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

Abstract

The invention relates to cold-rolled boron-containing DH steel capable of resisting secondary processing brittleness and a production method of the cold-rolled boron-containing DH steel. The steel comprises the following chemical components in percentage by weight: 0.10%-0.30% of C, 0.5%-3.5% of Mn, 0.15%-2.0% of Si, 0.02%-9.00% 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, 0.005%-0.50% of Ca, 0.005%-0.50% of Mg, 0.01%-0.50% of Ti and the balance of Fe and inevitable impurities. The boron-containing DH steel meets basic performance indexes such as high strength, high plasticity and high formability of a DH product and also has the characteristic of resisting secondary processing embrittlement, the tensile strength of the boron-containing DH steel is larger than or equal to 600 MPa, the percentage elongation after fracture is larger than or equal to 26%, the hole expansion rate is larger than or equal to 30%, and the density is 6.0-7.5 g / cm < 3 >; and the secondary processing brittle transition temperature is less than or equal to-160 DEG C.

Description

Technical Field

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

[0002] In recent years, as the automotive industry has increasingly higher requirements for material forming performance indicators, traditional dual-phase steel has been unable to meet the requirements of complex stamping parts for high drawability, and TRIP steel has limited its widespread use due to its high alloy content and expensive production costs. DH steel, which came into being, introduces a certain amount of residual austenite into traditional dual-phase steel, and through the TRIP effect, the material exhibits excellent forming performance, which can significantly overcome the shortcomings of DP steel and TRIP steel in the above-mentioned application process. DH steel has become one of the research hotspots in the field of automotive steel development.

[0003] Secondary processing brittleness (SWE) refers to the low-temperature brittleness characteristics of automotive cold-rolled steel sheets 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 pose a hidden danger to the safety of passenger cars, especially in areas with cold winter climates and large temperature differences between day and night. There is a great risk of automotive sheets 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 topic in the research and development of automotive steel products.

[0004] 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 at the same time adapt to the green and low-carbon product design of automotive high-strength steel under the background of dual carbon.

[0005] Therefore, this patent aims to develop a cold-rolled boron-containing DH steel that is resistant to secondary processing brittleness and a production method thereof. The invention achieves low-carbon, green, and lightweight design and development of automotive high-strength steel with a short-process, low-cost process path and an extremely cost-reducing alloy design, 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

[0006] In view of the above existing technical problems, the object of the present invention is to provide a cold-rolled boron-containing DH steel resistant to secondary processing brittleness and a production method thereof, which not only meets the basic performance indicators of DH products such as high strength, high plasticity and high formability, but also has the characteristics of resisting secondary processing brittleness. The boron-containing DH steel has a tensile strength of ≥600MPa, an elongation after fracture of ≥26%, a hole expansion rate of ≥30%, and a density of 6.0-7.5g / cm 3; Excellent performance of secondary processing brittle transition temperature ≤-160℃.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A secondary processing brittle cold-rolled boron-containing DH steel is characterized in that the chemical components in the steel are as follows by weight percentage: C: 0.10%-0.30%, Mn: 0.5%-3.5%, Si: 0.15%-2.0%, Al: 0.02%-9.00%, B: 0.0005%-0.02%, P≤0.005%, S≤0.005%, N≤0.005%, Ca: 0.005%-0.50%, Mg: 0.005%-0.50%, Ti: 0.01%-0.50%, and the balance is Fe and unavoidable impurities.

[0009] The reasons for the alloy design of the present invention are as follows:

[0010] 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 secondary processing embrittlement. Therefore, in the present invention, the content of C element is controlled to 0.10% to 0.30%.

[0011] 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 0.5% to 3.5%.

[0012] 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 0.15% to 2.0%.

[0013] Al: Aluminum can 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, etc. When the Al content is too low, the low-density design of the material cannot be achieved. Therefore, in the present invention, the Al content is controlled within the range of 0.02% to 9.0%.

[0014] 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%.

[0015] P: P is a harmful element in steel, which is very easy to segregate to the grain boundary and seriously reduce the plasticity and deformation performance of the steel. The lower its content, the better. Considering the cost, the content of P in the present invention is controlled to P≤0.005%.

[0016] S: S is a harmful element in steel. Sulfur and manganese are easily combined to form MnS inclusions. After rolling and deformation, the transverse properties of the material will be significantly reduced, which seriously affects the formability of the steel. The lower the content, the better. Considering the cost, the content of S in the present invention is controlled to S≤0.005%.

[0017] N: N element easily reacts with Ti to precipitate large TiN particles, which act as crack sources during deformation and are detrimental to the anti-hydrogen embrittlement performance. Therefore, the N element content in the steel must be strictly controlled. The present invention controls the N content to N≤0.005%.

[0018] Ca: A small amount of calcium is added because Ca is cheap. It can be added to the master alloy 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 0.500%.

[0019] 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. A small amount of magnesium can improve the carbide size and distribution of DH steel, promote the fine and uniform carbide particles, and also help to achieve low-density material design. In order to control production costs, the Mg content is controlled at 0.005% to 0.50% in the present invention.

[0020] 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 and significantly improve the strength and toughness of the material. In the present invention, the Ti element content is controlled at 0.01% to 0.50%.

[0021] The tensile strength of the DH steel is ≥600MPa, the elongation after fracture is ≥26%, the hole expansion rate is ≥30%, and the density is 6.0-7.5g / cm 3 ; Secondary processing brittle transition temperature ≤-160℃.

[0022] The microstructure of the DH steel is 20% to 40% ferrite, 50% to 70% martensite, 3% to 12% residual austenite, and 3% to 15% bainite by volume, the total of which is 100%; and the residual austenite in the product of the present invention is in two forms, namely, block and film, with a grain size between 0.05 and 1.00 μm, the block residual austenite is mainly distributed at the interface between martensite and ferrite and inside the ferrite, and the film residual austenite is mainly distributed between the martensite laths.

[0023] A production method of secondary processing resistant brittle cold-rolled boron-containing DH steel comprises converter smelting, medium-thin slab continuous casting and rolling, pickling cold rolling, continuous annealing or continuous hot-dip galvanizing aluminum-magnesium, and skin-passing.

[0024] The converter smelting comprises: the present invention selects 40wt% to 100wt% scrap steel as raw material, smelts through a converter, obtains molten steel that meets the following composition requirements by mass percentage: C: 0.10% to 0.30%, Mn: 0.5% to 3.5%, Si: 0.15% to 2.0%, Al: 0.02% to 9.00%, B: 0.0005% to 0.02%, P≤0.005%, S≤0.005%, N≤0.005%, Ca: 0.005% to 0.50%, Mg: 0.005% to 0.50%, Ti: 0.01% to 0.50%; the remainder is Fe and unavoidable impurities. The temperature of the molten steel is between 1600 and 1750°C.

[0025] The continuous casting and rolling of medium and thin slabs includes: using special protective slag for high aluminum steel (Li2O content ranges from 0.5% to 10.0%) for casting, the casting temperature is 1530 to 1600°C, the casting machine pulling speed is 1.0 to 5.5 m / min, the thickness of the continuous casting slab is between 60 and 115 mm, the starting rolling temperature is between 1000 and 1150°C, the final rolling temperature is above 880°C, and the coiling temperature is between 500 and 700°C.

[0026] The hot rolled coil thickness specification is 2.0 to 4.5 mm. The hot rolled microstructure of the product is composed of 20% to 60% ferrite, 20% to 50% pearlite, 5% to 20% bainite by volume, and the rest is a small amount of cementite and impurities; the total is 100%.

[0027] The pickling cold rolling: the hot rolled steel coil is subjected to acid to remove the surface iron oxide scale, and the cold rolling reduction rate is 45% to 70%. If the reduction rate is too high, the deformation resistance will be too large, and it will be difficult to roll to the target thickness; if the reduction rate is too low, the elongation of the cold rolled steel plate will decrease.

[0028] The continuous annealing includes controlling the belt speed at 60-180 m / min, the annealing temperature at 760-880°C, the annealing time at 10-600s, the slow cooling outlet temperature at 700-750°C, the rapid cooling rate at more than 25°C / s, the rapid cooling temperature at 320-500°C, the aging temperature at 310-500°C, and the aging time at 30-300s.

[0029] The annealing temperature is 760-880℃. If the annealing 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 annealing time is 10-600s. If the annealing time is too long, the grain size of the steel plate will be coarse. 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.

[0030] The continuous hot-dip galvanized aluminum-magnesium process includes: the belt speed is controlled at 60-180 m / min, the annealing temperature is between 760-880°C, the dew point temperature is controlled between -20--10°C, the annealing time is between 30-300s, the slow cooling outlet temperature is 680-760°C, the rapid cooling rate is greater than 20°C / s, and the rapid cooling outlet temperature is 450-470°C; the galvanizing temperature is 450-470°C, after the galvanizing is completed, the strip is first air-knife cooled to 400-420°C, and then air-cooled, and the top roller temperature of the cooling tower is controlled at 250-300°C. The annealing temperature is 760-880°C. If the annealing 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 phase ferrite ratio of the final material is too high, which will greatly reduce the strength of the material. The annealing time is 30 to 300 seconds. If the annealing time is too long, the grains of the steel plate will be coarse. 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.

[0031] The composition of the plating solution in the continuous hot-dip galvanizing aluminum-magnesium process is calculated by mass percentage: Al: 2.0% ~ 10.0%, Mg: 1.0% ~ 5.0%, Si: 0.001% ~ 0.1%, and the rest is Zn and unavoidable impurities; the weight of the zinc-aluminum-magnesium coating on the steel plate after continuous hot-dip galvanizing aluminum-magnesium is 50 ~ 200g / cm 2 .

[0032] The skin-finishing process adopts rolling force control, the rolling force is controlled at 1000-3500 kN, and the rolling tension is 500-2000 kN.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1) The chemical composition of the steel material of the present invention mainly includes C, Mn, Al, Si and B as main elements, and the initial cost is relatively low.

[0035] 2) The present invention adopts a new short-process, low-cost production process of "large proportion of scrap steel + converter smelting + continuous casting and rolling of medium and thin slabs", which can greatly reduce carbon emissions and save energy consumption.

[0036] 3) The cold-rolled boron-containing DH steel resistant to secondary processing brittleness produced by the present invention is a conventional cold-rolled dual-phase steel with a certain proportion of retained austenite added thereto, and under the action of transformation induced plasticity (TRIP) effect, the characteristics of high strength, high plasticity and formability are achieved.

[0037] 4) Adding a large amount of aluminum element to the cold-rolled boron-containing DH steel resistant to secondary processing brittleness produced by the present invention can achieve low density of high-strength steel, and adding a large amount of boron element can greatly improve the secondary processing brittleness of high-strength steel, meeting the personalized needs of automobile lightweight and service resistance.

[0038] 5) The cold-rolled boron-containing DH 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 continuous hot-dip galvanizing aluminum-magnesium due to the special design of composition and process, that is, one steel has multiple uses, which can significantly save product manufacturing costs.

[0039] 6) The cold-rolled boron-containing DH steel produced by the present invention can achieve a tensile strength of ≥600MPa, an elongation after fracture of ≥26%, a hole expansion rate of ≥30%, and a density of 6.0-7.5g / cm 3 ; Excellent performance of secondary processing brittle transition temperature ≤-160℃. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a typical engineering stress-strain curve diagram of Example 1-1 and Example 4-1. DETAILED DESCRIPTION

[0041] 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.

[0042] Table 1 lists the chemical composition of the example steel, Table 2 lists the continuous casting and rolling process parameters and hot rolling structure of the example steel, Table 3 lists the process parameters of continuous annealing of the example steel, Table 4 gives the process parameters of alloying hot-dip galvanizing of the example steel, and Table 5 gives the mechanical properties of the example steel. Typical engineering stress-strain curves of Example 1-1 and Example 4-1 are shown in Figure 1 .

[0043] Table 1 Chemical composition of the example steel, wt%

[0044] Example C Mn Si Al B P S N Ca Mg Ti 1 0.106 2.14 0.16 1.14 0.0029 0.002 0.001 0.002 0.037 0.027 0.057 2 0.157 1.66 0.24 2.58 0.0047 0.001 0.002 0.003 0.061 0.163 0.096 3 0.116 1.47 0.18 5.32 0.0061 0.003 0.003 0.001 0.054 0.224 0.182 4 0.193 1.35 0.86 4.51 0.0116 0.003 0.004 0.001 0.186 0.077 0.339 5 0.228 1.11 0.74 0.82 0.0082 0.002 0.001 0.004 0.274 0.365 0.218 6 0.241 0.84 1.23 0.46 0.0055 0.004 0.003 0.003 0.118 0.218 0.082

[0045] Table 2 Continuous casting and rolling process parameters and hot rolled structure of example steel

[0046]

[0047] Table 3 Cold rolling continuous annealing process of example steel

[0048]

[0049]

[0050] Table 4 Main process parameters of continuous hot-dip galvanizing aluminum-magnesium steel in the embodiment of the present invention

[0051]

[0052] Table 5 Mechanical properties and structure of steel plate

[0053]

[0054] It can be seen from the above embodiments that the composition design, continuous casting and rolling, and continuous hot-dip galvanizing process of the present invention can produce a product with a tensile strength of ≥600MPa, an elongation after fracture of ≥26%, a hole expansion rate of ≥30%, and a density of 6.0-7.5g / cm 3 ; Excellent performance of secondary processing brittle transition temperature ≤-160℃.

Claims

1. A secondary processing brittle cold-rolled boron-containing DH steel, characterized in that: The chemical composition of the steel by weight percentage is: C: 0.10% ~ 0.30%, Mn: 0.5% ~ 3.5%, Si: 0.15% ~ 2.0%, Al: 0.02% ~ 9.00%, B: 0.0005% ~ 0.02%, P ≤ 0.005%, S ≤ 0.005%, N ≤ 0.005%, Ca: 0.005% ~ 0.50%, Mg: 0.005% ~ 0.50%, Ti: 0.01% ~ 0.50%, and the balance is Fe and unavoidable impurities.

2. The cold-rolled boron-containing DH steel resistant to secondary processing brittleness according to claim 1, characterized in that: The tensile strength of the DH steel is ≥600MPa, the elongation after fracture is ≥26%, the hole expansion rate is ≥30%, and the density is 6.0-7.5g / cm 3 ; Secondary processing brittle transition temperature ≤-160℃.

3. The cold-rolled boron-containing DH steel resistant to secondary processing brittleness according to claim 1, characterized in that: The microstructure of the DH steel is as follows: 20% to 40% ferrite, 50% to 70% martensite, 3% to 12% retained austenite, and 3% to 15% bainite by volume, the total being 100%; wherein the retained austenite is in two forms, blocky and filmy, with a grain size between 0.05 and 1.00 μm, the blocky retained austenite is mainly distributed at the interface between martensite and ferrite and inside the ferrite, and the filmy retained austenite is mainly distributed between martensite laths.

4. A method for producing cold-rolled boron-containing DH steel resistant to secondary processing brittleness according to any one of claims 1 to 3, characterized in that: The process comprises converter smelting, continuous casting and rolling of medium and thin slabs, pickling and cold rolling, continuous annealing or continuous hot-dip galvanizing and aluminum-magnesium and skin pass. The continuous annealing comprises an annealing temperature of 760-880°C, an annealing time of 10-600s, a slow cooling outlet temperature of 700-750°C, a rapid cooling rate of greater than 25°C / s, a rapid cooling temperature of 320-500°C, an aging temperature of 310-500°C and an aging time of 30-300s.

5. The method for producing a secondary processing brittle cold-rolled boron-containing DH steel according to claim 4, characterized in that: The continuous hot-dip galvanized aluminum-magnesium process includes: an annealing temperature between 760 and 880°C, a dew point temperature controlled between -20 and -10°C, an annealing time between 30 and 300s, a slow cooling outlet temperature of 680 to 760°C, a rapid cooling rate greater than 20°C / s, and a rapid cooling outlet temperature of 450 to 470°C; a galvanizing temperature of 450 to 470°C, and after galvanizing, the strip is first air-knife cooled to 400 to 420°C, and then air-cooled, and the temperature of the cooling tower top roller is controlled at 250 to 300°C.

6. The method for producing a secondary processing brittle cold-rolled boron-containing DH steel according to claim 4, characterized in that: The converter smelting comprises: selecting 40wt% to 100wt% scrap steel as raw material, and the temperature of molten steel is between 1600 and 1750°C.

7. The method for producing a secondary processing brittle cold-rolled boron-containing DH steel according to claim 4, characterized in that: The continuous casting and rolling of medium and thin slabs includes: a casting temperature of 1530-1600°C, a casting machine pulling speed of 1.0-5.5m / min, a continuous casting slab thickness of 60-115mm, a starting rolling temperature of 1000-1150°C, a final rolling temperature of more than 880°C, and a coiling temperature of 500-700°C.

8. The method for producing a secondary processing brittle cold-rolled boron-containing DH steel according to claim 7, characterized in that: The hot rolled coil thickness specification is 2.0 to 4.5 mm. The hot rolled microstructure of the product is composed of 20% to 60% ferrite, 20% to 50% pearlite, 5% to 20% bainite by volume, and the rest is a small amount of cementite and impurities; the total is 100%.

9. The method for producing a secondary processing brittle cold-rolled boron-containing DH steel according to claim 4, characterized in that: The pickling cold rolling reduction rate is 45% to 70%.

10. The method for producing a secondary processing brittle cold-rolled boron-containing DH steel according to claim 4, characterized in that: The skin-pass rolling force is controlled at 1000-3500 kN, and the rolling tension is 500-2000 kN.

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