WO2022042622A1 - 一种具有超高屈强比的吉帕级贝氏体钢及其制造方法 - Google Patents

一种具有超高屈强比的吉帕级贝氏体钢及其制造方法 Download PDF

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WO2022042622A1
WO2022042622A1 PCT/CN2021/114658 CN2021114658W WO2022042622A1 WO 2022042622 A1 WO2022042622 A1 WO 2022042622A1 CN 2021114658 W CN2021114658 W CN 2021114658W WO 2022042622 A1 WO2022042622 A1 WO 2022042622A1
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ultra
steel
gipa
high yield
ratio
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PCT/CN2021/114658
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English (en)
French (fr)
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张瀚龙
张玉龙
刘春粟
金鑫焱
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宝山钢铁股份有限公司
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Priority to AU2021332868A priority Critical patent/AU2021332868A1/en
Priority to MX2023002204A priority patent/MX2023002204A/es
Priority to KR1020237009592A priority patent/KR20230058083A/ko
Priority to EP21860457.7A priority patent/EP4206347A4/en
Priority to JP2023513194A priority patent/JP2023538680A/ja
Priority to US18/043,034 priority patent/US20230357882A1/en
Publication of WO2022042622A1 publication Critical patent/WO2022042622A1/zh

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Definitions

  • the invention relates to a steel grade and a manufacturing method thereof, in particular to a gigapaste grade bainitic steel and a manufacturing method thereof.
  • jeeppa-grade high-strength steel has become one of the most concerned automotive structural materials by major automobile manufacturers.
  • the yield-strength ratio of gigapascal high-strength steels mentioned in the existing invention patents is generally not high.
  • the yield ratio of tensite steel, quenched and partitioned steel (Q&P steel), and complex phase steel is slightly increased, but it is only about 0.75 to 0.85.
  • the publication number is CN103361577A
  • the publication date is October 23, 2013,
  • the Chinese patent document entitled "High-yield-ratio high-strength steel plate with excellent workability” discloses a high-yield-ratio high-strength steel plate, which is The microstructure is mainly ferrite, martensite, tempered martensite and bainite, and the tensile strength can reach more than 980MPa, but the yield ratio is only ⁇ 0.68, which still does not meet the high yield strength of the auto parts market. Up-to-date requirements for gigapa grade steel plates.
  • the publication number is CN106170574A
  • the publication date is November 30, 2016,
  • the Chinese patent document entitled "high-yield-strength cold-rolled steel sheet and its manufacturing method” discloses a high-yield-strength ratio high Strength cold-rolled steel plate and its manufacturing method
  • the structure of the steel plate mainly contains ferrite, retained austenite, martensite and trace amounts of bainite and tempered ferrite, and its tensile strength can reach more than 980MPa, but the yield strength
  • the ratio is only ⁇ 0.75, and the highest is not more than 0.8, which still cannot meet the market demand for gigapa-grade high-strength steel with a yield ratio of ⁇ 0.9.
  • the publication number is CN102719736A
  • the publication date is October 10, 2012
  • the Chinese patent document entitled "A kind of ultra-fine grain slideway steel with a yield ratio ⁇ 0.9 and its production method” discloses a A steel sheet with a yield ratio ⁇ 0.9 is obtained by using the ultra-fine grain structure, but its tensile strength is only 700 MPa.
  • the matrix structure in the steel plate consists of a single bainite or a single martensite, and the matrix structure of multi-phase or multiple phases, such as the matrix structure, contains both ferrite and retained austenite. It is not easy to obtain high yield ratios for steel plates of solid, tempered martensite and martensite. In order for the strength of the steel plate to reach the gigapascal level, the cooperation of multiphase structures is often required, such as typical ferrite/martensitic dual-phase steels and advanced high-strength steels containing retained austenite that introduce the TRIP effect. This is the first layer of technical contradictions.
  • the yield ratio of the steel plate is difficult to be greater than or equal to 0.9 due to the dislocation slip and work hardening caused by the working strain.
  • a single martensite Or the yield ratio of the steel plate of the bainite matrix is about 0.8 to 0.9.
  • the publication number is CN101910436A
  • the publication date is December 8, 2010,
  • the Chinese patent document entitled "a high-strength cold-rolled steel sheet with excellent weather resistance and its preparation method" discloses a Solid solution alloys Cr, Zr, Co, W, etc., to improve the yield strength of materials.
  • One of the objectives of the present invention is to provide a Gipa-level bainitic steel with an ultra-high yield-strength ratio.
  • the present invention can obtain a Gipa-level bainitic steel with an ultra-high yield-strength ratio through reasonable chemical composition design.
  • the gipa-grade bainitic steel has a tensile strength of ⁇ 980MPa, a yield strength of ⁇ 900MPa, a yield-to-strength ratio of ⁇ 0.9, and a hole expansion ratio of ⁇ 55%. It has both ultra-high yield ratio, ultra-high strength and excellent hole expansion. and bending properties, can be used to prepare automobile structural parts, and has good promotion prospects and application value.
  • the present invention proposes a gipa-grade bainitic steel with ultra-high yield ratio, which in addition to Fe and inevitable impurities, also contains the following chemical elements in the following mass percentages:
  • At least one of Cr, Nb, Ti and Mo At least one of Cr, Nb, Ti and Mo, wherein Cr ⁇ 0.4%, Nb ⁇ 0.06%, Ti ⁇ 0.1%, Mo ⁇ 0.4%.
  • the mass percentage content of each chemical element is:
  • At least one of Cr, Nb, Ti and Mo At least one of Cr, Nb, Ti and Mo, wherein Cr ⁇ 0.4%, Nb ⁇ 0.06%, Ti ⁇ 0.1%, Mo ⁇ 0.4%;
  • the balance is Fe and other inevitable impurities.
  • C element is one of the key control elements of microstructure transformation in carbon steel.
  • element C has a great influence on the strength of the steel plate.
  • C element can form alloy carbides with other alloying elements, thereby improving the strength of the steel plate.
  • the mass percentage of C is controlled between 0.12 and 0.24%.
  • the mass percentage of element C can be controlled between 0.15 and 0.20%.
  • Si element is a necessary element for deoxidation in steelmaking, which has a certain solid solution strengthening effect, and also has a certain effect on bainite.
  • the formation of carbon-free bainite has a certain influence (the higher the content of B element in the steel, the easier it is to form carbon-free bainite). It should be noted that when the content of Si element in the steel is less than 0.2%, it is difficult to obtain sufficient deoxidation effect; and when the content of Si element in the steel is higher than 0.5%, it is easy to form an oxide scale or tiger skin stripe-like color difference, It is not conducive to the surface quality of steel plates for automobiles. Based on this, the mass percentage of Si is controlled between 0.2 and 0.5% in the Gipa-level bainitic steel with ultra-high yield ratio according to the present invention.
  • Mn element is the main additive element, and one of the key control elements for the microstructure transformation in the steel. It should be noted that Mn element has low cost, and it is not only an effective element for improving the strength of steel, but also an important solid solution strengthening element. However, it should be noted that the content of Mn element in the steel should not be too high. When the content of Mn element in the steel is too high, the corrosion resistance and welding performance will be deteriorated, and the grain coarsening tendency will also be aggravated, and the plasticity and toughness of the steel will be reduced. Based on this, in the Gipa bainitic steel with ultra-high yield strength ratio according to the present invention, the mass percentage of Mn is controlled between 1.3 and 2.0%.
  • the mass percentage of Mn element can be controlled between 1.6-2.0%.
  • element B In the Gipa-level bainitic steel with ultra-high yield ratio according to the present invention, element B is not only conducive to the formation of bainite in the steel, but also greatly affects the strength and hardness of the steel plate influence. It should be noted that if the content of element B in the steel is lower than 0.001%, the strength of the steel will not meet the target requirements; while when the content of element B in the steel is higher than 0.004%, brittle borides are easily formed, affecting the steel plate. reaming and bending properties. Based on this, the mass percentage of B is controlled between 0.001 and 0.004% in the Gipa bainitic steel with ultra-high yield strength ratio according to the present invention.
  • the Al element is only added to the steel as a deoxidizing element, which can remove the O element in the steel to ensure the performance and quality of the steel . Therefore, in the Gipa bainitic steel with ultra-high yield ratio according to the present invention, the mass percentage of Al is controlled between 0.01 and 0.05%.
  • Ti, Cr, Nb and Mo are optional alloying elements which can be added to the steel , so as to form the precipitation of fine and dispersed carbide second phase, and further improve the strength and yield ratio of the steel plate.
  • Cr and Mo elements can increase the incubation period of pearlite and ferrite in the CCT curve, inhibit the formation of pearlite and ferrite, and make it easier to obtain bainite structure during cooling, which is conducive to improving the The hole expansion ratio of steel.
  • the above four alloying elements have an influence on the structure control of the steel plate and the corresponding annealing process, and their influencing factors on the formation of carbides directly affect the formation ratio and morphology of carbides.
  • the mass percentages of Cr, Nb, Ti and Mo are respectively controlled as: Cr ⁇ 0.4%, Nb ⁇ 0.06%, Ti ⁇ 0.1%, Mo ⁇ 0.4%.
  • the Gipa bainitic steel with ultra-high yield ratio according to the present invention contains at least 0.1-0.4% Cr. In some preferred embodiments, the Gipa bainitic steel with ultra-high yield ratio according to the present invention contains at least 0.1-0.4% Mo. In some preferred embodiments, the Gipa bainitic steel with ultra-high yield ratio described in the present invention contains at least one or both of Cr and Mo. In some preferred embodiments, the Gipa bainitic steel with ultra-high yield ratio according to the present invention contains at least 0.1-0.4% Cr and 0.1-0.4% Mo.
  • the mass percentage of each chemical element satisfies at least one of the following items:
  • both P and S are impurity elements in the steel. If the technical conditions allow, in order to obtain a quenched and tempered steel with better performance and better quality, the content of the impurity elements in the steel should be reduced as much as possible.
  • the gipa-grade bainitic steel with ultra-high yield ratio according to the present invention, it also contains at least one of the following chemical elements:
  • the above-mentioned elements of Cu, Ni, V and Ce can further improve the performance of the Gipa-level bainitic steel with ultra-high yield ratio of the present invention.
  • M can be controlled to be 0.18 ⁇ M ⁇ 0.27, thereby ensuring the dispersion and precipitation of nanometer, submicrometer or micrometer granular carbides in the steel, and ensuring the maximum diameter size of the granular carbide precipitation phase.
  • Cb can also be preferably controlled to be 0.20 ⁇ Cb ⁇ 0.27 , so that the phase ratio of acicular lower bainite in the steel can be effectively guaranteed to be ⁇ 90 %.
  • the microstructure is mainly acicular lower bainite, and the phase ratio of acicular lower bainite is ⁇ 90%.
  • the microstructure also contains nano-, sub-micron or micron granular carbide precipitations dispersed and precipitated, and the granular
  • the total ratio of carbide precipitation phase + acicular lower bainite is greater than or equal to 99%.
  • the diameter of the largest granular carbide precipitation phase is ⁇ 2 ⁇ m.
  • the tensile strength of the gipa-grade bainitic steel with ultra-high yield-strength ratio according to the present invention is ⁇ 980 MPa, preferably ⁇ 1000 MPa, the yield strength is ⁇ 900 MPa, preferably ⁇ 950 MPa, and the yield-strength ratio is ⁇ 0.9, preferably ⁇ 0.95, the hole expansion ratio is ⁇ 55%, preferably ⁇ 60%.
  • the yield strength of the Gipa bainitic steel with ultra-high yield-strength ratio according to the present invention is ⁇ 950 MPa, and the yield-strength ratio is ⁇ 0.95; further preferably, its tensile strength is ⁇ 1000 MPa, and its hole expansion ratio is ⁇ 60 %.
  • the elongation of the Gepa-grade bainitic steel with ultra-high yield strength ratio according to the present invention is ⁇ 9.0%.
  • another object of the present invention is to provide the above-mentioned annealing process for the Gipa-grade bainitic steel with ultra-high yield ratio, which plays a key role in the performance of the steel. With the control of relevant process parameters, a Gipa-grade bainitic steel with ultra-high yield-strength ratio can be obtained.
  • the present invention proposes the above-mentioned annealing process of the Gipa-grade bainite steel with ultra-high yield ratio, which comprises the steps:
  • the above-mentioned annealing process includes a heating section, a soaking section, a slow cooling section, a fast cooling section, a self-returning temperature-controlled cooling section and an air cooling section, which are very important to the present invention.
  • the properties of the gipa-grade bainitic steel play a key role.
  • step (a) in the heating section, it is necessary to ensure that the heating rate is less than or equal to 50°C/s to the soaking temperature Ts: 840-900°C, preferably to a soaking stable temperature of 840-870°C.
  • the heating rate of the heating section should not be too high, otherwise the uniformity of the strip structure will be reduced.
  • the soaking temperature Ts is lower than the above soaking design temperature range, the strip cannot obtain ⁇ 90% acicular lower bainite structure; and if the soaking temperature Ts is higher than the above soaking design temperature range The temperature range will cause coarse grains of the strip, resulting in deterioration of the formability of the steel.
  • the heating rate of step (a) is 5-45°C/s.
  • the holding time is not less than 1 minute.
  • the holding time is from 1 minute to 4.5 minutes.
  • step (c) the slow cooling section needs to be cooled to (Ts-80) ⁇ (Ts-140)°C at a first cooling rate of ⁇ 15°C/s.
  • the first cooling rate of the slow cooling section should not be too high, otherwise it will not only cause energy waste, but also lead to uneven structure of the strip.
  • the first cooling rate in step (c) is 5-15°C/s, preferably 5-12°C/s.
  • the bainite transformation will occur in advance, and a high-temperature bainite structure (such as the above bainite or equiaxed bainite) will be formed.
  • step (e) in the self-return temperature control cooling section, if the strip steel can be executed according to the design parameters in the fast cooling section, the strip steel will realize the temperature self-return phenomenon due to the large release of latent heat of phase transformation.
  • the temperature return can achieve a rapid, uniform and efficient increase of the strip temperature by 50 to 120 °C, thereby promoting the uniform and dispersed precipitation of carbides.
  • the third cooling rate in the self-returning temperature control cooling section is too low or the cooling time is too long, which does not meet the above design requirements of the present invention, it is easy to cause carbide precipitation and coarsening, thereby deteriorating the hole expansion ratio. and bending properties; and if the third cooling rate is too high or the controlled cooling time period is too short, it is easy to cause insufficient carbide precipitation, so that the steel cannot obtain ultra-high yield-strength ratio performance with a yield-strength ratio ⁇ 0.9.
  • another object of the present invention is to provide the above-mentioned manufacturing method of the gipa-grade bainitic steel with ultra-high yield ratio, by which the ultra-high yield ratio of the present invention can be effectively obtained by using the manufacturing method.
  • the present invention proposes a method for manufacturing a Gipa bainite steel with an ultra-high yield ratio, which comprises the steps:
  • the operation steps in steps (1) to (4) of the pre-annealing process are mainly to obtain a steel plate or steel strip with uniform composition and original structure, so as to ensure When the subsequent annealing process is implemented, the uniform and stable structure and properties can be satisfied, and the annealing process in step (5) plays a key role in the performance of the steel sheet.
  • step (2) the heating temperature is controlled to be 1150-1260 °C; .
  • step (3) the cooling rate is controlled to be 30-150°C/s, and the coiling temperature is controlled to be 450-580°C.
  • step (4) the cold rolling reduction ratio is controlled to be greater than or equal to 50%.
  • the Gipa-level bainitic steel with ultra-high yield ratio is the Gipa-level bainitic steel with ultra-high yield ratio described in any of the embodiments herein body steel.
  • the gipa-grade bainitic steel with ultra-high yield ratio and the manufacturing method thereof of the present invention have the following advantages and beneficial effects:
  • the invention ensures that the matrix structure of the steel plate is a simple and single bainite structure through the optimal ratio of alloy elements and innovative adjustment of the annealing process.
  • the introduction of phase transformation latent heat release realizes the self-return of the steel strip, which not only reduces energy consumption, but also realizes fast, uniform and efficient strip temperature control, induces the dispersion and precipitation of fine second phases, so as to obtain ultra-high yield strength Gpa bainitic steel with good formability ratio and good formability.
  • the present invention can obtain Gipa-grade bainitic steel with ultra-high yield strength ratio, the tensile strength of which is ⁇ 980 MPa, the yield strength of ⁇ 900 MPa, the yield strength ratio of ⁇ 0.9, and the hole expansion ratio of ⁇ 55%.
  • This gigapaste grade bainitic steel has both ultra-high yield ratio, ultra-high strength and excellent hole reaming and bending properties. It can be used to prepare automobile structural parts and realize the new design concept of "green-safety" for automobiles. It has good promotion prospects and application value.
  • the annealing process of the present invention plays a key role in the performance of the steel.
  • the annealing process includes a heating section, a soaking section, a slow cooling section, a rapid cooling section, a self-returning temperature control cooling section and an air cooling section. Design and control of related process parameters can obtain Gipa-grade bainitic steel with ultra-high yield-to-strength ratio.
  • the manufacturing method of the present invention has a unique production process, which adopts the above-mentioned annealing process to ensure the performance of the prepared gipa-grade bainitic steel.
  • the obtained gipa-grade bainitic steel not only has ultra-high strength and yield ratio, but also has excellent hole expansion and bending properties.
  • FIG. 1 is a photo of the microstructure of the Gipa-level bainitic steel of Example 1 at a magnification of 3000 times.
  • FIG. 2 is a photo of the microstructure of the comparative steel of Comparative Example 7 at a magnification of 3000 times.
  • FIG. 3 is a photo of the microstructure of the comparative steel of Comparative Example 8 at a magnification of 1000 times.
  • the Gipa-grade bainitic steels with ultra-high yield ratios of Examples 1-14 were prepared by the following steps:
  • Hot rolling the heating temperature is controlled to be 1150-1260°C; the starting temperature of finishing rolling is 1100-1220°C, and the finishing rolling temperature is 900-950°C.
  • Cooling and coiling after rolling the cooling rate is controlled to be 30-150°C/s, and the coiling temperature is controlled to be 450-580°C.
  • step (5) the annealing process includes the following steps:
  • the comparative steels of Comparative Examples 1-10 were also prepared by the processes of smelting and casting, hot rolling, post-rolling cooling and coiling, pickling and cold rolling and annealing.
  • the chemical compositions and related process parameters of Comparative Examples 1-6 all have parameters that fail to meet the design requirements of the present invention.
  • the chemical compositions of Comparative Examples 7-10 meet the design requirements of the present invention, they all have parameters that fail to meet the design requirements of the present invention. Process parameters are required.
  • the chemical element composition of Comparative Example 7 is the same as that of Example 1
  • the chemical element composition of Comparative Example 8 is the same as that of Example 2
  • the chemical element composition of Comparative Example 9 is the same as that of Example 6.
  • the chemical element composition of Comparative Example 10 is the same as that of Example 11.
  • Table 1 lists the mass percentage ratio (%) of each chemical element of the Gipa bainitic steels with ultra-high yield ratios of Examples 1-14 and the comparative steels of Comparative Examples 1-10.
  • Table 2-1 and Table 2-2 list the specific process parameters of the Gipa-level bainitic steels with ultra-high yield ratios of Examples 1-14 and the comparative steels of Comparative Examples 1-10.
  • the obtained gipa-grade bainitic steel with ultra-high yield ratio of Examples 1-14 and the comparative steels of Comparative Examples 1-10 were sampled respectively, and the yield strength of the steel was measured by taking JIS 5# tensile specimens along the transverse direction. and tensile strength, the hole expansion ratio and bending properties of the steel were measured by taking the middle area of the plate.
  • the hole expansion rate of steel is measured by hole expansion test.
  • a punch to press the test piece with a hole in the center into the concave die, the center hole of the test piece is enlarged until the edge of the plate hole necks or penetrates the crack. Since the preparation method of the original hole in the center of the specimen and the quality of the corresponding original hole edge have a great influence on the hole expansion ratio test results, the test and test method are carried out according to the hole expansion ratio test method specified in the ISO/DIS 16630 standard.
  • the central original hole adopts the form of punching hole (corresponding to the processing method with the worst quality of the edge of the original hole).
  • Table 3 lists the test results of the mechanical properties of the Gipa bainitic steels with ultra-high yield ratios of Examples 1-14 and the comparative steels of Comparative Examples 1-10.
  • the gipa-grade bainitic steels with ultra-high yield-strength ratios in Examples 1-14 of the present invention have both ultra-high yield-strength ratios, ultra-high strengths, and excellent hole reaming and bending properties, and their tensile strengths are all ⁇ 980 MPa.
  • the yield strengths are all ⁇ 900MPa
  • the yield-strength ratios are all ⁇ 0.9
  • the hole expansion ratios are all ⁇ 55%.
  • the Gipa bainitic steel with ultra-high yield strength of Example 1 has a yield strength of ⁇ 950 MPa, a yield ratio of ⁇ 0.95, and an ultra-high yield strength ratio and ultra-high yield strength.
  • FIG. 1 is a photo of the microstructure of the Gipa-level bainitic steel of Example 1 at a magnification of 3000 times.
  • the gipa-grade bainite steel of Example 1 is cooled to the lower bainite phase region (the rapid cooling temperature meet the requirements of the invention), its microstructure matrix is acicular lower bainite, and because the cooling rate is appropriate in the self-returning temperature section (the third cooling rate meets the requirements of the invention), so the microstructure also contains finely dispersed and precipitated nanoscale, Submicron or micron granular carbide precipitates.
  • the phase ratio of acicular lower bainite is ⁇ 90%
  • the total phase ratio of granular carbide precipitation + acicular lower bainite is ⁇ 99%
  • the diameter of the largest granular carbide precipitation phase is ⁇ 2 ⁇ m.
  • FIG. 2 is a photo of the microstructure of the comparative steel of Comparative Example 7 at a magnification of 3000 times.
  • the comparative steel of Comparative Example 7 has insufficient cooling rate during cooling in the rapid cooling section (the second cooling rate does not meet the requirements of the invention), and the comparative steel is at a relatively low temperature when it is not cooled to the lower bainite phase region. Bainite transformation occurs at high temperature. Although it is finally cooled to a suitable lower bainite temperature, the microstructure is still dominated by massive equiaxed bainite and almost no acicular lower bainite. , and the carbide precipitation is not fine and uniform enough.
  • FIG. 3 is a photo of the microstructure of the comparative steel of Comparative Example 8 at a magnification of 1000 times.
  • the comparative steel of Comparative Example 8 has a suitable cooling rate in the rapid cooling section (the second cooling rate meets the requirements of the invention), the cooling temperature of the rapid cooling is too high (the cooling temperature in the rapid cooling section does not meet the requirements of the invention) ), so the microstructure is almost entirely massive equiaxed bainite structure, and almost no needle-shaped lower bainite is contained, and the carbide precipitation is not fine and uniform enough.
  • the present invention can obtain the gipa-grade bainitic steel with ultra-high yield strength ratio through reasonable chemical composition design, the tensile strength of which is ⁇ 980MPa, the yield strength is ⁇ 900MPa, and the yield strength ratio is ⁇ 0.9, hole expansion ratio ⁇ 55%, this gipa-grade bainitic steel has both ultra-high yield ratio, ultra-high strength and excellent hole expansion and bending properties. It can be used to prepare automotive structural parts and realize automotive "green”. -Safety" new design concept has good promotion prospects and application value.
  • the annealing process of the present invention plays a key role in the performance of the steel.
  • the annealing process includes a heating section, a soaking section, a slow cooling section, a rapid cooling section, a self-returning temperature control cooling section and an air cooling section. Design and control of related process parameters can obtain Gipa-grade bainitic steel with ultra-high yield-to-strength ratio.
  • the manufacturing method of the present invention has a unique production process, which adopts the above-mentioned annealing process to ensure the performance of the prepared gipa-grade bainitic steel.
  • the obtained gipa-grade bainitic steel not only has ultra-high strength and yield ratio, but also has excellent hole expansion and bending properties.

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Abstract

一种具有超高屈强比的吉帕级贝氏体钢，其除了Fe以外还含有质量百分含量如下的化学元素：C：0.12～0.24％；Si：0.2～0.5％；Mn：1.3～2.0％；B：0.001～0.004％；Al：0.01～0.05％；Cr、Nb、Ti和Mo中的至少一种，其中Cr≤0.4％，Nb≤0.06％，Ti≤0.1％，Mo≤0.4％。还包括用于上述钢的制造方法和退火工艺。

Description

一种具有超高屈强比的吉帕级贝氏体钢及其制造方法 技术领域
本发明涉及一种钢种及其制造方法，尤其涉及一种吉帕级贝氏体钢及其制造方法。
背景技术
在“绿色-安全”的新时代理念下，随着汽车对结构件和安全的件的强度要求变得越来越高，吉帕级高强钢已经成为各大汽车厂商最为关注的汽车结构材料之一。
近年来，越来越多的汽车结构件(如座舱和底盘体系的零部件)要求在服役时保证“零变形”，以保证汽车的正常使用和成员的安全，这就对材料的性能提出了相当高的要求，要求其屈服强度或屈强比越高越好。目前，高屈服强度或高屈强比材料越发受到汽车厂商的关注，高屈服强度或高屈强比吉帕钢的市场需求也变的日益广泛。
现有发明专利中提及的吉帕级高强钢的屈强比普遍不高，占吉帕级汽车高强钢市场份额90％的双相钢屈服强度仅有0.6～0.75，其余小部分产品如马氏体钢、淬火配分钢(Q&P钢)、复相钢等的屈强比略有提升，但也只有0.75～0.85左右。
例如：公开号为CN103361577A，公开日为2013年10月23日，名称为“加工性优异的高屈强比高强度钢板”的中国专利文献，公开了一种高屈强比高强度钢板，其微观组织以铁素体、马氏体、回火马氏体和贝氏体为主，抗拉强度可以达到980MPa以上，但是屈强比仅≥0.68，仍不满足汽车零部件市场对高屈强比吉帕级钢板的最新要求。
又例如：公开号为CN106170574A，公开日为2016年11月30日，名称为“高屈强比高强度冷轧钢板及其制造方法”的中国专利文献，公开了的一种高屈强比高强度冷轧钢板及其制造方法，钢板组织主要含有铁素体、残余奥氏体、马氏体和微量的贝氏体与回火铁素体，其抗拉强度可以达到980MPa以上，但是屈强比仅≥0.75，最高不超过0.8，仍然不能满足市场对屈强比≥0.9的吉帕级高强钢的需求。
另一方面，虽然有部分专利文献中公开了屈强比≥0.9的高屈强比钢板及其制造方法，但是这些专利文献中钢板的抗拉强度均无法达到980MPa级别。
例如：公开号为CN102719736A，公开日为2012年10月10日，名称为“一种屈强比≥0.9的超细晶粒滑道用钢及其生产方法”的中国专利文献，公开了一种利用超细晶粒组织获得屈强比≥0.9的钢板，但是其抗拉强度仅为700MPa级别。
由此可见，在现阶段，钢板的抗拉强度达到吉帕级与屈强比≥0.9以上属于相互矛盾的两项技术指标。这一矛盾背后的技术问题在于实现≥0.9的超高屈强比的组织调控技术具有较大难度。
首先高屈强比需要钢板内的基体组织较为均匀，如基体由单一的贝氏体或单一的马氏体组成，多相或复相的基体组织如基体组织同时含有铁素体、残余奥氏体、回火马氏体和马氏体的钢板不易获得高屈强比。而要想钢板的强度达到吉帕级，往往需要多相组织的相互配合，如典型的铁素体/马氏体双相钢和引入TRIP效应的含残余奥氏体的先进高强钢。这就是第一层技术矛盾。
但即便是单一的贝氏体或单一的马氏体组织，由于加工应变带来的位错滑移与加工硬化，钢板的屈强比也很难≥0.9，通常来说，单一的马氏体或贝氏体基体的钢板屈强比约为0.8～0.9。
因此，要想进一步获得超高屈强比的钢板，需要进一步设计复杂的成分工艺来阻止位错的滑移，以提升材料的屈服强度。例如：公开号为CN101910436A，公开日为2010年12月8日，名称为“一种具有优良耐候性的高强度冷轧钢板及其制备方法”的中国专利文献公开了一种通过引入大量昂贵的固溶合金Cr、Zr、Co、W等，以提升材料的屈服强度的方法。但考虑到现有吉帕级超高强钢的复杂的制备工艺和相对已经过高的合金添加量，上述用于进一步提升屈强比的复杂工艺技术或昂贵的合金添加是否适合引入现有的组织已经极为复杂的吉帕级超高强钢，尚存较大疑虑。这便是第二层技术矛盾。
因此如何获得屈强比≥0.9的吉帕级超高强钢，需要突破上述第一层技术矛盾和第二层技术矛盾等一系列技术难点，这在现有专利技术中尚无法实现。
基于此，为了解决上述问题，期望获得一种具有超高屈强比的吉帕级贝氏体钢，该吉帕级贝氏体钢同时兼备超高屈强比、超高强度与优良的扩孔和弯曲性能，其可以用于制备汽车结构件，实现汽车“绿色-安全”的新设计理念。
发明内容
本发明的目的之一在于提供一种具有超高屈强比的吉帕级贝氏体钢，本发明通过合理的化学成分设计，可以获得具有超高屈强比的吉帕级贝氏体钢，该吉帕级贝氏体钢抗拉强度≥980MPa，屈服强度≥900MPa，屈强比≥0.9，扩孔率≥55％，其同时兼备超高屈强比、超高强度与优良的扩孔和弯曲性能，可以用于制备汽车结构件，具有良好的推广前景和应用价值。
为了实现上述目的，本发明提出了一种具有超高屈强比的吉帕级贝氏体钢，其除了Fe和不可避免的杂质以外还含有质量百分含量如下的下述各化学元素：
C：0.12～0.24％；
Si：0.2～0.5％；
Mn：1.3～2.0％；
B：0.001～0.004％；
Al：0.01～0.05％；
Cr、Nb、Ti和Mo中的至少一种，其中Cr≤0.4％，Nb≤0.06％，Ti≤0.1％，Mo≤0.4％。
进一步地，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，其各化学元素质量百分含量为：
C：0.12～0.24％；
Si：0.2～0.5％；
Mn：1.3～2.0％；
B：0.001～0.004％；
Al：0.01～0.05％；
Cr、Nb、Ti和Mo中的至少一种，其中Cr≤0.4％，Nb≤0.06％，Ti≤0.1％，Mo≤0.4％；
余量为Fe和其他不可避免的杂质。
在本发明所述的技术方案中，各化学元素的设计原理具体如下所述：
C：在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，C元素是碳钢中组织相变的关键控制元素之一。同时C元素对钢板强度的影响也很大，C元素可以和其 他合金元素形成合金碳化物，从而提高钢板的强度。当钢中C元素含量低于0.12％，则会导致钢的强度达不到目标要求；而若钢中C元素含量高于0.24％时，则容易生成马氏体组织和粗大的渗碳体，恶化钢板的性能。基于此，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，将C的质量百分比控制在0.12～0.24％之间。
当然，在一些优选的实施方式中，为了得到更好的实施效果，C元素的质量百分比可以控制在0.15～0.20％之间。
Si：在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，Si元素是炼钢脱氧的必要元素，其具有一定的固溶强化作用，同时也会对贝氏体的形成有一定影响(钢中B元素含量越高越容易形成无碳贝氏体)。需要说明的是，当钢中Si元素含量低于0.2％时，难以获得充分的脱氧效果；而当钢中Si元素含量高于0.5％时，则容易形成氧化铁皮或虎皮纹条状色差，不利于汽车用钢板的表面质量。基于此，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，将Si的质量百分比控制在0.2～0.5％之间。
Mn：在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，Mn元素是主要添加元素，其钢中组织相变关键的控制元素之一。需要说明的是，Mn元素成本低廉，其不仅是提高钢的强度的有效元素，同时也是较为重要的固溶强化元素。但需要注意的是，钢中Mn元素含量不宜过高，当钢中Mn元素含量过高时，会恶化耐腐蚀性能和焊接性能，同时也加剧晶粒粗化趋势，降低钢的塑性和韧性。基于此，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，将Mn的质量百分比控制在1.3～2.0％之间。
当然，在一些优选的实施方式中，为了得到更好的实施效果，Mn元素的质量百分比可以控制在1.6～2.0％之间。
B：在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，B元素不仅有利于钢中贝氏体的形成，同时其还会对钢板的强度和硬度产生较大影响。需要注意的是，若钢中B元素含量低于0.001％，则会导致钢的强度达不到目标要求；而当钢中B元素含量高于0.004％时，则容易生成脆性硼化物，影响钢板的扩孔和弯曲性能。基于此，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，将B的质量百分比控制在0.001～0.004％之间。
Al：在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，Al元素仅作为脱氧 元素加入钢中，其可以出去钢中的O元素，以保证钢的性能和质量。因此，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，将Al的质量百分比控制在0.01～0.05％之间。虽然在一些现有技术中Al元素作为铁素体形成元素和抑制碳化物析出元素而大量(≥0.1％)加入钢中，以期望带来固溶强化，或通过Al的添加来改变相变温度、贝氏体形成动力学和碳化物析出动力学来改变钢材的相变，形成残余奥氏体或无碳贝氏体，并最终提高钢材强度，但本发明现有的成分控制和工艺调节已可以获得具有超高屈强比的吉帕级贝氏体钢，因此无需添加大量Al元素，以避免造成成本升高和炼钢制造难度的大幅增加。
Ti、Cr、Nb和Mo：在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，Ti、Cr、Nb和Mo作为可选合金元素，可以将其添加进钢中，从而形成细小弥散的碳化物第二相析出，进一步提升钢板的强度和屈强比。此外，需要说明的是，Cr和Mo元素可以使CCT曲线中珠光体和铁素体的孕育期增长，抑制珠光体的铁素体的形成，使冷却时易于得到贝氏体组织，有利于提高钢的扩孔率。
由此可见，上述四种合金元素对于钢板的组织调控及其对应的退火工艺均有影响，其对碳化物形成的影响因素直接影响了碳化物形成比例及形态。基于此，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，将Cr、Nb、Ti和Mo的质量百分比分别控制为：Cr≤0.4％，Nb≤0.06％，Ti≤0.1％，Mo≤0.4％。
此外，上述合金元素的加入会增加材料的成本，综合考虑到性能与成本控制，在本发明所述的技术方案中，可以优选地添加Cr、Nb、Ti和Mo中的至少一种加入钢中。在一些优选的实施方案中，本发明所述的具有超高屈强比的吉帕级贝氏体钢至少含有0.1-0.4％的Cr。在一些优选的实施方案中，本发明所述的具有超高屈强比的吉帕级贝氏体钢至少含有0.1-0.4％的Mo。在一些优选的实施方案中，本发明所述的具有超高屈强比的吉帕级贝氏体钢至少含有Cr和Mo中之一或两者。在一些优选的实施方案中，本发明所述的具有超高屈强比的吉帕级贝氏体钢至少含有0.1-0.4％的Cr和0.1-0.4％的Mo。
进一步地，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，其各化学元素的质量百分比满足下列各项的至少其中之一：
C：0.15～0.20％，
Mn：1.6～2.0％。
进一步地，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，在其他不可避免的杂质中：P≤0.015％并且/或者S≤0.004％。
在上述技术方案中，P和S均为钢中的杂质元素，在技术条件允许情况下，为了获得性能更好且质量更优的调质钢，应尽可能降低钢中杂质元素的含量。
进一步地，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，还含有下述化学元素的至少其中一种：
0＜Cu≤0.2％，0＜Ni≤0.2％，0＜V≤0.2％，0＜Ce≤0.2％。
在本发明所述的技术方案中，上述的Cu、Ni、V以及Ce元素均可以进一步提高本发明所述的具有超高屈强比的吉帕级贝氏体钢的性能。
进一步地，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，其满足0.18≤M≤0.27，其中M＝Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7，其中Cr、V、Nb、Ti和Mo表示各化学元素质量百分含量百分号前面的数值。
在上述技术方案中，需要说明的是，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，在控制单一化学元素的质量百分比的同时，为了获得更优性能及质量的吉帕级贝氏体钢，还可以优选的控制M为0.18≤M≤0.27，其中M＝Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7，Cr、V、Nb、Ti和Mo表示各化学元素质量百分含量百分号前面的数值。
需要说明的是，在本发明中，若M过高，则容易形成粗大的碳化物，恶化钢的扩孔率和弯曲性能；而若M过低，则无法形成足够的碳化物析出相，从而会导致钢的强度和屈强比不足。因此在本发明中可以将M控制为0.18≤M≤0.27，从而保证钢中含有纳米、亚微米或微米级的粒状碳化物弥散析出，保证最大的粒状碳化物析出相的直径尺寸。
进一步地，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，其满足0.20≤C b≤0.27，其中等效贝氏体碳元素含量C b＝C-(Mo+Nb)/8-(Ti+V)/4-Cr/12+Ni/10+Mn/20+B×10，式中的各元素均表示该种元素质量百分含量百分号前面的数值。
上述技术方案中，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，由于合金元素和M值影响碳化物的析出，所以会间接地影响钢中等效贝氏体碳元素含量C b。需要说明的是，在本发明中，若C b过低，则无法形成足量的单一贝氏体基体组 织；而若C b过高，则会导致贝氏体的硬度过大，导致钢的弯曲和扩孔性能恶化。因此，在本发明中，在控制单一化学元素的质量百分比的同时，还可以优选将C b控制为0.20≤C b≤0.27，从而可以有效保证钢中针状下贝氏体的相比例≥90％。
进一步地，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，其微观组织主要为针状下贝氏体，针状下贝氏体的相比例≥90％。
进一步地，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，其微观组织还含有弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相，粒状碳化物析出相+针状下贝氏体的相比例总量≥99％。
进一步地，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，最大的粒状碳化物析出相的直径≤2μm。
进一步地，本发明所述的具有超高屈强比的吉帕级贝氏体钢的抗拉强度≥980MPa、优选≥1000MPa，屈服强度≥900MPa、优选≥950MPa，屈强比≥0.9、优选≥0.95，扩孔率≥55％、优选≥60％。在优选的实施方案中，在本发明所述的具有超高屈强比的吉帕级贝氏体钢中，其抗拉强度≥1000MPa，屈服强度≥910MPa，屈强比≥0.9，扩孔率≥55％。
进一步地，本发明所述的具有超高屈强比的吉帕级贝氏体钢的屈服强度≥950MPa，屈强比≥0.95；进一步优选地，其抗拉强度≥1000MPa，扩孔率≥60％。
进一步地，本发明所述的具有超高屈强比的吉帕级贝氏体钢的延伸率≥9.0％。相应地，本发明的另一目的在于提供上述的具有超高屈强比的吉帕级贝氏体钢的退火工艺，该退火工艺对钢的性能起到关键作用，其通过合理的工艺设计和相关工艺参数控制，可以获得具有超高屈强比的吉帕级贝氏体钢。
为了实现上述目的，本发明提出了上述的具有超高屈强比的吉帕级贝氏体钢的退火工艺，其包括步骤：
(a)在加热段以≤50℃/s的加热速率加热至均热温度Ts，其中Ts为840～900℃；
(b)在均热段以温度Ts保温5min以下；
(c)在缓冷段以≤15℃/s的第一冷却速度冷却至(Ts-80)～(Ts-140)℃；
(d)在快冷段以≥(130-Q)℃/s的第二冷却速度冷却至(Ts-490)～(Ts-440)℃；其中Q＝C×180+Si×10+Mn×30+Ni×50+Cr×15+Mo×15+B×2000，式中的各元素均表示该种元素质量百分含量百分号前面的数值；
(e)在自返温控冷段，以第三冷却速度冷却10～40s，其中[(Q-80)/12]≤第三冷却速度≤[(Q-80)/8]；
(f)最后在空冷段，使得带钢空冷至室温。
在本发明所述的技术方案中，需要说明的是，上述的退火工艺包括加热段、均热段、缓冷段、快冷段、自返温控冷段和空冷段，其对本发明所述的吉帕级贝氏体钢的性能起关键作用。
在步骤(a)中，在加热段中，需要保证以≤50℃/s的加热速率加热至均热温度Ts：840～900℃，优选加热至均热稳定840-870℃。其中，加热段的加热速率不宜过高，否则会造成带钢组织的均匀性下降。此外，需要说明的是，若均热温度Ts低于上述均热设计温度范围，则带钢无法获得≥90％的针状下贝氏体组织；而若均热温度Ts高于上述均热设计温度范围，则会造成带钢晶粒粗大，导致钢的成形性能恶化。在一些实施方案中，步骤(a)的加热速率为5-45℃/s。
在步骤(b)中，优选地，保温时间不少于1分钟。例如，保温时间为1分钟到4.5分钟。
在步骤(c)中，缓冷段需以≤15℃/s的第一冷却速度冷却至(Ts-80)～(Ts-140)℃。其中，缓冷段的第一冷却速度不宜过高，否则不仅会造成能源浪费，还会导致带钢组织不均匀。此外，需要说明的是，若缓冷温度低于上述缓冷设计温度范围，则带钢无法获得≥90％的贝氏体组织；而若缓冷温度高于上述缓冷设计温度范围，则在后续快冷段会需要更高的冷却能力和更高的温度精度控制能力，容易出现因冷却能力或温度精度控制能力不足而导致带钢的组织均匀性恶化，进而恶化产品的性能。优选地，步骤(c)中的第一冷却速度为5-15℃/s，优选5-12℃/s。
在步骤(d)中，在快冷段中，需要以≥(130-Q)℃/s的第二冷却速度冷却至(Ts-490)～(Ts-440)℃；其中Q＝C×180+Si×10+Mn×30+Ni×50+Cr×15+Mo×15+B×2000。其中，若快冷段第二冷却速度不足，或冷却温度高于(Ts-440)℃，则会导致贝氏体相变提前发生，生成高温贝氏体组织(如上贝氏体或等轴贝氏体)，不仅无法保证钢中针状下贝氏体的相比例≥90％，还会导致相变潜热大幅降低，使得无法实现后续的自返温控冷，导致材料组织异常，使钢板和钢带无法获得超高屈强比。而若快冷段冷却温度低于(Ts-490)℃时，则会生成马氏体组织，导致钢的扩孔率和弯曲性能下降。
在步骤(e)中，在自返温控冷段中，若带钢在快冷段能够按照设计参数执行，则带钢就会因相变潜热的大量释放而实现温度自返温现象，自返温可以实现带钢温度快速、均匀、高效的升高50～120℃，从而促进碳化物均匀、弥散析出。为保证碳化物析出充分、析出尺寸细小，需要控制带钢温度以第三冷却速度冷却10～40s，其中[(Q-80)/12]≤第三冷却速度≤[(Q-80)/8]。
需要说明的是，若自返温控冷段中的第三冷却速度过低或控冷时间过长，不符合本发明上述的设计要求，则容易造成碳化物析出粗化，从而恶化扩孔率和弯曲性能；而若第三冷却速度过高或控冷时间段过短，则容易造成碳化物析出不充分，从而无法使钢获得屈强比≥0.9的超高屈强比性能。
此外，本发明的又一目的在于提供上述的具有超高屈强比的吉帕级贝氏体钢的制造方法，采用该制造方法可以有效制得本发明所述的具有超高屈强比的吉帕级贝氏体钢。
为了实现上述目的，本发明提出了一种具有超高屈强比的吉帕级贝氏体钢的制造方法，其包括步骤：
(1)冶炼和铸造；
(2)热轧；
(3)轧后冷却和卷取；
(4)酸洗和冷轧；
(5)如上述的退火工艺。
在本发明所述的技术方案中，在上述制造方法中，退火前工艺的步骤(1)-步骤(4)中的操作步骤主要是为了获得成分和原始组织均匀的钢板或钢带，以保证后续的退火工艺实施时能够满足组织和性能的均匀稳定，而对钢板的性能起到关键作用的是步骤(5)中的退火工艺。
进一步地，在本发明所述的制造方法中，在步骤(2)中，控制加热温度为1150～1260℃；精轧开轧温度为1100～1220℃，精轧终轧温度为900～950℃。
进一步地，在本发明所述的制造方法中，在步骤(3)中，控制冷却速度为30～150℃/s，控制卷取温度为450～580℃。
进一步地，在本发明所述的制造方法中，在步骤(4)中，控制冷轧压下率≥50％。
进一步地，在本发明所述的制造方法中，所述具有超高屈强比的吉帕级贝氏体 钢为本文任一实施方案所述的具有超高屈强比的吉帕级贝氏体钢。
本发明所述的具有超高屈强比的吉帕级贝氏体钢及其制造方法相较于现有技术具有如下所述的优点以及有益效果：
本发明在保证化学元素成分和工艺相对简洁可控的前提下，通过合金元素的最优配比与退火工艺的创新调节，在保证钢板的基体组织是简单且单一的贝氏体组织基础之上，引入相变潜热释放实现钢带自返温，不仅减少了能源消耗，也实现了快速、均匀、高效的带钢回温控制，诱发细小的第二相弥散析出，从而获得具有超高屈强比和良好成形性能的吉帕级贝氏体钢。
本发明通过合理的化学成分设计，可以获得具有超高屈强比的吉帕级贝氏体钢，其抗拉强度≥980MPa，屈服强度≥900MPa，屈强比≥0.9，扩孔率≥55％，该吉帕级贝氏体钢同时兼备超高屈强比、超高强度与优良的扩孔和弯曲性能，其可以用于制备汽车结构件，实现汽车“绿色-安全”的新设计理念，具有良好的推广前景和应用价值。
本发明所述的退火工艺对钢的性能起到关键作用，该退火工艺包括加热段、均热段、缓冷段、快冷段、自返温控冷段和空冷段，其通过合理的工艺设计和相关工艺参数控制，可以获得具有超高屈强比的吉帕级贝氏体钢。
相应的，本发明所述的制造方法生产工艺独特，其采用了上述的退火工艺，以保证制得的吉帕级贝氏体钢的性能。所获得的吉帕级贝氏体钢不仅具有超高的强度和屈强比，还具有优良的扩孔和弯曲性能。
附图说明
图1为实施例1的吉帕级贝氏体钢放大3000倍的微观组织照片。
图2为对比例7的对比钢放大3000倍的微观组织照片。
图3为对比例8的对比钢放大1000倍的微观组织照片。
具体实施方式
下面将结合说明书附图以及具体的实施例对本发明所述的具有超高屈强比的吉帕级贝氏体钢及其制造方法做进一步的解释和说明，然而该解释和说明并不对本发明的技术方案构成不当限定。
实施例1-14以及对比例1-10
实施例1-14的具有超高屈强比的吉帕级贝氏体钢采用以下步骤制得：
(1)按照表1所示的化学成分进行冶炼和铸造。
(2)热轧：控制加热温度为1150～1260℃；精轧开轧温度为1100～1220℃，精轧终轧温度为900～950℃。
(3)轧后冷却和卷取：控制冷却速度为30～150℃/s，控制卷取温度为450～580℃。
(4)酸洗和冷轧：控制冷轧压下率≥50％。
(5)退火。
需要说明的是，在步骤(5)中，退火工艺包括以下步骤：
(a)在加热段以≤50℃/s的加热速率加热至均热温度Ts，其中Ts为840～900℃。
(b)在均热段以温度Ts保温5min以下。
(c)在缓冷段以≤15℃/s的第一冷却速度冷却至(Ts-80)～(Ts-140)℃。
(d)在快冷段以≥(130-Q)℃/s的第二冷却速度冷却至(Ts-490)～(Ts-440)℃；其中Q＝C×180+Si×10+Mn×30+Ni×50+Cr×15+Mo×15+B×2000。
(e)在自返温控冷段，以第三冷却速度冷却10～40s，其中[(Q-80)/12]≤第三冷却速度≤[(Q-80)/8]。
(f)最后在空冷段，使得带钢空冷至室温。
此外，需要注意的是，本发明所述的实施例1-14的具有超高屈强比的吉帕级贝氏体钢均采用以上步骤制得，且其化学成分及相关工艺参数均满足本发明设计规范控制要求。
而对比例1-10的对比钢同样采用：冶炼和铸造、热轧、轧后冷却和卷取、酸洗和冷轧和退火的工艺制得。但其中对比例1-6的化学成分及相关工艺参数均存在未能满足本发明设计要求的参数，对比例7-10的化学成分虽然满足本发明设计要求，但均存在未能满足本发明设计要求工艺参数。
其中，在本发明的实施例和对比例中，对比例7与实施例1的化学元素成分相同，对比例8与实施例2的化学元素成分相同，对比例9与实施例6的化学元素成分相同，对比例10与实施例11的化学元素成分相同。
表1列出了实施例1-14的具有超高屈强比的吉帕级贝氏体钢以及对比例1-10的对比钢的各化学元素的质量百分配比(％)。
表1(余量为Fe和除了P和S以外的其他不可避免的杂质)
Figure PCTCN2021114658-appb-000001
Figure PCTCN2021114658-appb-000002
注：上表中，Cb＝C-(Mo+Nb)/8-(Ti+V)/4-Cr/12+Ni/10+Mn/20+B×10，式中的各元素均表示该种元素质量百分含量百分号前面的数值；M＝Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7，其中Cr、V、Nb、Ti和Mo表示各化学元素质量百分含量百分号前面的数值。
表2-1以及表2-2列出了实施例1-14的具有超高屈强比的吉帕级贝氏体钢以及对比例1-10的对比钢的具体工艺参数。
表2-1
Figure PCTCN2021114658-appb-000003
Figure PCTCN2021114658-appb-000004
表2-2
Figure PCTCN2021114658-appb-000005
Figure PCTCN2021114658-appb-000006
注：上表中，Q＝C×180+Si×10+Mn×30+Ni×50+Cr×15+Mo×15+B×2000，式中的各元素均表示该种元素质量百分含量百分号前面的数值。
对实施例1-14的具有超高屈强比的吉帕级贝氏体钢以及对比例1-10的对比钢进行相关的力学性能测试，将所得的各实施例和对比例的力学性能测试结果列于表3中，相关性能测试手段如下所述。
将得到的实施例1-14的具有超高屈强比的吉帕级贝氏体钢以及对比例1-10的对比钢分别取样，取沿横向JIS 5#拉伸试样测定钢的屈服强度及抗拉强度，取板中部区域测定钢的扩孔率和弯曲性能。
其中，钢的扩孔率采用扩孔试验测定，通过使用凸模把中心带孔的试件压入凹模，使试件中心孔扩大，直到板孔边缘出现颈缩或贯穿裂纹为止。由于试件中心原始孔的制备方式及对应的原始孔边部质量对扩孔率测试结果存在较大影响，因此，试验及测试方法按ISO/DIS 16630标准中规定的扩孔率测试方法执行，中心原始孔采用冲压孔形式(对应为原始孔边部质量最差的加工方式)。180°弯曲实验采用GB/T232-2010标准中弯曲性能的测定方法执行(弯曲直径d＝1a)。
表3列出了实施例1-14的具有超高屈强比的吉帕级贝氏体钢以及对比例1-10的 对比钢的力学性能测试结果。
表3
Figure PCTCN2021114658-appb-000007
Figure PCTCN2021114658-appb-000008
由表3可以看出，相较于对比例1-10的对比钢，本发明实施例1-14的具有超高屈强比的吉帕级贝氏体钢的力学性能明显更优。
本发明实施例1-14的具有超高屈强比的吉帕级贝氏体钢同时兼备超高屈强比、超高强度与优良的扩孔和弯曲性能，其抗拉强度均≥980MPa，屈服强度均≥900MPa，屈强比均≥0.9，扩孔率均≥55％。
在个别优选的实施方式中，如实施例1中，实施例1的具有超高屈强比的吉帕级贝氏体钢的屈服强度≥950MPa，屈强比≥0.95，具有超高的屈强比和超高的屈服强度。
图1为实施例1的吉帕级贝氏体钢放大3000倍的微观组织照片。
如图1所示，实施例1的吉帕级贝氏体钢因为在快冷段以足够快的冷速(第二冷速符合发明要求)冷却到下贝氏体相区(快冷冷却温度符合发明要求)，其微观组织基体为针状下贝氏体，且因为在自返温段控冷冷速合适(第三冷速符合发明要求)，因此组织还含有细小弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相。其中，针状下贝氏体的相比例≥90％，粒状碳化物析出相+针状下贝氏体的相比例总量≥99％，最大的粒状碳化物析出相的直径≤2μm。
图2为对比例7的对比钢放大3000倍的微观组织照片。
如图2所示，对比例7的对比钢因为在快冷段冷却时的冷速不足(第二冷速不满足发明要求)，对比钢在未冷却到下贝氏体相区时即在较高温度发生贝氏体相变，虽然最终也冷却到了合适的下贝氏体相区温度，但微观组织中仍然以块状的等轴贝氏体为主，几乎不含有针状下贝氏体，且碳化物析出也不够细小均匀。
图3为对比例8的对比钢放大1000倍的微观组织照片。
如图3所示，对比例8的对比钢虽然在快冷段冷却时的冷速合适(第二冷速符合发明要求)，但快冷冷却温度却过高(快冷段冷却温度不符发明要求)，因此微观组织为几乎全部为块状的等轴贝氏体组织，而几乎不含有针状下贝氏体，且碳化物析出也不够细小均匀。
综上所述可以看出，本发明通过合理的化学成分设计，可以获得具有超高屈强比的吉帕级贝氏体钢，其抗拉强度≥980MPa，屈服强度≥900MPa，屈强比≥0.9， 扩孔率≥55％，该吉帕级贝氏体钢同时兼备超高屈强比、超高强度与优良的扩孔和弯曲性能，其可以用于制备汽车结构件，实现汽车“绿色-安全”的新设计理念，具有良好的推广前景和应用价值。
本发明所述的退火工艺对钢的性能起到关键作用，该退火工艺包括加热段、均热段、缓冷段、快冷段、自返温控冷段和空冷段，其通过合理的工艺设计和相关工艺参数控制，可以获得具有超高屈强比的吉帕级贝氏体钢。
相应的，本发明所述的制造方法生产工艺独特，其采用了上述的退火工艺，以保证制得的吉帕级贝氏体钢的性能。所获得的吉帕级贝氏体钢不仅具有超高的强度和屈强比，还具有优良的扩孔和弯曲性能。
此外，本案中各技术特征的组合方式并不限本案权利要求中所记载的组合方式或是具体实施例所记载的组合方式，本案记载的所有技术特征可以以任何方式进行自由组合或结合，除非相互之间产生矛盾。
还需要注意的是，以上所列举的实施例仅为本发明的具体实施例。显然本发明不局限于以上实施例，随之做出的类似变化或变形是本领域技术人员能从本发明公开的内容直接得出或者很容易便联想到的，均应属于本发明的保护范围。

Claims (15)

  1. 一种具有超高屈强比的吉帕级贝氏体钢，其特征在于，其除了Fe以及不可避免的杂质以外还含有质量百分含量如下的下述各化学元素：
    C：0.12～0.24％；
    Si：0.2～0.5％；
    Mn：1.3～2.0％；
    B：0.001～0.004％；
    Al：0.01～0.05％；
    Cr、Nb、Ti和Mo中的至少一种，其中Cr≤0.4％，Nb≤0.06％，Ti≤0.1％，Mo≤0.4％。
  2. 如权利要求1所述的具有超高屈强比的吉帕级贝氏体钢，其特征在于，其各化学元素质量百分含量为：
    C：0.12～0.24％；
    Si：0.2～0.5％；
    Mn：1.3～2.0％；
    B：0.001～0.004％；
    Al：0.01～0.05％；
    Cr、Nb、Ti和Mo中的至少一种，其中Cr≤0.4％，Nb≤0.06％，Ti≤0.1％，Mo≤0.4％；
    余量为Fe和其他不可避免的杂质。
  3. 如权利要求1或2所述的具有超高屈强比的吉帕级贝氏体钢，其特征在于，其各化学元素的质量百分比含量满足下列各项的至少其中之一：
    C：0.15～0.20％，
    Mn：1.6～2.0％。
  4. 如权利要求1或2所述的具有超高屈强比的吉帕级贝氏体钢，其特征在于，在其他不可避免的杂质中：P≤0.015％并且/或者S≤0.004％。
  5. 如权利要求1或2所述的具有超高屈强比的吉帕级贝氏体钢，其特征在于，还含有下述化学元素的至少其中一种：
    0＜Cu≤0.2％，0＜Ni≤0.2％，0＜V≤0.2％，0＜Ce≤0.2％。
  6. 如权利要求5所述的具有超高屈强比的吉帕级贝氏体钢，其特征在于，其满足0.18≤M≤0.27，其中M＝Cr/2.5+Ti+V/5+Nb/1.7+Mo/1.7，其中Cr、V、Nb、Ti和Mo表示各化学元素质量百分含量百分号前面的数值；和/或，0.20≤C b≤0.27，其中等效贝氏体碳元素含量C b＝C-(Mo+Nb)/8-(Ti+V)/4-Cr/12+Ni/10+Mn/20+B×10，式中的各元素均表示该种元素质量百分含量百分号前面的数值。
  7. 如权利要求1或2所述的具有超高屈强比的吉帕级贝氏体钢，其特征在于，其微观组织主要为针状下贝氏体，针状下贝氏体的相比例≥90％。
  8. 如权利要求7所述的具有超高屈强比的吉帕级贝氏体钢，其特征在于，其微观组织还含有弥散析出的纳米级、亚微米级或微米级的粒状碳化物析出相，粒状碳化物析出相+针状下贝氏体的相比例总量≥99％；优选地，最大的粒状碳化物析出相的直径≤2μm。
  9. 如权利要求1所述的具有超高屈强比的吉帕级贝氏体钢，其特征在于，其抗拉强度≥980MPa，屈服强度≥900MPa，屈强比≥0.9，扩孔率≥55％；优选地，其屈服强度≥950MPa，屈强比≥0.95。
  10. 一种用于如权利要求1-9中任意一项所述的具有超高屈强比的吉帕级贝氏体钢的退火工艺，其特征在于，包括步骤：
    (a)在加热段以≤50℃/s的加热速率加热至均热温度Ts，其中Ts为840～900℃；
    (b)在均热段以温度Ts保温5min以下；
    (c)在缓冷段以≤15℃/s的第一冷却速度冷却至(Ts-80)～(Ts-140)℃；
    (d)在快冷段以≥(130-Q)℃/s的第二冷却速度冷却至(Ts-490)～(Ts-440)℃；
    (e)在自返温控冷段，以第三冷却速度冷却10～40s，其中[(Q-80)/12]≤第三冷却速度≤[(Q-80)/8]；
    (f)最后在空冷段，使得带钢空冷至室温；
    其中，Q＝C×180+Si×10+Mn×30+Ni×50+Cr×15+Mo×15+B×2000。
  11. 一种具有超高屈强比的吉帕级贝氏体钢的制造方法，其特征在于，其包括步骤：
    (1)冶炼和铸造；
    (2)热轧；
    (3)轧后冷却和卷取；
    (4)酸洗和冷轧；
    (5)如权利要求13所述的退火工艺。
  12. 如权利要求11所述的制造方法，其特征在于，在步骤(2)中，控制加热温度为1150～1260℃；精轧开轧温度为1100～1220℃，精轧终轧温度为900～950℃。
  13. 如权利要求11所述的制造方法，其特征在于，在步骤(3)中，控制冷却速度为30～150℃/s，控制卷取温度为450～580℃。
  14. 如权利要求11所述的制造方法，其特征在于，在步骤(4)中，控制冷轧压下率≥50％。
  15. 如权利要求11所述的制造方法，其特征在于，所述具有超高屈强比的吉帕级贝氏体钢如权利要求1-9中任一项所述。
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