Steel Plate and its Manufacturing Method

Abstract

The steel plate of the present invention has a specified chemical composition and metallic structure. In the crystal orientation distribution function of the texture at the 1 / 4 position of the plate thickness, the ratio A of the maximum value of the extreme density of Φ = 20-60° and φ1 = 30-90° in the φ2 = 45° section to the maximum value of the extreme density of Φ = 120-60° and φ1 = 30-90° in the same section is 1.50 or less, and the sum of the maximum value A and the maximum value B is 6.00 or less. The tensile strength of the steel plate is 1030 MPa or more.

Description

Technical Field

[0001] This invention relates to steel plates and methods for manufacturing the same.

[0002] This application claims priority based on Japanese Patent Application No. 2021-030349 filed on February 26, 2021, the contents of which are incorporated herein by reference. Background Technology

[0003] In recent years, efforts have been made to reduce the weight of automotive and mechanical components. This weight reduction is achieved by optimizing the shape of components while ensuring rigidity. Furthermore, for blank-formed components such as press-formed parts, weight reduction is achieved by reducing the thickness of the sheet material. However, to ensure the strength characteristics of components, such as static fracture strength and yield strength, while reducing sheet thickness, the use of high-strength materials becomes necessary. In particular, research has begun on the application of steel sheets with strengths exceeding 780 MPa for automotive running gear components such as lower control arms, connecting rods, and steering knuckles. These automotive running gear components are manufactured by performing processes such as inner edge flanging, stretching flanges, and bending forming on the steel sheets. Therefore, the steel sheets used in these automotive running gear components require excellent formability, especially excellent hole-expanding properties.

[0004] For example, Patent Document 1 discloses a hot-rolled steel plate in which the grain size and aspect ratio of the original austenite are controlled and anisotropy is reduced by setting the finishing temperature and reduction rate within a specified range during the hot rolling process.

[0005] Patent document 2 discloses a cold-rolled steel sheet that improves toughness by setting the rolling rate and average strain rate within an appropriate range within a specified finishing temperature range during the hot rolling process.

[0006] To further reduce the weight of automotive and mechanical components, there is a prospect of applying steel sheets of considerable thickness, based on cold-rolled steel sheets, to automotive running gear. The techniques described in Patent Documents 1 and 2 are effective in manufacturing automotive running gear using high-strength steel sheets. In particular, these techniques provide important insights into the formability and impact resistance of automotive running gear with complex shapes.

[0007] However, automotive running gear is constantly subjected to repeated loads from vibrations caused by its own weight, rotation, and impacts. Therefore, durability is an important characteristic of these components. As mentioned above, automotive running gear is subjected to various forming processes. Regarding the planar portion near the inner side of the R-section, which is formed by bending or bending recovery, there are many areas where the mold contact is weak. In such planar portions near the inner side of the R-section, due to the unevenness of the surface developed through forming, and the mold contact under weak loads, a surface characteristic with relatively sharp concave depressions is formed periodically (hereafter, such changes in surface characteristic are referred to as forming damage). Components containing portions where forming damage has occurred (forming damage areas) are prone to stress and strain concentration, resulting in reduced component strength. Therefore, the steel sheets formed for automotive running gear must be able to suppress the formation of forming damage.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent No. 5068688

[0011] Patent Document 2: Japanese Patent No. 3858146 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] In view of the above-mentioned actual situation, the object of the present invention is to provide a steel plate with high strength and excellent hole expansion properties, and a method for manufacturing the same, which can suppress the generation of forming damage.

[0014] Methods for solving problems

[0015] The inventors conducted groundbreaking research and observed that the occurrence of forming damage is related to the surface texture of the steel sheet. They discovered that forming damage is more likely to occur in the surface texture of the steel sheet when the density is extremely high and the symmetry is low. In particular, for steel sheets with a tensile strength of 1030 MPa or higher that utilize precipitation strengthening, recrystallization is less likely to occur during finishing rolling, resulting in a high density and low symmetry in the texture. The inventors also discovered that by preferably controlling the ratio and sum of the densities within a desired range in the surface texture of the steel sheet, the occurrence of forming damage can be suppressed.

[0016] Furthermore, the inventors have realized that, in order to preferably control the surface texture of the steel sheet, it is effective to apply the desired strain along the width direction of the slab before finishing rolling and to finish roll it under the desired conditions.

[0017] Based on the above insights, the main points of this invention are as follows.

[0018] (1) The chemical composition of the steel plate of one embodiment of the present invention, in mass percent, contains:

[0019] C: 0.030~0.180%

[0020] Si: 0.030~1.400%

[0021] Mn: 1.60~3.00%

[0022] Al: 0.010~0.700%

[0023] P: below 0.0800%

[0024] S: below 0.0100%

[0025] N: below 0.0050%

[0026] Ti: 0.020~0.180%

[0027] Nb: 0.010~0.050%

[0028] Mo: 0~0.600%

[0029] V: 0~0.300%

[0030] Total of Ti, Nb, Mo and V: 0.100–1.130%

[0031] B: 0–0.0030%, and

[0032] Cr: 0~0.500%,

[0033] The remaining portion contains Fe and impurities.

[0034] The metal structure, expressed as an area ratio, is as follows:

[0035] Bainite: 80.0% or more,

[0036] The total of newly formed martensite and tempered martensite is less than 20.0%, and...

[0037] The total content of pearlite, ferrite, and austenite is less than 20.0%.

[0038] In the crystal orientation distribution function of the texture at the 1 / 4 position of the plate thickness,

[0039] The ratio A / B of the maximum value of the extreme density A of Φ = 20~60° and φ1 = 30~90° in the φ2=45° section and the maximum value of the extreme density B of Φ = 120~60° and φ1 = 30~90° in the φ2=45° section, i.e., A / B, is less than 1.50.

[0040] The sum of the above maximum value A and the above maximum value B is less than 6.00.

[0041] The tensile strength of the steel plate is above 1030 MPa.

[0042] (2) According to the steel plate described in (1) above, the proportion of the area ratio of the tempered martensite in the total area ratio of the newly formed martensite and the tempered martensite is 80.0% or more.

[0043] (3) The steel plate according to (1) or (2) above, wherein the above chemical composition, in mass percent, contains one or more of the following elements:

[0044] Mo: 0.001~0.600%

[0045] V: 0.010~0.300%

[0046] B: 0.0001~0.0030%, and

[0047] Cr: 0.001~0.500%.

[0048] (4) Another aspect of the present invention is a method for manufacturing a steel plate, as described in (1) above, comprising the following steps:

[0049] The process of holding a slab with the chemical composition described in (1) above in a temperature range of 1200°C or above for more than 30 minutes;

[0050] For the above-mentioned slab after being held, a process of applying a strain of 3 to 15% along the width direction is performed.

[0051] For the slab subjected to the aforementioned strain, a finishing rolling process is performed in a temperature range where the final reduction rate is 24-60% and the finishing rolling temperature is 960-1060°C; and

[0052] The steel plate after precision rolling is cooled at an average cooling rate of 30°C / second or more in a temperature range of 900-650°C, and then coiled in a temperature range of 400-580°C.

[0053] (5) The steel plate manufacturing method according to (4) above may also include a step of holding the coiled steel plate in a temperature range of 600 to 750°C for 60 to 3010 seconds.

[0054] Invention Effects

[0055] According to the above-described embodiments of the present invention, a steel sheet with high strength and excellent hole-expanding properties, and a method for manufacturing the same, capable of suppressing forming damage, can be provided. Furthermore, according to a preferred embodiment of the present invention, a steel sheet with even better hole-expanding properties, and a method for manufacturing the same, can be provided. Attached Figure Description

[0056] Figure 1 This is a diagram illustrating the cap component manufactured in the embodiment. Detailed Implementation

[0057] The steel plate of this embodiment will now be described in detail. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the spirit of the invention.

[0058] It should be noted that for the numerical ranges specified below, indicated by “~”, both the lower and upper limits are included within the range. Values ​​expressed as “lower than” or “exceed” are not included within the numerical range. All “%” values ​​related to chemical composition refer to “mass %”.

[0059] The steel plate of this embodiment contains, by mass percent: 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%, the total of Ti, Nb, Mo, and V: 0.100–1.130%, and the remainder: Fe and impurities. The following is a detailed description of each element.

[0060] C: 0.030~0.180%

[0061] Carbon (C) is an essential element for obtaining the desired tensile strength of the steel sheet. If the C content is less than 0.030%, the desired tensile strength cannot be obtained. Therefore, the C content is set to 0.030% or more. The C content is preferably 0.060% or more, more preferably 0.080% or more, and even more preferably 0.085% or more, 0.090% or more, 0.095% or more, or 0.100% or more.

[0062] On the other hand, when the carbon content exceeds 0.180%, the combined area ratio of newly formed martensite and tempered martensite becomes excessive, and the porosity of the steel sheet deteriorates. Therefore, the carbon content is set to 0.180% or less. The carbon content is preferably 0.170% or less, and more preferably 0.150% or less.

[0063] Si: 0.030~1.400%

[0064] Si is an element that increases the tensile strength of steel plates through solid solution strengthening. When the Si content is below 0.030%, the desired tensile strength cannot be obtained. Therefore, the Si content is set to 0.030% or more. Preferably, the Si content is 0.040% or more, and more preferably 0.050% or more.

[0065] On the other hand, if the Si content exceeds 1.400%, the area ratio of retained austenite increases, and the porosity 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.

[0066] Mn: 1.60~3.00%

[0067] Mn is an essential element for improving the strength of steel plates. If the Mn content is less than 1.60%, the area fraction of ferrite becomes too high, and the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.60% or more. The Mn content is preferably 1.80% or more, and more preferably 2.00% or more.

[0068] On the other hand, if the Mn content exceeds 3.00%, the toughness of the cast slab deteriorates, making hot rolling 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.

[0069] Al: 0.010~0.700%

[0070] Al acts as a deoxidizer, improving the cleanliness of steel. If the Al content is below 0.010%, sufficient deoxidation cannot be achieved, resulting in a large number of inclusions (oxides) in the steel sheet. These inclusions deteriorate the workability of the steel sheet. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.020% or more, and more preferably 0.030% or more.

[0071] On the other hand, casting becomes difficult when the Al content exceeds 0.700%. 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.

[0072] P: below 0.0800%

[0073] Phosphorus (P) is an element that segregates in the central part of the steel plate's thickness. Furthermore, P is also an element that embrittles the weld. If the P content exceeds 0.0800%, the porosity of the steel plate 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.

[0074] A lower phosphorus (P) content is preferred, but 0% is ideal. Excessively reducing the P content significantly increases the cost of P removal. Therefore, the P content can also be set to 0.0005% or higher.

[0075] S: below 0.0100%

[0076] Sulfur (S) is an element that embrittles slabs by existing as sulfides. Furthermore, S also deteriorates the workability of steel sheets. If the S content exceeds 0.0100%, the porosity 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.

[0077] The lower the sulfur content, the better, but 0% is preferred. If the sulfur content is reduced excessively, the cost of desulfurization will increase significantly. Therefore, the sulfur content can also be set to 0.0005% or higher.

[0078] N: below 0.0050%

[0079] Nitrogen (N) is an element that forms coarse nitrides in steel, thus deteriorating the bending workability and elongation of the steel sheet. If the N content exceeds 0.0050%, the hole-forming ability of the steel sheet 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.

[0080] A lower nitrogen content is preferred, but 0% is ideal. Excessively reducing the nitrogen content significantly increases the cost of nitrogen removal. Therefore, the nitrogen content can also be set to 0.0005% or higher.

[0081] Ti: 0.020~0.180%

[0082] Ti is an element that increases the strength of steel sheets by forming fine nitrides in the steel. If the Ti content is less than 0.020%, the desired tensile strength cannot be obtained. Therefore, the Ti content is set to 0.020% or more. The Ti content is preferably 0.050% or more, and more preferably 0.080% or more.

[0083] On the other hand, if the Ti content exceeds 0.180%, the porosity 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.

[0084] Nb: 0.010~0.050%

[0085] Nitrogen (Nb) is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Furthermore, Nb also increases the strength of steel sheets by forming fine carbides. If the Nb content is below 0.010%, the desired tensile strength cannot be obtained. Therefore, the Nb content is set to 0.010% or more. The Nb content is preferably 0.013% or more, and more preferably 0.015% or more.

[0086] On the other hand, if the Nb content exceeds 0.050%, the toughness of the cast slab deteriorates, making hot rolling 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.

[0087] Total of Ti, Nb, Mo, and V: 0.100–1.130%

[0088] In this embodiment, the total content of Ti and Nb, as well as Mo and V (described later), is controlled. If the total content of these elements is less than 0.100%, the effect of forming fine carbides to improve the strength of the steel plate cannot be sufficiently obtained, and the desired tensile strength cannot be achieved. Therefore, the total content of these elements is set to 0.100% or more. It should be noted that it is not necessary to include all of Ti, Nb, Mo, and V; the above-mentioned effect can be obtained as long as the content of any one of them is 0.100% or more. The total content of these elements is preferably 0.150% or more, more preferably 0.200% or more, and even more preferably 0.230% or more.

[0089] On the other hand, if the total content of these elements exceeds 1.130%, the hole-expanding properties of the steel plate deteriorate. 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, more preferably 0.500% or less.

[0090] The remaining chemical composition of the steel plate in this embodiment may also consist of Fe and impurities. In this embodiment, impurities refer to components that have been introduced from the ore used as raw material, scrap iron, or the manufacturing environment, or components that are permissible within a range that do not adversely affect the steel plate of this embodiment.

[0091] The steel plate of this embodiment may also contain the following optional elements to replace a portion of the Fe. The minimum content of any optional element is 0%. The optional elements will be described below.

[0092] Mo: 0.001~0.600%

[0093] Mo is an element that increases the strength of steel sheets by forming fine carbides in the steel. To reliably achieve this effect, the Mo content is preferably set to 0.001% or more.

[0094] On the other hand, if the Mo content exceeds 0.600%, the porosity of the steel plate deteriorates. Therefore, the Mo content is set to be below 0.600%.

[0095] V: 0.010~0.300%

[0096] V is an element that increases the strength of steel plates by forming fine carbides in the steel. To reliably achieve this effect, the V content is preferably set to 0.010% or more.

[0097] On the other hand, if the V content exceeds 0.300%, the porosity of the steel plate deteriorates. Therefore, the V content is set to be below 0.300%.

[0098] B: 0.0001~0.0030%

[0099] Boron (B) is an element that inhibits the formation of ferrite during the cooling process and increases the strength of the steel plate. To reliably achieve this effect, the B content is preferably set to 0.0001% or more.

[0100] On the other hand, even if the content of B exceeds 0.0030%, the above effect becomes saturated. Therefore, the content of B is set to be below 0.0030%.

[0101] Cr: 0.001~0.500%

[0102] Cr is an element that exhibits effects similar to Mn. To reliably obtain the strength-enhancing effect of Cr in steel plates, the Cr content is preferably set to 0.001% or more.

[0103] On the other hand, even when Cr is present at concentrations exceeding 0.500%, the aforementioned effect becomes saturated. Therefore, the Cr content is set to be below 0.500%.

[0104] The chemical composition of the aforementioned steel plate can be analyzed using a spark discharge emission spectrometer or similar device. It should be noted that C and S are values ​​determined by infrared absorption analysis after combustion in an oxygen stream using a gas composition analyzer. Furthermore, N is a value determined by thermal conductivity analysis after melting a sample taken from the steel plate in a helium stream.

[0105] Next, the metal structure of the steel plate of this embodiment will be described.

[0106] The metal microstructure of the steel plate in this embodiment, in terms of area ratio, is as follows: bainite: 80.0% or more, the total of newly formed martensite and tempered martensite: 20.0% or less, and the total of pearlite, ferrite and austenite: 20.0% or less. In the crystal orientation distribution function of the texture at the 1 / 4 position of the plate thickness, the ratio of the maximum value A of the extreme density of Φ = 20 to 60° and φ1 = 30 to 90° in the φ2 = 45° section to the maximum value B of the extreme density of Φ = 120 to 60° and φ1 = 30 to 90° in the φ2 = 45° section, i.e., A / B, is 1.50 or less, and the total of the above maximum value A and the above maximum value B is 6.00 or less.

[0107] The following explains each regulation. It should be noted that all percentages regarding metal structure mentioned below are area percentages.

[0108] Bainite area ratio: 80.0% or higher

[0109] Bainite is a microstructure that possesses a specified strength and an excellent balance of ductility and porosity. If the area ratio of bainite is less than 80.0%, the desired tensile strength and / or porosity cannot be obtained. Therefore, the area ratio of bainite is set to 80.0% or more. Preferably, the area ratio of bainite is 81.0% or more, more preferably 82.0% or more, and even more preferably 83.0% or more.

[0110] There is no specific upper limit to the area ratio of bainite, but it can be set to below 100.0%, below 95.0%, or below 90.0%.

[0111] The total area ratio of newly formed martensite and tempered martensite is less than 20.0%.

[0112] Newly formed martensite and tempered martensite improve the strength of steel sheets, but due to their low local deformation capacity, the increased area ratio deteriorates the porosity of the steel sheet. If the combined area ratio of newly formed martensite and tempered martensite exceeds 20.0%, the porosity of the steel sheet deteriorates. Therefore, the combined area ratio of newly formed martensite and tempered martensite is set to 20.0% or less. The combined area ratio of newly formed martensite and tempered martensite is preferably 15.0% or less, more preferably 10.0% or less, and even more preferably 5.0% or less.

[0113] There is no particular lower limit to the total area ratio of newly formed martensite and tempered martensite, but it can be set to 0.0% or more, 0.5% or more, or 1.0% or more.

[0114] The proportion of tempered martensite area: ≥80.0% of the total area of ​​newly formed martensite and tempered martensite.

[0115] By increasing the proportion of tempered martensite in the total area ratio of newly formed martensite and tempered martensite, the porosity of the steel sheet can be further improved. Therefore, the proportion of tempered martensite in the total area ratio of newly formed martensite and tempered martensite can be set to 80.0% or more. A higher proportion of tempered martensite in the total area ratio of newly formed martensite and tempered martensite is preferred, more preferably 90.0% or more, and can also be set to 100.0%.

[0116] It should be noted that the area ratio of tempered martensite can be calculated by multiplying {area ratio of tempered martensite / (total area ratio of newly formed martensite and tempered martensite)} by 100.

[0117] The combined area ratio of pearlite, ferrite, and austenite is less than 20.0%.

[0118] Ferrite and austenite are microstructures that degrade the strength of steel sheets. Pearlite is a microstructure that degrades the porosity of steel sheets. If the total area fraction of these microstructures exceeds 20.0%, the desired tensile strength and / or porosity cannot be obtained. Therefore, the total area fraction of these microstructures is set to 20.0% or less. Preferably, the total area fraction of these microstructures is 17.0% or less, more preferably 15.0% or less.

[0119] There is no particular lower limit to the total area ratio of pearlite, ferrite and austenite, but it can be set to 0.0% or more, 5.0% or more, or 10.0% or more.

[0120] The following describes the methods for determining the area ratio of each tissue.

[0121] Test pieces were collected from the steel plate in a manner that allowed observation of the metal structure at a depth of 1 / 4 of the plate thickness (from 1 / 8 to 3 / 8 of the plate thickness) in a section parallel to the rolling direction, and at the center of the plate width.

[0122] The cross-section of the above-mentioned test piece was ground using #600 to #1500 silicon carbide paper, and then refined into a mirror finish using a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water. Next, it was ground at room temperature using colloidal silica without an alkaline solution to remove the strain introduced into the surface layer of the sample. At any position along the length of the sample cross-section, in a region 50 μm long and ranging from 1 / 8 to 3 / 8 of the plate thickness from the surface, with an observable depth of 1 / 4 of the plate thickness, electron backscatter diffraction was performed at measurement intervals of 0.1 μm to obtain crystal orientation information.

[0123] For the measurements, 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 level within the EBSD apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage was set to 15 kV, the irradiation current level to 13, and the electron beam irradiation level to 62. Using the obtained crystal orientation information, the "Phase Map" function of the "OIM Analysis" software (registered trademark) included in the EBSD analysis device was used to calculate the austenite area ratio. Thus, the austenite area ratio was obtained. It should be noted that microstructures with an fcc crystal structure are classified as austenite.

[0124] Next, the microstructure with a bcc crystal structure was identified as bainite, ferrite, pearlite, newly formed martensite, and tempered martensite. For these regions, the "Grain Orientation Spread" function of the "OIM Analysis" software (registered trademark) included in the EBSD analysis device was used. Under the condition that 15° grain boundaries were defined as grain boundaries, regions with a "Grain Orientation Spread" of less than 1° were extracted as ferrite. The area ratio of the extracted ferrite was obtained by calculating the area ratio of the extracted ferrite.

[0125] Next, in the remaining region (the region where the "Grain Orientation Spread" exceeds 1°), under the condition that the 5° grain boundary is defined as the crystal boundary, when the maximum value of the "Grain Average IQ" of the ferrite region is set to Iα, the region that exceeds Iα / 2 is extracted as bainite, and the region that is less than Iα / 2 is extracted as "pearlite, newly formed martensite, and tempered martensite". The area ratio of bainite is obtained by calculating the area ratio of the extracted bainite.

[0126] The extracted pearlite, newly formed martensite, and tempered martensite are distinguished using the following method.

[0127] To observe the same area as the EBSD measurement area using SEM, Vickers indentations were made near the observation location. Afterwards, the microstructure of the observation surface was preserved, surface contaminants were removed by grinding, and nitric acid ethanol etching was performed. Next, the same field of view as the EBSD observation surface was observed using SEM at 3000x magnification. Regions identified as "pearlite, nascent martensite, and tempered martensite" in the EBSD measurement, exhibiting intragranular substructure and cementite precipitating in multiple variants, were classified as tempered martensite. Regions where cementite precipitated in lamellar form were classified as pearlite. Regions with high brightness and where the substructure was not revealed by etching were classified as nascent martensite. The area ratios of tempered martensite, pearlite, and nascent martensite were calculated.

[0128] It should be noted that for removing dirt from the surface of the observation surface, it is sufficient to use methods such as polishing and grinding with alumina particles with a particle size of less than 0.1μm, or Ar ion sputtering.

[0129] Texture at 1 / 4 of the plate thickness: A / B is 1.50 or less, A+B is 6.00 or less.

[0130] In the crystal orientation distribution function of the texture at the 1 / 4 position of the plate thickness, if the ratio of the maximum value A of the extreme density of Φ = 20~60° and φ1 = 30~90° in the φ2 = 45° section to the maximum value B of the extreme density of Φ = 120~60° and φ1 = 30~90° in the same section (A / B) exceeds 1.50, or the sum of the maximum values ​​A and B (A+B) exceeds 6.00, then the desired porosity cannot be obtained, and / or the generation of forming damage cannot be suppressed. Therefore, A / B is set to 1.50 or less, and A+B is set to 6.00 or less.

[0131] The A / B ratio is preferably 1.40 or less, more preferably 1.30 or less, and even more preferably 1.20 or less. The lower limit of A / B is not particularly limited, but it can also be set to 1.00 or more.

[0132] A+B is preferably 5.50 or less, more preferably 5.00 or less, and even more preferably 4.50 or less. The lower limit of A+B is not particularly limited, but it can also be set to 2.00 or more or 3.00 or more.

[0133] The maximum values ​​A and B mentioned above were determined by the following method.

[0134] Samples were collected from the steel plate along a cross-section observable and parallel to the rolling direction. The cross-section perpendicular to the plate surface was mechanically ground, followed by strain removal via chemical or electrolytic grinding. For measurements, a combination of a scanning electron microscope and an EBSD analysis device, along with an OIM Analysis (trademarked) instrument manufactured by TSL, was used. The samples were analyzed using the EBSD (Electron Back Scattering Diffraction) method. The crystal orientation distribution function (ODF) was calculated from the obtained orientation data. It should be noted that the measurement range was set at 1 / 4 of the plate thickness (the region from 1 / 8 to 3 / 8 of the plate thickness depth from the surface).

[0135] The maximum value A is obtained by calculating the maximum values ​​of the polar densities of Φ = 20–60° and φ1 = 30–90° in the cross section of φ2 = 45° using the obtained crystal orientation distribution function. Furthermore, the maximum value B is obtained by calculating the maximum values ​​of the polar densities of Φ = 120–60° and φ1 = 30–90° in the cross section of φ2 = 45°.

[0136] Tensile strength: above 1030MPa

[0137] The steel plate in this embodiment has a tensile strength of 1030 MPa or higher. If the tensile strength is lower than 1030 MPa, it cannot be suitable for use in various automotive running gear components. The tensile strength can also be set to 1050 MPa or higher, or 1150 MPa or higher.

[0138] Higher tensile strength is preferred, but it can also be set to below 1450MPa.

[0139] Tensile strength was determined by tensile testing using test piece No. 5 according to JIS Z 2241:2011. The tensile test piece was collected at the center of the plate width direction, and the direction perpendicular to the rolling direction was defined as the length direction.

[0140] Hole expansion rate: 35% or more

[0141] The steel sheet in this embodiment can also have an expansion ratio of 35% or more. By setting the expansion ratio to 35% or more, forming fracture at the end of the inner edge flange of the cylinder can be suppressed. Therefore, it can be appropriately applied to automotive running gear components. To further increase the forming height of the inner edge flange of the cylinder, the expansion ratio can also be set to 40% or more, 45% or more, or 50% or more.

[0142] The porosity was determined by porosity testing in accordance with JIS Z 2256:2020.

[0143] The steel sheet of this embodiment can also be surface-treated by having a coating on its surface for purposes such as improving corrosion resistance. The coating can be an electroplated layer or a hot-dip galvanized layer. Examples of electroplated layers include electroplated zinc layers and electroplated Zn-Ni alloy layers. Examples of hot-dip galvanized layers 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, and hot-dip Zn-Al-Mg-Si alloy layers. There are no particular limitations on the coating amount; it is preferable to set it as in the conventional method. Furthermore, corrosion resistance can be further improved by performing appropriate chemical conversion treatment (e.g., coating and drying with a silicate-based chromium-free chemical conversion solution) after coating.

[0144] Next, a preferred manufacturing method for the steel plate of this embodiment will be described.

[0145] The preferred manufacturing method of the steel plate in this embodiment includes the following steps:

[0146] The process of holding a slab with the above chemical composition in a temperature range of 1200°C or higher for more than 30 minutes;

[0147] For the above-mentioned slab after being held, a process of applying a strain of 3 to 15% along the width direction is performed.

[0148] For the slab subjected to the aforementioned strain, a finishing rolling process is performed in a temperature range where the final reduction rate is 24-60% and the finishing rolling temperature is 960-1060°C; and

[0149] The steel plate after precision rolling is cooled at an average cooling rate of 30°C / second or more in a temperature range of 900-650°C, and then coiled in a temperature range of 400-580°C.

[0150] In addition to the above-mentioned processes, the following can also be added:

[0151] The process of holding the above-coiled steel sheet in a temperature range of 600-750°C for 60-3010 seconds.

[0152] The following is a description of each process.

[0153] The slab heating temperature is set to 1200°C or higher. Furthermore, the holding time in the temperature range above 1200°C is set to 30 minutes or higher. If the slab heating temperature is below 1200°C, or the holding time in the temperature range above 1200°C is less than 30 minutes, the coarse precipitates cannot be fully melted, resulting in a steel sheet with the desired tensile strength that cannot be obtained. There are no specific upper limits on the heating temperature and the holding time in the temperature range above 1200°C, but they can also be set to below 1300°C and below 300 minutes, respectively.

[0154] It should be noted that, apart from having the aforementioned chemical composition, there are no particular limitations on the slabs to be heated. For example, slabs manufactured by melting molten steel with the above chemical composition in a converter or electric furnace and then producing them through continuous casting can be used. Alternatively, ingot casting or thin slab casting can also be used instead of continuous casting.

[0155] Before finishing rolling, a strain of 3% to 15% is applied to the slab along its width direction (orthogonal rolling direction). If the strain applied along the width direction is less than 3% or more than 15%, it is not possible to preferably control the ratio of the maximum value A to the maximum value B, i.e., A / B. As a result, the desired hole expansion property cannot be obtained, and / or the generation of forming damage cannot be suppressed. Therefore, the strain applied along the width direction is set to 3% to 15%. The strain applied along the width direction is preferably 5% or more, more preferably 7% or more. Furthermore, the strain applied along the width direction is preferably 13% or less, more preferably 11% or less.

[0156] It should be noted that when the width direction length of the slab before strain is applied is set to w0, and the width direction length of the slab after strain is applied is set to w1, the strain applied along the width direction of the slab can be expressed as (1-w1 / w0)×100 (%). As a method of applying strain along the width direction of the slab, for example, a method of applying strain using a roller arranged so that the axis of rotation becomes perpendicular to the surface of the slab can be cited.

[0157] It should be noted that the heated slab can also be rough rolled using conventional methods. In the case of rough rolling, strain can be applied along the width direction under the conditions described above before, during, or after rough rolling.

[0158] After strain is applied along the width direction, finish rolling is performed. Finish rolling is carried out in a temperature range where the final reduction rate is 24-60% and the finish rolling temperature is 960-1060°C.

[0159] If the final reduction rate of finishing rolling is less than 24%, recrystallization will not be promoted, and it will be impossible to preferably control the sum of the maximum values ​​A and B, i.e., A+B. As a result, the desired porosity cannot be obtained, and / or the generation of forming damage cannot be suppressed. The final reduction rate of finishing rolling is preferably 30% or more. From the viewpoint of suppressing the increase of equipment load, the upper limit of the final reduction rate of finishing rolling is set to 60% or less.

[0160] When the thickness of the plate after the final pass of finishing rolling is set as t, and the thickness of the plate before the final pass is set as t0, the final reduction rate of finishing rolling can be expressed as (1-t / t0)×100 (%).

[0161] If the finishing rolling temperature (the surface temperature of the steel sheet exiting the final pass of finishing rolling) is below 960°C, recrystallization will not be promoted, and it will be impossible to preferably control the sum of the maximum values ​​A and B, i.e., A+B. As a result, the desired porosity cannot be obtained, and / or the generation of forming damage cannot be suppressed. The finishing rolling temperature is preferably 980°C or higher. From the viewpoint of suppressing grain size coarsening and suppressing the deterioration of the steel sheet's toughness, the upper limit of the finishing rolling temperature is set to 1060°C or lower.

[0162] After finishing rolling, the material is cooled at an average cooling rate of 30°C / second or higher within the 900–650°C temperature range. If the average cooling rate within this range is lower than 30°C / second, a large amount of ferrite and pearlite will form, making it impossible to obtain the desired tensile strength. The average cooling rate within the 900–650°C temperature range is preferably 50°C / second or higher, and more preferably 80°C / second or higher.

[0163] There is no particular upper limit to the average cooling rate in the temperature range of 900 to 650°C, but it can also be set to below 300°C / second or below 200°C / second.

[0164] It should be noted that the average cooling rate mentioned here is the value obtained by dividing the temperature difference between the start and end points of the set range by the elapsed time from the start to the end point. There are no particular limitations on the cooling process from cooling the temperature range of 900 to 650°C at the above average cooling rate until winding.

[0165] After the aforementioned cooling process, the steel sheet is wound in a temperature range of 400–580°C. This yields the steel sheet of this embodiment. If the winding temperature is below 400°C, excessive formation of newly formed martensite and tempered martensite occurs, deteriorating the porosity of the steel sheet. A winding temperature of 450°C or higher is preferred.

[0166] Furthermore, if the winding temperature exceeds 580°C, the ferrite content increases, making it impossible to obtain the desired tensile strength. The winding temperature is preferably below 560°C.

[0167] The steel plates manufactured using the above methods can be cooled to room temperature, or they can be rolled into coils and then water-cooled.

[0168] The coiled steel sheet can also be uncoiled and pickled, followed by light reduction. It should be noted that pickling and light reduction can also be omitted before proceeding with the heat treatment described later. If the cumulative reduction rate during light reduction is too high, the dislocation density may increase, deteriorating the porosity of the steel sheet. Therefore, when light reduction is performed, the cumulative reduction rate is preferably set to 15% or less.

[0169] When the thickness of the plate after light pressing is set to t, and the thickness of the plate before light pressing is set to t0, the cumulative pressing rate can be expressed as (1-t / t0)×100 (%).

[0170] Heat treatment can also be performed after coiling or light pressing. When heat treatment is performed, it is preferable to hold the temperature in the range of 600–750°C for 60–3010 seconds. By setting the heating temperature and holding time during heat treatment within the above range, it is possible to sufficiently increase the amount of fine precipitates and reduce the dislocation density. As a result, the proportion of tempered martensite in both newly formed martensite and tempered martensite can be increased, further improving the porosity of the steel sheet.

[0171] The steel sheet of this embodiment can be manufactured by the manufacturing method comprising the processes described above. Furthermore, by further comprising the preferred processes described above, the proportion of tempered martensite can be increased, and the porosity of the steel sheet can be further improved.

[0172] Example

[0173] Slabs with the chemical compositions shown in Table 1 were manufactured by continuous casting. Using the resulting slabs, steel plates with a thickness of 3.0 mm were manufactured under the conditions shown in Tables 2 and 3. Light reduction and / or heat treatment were performed as needed, under the conditions shown in Tables 2 and 3. It should be noted that, for the examples where light reduction was performed, pickling was carried out prior to the light reduction.

[0174] The empty columns in Table 1 indicate that the element was not intentionally present. Furthermore, Test No. 29 in Table 3 involved holding the slab at 1189°C for 46 minutes. Additionally, Test No. 10 in Table 3 did not undergo heat treatment.

[0175] For the obtained steel plate, the area fraction, maximum values ​​A and B, tensile strength, and porosity of each microstructure were determined using the methods described above. The results are shown in Tables 4 and 5.

[0176] It should be noted that in Tables 4 and 5, "A / B" represents the ratio of the maximum value A of the extreme density of Φ = 20~60° and φ1 = 30~90° in the φ2 = 45° section and the maximum value B of the extreme density of Φ = 120~60° and φ1 = 30~90° in the φ2 = 45° section, and "A+B" represents the sum of the maximum value A and the maximum value B.

[0177] “B” represents bainite, “α+P+γ” represents ferrite, pearlite and austenite, and “FM+TM” represents newly formed martensite and tempered martensite. “TM ratio” represents the proportion of tempered martensite in the total area ratio of newly formed martensite and tempered martensite.

[0178] Manufactured from the obtained steel plates Figure 1 The cap component shown.

[0179] right Figure 1 A load of 10 mm / s is applied to the center of face S of the cap component. If there is no load reduction leading to fracture at parts A, A', B, and B' up to the maximum load, the steel plate is deemed acceptable because it possesses sufficient component strength and suppresses forming damage; this is recorded as "OK" in the load reduction column of the table. Conversely, if there is a load reduction leading to fracture at parts A, A', B, and B' up to the maximum load, the steel plate is deemed unacceptable because it lacks sufficient component strength and cannot suppress forming damage; this is recorded as "NG" in the load reduction column of the table.

[0180] If the tensile strength is above 1030MPa, it is considered to have high strength and is therefore deemed to be qualified; if the tensile strength is below 1030MPa, it is considered to have low strength and is therefore deemed to be unqualified.

[0181] Furthermore, when the porosity is above 35%, the porosity is considered excellent and deemed acceptable; when the porosity is below 35%, the porosity is considered poor and deemed unacceptable. In particular, porosity above 45% is considered to have even better porosity.

[0182]

[0183]

[0184]

[0185]

[0186]

[0187] By observing Tables 4 and 5, it can be seen that the steel plate of the present invention has high strength and excellent hole-expanding properties, and suppresses the generation of forming damage. In the present invention, it is found that the steel plate with a tempered martensite area ratio of 80.0% or more in the total area ratio of newly formed martensite and tempered martensite has even better hole-expanding properties.

[0188] In contrast, it was found that one or more of the properties of the steel plate of the comparative example were inferior.

[0189] Industrial availability

[0190] According to the above-described embodiments of the present invention, a steel sheet with high strength and excellent hole-expanding properties, and a method for manufacturing the same, capable of suppressing forming damage, can be provided. Furthermore, according to a preferred embodiment of the present invention, a steel sheet with even better hole-expanding properties, and a method for manufacturing the same, can be provided.

Claims

1. A steel plate, characterized in that, Chemical composition, by mass%, contains: C：0.030～0.180%、 Si: 0.030~1.400% Mn: 1.60~3.00% Al：0.010～0.700%、 P: Below 0.0800% S: Below 0.0100% N: below 0.0050% 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: 0~0.0030%, and Cr：0～0.500%， The remaining part consists of Fe and impurities. The metal structure, expressed as an area ratio, is as follows: Bainite: more than 80.0%, The total of newly formed martensite and tempered martensite is less than 20.0%, and... The total content of pearlite, ferrite, and austenite is less than 20.0%. In the crystal orientation distribution function of the texture at the 1 / 4 position of the plate thickness, Φ in the cross section is 20~60°. The maximum value A of the extreme density and the Φ in the cross section is 60~120°. The ratio of the maximum value of the extreme density B, i.e., A / B, is below 1.

50. The sum of the maximum value A and the maximum value B is less than 6.

00. The tensile strength of the steel plate is above 1030 MPa.

2. The steel plate according to claim 1, characterized in that, The proportion of the area ratio of tempered martensite in the total area ratio of newly formed martensite and tempered martensite is 80.0% or more.

3. The steel plate according to claim 1 or 2, characterized in that, The chemical composition, expressed as a percentage by mass, contains one or more elements from the group consisting of: 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, It is a method for manufacturing the steel plate according to claim 1, comprising the following steps: The process of holding a slab having the chemical composition of claim 1 in a temperature range of 1200°C or higher for more than 30 minutes; The process of applying a strain of 3 to 15% along the width direction to the slab after it has been held. For the slab subjected to the strain, a finishing rolling process is performed in a temperature range of 24-60% with a final reduction rate of 960-1060°C; and The steel plate after precision rolling is cooled at an average cooling rate of 30°C / second or more in a temperature range of 900-650°C, and then coiled in a temperature range of 400-580°C.

5. The method for manufacturing a steel plate according to claim 4, characterized in that, It includes a process of holding the coiled steel sheet in a temperature range of 600-750°C for 60-3010 seconds.

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