120 kg class cold rolled low alloy annealed dual phase steel and its manufacturing method

Abstract

The present invention discloses a 120-kilogram class cold-rolled low-alloy annealed duplex steel, which contains Fe and inevitable impurity elements, and further contains the following chemical elements in mass percentage content: 0.13% < C ≤ 0.15%, Si: 0.5% - 0.8%, Mn: 1.8% - 2.0%, Al: 0.01% - 0.03%, Nb: 0.015 - 0.025%, Ti: 0.015 - 0.025%, B: 0.0020 - 0.0030%; the chemical elements do not contain Mo and Cr; the microstructure of the 120-kilogram class cold-rolled low-alloy annealed duplex steel is martensite + ferrite. Correspondingly, the present invention discloses a manufacturing method of the above 120-kilogram class cold-rolled low-alloy annealed duplex steel. The 120-kilogram class cold-rolled low-alloy annealed duplex steel obtained by this manufacturing method not only has good economic efficiency, but also has high strength, excellent elongation and hole expansion rate.

Description

[Technical Field]

[0001] The present invention relates to a metallic material and a method for manufacturing the same, and more particularly to a 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steel and a method for manufacturing the same. [Background technology]

[0002] In recent years, with the global energy crisis and worsening environmental problems, energy conservation and safety have become the main development trends in the automobile manufacturing industry, and vehicle weight reduction is one of the measures to save energy and reduce exhaust gas emissions. In practice, high-strength duplex stainless steels have good mechanical properties and usability, so they can be effectively applied in the production and manufacturing of vehicle structural components.

[0003] With the development of ultra-high strength steels and the current changes in the market, the market and users are looking forward to high strength steels with good economical efficiency and better performance. Currently, the use of 1180MPa-class low alloy steels is relatively small, but with the trend of lightweighting and energy saving in the automotive industry and the rapid progress of steel mills both at home and abroad, especially in China, the future development of duplex steels will inevitably be centered on low cost and high performance, and the applications of 1180MPa-class low alloy steels are likely to expand further.

[0004] In the prior art, some research has already been carried out on 1180 MPa class steel materials, and some research results have been obtained.

[0005] For example, the Chinese patent publication number CN108193139A, published on June 22, 2018, titled "1180 MPa-class cold-rolled high-strength dual-phase steel for automobiles and its manufacturing method" discloses an 1180 MPa-class cold-rolled high-strength dual-phase steel for automobiles and its manufacturing method, whose chemical composition and weight percentages are: C 0.10-0.13%, Si 0.45-0.68%, Mn 2.25-2.55%, P≦0.02%, S≦0.008%, Ti 0.10-0.14%, Cr 0.40-0.65%, Mo 0.17-0.21%, N≦0.0050%, Al 0.025-0.060%, with the remainder being Fe and unavoidable impurities. This dual-phase steel utilizes elements such as C, Mn, Cr, and Mo to improve the steel's hardenability and strip strength, while Ti is used for microalloying to enhance the yield strength through the fine grain strengthening effect, and Si is used to increase elongation. This rational composition allows for high strength while maintaining good elongation, avoiding the reduction in elongation caused by excessive Nb. However, the dual-phase steel utilizes high Cr and Mo contents in its chemical composition, which has been found to be costly.

[0006] For example, a Chinese patent document entitled "Low Carbon Equivalent High Hole Expansion Ratio 1180 MPa Class Cold Rolled Steel Sheet and Manufacturing Method Thereof," published on January 15, 2019, with publication number CN109207847A, discloses a low carbon, high hole expansion ratio 1180 MPa class cold rolled steel sheet and a manufacturing method therefor. The chemical composition, in mass percent, is 0.1%-0.15% C, 0.1%-0.4% Si, 1.5%-2.0% Mn, 0.01%-0.05% Al, 0.25-0.5% Mo, and 0.08-0.16% Ti, with the remainder being Fe and unavoidable impurities. The steel plate of this technical proposal has a rational chemical composition and manufacturing process design that ensures that the steel plate can reach a tensile strength of 1180 MPa under low carbon equivalent conditions, and contains nano-scale precipitates with uniform distribution throughout the structure, achieving high precipitation hardening and excellent hole expansion ratio.However, the steel of this technical proposal uses a high content of Mo in its chemical composition.

[0007] For example, a Chinese patent document, Publication Number CN109207841A, published on January 15, 2019, titled "Low-cost, highly formable 1180 MPa-class cold-rolled annealed dual-phase steel sheet and manufacturing method thereof," discloses a low-cost, highly formable 1180 MPa-class cold-rolled annealed dual-phase steel sheet and manufacturing method thereof, whose chemical composition, in mass percent, is 0.1%-0.125% C, 0.4%-0.8% Si, 2.6%-2.9% Mn, 0.01%-0.05% Al, 0.01-0.03% Nb, and 0.01-0.03% Ti, with the remainder being Fe and unavoidable impurities. The dual-phase steel plate of this technical proposal uses rational alloying elements and manufacturing process design to ensure that the steel plate reaches a strength of 1180 MPa at low cost, and has a fine and uniform martensite + ferrite dual-phase structure that ensures excellent elongation and cold bending properties, resulting in good formability.However, the dual-phase steel uses a high content of Mn in its chemical composition, which increases the cost and also causes serious banding and uneven mechanical properties.

[0008] As such, although some of the existing patented technologies for 1180MPa dual-phase stainless steels have better formability, most of them contain high alloying contents such as Cr, Mo, and Mn, which adversely affect the weldability, surface quality, and phosphate treatment performance of the steel, and also increase costs.

[0009] Therefore, in order to meet the current market needs, the present invention aims to develop a 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steel that is both economical and has excellent mechanical properties. Summary of the Invention [Problem to be solved by the invention]

[0010] One object of the present invention is to provide a 120 kgf / cm2 cold-rolled, low-alloy, annealed, duplex steel. This 120 kgf / cm2 cold-rolled, low-alloy, annealed, duplex steel is economical and has excellent mechanical properties. It has high strength, excellent elongation and hole expansion ratio without adding Mo or Cr, and has a yield strength of 820 MPa or more, a tensile strength of 1200 MPa or more, and an A50 The gauge elongation rate ≥ 8% and the hole expansion rate λ ≥ 45%, which have very excellent general applicability and application value. In the present invention, the 120-kilogram class and the 1180 MPa class mean that the tensile strength of the steel ≥ 1180 MPa, and the 1200 MPa class means that the tensile strength of the steel ≥ 1200 MPa.

Means for Solving the Problems

[0011] In order to achieve the above object, the present invention contains Fe and inevitable impurity elements, and further, the following chemical elements in mass percentage content: 0.13% < C ≤ 0.15%, Si: 0.5% - 0.8%, Mn: 1.8% - 2.0%, Al: 0.01% - 0.03%, Nb: 0.01 - 0.03%, Ti: 0.01 - 0.03%, B: 0.0020 - 0.0030%; The chemical elements do not contain Mo and Cr; The microstructure of the 120-kilogram class cold-rolled low-alloy annealed dual-phase steel is martensite + ferrite, and a 120-kilogram class cold-rolled low-alloy annealed dual-phase steel is provided.

[0012] Furthermore, in the 120-kilogram class cold-rolled low-alloy annealed dual-phase steel according to the present invention, the mass percentage content of each chemical element is as follows: 0.13% < C ≤ 0.15%, Si: 0.5% - 0.8%, Mn: 1.8% - 2.0%, Al: 0.01% - 0.03%, Nb: 0.01 - 0.03%, Ti: 0.01 - 0.03%, B: 0.0020 - 0.0030%, and the balance is Fe and other inevitable impurities.

[0013] In the present invention, by using a composition system mainly composed of C-Si-Mn, the cold-rolled low-alloy annealed dual-phase steel obtained can achieve a strength of 1200 MPa grade. In the chemical composition design of this dual-phase steel, precious alloy elements such as Mo and Cr are not added, and the economy is effectively maintained. Further, in the present invention, by adding a trace amount of high hardenability element B in the chemical composition design, the effect of further reducing the Mn content is achieved. In addition, by adding trace amounts of Nb and Ti to the steel, the effect of suppressing the growth of austenite crystal grains is achieved, thereby effectively refining the crystal grains.

[0014] In the 120-kilogram-class cold-rolled low-alloy annealed dual-phase steel according to the present invention, the design principle of each chemical element is as follows: C: In the 120-kilogram-class cold-rolled low-alloy annealed dual-phase steel according to the present invention, the addition of C can increase the strength of the steel and the hardness of martensite. If the mass percentage content of C in the steel is less than 0.13%, not only the strength of the steel sheet is affected, but it is also disadvantageous for the formation amount and stability of austenite; and if the mass percentage content of C in the steel exceeds 0.15%, the hardness of martensite is too high and the crystal grain size becomes coarse, so it is disadvantageous for the forming performance of the steel sheet. At the same time, the carbon equivalent is too high, which is also disadvantageous for welding use. Therefore, in order to maintain the performance of the steel material, in the 120-kilogram-class cold-rolled low-alloy annealed dual-phase steel according to the present invention, the mass percentage content of C is set to 0.13% < C ≦ 0.15%. For example, the mass percentage content of C is 0.131%, 0.135%, 0.14%, 0.145%, 0.15%, or it may also be within the range between any two of the above numerical values.

[0015] Si: In the 120 kgf / cm2 cold-rolled, low-alloy, annealed duplex steel of the present invention, the addition of Si improves the hardenability of the steel. The dissolved Si in the steel affects dislocation interactions, increasing the work-hardening rate, and appropriately improving the elongation of the duplex steel, which is beneficial for achieving better formability. However, it should be noted that the Si content in the steel should not be too high; too high a mass percent of Si in the steel is detrimental to controlling the surface quality of the steel sheet. Therefore, to maximize the beneficial effects of Si, the mass percent of Si in the 120 kgf / cm2 cold-rolled, low-alloy, annealed duplex steel of the present invention is set to 0.5% to 0.8%. For example, the mass percent of Si may be 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, or 0.8%, or may be within a range between any two of the above values.

[0016] Mn: In the 120 kgf / cm2 cold-rolled low-alloy annealed duplex steel according to the present invention, the addition of Mn not only improves the hardenability of the steel but also effectively increases the strength of the steel sheet. If the Mn content in the steel is less than 1.8%, the strength of the steel sheet is insufficient. However, if the Mn content in the steel is more than 2.0%, the strength of the steel sheet is too high, resulting in poor formability. Therefore, taking into account the beneficial effects of Mn, the Mn content in the 120 kgf / cm2 cold-rolled low-alloy annealed duplex steel according to the present invention is set to 1.8% to 2.0% by mass. For example, the Mn content in the steel may be 1.8%, 1.83%, 1.85%, 1.88%, 1.9%, 1.93%, 1.95%, 1.98%, or 2.0%, or may be within a range between any two of the above values.

[0017] Al: In the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steel according to the present invention, the addition of Al has deoxidizing and grain refining effects. Meanwhile, the lower the Al content in the steel, the more advantageous it is for castability during smelting. Therefore, to maximize the beneficial effects of Al, the present invention specifies an Al content of 0.01 to 0.03% by mass. For example, the Al content may be 0.01%, 0.015%, 0.02%, 0.025%, or 0.03%, or may be within a range between any two of the above values.

[0018] Nb: In the present invention, 120 kgf / cm2 cold-rolled low-alloy annealed duplex steel, Nb is an important element for grain refinement. Adding a small amount of Nb, a strong carbide-forming element, to microalloy steel induces strain during controlled rolling, resulting in the precipitation of phases. Through the effects of mass point pinning and subgrain boundary pinning, this significantly lowers the recrystallization temperature of deformed austenite, providing nucleation mass points, and significantly improving grain refinement. Furthermore, during continuous annealing and austenitization, the mass points of undissolved carbon and nitrides prevent the coarsening of soaked austenite grains through the grain boundary pinning mechanism, thereby effectively refining the grains. Therefore, to maximize the beneficial effects of Nb, the mass percent Nb content in the present invention is set to 0.01-0.03%. For example, the mass percent content of Nb can be 0.01%, 0.015%, 0.02%, 0.025%, 0.03%, or a range between any two of the above values.

[0019] Of course, in some preferred embodiments, the mass percentage of Nb may be more preferably 0.015 to 0.025% to obtain better effects.

[0020] Ti: In the 120 kgf / cm2 cold-rolled, low-alloy, annealed dual-phase steel according to the present invention, the addition of Ti, a strong carbide-forming element, exhibits a strong austenite grain growth suppression effect even at high temperatures. At the same time, the addition of Ti is also advantageous for grain refinement. Therefore, to maximize the beneficial effects of Ti, the Ti content in the present invention is set to 0.01 to 0.03% by mass. For example, the Ti content in the present invention may be 0.01%, 0.015%, 0.02%, 0.025%, 0.03%, or any range between any two of the above values.

[0021] Of course, in some preferred embodiments, the mass percentage of Ti may be more preferably 0.015 to 0.025% to obtain better effects.

[0022] B: In the 120 kgf / cm2 cold-rolled low-alloy annealed duplex steel according to the present invention, the addition of B not only improves the hardenability of the steel but also effectively increases the strength of the steel sheet. If the B content in the steel is less than 0.0020%, the strength of the steel sheet will be insufficient. However, if the B content in the steel is more than 0.0030%, the strength of the steel sheet will be too high, resulting in poor formability. Therefore, in the 120 kgf / cm2 cold-rolled low-alloy annealed duplex steel according to the present invention, the B content in the range of 0.0020 to 0.0030% by mass is specified. For example, the B content in the range of 0.0020%, 0.0023%, 0.0025%, 0.0028%, 0.0030%, or any range between any two of the above values.

[0023] In the above-mentioned composition design, the cold-rolled, low-alloy annealed duplex steel of the present invention does not contain valuable alloying elements such as Mo or Cr, making it extremely economical. At the same time, to ensure a tensile strength of 1200 MPa or higher for duplex steel at a normal continuous annealing gas cooling rate of 40 to 100°C / s, the chemical composition design must ensure the addition of alloying elements C, Mn, and B to provide sufficient hardenability. However, it should be noted that there are upper limits to the contents of alloying elements C, Mn, and B in duplex steel to ensure excellent weldability and formability and to ensure that the strength does not exceed the upper limit.

[0024] Furthermore, in the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steel according to the present invention, the unavoidable impurities are P≦0.01%, S≦0.002%, and N≦0.005%.

[0025] In the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steel according to the present invention, the elements P, S, and N are all impurities in the steel, and the lower the contents of the elements P, N, and S in the steel, the better the effect. Specifically, MnS formed by the element S has a significant impact on the formability of the steel, while the element N is likely to cause cracks and bubbles on the surface of the slab. Therefore, if circumstances permit, in order to obtain steel with better performance and quality, the contents of the elements P, S, and N in the steel should be reduced as much as possible, and the contents of P, S, and N in the steel should be controlled to satisfy the following conditions: P≦0.01%, S≦0.002%, and N≦0.005%. In some embodiments, in the 120 kg class cold-rolled low-alloy annealed dual-phase steel according to the present invention, the mass percent content of the element P is 0.001 to 0.01%, and / or the mass percent content of the element S is 0.001 to 0.002%, and / or the mass percent content of the element N is 0.001 to 0.005%.

[0026] Furthermore, in the 120 kg class cold-rolled low-alloy annealed dual-phase steel according to the present invention, the mass percent content of each chemical element satisfies at least one of the following conditions: Nb: 0.015 to 0.025%, Ti: 0.015~0.025%.

[0027] Furthermore, in the 120 kg cold-rolled low-alloy annealed dual-phase steel according to the present invention, the volume percent content of martensite is ≥ 75%, for example, the volume percent content of martensite may be 75%, 80%, 85%, 90%, 95%, 96%, 98%, or any range between any two of the above values.

[0028] Furthermore, in the 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steel according to the present invention, the hardenability coefficient YQ: 2.2≦Y Q≦2.6, but Y Q = Mn + 200 × B, where each chemical element in the formula is substituted with the numerical value before the mass percent symbol. In some embodiments, the hardenability coefficient Y Q is 2.2, 2.3, 2.4, 2.5, 2.6, or within a range between any two of the above values.

[0029] In the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steel according to the present invention, the synergistic effect of the elements B and Mn improves the strength of the steel. In order to ensure that the final strength meets the requirements, the present invention controls the mass percent contents of the individual chemical elements Mn and B, and also adjusts the mass percent contents of Mn and B to 2.2≦Y Q ≦2.6, provided that Y Q =Mn+200×B.

[0030] However, it should be noted that the Mn content has the greatest impact on the overall cost of the alloy. Therefore, the present invention utilizes the overall hardenability of Mn-B and adds an appropriate amount of B to further reduce the Mn content in the alloy. This is advantageous for cost reduction and further helps improve the manufacturability of on-site production.

[0031] Furthermore, in the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steel according to the present invention, the grain sizes of both martensite and ferrite are 5 μm or less. For example, the grain size of martensite may be 3 μm, 3.5 μm, 3.8 μm, 4 μm, 4.2 μm, 4.5 μm, 4.8 μm, 5 μm, or within a range between any two of the above values, and the grain size of ferrite may be 3 μm, 3.5 μm, 3.8 μm, 4 μm, 4.2 μm, 4.5 μm, 4.8 μm, 5 μm, or within a range between any two of the above values.

[0032] Furthermore, in the 120 kgf / cm2 cold-rolled, low-alloy, annealed dual-phase steel according to the present invention, numerous carbide precipitates with a grain size of ≦0.2 μm or less are present within the martensite grains after tempering. In some embodiments, the grain size of the precipitated carbides is 0.1 μm, 0.12 μm, 0.14 μm, 0.16 μm, 0.18 μm, 2 μm, or within a range between any two of the foregoing values.

[0033] Furthermore, the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steel according to the present invention has a yield strength of ≥ 820 MPa, a tensile strength of ≥ 1200 MPa, and an A 50 The gauge elongation at break is ≥ 8% and the hole expansion ratio λ is ≥ 45%. In some embodiments, the 120 kg class cold rolled low alloy annealed duplex steel according to the present invention has a yield strength of 820 MPa, 850 MPa, 900 MPa, 950 MPa, 1000 MPa, or a range between any two of the foregoing values. In some embodiments, the 120 kg class cold rolled low alloy annealed duplex steel according to the present invention has a tensile strength of 1200 MPa, 1220 MPa, 1250 MPa, 1280 MPa, 1300 MPa, 1350 MPa, or a range between any two of the foregoing values. In some embodiments, the 120 kg class cold rolled low alloy annealed duplex steel according to the present invention has a tensile strength of A 50 The gauge elongation at break is 8%, 9%, 10%, 11%, 12%, 13%, or any range between any two of the foregoing values. In some embodiments, the 120 kg cold rolled low alloy annealed duplex stainless steel according to the present invention has a hole expansion ratio λ of 45%, 50%, 55%, 60%, 65%, 70%, or any range between any two of the foregoing values.

[0034] Another object of the present invention is to provide a method for producing the above-mentioned 120 kgf / cm2 cold-rolled low-alloy annealed duplex steel. This method is convenient and simple to implement, and the 120 kgf / cm2 cold-rolled low-alloy annealed duplex steel obtained by this method has high strength and excellent elongation and bending properties, including a yield strength of ≥ 820 MPa, a tensile strength of ≥ 1200 MPa, and an A 50 The gauge breaking elongation is ≧8%, and the hole expansion ratio λ is ≧45%.

[0035] To achieve the above object, the present invention provides a method for producing the above-mentioned 120 kg / h cold-rolled low-alloy annealed dual-phase steel, which comprises the following steps: (1) Smelting and foundry; (2) Hot rolling; (3) cold rolling; (4) Annealing: The annealing temperature is 825-855°C, the annealing time is 40-200 seconds, and then the quenching is started at a rate of 3-5°C / s, and then the quenching is stopped at a rate of 40-100°C / s; however, the quenching start temperature is 735-760°C, and the quenching completion temperature is 265-290°C; (5) Tempering; (6) Leveling.

[0036] Furthermore, in the manufacturing method according to the present invention, the annealing soaking temperature is set to 830 to 840°C in step (4).

[0037] In some preferred embodiments of the technical solution according to the present invention, in order to obtain better effects, such as smaller grain size of the obtained crystal grains, suitable mechanical properties of the obtained steel material, and better formability, the more preferred annealing temperature can be between 830 and 840°C.

[0038] Furthermore, in the manufacturing method according to the present invention, in step (2), the continuously cast slab is first heated to 1160 to 1190°C, and is kept at that temperature for 150 minutes or more (for example, 150 to 250 minutes), and then subjected to finish hot rolling at 850 to 890°C. After rolling, the slab is rapidly cooled at a rate of 30 to 80°C / s and wound at a winding temperature of 500 to 540°C. Thereafter, the temperature inside the heat-insulating cover is set to 450 to 600°C, and the hot coil is kept at a temperature of 450 to 600°C using the heat-insulating cover. After the heat-insulating treatment is completed, the steel coil is air-cooled to room temperature.

[0039] In the above technical solution of the present invention, the hot coil is kept warm by the heat insulation cover. The keeping time can be determined according to the demand. For example, in some embodiments, the keeping time can be set to 1 to 4 hours.

[0040] Furthermore, in the manufacturing method according to the present invention, in step (3), the cold rolling reduction is set to 50 to 70%.

[0041] Furthermore, in the manufacturing method according to the present invention, in step (5), the tempering temperature is set to 265 to 290°C and the tempering time is set to 100 to 400 seconds.

[0042] Furthermore, in the manufacturing method according to the present invention, in step (6), the leveling reduction is set to ≦0.3%, for example, 0.1 to 0.3%.

[0043] Compared with the prior art, the 120 kg class cold-rolled low-alloy annealed dual-phase steel and its manufacturing method according to the present invention have the following advantages and beneficial effects: This invention has developed a new 120 kgf / cm2 cold-rolled, low-alloy, annealed duplex steel. By combining a rational chemical composition design with an optimized manufacturing process, we have been able to produce a martensite-ferrite dual-phase steel sheet with a tensile strength exceeding 1200 MPa without adding alloying elements such as Mo or Cr. This 120 kgf / cm2 cold-rolled, low-alloy, annealed duplex steel has a fine, uniform martensite-ferrite dual-phase structure, and after tempering, numerous carbide precipitates with a grain size of ≤0.2 μm are present within the martensite grains. The final steel sheet has excellent elongation and excellent punching and flanging properties.

[0044] The 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steel of the present invention is not only economical but also has high strength, excellent elongation and hole expansion ratio. Its yield strength is ≥ 820 MPa, tensile strength is ≥ 1200 MPa, and A 50 With a gauge breaking elongation of ≥ 8% and a hole expansion ratio λ of ≥ 45%, this 120 kgf / cm2 cold-rolled low-alloy annealed duplex stainless steel is easy to manufacture and has excellent general-purpose prospects and application value, effectively meeting the needs of the market and users. [Brief explanation of the drawings]

[0045] [Figure 1]FIG. 1 is a schematic view of a metal structure of the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steel in Example 1, taken under a scanning electron microscope. DETAILED DESCRIPTION OF THE INVENTION

[0046] The 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steel and its manufacturing method according to the present invention will be described in more detail below with reference to the drawings and specific examples, but the description does not limit the technical solution of the present invention.

[0047] Examples 1 to 6 and Comparative Examples 1 to 15 Table 1-1 shows the mass percent ratio of each chemical element in the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steels of Examples 1 to 6 and the comparative steels of Comparative Examples 1 to 15.

[0048] [Table 1-1]

[0049] Table 1-2 shows the hardenability coefficient Y of the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steels of Examples 1 to 6 and the comparative steels of Comparative Examples 1 to 15. Q Indicates the value of

[0050] [Table 1-2]

[0051] The 120 kgf / cm2 cold-rolled, low-alloy, annealed dual-phase steels of Examples 1 to 6 according to the present invention and the comparative steels of Comparative Examples 1 to 15 were all produced by the following steps: (1) Smelting and casting were carried out with the chemical compositions shown in Tables 1-1 and 1-2, and the contents of S and P were reduced as much as possible to obtain continuously cast slabs.

[0052] (2) Hot rolling: The continuous cast slab is first heated to 1160-1190°C and kept at this temperature for 150 minutes or more, and then finished by hot rolling at 850-890°C. After rolling, it is rapidly cooled at a rate of 30-80°C / s and wound at a winding temperature of 500-540°C. After that, the temperature inside the heat-insulating cover is set to 450-600°C, and the heat-insulating time can be 1-4 hours. The heat-insulating cover is used to keep the hot coil insulated so that the steel coil is continuously heated or cooled inside the heat-insulating cover. After the heat-insulating process is completed, the steel coil is air-cooled to room temperature and sent to the cold rolling unit.

[0053] (3) Cold rolling: The steel coil was cold rolled at a cold rolling reduction of 50 to 70%.

[0054] (4) Annealing: The annealing temperature was 825 to 855°C, preferably 830 to 840°C, and the annealing time was 40 to 200 seconds. The steel was cooled at a rate of 3 to 5°C / s to the quenching start temperature, and then rapidly cooled at a rate of 40 to 100°C / s. The quenching start temperature was 735 to 760°C, and the quenching completion temperature was 265 to 290°C.

[0055] (5) Tempering: The tempering temperature was 265 to 290°C, and the tempering time was 100 to 400 s.

[0056] (6) Leveling: The leveling reduction rate was ≦0.3% to obtain the finished dual-phase steel product. In the technical solution designed by the present invention, the chemical composition and related processes of the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steels of Examples 1 to 6 prepared by the present invention as described above all meet the design criteria requirements of the present invention.

[0057] In contrast, the comparative steel materials of Comparative Examples 1 to 15 were still produced by the above process according to the compositions in Tables 1-1 and 1-2, but in order to demonstrate the superiority of the technical solution of the present invention, the comparative steel materials of Comparative Examples 1 to 15 have parameters in their chemical compositions and / or related processes that do not meet the design requirements of the present invention.

[0058] Specifically, the comparative steel materials of Comparative Examples 1 to 6 have chemical composition parameters that do not satisfy the design requirements of the present invention. And, while the chemical compositions of the steel materials of Comparative Examples 7 to 15 satisfy the design requirements of the present invention, the related process parameters do not satisfy the design criteria of the present invention.

[0059] Tables 2-1 and 2-2 show specific parameters in the above process steps (1) to (6) for the 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steels of Examples 1 to 6 and the comparative steels of Comparative Examples 1 to 15.

[0060] [Table 2-1]

[0061] [Table 2-2]

[0062] It should be noted that in Table 2-2 above, the quenching completion temperature for each example and comparative example is equal to the tempering temperature, because in the actual process, tempering is performed immediately after quenching is completed.

[0063] Furthermore, after completing the above manufacturing process, the inventors sampled the produced dual-phase steel products of Examples 1 to 6 and Comparative Examples 1 to 15 to obtain corresponding sample steel sheets. The microstructures of the sample steel sheets of the Examples and Comparative Examples were observed and analyzed using an optical microscope, and it was found that the microstructures of the 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steels of Examples 1 to 6 and the comparative steels of Comparative Examples 1 to 15 were all martensite + ferrite.

[0064] In the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steels of Examples 1 to 6 and the comparative steel sheets of Comparative Examples 1 to 15, it was observed that numerous carbide precipitates were present in the martensite after tempering.

[0065] In this regard, the inventors further analyzed the microstructures of the steel sheets of each Example and Comparative Example, and obtained test results for the martensite phase fraction, martensite grain size, ferrite grain size, and grain size of the carbide precipitates present in large numbers within the martensite grains after tempering in the microstructures of the steel sheets of Examples 1 to 6 and Comparative Examples 1 to 15. Specifically, the relevant test results are shown in Table 3 below. In this specification, the phase fraction refers to the proportion of each phase in the microstructure measured by the area method, and the grain size refers to the grain size of each microstructure, based on the average values ​​in the horizontal and vertical directions. The phase fraction and grain size were observed using an optical microscope and measured using analysis software attached to the optical microscope.

[0066] [Table 3]

[0067] From Table 3 above, it can be seen that in the present invention, the microstructures of the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steels produced in Examples 1 to 6 are all martensite + ferrite, the volume percentage content (phase ratio) of martensite is 79 to 96%, the grain size of martensite is 3.8 to 4.8 μm, the grain size of ferrite is 3.6 to 5.0 μm, and the grain size of the carbide precipitates present in large numbers within the grains of martensite after tempering is 0.11 to 0.18 μm.

[0068] After completing the above observations and analyses, the 120 kgf / cm2 cold-rolled, low-alloy, annealed dual-phase steels of Examples 1 to 6 and the comparative steels of Comparative Examples 1 to 15 were sampled to obtain corresponding sample steel plates in order to confirm the performance of the steels of each Example and Comparative Example. The mechanical properties of the obtained sample steel plates of Examples 1 to 6 and Comparative Examples 1 to 15 were tested to obtain data on the mechanical properties of the steels of Examples 1 to 6 and Comparative Examples 1 to 15. The relevant test results are shown in Table 4 below.

[0069] Test methods for relevant mechanical properties are as follows: Tensile test: Using the room temperature tensile test method for metallic materials in GB / T228-2010, the yield strength, tensile strength, and A50 gauge breaking elongation of the steel materials obtained in Examples 1 to 6 and Comparative Examples 1 to 15 were determined. Note that the A50 gauge breaking elongation represents the breaking elongation when the parallel length*width of the tensile test sample is 50mm*25mm.

[0070] Hole expansion ratio test: GB / T24524-2021 Metallic material thin sheet and strip hole expansion test method was used, and the corresponding hole expansion ratio value was the average value of three locations: "end", "middle", and "other end" of the steel sheet. In this way, the hole expansion ratios of the steel materials of Examples 1 to 6 and Comparative Examples 1 to 15 were detected.

[0071] Table 4 shows the results of measuring the mechanical properties of the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steels of Examples 1 to 6 and the comparative steels of Comparative Examples 1 to 15.

[0072] [Table 4]

[0073] From Table 4, it can be seen that the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steels of Examples 1 to 6, which are manufactured by the technical solution designed by the present invention, have excellent mechanical properties, with a yield strength of 848 to 933 MPa, a tensile strength of 1245 to 1288 MPa, and an A 50 It can be seen that the gauge break elongation is 8.5 to 10.5%, and the hole expansion ratio is 48 to 60%. The dual-phase stainless steels of each example exhibited excellent performance, achieving tensile strengths exceeding 1200 MPa without the addition of valuable alloying elements such as Mo or Cr. All of them were 120 kgf / cm2 cold-rolled, low-alloy, annealed dual-phase stainless steels, and simultaneously exhibited good elongation and hole expansion ratios.

[0074] From Tables 1-1, 1-2, 2-1, 2-2, 3 and 4, it can be seen that, compared with the steels of Comparative Examples 1 to 15, the chemical compositions of the 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steels of Examples 1 to 6 of the present invention are within the claimed range, and at the same time, by combining them with optimal process parameters, dual-phase steels that combine low cost and high performance can be obtained.

[0075] Compared with the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steels of Examples 1 to 6, the comparative steels of Comparative Examples 1 to 15 had parameters in their chemical composition design and / or related manufacturing processes that did not meet the requirements of the present invention, and therefore their overall performance was clearly inferior.

[0076] In the present invention, the microstructure of the dual-phase steel corroded with 4% nitric acid alcohol in Example 1 was observed using a scanning electron microscope, and the volume fraction and size of martensite and ferrite were measured using image analysis software. The results are shown in Figure 1.

[0077] FIG. 1 is a schematic view of a metal structure of the 120 kgf / cm2 cold-rolled low-alloy annealed dual-phase steel in Example 1, taken under a scanning electron microscope.

[0078] From FIG. 1, it can be seen that in the embodiment of Example 1, the microstructure of the prepared 120 kgf cold-rolled low-alloy annealed dual-phase steel contains martensite and ferrite, the volume percentage content of martensite is more than 75%, the grain sizes of martensite and ferrite are both less than 5 μm, and at the same time, there are a large amount of carbide precipitates with a grain size of ≦0.2 μm in the grains of martensite after tempering.

[0079] It should be noted that the combination of the technical features in this application is not limited to the combinations described in the claims or the specific embodiments, and all the technical features described in this application may be freely combined or combined in any manner as long as they are not inconsistent with each other.

[0080] Furthermore, it should be noted that the above-mentioned embodiments are merely specific embodiments of the present invention, and the present invention is not limited to the above-mentioned embodiments, and similar changes and modifications thereof are directly obtained or easily conceived by those skilled in the art from the disclosure of the present invention, and therefore, all of them should fall within the protection scope of the present invention.

Claims

1. It contains Fe and unavoidable impurity elements, and further contains the following chemical elements in mass percent: 0.13%<C≦0.15%, Si: 0.5% to 0.8%, Mn: 1.8% to 2.0%, Al: 0.01% to 0.03%, Nb: 0.01 to 0.03%, Ti: 0.01 to 0.03%, B: 0.0020 to 0.0030%; Its chemical elements do not contain Mo and Cr; The microstructure of the 120 kgf / cm2 cold-rolled low-alloy annealed duplex steel is martensite + ferrite.

2. 2. The 120 kg class cold-rolled low-alloy annealed dual-phase steel according to claim 1, wherein the mass percent contents of each chemical element thereof are as follows: 0.13%<C≦0.15%, Si: 0.5% to 0.8%, Mn: 1.8% to 2.0%, Al: 0.01% to 0.03%, Nb: 0.01 to 0.03%, Ti: 0.01 to 0.03%, B: 0.0020 to 0.0030%, and the remainder is Fe and other unavoidable impurities.

3. 3. The 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steel according to claim 1, wherein the unavoidable impurities are P≦0.01%, S≦0.002%, and N≦0.005%.

4. 3. The 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steel according to claim 1 or 2, wherein the mass percent content of each chemical element satisfies at least one of the following conditions: Nb: 0.015-0.025%, Ti: 0.015-0.025%.

5. 3. The 120 kg class cold rolled low alloy annealed dual phase steel according to claim 1 or 2, wherein the volume percent content of martensite is ≥ 75%.

6. 3. The 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steel according to claim 1, wherein the hardenability coefficient YQ is 2.2≦YQ≦2.6, where YQ=Mn+200×B, and the numerical value before the mass percent symbol is substituted for each chemical element in the formula.

7. 3. The 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steel according to claim 1, wherein the grain sizes of martensite and ferrite are both 5 μm or less.

8. 3. The 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steel according to claim 1, wherein carbide precipitates having a grain size of 0.2 μm or less are present within the martensite grains after tempering.

9. Its yield strength ≥ 820 MPa, tensile strength ≥ 1200 MPa, A ５０ 3. The 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steel according to claim 1, wherein the gauge breaking elongation is ≥ 8% and the hole expansion ratio λ is ≥ 45%.

10. A method for producing a 120 kgf / cm2 class cold-rolled low-alloy annealed dual-phase steel according to any one of claims 1 to 9, comprising the following steps: (1) Smelting and foundry; (2) Hot rolling: The continuously cast slab is first heated to 1160-1190°C, kept at this temperature for 150 minutes or more, and then subjected to finish hot rolling at 850-890°C. After rolling, the slab is rapidly cooled at a rate of 30-80°C / s, and then wound at a winding temperature of 500-540°C. After that, the temperature inside the heat-insulating cover is set to 450-600°C, and the hot coil is kept at this temperature using the heat-insulating cover. After the heat-insulating process is completed, the steel coil is air-cooled to room temperature. (3) cold rolling; (4) Annealing: the annealing soaking temperature is 825 to 855°C, the annealing time is 40 to 200 seconds, and then the quenching is started at a rate of 3 to 5°C / s, and then rapidly cooled at a rate of 40 to 100°C / s; however, the quenching start temperature is 735 to 760°C, and the quenching completion temperature is 265 to 290°C; (5) Tempering; (6) Leveling.

11. The manufacturing method according to claim 10, wherein in step (4), the annealing soaking temperature is 830 to 840°C.

12. The method according to claim 10, wherein in step (3), the cold rolling reduction is 50 to 70%.

13. The manufacturing method according to claim 10, wherein in step (5), the tempering temperature is 265 to 290 ° C. and the tempering time is 100 to 400 s.

14. The method of manufacturing according to claim 10, wherein in step (6), the leveling reduction is ≦0.3%.

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