Forging heat treatment process for ultralow-temperature high-strength austenitic stainless steel forge piece

Abstract

The invention relates to a forging heat treatment process for an ultralow-temperature high-strength austenitic stainless steel forge piece. The forging heat treatment process comprises the following steps that S1, raw materials are smelted; s2, performing high-temperature diffusion annealing on the raw materials; step S3, forging: (1) heating for the first time, lightly pressing and rounding a steel ingot at the final forging temperature of more than or equal to 950 DEG C; (2) secondary heating is conducted, the steel ingot is upset and drawn out, and the finish forging temperature is larger than or equal to 900 DEG C; (3) the third heating number is carried out, the steel ingot is upset and drawn out, and the finish forging temperature is larger than or equal to 850 DEG C; s4, heat treatment: (1) water cooling treatment after forging; (2) performing rough machining on the forge piece; (3) carrying out solution treatment, wherein the solution temperature range is 1050-1120 DEG C; the forging method has the advantages that the internal quality of the forge piece is improved, grains are refined, non-uniformity is eliminated, and the overall mechanical performance is improved by improving the components and the structure of the steel ingot and the forging process.

Description

Technical Field

[0001] The invention relates to the field of alloy forging manufacturing, and in particular to a forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings. Background Art

[0002] Austenitic stainless steel is a type of alloy with chromium and nickel as the main elements, and its composition is basically all austenite. Among them, 316LN stainless steel is the representative, which is mainly composed of chromium, nickel, molybdenum and an appropriate amount of nitrogen. Its good corrosion resistance and mechanical properties enable it to be manufactured into various structures and components, including pipelines, storage tanks, heat exchangers and building elements.

[0003] However, austenitic stainless steel, represented by 316LN stainless steel, is prone to mixed crystals during the manufacturing process, resulting in uneven grains and even cracks in severe cases. On a macro scale, the low-temperature mechanical properties of the alloy are unstable and the product scrap rate is high.

[0004] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of the present disclosure, and therefore may include information that does not constitute the prior art known to ordinary technicians in the field. Summary of the invention

[0005] In view of the deficiencies in the prior art, the present invention discloses a forging heat treatment process for high-strength austenitic stainless steel forgings for ultra-low temperature use.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings comprises the following steps:

[0008] Step S1, smelting raw materials;

[0009] Step S2, high temperature diffusion annealing of raw materials;

[0010] Step S3, forging, comprises:

[0011] (1) In the first tempering, the steel ingot is gently pressed and rounded, and the final forging temperature is ≥950℃;

[0012] (2) In the second tempering, the steel ingot is upset and stretched, and the final forging temperature is ≥900℃;

[0013] (3) The third fire, the steel ingot is upset and stretched, and the final forging temperature is ≥850℃;

[0014] Step S4: heat treatment:

[0015] (1) Water cooling treatment after forging;

[0016] (2) Rough machining of forgings;

[0017] (3) Solution treatment: The solution temperature range is 1050-1120°C;

[0018] Step S5: finishing the forging.

[0019] Furthermore, in step S1, the raw materials are sequentially subjected to the processes of electric arc furnace, refining outside the furnace and electroslag remelting.

[0020] Furthermore, in step S2, raw materials are loaded into the furnace with a loading temperature range of 350-400°C, then heated to a temperature of 400-800°C, and then kept warm in stages, with the first stage keeping warm at a temperature range of 800-870°C; the second stage keeping warm at a temperature range of 1180-1220°C; the keeping warm time of each keeping warm stage is positively correlated with the thickness of the steel ingot, with the keeping warm time of 2.2-2.5mm for every 1mm thickness of the steel ingot.

[0021] Furthermore, in step S3, after the first firing is completed, the steel ingot is heated to 1180-1220°C and kept warm. The heating time is positively correlated with the thickness of the steel ingot. Each 200mm thick steel ingot is heated for 1 to 1.2 hours. After the insulation is completed, the next firing is carried out.

[0022] Further, in step S3, in the second firing, the upsetting ratio is greater than 2.2, and the drawing ratio is greater than 2.2. After completion, the steel ingot is heated to 1130-1150°C and kept warm. The heating time is positively correlated with the thickness of the steel ingot. Each 200mm thick steel ingot is heated for 1 to 1.2 hours. After the insulation is completed, the next firing is carried out.

[0023] Further, in step S3, in the third firing, the upsetting ratio is greater than 2.2, and the drawing ratio is greater than 2.2. After completion, the steel ingot is directly water-cooled.

[0024] Furthermore, in step S4, the solution holding time is positively correlated with the thickness of the forging, and the forging is held for 1.2 to 1.5 minutes for every 1 mm thickness.

[0025] Furthermore, in step S4, after the solutionizing is completed, the forging is placed in running water for cooling; the interval time from the forging being taken out of the furnace to being cooled is ≤2min; and the temperature of the cooling water after the solutionizing is ≤40°C.

[0026] Furthermore, in step S4, after the fixing cooling is completed, solid solution is performed again, and the number of repetitions is ≤2.

[0027] Furthermore, the weight percentage of the chemical elements in the forging is: C: ≤0.02%, Mn: ≤2.0%, P: ≤0.025%, S≤0.010%, Si: ≤0.6%, Cr: 16.5-18.0%, Ni: 13.20-14.0%, Mo: 2.2~2.8; Cu: ≤1.0%; Co: ≤0.05%; N: 0.18~0.21; B≤0.03; Nb≤0.05, and the balance is Fe and impurities.

[0028] The beneficial effects of the present invention are as follows:

[0029] 1. The three-fire process is adopted. The first fire makes the forging reach a larger forging ratio. The large forging ratio improves the internal quality of the steel and eliminates the cast structure. The internal structure of the forging is uniform, and the looseness and segregation are improved or eliminated. The internal inclusions of the material can also be effectively forged small or forged. At the same time, the grain size of the forging is further refined by controlling the heating between fires and the final forging temperature. The grain refinement of the forging can improve various performances such as improving toughness, increasing fatigue resistance and thus increasing service life.

[0030] 2. The forging heating step optimizes the uniformity of the chemical composition of the steel ingot, effectively reduces the free ferrite structure in the steel ingot, and refines the austenite grain size.

[0031] 3. In the heat treatment step after forging, timely water cooling after forging ensures that the grains of the forgings are more uniform and refined. During solid solution, the water entry time and cooling water temperature are controlled to ensure the solid solution effect of the forgings.

[0032] 4. Diffusion annealing is carried out before forging to effectively solve the segregation of materials, make the chemical composition as uniform as possible, and make the product reach a single austenite, thus ensuring the non-magnetic effect.

[0033] 5. During the solid solution strengthening process, carbides, γ' phases and other phases in the stainless steel matrix will be dissolved to obtain a uniform supersaturated solid solution, and a strengthening phase with fine particles and uniform distribution will be obtained, which will eliminate the stress and structural inhomogeneity inside the material, improve the consistency of material performance, and increase the grain size of the alloy.

[0034] 6. The contents of Cr, Ni, Mo and N were adjusted, and the contents of the above elements were appropriately increased and controlled, which effectively improved the uniformity of the metallographic structure of the material, thereby achieving higher strength and good toughness without magnetism and at low temperatures. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 The present invention is a schematic diagram of the steps of a forging heat treatment process for a high-strength austenitic stainless steel forging for ultra-low temperature use.

[0036] Figure 2 This is the mechanical properties test diagram of the forging.

[0037] Figure 3 This is a low-power micrograph of the forging.

[0038] Figure 4 This is a micrograph of the grain size of the forging. DETAILED DESCRIPTION

[0039] The specific implementation of the present invention will be described below in conjunction with the accompanying drawings.

[0040] In order to make the purpose, technical scheme and advantages of the present invention clearer, the device proposed by the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. According to the following description, the advantages and features of the present invention will be clearer. It should be noted that the accompanying drawings adopt a very simplified form and use non-precise proportions, which are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention. In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, please refer to the accompanying drawings. It should be noted that the structure, proportion, size, etc. illustrated by the accompanying drawings of this specification are only used to match the content disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the present invention, so they have no technical substantive significance. Any modification of the structure, change of the proportional relationship or adjustment of the size, without affecting the effect that the present invention can produce and the purpose that can be achieved, should still fall within the scope of the technical content disclosed by the present invention.

[0041] Example:

[0042] A forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings, such as Figure 1 As shown, the following steps are included:

[0043] Step S1, smelting raw materials, the raw materials are sequentially subjected to the processes of electric arc furnace, refining outside the furnace and electroslag remelting to obtain steel ingots, wherein the weight percentage of the chemical elements is: C: ≤0.02%, Mn: ≤2.0%, P: ≤0.025%, S≤0.010%, Si: ≤0.6%, Cr: 16.5-18.0%, Ni: 13.20-14.0%, Mo: 2.2~2.8; Cu: ≤1.0%; Co: ≤0.05%; N: 0.18~0.21; B≤0.03; Nb≤0.05, and the balance is Fe and impurities.

[0044] Step S2, high temperature diffusion annealing of raw materials. The raw materials are charged into a furnace with a charging temperature range of 350-400°C, and then heated to a temperature of 400-800°C, and then heat-insulated in stages. The first stage heat-insulating temperature range is 800-870°C; the second stage heat-insulating temperature range is 1180-1220°C; the heat-insulating time of each heat-insulating stage is positively correlated with the thickness of the steel ingot, and the heat-insulating time of each 1mm thick steel ingot is 2.2-2.5mm. Promote the segregation of the alloy structure, promote the uniformity of each element, and drive the alloy structure to be a single austenite.

[0045] Step S3, forging, comprises:

[0046] (1) In the first fire, the steel ingot is lightly pressed and rounded, and the final forging temperature is ≥950℃. After the first fire, the steel ingot is heated to 1180-1220℃ and kept warm. The heating time is positively correlated with the thickness of the steel ingot. The steel ingot with a thickness of 200mm is heated for 1 to 1.2 hours. After the insulation is completed, the next fire is carried out.

[0047] (2) In the second firing, the steel ingot is upset and drawn, with the upset ratio being greater than 2.2 and the drawing ratio being greater than 2.2. The final forging temperature is ≥900°C. After completion, the steel ingot is heated to 1130-1150°C and kept warm. The heating time is positively correlated with the thickness of the steel ingot. The steel ingot is heated for 1 to 1.2 hours for every 200 mm thickness. After the insulation is completed, the next firing is carried out.

[0048] (3) In the third tempering, the steel ingot is upset and drawn, the upset ratio is greater than 2.2, the drawing ratio is greater than 2.2, and the final forging temperature is ≥850℃.

[0049] Step S4: heat treatment:

[0050] (1) Water cooling treatment after forging: Place the forging in cooling water; the cooling water temperature must be less than 60°C.

[0051] (2) Perform rough mechanical processing on the forgings to remove the oxide scale on the surface of the forgings to ensure the consistency of the internal and external structures and prevent the oxide scale from affecting the heat conduction into the forgings.

[0052] (3) During the solution process, the solution temperature is 1050-1120℃. The holding time is positively correlated with the thickness of the forging. The forging is kept warm for 1.2-1.5 minutes for every 1mm thickness. After the holding period, the forging is taken out of the furnace and water-cooled. The forging is placed in running water for cooling. The interval time from the forging being taken out of the furnace to cooling is ≤2 minutes. The temperature of the cooling water after solution is ≤40℃. Samples are taken from the forgings after cooling for testing. If the performance does not meet the standard, the solution process is repeated, and the number of repetitions is ≤2.

[0053] Step S5: finishing the forging.

[0054] Forging test results:

[0055] The comprehensive mechanical properties of forgings are shown in Figure 2 shown.

[0056] Analysis of mechanical test results: the tensile strength exceeds the standard by about 10%, the yield strength exceeds the standard by about 17%, the elongation exceeds the standard by about 37.5%, the shrinkage exceeds the standard by about 65%, and the three-dimensional hardness of the sample meets the test standards.

[0057] Low-power microscopic inspection of forgings, see Figure 3 shown.

[0058] The forging structure contains no coarse grains or mixed grains.

[0059] Metallographic examination diagram of forgings, see Figure 4 shown.

[0060] No ferrite structure is found in the forging structure, and the austenite structure is uniform.

[0061] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0062] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings, characterized in that: The following steps are involved: Step S1, smelting raw materials; Step S2, high temperature diffusion annealing of raw materials; Step S3, forging, comprising: (1) In the first tempering, the steel ingot is lightly pressed and rounded, and the final forging temperature is ≥950℃; (2) In the second tempering, the steel ingot is upset and stretched, and the final forging temperature is ≥900℃; (3) The third fire, the steel ingot is upset and stretched, and the final forging temperature is ≥850℃; Step S4: heat treatment: (1) Water cooling treatment after forging; (2) Rough machining of forgings; (3) Solution treatment: The solution temperature range is 1050-1120°C; Step S5: The forging is finished.

2. The forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings according to claim 1 is characterized in that: In step S1, the raw materials are sequentially processed through an electric arc furnace, refining outside the furnace and electroslag remelting.

3. The forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings according to claim 1, characterized in that: In step S2, raw materials are loaded into a furnace with a loading temperature range of 350-400°C, then heated to a temperature of 400-800°C, and then kept warm in stages, with the first stage keeping warm at a temperature range of 800-870°C; the second stage keeping warm at a temperature range of 1180-1220°C; the keeping warm time of each keeping warm stage is positively correlated with the thickness of the steel ingot, with the keeping warm time of 2.2-2.5mm for every 1mm thickness of the steel ingot.

4. The forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings according to claim 1, characterized in that: In step S3, after the first firing is completed, the steel ingot is heated to 1180-1220°C and kept warm. The heating time is positively correlated with the thickness of the steel ingot. Each 200mm thick steel ingot is heated for 1 to 1.2 hours. After the insulation is completed, the next firing is carried out.

5. The forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings according to claim 1, characterized in that: In step S3, in the second firing, the upsetting ratio is greater than 2.2, and the drawing ratio is greater than 2.

2. After completion, the steel ingot is heated to 1130-1150°C and kept warm. The heating time is positively correlated with the thickness of the steel ingot. Each 200mm thick steel ingot is heated for 1 to 1.2 hours. After the insulation is completed, the next firing is carried out.

6. The forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings according to claim 1, characterized in that: In step S3, the third fire, the upsetting ratio>2.2, the drawing ratio>2.2, after completion, the steel ingot is directly water-cooled.

7. The forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings according to claim 1, characterized in that: In step S4, the solution holding time is positively correlated with the thickness of the forging, and the holding time is 1.2 to 1.5 minutes for every 1 mm thickness of the forging.

8. The forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings according to claim 7, characterized in that: In step S4, after the solution treatment is completed, the forging is placed in running water for cooling; the interval time from the forging being taken out of the furnace to cooling is ≤2min; the temperature of the cooling water after the solution treatment is ≤40°C.

9. The forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings according to claim 8, characterized in that: In step S4, after the fixing cooling is completed, solid solution is performed again, and the number of repetitions is ≤2.

10. The forging heat treatment process for ultra-low temperature high-strength austenitic stainless steel forgings according to claim 1, characterized in that: The weight percentage of each chemical element in the steel ingot is: C: ≤0.02%, Mn: ≤2.0%, P: ≤0.025%, S≤0.010%, Si: ≤0.6%, Cr: 16.5-18.0%, Ni: 13.20-14.0%, Mo: 2.2-2.8; Cu: ≤1.0%; Co: ≤0.05%; N: 0.18~0.21; B≤0.03; Nb≤0.05, the remainder is Fe and impurities.

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