A method for producing a thick plate of an f36 cryogenic steel

Abstract

A production method of F36 low-temperature steel thick plate, the chemical composition of the steel is as follows: C=0.10%-0.12%, Si=0.05%-0.15%, Mn=1.40%-1.60%, P<=0.010%, S<=0.003%, Al=0.02%-0.06%, Ti=0.008%-0.015%, Nb=0.020%-0.050%, Cr=0.20%-0.50%, and the balance is Fe and inevitable impurity elements; the key process steps include smelting, heating, controlled rolling and controlled cooling, quenching, normalizing and stress relief tempering; the composition design of low C, low Si, low P, low S, Nb addition and a small amount of Cr is adopted, the controlled rolling+relaxation+rapid cooling are carried out, and then the heat treatment process of quenching+normalizing+rapid cooling+stress relief tempering is carried out, the microstructure of ferrite+granular pearlite is obtained, the yield strength of the steel plate is >=360MPa, the tensile strength is >=490MPa, and the impact energy at-60 DEG C is >=120J.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of low alloy steel production and relates to a production method of F36 low-temperature steel thick plate. BACKGROUND

[0002] Due to long-term service in extremely cold environments, the performance and quality requirements of steel materials are extremely strict. In order to meet the impact toughness requirements of F-grade steel at-60 DEG C, the prior art usually adds a large amount of valuable elements such as Ni, Mo and even rare earth elements in the steel, such as Chinese patents CN102851623A, CN102703807A and the like. Chinese patent CN114134404A proposes an economical FH36 steel plate for icebreaker, and the low-temperature longitudinal impact toughness value at-60 DEG C is greater than or equal to 200J, but the thickness of the produced steel plate is only 20-40mm. At present, there are few technologies developed for F36 steel with a thickness of more than 100mm, but low-temperature steel for deep-sea offshore platforms, offshore wind turbine piles and the like is developing towards a larger thickness, and at present, it has been expanded to 150mm. SUMMARY

[0003] The purpose of the application is to provide a production method of F36 low-temperature steel thick plate, the thickness of the steel plate is 100-150mm, a microstructure mainly composed of ferrite and granular pearlite is obtained, the yield strength of the steel plate is greater than or equal to 360MPa, the tensile strength is greater than or equal to 490MPa, and the impact energy at-60 DEG C is greater than or equal to 120J.

[0004] The technical scheme of the application is as follows:

[0005] A production method of F36 low-temperature steel thick plate, the thickness of the produced steel plate is 100-150mm, the chemical composition of the steel by weight percentage is C=0.10%-0.12%, Si=0.05%-0.15%, Mn=1.40%-1.60%, P≤0.010%, S≤0.003%, Al=0.02%-0.06%, Ti=0.008%-0.015%, Nb=0.020%-0.050%, Cr=0.20%-0.50%, and the balance is Fe and inevitable impurity elements; the key process steps include:

[0006] (1) Smelting: the molten steel is subjected to LF refining, RH vacuum treatment, and then is protected casting into a 350-450mm thick continuous casting billet, no electromagnetic stirring at the end of the crystallizer is used in the continuous casting process, and the specific water quantity of the secondary cooling water is controlled to be 0.8±0.1L / kg;

[0007] (2) Heating: the hearth temperature is less than or equal to 1220 DEG C, the soaking temperature is 1150-1180 DEG C, and the soaking time is 60-90min;

[0008] (3) Controlled rolling and controlled cooling: two-stage controlled rolling is adopted, at least one pass reduction ratio is greater than or equal to 12% in the rough rolling process at a rolling temperature greater than or equal to 1000 DEG C, the rough rolling end temperature is controlled to be 960-1000 DEG C and the pass reduction ratio is greater than or equal to 15%; the open rolling temperature is less than or equal to 850 DEG C, the finish rolling temperature is 780-820 DEG C, the pass reduction ratio of each pass is less than or equal to 10%, and the interval time is greater than or equal to 8s; after rolling, relaxation is carried out for 30-60s, then rapid cooling is carried out to 250-350 DEG C at a cooling rate greater than or equal to 2 DEG C / s, and then stack cooling is carried out to room temperature;

[0009] (4) Quenching: the heating temperature is 890±10 DEG C, the heating-up time is 1.2-1.5 min / mm, and the holding time is 30-60 min, then air cooling is carried out to room temperature;

[0010] (5) Normalizing: the heating temperature is 860±5 DEG C, the heating-up time is 1.5-1.8 min / mm, and the holding time is 50-80 min; after heating, rapid cooling is carried out to 500-550 DEG C at a cooling rate greater than or equal to 2 DEG C / s, and then air cooling is carried out to room temperature;

[0011] (6) Stress relieving: the heating temperature is 500-550 DEG C, and the holding time is 50-70 min.

[0012] Principle of the application: the low-C and low-Si composition design is adopted to improve the toughness of the steel; Cr is added to improve the strength of the steel, improve the segregation of manganese, and Cr has the effect of delaying the bainite phase change, so that the ferrite and pearlite matrix structure is obtained; Cr also has the effect of increasing the intergranular misorientation, so that more and larger angle large-angle grains are obtained; the addition of Nb, Ti and other micro-alloy elements plays an important role in reducing the austenite grain size in the heating rolling process and the normalizing process. In order to ensure the compression ratio, a large thickness continuous casting billet is adopted, and the end electromagnetic stirring of the crystallizer is not used to prevent negative segregation near 1 / 4 thickness; in order to improve the dendritic segregation, strong secondary cooling is adopted in the continuous casting process; the main purpose of the heating and controlled rolling and controlled cooling process adopted in the application is to refine the original austenite grain size and control the quantity and morphology of the bainite transformation, so that the bainite obtained is small, diversified, has a large orientation and clear grain boundary. Such structure has heredity during subsequent heat treatment, especially when quenching at a large heating speed, very small and uniform original austenite grains can be obtained; the heat treatment process of quenching + normalizing + rapid cooling + stress relieving is adopted in the application, quenching can maintain the original small and uniform grains on one hand, and obtain a non-equilibrium structure with more defects on the other hand, so that the austenite grains obtained during subsequent normalizing are smaller, and then rapid cooling is carried out, so that the ferrite and granular pearlite generated are extremely small and uniformly distributed, and finally stress relieving is carried out to reduce stress concentration and make cementite and carbide precipitate, thereby improving the toughness of the ferrite matrix.

[0013] The beneficial effects of the present application are: (1) the present application adopts an economic component design, without adding valuable elements such as Ni and Mo, and the alloy cost is low; (2) the controlled rolling and controlled cooling and heat treatment process adopted by the present application is relatively complex, but it can be realized by the existing equipment and production line, and through such process, a large thickness F36 low temperature steel plate with a thickness of 100-150 mm is successfully produced, the yield strength of which is ≥360 MPa, the tensile strength of which is ≥490 MPa, and the impact energy at-60℃ of which is ≥120 J; (3) the thick plate produced by the present application has excellent strength and toughness, and the production method can also play a reference role in improving the strength and toughness of thin plates. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 The metallographic structure at the 1 / 4 thickness of the steel plate of Example 1 of the present application. DETAILED DESCRIPTION

[0015] The content of the present application will be further described below in combination with examples and drawings. Example 1

[0016] A production method of a F36 low temperature steel thick plate, the chemical composition of the steel is as follows: C=0.10%, Si=0.08%, Mn=1.49%, P=0.008%, S=0.002%, Al=0.035%, Ti=0.012%, Nb=0.036%, Cr=0.30%, and the balance is Fe and inevitable impurity elements; the thickness of the steel plate is 100 mm, and the key process steps include:

[0017] (1) Smelting: the molten steel is subjected to LF refining and RH vacuum treatment, and then is protected casting into a 350 mm thick continuous casting billet, without using electromagnetic stirring at the end of the crystallizer during the continuous casting process, and the specific water quantity of the secondary cooling water is about 0.75 L / kg;

[0018] (2) Heating: the highest furnace temperature during the heating process is 1215℃, the soaking temperature is 1160℃, and the soaking time is 75 min;

[0019] (3) Controlled rolling and controlled cooling: two-stage controlled rolling is adopted, the maximum pass reduction rate during the rough rolling process at a rolling temperature ≥1000℃ is 13.6%, the rough rolling end temperature is 968℃ and the pass reduction rate of this pass is 17.4%; the rough rolling start temperature is 840℃, the finish rolling temperature is 790℃, the maximum pass reduction rate of the finish rolling is 9.2%, and the minimum pass interval time is 9 s; after rolling, the relaxation is 50 s, and then the rapid cooling is carried out to 320-340℃ at a cooling rate of about 4.8℃ / s, and then the cooling is carried out to room temperature;

[0020] (4) Quenching: the heating temperature is 890 DEG C, the heating time is 135 min, the holding time is 40 min, and then air cooling to room temperature;

[0021] (5) Normalizing: the heating temperature is 860 DEG C, the heating time is 160 min, the holding time is 60 min; after heating, it is rapidly cooled to 535-550 DEG C at a cooling rate of about 3.6 DEG C / s, and then air cooling to room temperature;

[0022] (6) Stress relief annealing: the heating temperature is 520 DEG C, and the holding time is 60 min.

[0023] Figure 1 For the metallographic structure of the steel plate of example 1 at 1 / 4 thickness, it can be seen that the microstructure is mainly composed of fine ferrite and fine and dispersed granular pearlite, and the grain size is 9 levels; the sample at 1 / 4 thickness of the steel plate of example 1 is taken for tensile and charpy V impact test: the yield strength is 380 MPa, the tensile strength is 528 MPa, and the impact energy at-60 DEG C is 205 J. Example 2

[0024] A production method of F36 low-temperature steel thick plate, the chemical composition of the steel is as follows: C=0.12%, Si=0.10%, Mn=1.53%, P=0.007%, S=0.002%, Al=0.042%, Ti=0.013%, Nb=0.042%, Cr=0.40%, and the balance is Fe and inevitable impurity elements; the thickness of the steel plate is 120 mm, and the key process steps include:

[0025] (1) Smelting: the molten steel is subjected to LF refining, RH vacuum treatment, and then is protected casting into a 450 mm thick continuous casting blank, no electromagnetic stirring at the end of the crystallizer is used in the continuous casting process, and the specific water consumption of the secondary cooling water is about 0.85 L / kg;

[0026] (2) Heating: the highest hearth temperature in the heating process is 1210 DEG C, the soaking temperature is 1170 DEG C, and the soaking time is 80 min;

[0027] (3) Controlled rolling and controlled cooling: two-stage controlled rolling is adopted, the maximum pass reduction rate in the rough rolling process at a rolling temperature of 1000 DEG C or more is 12.5%, the rough rolling end temperature is 980 DEG C, and the pass reduction rate of this pass is 16.3%; the rough rolling temperature is 830 DEG C, the finish rolling temperature is 800 DEG C, the maximum pass reduction rate of the finish rolling is 8.5%, and the minimum pass interval time is 10 s; after rolling, the relaxation is 40 s, and then it is rapidly cooled to 300-320 DEG C at a cooling rate of about 3.5 DEG C / s, and then it is stack cooled to room temperature;

[0028] (4) Quenching: the heating temperature is 890 DEG C, the heating time is 160 min, the holding time is 45 min, and then air cooling to room temperature;

[0029] (5) normalizing: the heating temperature is 860℃, the heating time is 200min, and the holding time is 60min; after heating, it is rapidly cooled to 520~540℃ at a cooling rate of about 3.2℃ / s, and then air-cooled to room temperature;

[0030] (6) stress relief annealing: the heating temperature is 520℃, and the holding time is 60min.

[0031] The sample at 1 / 4 thickness of the steel plate of Example 2 is taken for tensile and Charpy V-notch impact tests: the yield strength is 375MPa, the tensile strength is 530MPa, and the impact energy at -60℃ is 175J. Example 3

[0032] A production method of a F36 low-temperature steel thick plate, the chemical composition of the steel is as follows: C=0.11%, Si=0.11%, Mn=1.56%, P=0.006%, S=0.001%, Al=0.033%, Ti=0.010%, Nb=0.045%, Cr=0.44%, and the balance is Fe and inevitable impurity elements; the thickness of the steel plate is 150mm, and the key process steps include:

[0033] (1) smelting: the molten steel is subjected to LF refining, RH vacuum treatment, and then protective casting to form a 450mm thick continuous casting billet, no electromagnetic stirring at the end of the crystallizer is used during the continuous casting process, and the specific water consumption of the secondary cooling water is about 0.85L / kg;

[0034] (2) heating: the maximum hearth temperature during the heating process is 1216℃, the soaking temperature is 1180℃, and the soaking time is 80min;

[0035] (3) controlled rolling and controlled cooling: two-stage controlled rolling is adopted, the maximum pass reduction rate during the rough rolling process at a rolling temperature≥1000℃ is 12.1%, the rough rolling end temperature is 985℃ and the pass reduction rate is 15.7%; the rough rolling start temperature is 820℃, the finish rolling temperature is 795℃, the maximum pass reduction rate of the finish rolling is 7.2%, and the minimum pass interval time is 9s; after rolling, it is relaxed for 40s, then rapidly cooled to 280~300℃ at a cooling rate of about 3.0℃ / s, and then stack-cooled to room temperature;

[0036] (4) quenching: the heating temperature is 890℃, the heating time is 210min, and the holding time is 50min, and then air-cooled to room temperature;

[0037] (5) normalizing: the heating temperature is 860℃, the heating time is 240min, and the holding time is 70min; after heating, it is rapidly cooled to 510~525℃ at a cooling rate of about 2.6℃ / s, and then air-cooled to room temperature;

[0038] (6) Stress relief annealing: heating temperature is 520°C, and holding time is 60 minutes.

[0039] Tensile and Charpy V-notch impact tests were performed on the sample taken at the 1 / 4 thickness of the steel sheet of Example 3: the yield strength is 369 MPa, the tensile strength is 512 MPa, and the -60°C impact energy is 138 J.

Claims

1. A method for producing F36 low-temperature steel thick plates, characterized in that: The chemical composition of F36 low-temperature steel thick plate, by weight percentage, is C=0.10%~0.12%, Si=0.05%~0.15%, Mn=1.40%~1.60%, P≤0.010%, S≤0.003%, Al=0.02%~0.06%, Ti=0.008%~0.015%, Nb=0.020%~0.050%, Cr=0.20%~0.50%, with the balance being Fe and unavoidable impurity elements; The thickness of the F36 low-temperature steel plate is 100~150mm, and its key process steps include: (1) Smelting: The molten steel is refined by LF and vacuum treated by RH, and then cast into a continuous casting billet with a thickness of 350~450mm under protection. During the continuous casting process, electromagnetic stirring at the end of the crystallizer is not used, and the water volume of the secondary cooling water is controlled to be 0.8±0.1L / kg. (2) Heating: Furnace temperature ≤1220℃, heat spread temperature 1150~1180℃, heat spread time 60~90min; (3) Controlled rolling and controlled cooling: Two-stage controlled rolling is adopted. During the rough rolling process with a rolling temperature ≥1000℃, at least one pass has a reduction rate ≥12%. The rough rolling end temperature is controlled at 960~1000℃ and the reduction rate of the rough rolling end pass is ≥15%. The finishing rolling start temperature is ≤850℃ and the finishing rolling temperature is 780~820℃. The reduction rate of each pass is ≤10% and the interval time is ≥8s. After rolling, relax for 30~60s and then cool rapidly to 250~350℃ at a cooling rate of ≥2℃ / s. After cooling, it is stacked to room temperature. (4) Quenching: The heating temperature is 890±10℃, the heating time is 1.2~1.5min / mm, the holding time is 30~60min, and then air-cooled to room temperature; (5) Normalizing: The heating temperature is 860±5℃, the heating time is 1.5~1.8min / mm, and the holding time is 50~80min; after heating, it is rapidly cooled to 500~550℃ at a cooling rate of ≥2℃ / s, and then air-cooled to room temperature; (6) Stress-relief tempering: The heating temperature is 500~550℃ and the holding time is 50~70min to obtain F36 low temperature steel thick plate with a microstructure mainly composed of ferrite + granular pearlite, and a yield strength ≥360MPa, tensile strength ≥490MPa, and impact energy ≥120J at -60℃.

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