Production method of easily formed super high strength steel plate for engineering machinery

Abstract

This invention discloses a method for producing easily formable ultra-high strength steel plates for engineering machinery, belonging to the field of metal material processing technology. This invention employs a medium-carbon + multi-element microalloying system, and through converter steelmaking + refining, continuous casting, billet heating, two-stage controlled rolling, and quenching + tempering heat treatment, produces ultra-high strength steel plates with a thickness of 5-20 mm. The resulting steel plate has a matrix structure of ultra-fine-grained lath martensite with an effective grain boundary size of 1.2-1.5 μm, a yield strength ≥1500 MPa, a tensile strength ≥1800 MPa, an elongation >10%, an impact energy >50 J at -40℃, and meets the requirements of D=6a and 180° bending without cracking. It possesses ultra-high strength, excellent low-temperature toughness, and cold formability, and can be directly used for forming key load-bearing structural components of engineering machinery. The process is simple and highly adaptable to industrial applications.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal material processing, and relates to a production method of an easy-to-form super-high-strength steel plate for engineering machinery. BACKGROUND

[0002] Low-alloy high-strength structural steel is a core raw material for engineering machinery. The high-strength structural steel widely used in the current industry has a yield strength of 1100 MPa or below. Although the high-strength structural steel has good plastic forming property and weldability, it cannot fully meet the development needs of lightweight engineering machinery. Under this background, low-alloy super-high-strength steel with a yield strength of 1300 MPa or above has become an important development direction of high-performance structural materials due to its advantages of super-high strength, moderate plasticity and toughness, and low cost.

[0003] However, the increase in strength is often accompanied by a decrease in plasticity and toughness, which not only restricts the forming property of the steel plate, but also reduces the service safety of the engineering structure. Therefore, it is a core problem in the research and development of super-high-strength structural steel to realize super-high yield strength while maintaining excellent plasticity and toughness. It is of great significance to develop a super-high-strength steel plate with a yield strength of 1500 MPa or above, excellent cold formability, and low-temperature toughness by precise composition design and rolling and heat treatment process control. SUMMARY

[0004] The purpose of the present application is to provide a production method of an easy-to-form high-strength high-toughness super-high-strength steel plate for engineering machinery. Through composition design and process control, a yield strength of 1500 MPa or above and a tensile strength of 1800 MPa or above are realized, and the performance requirements of D=6a cold bending without cracking and impact energy of-40℃>50J are met.

[0005] To achieve the above purpose, the technical solution adopted by the present application is as follows: A production method of an easy-to-form super-high-strength steel plate for engineering machinery, the chemical composition of the continuous casting billet is C=0.20%-0.30%, Si=0.20%-0.40%, Mn=0.80%-1.20%, S≤0.003%, P≤0.010%, Cr=0.30%-0.60%, Ni=0.60%-1.00%, Mo=0.45%-0.65%, W=0.20%-0.50%, V=0.03%-0.07%, B=0.001%-0.003%, and the balance is iron and unavoidable impurities; the thickness of the finished steel plate is 5-20mm; comprising the following process steps: 1) Smelting and continuous casting: adopting converter steelmaking and refining according to the chemical composition to form a casting billet.

[0006] 2) Heating: the heating temperature of the casting billet is 1200-1250℃, and the casting billet is kept at the heating temperature for ≥120min after the core of the casting billet reaches the heating temperature.

[0007] 3) Rolling: Rolling adopts two-stage controlled rolling. The reduction rate of the last three passes in the roughing stage is ≥20%, the starting temperature of the finishing rolling is 800~850℃, and the cumulative reduction rate in the finishing rolling stage is ≥60%.

[0008] 4) Heat treatment: Quenching treatment, quenching temperature is 850~900℃, holding time is 20~50min, and water cooling to room temperature after holding; Tempering treatment, tempering temperature is 160~260℃, holding time is 30~120min, and air cooling to room temperature after holding. The resulting ultra-high strength steel plate has a matrix structure of ultra-fine lath martensite with an effective grain boundary size of 1.2~1.5μm. When cold-bent to D=6a, it does not crack when bent at 180°.

[0009] Furthermore, in step 1), the billet thickness is 160~300mm, and the casting temperature is 5~20℃ above the liquidus temperature; Furthermore, in step 2), the holding time for the core of the billet after it reaches the set temperature is 120~180 minutes; Furthermore, in step 3), the cumulative reduction rate during the finishing rolling stage is ≥80%, and large reduction rolling is adopted to refine the original austenite grain size; Furthermore, in step 4), the tempering temperature is 200~220℃ and the holding time is 60~120min to achieve the best match between the strength and toughness of the steel plate. Furthermore, the finished steel plate has a yield strength of ≥1500MPa, a tensile strength of ≥1800MPa, an elongation of >10%, and an impact energy of >50J at -40℃.

[0010] Compared with the prior art, the technical progress achieved by this invention is as follows: 1) By adopting a medium carbon + multi-element microalloying system, and through reasonable composition design combined with large reduction controlled rolling, low temperature precision rolling and precise heat treatment, an ultra-fine grain lath martensitic structure is obtained, which gives full play to the strengthening effect of fine grains. While ensuring ultra-high strength and excellent toughness, the cold bending performance meets D=6a, and it does not crack when bent at 180°. It can be directly used for forming complex structural parts of engineering machinery.

[0011] 2) The process of this invention is simple, highly adaptable to industrialization, and has a short production cycle. The thickness of the finished product can reach 5~20mm, which falls into the category of medium and heavy plates, and has a wider range of applications. Attached Figure Description

[0012] Figure 1 This is a grain orientation diagram of the steel plate in Example 1.

[0013] Figure 2 This is a diagram showing the effective grain boundary size distribution of the steel plate in Example 1. Detailed Implementation

[0014] The present invention will be further described below with reference to specific embodiments. Example 1

[0015] A method for producing ultra-high strength steel plates for easily formable engineering machinery, wherein the chemical composition of the steel, by weight percentage, is C=0.26%, Si=0.30%, Mn=1.10%, Cr=0.36%, Ni=0.92%, Mo=0.48%, W=0.39%, V=0.057%, B=0.0018%, with the balance being Fe and unavoidable impurity elements. The process steps include: 1) Smelting and continuous casting: Steelmaking and refining are carried out in a converter according to the above composition. During smelting, the content of harmful elements S and P is strictly controlled, with S=0.002% and P=0.010%; casting is used to form a 220mm billet.

[0016] 2) Heating: Heat the billet to 1220℃, and after the core of the billet reaches 1200℃, start holding it at that temperature for 125 minutes.

[0017] 3) Rolling: The rolling process adopts a two-stage controlled rolling process. The reduction rates of the last three passes in the roughing stage are 20.6%, 24% and 31.6% respectively. The starting temperature of the finishing stage is 820℃. The finishing stage has a large reduction amount, and the cumulative reduction rate is 87.5%.

[0018] 4) Heat treatment: Quenching treatment, quenching temperature is 860℃, holding time is 25min, and water cooling to room temperature is performed after holding; Tempering treatment, tempering temperature is 220℃, holding time is 36min, and air cooling to room temperature is performed after holding.

[0019] The ultra-high strength steel plate with a thickness of 10mm produced in Example 1 has the following mechanical property test results: yield strength of 1524MPa, tensile strength of 1893MPa, elongation of 11.1%, impact energy at -40℃ of 62J, effective grain boundary size of 1.21 μm, and cold bending of 180° without cracking under the condition of bending indenter diameter D=6a. Example 2

[0020] A method for producing ultra-high strength steel plates for easily formable engineering machinery, wherein the chemical composition of the steel, by weight percentage, is C=0.24%, Si=0.24%, Mn=0.82%, Cr=0.43%, Ni=0.65%, Mo=0.45%, W=0.24%, V=0.039%, B=0.0012%, with the balance being Fe and unavoidable impurity elements. The process steps include: 1) Smelting and continuous casting: Steelmaking and refining are carried out in a converter according to the above composition. During smelting, the content of harmful elements S and P is strictly controlled, with S=0.002% and P=0.008%; casting is used to form 180mm billets.

[0021] 2) Heating: Heat the billet to 1220℃, and after the core of the billet reaches 1200℃, start holding it at that temperature for 124 minutes.

[0022] 3) Rolling: The rolling process adopts a two-stage controlled rolling process. The reduction rates of the last three passes in the roughing stage are 20.7%, 23.8% and 32.6%, respectively. The starting temperature of the finishing rolling is 850℃. The finishing rolling stage has a large reduction amount, with a cumulative reduction rate of 88%.

[0023] 4) Heat treatment: Quenching treatment, quenching temperature is 850℃, holding time is 20min, and water cooling to room temperature is performed after holding; Tempering treatment, tempering temperature is 220℃, holding time is 31min, and air cooling to room temperature is performed after holding.

[0024] The ultra-high strength steel plate with a thickness of 6mm produced in Example 2 has the following mechanical property test results: yield strength of 1514MPa, tensile strength of 1875MPa, elongation of 10.3%, impact energy of 59J at -40℃, and effective grain boundary size of 1.22μm. It can achieve cold bending of 180° without cracking under the condition of bending indenter diameter D=6a. Example 3

[0025] A method for producing ultra-high strength steel plates for easily formable engineering machinery, wherein the chemical composition by weight percentage is C=0.26%, Si=0.35%, Mn=1.14%, Cr=0.56%, Ni=0.26%. ： 0.92%, Mo=0.62%, W=0.43%, V=0.058%, B=0.0016%, balance being Fe and unavoidable impurity elements. The process steps include: 1) Smelting and continuous casting: Steelmaking and refining are carried out using a converter. According to the above composition, the content of harmful elements S and P is strictly controlled during smelting, with S=0.001% and P=0.009%; casting is used to form 300mm billets.

[0026] 2) Heating: Heat the billet to 1220℃, and after the core of the billet reaches 1200℃, start holding it at that temperature for 128 minutes.

[0027] 3) Rolling: Two-stage controlled rolling is adopted. The reduction rates of the last three passes in the roughing stage are 21.5%, 23.6% and 32.5% respectively. The starting temperature of the finishing rolling is 800℃. The finishing rolling stage has a large reduction amount, and the cumulative reduction rate is 87.5%.

[0028] 4) Heat treatment: After rolling, offline heat treatment is carried out. The quenching temperature is 860℃ and the holding time is 34 min. The tempering temperature is 200℃ and the holding time is 60 min.

[0029] Example 3 produced an ultra-high strength steel plate with a thickness of 20mm. The mechanical property test results were as follows: yield strength of 1518MPa, tensile strength of 1863MPa, elongation of 10.7%, impact energy at -40℃ of 78J, effective grain boundary size of 1.36μm, and cold bending of 180° without cracking under the condition of bending indenter diameter D=6a.

Claims

1. A method for producing ultra-high strength steel plates for easily formable engineering machinery, characterized in that: The chemical composition of the continuously cast billet by weight percentage is C=0.20%-0.30%, Si=0.20%-0.40%, Mn=0.80%-1.20%, S≤0.003%, P≤0.010%, Cr=0.30%-0.60%, Ni=0.60%-1.00%, Mo=0.45%-0.65%, W=0.20%~0.50%, V=0.03%-0.07%, B=0.001%-0.003%, with the balance being iron and unavoidable impurities; Finished steel plate thickness 5~20mm; The process includes the following steps: 1) Smelting and continuous casting: Steel is produced and refined in a converter according to the chemical composition, and then cast into a billet; 2) Heating: The billet is heated to 1200~1250℃. After the core of the billet reaches the heating temperature, it is held at the temperature for ≥120min. 3) Rolling: Rolling adopts a two-stage controlled rolling process. The reduction rate of the last three passes in the roughing stage is ≥20%, the starting temperature of the finishing rolling is 800~850℃, and the cumulative reduction rate in the finishing rolling stage is ≥60%. 4) Heat treatment: Quenching treatment, quenching temperature is 850~900℃, holding time is 20~50min, and water cooling to room temperature after holding; Tempering treatment, tempering temperature is 160~260℃, holding time is 30~120min, and air cooling to room temperature after holding. The resulting ultra-high strength steel plate has a matrix structure of ultra-fine lath martensite with an effective grain boundary size of 1.2~1.5μm. When cold-bent to D=6a, it does not crack when bent at 180°.

2. The method for producing an easily formable ultra-high strength steel plate for engineering machinery according to claim 1, characterized in that: In step 1), the thickness of the billet is 160~300mm, and the casting temperature is 5~20℃ above the liquidus temperature.

3. The method for producing an easily formable ultra-high strength steel plate for engineering machinery according to claim 1, characterized in that: In step 2), the holding time after the core of the billet reaches the desired temperature is 120~180 minutes.

4. The production method according to claim 1, characterized in that: In step 3), the cumulative reduction rate in the finishing rolling stage is ≥80%, and large reduction rolling is used to refine the original austenite grain size.

5. The method for producing an easily formable ultra-high strength steel plate for engineering machinery according to claim 1, characterized in that: In step 4), the tempering temperature is 200~220℃ and the holding time is 60~120min.

6. The method for producing an easily formable ultra-high strength steel plate for engineering machinery according to claim 1, characterized in that: The finished steel plate has a yield strength of ≥1500MPa, a tensile strength of ≥1800MPa, an elongation of >10%, and an impact energy of >50J at -40℃.

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