High-strength steel sheet and method for producing same

Abstract

The purpose of the present invention is to provide: a high-strength steel sheet having a tensile strength of 980 MPa or more and excellent ductility, fatigue strength, and punchability; and a method for producing the high-strength steel sheet. A high-strength steel sheet which has a prescribed component composition, has 85% or more by area ratio of a bainite phase containing upper bainite and granular bainite as the main phase, has an area ratio of less than 15% of a hard second phase containing primary martensite and / or retained austenite phase, and has a thickness of less than 20 [mu] m in the bainite phase. The area ratio of the granular bainite is 5-40% relative to the area ratio of the upper bainite, the difference (Hv1-Hv2) between the hardness (Hv1) at the position 1 / 2 of the sheet thickness and the hardness (Hv2) at the position 1 / 4 of the sheet thickness is 30% or less relative to 0.3 TS, and the average crystal grain size of the upper bainite phase is 7 [mu] m or less.

Description

Technical Field

[0001] This invention relates to high-strength steel sheets (especially hot-rolled steel sheets) suitable for use as automotive components, particularly those with improved fatigue resistance and punching properties in addition to strength and ductility, and methods for manufacturing the same. Background Technology

[0002] In recent years, from the perspective of global environmental protection, there has been a global demand to reduce CO2 emissions. In particular, there is a strong desire to improve automobile fuel efficiency, making lightweight vehicle bodies a clear direction. Increasing the strength of steel sheets used as raw materials for automotive components by reducing their wall thickness is one effective means of achieving lightweighting without compromising the strength of the vehicle body. Especially for steel sheets with a tensile strength of 980 MPa or higher, they are highly anticipated as a raw material for significantly improving fuel efficiency through thinner wall thickness.

[0003] On the other hand, increasing the tensile strength of a steel sheet reduces its ductility, thus worsening its press formability. For automotive parts, especially chassis components such as suspension components, complex shapes are required to ensure rigidity; therefore, the raw materials for automotive parts demand high press formability, particularly excellent ductility.

[0004] Furthermore, to ensure durability despite the decrease in durability due to the thinning of automotive components, it is necessary to improve the fatigue strength of the steel sheet. Automotive components, especially chassis components such as suspension components, bear repeated loads from the tires; therefore, if fatigue strength is low, the component's durability may fall short of the design expectations as the driving distance increases. However, even increasing the strength of the steel sheet does not necessarily improve fatigue strength. Moreover, increasing the material's strength can easily lead to defects with unevenness at the stamped end face, which can then trigger cracking, resulting in deterioration of fatigue resistance.

[0005] In steel sheets with tensile strength exceeding 980 MPa, excellent ductility, fatigue strength, and punching properties are required simultaneously. To date, various studies have been conducted to improve both the tensile strength and fatigue strength of steel sheets (Patent Documents 1-3).

[0006] Patent document 1 discloses a technology involving high-strength hot-rolled steel sheets with excellent formability and fatigue resistance by controlling the hot-rolling manufacturing conditions, using ferrite as the main phase, and controlling the shape and dispersion morphology of inclusions.

[0007] Patent document 2 discloses a technology involving high-strength hot-rolled steel sheets with excellent resistance to punching fatigue and good workability by controlling the shape and hardness of the martensite in the central part of the sheet thickness.

[0008] Patent document 3 discloses a technology that produces high-strength hot-rolled steel sheets with excellent hole-expanding and punching properties by controlling the hot-rolling manufacturing conditions, using bainite as the main phase and controlling the shape and dispersion morphology of the hard second phase, and further controlling the amount of precipitates.

[0009] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2014-31560 Patent Document 2: Japanese Patent Application Publication No. 2015-214718 Patent Document 3: Japanese Re-publication No. 2017-017933 Summary of the Invention

[0010] The problem that the invention aims to solve However, the prior art described in Patent Documents 1 to 3 has the following problems.

[0011] The techniques described in Patent Documents 1 and 2 cannot achieve a tensile strength of 980 MPa or higher.

[0012] The technology described in Patent Document 3 yields hot-rolled steel sheets with tensile strength of 980 MPa or more, excellent fatigue strength, and excellent punching properties. However, the technology described in Patent Document 3 does not address fatigue strength, which is the subject of this application, and therefore good fatigue strength may not be achievable.

[0013] As such, in the existing technology, there is no established technology for manufacturing high-strength hot-rolled steel sheets with tensile strength of over 980 MPa and excellent ductility, fatigue strength and punching properties.

[0014] Therefore, the purpose of this invention is to provide a high-strength hot-rolled steel sheet with a tensile strength of 980 MPa or higher, and excellent ductility, fatigue strength, and punching properties, as well as a method for manufacturing the same.

[0015] It should be noted that, in this invention, excellent ductility refers to having a uniform elongation of 6.0% or more. Furthermore, excellent fatigue strength refers to a strength of 2 × 10⁻⁶ in a plane bending test with a stress ratio of -1. 6 The ratio of planar bending fatigue strength to tensile strength (fatigue limit ratio) is greater than 0.50. Furthermore, excellent blanking performance refers to the ability to use 10mm... The punch, within a gap range of 10-20%, performs punching processing with three or more gaps including 10% and 20%, and there are no cracks, defects, brittle fractures, or double shearing surfaces on the end faces of each punch.

[0016] Methods for solving problems To achieve the aforementioned objectives, the inventors of this application conducted in-depth research to improve the ductility, fatigue strength, and punching properties of hot-rolled steel sheets while ensuring a tensile strength of 980 MPa or higher. The results showed that by using bainite as the main phase, the area ratio of soft granular bainite and upper bainite generated near the Bs point was controlled within a certain range. Furthermore, the primary martensite and retained austenite phases, which are hard second phases, were kept to less than 15%. Additionally, segregation was reduced by ensuring that the difference between the hardness Hv1 at the 1 / 2 position and the hardness Hv2 at the 1 / 4 position, |Hv1-Hv2|, was less than 30% relative to 0.3TS (MPa). Furthermore, the average grain size of the upper bainite was kept to be 7 μm or less. Thus, a steel sheet exhibiting excellent ductility, fatigue strength, and punching properties while ensuring a tensile strength of 980 MPa or higher can be obtained.

[0017] It should be noted that the upper bainite phase is an aggregate of lath bainitic ferrite. Specifically, it refers to a microstructure containing Fe-based carbides and / or retained austenite phases between the lath bainitic ferrite.

[0018] Unlike lamellar ferrite and polygonal ferrite in pearlite, lath bainitic ferrite has a lath shape and a high dislocation density inside, so the two can be distinguished using SEM (scanning electron microscope) and TEM (transmission electron microscope).

[0019] It should be noted that when there is retained austenite phase between laths, only the lath-shaped bainitic ferrite portion is considered as upper bainite and distinguished from the retained austenite phase. Primary martensite refers to martensite without Fe-based carbides.

[0020] Furthermore, granular bainite refers to a microstructure composed of granular bainitic ferrite, with Fe-based carbides and / or retained austenite phases interspersed within the granular bainitic ferrite. Granular bainite is defined as grains with an aspect ratio of 2 or less and grain boundaries exhibiting an axial-rotation relationship without orientation relationships of <15 16 18>-60°, <6 15 15>-56°, <6 7 7>-50°, <1 0 1>-60°, or <12 2>-17°. This crystal orientation can be determined using electron backscatter diffraction patterns (EBSD). Primary martensite and / or retained austenite phases have low clarity (IQ value). Primary austenite grains have large aspect ratios and are large; therefore, in this study, grains with an aspect ratio of 2 or more and a diameter of 10 μm or more are considered primary austenite grains. Therefore, the upper bainite grains are excluded from the grains with an orientation difference of 15° or more. Among the remaining grains, grains with a high IQ value and an aspect ratio of less than 2 and a diameter of less than 10 μm are identified as granular bainite.

[0021] Furthermore, the SEM images of primary martensite and / or retained austenite phases show brighter contrast compared to the upper bainite phase, lower bainite phase, and / or tempered martensite phase, as well as polygonal ferrite phase. Therefore, primary martensite and / or retained austenite phases can be distinguished from these microstructures using SEM.

[0022] The primary martensite phase and the retained austenite phase have the same contrast in SEM, but can be distinguished from each other by using electron beam reflection diffraction.

[0023] This invention was completed based on the above insights and further research, and its main points are as follows.

[0024] [1] A high-strength steel plate having the following composition, wherein the composition contains, by mass%, the following: C: 0.04~0.18% Si: 0.1–2.5% Mn: 1.0~3.0%, P: below 0.1% S: Below 0.01% Al: 0.010–1.0%, and N: below 0.01%, The balance of the composition is Fe and unavoidable impurities. In the microstructure of the high-strength steel plate, a bainitic phase comprising upper bainite and granular bainite, accounting for more than 85% by area, is used as the main phase. The area fraction of the hard second phase, comprising primary martensite and / or retained austenite, is less than 15%. Within the bainitic phase, the area ratio of granular bainite is between 5% and 40% relative to the area ratio of the upper bainite. The difference between the hardness Hv1 at the 1 / 2 position and the hardness Hv2 at the 1 / 4 position, |Hv1-Hv2|, is less than 30% relative to 0.3TS. The average crystal grain size of the upper bainitic phase is less than 7 μm.

[0025] [2] The high-strength steel plate as described in [1] further comprises, in addition to the aforementioned composition, one or more of groups a to e by mass%. Group a: Selected from one or more of the following: V: 0.005–0.5%, Ti: 0.005–0.2%, and Nb: 0.005–0.1%; Group b: Selected from one or more of the following: Cu: 0.005–0.5%, Ni: 0.005–0.5%, Cr: 0.005–1.0%, and Mo: 0.005–0.5%; Group C: B: 0.0002~0.005%; Group d: Selected from one or two of Sb: 0.001–0.1% and Sn: 0.001–0.1%; Group e: Selected from one or more of the following: Ca: 0.0001–0.005%, Mg: 0.0001–0.005%, and REM: 0.0001–0.005%.

[0026] [3] A method for manufacturing high-strength steel plates, which is the method for manufacturing high-strength steel plates described in [1] or [2], wherein, While casting, the molten steel with the aforementioned composition is rotated at a speed of 10 cm / s or more in the horizontal plane relative to the mold using an inductive electromagnetic stirring device. The obtained steel raw material is heated to above 1150℃. Steel plates are produced by rough rolling heated steel raw materials. The steel plate is precision rolled under conditions where the finishing temperature is above (RC-50)℃ and below (RC+100)℃. The finished steel plate is cooled under the following conditions: a time from the end of finishing rolling to the start of cooling of less than 2.0 s; an average cooling rate of 30 °C / s or more at the surface up to Bs; a residence time at a temperature of (Bs-100) °C or less than Bs °C of 3.0 s or more but less than 10.0 s; and a cooling stop temperature of (Bs-250) °C or less but less than (Bs-100) °C. The cooled steel sheet is then wound at a temperature between (Bs-250)℃ and (Bs-100)℃. Cool to below (Bs-400)℃ at an average cooling rate of less than 1℃ / s. Wherein, RC and Bs are defined by the following equations (1) and (2), respectively. RC (℃)=750+120×C+100×N+10×Mn+250×Ti+5000×B+10×Cr+50×Mo+750×Nb+150×V (1) Bs (℃)=830-270×C-90×Mn-70×Cr-37×Ni-83×Mo-20×Cu (2) In the above formulas (1) and (2), the symbols of each element represent the content (mass%) of each element, and are set to 0 when the element is not present.

[0027] Invention Effects According to the present invention, a high-strength steel plate with a tensile strength of 980 MPa or more, and excellent ductility, fatigue strength and punching properties, and a method thereof are provided.

[0028] When the high-strength steel plate of the present invention is applied to automotive chassis components, structural components, frame components, and chassis components such as suspensions, safety can be ensured and the automotive body can be made lighter, thus having a significant effect on the industry. Attached Figure Description

[0029] [ Figure 1 ] Figure 1 This is a schematic diagram showing the shape of the test piece for the planar bending fatigue test of the present invention. Detailed Implementation

[0030] Hereinafter, embodiments of the present invention will be described. Furthermore, the following description illustrates examples of preferred embodiments of the present invention, and the present invention is not limited to these embodiments.

[0031] The steel plate has the following composition. In the following description, unless otherwise specified, the unit "%" for the content of elements in the composition refers to "mass %".

[0032] <C:0.04~0.18%> C promotes the formation of bainite by increasing hardenability, thereby effectively increasing strength. Additionally, by lowering the Bs point, bainite transforms at a lower temperature, thus suppressing the formation of granular bainite. When the C content is less than 0.04%, such effects cannot be fully achieved, and a tensile strength of 980 MPa or more cannot be obtained. Therefore, the C content is 0.04% or more, preferably 0.05% or more, and more preferably 0.06% or more. On the other hand, if the C content exceeds 0.18%, hard second phases containing primary martensite and / or retained austenite phase are excessively formed, and sufficient fatigue strength cannot be obtained. Therefore, the C content is 0.18% or less, preferably 0.17% or less, and more preferably 0.15% or less.

[0033] <Si: 0.1 - 2.5%> Si solid-solution strengthens the steel, contributing to an increase in the strength of the steel. Therefore, the Si content is 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more. On the other hand, Si is an element that promotes ferrite formation. If the Si content exceeds 2.5%, ferrite is formed and the fatigue strength decreases. Therefore, the Si content is 2.5% or less, preferably 2.3% or less, and more preferably 2.0% or less.

[0034] <Mn: 1.0 - 3.0%> Mn is an element that stabilizes austenite and is effective in increasing strength by suppressing the formation of ferrite. Additionally, by lowering the Bs point, bainite transforms at a lower temperature, thus suppressing the formation of granular bainite. When the Mn content is less than 1.0%, such effects cannot be fully achieved, ferrite and granular bainite are formed, and a tensile strength of 980 MPa or more and excellent fatigue strength cannot be obtained. Therefore, the Mn content is 1.0% or more, preferably 1.2% or more, and more preferably 1.5% or more. On the other hand, if the Mn content exceeds 3.0%, hard second phases containing primary martensite and / or retained austenite phase increase, and sufficient fatigue strength cannot be obtained. Therefore, the Mn content is 3.0% or less, preferably 2.8% or less, and more preferably 2.5% or less.

[0035] <P: 0.1% or less> P deteriorates weldability, so it is preferably reduced as much as possible. In the present invention, the P content can be allowed up to 0.1%. Therefore, the P content is set to 0.1% or less. The lower limit is not particularly specified, but when the P content is less than 0.003%, the refining cost increases. Therefore, the P content is preferably 0.003% or more, and more preferably 0.005% or more.

[0036] <S: 0.01% or less> S deteriorates weldability, so it is preferable to reduce its amount as much as possible. In the present invention, the S content can be allowed up to 0.01%. Therefore, the S content is set to 0.01% or less. There is no particular limitation on the lower limit, but when the S content is less than 0.0001%, it causes a reduction in production efficiency, so it is preferably 0.0001% or more, more preferably 0.0005% or more.

[0037] <Al: 0.010 - 1.0%> Al acts as a deoxidizer and is effective in improving the cleanliness of steel. When the amount of Al is too small, its effect may not be sufficient. Therefore, the Al content is 0.010% or more, preferably 0.015% or more, more preferably 0.020% or more. On the other hand, Al is an element that promotes the formation of ferrite. If the Al content rate exceeds 1.0%, ferrite is generated and the fatigue strength is reduced. Therefore, the Al content is 1.0% or less, preferably 0.8% or less, more preferably 0.5% or less.

[0038] <N: 0.01% or less> N precipitates as nitrides by combining with elements that form nitrides, contributing to the refinement of crystal grains. However, N easily combines with Ti at high temperatures to form coarse nitrides, and excessive content reduces the fatigue strength. Therefore, the N content is 0.01% or less, preferably 0.008% or less, more preferably 0.006% or less. There is no particular limitation on the lower limit, but when the N content is less than 0.001%, it causes a reduction in production efficiency, so it is preferably 0.001% or more.

[0039] The balance is Fe and inevitable impurities.

[0040] The above components are the basic component composition of the high-strength steel plate of the present invention. As needed, the following elements can also be contained.

[0041] V, Ti, and Nb form carbides and are elements effective in improving strength by precipitation strengthening. Therefore, when V, Ti, and Nb are contained, it is preferable that the contents are V: 0.005 - 0.5%, Ti: 0.005 - 0.2%, and Nb: 0.005 - 0.1% respectively. If the contents of V, Ti, and Nb respectively exceed the above upper limits, the carbides may coarsen and the blanking property deteriorates. Therefore, the V content is preferably 0.05% or more, more preferably 0.1% or more, and more preferably 0.3% or less. The Ti content is more preferably 0.01% or more, and more preferably 0.1% or less. The Nb content is more preferably 0.01% or more, and more preferably 0.08% or less.

[0042] Cr, Ni, Cu, and Mo are elements that stabilize austenite and are effective in suppressing ferrite formation and increasing strength. Furthermore, by lowering the Bs point, bainite undergoes a phase transformation at a lower temperature, thereby suppressing the formation of granular bainite. Therefore, when Cr, Ni, Cu, and Mo are present, the preferred contents are Cr: 0.005–1.0%, Ni: 0.005–0.5%, Cu: 0.005–0.5%, and Mo: 0.005–0.5%, respectively. When the contents of Cr, Ni, Cu, and Mo exceed the above-mentioned upper limits, an excessive amount of hard second phase containing primary martensite and / or retained austenite may be formed, making it impossible to obtain the steel microstructure of the present invention. The Cr content is preferably 0.01% or more, more preferably 0.3% or more, and more preferably 0.8% or less. The Ni content is preferably 0.01% or more, more preferably 0.05% or more, and more preferably 0.3% or less. The Cu content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.3% or less. The Mo content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.3% or less.

[0043] When boron (B) is present, the content is preferably set to 0.0002% to 0.005%. B segregates at austenite grain boundaries, inhibiting ferrite formation and thereby further promoting the formation of upper bainite and granular bainite, making it an effective element for further improving the strength of the steel sheet. To exhibit these effects, the B content is preferably 0.0002% or more. Therefore, a B content of 0.0002% or more is preferred, 0.0005% or more, and more preferably 0.0007% or more. On the other hand, if the B content exceeds 0.005%, the above effects saturate. Therefore, a B content of 0.005% or less is preferred, 0.004% or less, and more preferably 0.003% or less.

[0044] Sb is an element that inhibits the removal of steel from its surface during heating, thus effectively preventing a decrease in steel strength. Therefore, when Sb is present, its content is preferably 0.001 to 0.1%. If the Sb content exceeds the above-mentioned upper limit, it may sometimes lead to embrittlement of the steel plate. The Sb content is more preferably 0.005% or more, and more preferably 0.05% or less.

[0045] Sn is an effective element for suppressing the formation of pearlite, thereby inhibiting the reduction of steel strength. To achieve this effect, when Sn is present, the content is preferably 0.001 to 0.1%. When the Sn content exceeds the above-mentioned upper limit, it sometimes leads to embrittlement of the steel plate. The Sn content is more preferably 0.005% or more, and more preferably 0.05% or less.

[0046] Ca, Mg, and REM are elements that are effective in improving punching performance by controlling the morphology of inclusions. To achieve this effect, when containing Ca, Mg, and REM, the preferred contents are Ca: 0.0001–0.005%, Mg: 0.0001–0.005%, and REM: 0.0001–0.005%, respectively. On the other hand, if the contents of Ca and REM exceed the above-mentioned upper limits, the amount of inclusions may increase and fatigue strength may deteriorate. The Ca content is more preferably 0.0005% or more, and more preferably 0.003% or less. The Mg content is more preferably 0.0005% or more, and more preferably 0.003% or less. The REM content is more preferably 0.0005% or more, and more preferably 0.003% or less. It should be noted that REM (rare earth elements) is a collective term for 15 elements, including Sc, Y, and lanthanum (La) from atomic number 57 to lutetium (Lu) from atomic number 71. The REM content mentioned here is the total content of these elements. It should be noted that for each component in groups a to e (V, Ti, Nb, Cr, Ni, Cu, Mo, B, Sb, Sn, Ca, Mg, REM), if the value is less than the lower limit mentioned above, the component is considered to be present as an unavoidable impurity.

[0047] Next, the microstructure of the high-strength steel plate of the present invention will be described.

[0048] The high-strength steel plate of the present invention has the following microstructure. A bainitic phase comprising upper bainite and granular bainite, comprising 85% or more of the area fraction, is used as the main phase. Furthermore, the area fraction of a hard second phase comprising primary martensite and / or retained austenite is less than 15%. Additionally, within the bainitic phase, the area fraction of granular bainite is 5% to 40% relative to the area fraction of upper bainite, and the average grain size of the upper bainite phase is 7 μm or less. The difference between the hardness Hv1 at the 1 / 2 position and the hardness Hv2 at the 1 / 4 position, |Hv1-Hv2|, is 30% or less relative to 0.3TS.

[0049] <Main phase: more than 85% bainite phase> The microstructure of the high-strength steel sheet of the present invention contains a bainite phase comprising upper bainite and granular bainite as the main phase. If the bainite phase is less than 85% in terms of area ratio, excellent punching properties cannot be obtained. Therefore, the bainite phase content is 85% or more, preferably 90% or more, and more preferably 95% or more.

[0050] <Hard secondary phase: Primary martensite and / or retained austenite phase less than 15%> If the hard second phase is controlled to be primary martensite and / or retained austenite, and its area fraction is less than 15%, macroscopic stress concentration will not occur at the phase interface during fatigue testing, and it will not become the initiation point for fatigue cracks, thus achieving excellent fatigue strength. Therefore, the area fraction of the hard second phase containing primary martensite and / or retained austenite is less than 15%, preferably less than 10%, and more preferably less than 5%.

[0051] It should be noted that the remaining microstructure other than upper bainite, granular bainite, primary martensite, and / or retained austenite is acceptable as long as it is below 3%. Examples of remaining microstructure include, for instance, lower bainite, ferrite, tempered martensite, and pearlite.

[0052] Within the bainitic phase, the area ratio of granular bainite relative to the area ratio of the upper bainite is between 5% and 40%. Granular bainite exhibits superior ductility compared to upper bainite. Within the bainitic phase, excellent ductility cannot be achieved when the area ratio of granular bainite relative to the area ratio of upper bainite is less than 5%. Therefore, within the bainitic phase, the area ratio of granular bainite relative to the area ratio of upper bainite is 5% or more, preferably 8% or more, and more preferably 10% or more. On the other hand, if the area ratio of granular bainite relative to the area ratio of upper bainite exceeds 40%, it leads to a significant deterioration in fatigue strength. Therefore, the area ratio of granular bainite relative to the area ratio of upper bainite is 40% or less, preferably 35% or less, and more preferably 30% or less.

[0053] The average grain size of the upper bainite phase is less than 7 μm. Grain boundaries have the effect of preventing the aforementioned sliding deformation from propagating to adjacent grains, thereby improving fatigue strength. Therefore, the average grain size of the upper bainite phase is 7 μm or less, preferably 6 μm or less. While no specific lower limit is specified, excessively small average grain size can sometimes lead to reduced ductility; therefore, an average grain size of 1 μm or more is preferred, and more preferably 2 μm or more.

[0054] The difference between the hardness Hv1 at the 1 / 2 thickness position and the hardness Hv2 at the 1 / 4 thickness position, |Hv1-Hv2|, is less than 30% relative to 0.3TS. If excessive segregation of Mn and other components occurs at the 1 / 2 position of the plate thickness, a hard second phase containing primary martensite and / or retained austenite will be locally generated, resulting in a hardness difference with the parent phase. This makes it prone to cracking and roughness during punching. When the difference between the hardness Hv1 at the 1 / 2 position and the hardness Hv2 at the 1 / 4 position, |Hv1-Hv2|, relative to 0.3TS, exceeds 30%, the punching performance deteriorates significantly. Therefore, the difference between the hardness Hv1 at the 1 / 2 position and the hardness Hv2 at the 1 / 4 position, |Hv1-Hv2|, relative to 0.3TS, is 30% or less, preferably 25% or less, and more preferably 20% or less. The lower limit is not particularly limited and can be 0%. TS refers to tensile strength. The hardness Hv1, Hv2, and TS are measured using the method shown in the examples.

[0055] The high-strength steel plate of this invention has a tensile strength of 980 MPa or higher, and also possesses excellent ductility, excellent fatigue strength, and excellent punching properties. Therefore, the high-strength steel plate of this invention has high tensile strength, ensuring safety even in thin-walled applications, and is suitable for components in trucks and passenger vehicles. It should be noted that in this invention, the area ratio and mechanical properties of the aforementioned structures are values ​​measured using the methods described in the examples.

[0056] Next, a method for manufacturing a high-strength steel plate according to one embodiment of the present invention will be described. It should be noted that, in the following description, unless otherwise specified, the temperature symbol "°C" refers to the surface temperature of the object (steel raw material or steel plate).

[0057] The high-strength steel plate of the present invention can be manufactured by performing the following processes (1) to (6) in sequence. The process will be described below.

[0058] (1) Smelting and casting (2) Heating (3) Hot rolling (4) Cooling (first cooling) (5) Winding (6) Cooling (secondary cooling) (1) Smelting and casting There are no particular limitations on the smelting method for the molten steel. For example, molten steel with the above-mentioned composition can be smelted using known methods such as a converter to obtain molten steel. It should be noted that scrap iron can also be used as raw material. The molten steel obtained is then processed into steel raw material by continuous casting. Induction electromagnetic stirring has the effect of reducing component segregation during solidification and promoting the formation of granular bainite by truncating the columnar crystals growing from the mold and generating equiaxed crystals from the 1 / 4 to 1 / 2 position of the plate thickness. When the gyration speed during electromagnetic stirring is less than 10 cm / s, the columnar crystals cannot be sufficiently truncated, and such an effect cannot be obtained. Therefore, the gyration speed in the horizontal plane is 10 cm / s or more, preferably 15 cm / s or more, and more preferably 20 cm / s or more.

[0059] On the other hand, there is no particular upper limit to the gyratory speed during electromagnetic stirring. From a manufacturing point of view, it is preferably 50 cm / s or less, more preferably 45 cm / s or less, and even more preferably 40 cm / s or less. It should be noted that the steel raw material can be directly supplied to the subsequent heating process after being manufactured by continuous casting. Alternatively, the steel raw material that has been cooled to become warm or cold sheets can also be supplied to the heating process.

[0060] In addition, the composition of the final high-strength steel plate is the same as that of the steel raw material used.

[0061] (2) Heating First, the steel raw material is heated to a temperature of 1150°C or higher. In the steel raw material cooled to a low temperature, most of the precipitate-forming elements exist unevenly as coarse precipitates. When the added elements exist as coarse and uneven precipitates, the effect of the added elements cannot be fully obtained, and the desired steel structure cannot be obtained. Therefore, the steel raw material must be heated before hot rolling to dissolve the coarse precipitates. Therefore, the heating temperature of the steel raw material is 1150°C or higher, preferably 1180°C or higher, and more preferably 1200°C or higher. On the other hand, if the heating temperature of the steel raw material is too high, it leads to the generation of slab defects and a decrease in yield due to oxide scale peeling. Therefore, the heating temperature of the steel raw material is preferably 1350°C or lower, more preferably 1300°C or lower, and even more preferably 1280°C or lower. From the viewpoint of homogenizing the temperature of the steel raw material during heating, it is preferable to raise the steel raw material to the aforementioned heating temperature and then maintain it at that heating temperature. The holding time at the heating temperature is not particularly limited, but from the viewpoint of improving the temperature uniformity of the steel raw material, it is preferably 1800 seconds or more. On the other hand, if the holding time exceeds 10000 seconds, the amount of oxide scale generated increases. As a result, oxide scale intrusion and other defects are more likely to occur in the subsequent hot rolling, leading to a decrease in yield due to poor surface quality. Therefore, the holding time is preferably 10000 seconds or less, and more preferably 8000 seconds or less. It should be noted that this heating process also includes cases where the steel raw material before hot rolling is directly supplied to the hot rolling mill (direct feeding rolling) after casting at a high temperature (i.e., at a temperature within the range of the above-mentioned heating temperature) after casting.

[0062] (3) Hot rolling Next, the heated (or cast and at high temperature) steel raw material is subjected to hot rolling, including rough rolling and finish rolling. Rough rolling is only required to ensure the desired slab size, and there are no particular limitations on its conditions.

[0063] First, the steel raw material is rough rolled to obtain a rough-rolled plate. Alternatively, before finishing rolling the obtained rough-rolled plate, high-pressure water spraying can be performed on the inlet side of the finishing mill for descaling (high-pressure water descaling).

[0064] Next, in this invention, during finishing rolling, when the temperature RC is defined using the following formula (1), the finishing rolling end temperature is set to be (RC-50) °C or higher and (RC+100) °C or lower. RC is the lower limit temperature for austenite recrystallization estimated by the composition. If the finishing rolling end temperature is lower than (RC-50) °C, it is difficult to alleviate the strain caused by recrystallization, and therefore cooling is carried out in a state where strain has accumulated in the austenite, and the formation of granular bainite is excessively promoted. Therefore, the finishing rolling end temperature is (RC-50) °C or higher, preferably (RC-30) °C or higher, and more preferably RC °C or higher.

[0065] On the other hand, if the finishing rolling temperature is higher than (RC+100)℃, the austenite grains become coarser, and the average grain size of the upper bainite increases, thus failing to obtain sufficient fatigue strength. Therefore, the finishing rolling temperature is (RC+100)℃ or lower, preferably (RC+80)℃ or lower, and more preferably (RC+50)℃ or lower. It should be noted that RC is defined by the following formula (1).

[0066] RC (℃)=750+120×C+100×N+10×Mn+250×Ti+5000×B+10×Cr+50×Mo+750×Nb+150×V (1) Here, the symbols of each element in the above formula (1) represent the content (mass%) of each element, and are set to 0 if the element is not present.

[0067] (4) Cooling (first cooling) Next, the obtained hot-rolled steel sheet is cooled (first cooling). The time from the end of hot rolling to the start of cooling (cooling start time) is set to be within 2.0 s after the end of finish rolling. If the cooling start time exceeds 2.0 s, the austenite grains coarsen, and the average grain size of the upper bainite increases, thus failing to obtain sufficient tensile strength and fatigue strength. Therefore, the cooling start time is set to be within 2.0 s, preferably within 1.5 s, and more preferably within 1.0 s.

[0068] Furthermore, the average cooling rate from the finishing rolling end temperature to the surface up to Bs is set to 30°C / s or more. If the average cooling rate from the finishing rolling end temperature to Bs is too slow, excessive ferrite will form, resulting in insufficient tensile strength and fatigue strength. Therefore, the average cooling rate is 30°C / s or more, preferably 40°C / s or more, and more preferably 50°C / s or more. On the other hand, there is no particular upper limit, but if it is too fast, it becomes difficult to manage the cooling stop temperature. Therefore, the average cooling rate is preferably 500°C / s or less, more preferably 300°C / s or less, and even more preferably 150°C / s or less.

[0069] Furthermore, during cooling, the residence time at temperatures above (Bs-100)℃ and below Bs℃ is set to 3.0 s or more and 10.0 s or less. If the residence time at temperatures above (Bs-100)℃ and below Bs℃ is less than 3.0 s, granular bainite cannot be sufficiently obtained. Therefore, the residence time at temperatures above (Bs-100)℃ and below Bs℃ is 3.0 s or more, preferably 4.0 s or more, and more preferably 5.0 s or more. If the residence time at temperatures above (Bs-100)℃ and below Bs℃ exceeds 10.0 s, excessive granular bainite is generated, and sufficient fatigue strength cannot be obtained. Therefore, the residence time at temperatures above (Bs-100)℃ and below Bs℃ is 10.0 s or less, preferably 9.0 s or less, and more preferably 8.0 s or less.

[0070] Furthermore, during cooling, forced cooling can be performed at the aforementioned average cooling rate, and the cooling method is not particularly limited. The cooling stop temperature is (Bs-250) °C or higher and (Bs-100) °C or lower. If the cooling stop temperature is lower than (Bs-250) °C, the microstructure becomes lower bainite. Lower bainite is a high-strength microstructure, but its ductility is poor. Therefore, the cooling stop temperature is (Bs-250) °C or higher, preferably (Bs-220) °C or higher, and more preferably (Bs-200) °C or higher. On the other hand, if the cooling stop temperature is higher than (Bs-100) °C, excessive granular bainite is generated, and therefore excellent fatigue strength cannot be obtained. Therefore, the cooling stop temperature is (Bs-100) °C or lower, preferably (Bs-120) °C or lower, and more preferably (Bs-150) °C or lower. It should be noted that Bs is defined by the following formula (2).

[0071] Bs (℃)=830-270×C-90×Mn-70×Cr-37×Ni-83×Mo-20×Cu (2) In the above formula (2), the symbols of each element represent the content (mass%) of each element, and are set to 0 in the case of elements that are not present.

[0072] (5) Winding Next, the cooled hot-rolled steel sheet is coiled at a coiling temperature of (Bs-250) °C or higher and (Bs-100) °C or lower. When the coiling temperature is below (Bs-250) °C, lower bainite and / or tempered martensite are formed, resulting in insufficient ductility. Therefore, the coiling temperature is (Bs-250) °C or higher, preferably (Bs-230) °C or higher, and more preferably (Bs-200) °C or higher. On the other hand, if the coiling temperature is higher than (Bs-100) °C, excessive granular bainite is formed, thus failing to obtain excellent fatigue strength. Therefore, the coiling temperature is (Bs-100) °C or lower, preferably (Bs-120) °C or lower, and more preferably (Bs-150) °C or lower.

[0073] (6) Cooling (secondary cooling) Next, the material is cooled to below (Bs-400)°C at an average cooling rate of 1°C / s or less (second cooling). If the average cooling rate from the winding temperature to below (Bs-400)°C exceeds 1°C / s, lower bainite and / or tempered martensite are formed, resulting in insufficient ductility. Therefore, the average cooling rate from the winding temperature to below (Bs-400)°C is 1°C / s or less, preferably 0.8°C / s or less, and more preferably 0.5°C / s or less. There is no particular reason to limit the lower limit of the average cooling rate, but from the viewpoint of manufacturing efficiency and the coarsening of carbides, it is preferably 0.001°C / s or more. Cooling can be performed at any temperature below (Bs-400)°C, but cooling to 50°C is preferred. It should be noted that cooling can be performed in any manner, for example, in the state of the wound roll.

[0074] The high-strength steel plate of this invention can be manufactured through the above steps. It should be noted that after coiling and subsequent cooling, conventional methods can be followed. For example, temper rolling can be performed, or pickling can be performed to remove the oxide scale formed on the surface.

[0075] Example The molten steel with the composition shown in Table 1 is melted in a converter and cast using a continuous casting method at the gyration speed shown in Table 2 to produce steel slabs as raw materials. The obtained steel raw materials are heated to the heating temperature shown in Table 2, and then subjected to hot rolling, including roughing and finishing rolling, to produce hot-rolled steel sheets. The finishing temperature during hot rolling is shown in Table 2. Next, the obtained hot-rolled steel sheets are cooled under the conditions of the average cooling rate and cooling stop temperature shown in Table 2 (first cooling). The cooled hot-rolled steel sheets are coiled at the coiling temperature shown in Table 2, and the coiled steel sheets are cooled at the average cooling rate shown in Table 2 (second cooling) to obtain high-strength steel sheets. After cooling, temper rolling is performed, followed by pickling. Pickling is carried out using a 10% by mass hydrochloric acid aqueous solution at a temperature of 85°C.

[0076] [Table 1] [Table 2] Test specimens were collected from the obtained high-strength steel plates, and the microstructure and mechanical properties were evaluated according to the steps described below.

[0077] <Microstructure> Microstructure observation specimens were collected from the high-strength steel plate with the thickness section parallel to the rolling direction as the observation surface. The surface of the specimens was ground and then etched using a 3% nitric acid ethanol solution to reveal the microstructure. Next, 10 fields of view were captured at 5000x magnification at 1 / 4 of the plate thickness to obtain SEM images of the microstructure. The SEM images were analyzed through image processing to quantify the area fractions of upper bainite (UB), polygonal ferrite (F), lower bainite, and / or tempered martensite (LB+TM). Furthermore, upper bainite (UB), granular bainite (GB), primary martensite (FM), and retained austenite (γ) were difficult to distinguish using SEM; therefore, electron beam reflection diffraction was used for identification to determine their respective area fractions and average grain size. The measured area fractions of each microstructure and the average grain size of upper bainite are shown in Table 3. It should be noted that Table 3 also records the total area ratio (B) of the upper bainite and granular bainite, as well as the total area ratio (FM+γ) of the primary martensite and retained austenite phases.

[0078] [Table 3] Tensile Test JIS5 tensile test specimens (JIS Z 2201) were collected from the obtained hot-rolled steel sheet with the tensile direction perpendicular to the rolling direction, and strained at a rate of 10... －3 Tensile strength and uniform elongation are determined by tensile testing according to JIS Z 2241. It should be noted that in this invention, a tensile strength of 980 MPa or higher is considered acceptable. Furthermore, a uniform elongation of 6.0% or higher is evaluated as excellent ductility.

[0079] <Plane Bending Fatigue Test> The hot-rolled steel sheet was sampled with the test piece length direction perpendicular to the rolling direction. Figure 1The test specimens of the shown dimensions and shape were subjected to planar bending fatigue tests according to JIS Z 2275. The stress loading mode was set to a stress ratio R = -1 and a frequency f = 25 Hz. The load stress amplitude was varied in 6 stages, and the stress cycles until fracture were measured to obtain the S-N curve and 2 × 10⁻⁶. 6 Fatigue strength (fatigue limit) under subconditions. In this invention, when the fatigue limit divided by the tensile strength obtained from the tensile test is 0.50 or higher, it is evaluated as excellent fatigue characteristics.

[0080] <Adhesiveness Evaluation> Test pieces were collected from the obtained hot-rolled steel sheet (dimensions: t (plate thickness) × 30 mm (width) × 30 mm (length)). A 10 mm diameter marker was used in the center of the collected test piece. A cylindrical punch is used to punch holes within a clearance range of 10% to 20% with three or more clearances, including 10% and 20%. The clearance is a percentage [%] relative to the thickness of the test piece. If no cracks, defects, brittle fracture surfaces, or secondary shear surfaces are visually observed on the end face of the punched hole, it is rated as having excellent punching performance (○); if these are observed, it is rated as ×.

[0081] <Hardness Measurement> Hardness samples were collected from the high-strength steel plate with the thickness section parallel to the rolling direction as the test section. The hardness (Hv1) at the 1 / 2 thickness position and the hardness (Hv2) at the 1 / 4 thickness position were measured. The Vickers hardness test conditions were a load of 100g and a holding time of 10s. Five measurements were taken at intervals of at least 250μm within a region of ±1 / 20 of the plate thickness, and the results were averaged.

[0082] Next, the percentage of the difference between Hv1 and Hv2 (|Hv1-Hv2|) relative to 0.3 times the tensile strength obtained in the tensile test is calculated and evaluated.

[0083] The inventive examples are all high-strength steel plates with a tensile strength of 980 MPa or higher, and excellent ductility, fatigue strength, and punching properties. On the other hand, comparative examples that deviate from the scope of the present invention either cannot obtain a tensile strength of 980 MPa or higher, or cannot obtain excellent ductility, fatigue strength, and punching properties.

Claims

1. A high-strength steel plate having the following composition, wherein the composition contains, by mass%,: C：0.04～0.18%、 Si: 0.1–2.5% Mn: 1.0~3.0%, P: below 0.1% S: Below 0.01% Al: 0.010–1.0%, and N: below 0.01%, The balance of the composition is Fe and unavoidable impurities. In the microstructure of the high-strength steel plate, a bainitic phase comprising upper bainite and granular bainite, accounting for more than 85% by area, is used as the main phase. The area fraction of the hard second phase, comprising primary martensite and / or retained austenite, is less than 15%. Within the bainitic phase, the area ratio of granular bainite is between 5% and 40% relative to the area ratio of the upper bainite. The difference between the hardness Hv1 at the 1 / 2 position and the hardness Hv2 at the 1 / 4 position, |Hv1-Hv2|, is less than 30% relative to 0.3TS. The average crystal grain size of the upper bainitic phase is less than 7 μm.

2. The high-strength steel plate as described in claim 1, wherein, In addition to the aforementioned components, it further comprises, by mass percent, one or more of groups a to e. Group a: Selected from one or more of the following: V: 0.005–0.5%, Ti: 0.005–0.2%, and Nb: 0.005–0.1%; Group b: Selected from one or more of the following: Cu: 0.005-0.5%, Ni: 0.005-0.5%, Cr: 0.005-1.0%, and Mo: 0.005-0.5%; Group C: B：0.0002～0.005%； Group d: Selected from one or two of Sb: 0.001–0.1% and Sn: 0.001–0.1%; Group e: Selected from one or more of the following: Ca: 0.0001–0.005%, Mg: 0.0001–0.005%, and REM: 0.0001–0.005%.

3. A method for manufacturing a high-strength steel plate, wherein the method for manufacturing a high-strength steel plate as described in claim 1 or 2, wherein, While casting, the molten steel with the aforementioned composition is rotated at a speed of 10 cm / s or more in the horizontal plane relative to the mold using an induction electromagnetic stirring device. The obtained steel raw material is heated to above 1150℃. Steel plates are produced by rough rolling heated steel raw materials. The steel plate is precision rolled under conditions where the finishing temperature is above (RC-50)℃ and below (RC+100)℃. The finished steel plate is cooled under the following conditions: a time from the end of finishing rolling to the start of cooling of less than 2.0 s; an average cooling rate of 30 °C / s or more at the surface up to Bs; a residence time at a temperature of (Bs-100) °C or less than Bs °C of 3.0 s or more but less than 10.0 s; and a cooling stop temperature of (Bs-250) °C or less but less than (Bs-100) °C. The cooled steel sheet is then wound at a temperature between (Bs-250)℃ and (Bs-100)℃. Cool to below (Bs-400)℃ at an average cooling rate of less than 1℃ / s. Wherein, RC and Bs are defined by the following equations (1) and (2), respectively. RC (℃)=750+120×C+100×N+10×Mn+250×Ti+5000×B+10×Cr+50×Mo+750×Nb+150×V (1) Bs (℃)=830-270×C-90×Mn-70×Cr-37×Ni-83×Mo-20×Cu (2) In the above formulas (1) and (2), the symbols of each element represent the content (mass%) of each element, and are set to 0 when the element is not present.

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