Steel plate and method for manufacturing the same

Abstract

To provide a steel material more excellent in ductility than that of conventional TRIP steels, and to provide a method for manufacturing the same.SOLUTION: A chemical composition of a steel material comprises, by mass: 1.00% or less of C; 0.80-3.00% of Si; 0.2-7.0% of Mn; 0.10% or less of P; 0.030% or less of S; 3.00% or less of Al; 0.010% or less of N; 0-10.0% of Ni; 0-3.0% of Cu; 0-10.0% of Cr; 0-1.0% of Ti; 0-1.0% of Nb; 0-1.0% of V; 0-2.0% of Mo; 0-1.0% of W; 0-0.01% of B; 0-1.0% of Co; 0-0.01% of Ca; 0-0.01% of Mg; 0-0.01% of REM; the balance comprising Fe and impurities; and 0.10-1.00 of Ceq, wherein: a metallic structure comprises a tempered martensite, bainite, and a residual γ, a volume rate of the residual γ being 5% or more, a total volume ratio of the tempered martensite and bainite being 70% or more, and a total volume ratio of the tempered martensite, bainite, and residual γ being 90% or more; and an area ratio of crystal grains having an aspect ratio of less than 3.0 is 70% or more in the total amount of the tempered martensite and bainite. The steel material has tensile strength of 1,150 MPa or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to steel materials and a method for manufacturing the same.

Background Art

[0002] Steel materials containing retained austenite in the structure of steel are known. Transformation-induced plasticity steel (TRIP steel) having a mixed structure containing retained austenite is used in various fields because it has high strength and excellent ductility. Therefore, many studies on structure control for improving the properties of TRIP steel have been made so far.

[0003] For example, Patent Document 1 discloses a high-strength thin steel sheet composed of bainitic ferrite and retained austenite by heating and holding a steel sheet subjected to hot rolling and cold rolling at a temperature of A3 point to (A3 point + 50°C) for 10 to 1800 seconds and then cooling and holding it at an average cooling rate of 3°C / s or more to a temperature of (Ms point - 100°C) to Bs point.

[0004] Further, Patent Document 2 discloses a steel sheet obtained by using a steel sheet subjected to hot rolling and cold rolling as a raw material, heating it in an annealing process in a temperature range of (Ac3 point - 50°C) to Ac3 point at a rate of 2°C / s or less, cooling it at a cooling rate of 20°C / s or more to a temperature range of Ms point - 100°C or less in the subsequent cooling process, and performing a treatment of reheating it in a temperature range of 300 to 600°C, and having a structure containing 60 to 95% tempered martensite and retained austenite and having a steel sheet of 1200 MPa or more.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the case of ordinary TRIP steel as disclosed in Patent Documents 1 and 2, sufficient consideration has not been given to the crystal grain size and shape of tempered martensite, bainite, ferrite (and retained austenite) that make up the metal structure. Therefore, there is still room for improvement in conventional TRIP steel from the viewpoint of improving ductility.

[0007] An object of the present invention is to provide a steel material having a metal structure containing retained austenite and having superior ductility compared to conventional TRIP steel, and a method for producing the same.

Means for Solving the Problems

[0008] The present invention has been made to solve the above problems, and the gist thereof is the following steel material and a method for producing the same.

[0009] (1) The chemical composition is, by mass%, C: 1.00% or less, Si: 0.80 to 3.00%, Mn: 0.2 to 7.0%, P: 0.10% or less, S: 0.030% or less, Al: 3.00% or less, N: 0.010% or less, Ni: 0 to 10.0%, Cu: 0 to 3.0%, Cr: 0 to 10.0%, Ti: 0 to 1.0%, Nb: 0 to 1.0%, V: 0 to 1.0%, Mo: 0 to 2.0%, W: 0 to 1.0%, B: 0 to 0.01%, Co: 0 to 1.0%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, REM: 0 to 0.01%, The balance: Fe and impurities, Ceq defined by the following formula (i) is 0.10 to 1.00, the metallographic structure contains tempered martensite, bainite, and retained austenite, the volume fraction of retained austenite is 5% or more, the total volume fraction of tempered martensite and bainite is 70% or more, the total volume fraction of tempered martensite, bainite, and retained austenite is 90% or more, among the total amount of tempered martensite and bainite, the area ratio of crystal grains with an aspect ratio of less than 3.0 is 70% or more, having a tensile strength of 1150 MPa or more, steel material. Ceq = C + 1 / 24Si + 1 / 6Mn + 1 / 40Ni + 1 / 5Cr + 1 / 4Mo + 1 / 14V ···(i) However, each element symbol in the formula represents the content (mass%) of each element contained in the steel material.

[0010] (2) The average crystal grain size of tempered martensite and bainite in the metallographic structure is 3.0 μm or less, the steel material according to (1) above.

[0011] (3) The chemical composition is, by mass, Ni: 0.1 to 10.0%, Cu: 0.3 to 3.0%, and Cr: 0.1 to 10.0%, contains one or more selected from the steel material according to (1) or (2) above.

[0012] (4) The chemical composition is, by mass, Ti: 0.01 to 1.0%, Nb: 0.01 to 1.0%, V: 0.01 to 1.0%, Mo: 0.05 to 2.0%, W: 0.05 to 1.0%, B: 0.0003 to 0.01%, and Co: 0.05 to 1.0%, containing one or more selected from the steel material according to any one of (1) to (3) above.

[0013] (5) The chemical composition is in mass%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, and REM: 0.0001 to 0.01%, containing one or more selected from the steel material according to any one of (1) to (4) above.

[0014] (6) A steel material having the chemical composition according to any one of (1) and (3) to (5) above and having a metal structure mainly composed of martensite is held at a temperature of Ac3 point or higher for 10 s or more, heat-treated by cooling to room temperature at an average cooling rate of 5°C / s or higher, heated to a temperature range of Ac3 point to Ac3 point + 100°C at an average heating rate of 500°C / s or higher, held for 5 to 30 s and then cooling is started, cooled to a temperature range of Ms point - 50°C to Ms point - 200°C at an average cooling rate of 10°C / s or higher and less than 100°C / s, reheated to a temperature range of 300 to 500°C and held for 30 to 500 s, and then cooled to room temperature. Method for manufacturing a steel material.

[0015] (7) Cold working is performed in advance on the steel material having a metal structure mainly composed of martensite. The method for manufacturing a steel material according to (6) above.

Advantages of the Invention

[0016] According to the present invention, by forming a metal structure containing a large number of crystal grains with a low aspect ratio and containing retained austenite, it becomes possible to obtain a steel material having superior ductility compared to conventional TRIP steel.

Embodiments for Carrying Out the Invention

[0017] As a result of intensive studies on a method for producing steel with even better ductility than conventional TRIP steel, the present inventors have obtained the following findings.

[0018] (a) Generally, in the microstructure of martensite and lower bainite, acicular-shaped structures such as packets or blocks develop, so the aspect ratio of crystal grains in the microstructure tends to be large.

[0019] (b) When steel with large-aspect-ratio crystal grains is processed and the crystal grains are deformed, they are likely to break in the width direction and generate fine cracks. Steel in which such cracks are likely to occur is considered to be easily fractured and it is difficult to obtain high ductility.

[0020] (c) On the other hand, massive crystal grains are less likely to break when the steel is processed. Therefore, steel having a metal microstructure composed of massive crystal grains is considered to be less likely to break even when subjected to high deformation and to have excellent ductility.

[0021] As a result of further studies on a method for producing steel having the above-described metal microstructure, the present inventors have obtained the following findings.

[0022] (d) When a steel material having an initial microstructure mainly composed of martensite or cold-worked martensite is heated ultra-rapidly and heated all at once to the austenite single-phase region, austenite grains are generated.

[0023] (e) At this time, due to the difference in the microstructure of the steel material before ultra-rapid heating, the degree of fineness after heating varies greatly. When a steel in which a large number of austenite nucleation sites exist in the metal microstructure is used as the steel material, a fine microstructure is likely to be obtained.

[0024] (f) When it is desired to obtain a fine microstructure, it is preferable to perform cold working on the steel material before ultra-rapid heating.

[0025] (g) The generated ultrafine austenite grains tend to grow into coarse grains at high temperatures. Therefore, after ultra-rapid heating, cooling is started within a certain period of time to maintain the ultrafine structure.

[0026] (h) To prevent the growth coarsening of ultrafine austenite grains, it is also effective to lower the transformation temperature. Since the movement of grain boundaries is due to the diffusion of atoms, if the temperature is lowered and the diffusion rate is reduced, it becomes possible to maintain fine grains.

[0027] (i) By adjusting the content of Mn etc., it becomes possible to lower the transformation temperature of the steel material.

[0028] (j) After generating ultrafine austenite grains, cooling and annealing are performed. As a result, a phase transformation occurs, and tempered martensite and bainite are generated from austenite. In addition, a part of the austenite becomes retained austenite.

[0029] (k) The shape of the blocks of tempered martensite and bainite at that time greatly depends on the grain size of the original austenite grains, and the finer the austenite grains, the more massive the structure is generated.

[0030] (l) From the ultrafine austenite grains obtained by the manufacturing method of the present invention, a structure is obtained in which the area ratio of crystal grains with an aspect ratio of less than 3.0 is 70% or more among the total amount of tempered martensite and bainite.

[0031] (m) Also, heat treatment is performed before ultra-rapid heating to promote the diffusion of Mn in the steel and reduce the concentration distribution of Mn in the structure. Thereby, the total volume ratio of tempered martensite, bainite, and retained austenite can be increased, and the generation of other structures can be suppressed. As a result, the ductility can be further improved.

[0032] (n) Furthermore, by suppressing the formation of structures other than tempered martensite, bainite, and retained austenite, it is also possible to increase the yield ratio.

[0033] The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

[0034] (A) Chemical composition The reasons for limiting each element are as follows. In the following description, "%" regarding the content means "mass %".

[0035] C: 1.00% or less C is an element that improves the strength of steel. The C content is selected according to the properties required for the steel, but if it exceeds 1.00%, the uniform elongation decreases. Therefore, the C content is set to 1.00% or less. The C content is preferably 0.50% or less, and more preferably 0.35% or less. In order to obtain the above effects, the C content is preferably 0.03% or more.

[0036] Si: 0.80 - 3.00% Si is a ferrite-forming element that dissolves in steel and expands the temperature range in which the ferrite phase stably exists. It also has the effect of improving hardness together with cementite. However, if its content exceeds 3.00%, the hot workability deteriorates and it is likely to crack during rolling. Therefore, the Si content is set to 0.80 - 3.00%. The Si content is preferably 1.00% or more and preferably 2.50% or less.

[0037] Mn: 0.2 - 7.0% Mn is an element that completely dissolves in steel, lowers the A1 transformation point, and reduces the austenite formation temperature range, thereby being effective in enhancing hardenability. Also, Mn is an element that is distributed to the austenite phase. Furthermore, it becomes an effective element when it is desired to utilize retained austenite. In order to suppress the growth coarsening of the ultrafine austenite structure, it is necessary to contain 0.2% or more. On the other hand, when the Mn content exceeds 7.0%, the effect of suppressing grain growth saturates. Therefore, the Mn content is set to 0.2 - 7.0%. The Mn content is preferably 5.0% or less, and more preferably 3.0% or less.

[0038] P: 0.10% or less P is generally an element contained as an impurity, but it is also an element having an effect of increasing strength by solid solution strengthening. Therefore, P may be positively contained. However, P is an element that is prone to segregation, and when its content exceeds 0.10%, the decrease in formability and toughness due to grain boundary segregation becomes significant. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.050% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. The lower limit of the P content does not particularly need to be specified, but when it is desired to obtain the above effects, it is preferably 0.001% or more.

[0039] S: 0.030% or less S is an element contained as an impurity, and forms sulfides in steel, thereby reducing the hot formability of the steel sheet. When the S content exceeds 0.030%, the decrease in formability becomes significant. Therefore, the S content is set to 0.030% or less. The S content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.001% or less. The lower limit of the S content does not particularly need to be specified, but from the viewpoint of suppressing the increase in refining cost, it is preferably 0.0001% or more.

[0040] Al: 3.00% or less Al is a ferrite-forming element, which forms nitrides in steel and plays an effective role in refining crystal grains. However, when its content exceeds 3.00%, the hot workability deteriorates and it is prone to cracking during rolling. Therefore, the Al content should be 3.00% or less. The Al content is preferably 2.50% or less, and more preferably 2.00% or less. In order to obtain the above effects, the Al content is preferably 0.01% or more, and more preferably 0.03% or more.

[0041] N: 0.010% or less N is an element contained as an impurity and has the effect of reducing the low-temperature toughness of the steel sheet. When the N content exceeds 0.010%, the decrease in toughness becomes significant. Therefore, the N content should be 0.010% or less. The N content is preferably 0.0080% or less, and more preferably 0.0070% or less. Although the lower limit of the N content does not need to be particularly specified, considering the case of promoting the precipitation of carbonitrides by containing one or more of Ti, Nb, and V as described later to refine the steel structure, the N content is preferably 0.0010% or more, and more preferably 0.0020% or more.

[0042] In addition to the above elements, the steel used in the manufacturing method of the present invention may further contain one or more elements selected from Ni, Cu, Cr, Ti, Nb, V, Mo, W, B, Co, Ca, Mg, and REM in the amounts shown below.

[0043] Ni: 0 - 10.0% Ni completely dissolves in steel and is an austenite-forming element effective in expanding the stable region of the austenite phase by lowering the A1 transformation point and reducing the austenite formation temperature range. Since it is effective in improving hardness, strength, and toughness, Ni may be contained as needed. However, when the Ni content exceeds 10.0%, the effect of suppressing grain growth saturates. Therefore, the Ni content should be 10.0% or less. The Ni content is preferably 5.0% or less. In order to obtain the above effects, the Ni content is preferably 0.1% or more.

[0044] Cu: 0 to 3.0% Cu is an element effective in expanding the stable region of the austenite phase by lowering the A1 transformation point and reducing the austenite formation temperature range. Also, Cu is an austenite forming element. In order to achieve the effect of improving the corrosion resistance of the steel material, Cu may be contained as necessary. However, when the Cu content exceeds 3.0%, brittle fracture is likely to occur. Therefore, the Cu content should be 3.0% or less. The Cu content is preferably 2.5% or less. In order to obtain the above effects, the Cu content is preferably 0.3% or more.

[0045] Cr: 0 to 10.0% Cr is a ferrite forming element that completely dissolves in steel. Also, Cr segregates at grain boundaries or precipitates in steel as carbides or nitrides, thus playing an effective role in suppressing grain growth of crystal grains and refining them. It is also effective in improving the corrosion resistance and high-temperature strength of the steel material. Therefore, Cr may be contained as necessary. However, when the Cr content exceeds 10.0%, an imbalance between strength and ductility or strength and toughness occurs. Therefore, the Cr content should be 10.0% or less. The Cr content is preferably 8.0% or less. In order to obtain the above effects, the Cr content is preferably 0.1% or more.

[0046] Ti: 0 to 1.0% Although the solubility of Ti in steel is small, Ti is a ferrite forming element that expands the temperature range in which the ferrite phase stably exists. Also, Ti segregates at grain boundaries or precipitates in steel as carbides or nitrides, thus playing an effective role in suppressing grain growth of crystal grains and refining them. It is also effective in improving the deep drawability of the steel material. Therefore, Ti may be contained as necessary. However, when the Ti content exceeds 1.0%, the A1 transformation point rises, hardenability deteriorates, and hard carbonitrides are likely to form, significantly inhibiting the workability of the steel material. Therefore, the Ti content should be 1.0% or less. The Ti content is preferably 0.5% or less. In order to obtain the above effects, the Ti content is preferably 0.01% or more.

[0047] Nb: 0 to 1.0% Nb is a ferrite-forming element. Also, since Nb segregates at grain boundaries or precipitates in the steel as carbides, it is an effective element for suppressing the growth coarsening of the ultrafine austenite structure by suppressing the grain growth of austenite in the annealing process. Therefore, Nb may be contained as necessary. However, when the Nb content exceeds 1.0%, brittle fracture is likely to occur. Therefore, the Nb content is set to 1.0% or less. The Nb content is preferably 0.5% or less. In order to obtain the above effects, the Nb content is preferably 0.01% or more.

[0048] V: 0 to 1.0% V is a ferrite-forming element that completely dissolves in the steel and expands the temperature range in which the ferrite phase stably exists. Also, since V segregates at grain boundaries or precipitates in the steel as carbides or nitrides, it is an effective element for suppressing the growth coarsening of the ultrafine austenite structure by suppressing the grain growth of austenite in the annealing process. Therefore, V may be contained as necessary. However, when the V content exceeds 1.0%, brittle fracture is likely to occur. Therefore, the V content is set to 1.0% or less. The V content is preferably 0.5% or less. In order to obtain the above effects, the V content is preferably 0.01% or more.

[0049] Mo: 0 to 2.0% Mo is a ferrite-forming element. Also, since Mo segregates at grain boundaries or precipitates in the steel as carbides or nitrides, it is an effective element for suppressing the growth coarsening of the ultrafine austenite structure by suppressing the grain growth of austenite in the annealing process. Therefore, Mo may be contained as necessary. However, when the Mo content exceeds 2.0%, the effect of suppressing grain growth saturates. Therefore, the Mo content is set to 2.0% or less. The Mo content is preferably 1.0% or less. In order to obtain the above effects, the Mo content is preferably 0.05% or more.

[0050] W: 0 to 1.0% W is a ferrite-forming element. Also, since W segregates at grain boundaries or precipitates in the steel as carbides or nitrides, it is an element effective in suppressing the growth coarsening of the ultrafine austenite structure by suppressing the grain growth of austenite in the annealing process. Therefore, W may be contained as needed. However, when the W content exceeds 1.0%, the effect of suppressing grain growth saturates. Therefore, the W content is set to 1.0% or less. The W content is preferably 0.5% or less. In order to obtain the above effects, the W content is preferably 0.05% or more.

[0051] B: 0 to 0.01% B is an element that improves hardenability and is an effective element for obtaining a structure containing martensite. Therefore, B may be contained as needed. However, since B hardly dissolves in the steel, when it exceeds 0.01%, brittle fracture is caused. Therefore, the B content is set to 0.01% or less. The B content is preferably 0.005% or less. In order to obtain the above effects, the B content is preferably 0.0003% or more.

[0052] Co: 0 to 1.0% Co is an austenite-forming element that completely dissolves in the steel and expands the region where the austenite phase stably exists. Therefore, Co may be contained as needed. However, when the Co content exceeds 1.0%, it has no effect on the expansion of the austenite region. Therefore, the Co content is set to 1.0% or less. The Co content is preferably 0.5% or less. In order to obtain the above effects, the Co content is preferably 0.05% or more.

[0053] Ca: 0 to 0.01% Mg: 0 to 0.01% REM: 0 to 0.01% Ca, Mg, and REM all have the effect of refining oxides and nitrides precipitated during the solidification process of molten steel, thereby enhancing the soundness of the slab. Therefore, one or more selected from these elements may be contained as necessary. However, when the content of each of these elements exceeds 0.01%, embrittlement occurs and workability deteriorates. Therefore, the content of each element is set to 0.01% or less. Also, when two or more are contained in combination, the total content may be 0.03%. To obtain the above effects, it is preferable to contain 0.0001% or more of one or more selected from Ca, Mg, and REM.

[0054] Here, REM refers to a total of 17 elements including Sc, Y, and lanthanoids, and the content of the REM means the total content of these elements.

[0055] In the chemical composition of the steel used in the manufacturing method of the present invention, the balance is Fe and impurities.

[0056] The term "impurities" means components that are mixed in due to raw materials such as ore and scrap, and various factors in the manufacturing process when the steel material is industrially manufactured, and are allowed within a range that does not adversely affect the present invention.

[0057] Ceq: 0.10 - 1.00 Ceq means carbon equivalent and is defined by the following formula (i). When Ceq is less than 0.10, sufficient strength of the steel material cannot be obtained. On the other hand, when Ceq exceeds 1.00, not only toughness and ductility deteriorate, but also weldability and weld joint characteristics deteriorate when welding is performed. Therefore, Ceq is set to 0.10 - 1.00. It is preferable that Ceq is 0.20 or more, and more preferably 0.30 or more. Also, it is preferable that Ceq is 0.90 or less. Ceq = C + 1 / 24Si + 1 / 6Mn + 1 / 40Ni + 1 / 5Cr + 1 / 4Mo + 1 / 14V ···(i) However, each element symbol in the formula represents the content (mass%) of each element contained in the steel material.

[0058] (B) Metallic structure The metallic structure of the steel material according to the present invention includes tempered martensite, bainite, and retained austenite. Further, the retained austenite is 5% or more, the total volume ratio of tempered martensite and bainite is 70% or more, and the total volume ratio of tempered martensite, bainite, and retained austenite is 90% or more. The reason for limiting the total volume ratio will be explained.

[0059] Retained austenite: 5% or more Retained austenite is a structure that improves the ductility of steel by the TRIP effect. Therefore, the volume ratio of retained austenite is set to 5% or more. Regarding the volume ratio of retained austenite, no upper limit is particularly provided, but it is substantially 30% or less in relation to other structures. Also, if the volume ratio of retained austenite is excessive, the strength may decrease. Therefore, the volume ratio of retained austenite is preferably 25% or less, and more preferably 20% or less.

[0060] The inclusion of retained austenite is effective in improving the uniform elongation of the steel sheet, and due to the fine particle size, the effect that the bendability or elongation flangeability (hole expansion rate) is less likely to decrease is obtained, and a high yield stress is obtained. If coarse retained austenite is included, it is likely to become the starting point of voids during elongation flange deformation, and the hole expansion rate becomes low.

[0061] Total volume ratio of tempered martensite and bainite: 70% or more The steel material according to the present invention has a tensile strength of 1150 MPa or more. Tempered martensite and bainite are both hard structures and improve the strength of steel. By setting the total volume ratio of tempered martensite and bainite to 70% or more, a tensile strength of 1150 MPa or more can be ensured. The total volume ratio of tempered martensite and bainite is preferably 75% or more, and more preferably 80% or more.

[0062] Total volume ratio of tempered martensite, bainite, and retained austenite: 90% or more In addition to tempered martensite and bainite, the inclusion of retained austenite enables the production of steel with excellent strength and ductility. Furthermore, by setting the total volume ratio of tempered martensite, bainite, and retained austenite to 90% or more, it is possible to reduce the formation of structures that lower the yield ratio, such as ferrite and hard martensite, as will be described later. Therefore, the yield ratio of the steel can be increased. The total volume ratio of tempered martensite, bainite, and retained austenite is preferably 95% or more, more preferably 97% or more.

[0063] Although the volume ratio of each individual metal structure is not particularly limited, tempered martensite and bainite preferably have the following volume ratios.

[0064] Tempered martensite is a hard structure that improves the strength of the steel. Therefore, the metal structure of the steel material according to the present invention includes tempered martensite. That is, the volume ratio of tempered martensite is more than 0%. The volume ratio of tempered martensite is preferably 20% or more, more preferably 40% or more, and even more preferably 50% or more. Since tempered martensite has appropriate toughness, unlike hard martensite, which is a harder structure, it has a relatively small impact on inhibiting ductility.

[0065] Bainite is a structure that is hard and rich in toughness. Therefore, the metal structure of the steel material according to the present invention includes bainite. That is, the volume ratio of bainite is more than 0%. The volume ratio of bainite is preferably 10% or more, more preferably 20% or more. Bainite is a mixed structure of bainitic ferrite, which has a body-centered cubic structure of iron, and cementite (cementite having the chemical composition of Fe3C). The bainitic ferrite among these is distinguished from the ferrite described later.

[0066] The organization other than those described above is not particularly limited, but in addition, it may contain one or more selected from organizations such as ferrite, hard martensite, and pearlite. The total volume ratio of one or more organizations selected from ferrite, hard martensite, pearlite, etc. is allowed to be 10% or less, preferably 5% or less, and more preferably 3% or less.

[0067] Ferrite is a soft structure that improves the ductility of steel. On the other hand, if its amount is excessive, it will not only deteriorate the strength of the steel but also may lower the yield ratio. Therefore, the volume ratio of ferrite is allowed to be less than 5%, preferably 4% or less. Note that ferrite may be generated when a part of austenite undergoes a phase transformation during the cooling process after rapid heating and holding.

[0068] As described above, hard martensite is a hard structure that lowers the yield ratio of steel and deteriorates its ductility. Therefore, the volume ratio of hard martensite is preferably 3% or less, more preferably 1% or less, and even more preferably 0%.

[0069] Also, in the metal structure, among the total amount of tempered martensite and bainite, the area ratio of crystal grains with an aspect ratio of less than 3.0 is 70% or more. This further enhances the ductility of the steel. Here, the crystal grains of tempered martensite and bainite refer to regions surrounded by grain boundaries with a crystal orientation difference of 15° or more (hereinafter also referred to as "BCC crystal grains") as measured by an electron backscatter diffraction device (EBSD) described later. In EBSD analysis, bainite is measured as bainitic ferrite in the BCC phase, not cementite. Crystal grains with an aspect ratio of less than 3.0 mean that the crystal grains are close to spherical. Spherical-like crystal grains are considered to be difficult to break during deformation and have an effect of improving ductility. The area ratio of crystal grains with an aspect ratio of less than 3.0 is preferably 75% or more.

[0070] Further, in the metal structure, it is preferable that the average crystal grain size of tempered martensite and bainite is 3.0 μm or less. The crystal grains of tempered martensite and bainite referred to herein are regions surrounded by grain boundaries with a crystal orientation difference of 15° or more as measured by EBSD, that is, BCC crystal grains. The average crystal grain size of tempered martensite and bainite greatly depends on the grain size of the original austenite grains, and the finer the austenite grains, the more a massive structure can be obtained.

[0071] In the present invention, among the volume fraction of each structure, the average crystal grain size of tempered martensite and bainite, and the total amount of tempered martensite and bainite, the area ratio of crystal grains with an aspect ratio of less than 3.0 shall be measured by the following method.

[0072] First, a sample is taken so that a cross-section parallel to the rolling direction and the plate thickness direction of the steel material becomes the observation surface. Then, the observation surface is mirror-polished, corroded with a nital corrosion solution, and then microstructure observation is performed using a scanning electron microscope (SEM).

[0073] At the position of 1 / 4 of the plate thickness depth of the above observation surface, an area of 130 μm × 130 μm is photographed at a magnification of 1000 times. The obtained microstructural photograph is subjected to black-and-white binarization processing and then image analysis is performed to identify tempered martensite, bainite, ferrite, and other structures, and the area ratio of each is obtained using the method based on the "Microscopic Test Method for Crystal Grain Size of Steel" defined in JIS G 0551 (2013) standard. Further, the conversion from the area ratio to the volume ratio is performed by the line segment method. The line segment method is based on the method described in, for example, co-edited by Robert T. DeHoff and Frederik N. Rhines (Quantitative Microscopy, 1968). However, when it is difficult to distinguish between tempered martensite and bainite, the total area ratio of tempered martensite and bainite is obtained.

[0074] In addition, since it is difficult to distinguish retained austenite from hard martensite by SEM, the volume fraction thereof is measured by X-ray diffraction method. Then, the volume fraction of the remainder (corresponding to hard martensite, pearlite, etc.) is obtained by subtracting the volume fraction of retained austenite from the volume fractions of other structures obtained by the above SEM observation.

[0075] Furthermore, measurement and analysis of crystal orientation by EBSD are performed. In the measurement by EBSD, crystal grains having a BCC structure surrounded by crystal grain boundaries with a crystal orientation difference of 15° or more are defined as BCC crystal grains. And the BCC crystal grain size is determined by calculating the average value of the equivalent circle diameters of the specified BCC crystal grains based on the following formula. However, in the following formula, Ai represents the area of the i-th BCC crystal grain, and di represents the equivalent circle diameter of the i-th BCC crystal grain.

[0076] [Number]

[0077] In the above measurement, only crystal grains with an equivalent circle diameter of 0.3 μm or more are targeted.

[0078] The aspect ratio of the above crystal grains is determined by the following method. The structure is measured by EBSD, and in tempered martensite and bainite, crystal grains having a BCC structure surrounded by boundaries (large-angle grain boundaries) with a crystal orientation difference of 15° or more, that is, BCC crystal grains, are specified. Using the obtained data, program processing is performed to obtain the aspect ratio of the specified crystal grains. By program processing, the average coordinates (centroid) of the crystal grains, the straight line passing through the centroid, and the length of the section cutting across the crystal grains are obtained. In the program processing, the average X coordinate of the crystal grains, the average Y coordinate of the crystal grains, and the straight line passing through the centroid are obtained by the following formulas.

[0079] Average X coordinate of crystal grains: x(ave) = Σ i (x i / n) Average Y coordinate of crystal grains: y(ave) = Σ i (y i / n) The straight line passing through the centroid: y - x(ave) = a(x - x(ave)) The slope a of the straight line passing through the centroid is a = tanθ, where θ takes values from 0 to 360°.

[0080] Among the obtained lengths of the intercepts, the ratio of the maximum length to the minimum length (= maximum length / minimum length) is defined as the aspect ratio of the crystal grains.

[0081] Determine the area ratio of the total area of the crystal grains with an aspect ratio less than 3.0 obtained by the above method to the entire field of view.

[0082] (C) Mechanical properties In the present invention, the tensile strength of the steel material is 1150 MPa or more. The tensile strength is preferably 1200 MPa or more. Also, the yield ratio is preferably 0.60 or more. The yield ratio is more preferably 0.65 or more, and even more preferably 0.70 or more. Further, from the viewpoint of ensuring the balance between strength and ductility, the product of the tensile strength and the uniform elongation is preferably 15000 MPa·% or more.

[0083] (D) Manufacturing method The steel material according to the present invention has the above-described chemical composition. For a steel material having a predetermined metallographic structure, first, a first annealing step is performed in a temperature range where austenite is a single phase. Then, after rapidly heating to a temperature range where austenite is a single phase, it is cooled below the Ms point, and then a second annealing step of reheating is performed, whereby it can be manufactured. Each condition will be described in detail below.

[0084] (D-1) Steel material As the steel material before heat treatment, one having a metallographic structure mainly composed of martensite is used.

[0085] Here, the structure “mainly composed of martensite” means a metallographic structure having a volume ratio of 95.0% or more. In the steel material, structures such as ferrite, pearlite, bainite, and retained austenite may be mixed, but these structures are acceptable as long as their total volume ratio is 5.0% or less.

[0086] In addition, regarding the manufacturing method of the steel material, there is no particular limitation as long as the metallographic structure satisfies the above regulations, and a general method may be used.

[0087] (D-2) First annealing process The above steel material is subjected to a first annealing process. The conditions of the first annealing process will be described in detail below.

[0088] The steel material having the aforementioned chemical composition and metallographic structure is first held at a temperature above Ac3 for 10 s or more and then cooled to room temperature at an average cooling rate of 5 °C / s or more. Thereby, Mn in the steel can be diffused and the concentration distribution of Mn can be reduced. In the first annealing process, if the diffusion of Mn in the steel is insufficient, ferrite is likely to be generated from the part with less Mn in the second annealing process.

[0089] Note that the purpose of this heat treatment process is to reduce the Mn concentration distribution. Therefore, coarsening of austenite grains to some extent is allowed rather than holding for 10 s or more at the heating temperature.

[0090] There is no particular limitation on the upper limit of the heating temperature, but in order to prevent the heat treatment furnace from being exposed to an excessive high temperature and the equipment from deteriorating, it is preferably 1000 °C or lower. Also, there is no particular limitation on the upper limit of the holding time, but from the viewpoint of productivity, it is preferably 5 h or less. After heating, the steel plate is cooled to room temperature at an average cooling rate of 5 °C / s or more in order to suppress the generation of ferrite during cooling. There is no particular limitation on the upper limit of the average cooling rate, but since an excessive cooling rate is not required, it is preferably 200 °C / s or lower.

[0091] (D-3) Second annealing process The steel material after the above heat treatment process is subjected to a second annealing process. The second annealing process can be further subdivided into four processes: a heating-up process, a holding process, a cooling process, and a reheating process. The conditions in each process will be described in detail below.

[0092] <Heating process> The steel material after the above first annealing process is first heated to the temperature range from the Ac3 point to Ac3 point + 100°C at an average heating rate of 500°C / s or more. By heating to the austenite single-phase region above the Ac3 point, a uniform structure can be obtained. On the other hand, if heated beyond Ac3 point + 100°C, the grain growth rate increases and it grows into coarse austenite grains.

[0093] For the refinement of austenite grains, the faster the heating rate, the more preferable. The average heating rate to the above temperature range is 500°C / s or more. It is desirable that the average heating rate is 1000°C / s or more. There is no particular limitation on the upper limit of the average heating rate, but it is preferably 20000°C / s or less as a practical range.

[0094] In the present invention, the Ac3 point is determined by the following method. Prepare a plurality of test pieces having the same chemical composition and metal structure, heat them to various temperatures at a predetermined heating rate, and then, with the holding time within 1 s, cool them from the above heating temperature to 70°C at an average cooling rate of 1000°C / s. Then, the heating temperature applied to the test piece whose hardness of the subsequent test piece becomes the maximum hardening hardness is defined as the Ac3 point. Also, the Ac3 point can be obtained from the measurement of thermal expansion during heating and the same result can be obtained.

[0095] <Holding process> Heat under the above conditions, hold for 5 to 30 s, and then start cooling. Cementite is dissolved by holding in the temperature range from the Ac3 point to Ac3 point + 100°C. If the holding time is less than 5 s, cementite is not sufficiently dissolved and remains until the final structure, and there is a possibility that a sufficient amount of retained austenite cannot be obtained. On the other hand, if the holding time exceeds 30 s, the austenite grains coarsen and the ratio of crystal grains with an aspect ratio of less than 3.0 becomes lower than 70%.

[0096] <Cooling process> In the cooling process, cooling is performed from the temperature range of the Ac3 point to Ac3 point + 100°C at an average cooling rate of 10°C / s or more and less than 100°C / s to the temperature range of Ms point - 50°C to Ms point - 200°C. During this cooling, a part of the austenite transforms into ferrite. However, in the present invention, since the Mn concentration distribution is reduced in the first annealing process, the formation of ferrite in this cooling process can be suppressed. Further, by reaching and holding in the temperature range of Ms point - 50°C to Ms point - 200°C, a part of the remaining austenite transforms into martensite. The holding time in the temperature range of Ms point - 50°C to Ms point - 200°C is not particularly limited, but it is preferably about 5 to 300 s.

[0097] Note that the Ms point is calculated according to the following formula. Ms (°C) = 521 - 353×C - 22×Si - 24×Mn - 17×Ni - 18×Cr - 16×Mo However, the element symbols in the above formula represent the content (% by mass) of each element contained in the steel sheet.

[0098] In this cooling process, the bainite and martensite transformed from fine-grained austenite are fine-grained, and a blocky structure with a small aspect ratio is formed. Therefore, the area ratio of crystal grains with an aspect ratio of less than 3.0 can be 70% or more. When the average cooling rate is less than 10°C / s, ferrite is formed in the structure. When the temperature range after cooling exceeds Ms point - 50°C, sufficient martensite does not appear and the strength decreases. Also, from the viewpoint of ensuring sufficient ductility, the temperature range after cooling is preferably Ms point - 150°C or higher.

[0099] <Reheating process> Thereafter, the steel material is reheated to 300 - 500 °C and held for 30 - 500 s. At this time, a part of the untransformed austenite transforms into bainite, thereby obtaining bainite. In addition, since the martensite is tempered, a structure with high strength and excellent toughness is obtained. Furthermore, during the holding at 300 - 500 °C, carbon diffuses into the remaining untransformed austenite, enhancing the stability of the austenite. Due to this phenomenon, more than 5% retained austenite is obtained in the final structure, and the uniform elongation of the steel can be increased. Moreover, since the formation of structures such as ferrite and hard martensite can be suppressed, the yield ratio of the steel material can also be increased. The steel plate after reheating is cooled to room temperature at an arbitrary cooling rate.

[0100] By performing these steps, it becomes possible to obtain a steel material having the above-described metal structure.

[0101] (D - 4) Cold working process When a finer structure is desired, it is desirable to perform a cold working process before the above first annealing process. The reasons are as follows.

[0102] In order to disperse a large number of fine austenite crystal grains during heating, it is necessary to obtain a large number of austenite nucleation sites in the metal structure in advance. Possible nucleation sites include the grain boundaries of the initial structure, the interfaces between precipitates such as carbides and the matrix grain boundaries, etc. The tempered martensite structure has lower structures such as packets, blocks, and laths within the old austenite grains, and their boundaries can also serve as nucleation sites.

[0103] Also, when cold working is applied to the martensite structure, the crystal grains become finer, so that the number of nucleation sites further increases and they are in a finely dispersed state in the metal structure. Therefore, if cold - worked martensite is used as the steel material, a fine structure can be obtained.

[0104] When heating martensite, prior to the transformation of ferrite to austenite, C dissolved in the martensite precipitates as carbide. The carbide also preferentially precipitates at crystal interfaces and the like within the metal structure, similar to austenite. Since the interface between the carbide precipitated as described above and the matrix structure is also an effective nucleation site, by starting with a cold-worked martensite structure and heating in such a way that austenite transformation begins after a process in which a large number of fine carbides are formed during the heating process, more nucleation sites can be obtained.

[0105] Note that there are no particular restrictions on the method of performing cold working. For example, when performing cold rolling, it is desirable to set the cold working degree to 20% or more.

[0106] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.

Examples

[0107] An 180-kg steel ingot having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and forged hot into a steel sheet with a thickness of 30 mm. The obtained slab was hot-rolled using a hot-rolling testing machine to obtain a hot-rolled steel sheet with a thickness of 2 mm.

[0108] Note that for Test Nos. 1 to 3, 5 to 8, 10 to 12, 14 to 18, 21 to 26, 28, and 29, after hot rolling, cold rolling was performed on the hot-rolled steel sheet after annealing treatment using a cold-rolling testing machine to obtain a cold-rolled steel sheet with a thickness of 1 mm. Thereby, cold-rolled steel sheets or hot-rolled steel sheets having the metal structures shown in Table 2 were produced and used as steel materials (Test Nos. 1 to 29).

[0109]

Table 1

[0110]

Table 2

[0111] Test pieces with a width of 50 mm, a length of 70 mm, and a thickness of 0.5 mm were taken from the obtained steel material. For each of the taken test pieces, a first annealing process was performed according to the conditions shown in Table 2. The cooling rate in the first annealing process was set to 40 °C / s for all test pieces. Further, a second annealing process was carried out according to the conditions shown in Table 2. In the second annealing process, the holding time in the temperature range of Ms point - 50 °C to Ms point - 200 °C was set to 25 s for all test pieces. Also, the holding time in the reheating process was set to 300 s for all test pieces. Heating was performed by electric heating, and cooling was carried out by injecting nitrogen gas. Note that Table 2 also shows the Ac3 points of each material. Each test piece before and after heat treatment was subjected to microstructure observation and tensile test.

[0112] The metal microstructure of the test pieces before and after heat treatment was measured by the following method.

[0113] First, observation samples were taken from the test pieces before and after heat treatment so that the cross-sections parallel to the rolling direction and the plate thickness direction would be the observation surfaces. Then, the observation surfaces were mirror-polished, etched with nital etching solution, and microstructure observation was carried out using SEM.

[0114] At the position of 1 / 4 depth of the plate thickness of the above observation surface, an area of 130 μm × 130 μm was photographed at a magnification of 1000 times. The obtained microstructural photographs were subjected to black-and-white binarization processing and then image analysis was performed to identify tempered martensite, bainite, ferrite, and other microstructures, and the area ratios of each were determined based on JIS G 0551 (2013) and converted into volume ratios by the line segment method. Also, the volume ratio of retained austenite was measured by X-ray diffraction method. However, since it was difficult to distinguish between tempered martensite and bainite, the total area ratio of tempered martensite and bainite was determined.

[0115] Also, the average BCC crystal grain size was determined by calculating the average value of the equivalent circle diameters of the BCC crystal grains identified in the measurement by EBSD.

[0116] Furthermore, the aspect ratios of the BCC crystal grains identified by the above EBSD were measured, and the area ratio of the total area of the crystal grains with an aspect ratio of less than 3.0 to the entire field of view was determined.

[0117] In addition, in the above measurement, only the retained austenite with an equivalent circle diameter of 0.3 μm or more was targeted.

[0118] [Table 3]

[0119] Referring to Tables 1 to 3, Test Nos. 1 to 15 that satisfy all the conditions defined in the present invention had a tensile strength of 1150 MPa or more, and the product of the tensile strength and the uniform elongation was 15000 MPa·% or more, resulting in an excellent balance between strength and ductility.

[0120] On the other hand, for Test Nos. 16 to 26, although the chemical composition of the steel satisfied the provisions of the present invention, the metallographic structure did not satisfy the provisions of the present invention due to inappropriate manufacturing conditions.

[0121] Specifically, for Test Nos. 16 and 19, since the heating rate in the second annealing process was less than 500 °C / s, the austenite during annealing coarsened, and the area ratio of crystal grains with an aspect ratio of less than 3.0 became less than 70%.

[0122] For Test No. 17, since the holding temperature in the second annealing process exceeded Ac3 + 100 °C, the austenite during annealing coarsened, and the area ratio of crystal grains with an aspect ratio of less than 3.0 became less than 70%.

[0123] For Test No. 18, since the temperature in the first annealing process was lower than the Ac3 point, the ferrite volume fraction increased.

[0124] In addition, for Test No. 20, since the metallographic structure of the steel material was mainly ferrite / pearlite, the austenite during the second annealing coarsened, and the area ratio of crystal grains with an aspect ratio of less than 3.0 became less than 70%.

[0125] For Test No. 21, since the cooling rate in the second annealing process was less than 10 °C / s, a large amount of ferrite was generated during cooling, and the total volume ratio of tempered martensite and bainite in the final structure decreased. At the same time, the BCC average crystal grain size exceeded 5.0 μm.

[0126] For Test No. 22, since the cooling stop temperature in the second annealing process was lower than Ms point - 200 °C, most of the austenite transformed into martensite. Therefore, a sufficient amount of bainite could not be obtained in the metallographic structure, and the area ratio of crystal grains with an aspect ratio of less than 3.0 became less than 70%.

[0127] For Test No. 23, since the cooling stop temperature in the second annealing process was higher than Ms point - 50 °C, the transformation of austenite to martensite did not occur sufficiently. Instead, a large amount of bainite transformed at the reheating temperature. Therefore, the total volume ratio of tempered martensite, bainite, and retained austenite in the final structure decreased.

[0128] For Test No. 24, since the first annealing process was not carried out, the volume ratio of ferrite increased.

[0129] For Test No. 25, since the reheating temperature in the second annealing process was lower than 300 °C, carbon did not diffuse into the austenite. As a result, the stability of the austenite could not be enhanced, and retained austenite could not be obtained in the final structure.

[0130] For Test No. 26, since the holding time of the second annealing process exceeded 5 s, the austenite during annealing coarsened, and the area ratio of crystal grains with an aspect ratio of less than 3.0 became less than 70%.

[0131] As a result, either the strength or the ductility of these steels was low, resulting in an inferior balance between strength and ductility.

[0132] Test No. 27 is an example where the Mn content is excessive and the value of Ceq is high. Therefore, the Mn concentration distribution in the metal structure is large, and even after the first annealing process, regions with high Mn concentration and regions with low Mn concentration coexist in the metal structure. As a result, the regions with low Mn concentration could not stably exist as retained austenite during the cooling process of the second annealing process and became hard martensite, resulting in a low balance between strength and uniform elongation.

[0133] Test No. 28 is an example where the C content is excessive and the value of Ceq is high. Therefore, the total volume fraction of tempered martensite, bainite, and retained austenite in the structure decreased, resulting in an inferior balance between strength and uniform elongation.

[0134] Test No. 29 has a low Mn content and a low value of Ceq, and has low hardenability, so the structure is composed only of ferrite. Therefore, the strength decreased.

Industrial Applicability

[0135] According to the present invention, by forming a metal structure containing a large number of crystal grains with a low aspect ratio and containing retained austenite, it is possible to obtain a steel material that is more ductile and has a higher yield ratio than conventional TRIP steels.

Claims

1. The chemical composition is by mass percentage: C: 0.50% or less, Si: 0.80 - 3.00%, Mn: 0.2 - 5.0%, P: 0.10% or less, S: 0.030% or less, Al: 3.00% or less, N: 0.010% or less, Ni: 0 - 10.0%, Cu: 0 - 3.0%, Cr: 0 - 10.0%, Ti: 0 - 1.0%, Nb: 0 - 1.0%, V: 0 - 1.0%, Mo: 0 - 2.0%, W: 0 - 1.0%, B: 0 - 0.01%, Co: 0 - 1.0%, Ca: 0 - 0.01%, Mg: 0 - 0.01%, REM: 0 - 0.01%, The balance is Fe and impurities, Ceq defined by the following formula (i) is 0.10 - 1.00, The metal structure contains tempered martensite, bainite, and retained austenite, The volume fraction of retained austenite is 5% or more, The total volume fraction of tempered martensite and bainite is 70% or more, The total volume fraction of tempered martensite, bainite, and retained austenite is 90% or more, Among the total amount of tempered martensite and bainite, the area ratio of crystal grains with an aspect ratio of less than 3.0 is 70% or more, Having a tensile strength of 1150 MPa or more, Steel plate. Ceq = C + 1 / 24Si + 1 / 6Mn + 1 / 40Ni + 1 / 5Cr + 1 / 4Mo + 1 / 14V... (i) However, each element symbol in the formula represents the content (mass percentage) of each element contained in the steel plate.

2. The average crystal grain size of tempered martensite and bainite in the metal structure is 3.0 μm or less, The steel plate according to Claim 1.

3. The chemical composition is by mass percentage: Ni: 0.1 - 10.0%, Cu: 0.3 - 3.0%, and Cr: 0.1 - 10.0%, Containing one or more selected from the above, The steel plate according to Claim 1 or Claim 2.

4. The chemical composition is by mass percentage: Ti: 0.01 - 1.0%, Nb: 0.01 - 1.0%, V: 0.01 - 1.0%, Mo: 0.05 - 2.0%, W: 0.05 - 1.0%, B: 0.0003 - 0.01%, and Co: 0.05 - 1.0%, Containing one or more selected from the above, The steel plate according to any one of Claims 1 to 3.

5. The chemical composition is by mass percentage: Ca: 0.0001 - 0.01%, Mg: 0.0001 - 0.01%, and REM: 0.0001 - 0.01%, Containing one or more selected from the above, The steel sheet according to any one of claims 1 to 4.

6. A method for manufacturing the steel sheet according to any one of claims 1 and 3 to 5, having the chemical composition according to any one of claims 1 and 3 to 5, A steel material having a metal structure mainly composed of martensite is held at a temperature of Ac ３ point or higher for 10 s or longer, and is heat-treated by cooling to room temperature at an average cooling rate of 5°C / s or higher. Then, it is heated to the temperature range from Ac ３ point to Ac ３ point + 100°C at an average heating rate of 500°C / s or higher. After that, it is held for 5 to 30 s and then cooling is started. It is cooled to the temperature range of Ms point - 50°C to Ms point - 200°C at an average cooling rate of 10°C / s or higher and less than 100°C / s. After that, it is reheated to the temperature range of 300 to 500°C and held for 30 to 500 s, and then cooled to room temperature. A method for manufacturing a steel sheet.

7. A method for manufacturing the steel sheet according to claim 2, wherein cold working is performed in advance on a steel material having a metal structure mainly composed of martensite, The method for manufacturing a steel sheet according to claim 6.

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