A kind of production method of 9Ni steel

Abstract

The invention discloses a production method of 9Ni steel, firstly, the steel ingot is rolled into an initial steel plate through pretreatment, then the initial steel plate is controlled to be rolled into a first-stage steel plate through thermo-mechanical control, then air-cooled to room temperature, and then sequentially quenched and tempered in two-phase regions to obtain a 9Ni steel plate. The invention adopts a production process of thermo-mechanical control + heat treatment, so that the structure undergoes phase change, and at the same time changes the thickness of the material, directly controls the required size of the steel plate, simplifies the production process, optimizes the production process, reduces energy consumption, improves the existing strength and low-temperature toughness, and produces 9Ni steel with better comprehensive performance.

Description

Technical Field

[0001] The invention relates to the field of nickel steel production, in particular to a production method of 9Ni steel. Background Art

[0002] In order to facilitate transportation and storage, natural gas needs to be cooled to -162℃ for liquefaction and compression. 9Ni steel is widely used in the preparation of liquid natural gas storage tanks and transport ships due to its good strength, low temperature toughness and crack arrest ability. The composition of 9Ni steel adopts low carbon and high nickel, and a small amount of alloying elements such as Si and Mn are added. Fine lath martensite structure is obtained by dislocation strengthening and fine grain strengthening to improve strength. Reverse transformation austenite exists at the grain boundary, and reverse transformation austenite has the effect of improving matrix toughness.

[0003] 9Ni steel is mostly produced by the traditional process of quenching and tempering or quenching + two-phase quenching + tempering to meet the requirements of strength and low-temperature toughness. The current mainstream production technology requires the material to be completely austenitized for pretreatment before heat treatment. The specific process flow is as follows: Figure 2 As shown. The quenching and tempering process is simple, but the low-temperature toughness is weak. Although the traditional process of quenching + two-phase quenching + tempering meets the requirements of good strength and low-temperature toughness, it requires offline heat treatment, which increases energy and time consumption. Therefore, a production process is needed that can meet the requirements of good strength and low-temperature toughness while simplifying the process. Summary of the invention

[0004] In view of the deficiencies in the prior art, the technical problem to be solved by the present invention is to provide a production method of 9Ni steel.

[0005] The technical solution of the present invention to solve the technical problem is to provide a production method of 9Ni steel, characterized in that the method comprises the following steps:

[0006] Step 1, heating the steel ingot to 1150-1250°C and performing isothermal treatment for 90-150 minutes, and then performing rough rolling to form an initial steel plate;

[0007] Step 2, placing the initial steel plate in a furnace above the austenitizing temperature for isothermal treatment for 30 to 80 minutes;

[0008] Step 3, setting the total deformation amount to 50% to 55%; the rolling process is multi-pass rolling, the specific number of passes is determined according to the thickness of the initial steel plate, the deformation amount of each pass is 17% to 22%, and the final rolling temperature is 0 to 50°C lower than the starting rolling temperature to obtain a first-stage steel plate;

[0009] Step 4: After rolling, the first-stage steel plate is cooled to room temperature by air cooling;

[0010] Step 5, placing the first-stage steel plate after air cooling in a furnace at 660-700° C. for isothermal treatment for 50-80 minutes, and then water cooling to obtain a second-stage steel plate;

[0011] Step 6: Place the second-stage steel plate in a non-recrystallization zone at 560-600° C. for isothermal treatment for 50-80 min, and then water-cool it to obtain a 9Ni steel plate.

[0012] Compared with the existing technical process, the beneficial effects of the present invention are:

[0013] (1) The present invention adopts a production process of thermo-mechanical control (TMCP) + heat treatment to cause a phase change in the structure and change the thickness of the material, directly controlling the required size of the steel plate, simplifying the production process, optimizing the production process, reducing energy consumption, improving the existing strength and low-temperature toughness, and producing 9Ni steel with better comprehensive performance.

[0014] (2) After rolling, low carbon steel is air-cooled, and the structure undergoes dynamic recrystallization and bainite phase transformation. During the phase transformation, more bainite structure is generated in the structure, and part of the ferrite is preferentially segregated in the dendrites and generated between the strips formed by the extension of the carbon-rich structure; after two-phase quenching, the strain energy stored by rolling is activated, and the high dislocation position undergoes martensite phase transformation, and the final structure is a bainite + martensite complex structure. The increase in rolling temperature causes more dynamic recrystallization of the structure, with a higher degree of refinement, which increases the strength. In addition, air cooling after rolling will produce MA islands in the structure; after tempering, the reversed austenite grows from the existing martensite lath grain boundaries, and the MA islands decompose into martensite structure. This part of the residual austenite becomes the core of the growth of reversed austenite, so that there is more reversed austenite in the structure, which is conducive to enhancing low-temperature toughness.

[0015] (3) The 9Ni steel produced by the present invention has a yield strength of 690-730 MPa, a tensile strength of 730-780 MPa, an elongation of more than 25%, and mechanical properties that meet national standards. In addition, the strength and low-temperature toughness of the 9Ni steel produced by the traditional process are higher. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The 9Ni steel production process flow chart of the present invention;

[0017] Figure 2 This is a process flow chart of producing 9Ni steel by using the traditional process of quenching + two-phase zone quenching + tempering in the prior art;

[0018] Figure 3 This is a microstructure diagram of Example 1 of the present invention;

[0019] Figure 4 This is a microstructure diagram of Example 2 of the present invention;

[0020] Figure 5 This is a microstructure diagram of Example 3 of the present invention;

[0021] Figure 6 This is the microstructure diagram of Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0022] The specific embodiments of the present invention are given below. The specific embodiments are only used to further illustrate the present invention in detail and do not limit the protection scope of the claims of the present invention.

[0023] The present invention provides a production method of 9Ni steel (hereinafter referred to as method), characterized in that the method comprises the following steps:

[0024] Step 1, pretreatment: heating the steel ingot to 1150-1250°C (preferably 1200°C) and isothermally treating it for 90-150min (preferably 120min), and then rough rolling it to form an initial steel plate;

[0025] Preferably, in step 1, the mass fraction of the steel ingot is as follows: C < 0.10%, Ni is 8.5-9.5%, Si is 0.10-0.15%, Mn is 0.6-0.65%, P < 0.005%, S < 0.005%, Mo < 0.01%, V < 0.01%, W < 0.01%, and the balance is Fe. Preferably, the mass fraction of the steel ingot is as follows: C is 0.05-0.10%, Ni is 9-9.5%, Si is 0.10-0.15%, Mn is 0.6-0.65%, P < 0.005%, S < 0.005%, Mo < 0.01%, V < 0.01%, W < 0.01%, and the balance is Fe.

[0026] Preferably, in step 1, the thickness of the initial steel plate is less than 50 mm.

[0027] Step 2, material heating: the initial steel plate is placed in a furnace above the austenitizing temperature (i.e. 780-1050° C.) for isothermal treatment for 30-80 min (preferably 60 min);

[0028] Step 3, controlled rolling: the total deformation is set to 50% to 55%; the rolling process is multi-pass rolling, the specific number of passes is determined according to the thickness of the initial steel plate, the deformation of each pass is 17% to 22%, the final rolling temperature is 0 to 50°C lower than the start rolling temperature, and a first-stage steel plate is obtained;

[0029] Preferably, in step 3, the rolling start temperature is the material heating temperature in step 2.

[0030] Preferably, in step 3, the final rolling temperature is the same as the start rolling temperature.

[0031] Step 4: Material cooling: After rolling, the first-stage steel plate is cooled to room temperature by air cooling;

[0032] Step 5, placing the first-stage steel plate after air cooling in a furnace at 660-700° C. (preferably 680° C.) for isothermal treatment for 50-80 min (preferably 60 min), and then water cooling to achieve two-phase zone quenching to obtain a second-stage steel plate;

[0033] Step 6: Place the second-stage steel plate in a non-recrystallization zone at 560-600° C. (preferably 580° C.) for isothermal treatment for 50-80 min (preferably 60 min), and then water-cool it to achieve tempering to obtain a 9Ni steel plate.

[0034] Preferably, in step 6, the thickness of the 9Ni steel plate is less than 25 mm.

[0035] Example 1

[0036] Step 1, pretreatment: heat the steel ingot with a size of 130×120×100 mm to 1200°C and perform isothermal treatment for 120 min, and then perform rough rolling to roll the initial steel plate with a thickness of 35 mm;

[0037] Step 2, material heating: the initial steel plate is placed in a furnace at 1000°C for isothermal treatment for 60 minutes;

[0038] Step 3, controlled rolling: the total deformation is set to 53%; the rolling process is set to 4 passes, the first pass is to roll the initial steel plate from 35mm to 28mm, the second pass is to roll the steel plate from 28mm to 22mm, the third pass is to roll the steel plate from 22mm to 18mm, and the fourth pass is to roll the steel plate from 18mm to 15mm (i.e. 35-28-22-18-15), the starting rolling temperature is 1000°C, the final rolling temperature is 960°C, and a first-stage steel plate is obtained;

[0039] Step 4: Material cooling: After rolling, the first-stage steel plate is cooled to room temperature by air cooling;

[0040] Step 5, the first-stage steel plate after air cooling is placed in a furnace at 680° C. for isothermal treatment for 60 minutes, and then water-cooled to achieve two-phase zone quenching to obtain a second-stage steel plate;

[0041] Step 6: Place the second-stage steel plate in a non-recrystallization zone at 580° C. for isothermal treatment for 60 minutes, and then water-cool it to achieve tempering, thereby obtaining a 9Ni steel plate with a thickness of 15 mm.

[0042] Depend on Figure 3It can be seen that when the rolling temperature is 1000℃, the deformation resistance is low, the microstructure growth mechanism is recrystallization to form equiaxed grains, more subgrain boundaries appear in the grains, the dislocation density and nucleation points are greatly reduced, the ferrite is obviously larger, and the microstructure refinement is the highest.

[0043] Example 2

[0044] Step 1, pretreatment: heat the steel ingot with a size of 130×120×100 mm to 1200°C and perform isothermal treatment for 120 min, and then perform rough rolling to roll the initial steel plate with a thickness of 35 mm;

[0045] Step 2, material heating: the initial steel plate is placed in a furnace at 900°C for isothermal treatment for 60 minutes;

[0046] Step 3, controlled rolling: the total deformation is set to 53%; the rolling process is set to 4 passes (i.e., 35-28-22-18-15), the starting rolling temperature is 900° C., the final rolling temperature is 860° C., and a first-stage steel plate is obtained;

[0047] Step 4: Material cooling: After rolling, the first-stage steel plate is cooled to room temperature by air cooling;

[0048] Step 5, the first-stage steel plate after air cooling is placed in a furnace at 680° C. for isothermal treatment for 60 minutes, and then water-cooled to achieve two-phase zone quenching to obtain a second-stage steel plate;

[0049] Step 6: Place the second-stage steel plate in a non-recrystallization zone at 580° C. for isothermal treatment for 60 minutes, and then water-cool it to achieve tempering, thereby obtaining a 9Ni steel plate with a thickness of 15 mm.

[0050] Depend on Figure 4 It can be seen that the dislocation content in the organization is relatively reduced, and the nucleation position is thereby reduced, so that the ferrite grains that undergo dendrite segregation can grow fully, reducing the degree of organization refinement.

[0051] Example 3

[0052] Step 1, pretreatment: heat the steel ingot with a size of 130×120×100 mm to 1200°C and perform isothermal treatment for 120 min, and then perform rough rolling to roll the initial steel plate with a thickness of 35 mm;

[0053] Step 2, material heating: the initial steel plate is placed in a furnace at 800°C for isothermal treatment for 60 minutes;

[0054] Step 3, controlled rolling: the total deformation is set to 53%; the rolling process is set to 4 passes (35-28-22-18-15), the starting rolling temperature is 800°C, the final rolling temperature is 760°C, and a first-stage steel plate is obtained;

[0055] Step 4: Material cooling: After rolling, the first-stage steel plate is cooled to room temperature by air cooling;

[0056] Step 5, the first-stage steel plate after air cooling is placed in a furnace at 680° C. for isothermal treatment for 60 minutes, and then water-cooled to achieve two-phase zone quenching to obtain a second-stage steel plate;

[0057] Step 6: Place the second-stage steel plate in a non-recrystallization zone at 580° C. for isothermal treatment for 60 minutes, and then water-cool it to achieve tempering, thereby obtaining a 9Ni steel plate with a thickness of 15 mm.

[0058] Depend on Figure 5 It can be seen that compared with the structures of Example 1 and Example 2, the degree of refinement is the lowest, which can indicate that as the rolling temperature decreases, the grains become larger.

[0059] The mechanical properties of each embodiment are shown in Table 1:

[0060] Table 1

[0061] Example Tensile strength / MPa Yield strength / MPa Elongation / % -196℃ low temperature impact energy / J 1 781 734 28.2 239 2 744 697 28.4 249 3 736 696 29.6 245

[0062] As can be seen from Table 1, Examples 1 to 3 simplify the production process and obtain good strength and low-temperature toughness.

[0063] Comparative Example 1

[0064] The traditional process is adopted: 800℃ isothermal treatment for 1h quenching + 680℃ isothermal treatment for 1h two-phase zone quenching + 580℃ isothermal treatment for 1h.

[0065] Step 1, pretreatment: heat the steel ingot with a size of 130×120×100 mm to 1200°C and perform isothermal treatment for 120 min, and then perform rough rolling to roll the initial steel plate with a thickness of 35 mm;

[0066] Step 2, placing the initial steel plate in a furnace at 800°C for isothermal treatment for 60 minutes, and then water cooling to achieve two-phase zone quenching to obtain a first-stage steel plate;

[0067] Step 3, placing the first-stage steel plate in a furnace at 680° C. for isothermal treatment for 60 minutes, and then water-cooling to achieve two-phase zone quenching to obtain a second-stage steel plate;

[0068] Step 4: place the second-stage steel plate in a non-recrystallization zone at 580° C. for isothermal treatment for 60 min, and then water-cool it to achieve tempering, thereby obtaining a 9Ni steel plate with a thickness of 35 mm.

[0069] Depend on Figure 6It can be seen that the grains are finer and denser. After two-phase quenching, the 9Ni steel forms tempered martensite on the basis of the original martensite, and the organization is highly refined, which helps to increase the hardness and strength.

[0070] and Figure 3 to Figure 5 By comparison, it can be seen that the 9Ni steel prepared by thermo-mechanical control (TMCP) + heat treatment process under appropriate processing conditions has a finer structure, and the structure has a dual-phase strengthening effect of martensite and bainite.

[0071] Any matters not described in the present invention are applicable to the prior art.

Claims

1. A method for producing 9Ni steel, characterized in that: The method comprises the following steps: Step 1, heating the steel ingot to 1150-1250°C and performing isothermal treatment for 90-150 minutes, and then performing rough rolling to form an initial steel plate; Step 2, placing the initial steel plate in a furnace above the austenitizing temperature for isothermal treatment for 30 to 80 minutes; Step 3, setting the total deformation amount to 50% to 55%; the rolling process is multi-pass rolling, the specific number of passes is determined according to the thickness of the initial steel plate, the deformation amount of each pass is 17% to 22%, and the final rolling temperature is 0 to 50°C lower than the starting rolling temperature to obtain a first-stage steel plate; Step 4: After rolling, the first-stage steel plate is cooled to room temperature by air cooling; Step 5, placing the first-stage steel plate after air cooling in a furnace at 660-700° C. for isothermal treatment for 50-80 minutes, and then water cooling to obtain a second-stage steel plate; Step 6: Place the second-stage steel plate in a non-recrystallization zone at 560-600° C. for isothermal treatment for 50-80 min, and then water-cool it to obtain a 9Ni steel plate.

2. The method for producing 9Ni steel according to claim 1, characterized in that: In step 1, the mass fraction of the steel ingot is as follows: C < 0.10%, Ni is 8.5-9.5%, Si is 0.10-0.15%, Mn is 0.6-0.65%, P < 0.005%, S < 0.005%, Mo < 0.01%, V < 0.01%, W < 0.01%, and the balance is Fe.

3. The method for producing 9Ni steel according to claim 1, characterized in that: In step 3, the final rolling temperature is the same as the start rolling temperature.

4. The method for producing 9Ni steel according to claim 1, characterized in that: In step 3, the rolling temperature is the temperature of step 2.

5. The method for producing 9Ni steel according to claim 1, characterized in that: The thickness of the initial steel plate is less than 50 mm, and the thickness of the 9Ni steel plate is less than 25 mm.

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