steel sheet member and a manufacturing method thereof.
Background technique
[0002]
　In the field of automotive steel sheets, against the background of tightening of recent environmental regulations and crash safety standards, in order to achieve both the fuel efficiency and collision safety, the application of high-strength steel sheet has expanded with a high tensile strength. However, since the press formability of the steel sheet with a high strength is lowered, it is possible to manufacture a product having a complicated shape becomes difficult. Specifically, the decrease in ductility of the steel sheet with a high strength, has arisen a problem that breakage of the high machining area. Moreover, springback and wall warpage occurs due to the residual stress after working, it is caused a problem that the dimensional accuracy is deteriorated. Accordingly, high strength, is not easy, especially a steel sheet having a tensile strength of at least 780 MPa, pressed into a product having a complicated shape. Incidentally, according to the roll forming rather than press forming, but easily processed steel plate high strength, its application target 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, as a technique for press-molding the molding is difficult materials such as high-strength steel sheet, hot stamping techniques are employed. The hot stamp technique, a hot forming technique for molding after heating the material to be subjected to molding. In this technique, for shaping after heating the material, at the time of molding, the steel has a good moldability soft. Thus, even a high-strength steel can be molded accurately into complex shapes. Further, since the simultaneous quenching and molding by the press die steel after molding it has sufficient strength.
[0004]
　For example, according to Patent Document 1, by a hot stamping technique, it is possible to impart a tensile strength of at least 1400MPa in steel after molding. Further, Patent Document 2, excellent toughness and tensile strength of the press molded article hot press-forming than 1.8GPa is disclosed. Further, Patent Document 3, has a very high tensile strength of more than 2.0 GPa, further steel with good toughness and ductility are disclosed.
CITATION
Patent Literature
[0005]
Patent Document 1: JP 2002-102980 Patent Publication
Patent Document 2: JP 2012-180594 Patent Publication
Patent Document 3: JP 2012-1802 JP
Summary of the Invention
Problems that the Invention is to Solve
[0006]
　For hot forming techniques such as the hot stamping is an excellent molding method capable of increasing the strength of the member while securing formability, that need to be heated to a temperature as high as 800 ~ 1000 ° C., there is a problem that the surface of the steel sheet is oxidized. Its scale comprising iron oxide which occurs when drops and productivity to adhere to the mold and dropping during pressing. Further, there is a problem that the appearance when scale product after pressing remains becomes poor.
[0007]
　Moreover, if the scale remains on the surface of the steel sheet, adhesion between the steel sheet and the coating film is deteriorated when the paint in the next step, lowering the corrosion resistance. So after press molding, the scale removal treatment of the shot blasting is required. Accordingly, the characteristics required for generation scale, without causing mold staining and spalling during pressing, is that likely to be easily peeled off at the time of the shot blasting treatment.
[0008]
　Further, as described above, the automobile steel sheet is also required collision safety. Crashworthiness of automobiles is evaluated by crushing strength and absorbed energy at impact test of the vehicle body all or steel members. In particular for crushing strength is largely dependent on the material strength, the demand for ultra-high strength steel sheet has increased dramatically. However, in general for high strength due to the reduction fracture toughness and deformability decreases, or break prematurely during a crash collapse of the automobile member, or deformation is broken at the site, such as concentrates, crushing strength is commensurate with the material strength not exhibited, the absorption energy decreases. Therefore, in order to improve collision safety, not only material strength, is an important indicator of fracture toughness and deformability of automobile members, it is important to improve the toughness and ductility of the material.
[0009]
　In the technique described in Patent Documents 1 and 2, but are described for tensile strength and toughness, it is not considered ductility. Further, according to the technique described in Patent Document 3, the tensile strength, it is possible to improve the toughness and ductility, are not made examined to obtain an appropriate scale characteristic, room for improvement is left ing.
[0010]
　The present invention has been made to solve the above problems, and has a good scale characteristic, it has a tensile strength of at least 1.4 GPa, and provides excellent heat treatment steel sheet member to toughness and ductility an object of the present invention is to. Incidentally, in particular hot-forming steel sheet member is often is a molded body rather than a flat plate, in the present invention, including the case of a molded body as "heat treatment steel sheet member". In addition, also referred to as a "heat-treated steel sheet for" a steel plate which is a heat treatment prior to the material of the heat-treated steel plate member.
Means for Solving the Problems
[0011]
　The present invention has been made to solve the above problems, and the gist of the heat treatment the steel plate member and a manufacturing method thereof below.
[0012]
　(1) chemical composition, in
　mass%,
　C: 0.05 ~
　0.50%, Si: 0.50 ~ 5.0%, Mn: 1.5
　~ 4.0%, P: 0.05%
　hereinafter, S: 0.05% or
　less, N: 0.01% or
　less,
　Ti: 0.01 ~ 0.10%, B: 0.0005
　~ 0.010%, Cr: 0 ~ 1.0%,
　Ni :
　0
　~
　2.0%, Cu: 0 ~ 1.0%, Mo: 0 ~
　1.0%, V: 0 ~ 1.0%, Ca: 0 ~
　0.01%, Al: 0 ~ 1.
　% 0,
　Nb: 0 ~ 1.0%, REM: 0 ~ 0.1%,
　the balance is Fe and impurities,
　mainly martensite and the volume fraction of retained austenite is 5.0% or more has a metallic structure,
　the number density of the steel plate member circle equivalent diameter 0.1μm or more residual carbides present in the 4.0 × 10 3 cells / mm 2 or
　less, regulations with ASTM E8 When using a plate specimen to be constant was measured mechanical properties below,
　the value of the strain-induced transformation parameter k is expressed by the following formula (i) is less than 20.0,
　tensile strength 1. and at 4GPa above,
　it is the total elongation of 8.0% or more,
　heat treated steel member.
　= k (logf [gamma] 0 -Logf gamma (0.02)) / 0.02 · · · (i)
　where the meaning of each symbol in the formula is as follows.
　f [gamma] 0 : steel member the volume fraction of retained austenite present in the
　f gamma (0.02): a true strain of 0.02 was assigned to the steel plate member, the residual austenite present in the member after dividing pressurized volume rate
[0013]
　(2) the chemical composition, in
　mass%,
　Cr: 0.01 ~
　1.0%, Ni: 0.1 ~ 2.0%, Cu: 0.1
　~ 1.0%, Mo: 0.1
　1.0%
　~,
　0.1 ~ 1.0%, Ca: 0.001 ~ 0.01%,
　Al: 0.01 ~ 1.0% Nb: 0.01 ~ 1.0%, and
　REM: 0.001 ~ 0.1%,
　containing one or more selected from,
　heat treatment steel sheet member according to the above (1).
[0014]
　(3) below (ii) Mn segregation ratio of the formula α is 1.6 or less,
　a heat treatment steel sheet member according to the above (1) or (2).
　alpha = [Maximum Mn concentration in the thickness center portion (mass%)] / [average Mn concentration (mass%) of 1/4 depth position of the sheet thickness from the surface] · · · (ii)
[0015]
　(4) the value of the cleanliness of the steel defined by JIS G 0555 (2003) is less than 0.10%,
　a heat treatment steel sheet member according to any one of (1) to (3).
[0016]
　(5)で
　mass%,
　C: 0.05 ~
　0.50%, Si: 0.50 ~ 5.0%, Mn:
　4.0 ~ for 1.5%, P: 0.05% or
　less, S: 0.05% or
　less, N: 0.01% or
　less,
　of Ti: 0.01 ~ 0.10%,
　B: 0.0005 ~ 0.010%, of Cr: 0 ~
　1.0%, of Ni: 0 ~ 2
　%
　.0,
　a Cu: 0 ~ 1.0%,
　of Mo: 0 ~ 1.0%, V: 0 ~ 1.0%,
　of Ca: 0 ~ 0.01%, of Al: 0 ~
　1.0%, of Nb :
　0 ~ 1.0%, the REM: 0 ~ 0.1%,
　remnants: Feおyoびimpuritiesでthou ru chemical composition wo have shi,
　surface niおke ru maximum height sa crude sa Rz ga 3.0 ~ 10.0μmでthou ri,
　yen ga equivalent diameter above 0.1μmののnumber density of carbides 10 ga × 8.0 . 3 th / mm 2 or lessでthou ru steel wo,
　. 5 deg.] C / S than the average heating rateでのAc . 3 point ~ Ac . 3 point + 200 ℃のma - de heating temperature range After xi ta, the former referred to the temperature range kara Ms point Ma - de upper critical thanでcooling rate shi,その, Ms point kara 100 ℃ Ma - de 5 ℃ / average cooling rateでcooling suru, s or lessの
　heat-treated steel memberのmanufacturing method.
[0017]
　(6) The chemical composition, in
　mass%,
　Cr: 0.01 ~
　1.0%, Ni: 0.1 ~ 2.0%, Cu: 0.1
　~ 1.0%, Mo: 0.1
　1.0%
　~,
　0.1 ~ 1.0%, Ca: 0.001 ~ 0.01%,
　Al: 0.01 ~ 1.0% Nb: 0.01 ~ 1.0%, and
　REM: 0.001 ~ 0.1%,
　containing one or more selected from,
　a method of manufacturing heat treated steel member according to (5).
[0018]
　(7) the number density of residual carbides present in the steel sheet member is 4.0 × 10 3 cells / mm 2 or less,
　the manufacturing method of the heat treatment steel sheet member according to the above (5) or (6).
[0019]
　(8) Mn segregation ratio represented by the following (ii) expression α is 1.6 or less,
　the manufacturing method of the heat treatment steel sheet member according to any one of (5) to (7).
　alpha = [Maximum Mn concentration in the thickness center portion (mass%)] / [average Mn concentration (mass%) of 1/4 depth position of the sheet thickness from the surface] · · · (ii)
[0020]
　(9) the value of the cleanliness of the steel defined by JIS G 0555 (2003) is less than 0.10%,
　the manufacturing method of the heat treatment steel sheet member according to any one of (5) to (8).
[0021]
　(10) After heating to the temperature range, prior to cooling to the Ms point is subjected to hot forming the steel sheet
　manufacturing method of the heat treatment steel sheet member according to any one of (5) to (9).
Effect of the invention
[0022]
　According to the present invention, it is possible to obtain a heat-treated steel member which is excellent in toughness and ductility and having a tensile strength of at least 1.4 GPa.
DESCRIPTION OF THE INVENTION
[0023]
　The present inventors, as well as have a good scale characteristic, extensive studies were carried out the results for chemical composition and organization relationship for obtaining the steel plate member having excellent toughness and ductility, leading to obtain the following findings.
[0024]
　Component of the heat treatment for steel that is produced in (a) and abroad are almost identical, C: 0.2 ~ 0.3% and Mn: containing about 1-2%, further containing Ti and B. In the heat treatment step, the steel sheet Ac 3 was heated to a temperature above points, ferrite is conveyed quickly not precipitated, by rapidly cooling by the mold press to martensitic transformation start temperature (Ms point), the intensity obtain a high martensite structure.
[0025]
　(B) However, the present inventors have conducted a detailed structure investigation, it was found that in steel sheet member after the heat treatment step, it may not be all the occupied martensite. As the cause, it is considered to be as follows. In Ms point below the temperature range of the quenching process, the cooling rate is lowered by heat generated by the transformation. Therefore, (back automatic broiler) generated martensite is tempered in situ, carbon to untransformed austenite is diffused and segregated, the austenite remains about 1-2%. Also, the carbide precipitated by hot rolling or annealing conditions, it is also conceivable to remain even after the heat treatment step.
[0026]
　(C) The present inventors have found that the residual austenite unavoidably contained in the heat treatment the steel plate member is the impact on the properties of the steel sheet member, was examined in more detail. Steel having a composite structure containing retained austenite to ferrite as the base is excellent in ductility by utilizing the TRIP (Transformation Induced Plasticity) effect processing-induced transformation of retained austenite. On the other hand, in the steel having a composite structure containing retained austenite as a base martensite such as ultra-high strength steel, for if it TRIP effect works effectively, it was not clear so far.
[0027]
　(D) The present inventors have investigated whether the changed how residual austenite during deformation of the steel material, even in the steel which is based on martensite, TRIP effect is expressed, clear that improved ductility became. Further, inventors have conducted toughness evaluation test such as Charpy impact test, it was found to be improved toughness as well as ductility by TRIP effect of the retained austenite.
[0028]
　(E) the amount of residual austenite in the structure of the steel sheet member, C, by optimizing the composition design of elements such as Si and Mn, it is possible to dramatically increase.
[0029]
　(F) In addition, conventionally, in order to improve collision safety, although uniform elongation had been noted, it is also important in suppressing the breakage at the time of collision to enhance the local elongation.
[0030]
　(G) coarse carbide is present in excess in the steel sheet for heat treatment, the carbide remains much in the grain boundary after the heat treatment, not only the toughness of the heat-treated steel member is deteriorated, local elongation is lowered, resulting in a collision safety is deteriorated. Therefore, it is necessary that the number density of residual carbides present in the steel sheet member to below the specified value.
[0031]
　(H) In addition, the scale characteristic, in the steel sheet before the heat treatment, by more than a conventional steel sheet Si content in steel, it becomes possible to obtain a desired scale characteristic.
[0032]
　(I) quantifying the Mn segregation ratio included in the heat treatment for steel, the toughness of the heat-treated steel plate member is further improved by reducing it.
[0033]
　(J) inclusions contained in the steel sheet member is great toughness of ultra high strength steel impact. For toughness improvement, it is preferable to reduce the value of the cleanliness of the steel defined by JIS G 0555 (2003).
[0034]
　The present invention has been made based on the above findings. It will be described in detail below each requirement of the present invention.
[0035]
　(A) Chemical composition of heat treated steel member and the heat treatment for steel
　reasons for limiting each element are as follows. Incidentally, "%" for the content in the following description means "mass%".
[0036]
　C: 0.05 ~ 0.50%
　C increases the hardenability of steel, and is an element that improves the strength of the steel sheet member after quenching. However, the C content is less than 0.05%, it is difficult to ensure sufficient strength in the steel sheet member after quenching. Therefore, C content is 0.05% or more. On the other hand, when the C content exceeds 0.50%, too high strength of the steel sheet member after quenching, degradation of the toughness becomes significant. Therefore, C content is 0.50% or less. C content is preferably at 0.08% or more, and preferably 0.45% or less.
[0037]
　Si: 0.50 ~
　5.0% Si increases the hardenability of steel, and is an element that improves the strength of steel by solid solution strengthening. Furthermore, Si is because hardly solved in carbides, suppressing the precipitation of carbides during hot molding, to promote concentration of C into untransformed austenite. As a result, Ms point is significantly lowered, and the solid solution strengthened austenitic many remain. Further, Si is, Fe on the surface of the steel sheet during the heat treatment 2 SiO 4 to generate, as well as inhibit scale formation, reduces the FeO in the scale role. The Fe 2 SiO 4 is a barrier layer, since the supply of Fe into the scale is interrupted, it is possible to reduce the scale thickness. Further scale thickness is thin, and difficult to peel at the time of hot-forming, there is a merit of easy peeling during scale removal treatment after molding.
[0038]
　In order to obtain these effects, it is necessary to contain Si 0.50% or more. Incidentally, the Si is 0.50% or more, in the residual carbides is reduced tendency. As described below, the carbide precipitated in the steel sheet before the heat treatment is large, the rest they dissolve during the heat treatment can not secure sufficient hardenability, since the low strength ferrite is precipitated, may become insufficient strength, in this sense Si is 0.50% or more.
[0039]
　However, when the Si content in the steel exceeds 5.0%, during the heat treatment, the heating temperature required for austenite transformation becomes remarkably high. Accordingly, or cause an increase in cost required for heat treatment, which may or inviting quenching shortage due to insufficient heating. Therefore, Si content is 5.0% or less. Si content is preferably 0.75% or more is preferably 4.0% or less.
[0040]
　Mn: 1.5 ~ 4.0% Mn
　enhances the hardenability of steel, and in order to secure the strength after quenching stable, an element with a very effective. Further Ac 3 lowers the point, an element which promotes lowering the quenching temperature. However, Mn content, the effect is not sufficient below 1.5%. On the other hand, the effect of Mn content exceeds 4.0% is saturated, further leading to toughness deterioration of the quenched portion. Therefore, Mn content is from 1.5 to 4.0%. Mn content is preferably 2.0% or more. Further, it is preferable Mn content is less 3.8%, and more preferably not more than 3.5%.
[0041]
　P: 0.05% or less
　P is an element degrading the toughness of the steel plate member after quenching. In particular, when the P content exceeds 0.05%, deterioration of toughness becomes remarkable. Accordingly, P content is 0.05% or less. P content is preferably 0.005% or less.
[0042]
　S: 0.05% or less
　S is an element degrading the toughness of the steel plate member after quenching. In particular, when the S content exceeds 0.05%, deterioration of toughness becomes remarkable. Thus, S content is 0.05% or less. S content is preferably 0.003% or less.
[0043]
　N: 0.01% or less
　N is an element degrading the toughness of the steel plate member after quenching. In particular, when the N content exceeds 0.01%, coarse nitrides in the steel are formed, the local deformability and toughness is remarkably deteriorated. Therefore, N content is 0.01% or less. It is not necessary to particularly limit the lower limit of the N content, since it is economically undesirable for the N content to less than 0.0002%, N content is preferably 0.0002% or more, 0 and more preferably to .0008% or more.
[0044]
　Ti: 0.01 ~ 0.10% Ti
　is a steel sheet Ac 3 suppresses the recrystallization during the heat treatment by heating to a temperature above points, to suppress the grain growth to form fine carbides in an element having an effect of the austenite grains fine. Therefore, by containing Ti, effects are obtained toughness of the steel plate member is greatly improved. Moreover, Ti can suppress the consumption of B by precipitation of BN by preferentially bind N in the steel, to promote the effect of improving hardenability by later-described B. The Ti content is less than 0.01%, not sufficiently obtained the above effects. Therefore, Ti content is 0.01% or more. On the other hand, if the Ti content exceeds 0.10%, the precipitation amount of TiC is consumed C increases, the strength of the steel sheet member after quenching is reduced. Therefore, Ti content is 0.10% or less. Ti content is preferably 0.015% or more is preferably not more than 0.08%.
[0045]
　B: 0.0005 ~ 0.010%
　B, so it has a dramatically enhancing effect the hardenability of steel even in trace amounts, is a very important element in the present invention. Further, B is that segregates at the grain boundaries, increasing the toughness to strengthen grain boundaries. Furthermore, B suppresses grain growth of austenite during heating of the steel sheet. The B content is less than 0.0005%, may not be sufficiently obtained the above effects. Therefore, B content is 0.0005% or more. On the other hand, when the B content exceeds 0.010%, precipitates are many coarse compounds, the toughness of the steel plate member is deteriorated. Thus the B content is 0.010% or less. B content is preferably at 0.0010% or more is preferably 0.008% or less.
[0046]
　Heat treatment for steel heat treatment steel member and the heat treatment prior to the present invention, in addition to the above elements, is selected Cr amount shown below, Ni, Cu, Mo, V, Ca, Al, of Nb and REM one or more elements may be contained.
[0047]
　Cr: 0 ~ 1.0% Cr
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel sheet member after hardening stable, it may be contained. Also as with Si, FeCr on the surface of the steel sheet during the heat treatment 2 O 4 serves to generate, as well as inhibit scale formation, to reduce the FeO in the scale. The FeCr 2 O 4 becomes a barrier layer, since the supply of Fe into the scale is interrupted, it is possible to reduce the scale thickness. Further scale thickness is thin, and difficult to peel at the time of hot-forming, there is a merit of easy peeling during scale removal treatment after molding. However, the above effects when the Cr content exceeds 1.0% is saturated, unnecessarily cause an increase in cost. Therefore, Cr content of the case of containing the 1.0%. Cr content is preferably at 0.80%. To obtain the above effect, Cr content is preferably at least 0.01%, and more preferably 0.05% or more.
[0048]
　Ni: 0 ~ 2.0% Ni
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel sheet member after hardening stable, may be contained. However, when the Ni content exceeds 2.0%, economic efficiency and the above effect is saturated is reduced. Therefore, Ni content in the case of incorporating is 2.0% or less. To obtain the above effect, it is preferable to contain Ni at least 0.1%.
[0049]
　Cu: 0 ~ 1.0% Cu
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel sheet member after hardening stable, may be contained. However, when the Cu content exceeds 1.0%, economic efficiency and the above effect is saturated is reduced. Therefore, Cu content in the case of incorporating is 1.0% or less. To obtain the above effect, it is preferable to contain Cu of 0.1% or more.
[0050]
　Mo: 0 ~ 1.0% Mo
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel sheet member after hardening stable, it may be contained. However, when the Mo content exceeds 1.0%, economic efficiency and the above effect is saturated is reduced. Therefore, Mo content in the case of incorporating is 1.0% or less. To obtain the above effect, it is preferable to contain Mo of 0.1% or more.
[0051]
　V: 0 ~ 1.0% V
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel sheet member after hardening stable, it may be contained. However, when the V content exceeds 1.0%, economic efficiency and the above effect is saturated is reduced. Therefore, V content in the case of incorporating is 1.0% or less. To obtain the above effect, it is preferable to contain V of 0.1% or more.
[0052]
　Ca: 0 ~ 0.01%
　Ca, in order to inclusions in the steel refining, which is an element having an effect of improving the toughness and ductility after quenching, may be contained. However, the effect when the Ca content exceeds 0.01% saturated, unnecessarily cause an increase in cost. Therefore, in the case of containing Ca content thereof is 0.01% or less. Ca content is preferably 0.004% or less. If it is desired to obtain the above effect, it is preferable that the Ca content is 0.001% or more, and more preferably 0.002% or more.
[0053]
　Al: 0 ~ 1.0% Al
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel sheet member after hardening stable, may be contained. However, if the Al content exceeds 1.0%, economic efficiency and the above effect is saturated is reduced. Therefore, Al content in the case of incorporating is 1.0% or less. To obtain the above effect, it is preferable to contain Al 0.01% or more.
[0054]
　Nb: 0 ~ 1.0% Nb
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel sheet member after hardening stable, may be contained. However, when the Nb content exceeds 1.0%, economic efficiency and the above effect is saturated is reduced. Therefore, Nb content case of containing the 1.0% or less. To obtain the above effect, it is preferable to contain Nb of 0.01% or more.
[0055]
　REM: 0 ~ 0.1%
　REM, since refining inclusions in like the Ca steel, is an element having an effect of improving the toughness and ductility after quenching, it may be contained. However, the effect when the REM content exceeds 0.1% is saturated, unnecessarily cause an increase in cost. Therefore, REM content case of containing is 0.1% or less. REM content is preferably 0.04% or less. If it is desired to obtain the above effect, it is preferable that the REM content is 0.001% or more, and more preferably 0.002% or more.
[0056]
　Here, REM is, Sc, refers to a total of 17 elements of Y and lanthanoids, the content of the REM means the total content of these elements. REM is added to the molten steel by using, for example, Fe-Si-REM alloy, this alloy, for example, Ce, La, Nd, includes Pr.
[0057]
　In the chemical composition of the heat-treated steel member and the heat treatment for the steel sheet of the present invention, the balance being Fe and impurities.
[0058]
　Here, the "impurities", in manufacturing the steel sheet industrially, ores, raw material scraps, a component mixed by various factors of the manufacturing process, is allowed to the extent that the present invention does not adversely affect means shall.
[0059]
　(B) heat treatment steel sheet member of metal structure
　heat treatment steel sheet member according to the present invention, mainly martensite and has a metal structure volume fraction of retained austenite is 5.0% or more. Note that martensite present in the steel plate member is an automatic tempered martensite. Further, to the "mainly martensite", the volume fraction of martensite means a metal structure is 90% or more. During the steel plate member, ferrite, pearlite, and in some cases tissue bainite or the like is mixed, these tissues are acceptable as long as more than 5.0% in total volume ratio.
[0060]
　Retained austenite: 5.0% or more
　residual austenite by martensite transformation during deformation, it promotes work hardening to prevent constriction, improve the ductility. In particular, if the volume fraction of residual O State is less than 5.0%, the ductility is reduced significantly, fracture risk ultrahigh strength heat-treated steel member is increased, collision safety is lowered. Therefore, the volume fraction of retained austenite is 5.0% or more. On the other hand, the upper limit of the volume fraction of residual austenite is not provided particularly limited. However, the volume fraction of retained austenite is reduced in strength and is excessive, preferably 10% or less.
[0061]
　As the method of measuring the phase fraction of the tissue, including a second phase that including retained austenite (volume ratio), methods using X-ray diffraction is common. This first phase (martensite structure, body-centered cubic lattice) and second phase (residual austenite phase, face-centered cubic lattice) measured by the detector of the diffracted X-ray intensity coming, each from the area ratio of the diffracted curve a method of measuring the volume fraction of the phases, it is possible to measure the volume% of retained austenite in the steel sheet member with high accuracy. In the case where ferrite is mixed in addition to the residual austenite, it is possible to distinguish easily with an optical microscope, it can also vol% of martensite which is the main structure of the steel sheet member is measured with high precision.
[0062]
　(C) Residual carbides: 4.0 × 10 3 cells / mm 2 or less
　when performing the heat treatment, it is possible to secure sufficient hardenability by the re-dissolution of carbides generally present in the steel. However, if a portion of the carbide remains without being dissolved again can not secure sufficient hardenability, low strength ferrite precipitates. Therefore, the more the residual carbides is small, improves the hardenability, it is possible to ensure high strength.
[0063]
　Also, when there are many residual carbide in the steel sheet before the heat treatment, not only hardenability decreases, residual carbides deposited old γ grain boundaries, thereby embrittling the grain boundaries. Furthermore, the amount of residual carbides is excessive, the residual carbides becomes void origin during deformation, since the connecting is facilitated, ductility of the steel sheet member, decreases especially local elongation, resulting in collision safety is deteriorated.
[0064]
　In particular, a circle equivalent diameter which is present in the steel sheet member after heat treatment the number density of more residual carbides 0.1μm is 4.0 × 10 3 cells / mm 2 by weight, the toughness and ductility of the steel sheet member after heat treatment Getting worse. Therefore, a circle equivalent diameter which is present during the heat treatment the steel plate member is the number density of more residual carbides 0.1μm is 4.0 × 10 3 cells / mm 2 or less. Note that the number density of the equivalent circle diameters of more than 0.1μm carbides present in the steel sheet before the heat treatment is 8.0 × 10 3 cells / mm 2 is preferably not more than. Also, the carbide refers to those particulate, specifically directed to what aspect ratio is 3 or less.
[0065]
　(D) the mechanical properties of the heat-treated steel member
　as described above, it is possible to obtain a high ductility by TRIP effect utilizing strain induced transformation of retained austenite. However, the residual austenite will be transformed at low strain, high ductility by TRIP effect can not be expected. That is, not only the amount of residual austenite, it is necessary to control its properties.
[0066]
　Specifically, the residual austenite at a low strain value of the strain-induced transformation parameter k is expressed by the following formula (i) is increased resulting in transformation. Therefore, it is necessary to a value less than 20.0 strain induced transformation parameter k.
　= k (logf [gamma] 0 -Logf gamma (0.02)) / 0.02 · · · (i)
　where the meaning of each symbol in the formula is as follows.
　f [gamma] 0 : steel member the volume fraction of retained austenite present in the
　f gamma (0.02): a true strain of 0.02 was assigned to the steel plate member, the residual austenite present in the member after dividing pressurized volume rate
[0067]
　Incidentally, to govern whether easily transformed in strain retained austenite have been given, is considered to be the amount of dissolved C in retained austenite, in a range of the Mn content in the present invention, the amount of retained austenite positive correlation between the solid solution C amount. Then, for example, the value of the k as a dissolved amount of C is about 0.8% becomes about 15, the value of the k amount of solid solution C is to be about 0.2% is about 53.
[0068]
　The heat treatment steel sheet member according to the present invention has a tensile strength of not less than 1.4 GPa, the total elongation is to be at 8.0% or more. By high tensile strength and total elongation of more than 1.4GPa to and a good ductility of 8.0% or more, it becomes possible to meet the demand for achieving both fuel consumption and collision safety.
[0069]
　It provides excellent ductility as described above, in order to improve collision safety, not only uniform elongation, local elongation may be desirable to increase. From this viewpoint, local elongation is preferably set to 2.8% or more.
[0070]
　In the present invention, the strain-induced transformation parameters k, tensile strength, a measurement of the mechanical properties including total elongation and local elongation, and the use of half-size plate-shaped test piece of ASTM Standard E8. Specifically, the tensile test was performed in conformity with the provisions of ASTM Standard E8, 1.2 mm thick, 32 mm parallel portion length, parallel portion plate width with respect to the plate-like test piece of 6.25mm performed at room temperature tensile test at a strain rate of 3 mm / min, to measure the maximum intensity (tensile strength). Alternatively, it is acceptable to put a previously 25mm scoring of the parallel portion of the tensile test, to measure the combined percent elongation per fracture samples (total elongation). Then, a local elongation as a value obtained by subtracting the plastic strain at maximum intensity from total elongation (uniform elongation).
[0071]
　Mn segregation ratio of (E) heat treating the steel plate member
　Mn segregation ratio alpha: 1.6 or less
　alpha = [Maximum Mn concentration in the thickness center portion (mass%)] / [the surface at 1/4 depth position of the sheet thickness average Mn concentration (mass%)] ··· (ii)
　in the steel plate having a plate thickness of the cross-sectional center portion, Mn is concentrated by center segregation occurs. Accordingly, MnS is concentrated in the center as inclusions, since easily available hard martensite, there is a possibility that a difference occurs in the hardness of the surrounding, the toughness is deteriorated. In particular, when the value of the segregation ratio α of Mn represented by the above (ii) expression is more than 1.6, there is a possibility that the toughness is deteriorated. Therefore, in order to improve the toughness, it is preferable to set the value of α of heat treatment the steel plate member and 1.6 or less. To further improve the toughness, it is more preferable that the value of α is 1.2 or less.
[0072]
　Since there is no possibility that the value of α by heat treatment or hot forming is greatly changed, the value of α of heat treatment for steel that within the above range, even to 1.6 below the value of α of heat treatment the steel plate member it is possible, that it is possible to improve the toughness of the heat treatment the steel plate member.
[0073]
　Maximum Mn concentration in the center of plate thickness is determined by the following method. Perform line analysis in the thickness direction perpendicular to the direction in the center of plate thickness of the steel sheet by an electron probe microanalyzer (EPMA), to select the three measurements in the descending order from the analysis result, and calculates the average value. The average Mn concentration at 1/4 depth position of the sheet thickness from the surface is determined by the following method. Also analyzed the 10 locations in the 1/4 depth position of the steel sheet by using EPMA, and calculates the average value.
[0074]
　Segregation of Mn in the steel sheet is mainly steel composition, and in particular impurity content is controlled by the continuous casting conditions, does not substantially change before and after the hot rolling and hot forming. Thus, by controlling the segregation conditions of heat treatment for steel plates, then even segregation conditions of the heat-treated steel member can be similarly controlled.
[0075]
　(F) heat treatment steel sheet member cleanliness
　cleanliness: 0.10% or less
　during the heat treatment the steel plate member JIS G 0555 A system according to (2003), the B system and C-type inclusions there are many, the inclusions cause of toughness degradation. Since the inclusions increases easily happen crack propagation, there is a possibility that the toughness deteriorates. In particular, in the case of heat treatment the steel plate member so as to have a tensile strength of at least 1.4 GPa, it is preferable to suppress the existence ratio of the inclusions low. When the value of the cleanliness of the steel defined by JIS G 0555 (2003) is more than 0.10%, the amount of inclusions is large, it is difficult to ensure practically sufficient toughness. Therefore, cleanliness values of heat treatment the steel plate member is preferably set to 0.10% or less. It is more preferable to further improve the toughness of less 0.06% the value of cleanliness. Incidentally, cleanliness value of the steel is obtained by calculating the area percentage occupied by the above A type, B type and C type inclusions.
[0076]
　Since not the value of cleanliness is greatly changed by heat treatment or hot forming, the value of the cleanliness of the heat treatment for the steel sheet is within the above range, even cleanliness values ​​of heat treatment the steel plate member 0.10 % can be below.
[0077]
　In the present invention, the cleanliness of the value of the heat treatment for steel or heat treatment steel sheet member can be determined by the following method. For the heat treatment for steel or heat-parts, cut out test pieces from the five locations. Then, for each position of each sample in the plate thickness 1 / 8t, 1 / 4t, 1 / 2t, 3 / 4t, 7 / 8t, to investigate the cleanliness at point algorithm. The (lowest cleanability) numeric value of cleanliness is greatest with respect to the plate thickness, the value of cleanliness of test materials.
[0078]
　(G) the surface roughness of the heat treatment for steel
　maximum height roughness Rz: 3.0 ~ 10.0 [mu] m
　the heat treatment the material before heat treatment steel sheet member according to the present invention, the surface roughness of the heat treatment for steel, especially limited not provided. However, in order to obtain a heat-treated steel member which is excellent in scale adhesion during hot molding, the surface of the steel sheet, JIS B 0601 (2013) with the maximum height roughness Rz is 3.0 ~ 10.0 [mu] m which is defined by it is preferable to use a certain steel. The maximum height roughness Rz of the steel sheet surface by more than 3.0 [mu] m, scale adhesion during hot-forming is improved by an anchor effect. On the other hand, when the maximum height roughness Rz is more than 10.0 [mu] m, at the stage of the scale removal treatment shot blasting, it may scale remained to cause the pushing flaws.
[0079]
　By the maximum height roughness Rz of 3.0 ~ 10.0 [mu] m at the surface of the steel sheet, it is possible to achieve both the scale peelability for scale adhesion and shot blasting time during pressing. In order to obtain a suitable anchoring effect as described above, of managing an arithmetic mean roughness Ra is insufficient, it is necessary to use the maximum height roughness Rz.
[0080]
　If the maximum height roughness Rz of the steel sheet surface is a more steel 3.0μm and hot forming, shows a tendency to the ratio of wustite iron oxide that forms on the surface increases. Specifically, the percentage of wustite is in area%, by a 30-70%, resulting excellent scale adhesion.
[0081]
　Wustite hematite, excellent plastic deformability at high temperatures than magnetite, scaling is also considered to exhibit the easy features to plastic deformation when plastic deformation of the steel sheet during hot-forming. The reason why the ratio of wustite is increased, but clearly is not known, the area of ​​the scale areas iron surface becomes large when the irregularity is present, outward diffusion of iron ions is accelerated during the oxidation, the ratio of iron it is believed that a high wustite is increased.
[0082]
　Further, the surface of the steel sheet during hot molding by containing Si Fe 2 SiO 4 is generated and suppressing scale formation is as described above. Scale thickness of the whole becomes thinner, and that the wustite ratio in the scale is increased, the scale adhesion during hot molding is thought to be improved. Specifically, by the scale thickness becomes 5μm or less, excellent scale adhesion can be obtained.
[0083]
　(H) the production method of heat treatment for steel
　is steel sheet before annealing step-parts according to the present invention, there is no particular restriction as to the production conditions of the heat treatment for the steel sheet, by using the manufacturing method described below, the above-mentioned tissue it is possible to produce a heat treatment for steel sheet having a. In the following manufacturing method, for example, it performs the hot rolling, pickling, cold rolling and annealing.
[0084]
　After melting the steel having the chemical composition described above in a furnace to produce a slab by casting. At this time, in order to suppress concentration of MnS which becomes a starting point of delayed fracture, it is desirable to center segregation reduction process to reduce the center segregation of Mn. The central segregation reduction treatment, in the non-solidified layer before the slab is completely solidified, and a method of discharging the molten steel Mn is enriched.
[0085]
　Specifically, electromagnetic stirring, by a process such unsolidified layer pressure applied, can be completely solidified before Mn is to discharge the molten steel enriched. The electromagnetic stirring process described above may be carried out by giving fluidity to the unsolidified molten steel at 250 to 1000 gauss, the unsolidified layer rolling process, the final solidified portion by reduction with a gradient of about 1 mm / m It can be carried out.
[0086]
　Against a slab obtained by the method described above, soaking (soaking) process may be carried out as necessary. Soaking treatment by performing, it is possible to reduce the segregation ratio to diffuse segregated Mn. Preferred soaking temperature in the case of performing the soaking treatment is 1200 ~ 1300 ° C., soaking time is 20 ~ 50h.
[0087]
　Furthermore, the cleanliness of the steel sheet to 0.10% or less, at the time of continuous casting of molten steel, the heating temperature of the molten steel temperature high 5 ° C. or more above the liquidus temperature of the steel, and, per unit time it is desirable to suppress the molten steel pouring amount below 6t / min.
[0088]
　When the amount of casting per unit of molten steel during continuous casting time exceeds 6t / min, for faster flow of molten steel in the mold, inclusions solidified shell is easily trapped, inclusions in the slab increases. Further, when the molten steel heating temperature is 5 ° C. higher than a temperature above the liquidus temperature, high viscosity of the molten steel, difficult to inclusions floating in a continuous casting machine, as a result, inclusions in the slab is increased cleanliness Te is likely to deteriorate.
[0089]
　On the other hand, the molten steel heating temperature from the molten steel liquidus temperature 5 ° C. or higher, and the molten steel pouring amount per unit time by casting as follows 6t / min, inclusions is hardly brought into the slab. As a result, the slab can effectively reduce the amount of inclusions in a step of manufacturing, it is possible to easily achieve the steel cleanness of 0.10% or less.
[0090]
　When the continuous casting of molten steel, the molten steel heating temperature of the molten steel is preferably set to be 8 ° C. or more above the liquidus temperature higher temperatures, also, it is preferable that the molten steel pouring amount per unit time below 5t / min. The molten steel heating temperature and liquidus temperature 8 ° C. or higher than the high temperature, and, by a molten steel pouring amount per unit time below 5t / min, it is easy to cleanliness and 0.06% or less for desirable.
[0091]
　Then, subjected to a hot rolling on the slab. Hot rolling conditions, from the viewpoint of more uniformly generate a carbide, a hot rolling start temperature to a temperature range of 1000 ~ 1300 ° C., the hot rolling completion temperature is preferably set to 950 ° C. or higher.
[0092]
　In the hot rolling step, after the rough rolling, it performs descaling is performed as necessary finally finish rolling. At this time, when the time until the rough rolling starts finish rolling from the end to the 10s or less, recrystallization austenite is suppressed, not only is consequently suppressed the growth of carbides, scale generated in a high temperature inhibition, it is possible to adjust the suppression of oxidation of the austenite grain boundaries, and the maximum height roughness on the surface of the steel sheet in an appropriate range. In addition, the inhibition of scale formation and grain boundary oxidation, since Si in the surface layer is likely to remain in a state of solid solution, easily generated fayalite when heated pressing, considered wustite be easily generated for the It is.
[0093]
　Coiling temperature after hot rolling is preferably higher from the viewpoint of workability, since too high yield by scale formation is reduced, preferably in the 500 ~ 650 ° C.. Also, better to the coiling temperature in the low temperature, carbides easily finely dispersed, and also reduced the number of carbides.
[0094]
　Form of carbides, in addition to the conditions of hot rolling, it is possible to also control by adjusting the subsequent annealing conditions. That is, the annealing temperature to a high temperature, after once a solid solution carbides in annealing step, cause the transformation at a low temperature is desirable. Since carbide is hard, in the cold rolling is not that the form is changed, the presence form of after the cold rolling also hot rolling is maintained.
[0095]
　Subjected to descaling by pickling or the like hot-rolled steel sheet obtained by hot rolling. To adjust the maximum height roughness on the surface of the steel sheet in an appropriate range, it is preferable to adjust the scarfing amount in pickling. Maximum height roughness Smaller scarfing amount is increased, while the maximum height roughness by increasing the scarfing amount is small. Specifically, it is preferable to scarfing amount 1.0 ~ 15.0 .mu.m by pickling, and more preferably to 2.0 ~ 10.0 [mu] m.
[0096]
　The heat treatment for steel in the present invention, it is possible to use a hot-rolled sheet or hot-rolled annealed steel sheet or cold-rolled steel sheet or cold-rolled annealed steel sheet. Processing step may be appropriately selected depending on the plate thickness accuracy required level of products.
[0097]
　That is, hot-rolled steel sheet descaling has been performed, a hot-rolled annealed steel sheet is subjected to annealing as needed. Further, hot-rolled steel sheet or hot-rolled annealed steel sheets described above, if necessary subjected to a cold rolling and cold-rolled steel sheet, further, cold-rolled steel sheet, a cold-rolled annealed steel sheet is subjected to annealing as needed . Incidentally, if the steel sheet subjected to cold rolling is rigid, it is preferable to increase the workability of the steel sheet subjected to annealing before the cold rolling is subjected to cold rolling.
[0098]
　Cold rolling may be performed using conventional methods. From the viewpoint of ensuring good flatness, reduction ratio in cold rolling is preferably 30% or more. Meanwhile, in order to avoid that the load becomes excessive, the rolling reduction in cold rolling is preferably 80% or less. Incidentally, there is no possibility that the maximum height roughness greatly varies at the surface of the steel sheet by cold rolling.
[0099]
　When producing the annealed hot-rolled steel sheet or annealed cold-rolled steel sheet as the heat-treating a steel sheet, performing annealing with respect to hot-rolled steel sheet or cold-rolled steel sheet. The annealing, for example, holding the hot-rolled steel sheet or cold-rolled steel sheet in a temperature range of 550 ~ 950 ° C..
[0100]
　By the temperature to hold at the annealing and 550 ° C. or higher, even when manufacturing any annealed hot-rolled steel sheet or annealed cold-rolled steel sheet, the difference in characteristics due to the difference in the hot rolling condition is reduced, after quenching can be further assumed that stable and characteristics. Also, when performing annealing cold-rolled steel sheet at 550 ° C. or more, cold-rolled steel sheet by recrystallization to soften, thereby improving the workability. In other words, it is possible to obtain the annealed cold-rolled steel sheet with good workability. Therefore, the temperature holding at the annealing is preferably set to 550 ° C. or higher.
[0101]
　On the other hand, if the temperature holding at the annealing is higher than 950 ° C., it is that the tissue is coarsening. Roughening of tissue may reduce the toughness after quenching. Further, even beyond 950 ° C. temperature to hold at the annealing, not be effective enough to raise the temperature obtained, the cost is increased and the productivity is only lowered. Therefore, the temperature holding at the annealing is preferably set to 950 ° C. or less.
[0102]
　After annealing, it is preferable to cool to 550 ° C. at an average cooling rate of 3 ~ 20 ℃ / s. With the average cooling rate 3 ° C. / s or more, is suppressed formation of coarse pearlite and coarse cementite, thereby improving the characteristics after quenching. Further, by the average cooling rate 20 ° C. / s or less, and suppress the occurrence of such intensity irregularity, it is easy to assume that stable the material of the annealed hot-rolled steel sheet or annealed cold-rolled steel sheet.
[0103]
　(I) the production method of the heat treatment the steel plate member
　by applying a heat treatment to the above heat treatment for steel, it is possible to obtain a heat-treated steel member which is excellent in toughness and ductility and having a high strength. Although not provided any special restriction on heat treatment conditions, for example, it can be subjected to a heat treatment comprising the following heating and cooling steps in this order.
[0104]
　Heating step
　at 5 ° C. / s or more an average heating rate, Ac 3 point ~ Ac 3 to heat the steel sheet to a temperature range of point + 200 ° C.. This heating step, the tissue of the steel sheet to austenite single phase. When or heating temperature rise speed is too slow in the heating step is too high, and coarsened γ grains, the strength of the steel sheet member after cooling may deteriorate. In contrast, by carrying out the heating step satisfying the above conditions, it is possible to prevent deterioration of the strength of the heat-treated steel member.
[0105]
　Cooling process
　was subjected to the heating step the steel sheet, does not occur diffusion transformation (i.e. ferrite does not precipitate) As described above, since the temperature range is cooled in the upper critical cooling rate or higher to Ms point, then, from the Ms point to 100 ° C. 5 ° C. / s cooling below the average cooling rate. The cooling rate from a temperature below 100 ° C. to room temperature, the cooling rate of the order of air cooling is preferred. By carrying out the cooling step satisfying the above conditions, can prevent the formation of ferrite in the cooling process, and in a temperature range of Ms point or lower, the carbon diffusion in the untransformed austenite by automatic tempering, thickened, plastic deformation stable retained austenite is generated for. Thereby, it becomes possible to obtain excellent heat-treated steel member toughness and ductility.
[0106]
　The upper critical cooling rate has a large influence of the Mn content in the range of Mn content defined in the present invention, the critical cooling rate speed becomes 1 ~ 30 ℃ / s approximately. For tissue formation, the lower limit of the average cooling rate to the Ms point is the component of the steel sheet may be 1 ° C. / s. However, from the viewpoint of formation of scale, it is necessary a certain degree of cooling rate, for example may be set to 10 ° C. / s or higher, or at 45 ° C. / s or higher as exemplified in the Examples.
[0107]
　Further, as described above, the maximum height roughness Rz of the steel sheet is adjusted to 3.0 ~ 10.0 [mu] m. When the maximum height roughness Rz is less than 3.0 [mu] m, heating, processing and reduced adhesion of the scale in the cooling of the process, scale for separating partially, variations in the cooling rate increases. The maximum height roughness Rz be greater than 10.0 [mu] m, variation in due to uneven cooling rate of the surface increases. Thus, by adjusting the maximum height roughness Rz to 3.0 ~ 10.0 [mu] m, variation in the characteristics of the product is reduced to improve the temperature control.
[0108]
　The above heat treatment can be carried out by any method, for example, it may be performed by high-frequency heating quenching. In the heating step, the steel sheet Ac 3 point ~ Ac 3 is time for maintaining a temperature range of point + 200 ° C., from the viewpoint of enhancing the hardenability of the steel by dissolving carbides complete the austenite transformation, be a 10s or preferable. Further, the holding time, from the viewpoint of productivity, it is preferably not more than 600s.
[0109]
　As the steel sheet subjected to heat treatment, it may be used annealed hot-rolled steel sheet or annealed cold-rolled steel sheet was subjected to annealing treatment in hot-rolled steel sheet or cold-rolled steel sheet.
[0110]
　In the heat treatment, Ac 3 point ~ Ac 3 after heating to a temperature range of point + 200 ° C., before cooling to the Ms point, may be subjected to hot forming, such as hot stamping described above. The hot forming, bending, drawing, bulging, hole expansion molding, and the flanging and the like. Further, if provided with means for cooling the steel sheet forming the same time or immediately after, the present invention may be applied to forming methods other than press forming, for example, roll forming.
[0111]
　The following examples illustrate the present invention more specifically, the present invention is not limited to these examples.
Example
[0112]
　The steel having the chemical components shown in Table 1 were melted in a test converter, implemented continuously cast in a continuous casting test machine to produce a width 1000 mm, a thickness of 250mm slab. At this time, in the conditions shown in Table 2 were molten steel pouring amount of the adjustment of the heating temperature and the unit of the molten steel time.
[0113]
[Table 1]

[0114]
　Control of the cooling rate of the slab was carried out by changing the amount of water of the secondary cooling spray zone. The center segregation reduction treatment using a roll in the coagulation end portion, implement soft reduction with a gradient of 1 mm / m, the concentrated molten steel in the final solidified portion was carried out by discharging. For some of the slab, then, 1250 ° C., was carried out soaking treatment under the conditions of 24h.
[0115]
　The obtained slab is subjected to hot rolling by hot rolling tester, it was hot rolled steel plate having a thickness of 3.0 mm. The hot rolling step performs descaling after the rough rolling was carried out last finish rolling. Then pickling the hot-rolled steel sheet at the laboratory. Further subjected to cold rolling at a cold rolling tester, a cold rolled steel sheet having a thickness of 1.4 mm, to obtain a heat-treated steel plate (steel No.1 ~ 18).
[0116]
　The resulting heat-treated steel plate, the maximum height roughness, arithmetic average roughness, the number density of the carbides was measured Mn segregation ratio and cleanliness. In the present invention, the maximum time of obtaining the height roughness Rz and the arithmetic average roughness Ra, the maximum height roughness Rz and the rolling direction of the arithmetic mean roughness Ra and the rolling vertical 2mm intervals using a surface roughness meter measured each 10 places in, and the average value was recorded.
[0117]
　In the equivalent circle diameter determined the number density of more carbides 0.1 [mu] m, the surface of the heat-treated steel plate, corrosion using picral solution, magnified 2000 fold with a scanning electron microscope, and observed multiple field-of-view It was. At this time, 1 mm by counting the number of viewing the circle equivalent diameter is present above carbides 0.1 [mu] m 2 was calculated number per.
[0118]
　Measurements of Mn segregation ratio was carried out by the following procedure. In mid-thickness portion of the heat-treated steel plate using EPMA, perform line analysis in the thickness direction perpendicular to the direction, after selecting the three measurement values ​​in the descending order from the analysis results, to calculate the average value, the thickness It was determined maximum Mn concentration in the center. Further, the surface of the heat-treated steel plate at 1/4 depth position of the sheet thickness, analyzed for 10 sites using EPMA, and the average value was calculated from the surface at 1/4 depth position of the sheet thickness to obtain an average Mn concentration. Then, the maximum Mn concentration in the above thickness center portion, by dividing the average Mn concentration in the 1/4 depth position of the sheet thickness from the surface, to determine the Mn segregation ratio alpha.
[0119]
　Cleanliness, for each position of thickness 1 / 8t, 1 / 4t, 1 / 2t, 3 / 4t, 7 / 8t, was measured at a point algorithm. Then, the (lowest cleanliness) Numerical highest value of cleanliness in each plate thickness, and the value of the cleanliness of the steel sheet.
[0120]
　As described above, since there is no possibility that the value of Mn segregation ratio and cleanliness by hot forming is greatly changed, the Mn segregation ratio α and cleanliness of the heat treatment the steel plate member the value of the above Mn segregation ratio α and cleanliness It was a value.
[0121]
　The presence or absence of central segregation reduction treatment and soaking treatment in the manufacturing process of the heat treatment for steel, the time from the rough rolling in the hot rolling process is finished to the start of the finish rolling, coiling the hot rolling completion temperature and hot-rolled steel sheet temperature, scarfing amount of pickling, and a maximum height roughness Rz of the thermal treatment for steel, the number density measurement results of the arithmetic average roughness Ra and carbides are also shown in Table 2, Mn segregation ratio α and cleanliness It shows the measurement results in Table 4 below.
[0122]
[Table 2]

[0123]
　Then, from the steel plate of the thickness: 1.4 mm, width: 30 mm, and length: samples were taken 200mm two by two. For one of the samples taken, in accordance with the heat treatment conditions shown in Table 3 below simulating the hot forming, and conduction heating and cooling. In Table 3, Ac of the steel plate 3 are also shown a point and Ms point. After cooling, cut out soaking site of each sample, the tensile test, Charpy impact test, X-rays diffraction analysis, were subjected to microscopic observation.
[0124]
　Tensile test, in accordance with the provisions of the ASTM standard E8, was carried out in the Instron tensile testing machine. After grinding the heat-treated sample to 1.2mm thick, so that the test direction is parallel to the rolling direction, half-size plate-shaped test piece of ASTM Standard E8 (parallel portion length: 32 mm, parallel portion plate width: 6.25 mm ) were collected. In electrical heating device cooling apparatus used in this embodiment, the soaking region derived from the length 200mm about the samples for limited, we decided to use the half-size plate-shaped test piece of ASTM Standard E8.
[0125]
　Then, strain gauge on each specimen (Kyowa Electronic Instruments Ltd. KFG-5, gauge length: 5 mm) Paste and then subjected at room temperature tensile test at a strain rate of 3 mm / min, to measure the maximum intensity (tensile strength). Further, the parallel portion of the tensile test previously put beforehand 25mm scoring of was measured combined elongation per fracture samples (total elongation). Then, to determine the local stretch as a value obtained by subtracting the plastic strain at maximum intensity from total elongation (uniform elongation).
[0126]
　The Charpy impact test, the ground to a thickness of the soaking region is 1.2 mm, to prepare a V notched specimen which was laminated three sheets, -80 ° C. performing Charpy impact test of the test piece the impact value was determined. In the present invention, 40 J / cm 2 was to evaluate the superior case having more impact value toughness.
[0127]
　The X-ray diffraction test was used hydrofluoric acid and hydrogen peroxide solution and the test piece was chemically polishing the surface of the heat treatment the sample to a depth of 0.1mm using (thickness 1.1 mm). Specifically, the test piece after chemical polishing, using a Co tube was measured in the range of 105 ° from 45 ° in 2 [Theta]. From the obtained X-ray diffraction spectrum residual austenite volume fraction f [gamma] 0 was determined.
[0128]
　Further, the heat treatment sample was processed into the tensile bars, certain plastic strain (true strain: ε = 0.02) was applied, to produce the X-ray diffraction test strip from dividing pressurized with tensile test pieces, residual austenite volume fraction f gamma was determined (0.02). Strain induced transformation parameter k shown from these by the following formula (i) was calculated and used as an index of high ductility according TRIP effect. Since k is the residual austenite in the lower the strain larger transforms, it prevents constriction at high strain, i.e. high ductility by TRIP effect can not be expected.
[0129]
　= k (logf [gamma] 0 -Logf gamma (0.02)) / 0.02 · · · (i)
　where the meaning of each symbol in the formula is as follows.
　f [gamma] 0 : steel member the volume fraction of retained austenite present in the
　f gamma (0.02): a true strain of 0.02 was assigned to the steel plate member, the residual austenite present in the member after dividing pressurized volume rate
[0130]
　Furthermore, after the surface mirror polishing of the heat treatment the sample was corroded with a picral solution, magnified 2000 fold with a scanning electron microscope, were observed multiple field-of-view. At this time, 1 mm by counting the number of viewing the circle equivalent diameter is present above the residual carbides 0.1 [mu] m 2 was calculated number per. Further, after mirror-finishing the surface of the heat treatment the sample was nital corrosion. Then, using an optical microscope, performs observation of the metal structure, by measuring the area ratio of martensite is the primary tissue, were the value and the volume fraction of martensite.
[0131]
　Also, among the samples taken, the other one, after electrically heating with the heat treatment conditions shown in Table 3 below simulating the hot forming, subjected to bending against the soaking region and then cooled. After cooling, cut out parts subjected to bending of each sample was subjected to scale characteristic evaluation test. Note that when subjected to bending is supported at both ends of the sample support, by pressing the jig from above the R10mm near the longitudinal center were bent U-shaped. Spacing support each other was 30 mm.
[0132]
　Scale characteristic evaluation test, and scale adhesion evaluation indicative of whether or not spalling during pressing, the divided on whether the index becomes scale peelability rating and can be easily separated and removed by shot blasting or the like went. First, observe whether peeling occurs due bending after electrical heating, was evaluated the scale adhesion by the following criteria. In the present invention, when the result is "○○" or "○", it was decided to determine that excellent scale adhesion.
　○○: No peeling
　　○: 1 ~ 5 one release strip falling
　　×: 6 ~ 20 pieces of release strip falling
　××: 21 or more release strip falling
[0133]
　Then, except for the sample became "××" in evaluation of the scale adhesion above for further bending point subjected to processing, was subjected to a tape peeling test of attaching and peeling the adhesive tape. Then, the scale is observed whether readily peeled adhered to the tape was evaluated for scale peelability on the following criteria. In the present invention, when the result is "○○" or "○", it was decided to determine that excellent scale peelability. When excellent in both of the scale adhesion and scale peeling resistance was the scale characteristics during hot forming is excellent.
　○○: all peeling
　　○: 1 ~ 5 one release strip remains
　　×: 6 ~ 20 pieces of release strip remains
　××: 21 or more release strip remains
[0134]
[table 3]

[0135]
　Tensile test, Charpy impact test, X-rays diffraction analysis, microscopic observation, the results of the scale characteristic evaluation test shown in Table 4.
[0136]
[Table 4]

[0137]
　Table 1-4 See, tested met all defined by the chemical composition and structure in the present invention No. In 1 ~ 5,10 ~ 16, 19 ~ 22, 25 and 26, a tensile strength of not less than 1.4 GPa, good ductility having a total elongation of at least 8.0%, and, 40 J / cm 2 or more excellent in toughness has an impact value, it resulted in excellent scale adhesion. Since these samples k values were less than 20 both, it can be seen that high ductility is realized by TRIP effect. Of these Mn value and cleanliness than 1.6 of segregation ratio α is less than 0.10% Test No. In 1, 3 to 5, 10 to 16 and 19 - 22, 50 J / cm 2 have more impact value, particularly resulted in excellent toughness.
[0138]
　On the other hand, test No. In 6-8,17,23 and 33, due to the cooling rate from the Ms point to 100 ° C. is too high, the volume fraction of retained austenite is less than 5.0%, as a result, the total elongation 8 becomes less than 2.0%, the desired ductility is not obtained. From the k value becomes 20 or more, TRIP effect can not sufficiently expressed, it is considered that it could not achieve therefore high ductility.
[0139]
　In addition, test No. In 9, 18 and 24, Ac 3 point ~ Ac 3 due to heating conditions in heating to a temperature range of point + 200 ° C. was inadequate, decarburization becomes remarkable, tensile least 1.4GPa it was not possible to ensure the strength.
[0140]
　Test does not satisfy the chemical composition of the present invention No. In 27 and 28, since the maximum value of the height roughness Rz is less than 3.0 [mu] m, a scale adhesion failure, in addition since the volume fraction of retained austenite is less than 5.0%, the total elongation It becomes less than 8.0% was inferior in ductility.
[0141]
　Test No. In 32 and 33, the time from rough rolling in the hot rolling process is completed until the start of finish rolling exceeds 10s. In addition, test No. In 34, lower than the range Si content defined in the present invention, also the coiling temperature was high. Due to these, test No. In 32-34, in addition to the maximum value of the height roughness Rz is less than 3.0 [mu] m, residual carbides number density of 4.0 × 10 3 cells / mm 2 because it exceeded the scale adhesion failure , and the and the impact value 40 J / cm 2 becomes less than the desired toughness is not obtained.
[0142]
　It should be noted that the test No. 29-31, the provisions of the present invention although satisfactory, is a reference example in which a heat treatment for steel sheet inferior in scale characteristic. Test No. In 29 and 31, scarfing amount in pickling process after hot rolling is due to insufficient, since the maximum value of the height roughness Rz exceeds 10.0 [mu] m, the scale peelability poor Met. Furthermore, the test No. In 30, due to scarfing amount was excessive in pickling process after hot rolling, since the maximum value of the height roughness Rz is less than 3.0 [mu] m, scale adhesion was poor It was.
[0143]
　Test No. In 29 and 31, since the maximum height roughness was too large, due to the irregular shape, partially uneven cooling occurs. In addition, test No. Even 30, for the adhesion of the scale is poor, partially uneven cooling occurs. Therefore, for these samples, variation occurs in the material. These trends, when actually performing hot forming was more pronounced.
Industrial Applicability
[0144]
　According to the present invention, heat treatment steel sheet against the heat-treating a steel sheet excellent in scale characteristic at the time of hot-forming, by heat treatment or hot forming process, is excellent in toughness and ductility and having a tensile strength of at least 1.4GPa it is possible to obtain a member. Heat treatment steel sheet member according to the present invention are particularly suitable for use as an anti-collision automotive parts.
The scope of the claims

[Claim 1]
　Chemical composition, in
　mass%,
　C: 0.05 ~
　0.50%, Si: 0.50 ~ 5.0%, Mn: 1.5
　~ 4.0%, P: 0.05% or
　less, S 0.05% or
　less, N: 0.01% or
　less,
　Ti: 0.01 ~ 0.10%, B: 0.0005
　~ 0.010%, Cr: 0 ~ 1.0%,
　Ni: 0 ~
　%
　2.0,
　Cu: 0 ~ 1.0%, Mo:
　0 ~ 1.0%, V: 0 ~ 1.0%, Ca:
　0 ~ 0.01%, Al: 0 ~ 1.0%,
　nb:
　0 ~ 1.0%, REM: 0 ~ 0.1%,
　the balance is Fe and impurities,
　mainly martensite and the metal structure volume fraction of residual austenite is 5.0% or more a,
　circle equivalent diameter present in the steel plate member is the number density of more residual carbides 0.1μm is 4.0 × 10 3 cells / mm 2 or
　less, defined by ASTM E8 That by using a plate specimen as measured mechanical properties below,
　the value of the strain-induced transformation parameter k is expressed by the following formula (i) is less than 20.0,
　a tensile strength of at least 1.4GPa , and the
　total elongation is 8.0% or more,
　heat treated steel member.
　= k (logf [gamma] 0 -Logf gamma (0.02)) / 0.02 · · · (i)
　where the meaning of each symbol in the formula is as follows.
　f [gamma] 0 : steel member the volume fraction of retained austenite present in the
　f gamma (0.02): a true strain of 0.02 was assigned to the steel plate member, the residual austenite present in the member after dividing pressurized volume rate
[Claim 2]
　The chemical composition, in
　mass%,
　Cr: 0.01 ~
　1.0%, Ni: 0.1 ~ 2.0%, Cu: 0.1
　~ 1.0%, Mo: 0.1 ~ 1.
　% 0,
　V:
　0.1 ~ 1.0%, Ca: 0.001 ~ 0.01%,
　Al: 0.01 ~ 1.0% Nb: 0.01 ~ 1.0%, and
　REM: 0 .001 to 0.1 percent,
　contains one or more selected from,
　heat treatment steel sheet member according to claim 1.
[Claim 3]
　Mn segregation ratio α represented by the following (ii) formula is 1.6 or less,
　a heat treatment steel sheet member according to claim 1 or claim 2.
　alpha = [Maximum Mn concentration in the thickness center portion (mass%)] / [average Mn concentration (mass%) of 1/4 depth position of the sheet thickness from the surface] · · · (ii)
[Claim 4]
　The value of the cleanliness of the steel defined by JIS G 0555 (2003) is less than 0.10%,
　a heat treatment steel sheet member according to any one of claims 1 to 3.
[Claim 5]
　で
　mass%,
　C: 0.05 ~
　0.50%, Si: 0.50 ~ 5.0%, Mn:
　4.0 ~ for 1.5%, P: 0.05% or
　less, S: 0.05 % or
　less, N: 0.01% or
　less,
　of Ti: 0.01 ~
　0.10%, B: 0.0005 ~ 0.010%, of Cr:
　0 ~ 1.0%, of Ni: 0 ~ 2.0% ,
　a
　Cu:
　0 ~ 1.0%,
　of Mo: 0 ~ 1.0%, V: 0 ~ 1.0%,
　of Ca: 0 ~ 0.01%, of Al: 0 ~
　1.0%, of Nb: 0 ~
　1.0%, the REM: 0 ~ 0.1%,
　remnants: Feおyoびimpuritiesでthou ru chemical composition wo have shi,
　surface niおke ru maximum height sa crude sa Rz ga 3.0 ~ 10.0μmでthou ri,
　yen equivalent diameter ga 0.1μm aboveののnumber density of carbides 10 ga × 8.0 . 3 th / mm 2 or lessでthou ru wo steel,
　the average temperature rise rate of 5 ℃ / s or moreのでAc . 3 point ~ Ac . 3 point + 200 ℃のtemperature range ma - de after heating shi ta , The former referred to the temperature domain kara the Ms Ma - de After the above upper critical cooling rateでcooling shi,その, Ms point kara 100 ℃ Ma - de 5 ℃ / average cooling rateでcooling suru, s or lessの
　heat-treated steel memberのmanufacturing method.
[Claim 6]
　The chemical composition, in
　mass%,
　Cr: 0.01 ~
　1.0%, Ni: 0.1 ~ 2.0%, Cu: 0.1
　~ 1.0%, Mo: 0.1 ~ 1.
　% 0,
　V:
　0.1 ~ 1.0%, Ca: 0.001 ~ 0.01%,
　Al: 0.01 ~ 1.0% Nb: 0.01 ~ 1.0%, and
　REM: 0 .001 to 0.1 percent,
　contains one or more selected from,
　a manufacturing method of a heat treatment steel sheet member according to claim 5.
[Claim 7]
　The number density of residual carbides present in the steel sheet member is 4.0 × 10 3 cells / mm 2 or less,
　the manufacturing method of the heat treatment steel sheet member according to claim 5 or claim 6.
[8.]
　Mn segregation ratio α represented by the following (ii) formula is 1.6 or less,
　the manufacturing method of the heat treatment steel sheet member according to any of claims 5 to claim 7.
　alpha = [Maximum Mn concentration in the thickness center portion (mass%)] / [average Mn concentration (mass%) of 1/4 depth position of the sheet thickness from the surface] · · · (ii)
[Claim 9]
　The value of the cleanliness of the steel defined by JIS G 0555 (2003) is less than 0.10%,
　the manufacturing method of the heat treatment steel sheet member according to any of claims 5 to claim 8.
[Claim 10]
　After heating to the temperature range, prior to cooling to the Ms point it is subjected to hot forming the steel sheet
　manufacturing method of the heat treatment steel sheet member according to any of claims 5 to claim 9.