A 600MPa-grade coating-free oxidation-resistant hot-formed steel and a preparation method thereof

Abstract

The present invention relates to the technical field of steel production. The present invention provides a 600MPa-level coating-free anti-oxidation hot-formed steel and a preparation method thereof, comprising: C 0.035%-0.058%, Mn 0.5%-0.8%, Si 0.3%-1.0%, Al 0.1%-0.8%, Ti 0.012%-0.024%, Cr 2.0%-3.2%, (La+Ce) 0.005%-0.05%, N 0.0020%-0.0081%, and the rest is Fe. The 600MPa-level coating-free anti-oxidation hot-formed steel provided in the present application can be used to prepare energy-absorbing safety parts with ultra-high toughness by adding elements and adjusting the content, so as to match with 1500MPa-level and above high-strength coating-free anti-oxidation hot-formed steel; and has good oxidation resistance and extremely high bending toughness.

Description

Technical Field

[0001] The present invention relates to the technical field of steel production, and in particular to a 600MPa-level coating-free oxidation-resistant hot-formed steel and a preparation method thereof. Background Art

[0002] Advanced high-strength steel is an important lightweight material for body-in-white (BIW) and can effectively improve the collision performance of the body. Cold stamping is usually used for the stamping of advanced high-strength steel to prepare body-in-white structural parts. However, when the tensile strength of advanced high-strength steel is greater than 1200MPa, problems such as high springback, poor formability, high stamping load and low die life will occur.

[0003] The hot stamping process usually involves heating the steel sheet blank to 900~950℃ for austenitization treatment, keeping it warm for a certain period of time, and then quickly transferring it to the mold. The stamping equipment then quickly stamps it into shape, and then keeps the pressure in the mold and quickly cools it. Since the steel sheet blank is stamped and formed under high temperature conditions, the steel sheet has the characteristics of low strength and high plasticity, which makes it have technical advantages such as low rebound, good formability, and low stamping load after stamping. The tensile strength of the stamped parts can easily reach 1500MPa and above, so hot-formed steel is an important lightweight material for white body.

[0004] The initial steel sheet blank before hot stamping can be divided into two types: bare sheet and aluminum-silicon coated sheet. Bare sheet is prone to serious high-temperature oxidation and decarburization problems during hot stamping. Stamping parts usually need to be shot peened, which affects the dimensional accuracy of stamping parts, and the iron oxide scale that falls off in the mold needs to be cleaned in time, resulting in reduced industrial production efficiency. The aluminum-silicon coating on the surface of the aluminum-silicon coated sheet can effectively solve the problems of high-temperature oxidation and decarburization on the surface of the steel sheet during hot stamping. However, the cost of aluminum-silicon coating is high, cracks are prone to occur in the coating, and commercial production is limited by patent protection. In recent years, the development of coating-free oxidation-resistant hot-formed steel has become a research hotspot in the steel industry and the automotive industry. At present, domestic steel companies have successfully developed coating-free oxidation-resistant hot-formed steels with strength levels of 1500MPa and 2000MPa, and high levels of chromium and silicon are usually added to their alloy components.

[0005] As we all know, when the tensile strength of hot-formed steel reaches above 1500MPa, its ductility and toughness are poor. However, in the actual automobile collision process, not only are the automobile structural parts required to have high strength to resist the invasion of collision objects (i.e., the use of anti-intrusion safety parts), but also the automobile structural parts are required to have ductility and toughness as a whole or in part to ensure high collision energy absorption during the collision process (i.e., the use of high-energy-absorbing safety parts). For this reason, the automobile industry usually uses laser welding technology to combine high-strength anti-intrusion safety parts with low-strength, high-toughness, high-energy-absorbing safety parts. For example, a B-pillar can be designed with a high-strength anti-intrusion safety part on the upper part and a low-strength, high-toughness, high-energy-absorbing safety part on the lower part. The upper and lower parts are combined by welding to achieve the effect of anti-intrusion on the upper part and high-energy absorption on the lower part. At present, coating-free, anti-oxidation hot-formed steels with strength levels of 1500MPa, 1700MPa and 2000MPa for use in the preparation of anti-intrusion safety parts have been successfully developed. However, there is a lack of research and development of matching low-strength, high-toughness coating-free, anti-oxidation hot-formed steels for the preparation of high-energy-absorbing safety parts.

[0006] In addition, the existing low-strength, high-toughness hot-formed steel bare plates with strength levels such as 500MPa and 600MPa usually need to be industrially prepared through a complex production process, including: smelting molten steel → continuous casting of thick slabs → cutting of ingots → high-temperature heating and insulation of ingots → dephosphorization → rough rolling → finishing rolling → laminar cooling → coiling → pickling → cold rolling → continuous annealing. This production process has high energy consumption, high carbon emissions, long production lines, and high investment costs. Therefore, it is of great significance to provide a coating-free, anti-oxidation hot-formed steel that has both low strength and high toughness and a simple preparation process. Summary of the invention

[0007] The technical problem solved by the present invention is to provide a 600MPa-grade coating-free hot-formed steel, which has the characteristics of high toughness and can be used in combination with 1500MPa-grade and above high-strength coating-free anti-oxidation hot-formed steel, and the preparation process of the hot-formed steel provided in this application is short.

[0008] In view of this, the present application provides a 600MPa grade coating-free oxidation-resistant hot-formed steel, which includes, by mass percentage: C 0.035%~0.058%, Mn 0.5%~0.8%, Si 0.3%~1.0%, Al 0.1%~0.8%, Ti 0.012%~0.024%, (Nb+V+Mo) 0.1%~0.15%, Cr 2.0%~3.2%, (La+Ce) 0.005%~0.05%, N 0.0020%~0.0081%, P≤0.0035%, O<0.0025%, and Mn+Si≤1.6%, 0.7%≤Al+Si≤1.6%, 2.8%≤Si+Cr≤3.5%, and the rest is Fe.

[0009] Preferably, the C content is 0.040%~0.055%.

[0010] Preferably, the content of Mn+Si is 1.0%~1.6%.

[0011] Preferably, 0.8%≤Al+Si≤1.4%.

[0012] Preferably, the Si content is 0.6% to 0.8%, the Ti content is 0.015% to 0.020%, (Nb+V+Mo) is 0.12% to 0.14%, Cr is 2.2% to 3.0%, (La+Ce) is 0.010% to 0.035%, and N is 0.0030% to 0.0068%.

[0013] The present application also provides a method for preparing the hot-formed steel, comprising the following steps:

[0014] S1) preparing the raw materials according to the composition ratio, smelting the obtained mixed raw materials to obtain molten steel; during the smelting process, a magnesium-based alloy is used for deoxidation, and the magnesium-based alloy includes 55-60wt% Mg, 18-25wt% Al, 15-22wt% Ca, and 2.3-4.0wt% Ba;

[0015] S2) pouring the molten steel into a tundish and a flow ladle in sequence and then into a twin-roller thin strip continuous casting device for continuous casting to obtain a cast strip;

[0016] S3) cooling the cast strip and then hot rolling it once, laminar cooling the obtained hot-rolled plate and then coiling it to obtain the coiled hot-rolled plate;

[0017] S4) removing the iron oxide scale from the coiled hot-rolled sheet and then hot stamping the sheet to obtain 600 MPa-grade coating-free oxidation-resistant hot-formed steel.

[0018] Preferably, in step S1), the magnesium-based alloy is added in the form of iron cored wire, the addition time is 25-35 seconds after the molten steel is tapped, and the amount of the iron cored wire added is 3.0-4.0 kg / t;

[0019] In step S2), the temperature of the molten steel is 1600-1620°C, and O in the molten steel is less than 0.0025%; the superheat of the molten steel in the tundish is 60-80°C, and the superheat of the molten steel on the upper surface of the molten pool between the casting rollers in the twin-roll thin strip continuous casting equipment is 10-40°C. The continuous casting is carried out under a protective atmosphere, and the thickness of the cast strip is 1.9-2.1 mm and the width is 1-1.2 m.

[0020] Preferably, in step S3), the starting rolling temperature of the hot rolling is 1000-1050°C, the final rolling temperature is 910-950°C, the thickness of the hot-rolled plate is 1.0-1.7 mm; the cooling rate of the laminar cooling is 15-20°C / s, and the temperature is 550-600°C; the oxide layer thickness of the hot-rolled plate after coiling is 5-7 μm;

[0021] In step S4), the method of removing the iron oxide scale is selected from one or both of pickling treatment and EPS production line treatment; the pickling passes through three pickling tanks in sequence, the acid concentrations are 50-60g / L, 80-90g / L, and 100-120g / L respectively, the pickling temperature is 80-90°C, and the pickling speed is 80-100m / min; the EPS production line treatment is carried out using recycled harmless medium water and steel grit.

[0022] Preferably, in step S4), the hot stamping is specifically:

[0023] The hot rolled plate after removing the iron oxide scale is heated to 900-950°C in a heating furnace, and the total time = (the thickness of the hot rolled plate after removing the iron oxide scale * 110 + 60-80) s, and then transferred to the mold in 3-10 s and quenched to 200-220°C, the pressure is 14-20 MPa, and the pressure is maintained for 6-15 s; the atmosphere of the heating furnace is dry compressed air, nitrogen or argon, and the dew point is -25--5°C;

[0024] The oxide layer thickness of the 600MPa-grade coating-free hot-formed steel is 5-20nm.

[0025] Preferably, the hot stamping further includes tempering, and the tempering temperature is 150-200° C. and the time is 10-30 min.

[0026] The present application provides a 600MPa-grade coating-free oxidation-resistant hot-formed steel, which comprises: C 0.035%-0.058%, Mn 0.5%-0.8%, Si 0.3%-1.0%, Al 0.1%-0.8%, Ti 0.012%-0.024%, (Nb+V+Mo) 0.1%-0.15%, Cr 2.0%-3.2%, (La+Ce) 0.005%-0.05%, N 0.0020%~0.0081%, P≤0.0035%, O<0.0025%, and Mn+Si≤1.6%, 0.7%≤Al+Si≤1.6%, 2.8%≤Si+Cr≤3.5%, and the rest is Fe; the 600MPa-level coating-free oxidation-resistant hot-formed steel provided in the present application adds Ti, Nb, V and Mo and limits their content so that they are combined with C and N to form nano-scale second phase particles during austenitization and coiling, and are evenly dispersed in the ferrite, thereby reducing the hardness difference at the interface between ferrite and martensite, and effectively improving the plasticity and bending toughness of the hot-formed steel; by adding Al+Si+Cr+ (La+Ce) elements and limiting their content, The content is beneficial to improve the high-temperature oxidation resistance of hot-formed steel, so that nitrogen or inert gas protection is not required in the heating furnace during hot stamping, and the stamping parts do not need shot peening. At the same time, reducing the C content can make the obtained martensite hardness lower, thereby reducing the stress difference at the interface between ferrite and martensite, and improving the plasticity and bending toughness of hot-formed steel. Furthermore, strictly controlling the mass percentages of O, Mn, Si, Al and Cr to meet O<0.0025%, Mn+Si≤1.6%, 0.7%≤Al+Si≤1.6%, 2.8%≤Si+Cr≤3.5%, can slow down the high-temperature oxidation degree of the hot-rolled plate, so that the oxide layer thickness of the hot-rolled plate after coiling is 5~7μm.

[0027] Furthermore, the present application also provides a method for preparing 600MPa-grade uncoated hot-formed steel, which includes smelting, pouring, continuous casting, one-pass hot rolling, laminar cooling, coiling, removal of iron oxide scale and hot stamping in sequence. By adopting a double-roll thin strip continuous casting process in the continuous casting process, the thick slab continuous casting, ingot cutting, ingot high-temperature heating, phosphorus removal, rough rolling, finish rolling and other processes in the traditional strip steel production process can be omitted. Therefore, the preparation method provided by the present application has obvious advantages in energy saving, low carbon and environmental protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a microstructure photograph of the 600MPa-grade coating-free hot-formed steel after hot stamping provided in Example 1. DETAILED DESCRIPTION

[0029] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0030] In response to the demand for matching 600MPa-level coating-free anti-oxidation hot-formed steel with high-level coating-free anti-oxidation hot-formed steel in the prior art, the present application provides a 600MPa-level coating-free anti-thermal oxidation hot-formed steel, which, by designing the addition of alloy elements and adjusting the content, makes the hot-formed steel can be used to prepare high-energy-absorbing safety parts, has the advantage of high toughness, and can be matched with the existing 1500MPa-level and above high-strength coating-free anti-oxidation hot-formed steel, and it also has good oxidation resistance; further, the present application also provides a method for preparing hot-formed steel, which introduces a double-roll thin strip continuous casting process to prepare hot-formed steel casting strips, omitting some steps, making the preparation process short and efficient. Specifically, an embodiment of the present invention discloses a 600MPa-grade coating-free oxidation-resistant hot-formed steel, which comprises, by mass percentage, C 0.035%~0.058%, Mn 0.5%~0.8%, Si 0.3%~1.0%, Al 0.1%~0.8%, Ti 0.012%~0.024%, (Nb+V+Mo) 0.1%~0.15%, Cr 2.0%~3.2%, (La+Ce) 0.005%~0.05%, N 0.0020%~0.0081%, P≤0.0035%, O<0.0025%, and Mn+Si≤1.6%, 0.7%≤Al+Si≤1.6%, 2.8%≤Si+Cr≤3.5%, and the rest is Fe.

[0031] In the 600MPa-grade coating-free oxidation-resistant hot-formed steel provided in the present application, the C content is 0.035-0.058%, specifically, the C content is 0.040-0.055%; for example, the C content in the present application is 0.036%, 0.037%, 0.038%, 0.039%, 0.041%, 0.042%, 0.043%, 0.044%, 0.045%, 0.046%, 0.047%, 0.048%, 0.049%, 0.050%, 0.051%, 0.052%, 0.053%, 0.054%, 0.056%, 0.057%. The present application controls the C content to be relatively low, and can obtain martensite with lower hardness in the hot-formed steel, thereby reducing the hardness difference at the interface between ferrite and martensite, and improving the plasticity and bending toughness of the hot-formed steel.

[0032] The content of Mn is 0.5-0.8%. For example, the content of Mn in the present application is 0.5%, 0.6%, 0.7%, and 0.8%.

[0033] The Si content is 0.3-1.0%, specifically, the Si content is 0.6-0.8%; for example, the Si content in this application is 0.4%, 0.5%, 0.7%, and 0.9%.

[0034] Furthermore, Mn+Si≤1.6%, specifically, 1.0%≤Mn+Si≤1.6%; illustratively, the content of Mn+Si is 1.1%, 1.2%, 1.3%, 1.4% or 1.5%.

[0035] The content of Al is 0.1-0.8%. For example, the content of Al in the present application is 0.12%, 0.15%, 0.20%, 0.25%, 0.3%, 0.4%, 0.5%, 0.6%, and 0.7%.

[0036] Furthermore, 0.7%≤Al+Si≤1.6%, specifically, 0.8%≤Al+Si≤1.4%; illustratively, the content of Al+Si is 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, and 1.5%.

[0037] The Cr content is 2.0~3.2%, specifically, the Cr content is 2.2~3.0%; by way of example, the Cr content in this application is 2.1%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, and 3.1%.

[0038] Furthermore, 2.8%≤Si+Cr≤3.5%. Specifically, the contents of Si+Cr are 2.9%, 3.0%, 3.1%, 3.2%, 3.3%, and 3.4%.

[0039] The Ti content is 0.012-0.024%, specifically, the Ti content is 0.015-0.020%; for example, the Ti content in this application is 0.013%, 0.014%, 0.015%, 0.016%, 0.017%, 0.018%, 0.019%, 0.021%, 0.022%, and 0.023%.

[0040] The content of N is 0.0020-0.0081%. Specifically, the content of N is 0.0035-0.0068%. For example, the content of N in the present application is 0.0021%, 0.0022%, 0.0023%, 0.0024%, 0.0025%, 0.0026%, 0.0028%, 0.0030%, 0.0031%, 0.0032%, 0.0033%, 0.0034%, 0.0035%, 0.0036%, 0.0037%, 0.0038%, 0.0039%, 0.0041%, 0.0042%, 0.0043%, 0.0045%, 0.0044%, 0.0046%, 0.0047%, 0. 0.0048%, 0.0049%, 0.0050%, 0.0051%, 0.0052%, 0.0053%, 0.0054%, 0.0055%, 0.0056%, 0.0057%, 0.0058%, 0.0059%, 0.0060%, 0.0061%, 0.0062%, 0.0063%, 0.0064%, 0.0065%, 0.0066%, 0.0067%, 0.0069%, 0.0070%, 0.0071%, 0.0072%, 0.0073%, 0.0074%, 0.0075%, 0.0076%, 0.0077%, 0.0078%, 0.0079%, 0.0080%.

[0041] La and Ce, as rare earth elements, can deeply reduce the harmful effects of low melting point elements such as S and P. At the same time, trace amounts of rare earth elements can also improve the high temperature oxidation resistance of the material. However, if too much La+Ce is added, the nozzle will be blocked during the continuous casting process. The content of La+Ce is 0.005~0.05%, specifically, the content of La+Ce is 0.008~0.035%, and more specifically, the content of La+Ce is 0.015~0.025%.

[0042] In the present application, one or more of Nb, V and Mo are added, that is, Nb, V and Mo can be added in one of the three, two of the three, or all three at the same time; the total addition amount of (Nb+V+Mo) is 0.1-0.15%, specifically, the total addition amount of (Nb+V+Mo) is 0.12-0.14%; for example, the total addition amount of (Nb+V+Mo) is 0.11% and 0.13%. In a specific embodiment, Nb and V are added at the same time, or Nb, V and Mo are added at the same time.

[0043] Furthermore, the present application also limits the contents of P and O, P≤0.0035%, O<0.0025%, so as to slow down the high temperature oxidation of hot-formed steel.

[0044] The present application also provides a method for preparing hot-formed steel, comprising the following steps:

[0045] S1) preparing the raw materials according to the composition ratio, smelting the obtained mixed raw materials to obtain molten steel; during the smelting process, a magnesium-based alloy is used for deoxidation, and the magnesium-based alloy includes 55-60wt% Mg, 18-25wt% Al, 15-22wt% Ca, and 2.3-4.0wt% Ba;

[0046] S2) pouring the molten steel into a tundish and a flow ladle in sequence and then into a twin-roller thin strip continuous casting device for continuous casting to obtain a cast strip;

[0047] S3) cooling the cast strip and then hot rolling it once, laminar cooling the obtained hot-rolled plate and then coiling it to obtain a coiled hot-rolled plate;

[0048] S4) removing the iron oxide scale from the coiled hot-rolled sheet and then hot stamping the sheet to obtain 600 MPa-grade coating-free hot-formed steel.

[0049] In the process of preparation, the present application firstly performs smelting, and in the process of smelting, a low-Ca+low-Al magnesium-based composite deoxidizing alloy is used for steelmaking deoxidation to avoid the formation of clustered or strip-shaped aluminum oxide after steelmaking Al deoxidation, thereby improving the quality of molten steel, and at the same time avoiding the problem of easy plugging of water outlets. The magnesium-based composite deoxidizing alloy includes 55-60wt% Mg, 18-25wt% Al, 15-22wt% Ca, and 2.3-4.0wt% Ba. Specifically, the magnesium-based composite deoxidizing alloy includes 56-58wt% Mg, 19-23wt% Al, 15-20wt% Ca, and 2.5-3.5wt% Ba. The magnesium-based composite deoxidizing alloy described in the present application is first prepared into alloy particles with a particle size of 65-85 mm, and then made into cored wire (the skin is iron, and the core is magnesium-based composite deoxidizing alloy) through a core-wrapping operation. After the steel tapping is completed, the cored wire is added to the molten steel through a wire feeding machine; the timing of adding the cored wire is 25-35 seconds after the molten steel is tapped, and the amount of the cored wire added is 3.0-4.0 kg / t.

[0050] The present application then pours the molten steel into the tundish, then flows from the tundish into the cloth ladle, and finally flows from the cloth ladle into the twin-roll thin strip continuous casting equipment for continuous casting to obtain a cast strip; the temperature of the molten steel is 1600~1620℃, specifically, the temperature of the molten steel is 1612~1615℃; the superheat of the molten steel in the tundish is 60~80℃, specifically, the superheat of the molten steel in the tundish is 62~70℃; the superheat of the molten steel on the upper surface of the molten pool between the casting rollers in the twin-roll thin strip continuous casting equipment is 10~40℃, specifically, the superheat of the molten steel on the upper surface of the molten pool between the casting rollers in the twin-roll thin strip continuous casting equipment is 20~30℃; the continuous casting is carried out under a protective atmosphere, and the thickness of the cast strip is 1.9~2.1mm and the width is 1~1.2m. The present application strictly controls the temperature and superheat of the molten steel in the above process, and can obtain a cast strip with good quality by continuous casting under a nitrogen protective atmosphere.

[0051] The above-mentioned twin-roll thin strip continuous casting is a potential low-carbon preparation process for producing low-strength, high-toughness hot-formed steel bare plates. Through the twin-roll thin strip continuous casting technology, liquid steel can be directly cast into two casting rolls rotating in opposite directions, and directly solidified and formed between the roll gaps of the casting rolls to prepare 1.9~2.1mm thin strip steel. This can eliminate the complex processes of thick slab continuous casting, ingot cutting, ingot high-temperature heating, dephosphorization, rough rolling, and finish rolling in the traditional hot-rolled strip steel production process, and has technical advantages such as low energy consumption, low carbon emissions, short production lines, and low investment costs.

[0052] The present application then performs a hot rolling after cooling the cast strip, and coils the obtained hot-rolled plate after laminar cooling to obtain the coiled hot-rolled plate; the starting rolling temperature of the hot rolling is 1000~1050℃, the final rolling temperature is 910~950℃, and the thickness of the hot-rolled plate is 1.0~1.7mm. Specifically, the starting rolling temperature of the hot rolling is 1010~1030℃, and the final rolling temperature is 920~930℃; the cooling rate of the laminar cooling is 15~20℃ / s, and the temperature is 550~600℃. Specifically, the cooling rate of the laminar cooling is 16~18℃ / s, and the temperature is 560~580℃; the oxide layer thickness of the coiled hot-rolled plate is 5~7μm.

[0053] After the hot-rolled plate is coiled, there is iron oxide scale, i.e., an oxide layer, on its surface. The present application requires that the oxide layer be removed; the method for removing the oxide layer is selected from one or both of pickling and EPS production line treatment, and the pickling passes through three pickling tanks in sequence, and the acid concentrations are 50-60 g / L, 80-90 g / L, and 100-120 g / L, respectively. The pickling temperature is 80-90°C, and the pickling speed is 80-100 m / min; specifically, the pickling concentrations are 55-58 g / L, 85-88 g / L, and 110-118 g / L, respectively, the pickling temperature is 85-88°C, and the pickling speed is 90-108 m / min. During the pickling process, as the pickling progresses, the acid concentration in the acid tank increases successively, and the oxide layer can be destroyed to varying degrees. Among them, the outermost oxide layer is easy to pickle away, and a lower concentration of acid is used, and then the acid concentration is increased successively to better remove the iron oxide layer from the outside to the inside. In this way, the efficiency of removing the iron oxide scale is better and more effective, and the disadvantage of not pickling cleanly in a local area of ​​the hot-rolled coil in a single acid tank can be avoided. After pickling, the EPS production line can be used for treatment according to the residual iron oxide scale to further remove the surface oxide scale and achieve rust prevention. The EPS production line treatment can be carried out using recycled harmless medium water and steel grit. The microstructure of the hot-rolled plate after coiling consists of widmanstatten ferrite, equiaxed ferrite and martensite, wherein the volume fraction of widmanstatten ferrite is 10%-15%, the volume fraction of equiaxed ferrite is 75%-85%, the volume fraction of martensite is 5%-10%, and the yield strength of the hot-rolled plate after coiling is 350-370MPa, the tensile strength is 400-450MPa, and the total elongation is 20%-25%.

[0054] According to the present invention, hot stamping is finally performed, and the hot stamping is specifically as follows:

[0055] The hot rolled plate after removing the iron oxide scale is heated to 900~950℃ in a heating furnace, and the total time = (thickness of the hot rolled plate after removing the iron oxide scale * 110 + 60~80)s, and then transferred to the mold in 3~10s and quenched to 200~220℃, the pressure is 14~20MPa, and the pressure is maintained for 6~15s; the atmosphere of the heating furnace is dry compressed air, argon or nitrogen. The hot stamping of this application is heated in dry compressed gas without oxidation, and the dew point is -25~-5℃; the oxide layer thickness of the 600MPa-level coating-free hot-formed steel is 5~20nm.

[0056] During the hot stamping process, the quenching temperature is 210-218°C, the pressure is 16-18 MPa, and the pressure is maintained for 10-14 seconds; the dew point is -15--10°C.

[0057] The microstructure of the oxidation-resistant hot-formed steel obtained after the above hot stamping is ferrite + martensite + bainite, wherein the volume fraction of ferrite is 80%~85%, the volume fraction of martensite is 5%~10%, and the volume fraction of bainite is 5%~10%; the yield strength is 500~550MPa, the tensile strength is 620~700MPa, the total elongation is 20%~25%, and the corresponding bending angle under the maximum load of VDA238-100 three-point bending is 115~146° (the thickness of the sheet is 1.6mm).

[0058] Furthermore, the present application also performs baking and coating treatment after hot stamping, that is, the hot deformed steel after hot stamping is tempered, and the tempering temperature is 150~200℃, and the time is 10~30min. Specifically, the tempering temperature is 170~190℃, and the time is 20~25min. The microstructure of the hot deformed steel after tempering is ferrite+martensite+bainite, wherein the volume fraction of ferrite is 80%~85%, the volume fraction of martensite is 5%~10%, and the volume fraction of bainite is 5%~10%; the yield strength is 480~530MPa, the tensile strength is 600~680MPa, the total elongation is 23%~28%, and the corresponding bending angle under the maximum load of VDA238-100 three-point bending is 120~150° (thickness is 1.6mm sheet).

[0059] The present application provides a 600MPa-grade coating-free oxidation-resistant hot-formed steel and a preparation method thereof. The mass percentages of O, Mn, Si, Al and Cr in molten steel are strictly controlled to meet the requirements of O < 0.0025%, Mn + Si ≤ 1.6%, 0.7% ≤ Al + Si ≤ 1.6%, and 2.8% ≤ Si + Cr ≤ 3.5%. This ratio can slow down the high-temperature oxidation degree of the hot-rolled plate, and achieve an oxide layer thickness of 5 to 7 μm for the hot-rolled plate after coiling. The high-temperature oxidation resistance of the hot-formed steel is improved by adding Al + Si + Cr + (La + Ce) elements, so that the hot-rolled plate can be made to be oxidized at a temperature of 5 to 7 μm. During the stamping process, no nitrogen or inert gas protection is required in the heating furnace, and the stamping parts do not need shot peening. Ti, Nb, V, and Mo elements are combined with C and N atoms in the steel to form nanometer-scale second phase particles during the coiling and austenitization process, and are evenly dispersed in the ferrite, thereby reducing the hardness difference at the interface between ferrite and martensite, and effectively improving the plasticity and bending toughness of the hot-formed steel. Furthermore, by controlling the lower carbon content, the hardness of the martensite obtained in the hot-formed steel is lower, thereby reducing the hardness difference at the interface between ferrite and martensite, and improving the plasticity and bending toughness of the hot-formed steel.

[0060] In the preparation process of hot-formed steel, by strictly controlling the mass percentage of the chemical composition of the deoxidized alloy, the temperature of the molten steel in the ladle, the superheat of the molten steel in the tundish, and the superheat of the molten steel on the upper surface of the molten pool between the casting rollers, a high-quality cast strip with a thickness of 1.9~2.1mm and a width of 1~1.2m can be obtained by casting and rolling under nitrogen protective atmosphere conditions; the double-roll thin strip continuous casting process is used to prepare hot-formed steel cast strips, which can eliminate the thick slab continuous casting, ingot cutting, ingot high-temperature heating, dephosphorization, rough rolling, finish rolling and other processes in the traditional strip steel production process, so it has obvious advantages in energy saving, low carbon and environmental protection.

[0061] In order to further understand the present invention, the 600MPa-level coating-free anti-oxidation hot-formed steel and its preparation method provided by the present invention are described in detail below in combination with embodiments. The protection scope of the present invention is not limited by the following embodiments.

[0062] The test standard for tensile strength, yield strength and total elongation in the embodiment of the present invention is GB / T228.1-2010, the gauge length of the tensile specimen is 50 mm, and the tensile rate is 2 mm / min;

[0063] The bending angle corresponding to the maximum load of VDA238-100 three-point bending is determined according to the VDA 238-100 three-point bending test standard formulated by the German Association of the Automotive Industry.

[0064] Example 1

[0065] In this embodiment, the short process preparation method of 600MPa-grade coating-free oxidation-resistant hot-formed steel is carried out according to the following steps:

[0066] 1) Molten steel is melted according to the set composition, and its chemical composition mass percentage is: 0.035% C, 0.5% Mn, 0.3% Si, 0.1% Al, 0.012% Ti, 0.1% (Nb + V), 3.2% Cr, 0.005% RE (La + Ce), 0.0020% N, 0.003% P, 0.0021% O, and the rest is Fe and unavoidable impurity elements;

[0067] During the smelting process, a low-Ca+low-Al magnesium-based composite deoxidizing alloy is used for steelmaking deoxidation. The chemical composition of the deoxidizing alloy is as follows: 55%Mg, 25%Al, 16%Ca, 4.0%Ba. The deoxidizing alloy is first prepared into alloy particles with a particle size of about 65mm. When the molten steel is tapped for 25s, a cored wire (the core is a deoxidizing alloy) with a weight of 3.0kg / t Fe is added into the molten steel to deoxidize the molten steel and modify the inclusions.

[0068] 2) Control the temperature of the molten steel in the ladle to 1612℃, pour the molten steel into the tundish; control the superheat of the molten steel in the tundish to 60℃, and then flow the molten steel from the tundish into the flow ladle; finally, the molten steel flows from the flow ladle into the twin-roll thin strip continuous casting equipment, control the superheat of the molten steel on the upper surface of the molten pool between the casting rolls to 10℃, and obtain a cast strip with a thickness of 2.1mm and a width of 1m through casting and rolling under nitrogen protective atmosphere;

[0069] 3) After cooling the cast strip to the start rolling temperature, hot rolling is performed in one pass, with the start rolling temperature being 1000°C and the final rolling temperature being 910°C to obtain a hot rolled plate with a thickness of 1.6 mm;

[0070] 4) After the hot-rolled plate leaves the hot rolling mill, it is cooled to 560°C at 16°C / s by a laminar cooling system for coiling to obtain a coiled hot-rolled plate, wherein the microstructure of the coiled hot-rolled plate consists of widmanstatten ferrite, equiaxed ferrite and martensite, wherein the volume fraction of widmanstatten ferrite is 10%, the volume fraction of equiaxed ferrite is 85%, the volume fraction of martensite is 5%, and the yield strength, tensile strength and total elongation of the coiled hot-rolled plate are 350MPa, 400MPa and 25% respectively; the oxide layer thickness of the coiled hot-rolled plate is 5μm;

[0071] 5) The hot-rolled plate is pickled after coiling: the pickling solution is hydrochloric acid, which passes through three pickling tanks with different concentrations of hydrochloric acid in sequence, and the corresponding hydrochloric acid concentrations are 50g / L, 80g / L and 100g / L respectively; the pickling temperature is adjusted to 80°C, and then the hot-rolled plate is pickled at a pickling speed of 80m / min; after pickling, the EPS production line is used to further remove the surface oxide scale and achieve rust prevention;

[0072] 6) Hot stamping process: The hot rolled sheet after the iron oxide scale removal treatment is heated to 900℃ in a box furnace for austenitization and kept warm for a certain period of time. The total heating time is 236s. Then it is transferred to the die for quenching to 200℃, the pressure is 14MPa, the holding time is 6s, and the transfer time is 3s. The atmosphere of the box furnace is dry compressed air, and the dew point is controlled at -25℃.

[0073] Through the hot stamping process, a 600MPa-level coating-free anti-oxidation hot-formed steel is obtained after in-die quenching; the yield strength of the obtained 600MPa-level coating-free anti-oxidation hot-formed steel after in-die quenching is 550MPa, the tensile strength is 700MPa, the total elongation is 20%, and the corresponding bending angle under the maximum load of VDA238-100 three-point bending is 146° (the thickness is 1.6mm sheet); the oxide layer thickness of the obtained 600MPa-level coating-free anti-oxidation hot-formed steel after in-die quenching is 5nm; the microstructure of the obtained 600MPa-level coating-free anti-oxidation hot-formed steel after in-die quenching is ferrite+martensite+bainite, such as Figure 1As shown, the volume fraction of ferrite is 85%, the volume fraction of martensite is 10%, and the volume fraction of bainite is 5%;

[0074] 7) Baking and coating treatment: The 600MPa-grade coating-free anti-oxidation hot-formed steel after in-mold quenching is tempered at 170°C for 20 minutes to obtain the baked and coated 600MPa-grade coating-free anti-oxidation hot-formed steel; the obtained baked and coated 600MPa-grade coating-free anti-oxidation hot-formed steel has a yield strength of 530MPa, a tensile strength of 680MPa, a total elongation of 23%, and a bending angle of 150° under the maximum load of VDA238-100 three-point bending (sheet thickness of 1.6mm); the obtained baked and coated 600MPa-grade coating-free anti-oxidation hot-formed steel has a microstructure of ferrite + martensite + bainite, wherein the volume fraction of ferrite is 85%, the volume fraction of martensite is 10%, and the volume fraction of bainite is 5%.

[0075] Example 2

[0076] In this embodiment, the short process preparation method of 600MPa-grade coating-free oxidation-resistant hot-formed steel is carried out according to the following steps:

[0077] 1) Molten steel is melted according to the set composition, and its chemical composition mass percentage is: 0.058%C, 0.8%Mn, 0.8%Si, 0.2%Al, 0.024%Ti, 0.15%(Nb+V+Mo), 2%Cr, 0.05%RE(La+Ce), 0.0068%N, 0.002%P, 0.0015%O, and the rest is Fe and unavoidable impurity elements;

[0078] During the smelting process, a low-Ca+low-Al magnesium-based composite deoxidizing alloy is used for steelmaking deoxidation. The chemical composition of the deoxidizing alloy is as follows: 60% Mg, 18% Al, 19.7% Ca, 2.3% Ba. The deoxidizing alloy is first prepared into alloy particles with a particle size of about 85 mm. When the molten steel is tapped for 35 seconds, a cored wire (the core is a deoxidizing alloy) with a weight of 3.5 kg / t Fe is added into the molten steel to deoxidize the molten steel and modify the inclusions.

[0079] 2) Control the temperature of the molten steel in the ladle to 1620°C, pour the molten steel into the tundish; control the superheat of the molten steel in the tundish to 80°C, and then flow the molten steel from the tundish into the flow ladle; finally, the molten steel flows from the flow ladle into the twin-roll thin strip continuous casting equipment, control the superheat of the molten steel on the upper surface of the molten pool between the casting rolls to 40°C, and obtain a cast strip with a thickness of 1.9 mm and a width of 1.2 m through casting and rolling under nitrogen protective atmosphere;

[0080] 3) After the cast strip is cooled to the start rolling temperature, hot rolling is performed in one pass, the start rolling temperature is 1030°C, the final rolling temperature is 950°C, and the thickness of the obtained hot rolled plate is 1.6 mm;

[0081] 4) After the hot-rolled plate leaves the hot rolling mill, it is cooled to 600°C at 20°C / s by a laminar cooling system for coiling to obtain a coiled hot-rolled plate, wherein the microstructure of the coiled hot-rolled plate consists of widmanstatten ferrite, equiaxed ferrite and martensite, wherein the volume fraction of widmanstatten ferrite is 15%, the volume fraction of equiaxed ferrite is 75%, the volume fraction of martensite is 10%, and the yield strength, tensile strength and total elongation of the coiled hot-rolled plate are 370MPa, 450MPa and 20% respectively; the oxide layer thickness of the coiled hot-rolled plate is 7μm;

[0082] 5) The hot-rolled plate is pickled after coiling: the pickling solution is hydrochloric acid, which passes through three pickling tanks with different concentrations of hydrochloric acid in sequence, and the corresponding hydrochloric acid concentrations are 60g / L, 90g / L and 120g / L respectively; the pickling temperature is adjusted to 90°C, and then the hot-rolled plate is pickled at a pickling speed of 100m / min; after pickling, the EPS production line is used to further remove the surface oxide scale and achieve rust prevention;

[0083] 6) Hot stamping process: The hot rolled sheet treated with iron oxide is heated to 935℃ in a roller hearth furnace for austenitization and kept warm for a certain period of time. The total heating time is 256s. Then it is transferred to the die and quenched to 220℃, the pressure is 20MPa, the holding time is 15s, and the transfer time is 8s. The atmosphere of the roller hearth furnace is dry compressed air, and the dew point is controlled at -5℃.

[0084] Through the hot stamping process, a 600MPa-level coating-free anti-oxidation hot-formed steel is obtained after in-die quenching; the yield strength of the obtained 600MPa-level coating-free anti-oxidation hot-formed steel after in-die quenching is 500MPa, the tensile strength is 620MPa, the total elongation is 25%, and the bending angle corresponding to the maximum load of VDA238-100 three-point bending is 115° (the thickness of the sheet is 1.6mm); the oxide layer thickness of the obtained 600MPa-level coating-free anti-oxidation hot-formed steel after in-die quenching is 20nm; the microstructure of the obtained 600MPa-level coating-free anti-oxidation hot-formed steel after in-die quenching is ferrite+martensite+bainite, wherein the volume fraction of ferrite is 80%, the volume fraction of martensite is 10%, and the volume fraction of bainite is 10%;

[0085] 7) Baking and coating treatment: The 600MPa-grade coating-free anti-oxidation hot-formed steel after in-mold quenching is tempered at 170°C for 20 minutes to obtain the baked and coated 600MPa-grade coating-free anti-oxidation hot-formed steel; the yield strength of the obtained baked and coated 600MPa-grade coating-free anti-oxidation hot-formed steel is 480MPa, the tensile strength is 600MPa, the total elongation is 28%, and the corresponding bending angle under the maximum load of VDA238-100 three-point bending is 120° (the thickness of the sheet is 1.6mm); the microstructure of the obtained baked and coated 600MPa-grade coating-free anti-oxidation hot-formed steel is ferrite + martensite + bainite, in which the volume fraction of ferrite is 80%, the volume fraction of martensite is 10%, and the volume fraction of bainite is 10%.

[0086] Example 3

[0087] In this embodiment, the short process preparation method of 600MPa-grade coating-free oxidation-resistant hot-formed steel is carried out according to the following steps:

[0088] 1) Molten steel is melted according to the set composition, and its chemical composition mass percentage is: 0.055% C, 0.6% Mn, 0.6% Si, 0.8% Al, 0.02% Ti, 0.12% (Nb + V + Mo), 2.2% Cr, 0.035% RE (La + Ce), 0.0047% N, 0.0025% P, 0.0018% O, and the rest is Fe and unavoidable impurity elements;

[0089] During the smelting process, a low-Ca+low-Al magnesium-based composite deoxidizing alloy is used for steelmaking and deoxidation. The mass percentage of the chemical composition of the deoxidizing alloy is: 56% Mg, 25% Al, 15% Ca, 4.0% Ba. The deoxidizing alloy is first prepared into alloy particles with a particle size of about 70 mm. 3.2 kg / t Fe cored wire (the core is a deoxidizing alloy) is added into the molten steel 30 seconds after the molten steel is tapped to deoxidize the molten steel and modify the inclusions.

[0090] 2) Control the temperature of the molten steel in the ladle to 1615℃, pour the molten steel into the tundish; control the superheat of the molten steel in the tundish to 70℃, and then flow the molten steel from the tundish into the flow bag; finally, the molten steel flows from the flow bag into the twin-roll thin strip continuous casting equipment, control the superheat of the molten steel on the upper surface of the molten pool between the casting rolls to 30℃, and obtain a cast strip with a thickness of 2.0mm and a width of 1.1m through casting and rolling under nitrogen protective atmosphere;

[0091] 3) After the cast strip is cooled to the start rolling temperature, hot rolling is performed in one pass, the start rolling temperature is 1010°C, the final rolling temperature is 930°C, and the thickness of the obtained hot rolled plate is 1.6 mm;

[0092] 4) After the hot-rolled plate leaves the hot rolling mill, it is cooled to 580°C at 18°C / s by a laminar cooling system for coiling to obtain a coiled hot-rolled plate, wherein the microstructure of the coiled hot-rolled plate consists of widmanstatten ferrite, equiaxed ferrite and martensite, wherein the volume fraction of widmanstatten ferrite is 12%, the volume fraction of equiaxed ferrite is 80%, the volume fraction of martensite is 8%, and the yield strength, tensile strength and total elongation of the coiled hot-rolled plate are 360MPa, 440MPa and 23% respectively; the oxide layer thickness of the coiled hot-rolled plate is 6μm;

[0093] 5) The hot-rolled plate is pickled after coiling: the pickling solution is hydrochloric acid, which passes through three pickling tanks with different concentrations of hydrochloric acid in sequence, and the corresponding hydrochloric acid concentrations are 55g / L, 85g / L and 110g / L respectively; the pickling temperature is adjusted to 85°C, and then the hot-rolled plate is pickled at a pickling speed of 90m / min; after pickling, the EPS production line is used to further remove the surface oxide scale and achieve rust prevention;

[0094] 6) Hot stamping process: The hot rolled sheet treated with iron oxide is heated to 925℃ in an induction furnace for austenitization and kept warm for a certain period of time. The total heating time is 240s. Then it is transferred to the die and quenched to 210℃, the pressure is 16MPa, the holding time is 10s, and the transfer time is 5s. The atmosphere of the induction furnace is dry compressed air, and the dew point is controlled at -15℃.

[0095] Through the hot stamping process, a 600MPa-level coating-free anti-oxidation hot-formed steel is obtained after in-die quenching; the yield strength of the obtained 600MPa-level coating-free anti-oxidation hot-formed steel after in-die quenching is 540MPa, the tensile strength is 680MPa, the total elongation is 21%, and the corresponding bending angle under the maximum load of VDA238-100 three-point bending is 126° (the thickness of the sheet is 1.6mm); the oxide layer thickness of the obtained 600MPa-level coating-free anti-oxidation hot-formed steel after in-die quenching is 15nm; the microstructure of the obtained 600MPa-level coating-free anti-oxidation hot-formed steel after in-die quenching is ferrite+martensite+bainite, wherein the volume fraction of ferrite is 82%, the volume fraction of martensite is 10%, and the volume fraction of bainite is 8%;

[0096] 7) Baking and coating treatment: The 600MPa-grade coating-free anti-oxidation hot-formed steel after in-mold quenching is tempered at 170°C for 20 minutes to obtain the baked and coated 600MPa-grade coating-free anti-oxidation hot-formed steel; the obtained baked and coated 600MPa-grade coating-free anti-oxidation hot-formed steel has a yield strength of 510MPa, a tensile strength of 650MPa, a total elongation of 26%, and a bending angle of 130° under the maximum load of VDA238-100 three-point bending (sheet thickness of 1.6mm); the obtained baked and coated 600MPa-grade coating-free anti-oxidation hot-formed steel has a microstructure of ferrite + martensite + bainite, in which the volume fraction of ferrite is 82%, the volume fraction of martensite is 10%, and the volume fraction of bainite is 8%.

[0097] The above embodiments are only used to help understand the method and core idea of ​​the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0098] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A 600MPa grade coating-free oxidation-resistant hot-formed steel, comprising, by mass percentage: C 0.035%~0.058%, Mn 0.5%~0.8%, Si 0.3%~1.0%, Al 0.1%~0.8%, Ti0.012%~0.024%, (Nb+V+Mo)0.1%~0.15%, Cr 2.0%~3.2%, (La+Ce)0.005%~0.05%, N 0.0020%~0.0081%, P≤0.0035%, O<0.0025%, and Mn+Si≤1.6%, 0.7%≤Al+Si≤1.6%, 2.8%≤Si+Cr≤3.5%, and the rest is Fe; The structure of the oxidation-resistant hot-formed steel is ferrite+martensite+bainite, wherein the volume fraction of ferrite is 80% to 85%, the volume fraction of martensite is 5% to 10%, and the volume fraction of bainite is 5% to 10%.

2. The hot-formed steel according to claim 1, characterized in that: The C content is 0.040% to 0.055%.

3. The hot-formed steel according to claim 1, characterized in that: The content of Mn+Si is 1.0% to 1.6%.

4. The hot-formed steel according to claim 1, characterized in that: 0.8%≤Al+Si≤1.4%.

5. The hot-formed steel according to claim 1, characterized in that: The content of Si is 0.6% to 0.8%, the content of Ti is 0.015% to 0.020%, (Nb+V+Mo) is 0.12% to 0.14%, Cr is 2.2% to 3.0%, (La+Ce) is 0.010% to 0.035%, and N is 0.0030% to 0.0068%.

6. The method for preparing the hot-formed steel according to claim 1, comprising the following steps: S1) preparing the raw materials according to the composition ratio, smelting the obtained mixed raw materials to obtain molten steel; during the smelting process, a magnesium-based alloy is used for deoxidation, and the magnesium-based alloy includes 55-60wt% Mg, 18-25wt% Al, 15-22wt% Ca, and 2.3-4.0wt% Ba; S2) pouring the molten steel into a tundish and a flow ladle in sequence and then into a twin-roller thin strip continuous casting device for continuous casting to obtain a cast strip; S3) cooling the cast strip and then hot rolling it once, and then laminar cooling the obtained hot-rolled plate and then coiling it to obtain the coiled hot-rolled plate; S4) removing the iron oxide scale from the hot rolled plate after coiling and then hot stamping to obtain 600MPa-grade coating-free oxidation-resistant hot-formed steel; The structure of the oxidation-resistant hot-formed steel is ferrite+martensite+bainite, wherein the volume fraction of ferrite is 80% to 85%, the volume fraction of martensite is 5% to 10%, and the volume fraction of bainite is 5% to 10%.

7. The preparation method according to claim 6, characterized in that: In step S1), the magnesium-based alloy is added in the form of iron cored wire, the addition time is 25 to 35 seconds after the molten steel is tapped, and the amount of the iron cored wire added is 3.0 to 4.0 kg / t; In step S2), the temperature of the molten steel is 1600-1620°C, and O in the molten steel is less than 0.0025%; the superheat of the molten steel in the tundish is 60-80°C, and the superheat of the molten steel on the upper surface of the molten pool between the casting rollers in the twin-roll thin strip continuous casting equipment is 10-40°C. The continuous casting is carried out under a protective atmosphere, and the thickness of the cast strip is 1.9-2.1 mm and the width is 1-1.2 m.

8. The preparation method according to claim 6, characterized in that: In step S3), the hot rolling start temperature is 1000-1050°C, the final rolling temperature is 910-950°C, the thickness of the hot rolled plate is 1.0-1.7 mm; the laminar cooling rate is 15-20°C / s, the temperature is 550-600°C; the oxide layer thickness of the hot rolled plate after coiling is 5-7 μm; In step S4), the method of removing the iron oxide scale is selected from one or both of pickling treatment and EPS production line treatment; the pickling passes through three pickling tanks in sequence, the acid concentrations are 50-60g / L, 80-90g / L, and 100-120g / L respectively, the pickling temperature is 80-90°C, and the pickling speed is 80-100m / min; the EPS production line treatment is carried out using recycled harmless medium water and steel grit.

9. The preparation method according to claim 6, characterized in that: In step S4), the hot stamping is specifically: The hot rolled plate after removing the iron oxide scale is heated to 900-950°C in a heating furnace, and the total time = (the thickness of the hot rolled plate after removing the iron oxide scale * 110 + 60-80) s, and then transferred to the mold in 3-10 s and quenched to 200-220°C, the pressure is 14-20 MPa, and the pressure is maintained for 6-15 s; the atmosphere of the heating furnace is dry compressed air, nitrogen or argon, and the dew point is -25--5°C; The oxide layer thickness of the 600MPa-grade coating-free hot-formed steel is 5 to 20nm.

10. The preparation method according to any one of claims 6 to 9, characterized in that: The hot stamping further includes tempering, wherein the tempering temperature is 150-200° C. and the time is 10-30 minutes.

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