Technical field
[0001]
　The present invention relates to a steel plate.
　The present application claims priority based on Japanese Patent Application No. 2017-215829 filed in Japan on November 08, 2017, the contents of which are incorporated herein by reference.
Background technology
[0002]
　In recent years, in order to protect the global environment, it is required to improve the fuel efficiency of automobiles. In order to improve the fuel efficiency of automobiles, higher strength is required for steel sheets for automobiles in order to secure the safety and reduce the weight of the vehicle body. As such a high-strength steel plate, a composite structure steel plate having a composite structure represented by a Dual-Phase steel (DP steel) in which a structure is a combination of a hard structure such as martensite and bainite and a soft ferrite is often used.
[0003]
　Since such DP steel is generally a high alloy, alloy elements such as Mn segregate in the direction parallel to the plate thickness direction in the melting process. Since the segregated portion is stretched by hot rolling or cold rolling, the segregated portion is continuous in a band-like layer (hereinafter, referred to as microsegregation). In the case of DP steel, a hard phase is generated in this microsegregated portion. As a result, the hard phase has a structure in which the hard phases are continuous. It is known that such a structure in which hard phases formed by microsegregation and connected in a band form significantly deteriorates hole expandability and bendability.
[0004]
　As a technique for solving the problem caused by the above-mentioned micro segregation in DP steel, for example, in Patent Document 1, 0.5 h or more and 5 h or less are held in a temperature range of 1200° C. or more and 1300° C. or less before a hot rolling process. A steel sheet is described in which the ratio C1/C2 of the upper limit value C1 and the lower limit value C2 of the Mn concentration in the section of the steel sheet in the plate thickness direction is set to 2.0 or less by diffusing Mn. It is disclosed that, in this steel sheet, the variation in stretch flangeability was significantly reduced by setting C1/C2 to 2.0 or less.
[0005]
　On the other hand, if the strength of the steel sheet is increased, the ductility is reduced and cold press forming becomes difficult. Therefore, there is a demand for a material that is relatively soft during molding and is easy to mold, and that has a large amount of baking and curing during coating baking after molding. That is, in order to further strengthen the strength of automobile parts, a steel plate having a high bake hardenability is required. Bake hardening is a strain that occurs when an interstitial element (carbon or nitrogen) is fixed during baking of a coating at a high temperature (150°C to 200°C) to a dislocation introduced by press molding (hereinafter also referred to as "prestrain"). It is an aging phenomenon.
　However, the DP steel sheet containing a large amount of ferrite as disclosed in Patent Document 1 has a problem that the bake hardenability is generally low.
[0006]
　As a technique for improving the bake hardenability, for example, in Patent Document 2, a hard bakenite and martensite as a main structure is used as a main structure, and a high bake hardenability is secured by limiting the fraction of ferrite to 5% or less. Cold rolled steel sheet is described.
[0007]
　However, as a result of examination by the present inventors, in the cold-rolled steel sheet described in Patent Document 2, although a certain bake hardenability is obtained when the prestrain is 1%, the prestrain is small (for example, 0.5%). It was found that in the case of (1), sufficient bake hardenability could not be obtained. That is, in the cold rolled steel sheet of Patent Document 2, it is necessary to increase the prestrain in order to obtain high bake hardenability.
Prior art documents
Patent literature
[0008]
Patent Document 1: Japanese Patent Laid-Open No. 2010-065307
Patent Document 2: Japanese Patent Laid-Open No. 2008-144233
Summary of the invention
Problems to be Solved by the Invention
[0009]
　As described above, in steel sheets for automobiles, in order to meet the demand for higher strength in the future, it is necessary to secure excellent bake hardenability. On the other hand, when it is necessary to increase the pre-strain in order to obtain high bake hardenability, it cannot be applied to a member having a low workability during press molding or the like. Further, if the pre-strain is increased, the ductility decreases, so that it is difficult to apply it to a member that requires excellent ductility.
[0010]
　The present invention has been made in view of the above problems. An object of the present invention is to provide a steel sheet excellent in bake hardenability that can obtain a sufficient bake hardenability even with a prestrain of 0.5%.
Means for solving the problems
[0011]
　The present inventors have earnestly studied to solve the above problems. As a result, although there are two types of segregation, center segregation and micro segregation, in order to improve the bake hardenability, the micro segregation of alloying elements is reduced, and the dislocation density is increased. Clarified that it is important to be an organization.
　Conventionally, in a structure containing 95% or more of a hard structure, the hard phase is not formed into a band shape, so that microsegregation has hardly been considered. However, the present inventors have found that, in a structure containing 95% or more of a hard structure, by reducing microsegregation, dislocations introduced by prestrain are made uniform, and further, by increasing the dislocation density during manufacturing, seizure It was found that the curability was improved.
[0012]
　The steel sheet of the present invention which has achieved the above object is as follows.
(1) The steel sheet according to one aspect of the present invention is, in mass %, C: 0.05 to 0.30%, Si: 0.2 to 2.0%, Mn: 2.0 to 4.0%, Al: 0.001 to 2.000%, P: 0.100% or less, S: 0.010% or less, N: 0.010% or less, Ti: 0 to 0.100%, Nb: 0 to 0. 100%, V: 0 to 0.100%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 1.00%, W: 0 to 0.005%, Ca: 0 to 0.005%, Mg: 0 to 0.005%, Rare Earth Element (REM): 0 to 0.010%, B: 0 to 0.0030% The balance has a chemical composition consisting of Fe and impurities, and the metallographic structure contains a hard structure of 95% or more and an austenite content of 0 to 5% in terms of area ratio, and in a mass% in the thickness direction cross section. , C1/C2, which is the ratio of the upper limit C1 of the Mn content to the lower limit C2 of the Mn content, is 1.5 or less, and the bake hardening amount BH is 150 MPa or more.
[0013]
(2) In the steel sheet according to (1) above, the chemical composition is, in mass%, Ti: 0.003 to 0.100%, Nb: 0.003 to 0.100%, V: 0.003 to. 0.100% of 1 type(s) or 2 or more types may be included, and a total content may be 0.100% or less.
[0014]
(3) In the steel sheet according to (1) or (2) above, the chemical composition is, in mass%, Cu: 0.005 to 1.00%, Ni: 0.005 to 1.00%, Mo: One or more of 0.005 to 1.00% and Cr: 0.005 to 1.00% may be included, and the total content may be 1.00% or less.
[0015]
(4) In the steel sheet according to any one of the above (1) to (3), the chemical composition is% by mass, W: 0.0003 to 0.005%, Ca: 0.0003 to 0.005% , Mg: 0.0003 to 0.005%, rare earth element (REM): 0.0003 to 0.010%, and the total content is 0.010% or less. Good.　
[0016]
(5) In the steel sheet according to any one of (1) to (4) above, the chemical composition may include B:00001 to 0.0030% by mass %.
Effect of the invention
[0017]
　According to the above aspect of the present invention, a steel sheet excellent in bake hardenability can be provided by controlling the microsegregation of alloying elements in the steel sheet and increasing the dislocation density in the hard structure. This steel sheet is excellent in press formability and is further strengthened by being baked during coating after press forming, so that it becomes suitable as a structural member for automobiles and the like. In the present invention, “excellent bake hardenability” means that the bake hardenable amount (BH amount) after heat treatment at 170° C. for 20 minutes after applying 0.5% prestrain is 150 MPa or more.
MODE FOR CARRYING OUT THE INVENTION
[0018]
　Bake hardening is a strain aging phenomenon that occurs when interstitial elements (carbon and nitrogen) are fixed to dislocations that have been introduced into steel beforehand by prestrain when heated to a high temperature (150°C to 200°C). .. In the case of a steel sheet for automobiles, it occurs when interstitial elements (carbon and nitrogen) are fixed to the dislocations introduced by a press or the like at the time of forming into parts, during baking for coating.
　The bake-hardening amount is controlled by the dislocation density and the amount of solute carbon, and becomes more prominent as both parameters increase. Further, since the hard structure has more solid solution carbon than ferrite, the bake hardenability is high. The inventors of the present invention have earnestly studied to further improve the bake hardening amount in a high-strength steel sheet having a hard structure as a main phase. As a result, in a high-strength steel sheet having a hard structure as a main phase, the Si content and the Mn content are relatively large, and these alloy elements are easily segregated, so that dislocations introduced by prestrain do not uniformly enter. There was found. It was also found that segregation of alloy elements easily causes a hardness difference in the hard structure, and the effect of this hardness difference does not improve the bake hardening amount.
　As a result of further study by the inventor, it became clear that the nonuniformity of hardness difference and prestrain is caused by microsegregation formed by stretching the segregated portion during solidification by hot rolling or cold rolling. It was Further, the present inventors have made uniform the dislocations introduced by pre-strain by reducing the micro-segregation of alloying elements, further, by increasing the dislocation density during manufacturing, the steel sheet having a hard structure as the main phase It was found that the bake hardenability was improved.
　Further, it was found that the optimization of hot rolling conditions is effective for reducing the above-mentioned microsegregation, and the temper rolling after the annealing step is effective for increasing the dislocation density. ..
　During casting, substitutional elements such as Si and Mn segregate parallel to the rolling direction at the central portion of the plate thickness. This is commonly called central segregation. Due to such center segregation, cracks occur at the center of the plate thickness of the slab, and uneven distribution of alloying elements makes it difficult to control the structure in the subsequent annealing step and makes the material unstable. As a result of the study by the present inventors, even if the center segregation is reduced, the bake hardenability is not improved unless the micro segregation is reduced. On the other hand, it was found that bake hardenability is improved if the microsegregation can be controlled even if there is central segregation.
[0019]
　Hereinafter, the steel sheet according to this embodiment will be described.
[0020]
　A steel plate according to an embodiment of the present invention (a steel plate according to the present embodiment) has a mass% of C: 0.05 to 0.30%, Si: 0.2 to 2.0%, and Mn: 2.0. Up to 4.0%, P: 0.100% or less, S: 0.010% or less, Al: 0.001 to 2.000%, N: 0.010% or less, and optionally Ti or Nb. , V, Cu, Ni, Mo, Cr, W, Ca, Mg, REM, B, and the balance has a chemical composition of Fe and impurities, and the metal structure has an area ratio of 95% or more of hard. The ratio C1/C2 between the upper limit value C1 (unit: mass%) and the lower limit value C2 (unit: mass%) of the Mn content in the thickness direction cross section of the steel sheet, which contains the structure and 0 to 5% of retained austenite, It is a steel sheet having excellent bake hardenability, which is 1.5 or less, the bake hardening amount BH is 150 MPa or more, and the tensile strength TS is preferably 900 MPa or more.
　The chemical components and the structure will be described below.
[0021]
(I): Chemical composition
　The steel sheet according to the present embodiment is characterized in that the structure morphology is controlled by a manufacturing method, but in order to obtain a steel sheet having excellent workability and further improved bake hardenability. It is preferable that the chemical composition is properly adjusted. Therefore, the chemical composition of the steel sheet according to this embodiment and the slab used for manufacturing the steel sheet will be described. In the following description, “%”, which is a unit of the content of each element contained in the steel plate and the slab, means “mass %” unless otherwise specified.
[0022]
(C: 0.05% to 0.30%)
　C is an element that enhances the hardenability of the steel sheet. Further, C is an element having the action of increasing the strength by including it in a hard structure such as a martensite structure. It is also an element that has the effect of enhancing the bake hardenability. In order to effectively exhibit the above-mentioned effects, the C content is set to 0.05% or more. Preferably it is 0.07% or more.
　On the other hand, if the C content exceeds 0.30%, the weldability deteriorates. Therefore, the C content is set to 0.30% or less, preferably 0.20% or less.
[0023]
(Si: 0.2% to 2.0%)
　Si is an element necessary for suppressing the formation of carbides and ensuring solid solution C necessary for bake hardening. If the Si content is less than 0.2%, sufficient effect may not be obtained. Further, it is an essential element for increasing the solid solution C and increasing the strength of the steel sheet excellent in bake hardenability. In order to effectively exhibit this effect, the Si content is 0.2% or more. It is more preferably 0.5% or more.
　On the other hand, if the Si content exceeds 2.0%, not only the surface properties are deteriorated, the effect of inclusion is saturated, but also the cost is increased. Therefore, the Si content is 2.0% or less, preferably 1.5% or less.
[0024]
(Mn: 2.0% to 4.0%)
　Mn is an element that contributes to the improvement of hardenability and is an element that is useful for increasing the strength of the steel sheet. In order to effectively exhibit such an effect, the Mn content is set to 2.0% or more. Preferably it is 2.3% or more.
　On the other hand, excessive Mn content causes a decrease in low temperature toughness due to precipitation of MnS. Therefore, the Mn content is set to 4.0% or less.
[0025]
(P: 0.100% or less)
　P is not an essential element, but is an element contained as an impurity in steel, for example. From the viewpoint of weldability, the lower the P content, the better. In particular, when the P content exceeds 0.100%, the weldability is significantly reduced. Therefore, the P content is 0.100% or less, preferably 0.030% or less.
　On the other hand, the smaller the P content, the more preferable it is, so it may be 0%. However, the reduction of the P content is costly, and if it is attempted to reduce it to less than 0.0001%, the cost is significantly increased. Therefore, the P content may be 0.0001% or more. Further, since P contributes to the improvement of strength, the P content may be 0.0001% or more.
[0026]
(S: 0.010% or less)
　S is not an essential element, but is an element contained as an impurity in steel, for example. From the viewpoint of weldability, the lower the S content, the better. The higher the S content, the more the amount of MnS precipitated and the lower the low temperature toughness. In particular, when the S content exceeds 0.010%, the weldability and the low temperature toughness are significantly reduced. Therefore, the S content is set to 0.010% or less, preferably 0.005% or less.
　On the other hand, the smaller the S content is, the more preferable it is, and therefore it may be 0%. However, reducing the S content is costly, and if it is attempted to reduce it to less than 0.0001%, the cost is significantly increased. Therefore, the S content may be 0.0001% or more.
[0027]
　(Al: 0.001% to 2.000%)
　Al is an element effective in deoxidizing and improving the yield of carbide forming elements. The Al content is set to 0.001% or more in order to effectively exhibit the above-mentioned effects. Preferably it is 0.010% or more.
　On the other hand, when the Al content exceeds 2.000%, the weldability is deteriorated, or the oxide-based inclusions are increased to deteriorate the surface quality. Therefore, the Al content is set to 2.000% or less. It is preferably 1.000% or less.
[0028]
(N: 0.010% or less)
　N is not an essential element and is contained as an impurity in steel, for example. From the viewpoint of weldability, the lower the N content, the better. In particular, when the N content exceeds 0.010%, the weldability is remarkably deteriorated. Therefore, the N content is set to 0.010% or less, preferably 0.006% or less.
　On the other hand, the smaller the N content, the more preferable it is, and therefore it may be 0%. However, the reduction of the N content is costly, and if it is attempted to reduce it to less than 0.0001%, the cost is significantly increased. Therefore, the N content may be 0.0001% or more.
[0029]
　The basic composition of the steel sheet according to the present embodiment is as described above, and the balance is Fe and impurities brought in depending on the situation of raw materials, materials, manufacturing equipment and the like. Furthermore, the steel sheet according to the present embodiment may contain the following optional elements, if necessary. Since the following optional elements are not necessarily contained, the lower limit thereof is 0%.
[0030]
(Ti: 0.100% or less, Nb: 0.100% or less, V: 0.100% or less)
　Ti, Nb and V are elements that contribute to the improvement of strength. Therefore, a plurality of Ti, Nb or V or any combination thereof may be contained. In order to sufficiently obtain this effect, the content of Ti, Nb or V, or the total content of any combination of two or more of these is preferably 0.003% or more.
　On the other hand, if the content of Ti, Nb, or V, or the total content of any combination of two or more kinds thereof exceeds 0.100%, hot rolling and cold rolling become difficult. Therefore, the Ti content, the Nb content, the V content, or the total content of any combination of two or more thereof is 0.100% or less. That is, the limiting range for each component alone is Ti: 0.003% to 0.100%, Nb: 0.003% to 0.100%, and V: 0.003% to 0.100%. At the same time, the total content in the case of arbitrarily combining these is also preferably 0.003 to 0.100%.
[0031]
(Cu: 1.00% or less, Ni: 1.00% or less, Mo: 1.00% or less, Cr: 1.00% or less)
　Cu, Ni, Mo and Cr are elements contributing to the improvement of strength. .. Therefore, Cu, Ni, Mo, or Cr, or any combination thereof may be contained. In order to sufficiently obtain this effect, the content of Cu, Ni, Mo and Cr is preferably 0.005 to 1.00% in the case of using each component alone, and any two or more of these may be arbitrarily combined. In that case, the total content is preferably 0.005% to 1.00%.
　On the other hand, if the content of Cu, Ni, Mo and Cr, or the total content of any two or more of them is more than 1.00%, the effect due to the above-mentioned action is saturated and the cost is unnecessarily high. Get higher Therefore, the upper limit of the content of Cu, Ni, Mo and Cr, or the total content of any two or more of these is 1.00%. That is, Cu: 0.005% to 1.00%, Ni: 0.005% to 1.00%, Mo: 0.005% to 1.00%, and Cr: 0.005% to 1.00%. In addition, the total content in the case of arbitrarily combining these is also preferably 0.005 to 1.00%.
[0032]
(W: 0.005% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.010% or less)
　W, Ca, Mg and REM contribute to fine dispersion of inclusions. , Is an element that enhances toughness. Therefore, one or more of W, Ca, Mg or REM or two or more of them in any combination may be contained. In order to sufficiently obtain the above effect, the total content of one kind or an arbitrary combination of two or more kinds of W, Ca, Mg and REM is preferably 0.0003% or more.
　On the other hand, if the total content of W, Ca, Mg and REM exceeds 0.010%, the surface properties deteriorate. Therefore, the total content of W, Ca, Mg and REM is set to 0.010% or less. That is, W: 0.0003 to 0.005%, Ca: 0.00030.005%, Mg: 0.0003 to 0.005%, REM: 0.0003 to 0.010%, and any of these It is preferable that the total content of two or more of the above is 0.0003 to 0.010%.
　REM (rare earth element) refers to a total of 17 kinds of elements of Sc, Y and lanthanoid, and “REM content” means the total content of these 17 kinds of elements. Lanthanoids are added industrially, for example in the form of misch metal.
[0033]
(B: 0.0030% or less)
　B is an element for improving hardenability and is an element useful for increasing the strength of a bake hardening steel sheet. B may be contained in an amount of 0.0001% (1 ppm) or more.
　On the other hand, even if the B content exceeds 0.0030% (30 ppm), the above effect is saturated and the cost is increased. Therefore, the B content is 0.0030% or less. It is preferably 0.0025% or less.
[0034]
(II): Structure of Steel The
　steel sheet according to the present embodiment targets a structure containing a hard structure and retained austenite. The steel sheet according to the present embodiment is characterized by improving the bake hardenability by controlling the microsegregation of Mn and increasing the dislocation density. The reason for defining the area ratio of each organization will be described.
[0035]
(Hard structure: 95% or more)
　The steel sheet according to the present embodiment has a great feature in that the hard structure is secured in the metal structure in an area ratio of 95% or more. Here, the hard structure refers to bainite and martensite. That is, in the steel sheet according to the present embodiment, the total area ratio of bainite and martensite is 95% or more. Thereby, the dislocation density at the time of manufacturing the steel sheet can be increased, and as a result, the bake hardenability can be enhanced. In order to further enhance such an effect, it is recommended to secure a hard structure of 97% or more. The area ratio of the hard structure is more preferably 99% or more, and may be 100%.
[0036]
(Retained Austenite) A
　trace amount of retained austenite may be generated depending on the steel composition and manufacturing method. If the area ratio of such retained austenite is 5% or less, not only the bake hardenability is not affected, but also the TRIP effect when deformed contributes to the improvement of ductility. Therefore, the steel sheet according to the present embodiment may contain retained austenite in an area ratio of 5% or less.
　However, in order to further improve the bake hardenability, the retained austenite is preferably 3% or less in area ratio, more preferably 1% or less, and further preferably 0%.
　As the residual structure other than the hard structure and the retained austenite, ferrite and pearlite may be generated, but the total area ratio (%) of these is preferably 1% or less, and more preferably 0%. ..
[0037]
　In the present embodiment, the area ratio of hard tissue is determined as follows. First, a sample is taken with a plate thickness cross section perpendicular to the rolling direction of the steel plate as an observation surface, the observation surface is polished, and corroded with nital, and a structure at a 1/4 position of the thickness of the steel plate is magnified at a magnification of 5000 times. Observe with SEM (scanning electron microscope). Image analysis is performed in a field of view of 100 μm×100 μm to measure the area ratio of ferrite and pearlite. Five visual fields are measured at the center of the plate width direction, and the average of these measured values ​​is obtained. The ferrite here refers to, for example, polygonal ferrite, pseudopolygonal ferrite, and Widmanstatten ferrite, and when carbide is present in the lath or at the lath boundary, it can be determined to be bainite or martensite.
　Then, the area ratio of retained austenite is calculated. The area ratio of retained austenite can be specified by, for example, X-ray diffraction measurement. In this method, for example, a portion from the surface of the steel sheet to 1/4 of the thickness of the steel sheet is removed by mechanical polishing and chemical polishing, and MoKα ray is used as the characteristic X-ray. Then, from the integrated intensity ratios of the diffraction peaks of (200) and (211) of the body-centered cubic lattice (bcc) phase and (200), (220) and (311) of the face-centered cubic lattice (fcc) phase, The volume ratio of retained austenite is calculated by using the equation below. Then, assuming that the volume ratio is equal to the area ratio, this is defined as the area ratio.
　A value obtained by subtracting the area ratio of ferrite and pearlite and the area ratio of retained austenite obtained by the above method from the whole (100%) is defined as the area ratio of the hard structure.
[0038]
　Sγ=(I 200f +I 220f +I 311f )/(I 200b +I 211b )×100
[0039]
　In the above formula, Sγ is the area ratio of retained austenite, I 200f , I 220f , and I 311f are the intensity of diffraction peaks of (200), (220), and (311) of the fcc phase, and I 200b and I 211b are, respectively. The intensity of the diffraction peaks of (200) and (211) of the bcc phase is shown, respectively.
[0040]
(C1/C2 is 1.5 or less)
　The ratio C1/C2 between the upper limit C1 (unit: mass%) and the lower limit C2 (unit: mass%) of the Mn concentration in the thickness direction cross section of the steel sheet is 1.5. Below. More preferably, C1/C2 is 1.3 or less. When C1/C2 is 1.5 or less, microsegregation of alloy elements is suppressed, particularly microsegregation of Mn is suppressed, and the structure becomes uniform. As a result, the bake hardening amount BH and the tensile strength can be increased.
　When the hard structure is the main phase and the ferrite fraction is 5% or less, a structure in which the hard structure is continuous in a band shape is not generated. In such a case, it is considered that the necessary hole expandability and bendability can be secured without eliminating the microsegregation. Also, if micro-segregation is eliminated in the hard structure, the yield ratio will be high, and there is a concern that the load required for forming will be large. Was not considered. On the other hand, even in such a case, sufficient local ductility may not be obtained. In the steel sheet according to the present embodiment, the local ductility is improved by setting the microsegregation represented by C1/C2 to 1.5 or less.
[0041]
　The degree of Mn microsegregation represented by C1/C2 is measured as follows.
　After adjusting so that a cross section in the plate thickness direction in which the rolling direction is the normal direction of the steel plate can be observed, it is mirror-polished and the surface of the steel plate in the plate thickness direction cross section of the steel plate is measured by an EPMA (electronic probe microanalyzer) device. To 100 μm in the plate thickness direction included in the region from the ⅜ position to the ¼ position of the thickness of the steel plate, 0.5 μm from one surface side to the other surface side along the steel plate thickness direction. The Mn content of 200 points is measured at intervals. At this time, the inclusions such as MnS are avoided and the Mn content is measured. The same measurement was performed on 5 lines that cover almost the entire widthwise area in the steel sheet cross section, and among the Mn contents measured on all 5 lines, the highest value was the upper limit value M1 of the Mn content (unit: unit). : Mass%), and the lowest value is defined as the lower limit C2 (mass %) of the Mn content, and the ratio C1/C2 is calculated. The measurement from the surface of the steel sheet to the area from ⅜ position to ¼ position of the thickness of the steel sheet is performed in the range where this range shows a typical structure of the steel sheet and is not affected by center segregation. Because there is.
[0042]
(Tensile Strength TS: 900 MPa or More)
　The steel sheet according to the present embodiment preferably has a tensile strength of 900 MPa. The tensile strength is set to 900 MPa or more in order to satisfy the demand for weight reduction of the automobile body. The tensile strength TS is more preferably 1000 MPa or more, further preferably 1100 MPa or more.
[0043]
(Bake hardening amount BH: 150 MPa or more) In the
　steel sheet according to this embodiment, a bake hardening amount BH after applying a 0.5% prestrain and performing heat treatment at 170° C. for 20 minutes is 150 MPa or more.
　When the bake hardening amount BH is less than 150 MPa, it is difficult to mold and the strength after molding is low, so that it cannot be said to be excellent bake hardenability. Therefore, BH is set to 150 MPa or more. It is more preferably 200 MPa or more, and most preferably 250 MPa or more.
　Further, the larger the pre-strain, the larger the amount of bake hardening. However, if the prestrain is increased in order to increase the amount of bake hardening, the ductility of the steel sheet after bake hardening is reduced. In the steel sheet according to the present embodiment, the bake hardening amount after applying a relatively small prestrain of 0.5% prestrain is 150 MPa or more.
[0044]
　The method for measuring the bake hardening amount BH is as follows.
　First, a No. 5 test piece defined in JIS Z 2241:2011 whose longitudinal direction is the direction perpendicular to the rolling direction is prepared from a steel plate. Then, a tensile load is applied to this test piece to apply a 0.5% prestrain, and then heat treatment is performed at 170° C. for 20 minutes. Next, the yield stress when the test piece after the heat treatment is re-tensioned is obtained, and the value obtained by subtracting the stress when 0.5% prestrain is applied from this yield stress is obtained, and this value is taken as the bake hardening amount BH.
[0045]
(III): Manufacturing Method
　Next, a method for manufacturing the steel sheet according to the present embodiment will be described. The following description is intended to exemplify the characteristic method for manufacturing the steel plate according to the present embodiment described above, and the steel plate according to the present embodiment is manufactured by a manufacturing method as described below. It is not intended to be limited to one.
　In the method for manufacturing a steel sheet according to the present embodiment,
(I) a homogenizing step of performing multiaxial deformation processing of a slab having the above chemical composition,
(II) a rolling step of performing hot rolling and cold rolling,
(III) The annealing process and the temper rolling process
are performed in this order. In the rolling step, pickling may be performed before cold rolling. In the method for manufacturing a steel sheet according to the present embodiment, multiaxial deformation processing is performed instead of rough rolling normally performed in hot rolling. In the multi-axial deformation process, the compression deformation process is performed not only in the thickness direction of the slab but also in the width direction of the slab, so that it is possible to eliminate the microsegregation of alloy elements (especially Mn).
[0046]
(Homogenization step) The
　slab to be subjected to the homogenization step can be manufactured by a continuous casting method, for example, by melting molten steel having the above chemical composition using a converter or an electric furnace. Instead of the continuous casting method, an ingot making method, a thin slab casting method or the like may be adopted.
[0047]
　The slab is heated to 1000°C to 1300°C before being subjected to multiaxial deformation processing. If the slab heating temperature is low, the finish rolling temperature will fall below the Ac 3 transformation point, and multiaxial deformation processing and subsequent rolling will be carried out in the two-phase region of ferrite and austenite, and the hot-rolled sheet structure will be inhomogeneous. There may be a mixed grain structure. In this case, the inhomogeneous structure cannot be resolved even after the cold rolling and annealing steps.
　Moreover, even if the slab heating temperature exceeds 1300° C., the effect of eliminating segregation of alloying elements is saturated. Therefore, the upper limit of the slab heating temperature is preferably set to 1300°C or lower.
　The heating and holding time is not particularly limited, but it is preferable to hold the heating temperature for 30 minutes or more in order to bring the slab center to a predetermined temperature. The heating and holding time is preferably 10 hours or less and more preferably 5 hours or less in order to suppress excessive scale loss.
[0048]
　Multi-axial deformation processing is performed on the heated slab. In the multi-axis deformation process, the compression deformation process in the width direction and the compression deformation process in the thickness direction are performed on the slab at 1000°C to 1250°C. Here, the width direction of the slab is a direction corresponding to the plate width direction of the steel plate as a product, and the thickness direction of the slab is a direction corresponding to the plate thickness direction of the steel plate as a product. Due to the multiaxial deformation process, a portion of the slab where the alloy element such as Mn is concentrated is subdivided or lattice defects are introduced. Therefore, microsegregation of alloying elements is suppressed during multiaxial deformation processing, and an extremely homogeneous structure can be obtained. In particular, compression deformation processing in the width direction of the slab is effective. That is, the multiaxial deformation process finely divides the concentrated portion of the alloy element existing in the width direction so that the alloy element is uniformly dispersed. As a result, it is possible to achieve homogenization of the structure in a short time, which cannot be achieved by simply diffusing the alloy elements by heating for a long time.
　The multiaxial deformation processing includes, for example, compression deformation processing in the width direction and compression deformation processing in the thickness direction.
[0049]
　The multiaxial deformation processing is preferably performed in the temperature range of 1000 to 1250°C. If the slab temperature during multiaxial deformation processing is less than 1000° C., the multiaxial deformation processing will be performed in the two-phase region of ferrite and austenite, and ferrite may precipitate in the metallurgical structure of the steel sheet, which is not preferable. .. Further, even if the slab temperature during multiaxial deformation exceeds 1250°C, the segregation effect of alloying elements is saturated, so the upper limit is preferably 1250°C or less. That is, the maximum temperature during multiaxial deformation processing is 1250°C or lower, and the minimum temperature is 1000°C or higher.
[0050]
　If the deformation rate per compression deformation process in the width direction is less than 3%, the amount of lattice defects introduced by plastic deformation is insufficient, and segregation of alloy elements cannot be suppressed. Therefore, the deformation rate per compression deformation process in the width direction is set to 3% or more, preferably 10% or more, and more preferably 30% or more.
　On the other hand, if the deformation rate per compression deformation process in the width direction exceeds 50%, slab cracking may occur or the slab shape may become non-uniform and the dimensional accuracy of the hot rolled steel sheet obtained by hot rolling may deteriorate. Or Therefore, the deformation rate per compression deformation process in the width direction is 50% or less, preferably 40% or less.
[0051]
　If the deformation rate per compression deformation process in the thickness direction is less than 3%, the amount of lattice defects introduced by plastic deformation is insufficient, and segregation of alloy elements cannot be suppressed. Further, due to the defective shape, there is a possibility that the slab may not be properly caught in the rolling roll during hot rolling. Therefore, the deformation rate per compression deformation process in the thickness direction is 3% or more, preferably 10% or more, more preferably 30% or more.
　On the other hand, if the deformation rate per compression deformation process in the thickness direction exceeds 50%, slab cracking may occur, or the slab shape may become uneven and the dimensional accuracy of the hot rolled steel sheet obtained by hot rolling may deteriorate. To do Therefore, the deformation rate per compression deformation process in the thickness direction is 50% or less, preferably 40% or less.
[0052]
　When the difference between the deformation ratio in the width direction and the deformation ratio in the thickness direction is excessively large, alloy elements such as Mn do not sufficiently diffuse in the direction perpendicular to the direction in which the deformation ratio is small, and microsegregation in the hard structure occurs. It may not be able to be reduced sufficiently. In particular, when the difference in deformation rate exceeds 20%, it is difficult to eliminate microsegregation. Therefore, the difference in deformation rate between the width direction and the thickness direction is preferably 20% or less.
[0053]
　If the multiaxial deformation process is performed at least once (the width direction process and the thickness direction process are performed once), segregation of alloy elements can be suppressed. However, the effect of suppressing segregation of alloy elements becomes remarkable by repeating the multiaxial deformation process. Therefore, the number of times of multiaxial deformation processing is set to 1 or more, preferably 2 or more. When the multiaxial deformation processing is performed twice or more, the slab may be reheated to a temperature range of 1000°C to 1250°C between the multiaxial deformation processing.
　On the other hand, if the number of times of the multi-axial deformation processing exceeds 5, the manufacturing cost will increase excessively, or the scale loss will increase and the yield will decrease. In addition, the thickness of the slab becomes uneven, which may make hot rolling difficult. Therefore, the number of times of multiaxial deformation processing is preferably 5 times or less, more preferably 4 times or less.
[0054]
　The deformation rate in multiaxial deformation processing is defined as follows. The deformation rate in the case of performing the multiaxial deformation process involving the compressive deformation process in the width direction and the thickness direction once on the slab is the width dimension w 1 and the thickness dimension t 1 of the slab before the multiaxial deformation process. Then , based on the width dimension w 2 and the thickness dimension t 2 of the slab after the multiaxial deformation processing, the deformation rate is obtained from the following equation. Further, when the multiaxial deformation processing is performed a plurality of times, the deformation rate is obtained from the width dimension and the thickness dimension before and after the respective multiaxial deformation processing.
[0055]
　Deformation ratio in the width direction (%)=(w 2 −w 1 )/w 1 ×100
　Deformation ratio in the thickness direction (%)=(t 2 −t 1 )/t 1 ×100
[0056]
(Rolling Step)
　Hot rolling is performed as finish rolling on the slab after the multiaxial deformation processing. Further, cold rolling is performed after pickling the hot-rolled steel sheet after hot rolling, if necessary. In the method for manufacturing a steel sheet according to the present embodiment, as hot rolling, so-called rough rolling is not performed, but finish rolling is performed on the slab after the multiaxial deformation processing.
[0057]
　For hot rolling, a slab after multiaxial deformation is used as the material, this slab is heated to 1000°C or higher, and the total rolling reduction (cumulative rolling reduction) of the heated slab is 50% or less, and hot rolling is completed. Hot rolling is performed at a temperature (FT) of 800° C. or higher. Then, it is air-cooled and wound at a winding temperature (CT) of 500° C. or higher and 700° C. or lower. By performing hot rolling under such conditions, Mn subdivided by the multiaxial deformation process is further diffused, and micro segregation of Mn can be eliminated.
　If the total rolling reduction is more than 50%, the austenite is stretched, Mn is concentrated, and microsegregation cannot be eliminated. Therefore, the total rolling reduction is 50% or less. When the hot rolling finish temperature is 800° C. or lower, recrystallization becomes insufficient and unrecrystallized austenite remains, so that Mn is concentrated and microsegregation cannot be eliminated. Therefore, the hot rolling finish temperature is 800° C. or higher, preferably 850° C. or higher.
　If the winding temperature is higher than 700° C., pearlite is generated, Mn is concentrated, and microsegregation cannot be eliminated. Therefore, the winding temperature is 700° C. or lower, preferably 650° C. or lower. On the other hand, if the winding temperature is less than 500° C., the alloying elements do not diffuse during winding, and the Mn microsegregation cannot be eliminated. Therefore, the winding temperature is 500°C or higher, preferably 550°C or higher.
[0058]
　As for cold rolling, the total reduction ratio of cold rolling is preferably 50% or more from the viewpoint of homogenizing and refining the structure.
[0059]
(Continuous annealing step)
　The steel sheet (cold rolled steel sheet) obtained through the rolling step is annealed. The annealing is performed by heating in a temperature range of Ac 3 to 1200° C. and holding for 10 to 1000 seconds. The annealing temperature is the surface temperature of the steel sheet. This temperature range and the annealing time are for austenitic transformation of the entire steel sheet.
　If the annealing time exceeds 1000 seconds, the productivity will deteriorate. Therefore, the annealing time is set to 10 to 1000 seconds. If the annealing temperature is less than Ac 3 or the annealing time is less than 10 seconds, ferrite tends to precipitate. If the annealing temperature exceeds 1200°C, the austenite grain size becomes coarse, a hard structure with a large lath width is formed, and the toughness decreases.
　The Ac 3 point is calculated by the following formula. The mass% of the element is substituted for the element symbol in the following formula. 0 mass% is substituted for the elements not contained.
[0060]
　Ac 3 =937-477×C+56×Si-20×Mn-16×Cu-27×Ni-5×Cr+38×Mo+125×V+136×Ti-19×Nb+198×Al+3315×B
[0061]
　After holding at the annealing temperature (soaking temperature) for 10 to 1000 seconds, it is cooled at an average cooling rate of 10° C./second or more. In order to freeze the structure and efficiently cause the martensitic transformation, the higher the average cooling rate is, the better. If it is less than 10° C./sec, ferrite is generated and the desired structure cannot be controlled. Therefore, the average cooling rate is set to 10° C./second or more. It is preferably 40° C./second or more.
　In order to sufficiently generate a hard structure, the cooling stop temperature is 400°C or lower. Thereafter, the hard structure may be tempered to improve the toughness. For tempering, the cooling is stopped at 400° C. or lower, the air is cooled slowly by 0.5° C./sec or less, or the temperature is kept at 200 to 400° C. for 10 to 1000 sec. You may go.
[0062]
　The average cooling rate is a value obtained by dividing the temperature drop width of the steel sheet from the start of cooling to the end of cooling by the time required from the start of cooling to the end of cooling. The start of cooling is, for example, when the steel sheet is introduced into the cooling equipment, and the end of cooling is when the steel sheet is taken out from the cooling equipment. The above cooling end temperature is the surface temperature of the steel sheet immediately after being drawn out from the cooling equipment. Further, the cooling is preferably cooling with water as a cooling medium.
[0063]
(Skin pass rolling step) The
　final skin pass rolling is performed on the cooled steel sheet. Thereby, the dislocation density can be increased and the bake hardenability can be improved. The rolling reduction is 0.1% or more in order to uniformly introduce strain into the steel sheet. On the other hand, if the reduction ratio becomes high, it becomes difficult to control the plate thickness, so 0.5% is made the upper limit. For the above reasons, the rolling reduction in the skin pass rolling process is set to 0.1% or more and 0.5% or less.
　In this way, the steel sheet according to the embodiment of the present invention can be manufactured.
[0064]
　It should be noted that each of the above-described embodiments is merely an example of an embodiment when carrying out the present invention, and the technical scope of the present invention is not limitedly interpreted by these. That is, the present invention can be implemented in various forms without departing from the technical idea or the main features thereof.
Example
[0065]
　Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one condition example. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
[0066]
　A slab having the chemical composition shown in Table 1 was produced, the slab was heated at a temperature of 1000° C. or higher and 1300° C. or lower for 1.0 to 1.5 hours, and then subjected to multiaxial deformation under the conditions shown in Table 2-1. The test was performed (however, the test materials Nos. 24 and 26 were unidirectionally compressed and deformed). Table 2-1 shows the temperature of the slab at the time of multiaxial deformation processing as the maximum temperature and the minimum temperature. Then, the slab was reheated to 1250° C. and hot rolled under the conditions shown in Table 2-1 to obtain a hot rolled steel sheet. In the hot rolling, hot rolling was performed at the total reduction ratio shown in Table 2-1. After winding, the winding temperature was maintained for 1 hour. In Table 2-1, FT is the hot rolling finish end temperature, CT is the winding temperature, which is the surface temperature of the steel sheet. Then, the hot-rolled steel sheet was pickled and cold-rolled at a reduction rate shown in Table 2-2 to obtain a cold-rolled steel sheet. Subsequently, continuous annealing was carried out at the temperature and time shown in Table 2-2, and cooled to 400° C. or less at the average cooling rate shown in Table 2-2. Some of them were kept heated after cooling was stopped. Then, temper rolling was performed. The underline in Table 1 indicates that the value is out of the desired range. Each temperature shown in Table 2-1 and Table 2-2 is the surface temperature of the steel sheet.
　A c3 in Table 2-2 was calculated by the formula shown below. Mass% of the element was substituted for the element symbol in the following formula. 0 (mass %) was substituted for elements not containing.
[0067]
　Ac 3 =937-477×C+56×Si-20×Mn-16×Cu-27×Ni-5×Cr+38×Mo+125×V+136×Ti-19×Nb+198×Al+3315×B
[0068]
[table 1]

[0069]
[Table 2-1]

[0070]
[Table 2-2]

[0071]
　The steel structure of the obtained cold-rolled steel sheet was observed, and the area ratio of hard structure and austenite, and the area ratio of other structures (ferrite, pearlite) were obtained.
　The area ratio of each tissue was determined as follows.
　A sample is taken with a plate thickness cross section perpendicular to the rolling direction of the steel plate as an observation surface, the observation surface is polished, and nital corrosion is performed. It was observed with an electron microscope). The area ratio of ferrite and pearlite was measured by image analysis in a visual field of 100 μm×100 μm. Five fields of view were measured at the center of the plate width direction, and the average of these measured values ​​was obtained.
[0072]
　Then, the area ratio of retained austenite was obtained.
　The area ratio of austenite was measured by the X-ray diffraction method as follows. A portion from the surface of the steel sheet to 1/4 of the thickness of the steel sheet was removed by mechanical polishing and chemical polishing, and MoKα ray was used as the characteristic X-ray. Then, from the integrated intensity ratios of the diffraction peaks of (200) and (211) of the body-centered cubic lattice (bcc) phase and (200), (220) and (311) of the face-centered cubic lattice (fcc) phase, The volume ratio of the retained austenite was calculated using the formula (1), and this was regarded as the area ratio. In the following formula, Sγ is the area ratio of retained austenite, I 200f , I 220f , and I 311f are the intensity of diffraction peaks of (200), (220), and (311) of the fcc phase, and I 200b and I 211b are, respectively. The intensity of the diffraction peaks of (200) and (211) of the bcc phase is shown, respectively.
[0073]
　Sγ=(I 200f +I 220f +I 311f )/(I 200b +I 211b )×100
[0074]
　The area ratio of hard structure was obtained by subtracting the area ratio of ferrite and pearlite and the area ratio of retained austenite obtained by the above method from the whole.
　The results are shown in Table 3.
[0075]
　Further, the tensile strength TS, the elongation at break EL and the bake hardening amount BH of the obtained cold rolled steel sheet were measured. In the measurement of the tensile strength TS, the elongation at break EL, and the bake hardening amount BH, a JIS No. 5 tensile test piece having a longitudinal direction perpendicular to the rolling direction was taken, and a tensile test was conducted in accordance with JIS Z 2241:2011. ..
　BH was a value obtained by subtracting the stress at the time of adding 0.5% prestrain from the yield stress when the test piece heat-treated at 170° C. for 20 minutes was re-pulled after applying 0.5% prestrain. The steel sheet is a steel sheet having high bake hardenability for BH at 0.5% prestrain. By adopting BH at 0.5% prestrain as an evaluation index, the ductility after the steel sheet is formed into a component molded article is secured.
　When the tensile strength was 900 MPa or more, it was determined that the preferable strength was obtained in order to satisfy the demand for weight reduction of the automobile body. It is preferably 1000 MPa or more, more preferably 1100 MPa or more.
　Further, when it is assumed that press molding or the like is performed, the elongation is preferably 10% or more.
　Regarding BH, if it is less than 150 MPa, it is difficult to mold and the strength after molding becomes low. Therefore, if it is 150 MPa or more, it is judged that the bake hardenability is excellent. It is more preferably 200 MPa or more, and most preferably 250 MPa or more.
　The underline in Table 3 indicates that the value is out of the desired range.
[0076]
　The degree of microsegregation of Mn represented by C1/C2 was measured as follows. After adjusting the steel plate so that the cross-section in the plate thickness direction in which the rolling direction is the normal direction can be observed, mirror-polishing is performed, and the EPMA (Electron Probe Microanalyzer) device is used to measure the cross-section in the thickness direction of the steel plate from the surface of the steel plate. For the range of 100 μm in the plate thickness direction included in the region from the ⅜ position to the ¼ position of the thickness of the steel plate, at 0.5 μm intervals from one surface side to the other surface side along the steel plate thickness direction. The Mn content at 200 points was measured. At this time, the inclusions such as MnS were avoided and the Mn content was measured. The same measurement was performed on 5 lines that cover almost the entire widthwise region in the steel sheet cross section, and among the Mn contents measured on all 5 lines, the highest value was the upper limit C1 of the Mn content (unit: : Mass %), and the lowest value was defined as the lower limit C2 (mass %) of the Mn content, and the ratio C1/C2 was calculated.
[0077]
[Table 3]

[0078]
　[Evaluation Results] As
　shown in Table 3, in Sample Nos. 1, 3, 5, 6, 9, 13, 16, 18, 20 to 22, 25, 27, 28, and 31 which are within the scope of the present invention, excellent results are obtained. The tensile strength and BH could be obtained. In all cases, the tensile strength was 900 MPa or more and the BH was 150 MPa or more, and it was shown that the strength was high and the bake hardenability was excellent. Further, in the examples of the present invention, no phase or structure other than martensite and austenite was observed.
[0079]
　On the other hand, sample No. In No. 2, since the final skin pass step was not performed, the dislocation density in the structure was low and the BH was low.
　In sample No. 4, since the retained austenite was too much, the bake hardening of martensite was not sufficiently exhibited, and the BH was low.
　Sample No. In No. 7, since the annealing temperature was too low, a large amount of ferrite was generated and BH was low. The TS was also low.
　Sample No. In No. 8, since the annealing time was too short, a large amount of ferrite was generated and BH was low.
　Sample No. In No. 10, since the cooling rate after annealing was too slow, a hard structure could not be obtained sufficiently and BH was low. The TS was also low.
　Sample No. In No. 11, the C content was low and the BH was low.
　In Sample No. 12, the Si content was low and the BH was low.
　In Sample No. 14, since the temperature range of the multiaxial deformation processing was low, Mn microsegregation occurred and BH was low.
　In Sample No. 15, the winding temperature was low. As a result, Mn was not sufficiently diffused, microsegregation occurred, and BH was low.
　In sample No. 17, the BH was low because the Mn content was too low. Also, TS was low.
　In sample No. 19, the deformation rate of the multiaxial deformation processing was low. As a result, Mn microsegregation occurred and BH was low.
　Sample No. In No. 23, the total reduction rate of finish rolling was high. As a result, austenite was stretched, Mn microsegregation occurred, and BH was low.
　Sample No. In No. 24, the slab was rolled without performing the multiaxial deformation process. As a result, Mn microsegregation occurred and BH was low.
　Sample No. 26 did not have the multi-axial deformation processing step and the final skin pass step. As a result, Mn microsegregation occurred, the dislocation density was low, and the BH was low.
　In Sample No. 29, the hot rolling finish temperature was low. As a result, Mn microsegregation occurred in the portion of unrecrystallized austenite, and BH was low.
　Sample No. 30 had a high winding temperature. As a result, pearlite was generated, Mn microsegregation was generated, and BH was low.
Industrial availability
[0080]
　INDUSTRIAL APPLICABILITY The steel sheet of the present invention can be used as an original plate for automobile structural materials, particularly in the automobile industry field.
claims
[Claim 1]
　% By mass,
C: 0.05 to 0.30%,
Si: 0.2 to 2.0%,
Mn: 2.0 to 4.0%,
Al: 0.001 to 2.000%,
P: 0.100% or less,
S: 0.010% or less,
N: 0.010% or less,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
V: 0 to 0.100%,
Cu : 0 to 1.00%,
Ni: 0 to 1.00%,
Mo: 0 to 1.00%,
Cr: 0 to 1.00%,
W: 0 to 0.005%,
Ca: 0 to 0. 005%,
Mg: 0 to 0.005%,
rare earth element (REM): 0 to 0.010%,
B: 0 to 0.0030%,
with the balance being Fe and impurities. The
　metal structure contains 95% or more of hard structure and 0 to 5% of retained austenite in
　terms of area ratio, and the upper limit C1 of Mn content in the mass% in the thickness direction cross section and the Mn content. C1/C2, which is the ratio to the lower limit value C2 of
　A steel sheet having a bake hardening amount BH of 150 MPa or more.
[Claim 2]
　The chemical composition includes, by mass , one or more of
Ti: 0.003 to 0.100%,
Nb: 0.003 to 0.100%, and
V: 0.003 to 0.100%. The steel sheet according to claim 1, wherein the
total content is 0.100% or less
.
[Claim 3]
　The chemical composition is, by mass%,
Cu: 0.005 to 1.00%,
Ni: 0.005 to 1.00%,
Mo: 0.005 to 1.00%,
Cr: 0.005 to 1. The steel sheet according to claim 1 or 2,
which contains one or more of 00% and has a total content of 1.00% or less
.
[Claim 4]
　The chemical composition is% by mass,
W: 0.0003 to 0.005%,
Ca: 0.0003 to 0.005%,
Mg: 0.0003 to 0.005%,
rare earth element (REM): 0. The steel sheet according to any one of claims 1 to 3,
containing one or more of 0003 to 0.010% and a total content of 0.010% or less
.
[Claim 5]
The steel sheet according to any one of claims 1 to 4, 　wherein the chemical composition
contains B:00001 to 0.0030% by mass
.