Cold-rolled boron-containing steel capable of resisting secondary processing brittleness and production method of cold-rolled boron-containing steel

Abstract

The invention relates to cold-rolled boron-containing steel capable of resisting secondary processing brittleness and a production method thereof. The steel comprises the following chemical elements in percentage by weight: 0.005%-0.10% of C, 0.4%-1.5% of Mn, 0.05%-0.8% of Si, 0.02%-0.80% 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, 0.005%-0.10% of Ca, 0.002%-0.10% of Mg and the balance of Fe and inevitable impurities. According to the method, the basic performance indexes of products are met, meanwhile, the secondary machining embrittlement resistance characteristic is achieved, and reliable technical support can be provided for vast iron and steel companies and automobile manufacturers in solving the secondary machining embrittlement problem of automobile steel.

Description

Technical Field

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

[0002] With the rapid development of automobile lightweighting, 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 are manifested by their 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 automobile steel products has become a hot spot in the research and development of automobile 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, which enhances the grain boundary energy and reduces the concentration of internal stress, thereby reducing the possibility of cracks. It can significantly improve the low-temperature toughness of steel materials. In addition, adding a small amount of boron to steel can save a large amount of precious elements. The country encourages the development of boron-containing steel and provides certain preferential treatment in export tax rebate policies.

[0003] Invention document CN103160738A discloses a low-cost boron-containing steel and a manufacturing method thereof, wherein the main chemical components are: C: 0.05% to 0.08%, Si: 0.10% to 0.25%, Mn: 1.60% to 1.80%, Als: 0.03% to 0.045%, Ti: 0.032% to 0.045%, B: 0.0052% to 0.009%, P≤0.025%, S≤0.015%, O≤0.0015%, N≤0.0025%, and the rest is Fe and unavoidable impurities. The invention produces boron-containing steel with low cost characteristics, but the product of the invention is a hot-rolled product, and the secondary processing brittleness of the product is not considered.

[0004] In view of the above research status, the present invention aims to develop a cold-rolled boron-containing steel that is 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 resistance to secondary processing brittleness. The technology of the present invention can provide reliable technical support for steel companies and automobile manufacturers in solving the secondary processing brittleness of automobile steel. Summary of the invention

[0005] The present invention provides a cold-rolled boron-containing 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 characteristic of resisting secondary processing brittleness. The technology of the present invention can provide reliable technical support for steel companies and automobile manufacturers in solving the problem of secondary processing brittleness of automobile steel.

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

[0007] The invention discloses a secondary processing brittle cold-rolled boron-containing steel. The chemical elements in the steel are as follows by weight percentage: C: 0.005%-0.10%, Mn: 0.4%-1.5%, Si: 0.05%-0.8%, Al: 0.02%-0.80%, B: 0.0005%-0.02%, P≤0.005%, S≤0.005%, N≤0.005%, Ca: 0.005%-0.10%, Mg: 0.002%-0.10%, 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 and help 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 0.4% to 1.5%.

[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 0.8%.

[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 the coordinated deformation ability. Too high Al content will not only increase production costs, but also cause difficulties in continuous casting production. Therefore, in the present invention, the Al content is controlled within the range of 0.02% to 0.80%.

[0013] Adding boron to B 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 the steel, has a good effect of strengthening the grain boundary, enhances the grain boundary energy, reduces the concentration of internal stress, thereby reducing the possibility of cracks, and significantly improves the low-temperature toughness of steel materials. In addition, boron is easy to combine with sulfides and oxides to organize their further growth, and makes 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, the content of B element is controlled within the range of 0.0005% to 0.02% in the present invention.

[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] Ca: A small amount of calcium is added because Ca is cheap. It can be added to ferroalloys as a deoxidizer and inoculant, which plays a role in microalloying and can significantly refine the grains, improve the comprehensive properties of steel such as plasticity and welding performance; at the same time, Ca has a good desulfurization effect, can change the composition, quantity and form of non-metallic inclusions, can improve the resistance to hydrogen-induced cracking and lamellar tearing, and can extend the service life of parts. In order to control production costs, the content of Ca in the present invention is controlled at 0.005% to 0.10%.

[0018] Mg: Magnesium is a good deoxidizer, desulfurizer and spheroidizer in steel. Mg can reduce the number of inclusions in steel, reduce their size, make their distribution uniform and improve their morphology. Trace magnesium can improve the carbide size and distribution of boron-containing steel and promote the fine and uniform carbide particles. In order to control production costs, the Mg content is controlled at 0.002% to 0.10% in the present invention.

[0019] The tensile strength of the steel material of the present invention is ≥270MPa, A 80 Elongation after fracture ≥35.0%, hole expansion rate ≥60%; secondary processing brittle transition temperature ≤-120℃; meet the personalized needs of low cost and resistance to secondary processing brittleness of automobiles.

[0020] A production method of secondary processing resistant brittle cold-rolled boron-containing steel, comprising large proportion of scrap steel + converter smelting, medium and thin slab continuous casting and rolling, pickling cold rolling, continuous annealing or hood annealing;

[0021] Converter smelting: smelting is carried out by a converter, and 40wt% to 90wt% scrap steel is selected as a raw material for converter smelting; molten steel is obtained that meets the following composition requirements by weight percentage: C: 0.005% to 0.10%, Mn: 0.4% to 1.5%, Si: 0.05% to 0.8%, Al: 0.02% to 0.80%, B: 0.0005% to 0.02%, P≤0.005%, S≤0.005%, N≤0.005%, Ca: 0.005% to 0.10%, Mg: 0.002% to 0.10%, and the remainder is Fe and unavoidable impurities. The temperature of the molten steel is between 1600 and 1700°C.

[0022] The continuous casting and rolling process of medium and thin slabs includes: casting temperature of 1530-1580℃, casting machine pulling speed of 2.5-5.5m / min, continuous casting slab thickness of 60-115mm; the slab is directly put into the furnace after being pulled out and cut, the furnace temperature is controlled at 1100-1300℃, the holding time is 30-120min, the starting rolling temperature is between 1000-1150℃, the final rolling temperature is above 910℃, and the coiling temperature is between 550-700℃. The thickness specification of hot rolled coil is 2.0-4.5mm.

[0023] In the pickling cold rolling process, the hot rolled steel coil is subjected to acid to remove the surface iron oxide scale before cold rolling, and the cold rolling reduction rate is 50% to 85%. If the reduction rate is too high, the deformation resistance will be too large, making it difficult to roll to the target thickness; if the reduction rate is too low, the elongation of the cold rolled steel sheet will decrease.

[0024] The continuous annealing process includes: the belt speed is controlled at 60-240m / min, the furnace temperature of the soaking section is 780-900℃, the soaking time is 10-600s, the slow cooling outlet temperature is 700-750℃, the rapid cooling rate is greater than 25℃ / s, the rapid cooling outlet temperature is 400-550℃, no aging treatment is required and direct air cooling enters the skin finishing. The skin finishing process adopts rolling force control, the rolling force is controlled at 1000-2500kN, and the rolling tension is 300-1000kN.

[0025] The furnace temperature in the soaking section is 780-900℃. If the soaking section temperature is too high, the ductility of the steel will be reduced due to the complete austenitization and insufficient ferrite ratio. If the annealing temperature is too low, the soft ferrite ratio of the final material will be too high, which will greatly reduce the strength of the material. The soaking time is 10-600s. If the soaking time is too long, the steel plate will have coarse grains. If the annealing time is too short, the steel plate will not have enough time to complete the annealing and recrystallization process, resulting in a decrease in the elongation of the steel plate.

[0026] The hood annealing process comprises: controlling the annealing target temperature of the steel plate processed by the pickling and cold rolling steps to 600-700°C, adopting furnace-based segmented heating and cooling, and the segmented heating process is set as follows: the first segment: furnace temperature 400±20°C, insulation time 2±0.2h; the second segment: furnace temperature 500±20°C, insulation time 2±0.2h; the third segment: furnace temperature 600±20°C, insulation time 2±0.2h, the fourth segment: furnace temperature 650±50°C, insulation time 6-10h; the cooling rate is ≤15°C / s, and the skin-passing adopts elongation closed-loop control, and the skin-passing elongation is 1.0%-2.0%.

[0027] Through the above method, the tensile strength can be ≥270MPa, A 80 Cold-rolled boron-containing steel resistant to secondary processing brittleness, with elongation after fracture ≥35.0%, hole expansion rate ≥60% and secondary processing brittle transition temperature ≤-120°C.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1) The cold-rolled boron-containing steel resistant to secondary processing brittleness 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, Mo, Nb, and V, and the alloy cost is very low.

[0030] 2) The cold-rolled boron-containing steel resistant to secondary processing brittleness produced by the present invention adopts the production process of converter smelting - medium-thin slab continuous casting and rolling - pickling 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 and the aging process is saved in the continuous annealing process, which significantly shortens the process flow of boron-containing steel, stabilizes production, and greatly reduces the product manufacturing cost.

[0031] 3) The cold-rolled boron-containing steel 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 hood annealing due to the special design of composition and process, that is, one steel has multiple uses, which can significantly save product manufacturing costs.

[0032] 4) The tensile strength of the secondary processing brittle cold-rolled boron-containing steel produced by the present invention is ≥270MPa, A 80 The elongation after fracture is ≥35.0%, the hole expansion rate is ≥60%; the secondary processing brittle transition temperature is ≤-120℃; while ensuring the short process and low cost of this product, it also takes into account the advantages of resistance to secondary processing brittleness and high hole expansion, meeting the personalized needs of low cost, high hole expansion and resistance to secondary processing brittleness of automobiles. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the engineering stress-strain curve of Example 1-1. DETAILED DESCRIPTION

[0034] 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.

[0035] 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 process parameters of cold rolling and hood annealing of the example steel, 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 .

[0036] Table 1 Chemical composition of the example steel, wt%

[0037] Example C Mn Si Al B P S N Ca Mg 1 0.009 0.85 0.33 0.26 0.0009 0.004 0.001 0.002 0.008 0.022 2 0.017 1.08 0.22 0.54 0.0016 0.003 0.003 0.001 0.065 0.005 3 0.012 0.41 0.51 0.23 0.0027 0.005 0.001 0.003 0.029 0.043 4 0.022 0.72 0.46 0.55 0.0037 0.004 0.002 0.002 0.048 0.056 5 0.007 1.17 0.09 0.72 0.0013 0.003 0.002 0.003 0.035 0.072 6 0.055 0.55 0.17 0.35 0.0008 0.002 0.001 0.001 0.026 0.061

[0038] Table 2 Continuous casting and rolling process of example steel

[0039]

[0040]

[0041] Table 3 Cold rolling continuous annealing process of example steel

[0042]

[0043] Table 4 Cold rolling hood annealing process of example steel

[0044]

[0045] Table 5 Mechanical properties of example steel

[0046]

[0047]

[0048] It can be seen from the above embodiments that the tensile strength of the cold-rolled boron-containing steel resistant to secondary processing brittleness produced by the alloy composition, smelting, continuous casting and rolling, pickling cold rolling and continuous annealing / hood annealing process of the present invention is ≥270MPa, A 80 Elongation after fracture ≥35.0%, hole expansion rate ≥60%; secondary processing brittle transition temperature ≤-120℃; meet the personalized needs of low cost, high hole expansion and resistance to secondary processing brittleness of automobiles.

Claims

1. A cold-rolled boron-containing steel resistant to secondary processing brittleness, characterized in that: The chemical elements in steel by weight percentage are: C: 0.005% ~ 0.10%, Mn: 0.4% ~ 1.5%, Si: 0.05% ~ 0.8%, Al: 0.02% ~ 0.80%, B: 0.0005% ~ 0.02%, P ≤ 0.005%, S ≤ 0.005%, N ≤ 0.005%, Ca: 0.005% ~ 0.10%, Mg: 0.002% ~ 0.10%, and the balance is Fe and unavoidable impurities.

2. The cold-rolled boron-containing steel resistant to secondary processing brittleness according to claim 1, characterized in that: The secondary processing brittle transition temperature of the boron-containing steel is ≤-120°C.

3. The cold-rolled boron-containing steel resistant to secondary processing brittleness according to claim 1, characterized in that: The tensile strength of the boron-containing steel is ≥270MPa, A 80 Elongation after fracture ≥35.0%, hole expansion rate ≥60%.

4. A method for producing cold-rolled boron-containing steel resistant to secondary processing brittleness according to 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 hood annealing; the continuous annealing process specifically includes: the furnace temperature of the soaking section is 780-900°C, the soaking time is 10-600s, the slow cooling outlet temperature is 700-750°C, the rapid cooling rate is greater than 25°C / s, the rapid cooling outlet temperature is 400-550°C, and there is no need for aging treatment and direct air cooling to enter the skin finishing.

5. The method for producing a secondary processing brittle cold-rolled boron-containing steel according to claim 4, characterized in that: The skin finishing process adopts rolling force control, the rolling force is controlled at 1000-2500 kN, and the rolling tension is 300-1000 kN.

6. The method for producing a secondary processing brittle cold-rolled boron-containing steel according to claim 4, characterized in that: The continuous casting and rolling process of medium and thin slabs includes: a casting temperature of 1530-1580°C, a casting machine pulling speed of 2.5-5.5 m / min, and a continuous casting slab thickness of 60-115 mm; the slab is directly put into the furnace after being pulled out and cut, the furnace temperature is controlled at 1100-1300°C, the insulation time is 30-120 min, the start rolling temperature is between 1000-1150°C, the final rolling temperature is above 910°C, and the coiling temperature is between 550-700°C.

7. The method for producing a secondary processing brittle cold-rolled boron-containing steel according to claim 6, characterized in that: The thickness specification of hot rolled coil is 2.0~4.5mm.

8. The method for producing a secondary processing brittle cold-rolled boron-containing steel according to claim 4, characterized in that: In the pickling and cold rolling process, the cold rolling reduction rate is 50% to 85%.

9. The method for producing a secondary processing brittle cold-rolled boron-containing steel according to claim 4, characterized in that: The temperature of the molten steel in the converter is 1600-1700° C., and 40wt%-90wt% scrap steel is used as raw material in the converter.

10. The method for producing a secondary processing brittle cold-rolled boron-containing steel according to claim 4, characterized in that: The hood annealing process comprises: heating and cooling the steel plate processed by the pickling and cold rolling steps in sections along with the furnace, and the section heating process is set as follows: Section 1: furnace temperature 400±20°C, insulation time 2±0.2h; Section 2: furnace temperature 500±20°C, insulation time 2±0.2h; Section 3: furnace temperature 600±20°C, insulation time 2±0.2h; Section 4: furnace temperature 650±50°C, insulation time 6-10h; skin finishing adopts elongation closed-loop control, and the skin finishing elongation is 1.0%-2.0%.

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