Steel material and its manufacturing method

Abstract

To provide a steel having high ductility and a high yield ratio.SOLUTION: A steel has a chemical composition consisting of, in mass%, C: 1.00% or less, Si: 3.00% or less, Mn: 0.2-7.0%, P: 0.10% or less, S: 0.030% or less, Al: 3.00% or less, N: 0.010% or less with the balance being Fe and impurities, and optionally contains Ni, Cu, Cr, Ti, Nb, V, Mo, W, B, Co, Ca, Mg, or REM, with Ceq of 0.10-1.00. The metallographic structure consists of, in vol.%, retained austenite: 5.0-30.0%, the total of martensite and bainite: 2.5-50.0%, and the total of retained austenite, martensite, tempered martensite, bainite and ferrite: 90.0% or more. The retained austenite has an average crystal grain size of 2.0 μm or less, and the martensite and bainite have an average crystal grain size of 3.5 μm or less. HIT1-HIT2≤3.5 holds true.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a steel material and a method for producing the same. [Background technology]

[0002] It is known that the finer the steel structure, the higher its strength, ductility, and toughness. For example, in a multi-phase steel in which hard martensite is dispersed in a soft matrix, the steel with many finely dispersed martensite particles has superior ductility and toughness compared to a steel with the same martensite volume fraction in which the martensite particles are coarsened and connected.

[0003] Furthermore, when retained austenite is contained in the structure, the austenite undergoes strain-induced martensitic transformation when strain is introduced. Therefore, steel containing retained austenite in the structure has excellent ductility and high strength, resulting in an excellent balance between strength and ductility.

[0004] DP steel or retained austenite steel is known as a material with an excellent balance of strength and ductility. For example, Patent Document 1 discloses a high-strength thin steel plate with excellent rigidity, which has a tensile strength of 590 MPa or more, a yield ratio of 0.65 or more, and a Young's modulus of 225 GPa or more.

[0005] Patent Document 2 discloses a high-strength cold-rolled steel sheet that has high absorbed energy in a low strain range and excellent crash resistance even without the introduction of strain by press working, for use in automobiles, home appliances, machine structures, etc. Furthermore, Patent Document 3 discloses a high-strength cold-rolled steel sheet that has excellent elongation and stretch flangeability and a high yield ratio. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2007-92131 A [Patent Document 2] JP 2008-231480 A [Patent Document 3] JP 2015-34326 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, the steels described in Patent Documents 1 to 3 have a problem in that the composition must be strictly restricted in order to control the structure, and therefore there is a concern that the freedom in the chemical composition is low, leading to an increase in production costs.

[0008] In addition, the steels described in Patent Documents 1 to 3 contain soft ferrite in order to ensure ductility. Therefore, although the above steels have excellent formability, they have low deformation resistance in a low strain range, and when used as shock absorbing members for automobiles, for example, they have a problem of poor energy absorption ability at the initial stage of collision (deformation). In addition, voids are likely to occur at the interface between the soft ferrite layer and the hard martensite layer, and there is also a problem in terms of local deformability.

[0009] The present invention has been made to solve the above problems, and has an object to provide a steel material having excellent ductility and a high yield ratio. [Means for solving the problem]

[0010] The present invention relates to the following steel material and a method for producing the same.

[0011] (1) Chemical composition, in mass%, C: 1.00% or less, Si:3.00% or less, Mn: 0.2-7.0%, 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 to 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 to 0.01%, REM: 0~0.01%, The balance is Fe and impurities. Ceq defined by the following formula (i) is 0.10 to 1.00, The metal structure is, by volume percent, Retained austenite: 5.0 to 30.0%, and Total volume fraction of martensite and bainite: 2.5 to 50.0%; The total volume fraction of retained austenite, martensite, tempered martensite, bainite and ferrite is 90.0% or more, The average grain size of the retained austenite is 2.0 μm or less. The average grain size of martensite and bainite is 3.5 μm or less, The following formula (ii) is satisfied: Steel material. Ceq=C+1 / 24Si+1 / 6Mn+1 / 40Ni+1 / 5Cr+1 / 4Mo+1 / 14V (i) H IT1 -H IT2 ≦3.5 (ii) In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and when no element is contained, the value is set to zero. Each element symbol in the above formula is defined as follows. H IT1 : Average nanohardness of martensite and bainite (GPa) H IT2 : Average nano-hardness of tempered martensite and ferrite (GPa)

[0012] (2) The chemical composition is, in mass%, Ni: 0.1 to 10.0%, Cu: 0.3-3.0%, and Cr: 0.1-10.0%, Contains one or more selected from The steel material described in (1) above.

[0013] (3) The chemical composition is, in mass%, Ti: 0.01 to 1.0%, Nb: 0.01 to 1.0%, V: 0.01 to 1.0%, Mo: 0.05-2.0%, W: 0.05 to 1.0%, B: 0.0003 to 0.01%, and Co: 0.05-1.0%, Contains one or more selected from The steel material according to (1) or (2) above.

[0014] (4) The chemical composition is, in mass%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, and REM: 0.0001~0.01%, Contains one or more selected from The steel material according to any one of (1) to (3) above.

[0015] (5) The average nanohardness of tempered martensite and ferrite in the metal structure is 3.0 GPa or more. A steel material according to any one of (1) to (4) above.

[0016] (6) Having a chemical composition according to any one of (1) to (4) above, For steel materials having a metal structure mainly composed of cold worked martensite or cold worked tempered martensite, A first heat treatment step and a second heat treatment step are carried out in sequence, In the first heat treatment step, the rapid heating start temperature T to Ac 1 The average heating rate at each point is set to 150℃ / s or more.3 Ac above points 3 After heating to a temperature range below +100°C, start cooling within 2.0 seconds, and cool to a temperature of 100°C or less so that the average cooling rate from 800 to 500°C is 150°C / s or more. In the second heat treatment step, Ac 1 ≥ 100, and (Ac 1 Point+Ac 3 After heating to a two-phase temperature range in a range of less than 1 / 2, cooling is started within 2.0 seconds, and the temperature is cooled to 100°C or less so that the average cooling rate from the two-phase temperature range to 300°C is 20 to 150°C / s. Steel manufacturing method. However, the rapid heating start temperature T is calculated as follows. Ac 1 If ≧700℃, T=600℃ Ac 1 <700℃, T=Ac 1 -100℃ Effect of the Invention

[0017] According to the present invention, it is possible to obtain a steel material having excellent ductility and a high yield ratio. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present inventors have conducted extensive research into a method for obtaining a steel material having excellent ductility and a high yield ratio, and as a result have reached the following findings.

[0019] (a) In order to obtain excellent ductility and a high yield ratio, it is preferable that the metal structure contains finely dispersed retained austenite, martensite, and bainite, and the other base parts are mainly composed of relatively hard tempered martensite.

[0020] (b) In order to obtain a steel material having such a metal structure, the steel material is subjected to a first heat treatment process and a second heat treatment process in this order. In the first heat treatment process, the steel material is rapidly heated to the austenite single phase region to generate austenite grains. Then, by immediately rapidly cooling from that state, fine martensite is obtained.

[0021] (c) The degree of fineness after heating varies greatly depending on the structure of the steel material. It is preferable to use steel materials that have many austenite nucleation sites in the metal structure. Therefore, it is preferable to use steel materials that have a metal structure mainly composed of cold-worked martensite or cold-worked tempered martensite.

[0022] (d) Many austenite nucleation sites are formed in the fine martensite obtained by the first heat treatment step, and therefore, by carrying out the second heat treatment step, retained austenite, martensite, and bainite can be finely and abundantly dispersed in the matrix.

[0023] (e) In the second heat treatment step, ultrafine austenite grains are generated in part of the metal structure at high temperatures by heating to the low temperature side of the ferrite / austenite two-phase region, and then immediately cooled from that state to obtain finely dispersed retained austenite, martensite, and bainite.

[0024] (f) The untransformed region after heating, i.e., the region in which the martensite obtained in the first heat treatment step remains, is tempered to become tempered martensite.

[0025] (g) When heating is performed at a fast heating rate, the transformation to austenite completion temperature (Ac 3 Since the temperature rises, the initial martensite can be tempered in a short time at high temperatures.

[0026] (h) The ultrafine austenite grains that are generated tend to grow into coarse grains due to grain boundary migration at high temperatures. Therefore, cooling is started immediately after heating to allow the material to cool to room temperature while maintaining the ultrafine structure.

[0027] (i) By slowing down the cooling rate to a certain extent, martensite and bainite are softened. Furthermore, carbon diffuses during the cooling process, so carbon is concentrated in the austenite, which has the effect of stabilizing the retained austenite.

[0028] (j) In order to prevent the growth and coarsening of ultrafine austenite grains, it is also effective to lower the transformation temperature. Since the movement of grain boundaries is caused by the diffusion of atoms, lowering the temperature and slowing down the diffusion rate makes it possible to maintain fine grains.

[0029] (k) By adjusting the content of Mn, etc., it is possible to lower the transformation temperature of the steel material.

[0030] (l) The metal structure obtained by carrying out the first heat treatment step and the second heat treatment step can reduce the difference in hardness between the base part and the martensite and bainite, thereby suppressing the generation of voids and improving ductility and local deformability.

[0031] The present invention has been made based on the above findings. Each feature of the present invention will be described in detail below.

[0032] (A) Chemical composition The reasons for limiting the content of each element are as follows. In the following description, "%" for the content means "mass %."

[0033] 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 of the steel, but if it exceeds 1.00%, the Mf point drops too much, and some or all of the austenite generated during heating does not transform during cooling, so the required amount of martensite is not obtained and sufficient strength cannot be obtained. 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. To obtain the above effects, the C content is preferably 0.03% or more.

[0034] Si:3.00% or less Silicon is an element that is distributed to the ferrite phase, and in order to suppress the growth and coarsening of the ultrafine austenite structure, it is necessary to add more silicon than is normally added for deoxidation. However, if the silicon content exceeds 3.00%, the hot workability deteriorates and the steel is prone to cracking during rolling. Therefore, the silicon content is set to 3.00% or less. The silicon content is preferably 2.50% or less. In order to obtain the above effects, the silicon content is preferably 0.01% or more, and more preferably 0.03% or more.

[0035] Mn: 0.2-7.0% Mn is A 1 Mn is an effective element for lowering the growth and coarsening rate of the austenite phase by lowering the transformation point and lowering the austenite formation temperature range. Mn is also an element that is distributed to the austenite phase. Furthermore, it is an effective element when retained austenite is to be utilized. In order to suppress the growth and coarsening of the ultrafine austenite structure, it is necessary to contain 0.2% or more. On the other hand, if the Mn content exceeds 7.0%, the Mf point is too low, and part or all of the austenite generated during heating does not transform during cooling, so that the required amount of martensite cannot be obtained and sufficient strength cannot be obtained. Therefore, the Mn content is set to 0.2 to 7.0%. The Mn content is preferably 5.0% or less, more preferably 3.0% or less.

[0036] P:0.10% or less P is generally contained as an impurity, but it is also an element that has the effect of increasing strength by solid solution strengthening. Therefore, P may be actively contained. However, P is an element that easily segregates, and if its content exceeds 0.10%, the deterioration of 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 0.05% or less, more preferably 0.03% or less, and even more preferably 0.02% or less. There is no need to specify a lower limit for the P content, but if the above effect is to be obtained, it is preferable to set it to 0.001% or more.

[0037] S: 0.030% or less S is an element contained as an impurity, and forms sulfide-based inclusions in steel, thereby reducing the formability of the steel sheet. If the S content exceeds 0.030%, the formability is significantly reduced. 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. There is no particular need to specify a lower limit for the S content, but from the viewpoint of suppressing an increase in refining costs, it is preferably set to 0.0001% or more.

[0038] Al: 3.00% or less Al is an element distributed to the ferrite phase, and in order to suppress the growth and coarsening of the ultrafine austenite structure, it is necessary to add more Al than the amount normally contained for deoxidation. However, if the content exceeds 3.00%, the hot workability deteriorates and the steel is prone to cracking during rolling. Therefore, the Al content is set to 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 effect, the Al content is preferably 0.01% or more, and more preferably 0.03% or more.

[0039] N: 0.010% or less N is an element contained as an impurity and has the effect of reducing the formability of steel sheet. If the N content exceeds 0.010%, the formability is significantly reduced. Therefore, the N content is set to 0.010% or less. The N content is preferably 0.008% or less, and more preferably 0.007% or less. There is no need to specify a lower limit for the N content, but in consideration of the case where one or more of Ti, Nb, and V are contained to refine the steel structure as described below, the N content is preferably 0.0010% or more, and more preferably 0.0020% or more in order to promote the precipitation of carbonitrides.

[0040] In addition to the above elements, the steel material according to 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.

[0041] Ni: 0 to 10.0% Ni is A 1 Ni is an effective element for lowering the rate of growth and coarsening of the austenite phase by lowering the transformation point and lowering the austenite formation temperature range. Ni is also an element that is distributed to the austenite phase. Therefore, Ni may be contained as necessary. However, when the Ni content exceeds 10.0%, the effect of inhibiting grain growth becomes saturated. Therefore, the Ni content is set to 10.0% or less. The Ni content is preferably set to 5.0% or less. In order to obtain the above effect, the Ni content is preferably set to 0.1% or more.

[0042] Cu: 0-3.0% Cu is A 1Cu is an effective element for lowering the rate of growth and coarsening of the austenite phase by lowering the transformation point and lowering the austenite formation temperature range. Cu is also an element that is distributed to the austenite phase. Therefore, Cu may be contained as necessary. However, if the Cu content exceeds 3.0%, the workability deteriorates and the material is prone to cracking during rolling. Therefore, the Cu content is set to 3.0% or less. The Cu content is preferably set to 2.5% or less. In order to obtain the above effect, the Cu content is preferably set to 0.3% or more.

[0043] Cr: 0~10.0% Cr is an element that is distributed to the austenite phase and is effective in suppressing the growth and coarsening of ultrafine austenite structures. Therefore, Cr may be contained as necessary. However, if the Cr content exceeds 10.0%, an imbalance occurs between strength and ductility or strength and toughness. Therefore, the Cr content is set to 10.0% or less. The Cr content is preferably set to 8.0% or less. To obtain the above effects, the Cr content is preferably set to 0.1% or more.

[0044] Ti: 0-1.0% Ti is an element that distributes to the ferrite phase and diffuses slowly, and is effective in suppressing the growth and coarsening of ultrafine austenite structures. Therefore, Ti may be contained as necessary. However, if the Ti content exceeds 1.0%, the steel becomes embrittled. Therefore, the Ti content is set to 1.0% or less. The Ti content is preferably set to 0.5% or less. To obtain the above effect, the Ti content is preferably set to 0.01% or more.

[0045] Nb: 0-1.0% Nb is an element that distributes to the ferrite phase and diffuses slowly, and is effective in suppressing the growth and coarsening of ultrafine austenite structures. Therefore, Nb may be contained as necessary. However, if the Nb content exceeds 1.0%, the steel becomes embrittled. Therefore, the Nb content is set to 1.0% or less. The Nb content is preferably set to 0.5% or less. To obtain the above effects, the Nb content is preferably set to 0.01% or more.

[0046] V: 0 to 1.0% V is an element that is distributed to the ferrite phase and is effective in suppressing the growth and coarsening of ultrafine austenite structures. Therefore, V may be contained as necessary. However, if the V content exceeds 1.0%, the steel becomes embrittled. Therefore, the V content is set to 1.0% or less. The V content is preferably set to 0.5% or less. To obtain the above effects, the V content is preferably set to 0.01% or more.

[0047] Mo: 0-2.0% Mo is an element that distributes to the ferrite phase and diffuses slowly, and is effective in suppressing the growth and coarsening of ultrafine austenite structures. Therefore, Mo may be added as necessary. However, when the Mo content exceeds 2.0%, the effect of suppressing grain growth becomes saturated. Therefore, the Mo content is set to 2.0% or less. The Mo content is preferably set to 1.0% or less. To obtain the above effect, the Mo content is preferably set to 0.05% or more.

[0048] W: 0~1.0% W is an element that distributes to the ferrite phase and diffuses slowly, and is effective in suppressing the growth and coarsening of ultrafine austenite structures. Therefore, W may be contained as necessary. However, when the W content exceeds 1.0%, the effect of suppressing grain growth becomes saturated. Therefore, the W content is set to 1.0% or less. The W content is preferably set to 0.5% or less. To obtain the above effect, the W content is preferably set to 0.05% or more.

[0049] B: 0 to 0.01% B is an element that improves hardenability and is effective in obtaining a structure containing martensite. Therefore, B may be contained as necessary. However, if the B content exceeds 0.01%, toughness deteriorates. Therefore, the B content is set to 0.01% or less. The B content is preferably set to 0.005% or less. To obtain the above effects, the B content is preferably set to 0.0003% or more.

[0050] Cobalt: 0-1.0% Co is an element that is distributed to the ferrite phase and is effective in suppressing the growth and coarsening of the ultrafine austenite structure. Therefore, Co may be contained as necessary. However, when the Co content exceeds 1.0%, the effect of suppressing grain growth becomes saturated. Therefore, the Co content is set to 1.0% or less. The Co content is preferably set to 0.5% or less. To obtain the above effect, the Co content is preferably set to 0.05% or more.

[0051] Ca: 0-0.01% Magnesium: 0 to 0.01% REM: 0~0.01% Ca, Mg and REM have a pinning effect to suppress austenite grain growth and have the effect of refining austenite grains. Therefore, one or more selected from these elements may be contained as necessary. However, if 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. In addition, when two or more elements are contained in combination, the total content may be 0.03%. In order to obtain the above effect, it is preferable to contain 0.0001% or more of one or more selected from Ca, Mg and REM.

[0052] 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.

[0053] In the chemical composition of the steel material according to the present invention, the balance is Fe and impurities. Note that the term "impurities" refers to components that are mixed in due to various factors in raw materials such as ores and scraps and manufacturing processes during industrial production of steel material, and are permissible within a range that does not adversely affect the present invention.

[0054] Ceq: 0.10 to 1.00 Ceq means carbon equivalent and is defined by the following formula (i). If Ceq is less than 0.10, no martensite structure can be obtained even if quenching is performed. On the other hand, if Ceq exceeds 1.00, not only does toughness and ductility deteriorate, but also weldability and welded portion properties deteriorate when welding is performed. Therefore, Ceq is set to 0.10 to 1.00. Ceq is preferably 0.20 or more, and more preferably 0.30 or more. Ceq=C+1 / 24Si+1 / 6Mn+1 / 40Ni+1 / 5Cr+1 / 4Mo+1 / 14V (i) In the formula, each element symbol represents the content (mass%) of each element contained in the steel, and when the element is not contained, it is set to zero.

[0055] (B) Metal structure of steel The metal structure of the steel material according to the present invention is mainly a multi-phase structure of retained austenite, martensite, tempered martensite, bainite and ferrite. Specifically, the metal structure has a total volume fraction of retained austenite, martensite, tempered martensite, bainite and ferrite of 90.0% or more. The volume fraction of ferrite is preferably 5% or less. Furthermore, the steel material may contain structures such as pearlite, but the total volume fraction of these structures is acceptable as long as it is less than 10.0%.

[0056] In the following description, newly precipitated martensite is sometimes referred to as "new martensite" to distinguish it from tempered martensite, which is initial martensite that has been tempered. In addition, the soft tempered martensite and ferrite are sometimes collectively referred to as the "base structure," and the newly precipitated martensite and bainite are sometimes collectively referred to as the "hard structure."

[0057] The volume fraction of the retained austenite in the metal structure is 5.0-30.0%, and the average grain size is 2.0 μm or less. The total volume fraction of the newly formed martensite and bainite is 2.5-50.0%, and the average grain size is 3.5 μm or less. By dispersing predetermined amounts of fine retained austenite, newly formed martensite, and bainite in the metal structure, it is possible to suppress a decrease in elongation while maintaining high strength.

[0058] The mechanical properties of steel improve with the refinement of the grain size, and the improvement is particularly remarkable when the average grain size of the newly formed martensite and bainite is 3.5 μm or less. The average grain size is preferably 3.0 μm or less. The grain size of the newly formed martensite and bainite here refers to either the prior austenite grain size or the packet grain size of the martensite and bainite, whichever can be specified.

[0059] In the present invention, the volume fraction and average crystal grain size of each structure are measured by the following method.

[0060] First, a sample is taken so that the cross section parallel to the rolling direction and plate thickness direction of the steel material becomes the observation surface. The observation surface is then mirror-polished and etched with a nital etching solution, after which the structure is observed using a scanning electron microscope (SEM).

[0061] At a depth position of 1 / 4 of the plate thickness of the above observation surface, an area of ​​300μm x 300μm is photographed at 1000x magnification. The microstructure photograph obtained is binarized to black and white and then image analyzed to identify areas judged to be newly formed martensite, bainite, or retained austenite, and the total area ratio of these is calculated using a method based on the "Microscopic Test Method for Steel-Crystal Grain Size" specified in the JIS G 0551 (2013) standard.

[0062] Next, in the same field of view (photograph), from the areas determined to be other than areas determined to be new martensite, bainite, or retained austenite, areas that are tempered martensite or ferrite are identified. Ferrite, pearlite, and tempered martensite are identified based on the presence or absence of carbide precipitation. In addition, pearlite and tempered martensite are identified based on the shape and position of the carbides. Then, in image processing, the total area ratio of the identified areas that are tempered martensite or ferrite is calculated.

[0063] Next, crystal orientation is measured and analyzed using an electron backscatter diffraction (EBSD) device in the same field of view as that of the SEM observation of the same sample. The measurement is performed in an area of ​​30 μm x 30 μm or more, with steps of 0.05 μm or less. Then, the FCC phase is identified from the analysis results, and its area ratio is calculated based on JIS G 0551 (2013) as above, and is regarded as the area ratio of retained austenite.

[0064] The area ratio of each structure obtained by the above method is converted to a volume ratio by a line segment method based on the method described in, for example, Quantitative Microscopy, co-edited by Robert T. DeHoff and Frederik N. Rhines (1968).

[0065] The total volume fraction of the newly formed martensite and bainite is then calculated by subtracting the volume fraction of the retained austenite from the total volume fraction of the newly formed martensite, bainite and retained austenite. The total volume fraction of the newly formed martensite, bainite and retained austenite is then added to the total volume fraction of the tempered martensite and ferrite to calculate the total volume fraction of the retained austenite, newly formed martensite, bainite, tempered martensite and ferrite.

[0066] In addition, the crystal grains identified as FCC phase are approximated as spheroids to determine their diameter, and the average grain size of the retained austenite is calculated by averaging the diameters of all the crystal grains in the field of view.

[0067] Furthermore, from the photographs taken using the above-mentioned SEM, the parts identified as retained austenite in the same field of view by EBSD measurement are removed and the image is processed to identify the structure determined to be newly formed martensite or bainite. The identified structure is then approximated as a spheroid by image analysis to determine its diameter. The average diameters of all crystal grains in the field of view are then averaged to calculate the average grain size of the newly formed martensite and bainite.

[0068] In addition, in the steel material of the present invention, the relationship between the average nano-hardness of the newly formed martensite and bainite and the average nano-hardness of the matrix structure satisfies the following formula (ii). H IT1 -H IT2 ≦3.5 (ii) Each symbol in the above formula is defined as follows. H IT1 : Average nanohardness of martensite and bainite (GPa) H IT2 : Average nano-hardness of tempered martensite and ferrite (GPa)

[0069] Tempered martensite, which is the initial martensite that has been tempered, is softer than newly formed martensite and bainite. Also, the ferrite that is formed when austenite that appears during heating transforms into ferrite during cooling is also soft. However, if there is a large difference in hardness between the newly formed martensite and bainite and the base structure, voids will occur at the boundary between the hard layer and the soft layer during processing, causing a deterioration in ductility and local deformability.

[0070] Therefore, the value on the left side of equation (ii) is 3.5 GPa or less. The value on the left side of equation (ii) is preferably 3.0 GPa or less. The smaller the value on the left side of equation (ii), the more improved the ductility can be, but the value on the left side of equation (ii) for the steel material produced by the manufacturing method of the present invention is 0 GPa or more. In addition, the average nano-hardness of the tempered martensite and ferrite (base structure) is preferably 3.0 GPa or more, and more preferably more than 3.2 GPa.

[0071] The average nanohardness of the base structure is measured using the nanoindentation method. Specifically, the measurement is performed under the following conditions. A diamond cube-corner type indenter is used as the indenter, and the continuous stiffness method is adopted as the indentation method. The load is 500 μN, and measurements are performed at 10 locations (at intervals of 5 μm or more) at room temperature. The average value of these measurements is taken as the average nanohardness. The average nanohardness of the hard structure is measured in the same manner as the average nanohardness of the base structure.

[0072] (C) Manufacturing method The steel material according to the present invention can be manufactured by sequentially subjecting a steel material having the above-mentioned chemical composition and a predetermined metal structure to a first heat treatment and a second heat treatment. Each condition will be described in detail below.

[0073] (C-1) Steel material The steel material to be used before the heat treatment has a metal structure mainly composed of cold worked martensite or cold worked tempered martensite for the following reasons.

[0074] In order to disperse many fine austenite grains during heating, it is necessary to obtain many austenite nucleation sites in advance in the metal structure. Possible nucleation sites include the grain boundaries of the initial structure and the interfaces between precipitates such as carbides and the grains of the base material. Martensite structures have substructures such as packets, blocks, and laths within prior austenite grains, and the boundaries between these structures can also become nucleation sites. Therefore, compared to ferrite / pearlite structures, martensite structures have more austenite nucleation sites, and a fine structure can be obtained during heating.

[0075] In addition, when the martensite structure is cold worked, the crystal grains become finer, so the number of nucleation sites increases and the grains become finely dispersed in the metal structure. Therefore, a fine structure can be obtained by using the cold worked martensite as a steel material.

[0076] When martensite is heated, the C dissolved in the martensite precipitates as carbides before the ferrite transforms into austenite. Like austenite, carbides also precipitate preferentially at crystal interfaces within the metal structure. As mentioned above, the interfaces between the precipitated carbides and the base structure are also effective nucleation sites, so by starting with a cold-worked martensite structure and heating it so that austenite transformation begins after a process in which many fine carbides are formed during the heating process, more nucleation sites can be obtained.

[0077] In addition, since martensite is hard and has poor ductility, it may not be possible to perform cold working depending on the composition. In that case, tempering may be performed before cold working to obtain tempered martensite. Even if the starting structure is a cold worked tempered martensite structure, it is possible to obtain many nucleation sites.

[0078] Here, "mainly cold worked martensite or cold worked tempered martensite" means a metal structure in which the total volume fraction of cold worked martensite and cold worked tempered martensite is 95.0% or more. Although structures such as ferrite, pearlite, bainite, and retained austenite may be mixed in the steel material, these structures are acceptable as long as their total volume fraction is less than 5.0%.

[0079] The manufacturing method of the steel material is not particularly limited as long as it is possible to control the metal structure to the above structure, and a general method may be used. The method of cold working the steel material is also not particularly limited, and for example, when cold rolling is performed, it is preferable to set the condition such that the degree of cold working is 20% or more. Furthermore, there is no particular limit to the tempering treatment conditions, and it is sufficient to perform the tempering treatment under conditions that reduce the hardness to an extent that cold working can be performed.

[0080] (C-2) First heat treatment process The above steel material is subjected to a first heat treatment step. The conditions of the first heat treatment step are described in detail below.

[0081] <Heating process> The steel material having the above-mentioned chemical composition and metal structure is heated at a temperature of T~Ac 1 The average heating rate at each point is set to 150℃ / s or more. 3 Ac above points 3 Heat to a temperature range below +100℃. 3 A uniform structure can be obtained by heating to the austenite single phase region above the Ac point. 3 At temperatures above +100°C, the grain growth rate increases and the residence time in the austenite single phase temperature range, including during cooling, becomes longer, causing the austenite grains to grow into coarse grains. 3 Ac above points 3 Below +100℃.

[0082] In addition, the rapid heating start temperature T~Ac 1The average heating rate at each point is 150℃ / s or more. 1 By setting the average heating rate at the points to 150°C / s or more, the temperature range where recrystallization progresses can be rapidly heated and the disappearance of nucleation sites can be suppressed. The average heating rate is preferably 500°C / s or more, and more preferably 1000°C / s or more. There is no particular upper limit to the average heating rate, but a practical range of 20000°C / s or less is desirable.

[0083] In the present invention, Ac 1 Points and Ac 3 The Ac point is determined by the following method. Several test pieces having the same chemical composition and metal structure are prepared, heated to various temperatures at a specified heating rate, and then cooled from the heating temperature to 70°C at an average cooling rate of 1000°C / s with a holding time of 1 s or less. Each test piece is then observed under a microscope, and the lowest heating temperature at which newly formed martensite is observed in the metal structure is defined as Ac. 1 Furthermore, the hardness of each test piece is measured, and the heating temperature applied to the test piece at which the hardness becomes the maximum quenching hardness is defined as Ac 3 Also, Ac 1 Points and Ac 3 The same results can be obtained by measuring the thermal expansion during heating. The rapid heating start temperature T is calculated as follows: Ac 1 If ≧700℃, T=600℃ Ac 1 <700℃, T=Ac 1 -100℃

[0084] <Holding process> After heating under the above conditions, cooling should begin within 2.0 seconds. If the holding time in the above temperature range exceeds 2.0 seconds, austenite will grow during holding, making it difficult to obtain a fine martensite structure. The above holding time is preferably 1.0 seconds or less.

[0085] <Cooling process> In the cooling process, the above-mentioned heating temperature is cooled to a temperature of 100°C or less so that the average cooling rate from 800 to 500°C is 150°C / s or more. If the average cooling rate is less than 150°C / s, ferrite transformation, pearlite transformation, and bainite transformation occur during cooling, and some of the austenite grains become structures other than martensite, making it difficult to make the structure after the first heat treatment process fine martensite. In addition, ferrite remains until the second heat treatment process, making it difficult to make a hard base structure mainly composed of tempered martensite, so the average cooling rate is 150°C / s or more. The average cooling rate is preferably 300°C / s or more, and more preferably 500°C / s or more. There is no particular limit to the upper limit of the average cooling rate, but it is preferable that it is 20,000°C / s or less as a practical range.

[0086] By subjecting the steel material to the first heat treatment step under the above conditions, fine martensite can be obtained. Specifically, it is preferable that the average crystal grain size of the prior austenite grains in the fine martensite is 15 μm or less, and the area ratio of the martensite is 90% or more. Since many austenite nucleation sites are formed in the fine martensite, by subjecting the steel material to the second heat treatment step described later, it is possible to finely and abundantly disperse retained austenite, martensite, and bainite in the base material.

[0087] Furthermore, the fine martensite obtained by carrying out the first heat treatment step is tempered by carrying out the second heat treatment step to become fine tempered martensite, so that a hard matrix structure can be obtained.

[0088] (C-3) Second heat treatment process The steel material after the first heat treatment is subjected to a second heat treatment process. The conditions of the second heat treatment process are described in detail below.

[0089] <Heating process> The steel material having the above-mentioned chemical composition and metal structure is first 1 ≥ 100, and (Ac 1 Point+Ac3 Heating to the above temperature range allows fine austenite to be generated in part of the metal structure. In addition, the area where the martensite obtained in the first heat treatment process remains is tempered at high temperature for a short time, and becomes tempered martensite with fine carbides dispersed therein.

[0090] There is no particular restriction on the heating rate. However, in order to prevent excessive softening of the matrix structure as the tempering progresses, and to prevent coarsening of newly formed martensite and bainite in the final structure due to the growth of austenite formed during the heating process, the average heating rate up to the above temperature range is preferably 150°C / s or more, and more preferably 500°C / s or more. There is no particular upper limit on the average heating rate, but a practical range of 20,000°C / s or less is desirable.

[0091] In the present invention, Ac 1 Points and Ac 3 The Ac point is determined by the following method. Several test pieces having the same chemical composition and metal structure are prepared, heated to various temperatures at a specified heating rate, and then cooled from the heating temperature to 70°C at an average cooling rate of 1000°C / s with a holding time of 1 s or less. Each test piece is then observed under a microscope, and the lowest heating temperature at which newly formed martensite is observed in the metal structure is defined as Ac. 1 Furthermore, the hardness of each test piece is measured, and the heating temperature applied to the test piece at which the hardness becomes the maximum quenching hardness is defined as Ac 3 Also, Ac 1 Points and Ac 3 Similar results can be obtained by measuring the thermal expansion of the points during heating.

[0092] <Holding process> After heating under the above conditions, cooling is started within 2.0 seconds. If the holding time in the above temperature range exceeds 2.0 seconds, austenite grows during holding, making it difficult to obtain fine newly formed martensite, bainite, and retained austenite. In addition, in the process in which austenite grows during holding, the carbon concentration in the austenite decreases as the volume fraction of austenite increases, which impairs the stability of the austenite and reduces the amount of retained austenite obtained after cooling. The above holding time is preferably 1.0 seconds or less.

[0093] <Cooling process> In the cooling process, the material is cooled to a temperature of 100°C or less so that the average cooling rate from the heating temperature to 300°C is 20 to 150°C / s. By cooling under such conditions, a part of the austenite is transformed into martensite, and then carbon is diffused during cooling, and the newly formed martensite is tempered by auto-tempering, softening it. In addition, a part of the austenite is transformed into bainite, and soft bainite is obtained.

[0094] If the cooling rate is less than 20°C / s, most of the austenite will transform into ferrite or pearlite during cooling, and ultrafine martensite and bainite will not be obtained. On the other hand, if the cooling rate exceeds 150°C / s, most of the austenite will transform into martensite during cooling, reducing the amount of retained austenite, and the newly formed martensite and bainite will not be softened sufficiently, making it difficult to achieve a hardness difference of 3.5 GPa or less. Furthermore, since the austenite that existed before the start of cooling will transform into martensite with almost the same shape, the average grain size of the newly formed martensite and bainite in the final structure may become coarse.

[0095] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. EXAMPLES

[0096] A 180 kg steel ingot having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and hot forged into a 40 mm thick steel slab. The resulting slab was hot rolled in a hot rolling test machine to produce a 2 mm thick hot-rolled steel sheet.

[0097] Thereafter, the decarburized layer on the surface of the hot-rolled steel sheet was mechanically ground to produce a steel sheet with a thickness of 2 mm, which was then subjected to various heat treatments so that the main metal structure would become the structure shown in Table 2. Furthermore, the sheet was cold-rolled using a cold-rolling test machine to produce a cold-rolled steel sheet with a thickness of 1 mm, which was used as the steel material.

[0098] [Table 1]

[0099] [Table 2]

[0100] Test pieces measuring 30 mm in width, 200 mm in length, and 1 mm in thickness were cut from the obtained steel material. Each test piece was heat treated according to the conditions shown in Tables 2 and 3. Heating was performed by electrical resistance heating. After the target temperature was reached, the power supply for electrical resistance heating was turned off and cooling water or gas was immediately sprayed to cool the material to room temperature. Tables 2 and 3 show the Ac of each material. 1 Points and Ac 3 The heating rate in Table 2 is the rate from the start of rapid heating T to Ac 1 The cooling rate in Table 3 means the average cooling rate from 800 to 500° C. The cooling rate in Table 3 means the average cooling rate from the heating temperature to 300° C.

[0101] [Table 3]

[0102] The metal structures of the test pieces before and after the heat treatment were measured by the following method.

[0103] First, specimens were taken from the test pieces before and after the heat treatment so that the cross sections parallel to the rolling direction and the plate thickness direction were the observation surfaces. The observation surfaces were then mirror-polished and etched with a nital etchant, after which the structure was observed using a SEM.

[0104] An image of a 300μm x 300μm area was taken at 1000x magnification at a depth of 1 / 4 of the plate thickness on the above observation surface. The microstructure image obtained was binarized to black and white and then image analyzed to identify areas judged to be newly formed martensite, bainite, or retained austenite. The total area ratio of these was calculated based on JIS G 0551 (2013) and converted to a volume ratio using the line segment method.

[0105] Next, in the same field of view (photograph), the areas that were determined to be tempered martensite or ferrite were identified from the areas that were determined to be other than areas determined to be newly formed martensite, bainite, or retained austenite. Ferrite, pearlite, and tempered martensite were identified based on the presence or absence of carbide precipitation, while pearlite and tempered martensite were identified based on the shape and position of the carbides. Then, in image processing, the total area ratio of the identified tempered martensite and ferrite (base structure) based on JIS G 0551 (2013) was calculated and converted to a volume ratio using the line segment method.

[0106] Next, the crystal orientation was measured and analyzed by EBSD in the same field of view as the SEM observation of the same sample. The measurement was performed in an area of ​​30 μm x 30 μm or more in steps of 0.05 μm or less. From the analysis results, the FCC phase was identified, its area ratio was calculated based on JIS G 0551 (2013), and converted to the volume fraction of retained austenite using the line segment method. In addition, the crystal grains identified as FCC phase were approximated as spheroids to determine their diameter, and the average diameter of all crystal grains in the field of view was used to calculate the average grain size of retained austenite.

[0107] The total volume fraction of the newly formed martensite and bainite was calculated by subtracting the volume fraction of the retained austenite from the total volume fraction of the newly formed martensite, bainite, and retained austenite. Furthermore, the total volume fraction of the retained austenite, newly formed martensite, bainite, tempered martensite, and ferrite (complex phase structure) was calculated by adding the total volume fraction of the newly formed martensite, bainite, and retained austenite to the total volume fraction of the tempered martensite and ferrite.

[0108] Furthermore, newly formed martensite or bainite was identified by excluding the areas identified as retained austenite in the EBSD measurement in the same field of view from the photographs taken using the above-mentioned SEM and processing the images. The identified newly formed martensite or bainite was then approximated as a spheroid by image analysis to determine its diameter, and the average grain size of the newly formed martensite and bainite was calculated by averaging the diameters of all crystal grains in the field of view.

[0109] The average nanohardness of the matrix structure was measured using the nanoindentation method under the following conditions. A diamond cube-corner type indenter was used as the indenter, and the continuous stiffness method was adopted as the indentation method. The load was 500 μN, and measurements were taken at 10 locations (with intervals of 5 μm or more) at room temperature. The average value of these measurements was taken as the average nanohardness. The average nanohardness of the hard structure was also measured in the same manner as the average nanohardness of the matrix structure.

[0110] The results are shown in Table 4.

[0111] [Table 4]

[0112] Furthermore, some of these test pieces were subjected to a tensile test to measure the mechanical properties.

[0113] The tensile test was carried out using an Instron tensile tester in accordance with the provisions of ASTM standard E8. From the above test pieces, an Instron-type tensile test piece (parallel part length: 30 mm, parallel part plate width: 6.0 mm) was taken so that the test direction was parallel to the rolling direction. In addition, since the electric heating device and cooling device used in this embodiment limited the uniform heating area obtained from a sample with a length of about 200 mm, it was decided to adopt a half-size plate-shaped test piece of ASTM standard E8.

[0114] The results are shown in Table 5. In this example, it was determined that a steel sheet had excellent ductility and a high yield ratio when TS×EL was 17000 MPa·% or more and the yield ratio was 0.60 or more.

[0115] [Table 5]

[0116] As shown in Table 5, test numbers 1, 2, and 29 to 31, which satisfy the requirements of the present invention, were excellent in ductility and had a high yield ratio of 0.60 or more. On the other hand, test numbers 15 and 32 to 40, which are comparative examples that do not satisfy the requirements of the present invention, were inferior in at least one of TS×EL and the yield ratio. [Industrial Applicability]

[0117] According to the present invention, it is possible to obtain a steel material having excellent ductility and a high yield ratio.

Claims

1. The chemical composition, in mass%, is C: 1.00% or less, Si: 3.00% or less, Mn: 0.2-7.0%, 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: 0-3.0%, Cr: 0-10.0%, Ti: 0 to 1.0%, Nb: 0 to 1.0%, V: 0 to 1.0%, Mo: 0-2.0%, W: 0 to 1.0%, B: 0 to 0.01%, Co: 0 to 1.0%, Ca: 0-0.01%, Mg: 0 to 0.01%, REM: 0-0.01%, The balance is Fe and impurities. Ceq defined by the following formula (i) is 0.10 to 1.00, The metal structure is, in volume percent, Retained austenite: 5.0 to 30.0%; and Total volume fraction of martensite and bainite: 2.5 to 50.0%; The total volume fraction of retained austenite, martensite, tempered martensite, bainite and ferrite is 90.0% or more, The average grain size of the retained austenite is 2.0 μm or less. The average grain size of martensite and bainite is 3.5 μm or less, The following formula (ii) is satisfied: Steel material. Ceq=C+1 / 24Si+1 / 6Mn+1 / 40Ni+1 / 5Cr+1 / 4Mo+1 / 14V...(i) H ＩＴ１  ＩＴ２ ≦... ・・・(ii) In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and when no element is contained, the value is set to zero. Each element symbol in the above formula is defined as follows. H ＩＴ１ : Average nanohardness of martensite and bainite (GPa) H ＩＴ２ : Average nanohardness of tempered martensite and ferrite (GPa)

2. The chemical composition, in 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 claim 1.

3. The chemical composition, in mass%, Ti: 0.01 to 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 to 0.01%, and Co: 0.05-1.0%, Contains one or more selected from The steel material according to claim 1 or 2.

4. The chemical composition, in mass%, Ca: 0.0001-0.01%, Mg: 0.0001 to 0.01%, and REM: 0.0001-0.01%, Contains one or more selected from The steel material according to any one of claims 1 to 3.

5. The average nanohardness of tempered martensite and ferrite in the metal structure is 3.0 GPa or more. The steel material according to any one of claims 1 to 4.

6. A method for producing a steel material according to any one of claims 1 to 5, comprising: The chemical composition according to any one of claims 1 to 4, For steel materials having a metal structure mainly composed of cold worked martensite or cold worked tempered martensite, A first heat treatment step and a second heat treatment step are carried out in sequence, In the first heat treatment step, the rapid heating start temperature T to Ac １ The average heating rate at each point is set to 150° C. / s or more. ３ Ac above points ３ After heating to a temperature range of less than +100 ° C., cooling is started within 2.0 seconds, and the temperature is cooled to 100 ° C. or less so that the average cooling rate from 800 to 500 ° C. is 150 ° C. / s or more; In the second heat treatment step, Ac １ point or more, and (Ac １ Point + Ac ３ After heating to a two-phase temperature range in a range of less than 100° C. / s, cooling is started within 2.0 s, and the average cooling rate from the two-phase temperature range to 300° C. is 20 to 150° C. / s, Steel manufacturing method. However, the rapid heating start temperature T is determined as follows. A.C. １ If ≧700° C., T=600° C. A.C. １ <700°C, T = Ac １ -100℃

Images