Steel plate and method for manufacturing the same

Abstract

The steel plate has a predetermined chemical composition, and in the metal structure, the number density of alloy carbides with a long diameter of 10 to 100 nm existing at the crystal grain boundaries is 1.0×10 8 ~1.0×10 11 pieces / cm 2 The number density of alloy carbides with a long diameter of less than 10 nm in the grains is 1.0×10 16 ~1.0×10 19 pieces / cm 3 , the tensile strength of the steel plate is above 1030 MPa.

Description

Technical Field

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

[0002] This application claims priority based on Japanese Patent Application No. 2021-030350 filed in Japan on February 26, 2021, the contents of which are incorporated herein by reference. Background Art

[0003] In recent years, lightweighting of automotive and mechanical components has been a growing trend. This is achieved by optimizing component shapes to ensure rigidity. Furthermore, for blank-formed components, such as press-formed parts, lightweighting is possible by reducing the thickness of the component material. However, in order to maintain component strength characteristics such as static fracture strength and yield strength while reducing the thickness, the use of high-strength materials has become necessary. In particular, research is underway into the use of higher-strength steel sheets for automotive chassis.

[0004] Automotive chassis components are manufactured by subjecting steel sheets to processes such as flanging, stretch flanges, and bending. Therefore, steel sheets suitable for these components must not only possess high strength but also excellent formability, particularly hole expandability. Furthermore, post-processing bendability degradation must be minimized.

[0005] For example, Patent Document 1 discloses a high-strength thin steel sheet with excellent delayed fracture resistance on a shear surface, wherein the main phase is a ferrite phase that accounts for 95% or more of the area ratio, and the ferrite phase has the following structure: a ratio dN / dL of an average ferrite grain size dN in the thickness direction to an average ferrite grain size dL in the rolling direction is 0.5 or more, an average grain size defined by (2×dL×dN) / (dL+dN) is 5 μm or less, and a precipitation density of precipitates smaller than 10 nm is 1.0×10 5 pieces / μm 3 above.

[0006] Patent Document 2 discloses an alloyed hot-dip galvanized steel sheet containing, by area ratio, 10% to 90% ferrite and 10% or more tempered martensite and tempered bainite, wherein the total of the ferrite, the tempered martensite and the tempered bainite is 90% or more, and the ferrite grains are arranged at a density of 20 / μm. 2 The number density above contains carbides with a long diameter of 50nm to 300nm. According to formula (1) (S=Sy 2 / Sx 2 )The two-dimensional homogeneous dispersion ratio S defined by is 0.75 to 1.30.

[0007] However, deterioration of bendability after working is not considered in Patent Documents 1 and 2. Furthermore, the inventors of the present invention have recognized that the techniques described in Patent Documents 1 and 2 require further improvements in strength and hole expandability.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2015-147957

[0011] Patent Document 2: Japanese Patent No. 6690804 Summary of the Invention

[0012] Problems to be solved by the invention

[0013] An object of the present invention is to provide a steel plate having high strength and excellent hole expandability and having minimal deterioration in bendability after working. Another object of the present invention is to provide a method for producing the steel plate.

[0014] Means for solving problems

[0015] The inventors of the present invention have studied methods for obtaining the above-mentioned steel sheet and have found that by strictly controlling the chemical composition and the number density of alloy carbides present at grain boundaries and within grains, it is possible to achieve high strength and excellent hole expandability while minimizing the deterioration of bendability after working. Furthermore, the inventors of the present invention have found that the above-mentioned steel sheet can be produced by strictly controlling the conditions in the rough rolling and reheating steps, in particular.

[0016] The gist of the present invention made based on the above findings is as follows.

[0017] (1) The chemical composition of the steel sheet according to one embodiment of the present invention contains, in mass %, the following:

[0018] C: 0.030~0.180%,

[0019] Si: 0.030~1.400%,

[0020] Mn: 1.60~3.00%,

[0021] Al: 0.010~0.700%,

[0022] P: 0.0800% or less,

[0023] S: 0.0100% or less,

[0024] N: 0.0050% or less,

[0025] Ti: 0.020~0.180%,

[0026] Nb: 0.010~0.050%,

[0027] Mo: 0~0.600%,

[0028] V: 0~0.300%

[0029] Total of Ti, Nb, Mo and V: 0.100-1.130%,

[0030] B: 0 to 0.0030%, and

[0031] Cr: 0~0.500%,

[0032] The rest contains Fe and impurities.

[0033] The metal structure is calculated in area % as follows:

[0034] Bainite: more than 80.0%;

[0035] Total of fresh martensite and tempered martensite: less than 20.0%;

[0036] Total of pearlite, ferrite and austenite: less than 20.0%,

[0037] The number density of alloy carbides with a long diameter of 10 to 100 nm existing at the grain boundaries is 1.0×10 8 ~1.0×10 11 pieces / cm 2 ,

[0038] The number density of alloy carbides with a long diameter of less than 10 nm in the grains is 1.0×10 16 ~1.0×10 19 pieces / cm 3 ,

[0039] The tensile strength of the steel plate is greater than 1030 MPa.

[0040] (2) In the steel sheet according to (1), the ratio of the area ratio of the tempered martensite to the total area ratio of the fresh martensite and the tempered martensite may be 80.0% or more.

[0041] (3) The steel sheet according to (1) or (2) above, wherein the chemical composition may contain, in mass %, one or more elements selected from the group consisting of:

[0042] Mo: 0.001~0.600%,

[0043] V: 0.010~0.300%,

[0044] B: 0.0001~0.0030%, and

[0045] Cr: 0.001~0.500%.

[0046] (4) A method for manufacturing a steel plate according to another embodiment of the present invention is the method for manufacturing a steel plate according to (1) above, comprising the following steps:

[0047] A rough rolling step, wherein the slab having the chemical composition described in (1) is heated and rough rolled in a temperature range of 1000 to 1300° C. for four or more passes;

[0048] a finishing rolling step, after the rough rolling, performing finishing rolling at a final reduction ratio of 24 to 60% and a finishing rolling temperature in the temperature range of 960 to 1060° C.;

[0049] A cooling step of cooling after the finish rolling so that the average cooling rate in the temperature range of 900 to 650° C. becomes 30° C. / second or more;

[0050] a coiling step of coiling the product in a temperature range of 400 to 580° C. after the cooling; and

[0051] a reheating step, after the coiling, heating to a temperature range of 600 to 750°C at an average heating rate of 0.2 to 5.0°C / second, maintaining the temperature range of 600 to 750°C for 60 to 3010 seconds, and then cooling so that the average cooling rate in the temperature range of 500 to 700°C becomes 10°C / second or more;

[0052] The following settings are made in the above rough rolling process:

[0053] The temperature difference between the final pass and the previous pass is set to 50°C or less.

[0054] The reduction ratio of the first to third passes is set to 10-30%.

[0055] The reduction ratio after the fourth pass is set to 15 to 50%.

[0056] Effects of the Invention

[0057] According to the above aspects of the present invention, it is possible to provide a steel sheet having high strength and excellent hole expandability and having little deterioration in bendability after working.

[0058] Furthermore, according to the above-mentioned other aspect of the present invention, a method for manufacturing a steel plate capable of manufacturing the above-mentioned steel plate can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 It is a figure for demonstrating the manufacturing method of the hat-shaped component in Example. DETAILED DESCRIPTION

[0060] Hereinafter, the steel sheet and the manufacturing method thereof according to the present embodiment will be described in detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention.

[0061] For the numerical ranges described below with "to", the lower and upper limits are included in the range. For numerical values ​​expressed as "below" or "exceed", the values ​​are not included in the numerical range. All "%" in chemical composition refers to "mass %".

[0062] The steel sheet of this embodiment contains C: 0.030-0.180%, Si: 0.030-1.400%, Mn: 1.60-3.00%, Al: 0.010-0.700%, P: 0.0800% or less, S: 0.0100% or less, N: 0.0050% or less, Ti: 0.020-0.180%, Nb: 0.010-0.050%, and the balance: Fe and impurities. Each element is described in detail below.

[0063] C: 0.030~0.180%

[0064] C is an element necessary to achieve the desired tensile strength of the steel sheet. If the C content is less than 0.030%, the desired tensile strength cannot be achieved. Therefore, the C content is set to 0.030% or higher. The C content is preferably 0.060% or higher, more preferably 0.080% or higher, and even more preferably 0.085% or higher, 0.090% or higher, 0.095% or higher, or 0.100% or higher.

[0065] On the other hand, if the C content exceeds 0.180%, the combined area ratio of fresh martensite and tempered martensite becomes excessive, deteriorating the hole expandability of the steel sheet. Therefore, the C content is set to 0.180% or less. The C content is preferably 0.170% or less, and more preferably 0.150% or less.

[0066] Si: 0.030~1.400%

[0067] Si is an element that increases the tensile strength of steel sheets through solid solution strengthening. If the Si content is less than 0.030%, the desired tensile strength cannot be achieved. Therefore, the Si content is set to 0.030% or more. The Si content is preferably 0.040% or more, and more preferably 0.050% or more.

[0068] On the other hand, if the Si content exceeds 1.400%, the area ratio of retained austenite increases, and the hole expandability of the steel sheet deteriorates. Therefore, the Si content is set to 1.400% or less. The Si content is preferably 1.100% or less, and more preferably 1.000% or less.

[0069] Mn: 1.60~3.00%

[0070] Mn is an element necessary to increase the strength of steel sheets. If the Mn content is less than 1.60%, the area ratio of ferrite becomes too high, and the desired tensile strength cannot be achieved. Therefore, the Mn content is set to 1.60% or higher. The Mn content is preferably 1.80% or higher, and more preferably 2.00% or higher.

[0071] On the other hand, if the Mn content exceeds 3.00%, the toughness of the cast slab deteriorates and hot rolling becomes impossible. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.70% or less, and more preferably 2.50% or less.

[0072] Al: 0.010~0.700%

[0073] Al acts as a deoxidizer, improving the cleanliness of steel. If the Al content is less than 0.010%, sufficient deoxidation is not achieved, resulting in the formation of numerous inclusions (oxides) in the steel sheet. These inclusions degrade the steel sheet's hole expandability. Therefore, the Al content is set to 0.010% or higher. The Al content is preferably 0.020% or higher, and more preferably 0.030% or higher.

[0074] On the other hand, if the Al content exceeds 0.700%, casting becomes difficult. Therefore, the Al content is set to 0.700% or less. The Al content is preferably 0.600% or less, and more preferably 0.100% or less.

[0075] P: 0.0800% or less

[0076] P is an element that segregates in the center of the steel plate's thickness. It also embrittles welds. If the P content exceeds 0.0800%, the steel plate's hole expandability deteriorates. Therefore, the P content is set to 0.0800% or less. The P content is preferably 0.0200% or less, and more preferably 0.0100% or less.

[0077] The lower the P content, the better, preferably 0%. However, if the P content is reduced too much, the P removal cost increases significantly. Therefore, the P content may be set to 0.0005% or more.

[0078] S: 0.0100% or less

[0079] S is an element that embrittles slabs by existing as sulfides. Furthermore, S also degrades the workability of steel sheets. If the S content exceeds 0.0100%, the hole expandability of the steel sheet deteriorates. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, and more preferably 0.0050% or less.

[0080] The lower the S content, the better, preferably 0%. However, if the S content is reduced too much, the cost of removing the S will increase significantly. Therefore, the S content can be set to 0.0005% or more.

[0081] N: 0.0050% or less

[0082] Nitrogen (N) is an element that forms coarse nitrides in steel, degrading the workability of the steel sheet. If the N content exceeds 0.0050%, the steel sheet's hole expandability deteriorates. Therefore, the N content is set to 0.0050% or less. The N content is preferably 0.0040% or less, and more preferably 0.0035% or less.

[0083] The lower the N content, the better, preferably 0%. However, if the N content is reduced too much, the cost of removing N increases significantly. Therefore, the N content may be set to 0.0005% or more.

[0084] Ti: 0.020~0.180%

[0085] Ti is an element that increases the strength of steel sheets by forming fine nitrides in steel. If the Ti content is less than 0.020%, the desired tensile strength cannot be achieved. Therefore, the Ti content is set to 0.020% or higher. The Ti content is preferably 0.050% or higher, and more preferably 0.080% or higher.

[0086] On the other hand, if the Ti content exceeds 0.180%, the hole expandability of the steel sheet deteriorates. Therefore, the Ti content is set to 0.180% or less. The Ti content is preferably 0.160% or less, and more preferably 0.150% or less.

[0087] Nb: 0.010~0.050%

[0088] Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Furthermore, Nb increases the strength of the steel sheet by forming fine alloy carbides. If the Nb content is less than 0.010%, the desired tensile strength cannot be achieved. Therefore, the Nb content is set to 0.010% or higher. The Nb content is preferably 0.013% or higher, and more preferably 0.015% or higher.

[0089] On the other hand, if the Nb content exceeds 0.050%, the toughness of the cast slab deteriorates and hot rolling becomes impossible. Therefore, the Nb content is set to 0.050% or less. The Nb content is preferably 0.040% or less, and more preferably 0.035% or less.

[0090] Total of Ti, Nb, Mo and V: 0.100-1.130%

[0091] In this embodiment, the combined content of the aforementioned Ti and Nb, as well as the later-described Mo and V, is controlled. If the combined content of these elements is less than 0.100%, the effect of increasing the strength of the steel sheet by forming fine alloy carbides cannot be fully achieved, and the desired tensile strength cannot be achieved. Therefore, the combined content of these elements is set to 0.100% or greater. It should be noted that it is not necessary to include all of Ti, Nb, Mo, and V; as long as the content of any one of these is 0.100% or greater, the aforementioned effects can be achieved. The combined content of these elements is preferably 0.150% or greater, more preferably 0.200% or greater, and even more preferably 0.230% or greater.

[0092] On the other hand, if the total content of these elements exceeds 1.130%, the hole expandability of the steel plate deteriorates. Therefore, the total content of these elements is set to 1.130% or less. The total content of these elements is preferably 1.000% or less, and more preferably 0.500% or less.

[0093] The remainder of the chemical composition of the steel sheet of this embodiment may be Fe and impurities. In this embodiment, impurities refer to substances that are mixed from raw materials such as ore and scrap, or the manufacturing environment, or substances that are allowed within a range that does not adversely affect the steel sheet of this embodiment.

[0094] The steel sheet of this embodiment may contain the following optional elements in place of a portion of Fe. When no optional elements are contained, the lower limit of the content is 0%. Each optional element will be described below.

[0095] Mo: 0.001~0.600%

[0096] Mo is an element that increases the strength of the steel sheet by forming fine alloy carbides in the steel. To reliably achieve this effect, the Mo content is preferably set to 0.001% or more.

[0097] On the other hand, if the Mo content exceeds 0.600%, the hole expandability of the steel sheet deteriorates. Therefore, the Mo content is set to 0.600% or less.

[0098] V: 0.010~0.300%

[0099] V is an element that increases the strength of the steel sheet by forming fine alloy carbides in the steel. To reliably achieve this effect, the V content is preferably set to 0.010% or more.

[0100] On the other hand, if the V content exceeds 0.300%, the hole expandability of the steel sheet deteriorates. Therefore, the V content is set to 0.300% or less.

[0101] B: 0.0001~0.0030%

[0102] B is an element that suppresses the formation of ferrite during the cooling process and improves the strength of the steel sheet. In order to reliably obtain this effect, the B content is preferably set to 0.0001% or more.

[0103] On the other hand, even if the B content exceeds 0.0030%, the above-mentioned effect is saturated. Therefore, the B content is made 0.0030% or less.

[0104] Cr: 0.001~0.500%

[0105] Cr is an element that exhibits effects similar to those of Mn. In order to reliably obtain the effect of increasing the strength of the steel sheet due to the inclusion of Cr, the Cr content is preferably set to 0.001% or more.

[0106] On the other hand, even if the Cr content exceeds 0.500%, the above-mentioned effect is saturated. Therefore, the Cr content is set to 0.500% or less.

[0107] The chemical composition of the steel plate described above can be analyzed using a spark discharge emission spectrometer or the like. C and S are determined by burning the steel plate in an oxygen stream using a gas composition analyzer or the like and measuring it using infrared absorption. N is determined by melting a test piece taken from the steel plate in a helium stream and measuring it using thermal conductivity.

[0108] Next, the metal structure of the steel sheet according to this embodiment will be described.

[0109] The metal structure of the steel sheet of the present embodiment is as follows in terms of area %: bainite: 80.0% or more; the total of fresh martensite and tempered martensite: 20.0% or less; the total of pearlite, ferrite, and austenite: 20.0% or less; the number density of alloy carbides with a long diameter of 10 to 100 nm existing at the crystal grain boundaries is 1.0×10 8 ~1.0×10 10 pieces / cm 2 The number density of alloy carbides with a long diameter of less than 10 nm in the grains is 1.0×10 16 ~1.0×10 19 pieces / cm3 .

[0110] Note that in this embodiment, the metal structure at a position 1 / 4 of the plate thickness from the surface (the region from 1 / 8 to 3 / 8 of the plate thickness from the surface) is specified because the metal structure at this position represents the representative metal structure of the steel plate.

[0111] Bainite: 80.0% or more

[0112] Bainite is a structure that has a predetermined strength and excellent hole expandability. If the area fraction of bainite is less than 80.0%, the desired tensile strength and / or hole expandability cannot be achieved. Therefore, the area fraction of bainite is set to 80.0% or higher. The area fraction of bainite is preferably 81.0% or higher, more preferably 82.0% or higher, and even more preferably 83.0% or higher.

[0113] The upper limit of the area ratio of bainite is not particularly limited, but may be set to 100.0% or less, 95.0% or less, or 90.0% or less.

[0114] Total of fresh martensite and tempered martensite: 20.0% or less

[0115] Fresh martensite and tempered martensite have the effect of increasing the strength of the steel sheet, but due to their low local deformability, the increased area ratio degrades the steel sheet's hole expandability. If the combined area ratio of fresh martensite and tempered martensite exceeds 20.0%, the steel sheet's hole expandability deteriorates. Therefore, the combined area ratio of fresh martensite and tempered martensite is set to 20.0% or less. The combined area ratio of fresh martensite and tempered martensite is preferably 15.0% or less, more preferably 10.0% or less, and even more preferably 5.0% or less.

[0116] The lower limit of the total area ratio of fresh martensite and tempered martensite is not particularly limited, but may be set to 0.0% or more, 0.5% or more, or 1.0% or more.

[0117] Area ratio of tempered martensite: 80.0% or more of the total area ratio of fresh martensite and tempered martensite

[0118] By increasing the area ratio of tempered martensite within the total area ratio of fresh martensite and tempered martensite, the hole expandability of the steel plate can be further improved. Therefore, the area ratio of tempered martensite within the total area ratio of fresh martensite and tempered martensite can be set to 80.0% or greater. A higher area ratio of tempered martensite within the total area ratio of fresh martensite and tempered martensite is more preferred, more preferably 90.0% or greater, and can also be set to 100.0%.

[0119] The ratio of the area ratio of tempered martensite can be obtained by {area ratio of tempered martensite / (total area ratios of fresh martensite and tempered martensite)}×100.

[0120] Total of pearlite, ferrite and austenite: less than 20.0%

[0121] Ferrite and austenite are microstructures that degrade the strength of the steel sheet. Pearlite is a microstructure that degrades the hole expandability of the steel sheet. If the combined area ratio of these microstructures exceeds 20.0%, the desired tensile strength and / or hole expandability cannot be achieved. Therefore, the combined area ratio of these microstructures is set to 20.0% or less. The combined area ratio of these microstructures is preferably 17.0% or less, and more preferably 15.0% or less.

[0122] The lower limit of the total area ratio of pearlite, ferrite, and austenite is not particularly limited, but may be set to 0.0% or more, 5.0% or more, or 10.0% or more.

[0123] Hereinafter, the method for measuring the area ratio of each structure will be described.

[0124] Test specimens were collected from the steel plate in the following manner: in a cross section parallel to the rolling direction, the metal structure at a depth of 1 / 4 of the plate thickness from the surface (the area from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface) and at the center of the plate width direction was observed.

[0125] The cross section of the test piece is polished using #600 to #1500 silicon carbide paper and then finished to a mirror finish using a solution obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water. Next, the sample is polished at room temperature using colloidal silica that does not contain an alkaline solution to remove strain introduced into the surface layer of the sample. At any position along the length of the sample cross section, an area with a length of 50 μm and a depth of 1 / 8 to 3 / 8 of the thickness from the surface is measured using electron beam backscatter diffraction at a measurement interval of 0.1 μm, so that a position with a depth of 1 / 4 of the thickness from the surface can be observed. This provides crystal orientation information.

[0126] For the measurement, an EBSD apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) was used. The vacuum degree in the EBSD apparatus was set to 9.6 × 10 -5Pa or less, with an accelerating voltage of 15 kV, an irradiation current level of 13, and an electron beam irradiation level of 62. The resulting crystal orientation information was used to calculate the area fraction of austenite using the "Phase Map" function in the "OIM Analysis (registered trademark)" software included with the EBSD analyzer. This yielded the area fraction of austenite. It should be noted that crystals with an fcc structure were identified as austenite.

[0127] Next, those with a bcc crystal structure were classified as bainite, ferrite, pearlite, fresh martensite, and tempered martensite. Using the "Grain Orientation Spread" function in the "OIM Analysis (registered trademark)" software included with the EBSD analysis device, regions with a "Grain Orientation Spread" of 1° or less were extracted as ferrite, assuming 15° grain boundaries as crystal grain boundaries. The area ratio of the extracted ferrite was calculated to obtain the ferrite area ratio.

[0128] Next, in the remaining region (the region where the "Grain Orientation Spread" exceeds 1°), assuming the 5° grain boundary as a crystal grain boundary and setting the maximum value of the "Grain Average IQ" in the ferrite region to Iα, the region exceeding Iα / 2 is extracted as bainite, and the region below Iα / 2 is extracted as "pearlite, primary martensite, and tempered martensite." The area ratio of the extracted bainite is calculated to obtain the area ratio of bainite.

[0129] The extracted “pearlite, fresh martensite, and tempered martensite” were distinguished by the following method.

[0130] In order to observe the same area as the EBSD measurement area by SEM, a Vickers indentation is made near the observation position. Afterwards, the structure of the observation surface is retained, the dirt on the surface is ground off, and nitric alcohol etching is performed. Next, the same field of view as the EBSD observation surface is observed by SEM at a magnification of 3000 times. In the EBSD measurement, in the area identified as "pearlite, primary martensite and tempered martensite", the area with a lower structure in the grain and cementite precipitated in multiple variants is judged to be tempered martensite. The area where cementite precipitates in lamellar form is judged to be pearlite. The area with high brightness and no lower structure by etching is judged to be primary martensite. By calculating the respective area ratios, the area ratios of tempered martensite, pearlite and primary martensite are obtained.

[0131] Furthermore, the surface layer of the observation surface can be cleaned by polishing using aluminum oxide particles having a particle size of 0.1 μm or less, or by Ar ion sputtering.

[0132] The number density of alloy carbides with a long diameter of 10 to 100 nm existing at the crystal grain boundaries is 1.0×10 8 ~1.0×10 11 pieces / cm 2

[0133] There are spherical alloy carbides at the crystal grain boundaries. When subjected to deformations such as bending, if the dislocation density accumulated due to deformation reaches a critical amount, then at the interface between the alloy carbides present at the crystal grain boundaries and the parent phase (around the alloy carbides present at the crystal grain boundaries), microvoids are produced. If microvoids are produced in a large number at the crystal grain boundaries, bendability is significantly deteriorated. By making alloy carbides finely dispersed in large quantities at the crystal grain boundaries, the accumulation sites of dislocations can be dispersed. As a result, even if microvoids are produced, stress concentration can be relaxed, and the bendability degradation after processing can be reduced.

[0134] If the number density of alloy carbides with a long diameter of 10 to 100 nm existing at the grain boundaries is less than 1.0×10 8 pieces / cm 2 Therefore, the number density of the alloy carbides is set to 1.0×10 8 pieces / cm 2 More than 2.0×10 8 pieces / cm 2 Above, 5.0×10 8 pieces / cm 2 Above or 1.0×10 9 pieces / cm 2 above.

[0135] If the number density of the above alloy carbides exceeds 1.0×10 11 pieces / cm 2 Therefore, the number density of the alloy carbides is set to 1.0×10 11 pieces / cm 2 Below. Preferably 5.0×10 10 pieces / cm 2 Below, 1.0×10 10 pieces / cm 2 the following.

[0136] In this embodiment, alloy carbides refer to carbides containing one or more of Ti, Nb, Mo, and V. Furthermore, grain boundaries refer to boundaries where the crystal orientation difference is 1.0° or more in the analysis using EBSD described later.

[0137] In this embodiment, for the crystal grain boundaries, since the minimum long diameter of the alloy carbides that can be observed in the measurement method described later is 10 nm, the number density of alloy carbides with a long diameter of 10 nm or more is specified. In addition, if there are coarse alloy carbides with a long diameter exceeding 100 nm at the crystal grain boundaries, microvoids are formed in the early stage of deformation, resulting in necking. Therefore, the number density of alloy carbides with a long diameter exceeding 100 nm is preferably low. However, if the number density of alloy carbides with a long diameter of 10 to 100 nm present at the crystal grain boundaries is within the above range, alloy carbides with a long diameter exceeding 100 nm will not precipitate to a degree that adversely affects the steel plate of this embodiment, so there is no need to specify the number density of alloy carbides with a long diameter exceeding 100 nm.

[0138] The number density of alloy carbides having a major axis of 10 to 100 nm and existing at the crystal grain boundaries is measured by the following method.

[0139] The test piece is collected in such a way that the cross section of the plate thickness parallel to the rolling direction becomes the observation surface. After the observation surface of the test piece is polished, it is etched with nitric alcohol. In the area of ​​the observation surface from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface, the crystal orientation is analyzed in more than 5 fields of view by using the electron beam backscatter diffraction method (EBSD: Electron Back Scatter Diffraction) of a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microsope). Each field of view is set as a continuous area. According to the obtained crystal orientation map, the boundary with a crystal orientation difference of more than 1.0° is regarded as a crystal grain boundary.

[0140] For the same area as the observation field obtained by EBSD, SEM (scanning electron microscope) was used to observe at a magnification of 5000 to 30000 times. For each field of view, the number of alloy carbides with a long diameter of 10 to 100 nm present on the boundary identified as the crystal grain boundary by EBSD was calculated. The number density of alloy carbides with a long diameter of 10 to 100 nm present at the crystal grain boundary was obtained by dividing the number of alloy carbides obtained by the total observation area.

[0141] It should be noted that, as to whether the observed precipitates are alloy carbides, for particles whose brightness is lower than that of the iron matrix phase in the field of view of the secondary electron image obtained by SEM observation, point analysis is performed using SEM-EDS, and precipitates whose sum of the peak intensities of Ti (Kα, Kβ), Nb (Kα), Mo (Lα), and V (Kα) is greater than the peak intensity of Fe (Kα) are judged to be alloy carbides.

[0142] The number density of alloy carbides with a long diameter of less than 10 nm existing in the grains is 1.0×10 16 ~1.0×10 19 pieces / cm 3

[0143] Plate-shaped alloy carbides exist within the grains. By dispersing a large amount of fine alloy carbides within the grains, ferrite, bainite, primary martensite, and tempered martensite are precipitation-strengthened.

[0144] If the number density of alloy carbides with a long diameter of 10 nm or less in the grains is less than 1.0 × 10 16 pieces / cm 3 Therefore, the number density of alloy carbides with a long diameter of 10 nm or less existing in the grains is set to 1.0×10 16 pieces / cm 3 More than 5.0×10 16 pieces / cm 3 Above or 1.0×10 17 pieces / cm 3 above.

[0145] If the number density of the above alloy carbides exceeds 1.0×10 19 pieces / cm 3 Therefore, the number density of the alloy carbides is set to 1.0×10 19 pieces / cm 3 Below. Preferably 5.0×10 18 pieces / cm 3 Less than or 1.0×10 18 pieces / cm 3 the following.

[0146] The number density of alloy carbides having a major diameter of 10 nm or less existing in the grains is measured by the following method.

[0147] For the same area as the observation field obtained by EBSD, TEM (transmission electron microscope) was used to observe at a magnification of 100,000 to 1,000,000 times. For each field of view, the number of alloy carbides with a major diameter of 10 nm or less present in the boundary identified as the crystal grain boundary by EBSD was calculated. The number density of alloy carbides with a major diameter of 10 nm or less present in the grain was obtained by dividing the number of alloy carbides obtained by the total observation volume excluding the crystal grain boundary. In addition, a thin film sample was collected from the test piece when observing using TEM.

[0148] It should be noted that whether the observed precipitates are alloy carbides, since ferrite and precipitates have a Baker-Nutting orientation relationship, the excitation condition is set to g by incident electron beams from the αFe

[100] direction. MC The determination was made by dark field observation at 200. The thickness of the sample was measured perpendicularly to the film surface using SEM.

[0149] Tensile strength TS: 1030MPa or more

[0150] The tensile strength of the steel sheet of this embodiment is 1030 MPa or higher. If the tensile strength is lower than 1030 MPa, it cannot be suitably used in various automotive chassis parts. The tensile strength is preferably 1050 MPa or higher or 1150 MPa or higher.

[0151] The higher the tensile strength, the more preferable it is, but it may be set to 1450 MPa or less.

[0152] The tensile strength was measured by a tensile test using a No. 5 test piece according to JIS Z 2241: 2011. The tensile test piece was collected at the center of the sheet width, with the longitudinal direction perpendicular to the rolling direction.

[0153] Hole expansion rate λ: more than 30%

[0154] The hole expansion ratio of the steel plate of this embodiment may be 30% or more, 35% or more, 40% or more, or 45% or more.

[0155] The hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.

[0156] The steel sheet of the present embodiment can also be provided with a coating on the surface for the purpose of improving corrosion resistance to make a surface-treated steel sheet. The coating can be an electroplated layer or a hot-dip coated layer. As the electroplated layer, examples include electrogalvanized layers, electroplated Zn-Ni alloy layers, etc. As the hot-dip coated layer, examples include hot-dip galvanized layers, alloyed hot-dip galvanized layers, hot-dip aluminized layers, hot-dip Zn-Al alloy layers, hot-dip Zn-Al-Mg alloy layers, hot-dip Zn-Al-Mg-Si alloy layers, etc. The coating adhesion amount is not particularly limited and can be set to the same as in the past. In addition, after plating, suitable chemical conversion treatment (such as coating and drying of the chromium-free chemical conversion treatment solution of silicate system) can be implemented to further improve corrosion resistance.

[0157] Next, the method for manufacturing the steel plate according to the present embodiment will be described.

[0158] The method for manufacturing a steel plate according to this embodiment includes the following steps:

[0159] A rough rolling step, wherein the slab having the above-mentioned chemical composition is heated and rough rolled in a temperature range of 1000 to 1300° C. for four or more passes;

[0160] a finishing rolling step, after the rough rolling, performing finishing rolling at a final reduction ratio of 24 to 60% and a finishing rolling temperature in the temperature range of 960 to 1060° C.;

[0161] A cooling step of cooling after the finish rolling so that the average cooling rate in the temperature range of 900 to 650° C. becomes 30° C. / second or more;

[0162] a coiling step of coiling the product in a temperature range of 400 to 580° C. after the cooling; and

[0163] The reheating step is to heat the steel sheet to a temperature range of 600 to 750°C at an average heating rate of 0.2 to 5.0°C / second after coiling, maintain the steel sheet in the temperature range of 600 to 750°C for 60 to 3010 seconds, and then cool the steel sheet so that the average cooling rate in the temperature range of 500 to 700°C becomes 10°C / second or more.

[0164] In addition, the following settings are made in the above-mentioned rough rolling process:

[0165] The temperature difference between the final pass and the previous pass is set to 50°C or less.

[0166] The reduction ratio of the first to third passes is set to 10-30%.

[0167] The reduction ratio after the fourth pass is set to 15 to 50%.

[0168] Hereinafter, each step will be described.

[0169] Rough rolling process

[0170] In the rough rolling process, the slab having the above-described chemical composition is heated and rough rolled in four or more passes in a temperature range of 1000-1300°C. Furthermore, in the rough rolling process, the temperature difference between the final pass and the pass immediately preceding the final pass is set to 50°C or less, the reduction ratio for the first to third passes is set to 10-30%, and the reduction ratio for the fourth and subsequent passes is set to 15-50%.

[0171] If the rough rolling temperature is below 1000°C, the precipitation of alloy carbides will progress, and after the subsequent reheating process, excessive alloy carbides will precipitate at grain boundaries. As a result, the deterioration of bendability after working cannot be minimized. Therefore, rough rolling is performed in the temperature range of 1000°C or above.

[0172] On the other hand, if rough rolling is performed at 1300° C. or higher, fuel costs will increase, so rough rolling is performed in a temperature range of 1300° C. or lower.

[0173] If the rough rolling process is performed in the temperature range of 1000-1300°C for less than four passes, the reduction ratio per pass increases, which increases the load on the rough rolling mill. Therefore, rough rolling is performed in the temperature range of 1000-1300°C for four or more passes.

[0174] The upper limit is not particularly specified, but the rough rolling performed in the temperature range of 1000 to 1300° C. may be performed for, for example, 6 passes or less.

[0175] During the rough rolling process, if the temperature difference between the final pass and the pass immediately preceding the final pass exceeds 50°C, the austenite grain size becomes uneven, and the coarsening of alloy carbides progresses during the subsequent reheating process. As a result, sufficient amounts of alloy carbides cannot precipitate at the grain boundaries, failing to minimize post-processing bendability degradation. Therefore, the temperature difference between the final pass and the pass immediately preceding the final pass is set to 50°C or less. Preferably, it is 45°C or less, or 40°C or less.

[0176] The temperature difference here specifically refers to the difference between the slab surface temperature at the exit side of the final pass and the slab surface temperature at the exit side of the pass immediately preceding the final pass.

[0177] During the rough rolling process, if the reduction ratio in the first to third passes is less than 10%, or the reduction ratio in the fourth and subsequent passes is less than 15%, the grains will coarsen, and sufficient alloy carbides will not precipitate at the grain boundaries, failing to minimize the deterioration in bendability after working. Therefore, the reduction ratio in the first to third passes is set to 10% or more, and the reduction ratio in the fourth and subsequent passes is set to 15% or more.

[0178] Furthermore, if the reduction rate exceeds 30% in the first to third passes, or exceeds 50% in the fourth and subsequent passes, alloy carbides precipitate, which then coarsen during the subsequent reheating process. Consequently, sufficient amounts of alloy carbides cannot precipitate at grain boundaries, failing to minimize post-processing bendability degradation. The reduction rate for the first to third passes should be set to 30% or less, and the reduction rate for the fourth and subsequent passes should be set to 50% or less.

[0179] It should be noted that the reduction rate here does not refer to the cumulative reduction rate, but the reduction rate per pass.

[0180] Finishing process

[0181] After the rough rolling, finish rolling is performed so that the final reduction ratio (reduction ratio in the final pass) is 24 to 60% and the finish rolling temperature is in the temperature range of 960 to 1060°C.

[0182] If the reduction rate of the final pass is less than 24%, recrystallization will not proceed sufficiently, the alloy carbides precipitated at the crystal grain boundaries will coarsen, and the desired number density cannot be obtained at the crystal grain boundaries. As a result, the desired hole expandability cannot be obtained and / or the deterioration of bendability after processing cannot be reduced. Therefore, the reduction rate of the final pass is set to 24% or more. The final reduction rate of the finishing rolling is preferably 30% or more. From the perspective of suppressing the increase in equipment load, the upper limit of the final reduction rate of the finishing rolling is set to 60% or less.

[0183] When the plate thickness after the final pass of finish rolling is set as t and the plate thickness before the final pass is set as t0, the final reduction ratio of finish rolling can be expressed by (1-t / t0)×100(%).

[0184] If the finishing temperature (the surface temperature of the steel sheet at the exit of the final pass of finishing) is lower than 960°C, recrystallization will not proceed sufficiently, the alloy carbides precipitated at the grain boundaries will coarsen, and the desired number density cannot be obtained at the grain boundaries. As a result, the desired hole expandability cannot be obtained, and / or the deterioration of bendability after processing cannot be reduced. The finishing temperature is preferably 980°C or higher. From the perspective of suppressing the coarsening of grain size and suppressing the deterioration of the toughness of the steel sheet, the upper limit of the finishing temperature is set to 1060°C or lower.

[0185] Cooling process

[0186] After finish rolling, cooling is performed so that the average cooling rate in the temperature range of 900-650°C is 30°C / second or higher. If the average cooling rate in the temperature range of 900-650°C is lower than 30°C / second, a large amount of ferrite and pearlite will be generated, and the desired tensile strength cannot be achieved. Therefore, the average cooling rate in the temperature range of 900-650°C is set to 30°C / second or higher. It is preferably 50°C / second or higher, and more preferably 80°C / second or higher.

[0187] The upper limit of the average cooling rate in the temperature range of 900 to 650° C. is not particularly limited, but may be set to 300° C. / s or less or 200° C. / s or less.

[0188] In addition, the average cooling rate in the present embodiment refers to a value obtained by dividing the temperature difference between the starting point and the end point of a set range by the elapsed time from the starting point to the end point.

[0189] The cooling after cooling in the temperature range of 900 to 650° C. at the above-mentioned average cooling rate and before coiling is not particularly limited.

[0190] Coiling process

[0191] After the aforementioned cooling, the steel sheet is coiled in a temperature range of 400-580°C. If the coiling temperature is lower than 400°C, excessive formation of fresh martensite and tempered martensite occurs, deteriorating the hole expandability of the steel sheet. Therefore, the coiling temperature is set to 400°C or higher, preferably 450°C or higher.

[0192] Furthermore, if the coiling temperature exceeds 580°C, the amount of ferrite increases, and the desired tensile strength cannot be achieved. Furthermore, the desired number density within the grains cannot be achieved. Therefore, the coiling temperature is set below 580°C. The coiling temperature is preferably 560°C or lower.

[0193] The steel plate produced by the above method may be left to cool until it reaches room temperature, or may be water-cooled after being coiled into a coil.

[0194] After coiling, the coil can be unwound, pickled, and then subjected to soft reduction. If the cumulative reduction rate during soft reduction is too high, the dislocation density may increase, potentially degrading the hole expandability of the steel sheet. Therefore, when soft reduction is performed, the cumulative reduction rate is preferably set to 15% or less.

[0195] When the plate thickness after soft reduction is set as t and the plate thickness before soft reduction is set as t0, the cumulative reduction rate of soft reduction can be expressed by (1-t / t0)×100(%).

[0196] Reheating process

[0197] After coiling or light pressing, heat to a temperature range of 600-750°C at an average heating rate of 0.2-5.0°C / second, maintain in this temperature range for 60-3010 seconds, and then cool to an average cooling rate of 10°C / second or more from 500-700°C.

[0198] If the holding temperature in the reheating step is lower than 600°C, a sufficient amount of alloy carbides cannot be precipitated in the grains, and the desired strength cannot be obtained. Therefore, the holding temperature is set to 600°C or higher.

[0199] On the other hand, if the holding temperature exceeds 750°C, the alloy carbides within the grains will coarsen, and the number density of the alloy carbides within the grains will decrease. As a result, the desired strength cannot be achieved. Therefore, the holding temperature is set to 750°C or less.

[0200] If the holding time is less than 60 seconds, a sufficient amount of alloy carbides cannot be precipitated in the grains, and the desired strength cannot be obtained. Therefore, the holding time is set to 60 seconds or more.

[0201] On the other hand, if the holding time exceeds 3010 seconds, the alloy carbides in the crystal grains are coarsened, and the number density of the alloy carbides in the crystal grains is reduced. As a result, the desired strength cannot be obtained. Therefore, the holding time is set to below 3010 seconds.

[0202] If the average heating rate up to the temperature range of 600-750°C is lower than 0.2°C / s, dislocation recovery occurs, the desired strength cannot be obtained, and productivity is reduced. Therefore, the average heating rate up to the temperature range of 600-750°C is set to 0.2°C / s or higher.

[0203] On the other hand, if the average heating rate to the temperature range of 600-750°C exceeds 5.0°C / second, the fuel cost required for heating increases. Therefore, the average heating rate to the temperature range of 600-750°C is set to 5.0°C / second or less.

[0204] After the above-mentioned holding, the steel is cooled to a temperature range of, for example, 100°C or less. During this cooling, the steel is cooled so that the average cooling rate in the temperature range of 500 to 700°C is 10°C / second or more. If the average cooling rate in the temperature range of 500 to 700°C is lower than 10°C / second, the alloy carbides in the grains coarsen and the number density of the alloy carbides in the grains decreases. As a result, the desired strength cannot be obtained. Therefore, the average cooling rate in the temperature range of 500 to 700°C is set to 10°C / second or more.

[0205] The upper limit of the average cooling rate in the temperature range of 500 to 700° C. is not particularly specified, but may be set to 200° C. / second or less from the viewpoint of suppressing the increase in cooling equipment.

[0206] Example

[0207] Slabs having the chemical compositions shown in Table 1 were produced by continuous casting. The resulting slabs were used to produce steel plates having a thickness of 3.0 mm under the conditions shown in Tables 2A to 3B. In the rough rolling step, 4 to 6 passes of rough rolling were performed.

[0208] In addition, blank columns in Table 1 indicate that the element is not intentionally contained.

[0209] The area ratio of each microstructure, the number density of alloy carbides, the tensile strength TS, and the hole expansion ratio λ were determined for the resulting steel plates using the aforementioned methods. The results are shown in Tables 4A and 4B. Test No. 10 in Table 3A did not undergo a reheating step.

[0210] When the tensile strength TS is 1030 MPa or more, the strength is determined to be high and the test is acceptable. On the other hand, when the tensile strength TS is less than 1030 MPa, the strength is determined to be low and the test is unacceptable.

[0211] When the obtained hole expansion ratio λ is 30% or more, the hole expansion property is judged as excellent and acceptable. On the other hand, when the hole expansion ratio λ is less than 30%, the hole expansion property is judged as poor and unacceptable.

[0212] Furthermore, the bendability deterioration ratio after working of the obtained steel sheet was obtained by the following method: In this example, draw bending was performed as the working.

[0213] Drawing and bending process is achieved by utilizing d Figure 1 This was done by forming a hat-shaped part under the conditions shown in . During the forming of the hat-shaped part, the steel sheet undergoes bending and recovery deformation while in contact with the punch during the formation of the longitudinal wall. This allows the reproduction of the concave portion formed in the flat R-shaped portion near the longitudinal wall of the automotive chassis. The test piece used for forming was oriented in the L direction of the steel sheet, with dimensions of 240 mm in length and 50 mm in width. In the bending test described below, the test piece was collected so that the longitudinal wall of the hat-shaped part would form a curved portion.

[0214] A 100 mm x 30 mm strip test piece was cut from the halfway point in the width direction of the steel plate. A bending test was conducted using the V-block method (bending angle θ of 90°) according to JIS Z 2248:2006, with the bending ridgeline parallel to the rolling direction (L direction) (L-axis bending). The minimum bending radius R that does not cause cracking was calculated and divided by the plate thickness t to obtain the limiting bending value R / t.

[0215] The presence of cracks was determined by observing the bent surface of the test piece after the bending test with a magnifying glass or an optical microscope at a magnification of 10 times or more. Cracks were considered to be present when the length of cracks observed on the bent surface of the test piece exceeded 0.5 mm.

[0216] Bending tests were performed using the above method before and after the drawing and bending process, respectively, to determine the R / t before drawing and bending and the R / t of the processed portion after drawing and bending. If the value obtained by dividing the R / t before drawing and bending by the R / t of the processed portion after drawing and bending was 0.5 or greater, the bendability degradation after working was minimal, and the test was deemed acceptable and recorded as "Good" in the table. On the other hand, if the value was 0.5 or less, the bendability degradation after working was significant, and the test was deemed unacceptable and recorded as "Poor" in the table.

[0217] [Table 1]

[0218]

[0219] [Table 2A]

[0220]

[0221] The underlined elements are outside the scope of the present invention.

[0222] [Table 2B]

[0223]

[0224] The underlined elements are outside the scope of the present invention.

[0225] [Table 3A]

[0226]

[0227] The underlined elements are outside the scope of the present invention.

[0228] [Table 3B]

[0229]

[0230] The underlined elements are outside the scope of the present invention.

[0231] [Table 4A]

[0232]

[0233] [Table 4B]

[0234]

[0235] As can be seen from Table 4A and Table 4B, the steel sheets of the examples of the present invention have high strength and excellent hole expandability, and also have little deterioration in bendability after working.

[0236] On the other hand, it was found that the steel sheets of the comparative examples were inferior in one or more of the characteristics.

[0237] Industrial applicability

[0238] According to the above aspects of the present invention, a steel plate having high strength and excellent hole expandability with minimal deterioration in bendability after processing and a method for manufacturing the same can be provided. Furthermore, according to a preferred aspect of the present invention, a steel plate having even better hole expandability and a method for manufacturing the same can be provided.

Claims

1. A steel plate, characterized in that: The chemical composition contains in mass %: C：0.030～0.180％、 Si: 0.030~1.400%, Mn: 1.60-3.00%, Al：0.010～0.700％、 P: 0.0800% or less, S: 0.0100% or less, N: 0.0050% or less, Ti: 0.020~0.180%, Nb: 0.010~0.050%, Mo: 0~0.600%, V：0～0.300％、 Total of Ti, Nb, Mo and V: 0.100-1.130%, B: 0 to 0.0030%, and Cr：0～0.500％， The rest is Fe and impurities. The metal structure is calculated in area % as follows: Bainite: more than 80.0%; Total of fresh martensite and tempered martensite: less than 20.0%; Total of pearlite, ferrite and austenite: less than 20.0%, The number density of alloy carbides with a long diameter of 10 to 100 nm existing at the grain boundaries is 1.0×10 8 ~1.0×10 11 pieces / cm 2 , The number density of alloy carbides with a long diameter of less than 10 nm in the grains is 1.0×10 16 ~1.0×10 19 pieces / cm 3 , The tensile strength of the steel plate is greater than 1030 MPa.

2. The steel plate according to claim 1, wherein The ratio of the area ratio of the tempered martensite to the total area ratio of the fresh martensite and the tempered martensite is 80.0% or more.

3. The steel plate according to claim 1 or 2, characterized in that: The chemical composition contains one or more elements selected from the following elements in terms of mass %: Mo: 0.001~0.600%, V：0.010～0.300％、 B: 0.0001~0.0030%, and Cr：0.001～0.500％。 4. A method for manufacturing a steel plate, characterized in that: The method for manufacturing a steel plate according to claim 1 comprises the following steps: a rough rolling step of heating the slab having the chemical composition of claim 1 and performing rough rolling in a temperature range of 1000 to 1300° C. for four or more passes; a finishing rolling step, after the rough rolling, performing finishing rolling at a final reduction ratio of 24 to 60% and a finishing rolling temperature in the temperature range of 960 to 1060° C.; a cooling step of cooling after the finish rolling so that the average cooling rate in the temperature range of 900 to 650° C. becomes 30° C. / second or more; a coiling step of coiling the product in a temperature range of 400 to 580° C. after the cooling; and a reheating step, after the coiling, heating to a temperature range of 600 to 750°C at an average heating rate of 0.2 to 5.0°C / second, maintaining the temperature range of 600 to 750°C for 60 to 3010 seconds, and then cooling so that the average cooling rate in the temperature range of 500 to 700°C becomes 10°C / second or more; The following settings are made in the rough rolling process: The temperature difference between the final pass and the previous pass is set to 50°C or less. The reduction ratio of the first to third passes is set to 10-30%. The reduction ratio after the fourth pass is set to 15 to 50%.

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