A production method of low alloy cost high strength z-direction steel by direct quenching and tempering

Abstract

The application discloses a production method of low-alloy cost high-strength Z-direction steel direct quenching+tempering, which comprises the following steps: molten steel smelting and continuous casting; heating and rolling: the continuous casting blank is heated to 1250-1300 DEG C, and the soaking time is 30-90 min; the rough rolling outlet temperature is controlled to be 1020-1060 DEG C, the rough rolling stage adopts large reduction rolling, and the pass rolling reduction rate is greater than or equal to 15 %; the finish rolling temperature is controlled to be 770-800 DEG C, and the flattening is less than 2 passes; online quenching treatment: the steel plate directly enters an online quenching device after leaving the rolling mill, the steel plate directly enters an online quenching device after leaving the rolling mill, and the water inlet temperature is controlled to be greater than or equal to 96 % of the finish rolling temperature; tempering treatment: the quenched steel plate is subjected to tempering heat treatment, the tempering temperature is 550-600 DEG C, the holding time is T=3-4.0 min / mm, and the steel plate is air-cooled after tempering. Through optimization of the chemical composition of the steel, especially control of the C, S and P contents at a low level, and addition of appropriate amounts of Ti, Ca and other elements, the Z-direction tensile property meets the requirements of Z35 in the steel plate thickness direction property GB / T5313.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy, and in particular to a production method for low-alloy, high-strength Z-axis steel by direct quenching and tempering, which is applicable to the production of high-strength structural steel requiring excellent Z-axis properties. Background Technology

[0002] With increasingly stringent national requirements for major projects, large-scale energy equipment, and environmental protection, the application of steel plates with large thickness, high unit weight, and high internal quality and resistance to lamellar tearing is growing. Z-direction performance, i.e., the tensile strength in the thickness direction of the steel, is an important indicator for evaluating the steel's resistance to lamellar tearing. When the steel plate thickness is not less than 40 mm and is subjected to tensile force along the thickness direction, lamellar tear-resistant steel must be used to avoid lamellar tearing during welding.

[0003] Currently, high-strength steel plates are mainly produced through two process routes: one is the traditional rolling + offline heat treatment (quenching + tempering) process; the other is the direct quenching + tempering (DQ+T) process. Compared with the traditional process, the DQ+T process has advantages such as high production efficiency and low energy consumption. However, existing DQ+T high-strength steels generally suffer from high alloy costs and poor Z-axis properties. The main reasons include: high alloy costs, requiring the addition of large amounts of expensive alloying elements such as Ni and Mo; the traditional process's pursuit of high-temperature immersion to improve hardenability, resulting in coarse grains due to high-temperature rolling; improper control of quenching process parameters, leading to uneven plate shape and uneven internal stress distribution; and low tempering efficiency and high energy consumption. Therefore, developing a production method for DQ+T high-strength steel with low alloy costs and high Z-axis properties is of great significance. Summary of the Invention

[0004] To address the aforementioned technical needs, the purpose of this invention is to provide a production method for low-alloy, high-strength Z-axis steel by direct quenching and tempering. Through composition optimization and online quenching process adjustment, this method solves the problem of generally poor Z-axis performance in high-strength steel under quenching and tempering conditions.

[0005] To achieve the above objectives, the technical solution adopted by this invention is as follows: A production method for low-alloy, high-strength Z-axis steel by direct quenching and tempering, comprising the following steps: 1) Steel smelting and continuous casting: The chemical composition of the steel is controlled according to the following mass percentages: C: 0.06-0.20%, Si: ≤0.05%, Mn: 0.5-1.8%, Cr: 0.1-0.8%, Ti: 0.10-0.20%, Al: 0.015-0.06%, Ca: 0.001-0.004%, S≤0.003%, P≤0.015%, N≤0.005%, with the balance being Fe and unavoidable impurities; calcium treatment technology is adopted in the smelting process, and electromagnetic stirring technology is adopted in the continuous casting process to refine the billet structure and reduce segregation.

[0006] 2) Heating and rolling: The continuously cast billet is heated to 1220-1260℃, and the soaking time is 30-90min; the exit temperature of the roughing mill is controlled at 1000-1060℃, and a large reduction rolling is adopted in the roughing stage, with a reduction rate of ≥15% per pass; The finishing rolling temperature is controlled at 770-800℃, which is lower than the traditional process of 900-950℃, in order to refine the grains; Leveling should be done in less than 2 passes to reduce unevenness in the thickness direction; 3) Online Quenching Treatment: The steel plate enters the online quenching device directly after exiting the rolling mill. The water inlet temperature is controlled at above 96% of the finishing rolling temperature, i.e., 740-770℃, lower than the water inlet temperature of the traditional DQ process. The quenching cooling rate is ≥60℃ / s, and the final cooling temperature is (Ms-Ms+70)℃, where Ms is the martensitic transformation initiation temperature. Ms=539-423C-11.0Si-30.4Mn-12.1Cr; By adjusting the water volume and ratio of the upper and lower nozzles of the quenching machine and the speed of the roller conveyor, the red-hot temperature of the steel plate can be controlled between 330-400℃ to obtain good plate shape and uniformity of structure. 4) Tempering treatment: The quenched steel plate is subjected to tempering heat treatment at a temperature of 550-600℃ and a holding time of T=3-4.0min / mm. After tempering, it is air-cooled.

[0007] Furthermore, the chemical composition of the steel in step 1) also satisfies: Mn+2Cr≥2.0%, Ti-3.5N≥0.08%, 1.0≤Ca / S≤3.0.

[0008] Unavoidable impurities in step 1): P≤0.02%, S≤0.01%, O≤0.008%.

[0009] The microstructure of the steel consists of tempered martensite, in which TiC nanoprecipitates are dispersed; the grain size of the TiC nanoprecipitates is ≤10nm.

[0010] The tempering process in step 4) adopts a continuous heat treatment process.

[0011] The beneficial effects of this invention are: by optimizing the chemical composition of steel, especially by controlling the content of C, S and P at a low level, and by adding appropriate amounts of Ti, Ca and other elements, the Z-direction properties of the steel are effectively improved, and the Z-direction tensile properties meet the requirements of Z35 in GB / T5313 thickness direction performance steel plates.

[0012] By reducing the finishing rolling temperature to 770-800℃, the grain structure is refined, thereby improving the strength and toughness of the steel.

[0013] By reducing the leveling process to less than two passes, the non-uniformity of the microstructure in the thickness direction was reduced, and the performance in the Z direction was improved.

[0014] By increasing the water temperature and controlling the reddening temperature between 330-400℃, good plate shape and uniform structure were obtained, with longitudinal yield strength reaching 800-950MPa, tensile strength reaching 850-1000MPa, and elongation ≥14%.

[0015] Compared with the traditional rolling + offline heat treatment process, the direct quenching + tempering process increases production efficiency by more than 20% and reduces energy consumption by more than 15%. Attached Figure Description

[0016] Figure 1 It is the metallographic structure (surface S-reflection) on the thickness section of the steel plate produced by this invention.

[0017] Figure 2 It is the metallographic structure (B+F at 1 / 4 thickness) on the cross section of the steel plate produced by this invention. Detailed Implementation Example

[0018] Molten steel is prepared according to the following mass percentages: C: 0.14%, Si: 0.20%, Mn: 1.3%, Cr: 0.4%, Ti: 0.15%, Al: 0.03%, Ca: 0.002%, S: 0.002%, P: 0.012%, N: 0.003%, with the balance being Fe and unavoidable impurities.

[0019] The smelting process adopts the converter-LF-VD process, with a calcium processing capacity of 0.3 kg / t. The continuous casting process uses electromagnetic stirring technology with a stirring intensity of 300 A.

[0020] The continuously cast billet was heated to 1280℃ and the soaking time was 60 minutes. The exit temperature of the roughing mill is controlled at 1040℃, and the reduction rate per pass in the roughing mill stage is 15%. The finishing rolling temperature is controlled at 790℃, and the leveling passes are 2. After exiting the rolling mill, the steel plate directly enters the online quenching device. The water temperature is 765℃, the quenching cooling rate is 60℃ / s, the final cooling temperature is 350℃, and the reddening temperature is controlled at 380℃. The quenched steel plate is subjected to tempering heat treatment in a continuous furnace at a tempering temperature of 570℃ and a holding time of 160min.

[0021] The final steel plate has a thickness of 40mm, a longitudinal yield strength of 905MPa, a tensile strength of 986MPa, an elongation of 14%, an average impact energy of 65J at -20℃, Z-direction tensile properties that meet the Z35 grade requirements, and a section reduction rate of 38%. Example

[0022] Molten steel is prepared according to the following mass percentages: C: 0.18%, Si: 0.25%, Mn: 1.5%, Cr: 0.6%, Ti: 0.18%, Al: 0.04%, Ca: 0.003%, S: 0.0015%, P: 0.010%, N: 0.002%, with the balance being Fe and unavoidable impurities.

[0023] The smelting process adopts the converter-LF-RH process, with a calcium processing capacity of 0.4 kg / t. The continuous casting process uses electromagnetic stirring technology with a stirring intensity of 350 A.

[0024] The continuously cast billet was heated to 1270℃ and the soaking time was 50 minutes. The exit temperature of the roughing mill is controlled at 1030℃, and the reduction rate per pass in the roughing mill stage is 15.5%. The finishing rolling temperature is controlled at 770℃, and the leveling pass is 1. After exiting the rolling mill, the steel plate directly enters the online quenching device. The water temperature is 760℃, the quenching cooling rate is 53℃ / s, the final cooling temperature is 300℃, and the red-hot temperature is controlled at 340℃. The quenched steel plate is subjected to tempering heat treatment in a continuous furnace at a tempering temperature of 600℃ and a holding time of 200min.

[0025] The final steel plate has a thickness of 50 mm, a longitudinal yield strength of 835 MPa, a tensile strength of 887 MPa, an elongation of 16%, an average impact energy of 126 J at -20℃, Z-direction tensile properties that meet the Z35 grade requirements, and a section reduction rate of 40%.

Claims

1. A production method for low-alloy, high-strength Z-axis steel by direct quenching and tempering, characterized in that... Includes the following steps: 1) Steel smelting and continuous casting: The chemical composition of the steel is controlled according to the following mass percentages: C: 0.06-0.20%, Si: 0.05-0.30%, Mn: 0.5-1.8%, Cr: 0.1-0.8%, Ti: 0.10-0.20%, Al: 0.015-0.06%, Ca: 0.001-0.004%, S≤0.003%, P≤0.015%, N≤0.005%, with the balance being Fe and unavoidable impurities; calcium treatment technology is used in the smelting process, and electromagnetic stirring technology is used in the continuous casting process; 2) Heating and rolling: The continuously cast billet is heated to 1250-1300℃ and the soaking time is 30-90min; the exit temperature of the roughing mill is controlled at 1020-1060℃, and a large reduction is adopted in the roughing stage, with a reduction rate of ≥15% per pass; the finishing mill temperature is controlled at 770-800℃, and the leveling is less than 2 passes; 3) Online quenching treatment: The steel plate enters the online quenching device directly after exiting the rolling mill. The water temperature is controlled at more than 96% of the finishing rolling temperature; the quenching cooling rate is ≥60℃ / s, and the final cooling temperature is (Ms-Ms+70)℃, where Ms is the martensitic transformation start temperature and Ms=539-423C-11.0Si-30.4Mn-12.1Cr; the steel plate reddening temperature is controlled between 330-400℃ by adjusting the water volume and water ratio of the upper and lower nozzles of the quenching machine and the speed of the roller table. 4) Tempering treatment: The quenched steel plate is subjected to tempering heat treatment at a temperature of 550-600℃ and a holding time of T=3-4.0min / mm. After tempering, it is air-cooled.

2. The production method according to claim 1, characterized in that, In step 1), the chemical composition of the steel also satisfies the following conditions: Mn+2Cr≥2.0%, Ti-3.5N≥0.08%, 1.0≤Ca / S≤3.

0.

3. The production method according to claim 1, characterized in that, Unavoidable impurities in step 1): P≤0.02%, S≤0.01%, O≤0.008%.

4. The production method according to claim 1, characterized in that, The microstructure of the steel consists of tempered martensite, in which TiC nanoprecipitates are dispersed; the grain size of the TiC nanoprecipitates is ≤10nm.

5. The production method according to claim 1, characterized in that, The tempering process in step 4) adopts a continuous heat treatment process.

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