CN116875895B - A heat-treatment-free 1350MPa-grade hot-rolled steel plate for automobile upper body and production method thereof - Google Patents

Abstract

The invention discloses a heat-treatment-free 1350MPa-grade hot-rolled steel plate for automobile upper parts and a production method, belonging to the technical field of metal materials. The chemical composition and mass percentage of the hot-rolled steel plate of the invention are as follows: C: 0.12%-0.19%, Si: 0.10%-0.40%, Mn: 1.00%-1.57%, S: 0.009%-0.022%, Al: 0.020%-0.062%, Ti: 0.020%-0.055%, V: 0.020%-0.065%, Cr: 0.10%-0.25%, B: 0.0015%-0.0033%, Mo: 0.10%-0.16%, Cu: 0.25%-0.55%, Ni: 0.20%-0.30%, Bi: 0.010%-0.055%, Sb: 0.035%-0.060%, rare earth: 0.030%-0.053%, Ca: 0.004%-0.018%, P≤0.018%, N≤0.005%, the balance is Fe and inevitable impurities, through smelting, refining, continuous casting, heating, hot rolling, rapid cooling, coiling and hood slow cooling process, the obtained hot rolled steel plate is completely suitable for dump truck and modified vehicle loading steel.

Description

A heat-treatment-free 1350MPa-grade hot-rolled steel plate for automobile upper body and production method thereof

Technical Field

The invention belongs to the technical field of metal materials, and particularly relates to a heat-treatment-free 1350MPa-grade hot-rolled steel plate for automobile upper parts and a production method thereof.

Background Art

The automobile industry, steel industry, petrochemical industry and construction industry are the four pillar industries of the national economy. The scale of the automobile industry and the steel industry is often a sign of a country's industrial development. The world's major industrially developed countries regard the automobile industry as a pillar industry of the national economy. At the same time, the automobile industry is a major user of steel products. The research and development and application of steel products are closely related to the development trend of the automobile industry. In today's society, the automobile industry is developing in the direction of low cost, high reinforcement, green environmental protection and high safety, and the demand for hot-rolled high-strength steel is increasing.

The Chinese patent application with publication number CN114737136A discloses a method for producing high-strength and high-toughness hot-rolled thin steel plates with a Brinell hardness of 400 HBW. The method uses a common C-Mn component system to add a certain amount of Nb, Ti, Cr, B and other elements, and improves the strength through solid solution strengthening of carbon and manganese elements and fine grain strengthening of niobium and titanium elements in combination with subsequent offline heat treatment. The production process is complicated, and the elongation of the steel plate after fracture is ≥12%, which does not meet the requirements for difficult-to-form steel for automobile body parts, and does not have machinability and corrosion resistance. The Chinese patent application with publication number CN114672731A discloses a method for producing a high-strength and high-toughness hot-rolled thin steel plate with a Brinell hardness of 360 HBW. The method uses a common C-Mn component system to add a certain amount of Nb, Ti, Cr, B and other elements, and improves the strength through solid solution strengthening of carbon and manganese elements and fine grain strengthening of niobium and titanium elements and subsequent offline heat treatment. The production process is complicated, and the elongation of the steel plate is ≥14%, which does not meet the requirements of difficult-to-form automobile upper body steel, and does not have easy cutting and corrosion resistance. Therefore, research and development of a method for producing a 1350MPa grade hot-rolled steel plate with a simple production process and meeting the requirements of automobile upper body steel has become an important topic that needs to be studied urgently.

Summary of the invention

In view of the development demand of automobile steel, the purpose of the present invention is to provide a heat-treatment-free 1350MPa grade hot-rolled steel plate for automobile body and a production method. The yield strength of the obtained hot-rolled steel plate is ≥1050MPa, the tensile strength is ≥1350MPa, the transverse elongation _A50 is ≥19%, the Brinell hardness HBW is ≥402, the thickness is ≥2mm, the finished steel plate with a thickness of 6.0-10.0mm has an impact energy of ≥65J at -40℃, and the structure is martensite.

The object of the present invention is to achieve the following:

The present invention provides a heat-treatment-free 1350MPa grade hot-rolled steel plate for automobile upper parts. The chemical composition and mass percentage of the hot-rolled steel plate are as follows: C: 0.12%-0.19%, Si: 0.10%-0.40%, Mn: 1.00%-1.57%, S: 0.009%-0.022%, Al: 0.020%-0.062%, Ti: 0.020%-0.055%, V: 0.020%-0.065%, Cr: 0.10%- 0.25%, B: 0.0015%-0.0033%, Mo: 0.10%-0.16%, Cu: 0.25%-0.55%, Ni: 0.20%-0.30%, Bi: 0.010%-0.055%, Sb: 0.035%-0.060%, rare earth: 0.030%-0.053%, Ca: 0.004%-0.018%, P≤0.018%, N≤0.005%, the balance is Fe and unavoidable impurities.

Based on the above technical solution, further, the rare earth is La+Ce.

The mechanism of action of each alloy component in the hot-rolled steel sheet of the present invention is as follows:

C: Carbon is mainly used to form the required amount of martensite and ensure the strength of steel. It is also the most important solid solution strengthening element in steel. The increase of carbon element is beneficial to increase the hardenability of steel. The optimal range of carbon in the present invention is 0.12%-0.19%.

Si: Silicon is a solid solution strengthening element, which can improve the strength of the steel plate through solid solution strengthening. It also has the effect of improving the hardenability of the steel plate and promoting the enrichment of carbon into the untransformed austenite, thereby improving the stability of the austenite. The content of silicon in the present invention is 0.10%-0.40%.

Mn: Manganese can form substitutional solid solution in steel, play a strong solid solution strengthening role, and increase the yield strength and tensile strength linearly. The content of this element increases the strength of steel within a certain range without reducing the plasticity and toughness of steel. It is also an austenite stabilizing element. It can also improve the hardenability of steel and significantly delay the pearlite transformation. However, if the manganese content is too high, the carbon equivalent of steel will increase, and the uniformity of the steel plate structure will deteriorate during the smelting and hot rolling process, which is easy to cause serious banded structure defects in the structure. Therefore, the manganese content is selected to be 1.00%-1.57%.

P: Phosphorus can increase the formation temperature of α phase, expand the temperature range of α phase, delay the formation of cementite, and increase the stability of austenite; at the same time, phosphorus significantly changes the morphology of martensite islands, making the size of martensite islands smaller and more evenly distributed. However, excessive phosphorus content will deteriorate the processability of steel plates. In order to obtain a higher elongation, the phosphorus content is selected to be ≤0.018%.

S: Sulfur can form sulfide inclusions such as MnS in steel. Such inclusions can interrupt the continuity of the base metal, causing the chips to form a small and short curling radius during cutting, which is easy to remove, reducing tool wear, reducing the roughness of the processed surface, and increasing the tool life, achieving easy cutting. Generally, the machinability of steel increases with the increase of sulfur content in the steel. However, when the sulfur content in the steel is too high, it will cause hot brittleness, making it difficult to hot process the steel and deteriorating the mechanical properties of the steel. Therefore, the sulfur content in the present invention is controlled to be 0.009%-0.022%.

Al: Aluminum is a commonly used deoxidizer in steel. It plays a role in deoxidation and nitrogen determination during the smelting process and can effectively improve the anti-oxidation performance of the steel plate. Adding a certain amount of Al to the steel can form AlN precipitation, which plays a certain role in refining the grains. Therefore, in the present invention, the Al content is limited to 0.020%-0.062%.

Ti: Titanium can effectively delay the recrystallization of deformed austenite, prevent austenite grain growth, increase the austenite recrystallization temperature, refine the grains, and improve the strength and toughness of steel. Moreover, Ti is a strong carbon and nitride forming element, which can combine with carbon and nitrogen to form stable and fine carbon and nitride, playing a significant precipitation strengthening role, and can also inhibit BN precipitation, avoiding the strength reduction caused by B precipitation. Therefore, the titanium content of the present invention is selected to be 0.020%-0.055%.

V: Vanadium has significant precipitation strengthening and fine grain strengthening effects. The effect of vanadium is mainly achieved by forming precipitates with carbon and nitrogen. In particular, the precipitation of VN formed with nitrogen can greatly improve the strength of the steel plate, and can inhibit the precipitation of BN, avoiding the reduction of strength caused by the precipitation of B. In addition, the addition of V can also be combined with H to improve the delayed fracture resistance of the steel plate. In the present invention, the addition amount of V is 0.020%-0.065%.

Cr: Chromium is a carbide-forming element that can delay pearlite transformation and improve the hardenability of steel, thereby facilitating the formation of martensite structure and refining the structure to achieve a strengthening effect. Too high a chromium content will deteriorate the processing and formability of the material. The principle of selecting the chromium content is to promote the formation of martensite, so the chromium content of the present invention is selected to be 0.10%-0.25%.

B: Boron can significantly improve the hardenability of steel and is beneficial to the formation of martensite structure. When the B content is higher than 0.0050%, the excess B forms B compounds with N in the steel, reducing the performance of the steel plate. Therefore, the boron content of the present invention is in the range of 0.0015%-0.0033%.

Mo: Mo is a carbide-forming element that can improve the strength and toughness of steel plates. Mo can significantly improve austenite stability, increase steel hardenability, and facilitate the formation of martensite structure. Therefore, the Mo content of the present invention is selected to be 0.10%-0.16%.

Cu: Copper can expand the austenite phase, improve the weather resistance of steel, improve the weldability of steel, and effectively improve the corrosion resistance of steel. However, when the Cu content is high, it causes hot brittleness and deteriorates the surface performance of the steel plate. In addition, at a certain Cu content, it is beneficial to the strength and hot workability of the steel plate, effectively reduces the tendency of hot rolling edge cracking of the steel plate, and significantly improves the surface quality of the steel plate. Cu also has the effect of reducing work hardening and improving the plasticity of the steel plate. Therefore, the copper content of the present invention is selected between 0.25% and 0.55%.

Ni: Nickel has no adverse effect on the hardenability and toughness of the welding heat affected zone of steel, and can improve the corrosion resistance and hardenability of steel, which is beneficial to the formation of martensitic structure. Therefore, the Ni content in the steel of the present invention is selected to be 0.20%-0.30%.

Bi: Bismuth is mainly distributed in the grain boundaries and inside the grains in steel, which improves the strength of the steel plate, reduces the diffusion rate of carbon, oxygen and other elements on the grain boundaries, reduces decarburization and oxidation, and improves the surface and mechanical properties of the steel plate. Therefore, the present invention limits the Bi content to 0.010%-0.055%.

Sb: Antimony is mainly distributed in the grain boundaries and grain interiors in steel, which improves the strength of the steel plate, reduces the diffusion rate of elements such as carbon and oxygen on the grain boundaries, reduces decarburization and oxidation, and improves the surface and mechanical properties of the steel plate. The addition of antimony will cause a uniform and dense oxide film (rich in Sb, Cu, Cr and other elements) to form on the surface of the steel plate matrix, which can effectively prevent air moisture and oxygen from continuing to enter the matrix to form a corrosive internal environment, improve the corrosion resistance of the matrix, and enhance the corrosion resistance of the steel plate. If the Sb content is too low, the passivation film will be unevenly dispersed and the overall anti-corrosion effect will not be achieved; when it exceeds the design upper limit, it has an anti-corrosion effect, but significantly reduces the hot working performance. Therefore, the present invention limits the Sb content to 0.035% to 0.060%.

Rare earth: Rare earth RE has strong deoxidation and desulfurization capabilities. The spherical sulfide or sulfur oxide formed replaces the long strip manganese sulfide inclusions, which can improve the plasticity and anisotropy of the steel plate. Rare earth can improve the fatigue performance of the steel plate, improve the welding performance of the steel plate, and improve the cold forming performance of the steel plate. Rare earth has a strong affinity with other impurity elements in steel, which can reduce the content of sulfur, oxygen, phosphorus, hydrogen and other elements in the steel and eliminate their harmful effects. Therefore, the present invention limits the content of rare earth RE to 0.030%-0.053%.

Ca: Calcium can change the form of steel sulfide (MnS), prevent the formation of long strips of MnS inclusions, and improve the plasticity, toughness and fatigue performance of the steel plate. Therefore, the present invention controls the Ca content to 0.004%-0.018%.

N: Nitrogen is an impurity element in the present invention, and the smaller its content, the better. For B-containing steel, the lower the N content, the better. However, too low a content will lead to production difficulties and increase costs. Therefore, the N content in the present invention is ≤0.005%.

Based on the above technical solution, further, the microstructure of the hot-rolled steel plate is martensite, the yield strength is ≥1050MPa, the tensile strength is ≥1350MPa, the transverse elongation A ₅₀ is ≥19%, and the Brinell hardness HBW is ≥402.

The present invention also provides a method for producing the above-mentioned heat-treatment-free 1350MPa grade hot-rolled steel plate for automobile upper body, which mainly includes smelting, refining, continuous casting, heating, hot rolling, rapid cooling, coiling and hood slow cooling process; wherein,

During the heating process, the heating temperature is 1310-1350°C and the holding time is 166-192min; appropriate heating temperature and suitable holding time can make the alloy elements in the slab completely dissolved, the slab composition uniform, and play a role in controlling the original austenite grain size.

The hot rolling process is divided into two stages: rough rolling and finishing rolling. The rough rolling adopts a 3+3 rolling process (R1 adopts 3 passes, R2 adopts 3 passes), with a total of 6 passes, and the rough rolling outlet temperature is 1115-1180°C. The finishing rolling adopts 7-stand continuous rolling, the finishing rolling inlet temperature is 1080-1115°C, and the final rolling temperature is 840-952°C.

The cooling rate of the rapid cooling process is 155-175°C/s.

Based on the above technical solution, further, the refining adopts LF+RH process to control H≤0.0002%, O≤0.0015%, and calcium treatment is performed during the refining process to ensure w(Ca)/w(Al)＝0.09-0.14.

Based on the above technical solution, electromagnetic stirring and light reduction technology are further used in the continuous casting process, the billet pulling speed is ≤1.1m/min, and the billets are stacked and slowly cooled after they come off the line.

Based on the above technical solution, further, the heating process is always dynamically controlled in a slightly positive pressure state, and the positive pressure value is controlled at 3 to 15Pa.

Based on the above technical solution, further, during the hot rolling process, the intermediate billet thickness is 45 to 68 mm, and an insulation cover is used to keep the intermediate billet warm before entering the finishing mill to reduce the temperature drop of the intermediate billet on the delay roller and the temperature difference between the head and tail and the plate width direction.

Based on the above technical solution, high-pressure water descaling is further performed before finish rolling.

Based on the above technical solution, further, the coiling temperature in the coiling process is 88-123°C. The purpose of ultra-fast cooling to the coiling temperature is to cool to the martensite zone at a cooling rate greater than the critical cooling rate for the transformation of pearlite and bainite, avoiding the pearlite and bainite formation zone, while inhibiting grain growth, the martensite content is also guaranteed, thereby refining the martensite grains.

Based on the above technical solution, further, the slow cooling cover temperature is 150-255°C, and the insulation time is 30-60min.

Based on the above technical solution, further, after the slow cooling is completed, the steel coil is taken out and air-cooled to room temperature.

The beneficial effects of the present invention compared with the prior art are as follows:

The present invention cooperates with the composition design and the process innovation to prepare the heat-treatment-free 1350MPa-grade hot-rolled steel plate for automobile body, which has excellent mechanical properties, yield strength ≥1050MPa, tensile strength ≥1350MPa, transverse elongation _A50 ≥19%, Brinell hardness HBW ≥402, thickness ≥2mm, the finished steel plate with a thickness of 6.0-10.0mm has an impact energy of more than 65J at -40°C, and the structure is 100% martensite. The steel plate of the present invention is completely suitable for steel for dump trucks and modified vehicles.

DETAILED DESCRIPTION

The present invention is described in detail below with reference to the embodiments, but the embodiments of the present invention are not limited thereto. Obviously, the embodiments described below are only partial embodiments of the present invention. For those skilled in the art, other similar embodiments obtained without creative labor all fall within the protection scope of the present invention.

Examples 1-6

This embodiment provides a method for producing a heat-treatment-free 1350MPa grade hot-rolled steel plate for automobile upper parts. The chemical composition and weight percentage of the hot-rolled steel plate in the embodiment are shown in Table 1;

Table 1 Chemical composition of hot-rolled steel sheets of the present invention (wt, %)

The following steps are involved:

(1) Smelting and refining: LF+RH process is adopted, H and O contents are strictly controlled, H≤0.0002%, O≤0.0015%, and calcium treatment is performed in the refining process to ensure w(Ca)/w(Al)＝0.09-0.14;

(2) Continuous casting: Electromagnetic stirring and soft pressure reduction technology are used in the continuous casting process, and the billet pulling speed is ≤1.1m/min. The billets are stacked and slowly cooled after they come off the line;

(3) Heating: The continuous casting slab is sent to a walking beam heating furnace for heating. The heating temperature and holding time parameters are shown in Table 2. The furnace pressure is always dynamically controlled in a slightly positive pressure state, and the positive pressure value is controlled at 3-15 Pa to reduce oxidation and burning.

(4) Hot rolling: Hot rolling is divided into two stages: rough rolling and finishing rolling. Rough rolling adopts a 3+3 rolling process (R1 adopts 3 passes, R2 adopts 3 passes), with a total of 6 passes. The rough rolling outlet temperature is the parameter shown in Table 2. The intermediate billet thickness is 45-68 mm, and the width is 1510-1850 mm. The intermediate billet is kept warm by a heat preservation cover before entering the hot rolling and finishing mill to reduce the temperature drop of the intermediate billet on the delay roller and the temperature difference between the head and tail and the plate width direction. Finishing rolling is a 7-stand continuous rolling. High-pressure water descaling is performed before finishing rolling. The parameters of finishing rolling inlet temperature and final rolling temperature are shown in Table 2.

(5) Rapid cooling: After final rolling, ultra-fast cooling mode is adopted. The ultra-fast cooling rate parameters are shown in Table 2;

(6) Coiling: Coil the steel plate when it is cooled to the coiling temperature (see Table 2);

(7) Slow cooling in the hood: After coiling, the steel coil is immediately placed in a heated slow cooling hood. The temperature and insulation time of the slow cooling hood are shown in Table 2. The steel coil is taken out and air cooled to room temperature.

Table 2 Production process parameters of Examples 1-6

The convexity control accuracy of the steel plate produced by the above production method is ±40μm, the flatness is controlled within 18I, the thickness control accuracy is ±40μm, the finished steel plate thickness, the steel plate performance and the tissue volume percentage are shown in Table 3.

Table 3 Mechanical properties parameters and tissue volume percentage of the steel plates of Examples 1-6

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims (6)

1. A heat treatment-free 1350 MPa-grade hot rolled steel plate for automobile upper mounting is characterized in that the hot rolled steel plate comprises the following ：C：0.12%-0.19%、Si：0.10%-0.40%、Mn：1.00%-1.57 %、S：0.009%-0.022%、Al：0.020%-0.062%、Ti：0.020%-0.055%、V：0.020%-0.065%、Cr:0.10%-0.25%、B:0.0015%-0.0033%、Mo：0.10%-0.16%、Cu：0.25%-0.55%、Ni：0.20%-0.30%、Bi:0.010%-0.055%、Sb：0.035%-0.060%、 rare earth 0.030-0.053% by mass percent: 0.004% -0.018%, P is less than or equal to 0.018%, N is less than or equal to 0.005%, and the balance is Fe and unavoidable impurities;

the structure of the hot rolled steel plate is martensite, the yield strength is more than or equal to 1050MPa, the tensile strength is more than or equal to 1350MPa, the transverse elongation A ₅₀ is more than or equal to 19%, and the Brinell hardness HBW is more than or equal to 402;

The production method of the hot rolled steel plate for the automobile upper mounting of 1350MPa without heat treatment mainly comprises the technical processes of smelting, refining, continuous casting, heating, hot rolling, quick cooling, coiling and cover feeding slow cooling; wherein,

Heating temperature is 1310-1350 ℃ and heat preservation time is 166-192 min in the heating process;

The hot rolling process is divided into two stages of rough rolling and finish rolling, wherein the outlet temperature of rough rolling is 1115-1180 ℃, the finish rolling adopts continuous rolling, the inlet temperature of finish rolling is less than or equal to 1115 ℃, and the finish rolling temperature is 840-952 ℃;

The cooling rate of the rapid cooling process is more than or equal to 155 ℃/s;

the coiling temperature in the coiling process is less than or equal to 123 ℃;

Immediately after coiling, the coiled material is put into a slow cooling cover with heating, the temperature of the slow cooling cover is 150-255 ℃, and the heat preservation time is 30-60min.

2. The hot rolled steel sheet as claimed in claim 1 wherein the rare earth is La+Ce.

3. The hot rolled steel sheet according to claim 1, wherein the refining is performed by an lf+rh process, wherein H is 0.0002% or less, O is 0.0015% or less, and calcium treatment is performed during the refining process to ensure w (Ca)/w (Al) =0.09 to 0.14.

4. The hot rolled steel sheet according to claim 1, wherein the continuous casting process adopts electromagnetic stirring and soft reduction technology, the casting blank pulling speed is less than or equal to 1.1m/min, and the casting blank is stacked and slowly cooled after being taken off line.

5. The hot rolled steel sheet as claimed in claim 1, wherein the heating process is dynamically controlled at a micro positive pressure state at all times, and the positive pressure value is controlled at 3 to 15Pa.

6. The hot rolled steel sheet according to claim 1, wherein the thickness of the intermediate billet in the hot rolling process is 45-68 mm, and the intermediate billet is insulated by an insulation cover before entering the finishing mill group; and (5) removing scales by high-pressure water before finish rolling.