Boron-containing low-alloy high-strength steel resistant to secondary machining brittleness and production method thereof

Abstract

The invention relates to secondary processing brittleness resistant boron-containing low-alloy high-strength steel and a production method thereof. The steel comprises the following chemical components in percentage by weight: 0.005%-0.10% of C, 0.2%-1.0% of Mn, 0.05%-0.4% of Si, 0.02%-5.00% of Al, 0.0005%-0.02% of B, less than or equal to 0.005% of P, less than or equal to 0.005% of S, less than or equal to 0.005% of N, less than or equal to 0.10% of Nb, 0.002%-0.20% of Ti and the balance of Fe and inevitable impurities. The steel has the characteristic of secondary processing embrittlement resistance while meeting basic performance indexes of products, the yield strength of the steel is 420-520 MPa, the tensile strength is 470-600 MPa, the A80 percentage elongation after fracture is larger than or equal to 17.0%, and the hole expansion rate is larger than or equal to 60%; and the secondary processing brittle transition temperature is less than or equal to-80 DEG C.

Description

Technical Field

[0001] The invention belongs to the technical field of cold-rolled steel, and particularly relates to a secondary processing resistant brittle boron-containing low-alloy high-strength steel and a production method thereof. Background Art

[0002] Cold-rolled low-alloy high-strength steel is based on low-carbon manganese or silicon-manganese systems. By adding a small amount of alloying elements, it forms carbides and nitrides with carbon, nitrogen and other elements and precipitates on the ferrite matrix, thereby improving the strength of the steel; it is mainly used in automotive interior and exterior covers, structural parts, etc. With the rapid development of lightweight automobiles, automobile manufacturers have gradually increased their requirements for the comprehensive performance of steel products. Among them, the secondary processing brittleness of automobile steel plates has been highly valued by automobile manufacturers and steel manufacturers. Secondary processing performance. Secondary processing brittleness (SWE) refers to the low-temperature brittleness characteristics of automobile cold-rolled steel plates after stamping, which is manifested by the ability to withstand impact loads at low temperatures. When the secondary processing brittle transition temperature (SWET) is too high, it will bring hidden dangers to the safety of passenger cars. Especially for areas with cold winter climates and large temperature differences between day and night, there is a great risk of automobile plates being broken by low-temperature impact during use after stamping. How to effectively solve the secondary processing brittleness problem of automotive steel products has become a hot spot in the research and development of automotive steel products. Studies have found that adding boron to steel can significantly improve its hardenability. Boron can easily combine with sulfides and oxides to organize their further growth, and make the shape of these inclusions tend to be small spherical and evenly distributed on the grain boundaries, thereby enhancing the grain boundary energy, reducing the concentration of internal stress, thereby reducing the possibility of cracks, and significantly improving the low-temperature toughness of steel materials.

[0003] At the same time, the steel industry is actively promoting energy conservation, environmental protection and green transformation and development. How to develop green, low-carbon and high-strength automotive products has become a research hotspot for major steel suppliers. Based on the above research status, it is urgent to solve the problems of poor forming performance and secondary processing brittleness of automotive high-strength steel, and adapt to the design of green and low-carbon products of automotive high-strength steel.

[0004] In view of the above research status, this patent aims to develop a boron-containing low-alloy high-strength steel that is resistant to secondary processing brittleness and its production method, which not only meets the basic performance indicators of the product, but also has the characteristics of resistance to secondary processing embrittlement. The present invention uses a short-process low-cost process path and an extremely cost-effective alloy design to achieve low-carbon, green, and lightweight design and development of automotive high-strength steel, while taking into account the personalized needs of high-strength steel for resistance to secondary processing brittleness, high plasticity and high formability, providing reliable technical solutions for the majority of automobile manufacturers and steel companies. Summary of the invention

[0005] The purpose of the present invention is to provide a boron-containing low-alloy high-strength steel resistant to secondary processing brittleness and a production method thereof, which not only meets the basic performance indicators of the product, but also has the characteristics of resisting secondary processing brittleness. The yield strength of the steel of the present invention is 420-520MPa, the tensile strength is 470-600MPa, and the A 80 Elongation after fracture ≥17.0%, hole expansion rate ≥60%; secondary processing brittle transition temperature ≤-80℃.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A secondary processing brittle boron-containing low-alloy high-strength steel, the chemical composition of the steel is as follows by weight percentage: C: 0.005%-0.10%, Mn: 0.2%-1.0%, Si: 0.05%-0.4%, Al: 0.02%-5.00%, B: 0.0005%-0.02%, P≤0.005%, S≤0.005%, N≤0.005%, Nb≤0.10%, Ti: 0.002%-0.20%, and the balance is Fe and unavoidable impurities.

[0008] The reasons for the alloy design of the present invention are as follows:

[0009] C: Carbon element ensures the strength requirement of steel through solid solution strengthening. A sufficient amount of carbon element helps stabilize austenite, thereby improving the formability of steel. If the content of C element is too low, the mechanical properties of the steel in the present invention cannot be obtained; if the content is too high, the steel will become brittle and there is a risk of delayed fracture. Therefore, in the present invention, the content of C element is controlled to 0.005% to 0.10%.

[0010] Mn: Manganese is an austenite stabilizing element in steel. It can expand the austenite phase area and reduce the critical quenching rate of steel. At the same time, it can also refine the grains, which helps to improve the strength by solid solution strengthening. If the Mn content is too low, the supercooled austenite is not stable enough, which reduces the processing properties such as plasticity and toughness of the steel plate; if the Mn content is too high, the welding performance of the steel plate will deteriorate and the production cost will increase, which is not conducive to industrial production. Therefore, in the present invention, the Mn content is controlled to 0.2% to 1.0%.

[0011] Si: Silicon has a certain solid solution strengthening effect in ferrite, ensuring that the steel has sufficient strength. At the same time, Si can also inhibit the decomposition of residual austenite and the precipitation of carbides, reducing inclusions in the steel. If the Si content is too low, it will not play a strengthening role; if the Si content is too high, the surface quality and welding performance of the steel plate will be reduced. Therefore, in the present invention, the content of Si is controlled to 0.05% to 0.4%.

[0012] Al: Aluminum helps deoxidize molten steel. It can also inhibit the decomposition of residual austenite and the precipitation of carbides, and accelerate bainite transformation to improve coordinated deformation ability. The density of aluminum is much lower than that of Fe. Adding an appropriate amount of Al to steel can significantly reduce the density of steel, which is conducive to the lightweight development of steel. At the same time, Al has an antioxidant effect. Adding it in combination with Si can effectively improve the surface quality of steel, which is conducive to the design of one steel for multiple uses. Too high Al content will not only increase production costs, but also lead to difficulties in continuous casting production. Therefore, in the present invention, the content of Al is controlled within the range of 0.02% to 5.00%.

[0013] B: Adding boron to steel can significantly improve its hardenability. Due to the small radius of boron atoms, it gathers in the grain boundary in the form of free state in steel, which has a good effect of strengthening the grain boundary, enhancing the grain boundary energy, reducing the concentration of internal stress, thereby reducing the possibility of cracks, and significantly improving the low-temperature toughness of steel materials. In addition, boron can easily combine with sulfides and oxides to organize their further growth, and make the shape of these inclusions tend to be fine spherical and evenly distributed on the grain boundary. When the boron content exceeds 0.02%, the hardenability decreases and the brittleness increases due to the presence of borides in the steel. Therefore, in the present invention, the content of B element is controlled within the range of 0.0005% to 0.02%.

[0014] P: P is a harmful element in steel, which seriously reduces the plasticity and deformation performance of steel. The lower its content, the better. Considering the cost, the content of P in the present invention is controlled to P≤0.005%.

[0015] S: S is a harmful element in steel, which seriously affects the formability of steel. The lower its content, the better. Considering the cost, the content of S in the present invention is controlled to S≤0.005%.

[0016] N: N is a harmful element in steel, which seriously affects the comprehensive performance of steel. The lower its content, the better. Considering the cost, the content of N in the present invention is controlled to N≤0.005%.

[0017] Nb: The microalloying element Nb improves the comprehensive performance of the material by strengthening the grain size. No more than 0.1% Nb can be added as appropriate according to the actual situation. In order to control the production cost, Nb microalloying element may not be added.

[0018] Ti: A small amount of Ti element can refine the grain size, and the precipitates can pin dislocations to delay the expansion of crack sources at the weld and significantly improve the strength and toughness of the material. In the present invention, the Ti element content is controlled at 0.002% to 0.20%.

[0019] The microstructure of the secondary processing resistant brittle boron-containing low-alloy high-strength steel comprises ferrite and pearlite; calculated by volume percentage, the ferrite is 70% to 95% and the pearlite is 5% to 30%.

[0020] The yield strength of the secondary processing brittle boron-containing low-alloy high-strength steel is 420-520 MPa, the tensile strength is 470-600 MPa, and the 80 Elongation after fracture ≥17.0%, hole expansion rate ≥60%; secondary processing brittle transition temperature ≤-80℃;

[0021] The present invention also provides a method for producing a secondary processing brittle boron-containing low-alloy high-strength steel, comprising converter smelting, medium-thin slab continuous casting and rolling, pickling and cold rolling, continuous annealing or continuous hot-dip galvanizing, and skin-passing, wherein the following processes are controlled:

[0022] 1) Converter smelting: The present invention uses 40% to 80% scrap steel as raw material, which can greatly save energy consumption and smelting costs. The smelting is performed in a converter to obtain molten steel that meets the set composition requirements. The molten steel temperature is 1600 to 1700°C.

[0023] 2) Continuous casting and rolling of medium and thin slabs: The casting temperature is 1550-1600℃, and the casting machine pulling speed is 1.0-6.0m / min; the slab is directly put into the furnace for heating after being pulled out and cut, and the heating temperature is controlled at 1100-1300℃, and the holding time is 60-150min; the starting rolling temperature is 1000-1150℃, the final rolling temperature is above 920℃, and the coiling temperature is 600-700℃. The thickness of the continuous casting slab is 90-160mm. The thickness of the hot rolled coil is 2.5-4.5mm.

[0024] 3) Pickling cold rolling: The hot rolled steel coil is pickled to remove the surface iron oxide scale before cold rolling, and the cold rolling reduction rate is 40% to 80%.

[0025] 4) Continuous annealing: The preheating temperature of the steel plate is controlled at 450-650°C, and the belt speed is controlled at 90-250m / min; the soaking temperature is 760-860°C, and the soaking time is 60-600s; the slow cooling outlet temperature is 630-730°C; the rapid cooling rate is greater than 25°C / s, and the rapid cooling temperature is 380-580°C; the over-aging temperature is 350-550°C, and the over-aging time is 100-1000s; in the continuous annealing process, the rapid cooling outlet temperature is required to be higher than the over-aging temperature by more than 10°C, so as to avoid the deterioration of the plate shape caused by the large degree of supercooling during the rapid cooling process, reduce the internal stress caused by the uneven plate shape of the steel plate at room temperature, and thus improve the secondary processing brittleness of the steel plate.

[0026] Continuous hot-dip galvanizing: the strip speed is controlled at 80-250m / min, the annealing temperature is between 750-820℃, the dew point temperature is controlled between -20 and -10℃, the annealing time is between 30 and 300s, the slow cooling outlet temperature is 650-700℃, the rapid cooling rate is greater than 20℃ / s, and the rapid cooling outlet temperature is 450-470℃; the galvanizing temperature is 450-470℃. After galvanizing, the strip is first air-knife cooled to 400-420℃, and then air-cooled. The temperature of the cooling tower top roller is controlled at 250-300℃.

[0027] The composition of the plating solution in the continuous hot-dip galvanizing process is calculated by mass percentage: Al: 0.16%~0.25%, the rest is Zn and unavoidable impurities; the weight of the zinc layer of the steel plate after continuous hot-dip galvanizing is 60~200g / cm 2 .

[0028] 5) Finishing: The finishing elongation is controlled at 0.5% to 1.2%.

[0029] The thickness of the finished steel plate is 0.8 to 2.0 mm.

[0030] The yield strength can be obtained by the above method, which is 420-520MPa, the tensile strength is 470-600MPa, and A 80 Elongation after fracture ≥17.0%, hole expansion rate ≥60%; secondary processing brittle transition temperature ≤-80℃, low-alloy high-strength boron-containing steel that resists secondary processing brittleness.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1) The secondary processing brittle boron-containing low-alloy high-strength steel produced by the present invention is designed by optimizing the alloy composition, with C, Mn, Si, Al, and B as the main elements, without adding precious metal elements such as Cr and Mo, and the alloy cost is very low.

[0033] 2) The brittle boron-containing low-alloy high-strength steel resistant to secondary processing produced by the present invention adopts the production process of converter smelting - medium-thin slab continuous casting and rolling - pickling and cold rolling - continuous annealing, and the industrial production of boron-containing steel for automobiles can be realized on the traditional production line without adding new production equipment; however, in the present invention, the medium-thin slab continuous casting and rolling process is used to replace the original continuous casting-hot delivery and hot loading-heating-hot rolling-coiling process, which significantly shortens the process flow of boron-containing steel and stabilizes the production, and greatly reduces the product manufacturing cost.

[0034] 3) The boron-containing low-alloy high-strength steel resistant to secondary processing produced by the present invention is made by adding an appropriate amount of aluminum element to the traditional low-alloy high-strength steel. By replacing silicon with aluminum, the product has the properties of anti-rebound elasticity, high hole expansion and high surface quality, which meets the user's requirements for cold-rolled high-strength automotive steel with anti-rebound elasticity, high strength and high plasticity and excellent forming performance.

[0035] 4) The cold-rolled boron-containing dual-phase steel sheet resistant to secondary processing brittleness produced by the present invention can realize a set of alloy systems to meet the two diversified product requirements of continuous annealing and continuous hot-dip galvanizing due to the special design of composition and process, that is, one steel has multiple uses, which can significantly save product manufacturing costs;

[0036] 5) The yield strength of the brittle boron-containing low-alloy high-strength steel produced by the present invention is 420-520MPa, the tensile strength is 470-600MPa, and the A 80 The elongation after fracture is ≥17.0%, the hole expansion rate is ≥60%; the secondary processing brittle transition temperature is ≤-80℃, which ensures the short process and low cost of this product while taking into account the advantages of resistance to secondary processing brittleness and high hole expansion. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a typical engineering stress-strain curve diagram of Example 1-1. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the specific implementation methods of the present invention are further described below in conjunction with embodiments. The following embodiments are used to specifically illustrate the contents of the present invention. These embodiments are only general descriptions of the contents of the present invention and do not limit the contents of the present invention.

[0039] Table 1 lists the chemical composition of the example steel, Table 2 lists the continuous casting and rolling process parameters of the example steel, Table 3 lists the process parameters of cold rolling and continuous annealing of the example steel, Table 4 lists the main process parameters of continuous hot-dip galvanizing of the example steel of the present invention, and Table 5 gives the mechanical properties of the example steel. The engineering stress-strain curve of Example 1-1 is shown in Figure 1 .

[0040] Table 1 Chemical composition of the example steel, wt%

[0041] Example C Mn Si Als P S Nb Ti B 1 0.072 0.63 0.38 0.45 0.001 0.0023 0.047 0.048 0.0031 2 0.053 0.38 0.26 2.57 0.002 0.0035 0.068 0.031 0.0083 3 0.064 0.97 0.07 4.82 0.001 0.0013 0.038 0.02 0.0108 4 0.045 0.82 0.11 3.28 0.001 0.0021 0.021 0.035 0.0005 5 0.087 0.41 0.26 1.14 0.003 0.0036 0.013 0.071 0.0025 6 0.038 0.25 013 0.92 0.002 0.0041 0.046 0.039 0.0053

[0042] Table 2 Continuous casting and rolling process parameters of example steel

[0043]

[0044] Table 3 Cold rolling and continuous annealing process parameters of the steel plate of the embodiment

[0045]

[0046] Table 4 Main process parameters of continuous hot-dip galvanizing of steel in the embodiment of the present invention

[0047]

[0048] Table 5 Microstructure and mechanical properties of finished steel plates of the embodiment

[0049]

[0050] As can be seen from the above embodiments, the yield strength of the brittle boron-containing low-alloy high-strength steel resistant to secondary processing produced by the alloy composition, smelting, continuous casting and rolling, pickling and cold rolling, and continuous annealing or continuous hot-dip galvanizing process of the present invention is 420-520MPa, the tensile strength is 470-600MPa, and the A 80 Elongation after fracture ≥17.0%, hole expansion rate ≥60%; secondary processing brittle transition temperature ≤-80℃; meet the personalized needs of low cost, high hole expansion and resistance to secondary processing brittleness of automobiles.

Claims

1. A secondary processing resistant brittle boron-containing low alloy high-strength steel, characterized in that: The chemical composition of the steel by weight percentage is: C: 0.005% ~ 0.10%, Mn: 0.2% ~ 1.0%, Si: 0.05% ~ 0.4%, Al: 0.02% ~ 5.00%, B: 0.0005% ~ 0.02%, P ≤ 0.005%, S ≤ 0.005%, N ≤ 0.005%, Nb ≤ 0.10%, Ti: 0.002% ~ 0.20%, and the balance is Fe and unavoidable impurities.

2. The secondary processing resistant brittle boron-containing low alloy high strength steel according to claim 1, characterized in that: The yield strength of the high-strength steel is 420-520 MPa, the tensile strength is 470-600 MPa, 80 The elongation after fracture is ≥17.0%, the hole expansion rate is ≥60%; the secondary processing brittle transition temperature is ≤-80℃; the microstructure includes ferrite and pearlite; the volume percentage is: ferrite 70%~95%, pearlite 5%~30%.

3. The secondary processing resistant brittle boron-containing low alloy high strength steel according to claim 1, characterized in that: The thickness of the finished steel plate is 0.8 to 2.0 mm.

4. A method for producing a secondary processing resistant brittle boron-containing low alloy high strength steel as claimed in any one of claims 1 to 3, characterized in that: It includes converter smelting, continuous casting and rolling of medium and thin slabs, pickling and cold rolling, continuous annealing or continuous hot-dip galvanizing, and skin pass. The continuous annealing specifically includes: the preheating temperature of the steel plate is controlled at 450-650°C, and the belt speed is controlled at 90-250m / min; the soaking temperature is 760-860°C, and the soaking time is 60-600s; the slow cooling outlet temperature is 630-730°C; the rapid cooling rate is greater than 25°C / s, and the rapid cooling temperature is 380-580°C; the over-aging temperature is 350-550°C, and the over-aging time is 100-1000s; and the rapid cooling outlet temperature is more than 10°C higher than the over-aging temperature.

5. The method for producing a secondary processing resistant brittle boron-containing low alloy high strength steel according to claim 4, characterized in that: The continuous hot-dip galvanizing specifically includes: the belt speed is controlled at 80-250m / min, the annealing temperature is between 750-820℃, the dew point temperature is controlled between -20 and -10℃, the annealing time is between 30 and 300s, the slow cooling outlet temperature is 650-700℃, the rapid cooling rate is greater than 20℃ / s, and the rapid cooling outlet temperature is 450-470℃; the galvanizing temperature is 450-470℃, after the galvanizing is completed, the strip is first air-knife cooled to 400-420℃, and then air-cooled, and the temperature of the cooling tower top roller is controlled at 250-300℃.

6. The method for producing a secondary processing resistant brittle boron-containing low alloy high strength steel according to claim 4, characterized in that: The converter smelting uses 40wt% to 80wt% scrap steel as raw material, and the temperature of the molten steel is 1600 to 1700°C.

7. The method for producing a secondary processing resistant brittle boron-containing low alloy high strength steel according to claim 4, characterized in that: The continuous casting and rolling of medium and thin slabs includes: a casting temperature of 1550-1600°C, a casting machine pulling speed of 1.0-6.0 m / min; the ingot is directly put into a furnace for heating after being pulled out and cut, the heating temperature is controlled at 1100-1300°C, and the insulation time is 60-150 min; the starting rolling temperature is 1000-1150°C, the final rolling temperature is above 920°C, and the coiling temperature is 600-700°C.

8. The method for producing a secondary processing resistant brittle boron-containing low alloy high strength steel according to claim 7, characterized in that: The thickness of the continuous casting slab is 90-160 mm; the thickness of the hot rolled coil is 2.5-4.5 mm.

9. The method for producing a secondary processing resistant brittle boron-containing low alloy high strength steel according to claim 4, characterized in that: The pickling cold rolling reduction rate is 40% to 80%.

10. The method for producing a secondary processing resistant brittle boron-containing low alloy high strength steel according to claim 4, characterized in that: The skin-finishing elongation is controlled at 0.5% to 1.2%.

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