Production method of 450MPa-grade micro-copper low-temperature high-toughness steel plate

Abstract

The invention discloses a production method of a 450MPa-grade micro-copper low-temperature high-toughness steel plate, and relates to the field of steel production. The process comprises the steps of (1) converter smelting, (2) LF refining, (3) vacuum degassing, (4) continuous casting, (5) heating, (6) rolling and (7) heat treatment, a steel plate comprises the following alloy components in percentage by weight: 0.07%-0.09% of C, 0.15%-0.25% of Si, 1.05%-1.15% of Mn, less than or equal to 0.007% of P, less than or equal to 0.002% of S, 0.015%-0.030% of Al, 0.020%-0.028% of Nb, 0.035%-0.045% of V, 1.15%-1.25% of Ni, 0.05%-0.10% of Mo, 0.55%-0.63% of Cr, 0.010%-0.018% of Ti, 0.28%-0.33% of Cu and the balance of Fe and inevitable

Description

Technical Field

[0001] The present invention belongs to the technical field of steel production, and relates to a production method of a 450MPa grade micro-copper low-temperature high-toughness steel plate. Background Art

[0002] Micro-copper low-temperature high-toughness steel plates are usually used in various ships and ocean structure projects, such as icebreakers or new ships on the Arctic route. Due to the extremely low environmental temperature, ordinary steel is prone to brittleness and fracture, while high-toughness steel can better ensure the safety of the hull structure. The 450MPa grade enables the hull structure to be lighter, while ensuring strength and improving the load capacity or fuel efficiency of the ship.

[0003] In the actual production process, it is found that the strength and low-temperature impact toughness cannot be well matched during the production of 16mm quenching and tempering or normalizing + tempering. Summary of the Invention

[0004] In order to solve the above problems, the present invention provides a production method of a 450MPa grade micro-copper low-temperature high-toughness steel plate. The thickness specification of the produced steel plate is 16mm, the yield strength of the steel plate is 450 - 475Mpa, the tensile strength is 570 - 620Mpa, the elongation after fracture is ≥35%, the reduction of area is ≥70%, and the impact at -80°C is above 100J.

[0005] The technical solution adopted by the present invention: A production method of a 450MPa grade micro-copper low-temperature high-toughness steel plate, the technological steps include 1) converter smelting, 2) LF refining, 3) vacuum degassing, 4) continuous casting, 5) heating, 6) rolling, 7) heat treatment. It is characterized in that: the alloy composition and percentage of the steel plate are C = 0.07% - 0.09%, Si = 0.15% - 0.25%, Mn = 1.05 - 1.15%, P ≤ 0.007, S ≤ 0.002, Al = 0.015% - 0.030%, Nb = 0.020% - 0.028%, V = 0.035% - 0.045%, Ni = 1.15% - 1.25%, Mo = 0.05% - 0.10%, Cr = 0.55% - 0.63%, Ti = 0.010% - 0.018%, Cu = 0.28% - 0.33%, and the rest are Fe and inevitable impurities; the yield strength of the steel plate is ≥450Mpa, the tensile strength is 570 - 685Mpa, the elongation after fracture is ≥20%, the reduction of area is ≥70%, and the impact at -80°C is above 100J; the heat treatment in the technological step 7) is normalizing + tempering at high temperature in the delivery state, air cooling after normalizing at 900°C and then water cooling to room temperature after air cooling to 600°C; the steel plate after normalizing is tempered, the tempering temperature is 640°C, and air cooling is carried out after tempering; Further, in step 1) converter smelting: Endpoint control targets: tapping temperature ≥ 1600 °C, tapping phosphorus ≤ 0.008%, dissolved oxygen at tapping ≥ 500 ppm; Further, in step 2) LF refining: The dephosphorization and slagging process is adopted. After the molten steel is slagged off, slag making and deoxidation are carried out, and the white slag holding time shall not be less than 20 min; Further, in step 3) vacuum degassing: The vacuum pumping target is below 0.5 tor, the vacuum holding time ≥ 15 minutes, the soft blowing time ≥ 15 minutes, and the dissolved hydrogen of the molten steel at the station ≤ 1.5 ppm; Further, in step 4) continuous casting: The superheat of the tundish ≤ 20 °C, the maximum casting speed 0.85 m / min; Further, in step 5) heating: The heating temperature is 1160 °C to 1190 °C, and the soaking time ≥ 50 minutes; Further, in step 6) rolling: The two-stage rolling process is adopted. The rolling start temperature in the rough rolling stage is 1020 ± 20 °C, the final rolling temperature in the finish rolling stage is 720 ± 20 °C, and air cooling is carried out after rolling.

[0006] Principle of the present invention: The functions of alloying elements in the present invention are as follows: Cu atoms are dissolved in ferrite or austenite, and the strength of the steel is improved through lattice distortion; copper and nickel form eutectic strengthening to promote the precipitation of AlN and refine grains; copper, chromium, and phosphorus act synergistically to form a stable rust layer; Cr atoms dissolve into the ferrite or austenite lattice, causing lattice distortion and increasing the hardness and strength of the steel; Cr inhibits the grain boundary migration at high temperatures through solid solution strengthening and carbide precipitation, improving the creep fracture strength; Cr reduces the critical cooling rate of the transformation from austenite to pearlite and expands the hardenability layer depth; Ni can be infinitely dissolved with iron, expanding the austenite phase region, which is beneficial to the formation and stabilization of austenite; nickel can strengthen ferrite by solid solution strengthening and refine and increase pearlite; nickel can improve the low-temperature toughness of the steel; nickel can improve the weldability of the steel, reduce the precipitation of intermetallic compounds, and prevent and reduce their harmful effects; nickel can increase the stacking fault energy of the steel, increase the mobile dislocations during the deformation process, and thus improve the plastic toughness of the steel.

[0007] For the micro-copper low-temperature high-toughness steel plate of the present invention, the yield strength level is 450 MPa, the thickness specification is 16 mm, the normalizing + tempering process is adopted, and the required microstructure type is ferrite + pearlite. It is difficult to reach this yield strength level by using the traditional normalizing air cooling + tempering process, while the microstructure type of the normalizing rapid water cooling + tempering process does not meet ferrite + pearlite. Therefore, the chemical composition of the present invention adopts the micro-Cu, Cr, Ni alloy design, and its adjustable space of chemical composition is small. A series of heat treatment processes have been tested, and finally a production process that simultaneously meets the strength index and microstructure type has been explored, forming a 450 MPa grade high-toughness steel plate with the normalizing + tempering process capable of industrial production.

[0008] Advantages of the present invention: Alloy copper is added during the smelting process. Through the solution strengthening effect of copper, the yield strength and tensile strength are improved. At the same time, the toughness is maintained by refining the grains. Through narrow composition control and temperature control by air cooling, air quenching, and water quenching in stages during the heat treatment process, the relationship between strength and high low-temperature toughness is significantly balanced, and the Cr and Ni contents can be further reduced to achieve cost reduction. The key innovation of the invention lies in the heat treatment method being normalizing + tempering at high temperature in the delivery state. After normalizing at 900°C, it is taken out of the furnace and air cooled, and then water cooled to room temperature after air cooling to 600°C; the normalized steel plate is tempered, the tempering temperature is 640°C, and it is air cooled after tempering; Description of the Drawings

[0009] Figure 1 It is the microstructure of the core of the 16-mm-thick 450-MPa-grade micro-copper low-temperature high-toughness steel plate in Example 1 after normalizing at 900°C (air cooled to 640°C and then water quenched) + tempering at 600°C.

[0010] Figure 2 It is the microstructure of the core of the 16-mm-thick 450-MPa-grade micro-copper low-temperature high-toughness steel plate in Example 2 after normalizing at 920°C (air cooled to 650°C and then water quenched) + tempering at 600°C.

[0011] Figure 3 It is the microstructure of the core of the 16-mm-thick 450-MPa-grade micro-copper low-temperature high-toughness steel plate in Comparative Example 1 after normalizing at 900°C + tempering at 600°C.

[0012] Figure 4 It is the microstructure of the core of the 16-mm-thick 450-MPa-grade micro-copper low-temperature high-toughness steel plate in Comparative Example 2 after normalizing at 820°C + tempering at 600°C.

[0013] Figure 5 It is the microstructure of the core of the 16-mm-thick 450-MPa-grade micro-copper low-temperature high-toughness steel plate in Comparative Example 3 after tempering at 600°C. Detailed Embodiments

[0014] The present invention will be further described in detail below with reference to the drawings and specific embodiments. Example 1

[0015] The thickness of the 450-MPa-grade micro-copper low-temperature high-toughness steel plate in this example is 16 mm, and its chemical composition and carbon equivalent percentage are shown in Table 1.

[0016] The production steps of the 450-MPa-grade micro-copper low-temperature high-toughness steel plate in this example include converter smelting, LF refining, vacuum degassing, continuous casting, heating, rolling, and heat treatment. The specific parameters of the rolling and heat treatment processes are as follows: (1) Rolling process: Two-stage rolling is adopted. The rough rolling starting temperature is 1020°C, and the finish rolling starting temperature is 920°C. After rolling, air cooling is carried out. The conditions of each rolling pass are shown in Table 2; (2) Heat treatment process: Normalizing at 900°C (air cooling to 640°C and then water cooling) + tempering at 600°C. The normalizing temperature is 900°C, the holding time in the furnace is 45 min, and it is air cooled to 640°C and then put into water 3 minutes and 25 seconds after leaving the furnace; the tempering temperature is 600°C, the holding time coefficient is 48 min, and air cooling is carried out after tempering.

[0017] The microstructure at the 1 / 2 position of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this example is shown in Figure 2 , and it can be seen from Figure 2 that the microstructure is ferrite + pearlite structure, and the mechanical property indexes are shown in Table 4 and Table 5. Example 2

[0018] The thickness of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this example is 16 mm, and its chemical composition and carbon equivalent percentage are shown in Table 1.

[0019] The production steps of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this example include converter smelting, LF refining, vacuum degassing, continuous casting, heating, rolling, and heat treatment. The specific parameters of the rolling and heat treatment processes are as follows: (1) Rolling process: Two-stage rolling is adopted. The rough rolling starting temperature is 1020°C, and the finish rolling starting temperature is 920°C. After rolling, air cooling is carried out. The conditions of each pass are shown in Table 3; (2) Heat treatment process: Normalizing at 900°C (air cooling to 640°C and then water cooling) + tempering at 600°C. The normalizing temperature is 900°C, the holding time in the furnace is 45 min, and it is air cooled to 640°C and then put into water 3 minutes and 25 seconds after leaving the furnace; the tempering temperature is 600°C, the holding time coefficient is 48 min, and air cooling is carried out after tempering.

[0020] The microstructure at the 1 / 2 position of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this example is shown in Figure 5 , and it can be seen from Figure 5 that the microstructure is ferrite + pearlite structure, and the mechanical property indexes are shown in Table 4 and Table 5.

[0021] Comparative Example 1 The thickness of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this comparative example is 16 mm, and its chemical composition and carbon equivalent percentage are shown in Table 1.

[0022] The production steps of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this comparative example include converter smelting, LF refining, vacuum degassing, continuous casting, heating, rolling, and heat treatment. The specific parameters of the rolling and heat treatment processes are as follows: (1) Rolling process: Two-stage rolling is adopted. The rough rolling starting temperature is 1020°C, and the finish rolling starting temperature is 920°C. After rolling, it is air-cooled. The conditions of each rolling pass are shown in Table 2; (2) Heat treatment process: Normalizing at 900°C + tempering at 600°C. The normalizing temperature is 900°C, and the holding time in the furnace is 45 minutes. After taking out of the furnace, it is air-cooled; the tempering temperature is 600°C, the holding time coefficient is 48 minutes, and after tempering, it is air-cooled.

[0023] The microstructure at the 1 / 2 position of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this comparative example is shown in Figure 1 , and it can be seen from Figure 1 that the microstructure is ferrite + pearlite structure, and the mechanical property indexes are shown in Tables 4 and 5.

[0024] Comparative Example 2 The thickness of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this comparative example is 16 mm, and its chemical composition and carbon equivalent percentage are shown in Table 1.

[0025] The production steps of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this comparative example include converter smelting, LF refining, vacuum degassing, continuous casting, heating, rolling, and heat treatment. The specific parameters of the rolling and heat treatment processes are as follows: (1) Rolling process: Two-stage rolling is adopted. The rough rolling starting temperature is 1020°C, and the finish rolling starting temperature is 920°C. After rolling, it is air-cooled. The conditions of each rolling pass are shown in Table 3; (2) Heat treatment process: Normalizing at 820°C + tempering at 600°C. The normalizing temperature is 820°C, and the holding time in the furnace is 45 minutes. After taking out of the furnace, it is air-cooled; the tempering temperature is 600°C, the holding time coefficient is 48 minutes, and after tempering, it is air-cooled.

[0026] The microstructure at the 1 / 2 position of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this comparative example is shown in Figure 3 , and it can be seen from Figure 3 that the microstructure is ferrite + pearlite structure, and the mechanical property indexes are shown in Tables 4 and 5. Comparative Example 3 The thickness of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this comparative example is 16 mm, and its chemical composition and carbon equivalent percentage are shown in Table 1.

[0027] The production steps of the 450 MPa grade micro-copper low-temperature high-toughness steel plate in this comparative example include converter smelting, LF refining, vacuum degassing, continuous casting, heating, rolling, and heat treatment. The specific parameters of the rolling and heat treatment processes are as follows: (1) Rolling process: Two-stage rolling is adopted. The rough rolling starting temperature is 1020°C, and the finish rolling starting temperature is 920°C. After rolling, it is air-cooled. The conditions of each rolling pass are shown in Table 3; (2) Heat treatment process: tempering at 600°C. The tempering temperature is 600°C, the holding time coefficient is 48 min, and air cooling is carried out after tempering is completed.

[0028] The microstructure at the 1 / 2 position of the 450 MPa grade micro-copper low-temperature high-toughness steel plate of this comparative example is shown in Figure 4 , and it can be seen from Figure 4 that the microstructure is ferrite + pearlite structure, and the mechanical property indexes are shown in Table 4 and Table 5.

[0029] Table 1 Chemical composition and percentage (%) .

[0030] Table 2 Rolling pass table .

[0031] Table 3 Rolling pass table .

[0032] Table 4 Tensile properties of the steel plate after heat treatment .

[0033] Table 5 Impact properties and metallographic structure of the steel plate after heat treatment .

[0034] It can be seen from the above data that in Comparative Example 1 and Comparative Example 2 that do not adopt the technical solution of the present invention, the yield strength is low and does not meet the requirements, and in Comparative Example 3, the impact performance is low and does not meet the requirements. However, in Example 1 and Example 2 that adopt the technical solution of the present invention, the tensile and impact properties both meet the requirements.

Claims

1. A production method of a 450MPa grade micro-copper low-temperature high-toughness steel plate, the technological steps include 1) converter smelting, 2) LF refining, 3) vacuum degassing, 4) continuous casting, 5) heating, 6) rolling, 7) heat treatment, and it is characterized in that: The alloy composition and percentage of the steel plate are as follows: C = 0.07% - 0.09%, Si = 0.15% - 0.25%, Mn = 1.05 - 1.15%, P ≤ 0.007, S ≤ 0.002, Al = 0.015% - 0.030%, Nb = 0.020% - 0.028%, V = 0.035% - 0.045%, Ni = 1.15% - 1.25%, Mo = 0.05% - 0.10%, Cr = 0.55% - 0.63%, Ti = 0.010% - 0.018%, Cu = 0.28% - 0.33%, N ≤ 50 ppm, O ≤ 15 ppm, and the rest is Fe and unavoidable impurities; The yield strength of the steel plate is ≥450 Mpa, the tensile strength is 570 - 685 Mpa, the elongation after fracture is ≥20%, the reduction of area after fracture is ≥70%, and the impact at -80°C is above 100 J; the heat treatment in process step 7) is normalizing + tempering at high temperature in the delivered state, air cooling after normalizing at 900°C and then water cooling to room temperature after air cooling to 600°C; the steel plate after normalizing is tempered, the tempering temperature is 640°C, and air cooling is carried out after tempering.

2. The production method of a 450MPa grade micro-copper low-temperature high-toughness steel plate according to claim 1, characterized in that: Step 1) Converter smelting: The end-point control targets are tapping temperature ≥1600°C, tapping phosphorus ≤0.008%, and tapping dissolved oxygen ≥500 ppm.

3. The production method of a 450MPa grade micro-copper low-temperature high-toughness steel plate according to claim 1, characterized in that: Step 2) LF refining: The dephosphorization and slag-skimming process is adopted. After the molten steel is skimmed, slag-making and deoxidation are carried out, and the white slag holding time shall not be less than 20 min.

4. The production method of a 450MPa grade micro-copper low-temperature high-toughness steel plate according to claim 1, characterized in that: Step 3) Vacuum degassing: The vacuum pumping target is below 0.5 tor, the vacuum holding time is ≥15 minutes, the soft blowing time is ≥15 minutes, and the hydrogen content of the molten steel when leaving the station is ≤1.5 ppm.

5. The production method of a 450 MPa grade micro - copper low - temperature high - toughness steel plate according to claim 1, characterized in that: Step 4) Continuous casting: The superheat of the tundish is ≤20°C, and the maximum casting speed is 0.85 m / min.

6. The production method of a 450 MPa grade micro - copper low - temperature high - toughness steel plate according to claim 1, characterized in that Step 5) Heating: The heating temperature is 1160 - 1190°C, and the soaking time is ≥50 minutes.

7. The production method of a 450MPa grade micro-copper low-temperature high-toughness steel plate according to claim 1, characterized in that Step 6) Rolling: A two-stage rolling process is adopted. The starting rolling temperature in the rough rolling stage is 1020 ± 20°C, the finishing rolling temperature in the finish rolling stage is 720 ± 20°C, and air cooling is carried out after rolling.

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