Forging and Heat Treatment Process of a 925A Marine Pressure Resistant Shell Steel Forging

Abstract

The present invention discloses a forging and heat treatment process for a 925A marine pressure-resistant hull steel forging. The technical solution includes: Step S1, melting the raw materials according to the designed alloy components; Step S2, forging preheating treatment; Step S3, forging, including three heating times: (1) The first heating time, forging heating, the forging temperature is 1130 - 1150 °C, and then the forging is successively drawn out and upset; (2) The second heating time, forging heating, the forging temperature is 1130 - 1150 °C, and then the forging is successively drawn out, upset, and punched into shape; (3) The third heating time, forging heating, the forging temperature is 1100 - 1130 °C. Step S4, heat treatment of the forging, including the following steps: (1) Post-forging hydrogen-expansion annealing treatment; (2) Rough machining the forging mechanically; (3) Tempering treatment. The advantages of the present invention are to reduce the generation of Widmanstatten structure, eliminate the as-cast structure, make the internal structure of the forging uniform, improve the porosity and segregation phenomena, control the element components, increase the quenched layer of the alloy structure, and thus improve the impact toughness of the product.

Description

Technical Field

[0001] The present invention relates to the field of alloy steel forging treatment processes, and particularly to a forging and heat treatment process for 925A marine pressure-resistant hull steel forgings. Background Art

[0002] The main material for modern shipbuilding is high-strength alloy steel. Both the previous-generation Nimitz-class aircraft carriers and nuclear submarines of the US Navy were built using HY-80 and HY-100 series high-strength steels. After the 1990s, the US began to use HSLA-80 and HSLA-100 series steels to build new-generation warships. The HSLA-80 and HSLA-100 series steels are characterized by allowing welding without preheating and at a lower preheating temperature on the basis of comparable strength, toughness and other properties to HY-80 and HY-100, reducing the cost of shipbuilding.

[0003] Currently, as ships gradually move into the deep blue, there are higher construction requirements for ships. Alloys face very severe corrosion problems in the marine environment. Currently, the low-carbon alloy structural steel 925A has the potential to be used as an alloy for making pressure-resistant hulls due to its good physical and chemical properties.

[0004] However, the current low-carbon alloy structural steel 925A has the following problems:

[0005] 1. The quenched layer in the microstructure of the alloy is small, resulting in insufficient hardenability of the alloy macroscopically and affecting the impact toughness of the product.

[0006] 2. With the current traditional heat treatment and forging methods, a large amount of Widmanstatten structure exists in the alloy forgings, resulting in poor internal structure uniformity of the alloy forgings and the presence of porosity and segregation phenomena.

[0007] 3. The austenitization degree of the alloy is insufficient, resulting in the yield strength and fracture fiber ratio of the traditional low-carbon alloy structural steel 925A not meeting the increasingly strict requirements. Summary of the Invention

[0008] Aiming at the above-mentioned disadvantages of the prior art, the purpose of the present invention is to provide a forging and heat treatment process for marine pressure-resistant hull steel forgings, which has the advantages of reducing the generation of Widmanstatten structure, eliminating the as-cast structure, making the internal structure of the forgings uniform, improving the porosity and segregation phenomena, and increasing the quenched layer of the alloy structure by controlling the element composition, thereby improving the impact toughness of the product.

[0009] The above technical purpose of the present invention is achieved through the following technical solutions:

[0010] A forging and heat treatment process for 925A marine pressure-resistant hull steel forgings includes the following steps:

[0011] Step S1: Melting the raw materials according to the designed alloy composition.

[0012] Step S2: Forging preheating treatment, including the following steps: 1. Loading the furnace at a low temperature; 2. Heating stage, the temperature range is 400 - 850 °C; 3. Insulating in stages: the first-stage insulation temperature of the blank is 850 - 870 °C, and the second-stage insulation temperature of the blank is 1140 - 1180 °C.

[0013] Step S3: Forging, including three heats:

[0014] (1) The first heat: Forging heating, the forging temperature is 1130 - 1150 °C. After that, the forging is drawn out and upset in sequence, and then the forging is put into the forging heating furnace for post-forging heating, and the heating temperature range is 1150 - 1170 °C.

[0015] (2) The second heat: Forging heating, the forging temperature is 1130 - 1150 °C. After that, the forging is drawn out, upset, and punched into shape in sequence, and then the forging is put into the forging heating furnace for post-forging heating, and the heating temperature range is 1130 - 1150 °C.

[0016] (3) The third heat: Forging heating, the forging temperature is 1100 - 1130 °C. After that, the forging is upset, rolled, and punched into shape, and the forging is cooled.

[0017] Step S4: Heat treatment of the forging, including the following steps:

[0018] (1) Post-forging hydrogen-expansion annealing treatment: Cooling the forging in the furnace to 500 °C - 600 °C, then immediately heating to 940 °C, and then air-cooling to 550 °C - 600 °C and immediately putting it into the furnace to heat to 670 °C for insulation.

[0019] (2) Rough machining the forging mechanically.

[0020] (3) Quenching and tempering treatment: Heating the forging to the austenitizing temperature range of 900 - 920 °C, then insulating. After the insulation ends, taking the forging out of the furnace and water-cooling it. After the cooling ends, tempering the forging, the tempering temperature is 650 - 670 °C, then carrying out tempering insulation. After the insulation ends, taking the forging out of the furnace and cooling it in still air.

[0021] Furthermore, in Step S1, the weight percentage of each chemical element in the alloy is: C: 0.13 - 0.18%, Mn: 0.3 - 0.6%, P: ≤0.020%, S ≤0.015%, Si: 0.17 - 0.37%, Cr: 0.9 - 1.2%, Ni: 2.6 - 3.0%, Cu: ≤0.25%; Mo: 0.20 - 0.27%, V: 0.03 - 0.08%, and the balance is Fe and impurities.

[0022] Further, in the low-temperature furnace loading in step S2, the furnace loading temperature of the blank ≤ 500°C. In the staged heat preservation in step S2, the heat preservation time of each heat preservation stage is positively correlated with the thickness of the forging, and for a forging with a thickness of every 200 mm, the heat preservation time is 1.2 - 1.5 h.

[0023] Further, in the first heat in step S3, the total drawing ratio > 2.2; the total upsetting ratio > 2.2, and the final forging temperature ≥ 850°C.

[0024] Further, in the first heat in step S3, the heating time of post-forging heating is positively correlated with the thickness of the forging, and for a forging with a thickness of every 200 mm, the heating time is 1 - 1.2 h.

[0025] Further, in the second heat in step S3, the drawing ratio > 2.2, the upsetting ratio > 2.2, and the final forging temperature ≥ 850°C.

[0026] Further, in the second heat in step S3, the heating time of post-forging heating is positively correlated with the thickness of the forging, and for a forging with a thickness of every 200 mm, the heating time is 1 - 1.2 h.

[0027] Further, in the third heat in step S3, the upsetting ratio > 2.2, and the forging is cooled by water cooling.

[0028] Further, in the post-forging hydrogen-expansion annealing treatment in step S4, the heat preservation time is positively correlated with the thickness of the forging, and for a forging with a thickness of every 1 mm, the heat preservation time is 2.3 - 2.5 min.

[0029] Further, in the quenching and tempering process in step S4, the heat preservation time is positively correlated with the thickness of the forging, and for a forging with a thickness of every 1 mm, the heat preservation time is 1.5 - 1.7 min, and the water temperature for water cooling < 40°C; the tempering heat preservation time is positively correlated with the thickness of the forging, and for a forging with a thickness of every 1 mm, the heat preservation time is 2.3 - 2.5 min.

[0030] In summary, the present invention has the following beneficial effects:

[0031] 1. By strictly controlling the types and contents of elements in the alloy, especially precisely defining the contents of Cr, Ni, Mn, and V, and by controlling the hardenability layer-forming elements, the hardenability layer of the alloy is promoted to increase, and the effect of increasing the hardenability of the product is achieved, which is beneficial to improving the impact toughness of the alloy.

[0032] 2. By reasonably setting multiple forging steps and selecting the best heating and heat preservation windows in each forging step, the formation of Widmanstatten structure in the ingot is effectively reduced, the metallographic structure is made more fine and uniform, and the forging performance of the ingot is optimized.

[0033] 3. By selecting an appropriate large forging ratio, the best processing window of the alloy can be obtained, effectively forging small or breaking up internal inclusions, eliminating the as-cast structure, improving or eliminating porosity and segregation phenomena. At the same time, by controlling the heating between forging passes and the final forging temperature, the grain size of the forgings can be further refined.

[0034] 4. Timely post-forging hydrogen-expansion annealing plays a role in hydrogen expansion, effectively removing hydrogen. Strictly control the post-forging cooling, austenitizing temperature and cooling temperature to obtain a large number of austenite strengthening phases. During quenching and tempering, strictly control the tempering temperature according to the hardness after quenching to control the yield of the product to meet the requirements and ensure that the fiber rate of the fracture reaches 100%. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the steps of the forging and heat treatment process of 925A marine pressure-resistant hull steel forgings.

[0036] Figure 2 It is the metallographic inspection diagram of Example 1.

[0037] Figure 3 It is the metallographic inspection diagram of Example 2.

[0038] Figure 4 It is the metallographic inspection diagram of Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0039] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further details the device proposed by the present invention in combination with the 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 drawings are in a very simplified form and all use non-precise scales, only for the purpose of facilitating and clearly assisting in explaining the embodiments of the present invention. In order to make the objectives, features and advantages of the present invention more obvious and understandable, please refer to the drawings. It should be noted that the structures, ratios, sizes, etc. shown in the drawings of this specification are only used to cooperate with the content disclosed in the specification for those skilled in this technology to understand and read, and are not used to limit the limiting conditions of the implementation of the present invention. Therefore, they do not have technical essence. Any modification of the structure, change of the proportional relationship or adjustment of the size, without affecting the effects that the present invention can produce and the objectives that can be achieved, should still fall within the scope that the technical content disclosed by the present invention can cover.

[0040] Example 1:

[0041] A forging and heat treatment process for 925A marine pressure-resistant hull steel forgings, as Figure 1 shown, includes the following steps:

[0042] Step S1: Melting the raw materials according to the designed alloy components. Specifically, the weight percentages of the chemical elements in the alloy are as follows: C: 0.13 - 0.18%, Mn: 0.3 - 0.6%, P: ≤0.020%, S ≤0.015%, Si: 0.17 - 0.37%, Cr: 0.9 - 1.2%, Ni: 2.6 - 3.0%, Cu: ≤0.25%; Mo: 0.20 - 0.27%, V: 0.03 - 0.08%, and the balance is Fe and impurities.

[0043] Step S2: Forging preheating treatment, including the following steps:

[0044] 1. Loading the furnace at a low temperature, and the loading temperature of the blank ≤500°C.

[0045] 2. Heating-up stage, the temperature range is 400 - 850°C, and during the heating-up process, heat up at the maximum power in the furnace.

[0046] 3. Insulating in stages: The insulating temperature of the blank in the first stage is 850 - 870°C, and the insulating temperature of the blank in the second stage is 1140 - 1180°C. The insulating time in each insulating stage is positively correlated with the thickness of the forging. For forgings with a thickness of every 200mm, insulate for 1.2 - 1.5h.

[0047] Step S3: Forging, including three heats:

[0048] (1) The first heat: Forging heating, the forging temperature is 1130°C. After that, draw out and upset the forging in sequence, with the total draw ratio > 2.2; the total upset ratio > 2.2, and the final forging temperature ≥850°C. Then put the forging into the forging heating furnace for post-forging heating, and the heating temperature range is 1150°C. The heating time of the post-forging heating is positively correlated with the thickness of the forging. For forgings with a thickness of every 200mm, heat for 1 - 1.2h.

[0049] (2) The second heat: Forging heating, the forging temperature is 1130°C. After that, draw out, upset, and punch the forging into shape in sequence, with the draw ratio > 2.2, the upset ratio > 2.2, and the final forging temperature ≥850°C. Then put the forging into the forging heating furnace for post-forging heating, and the heating temperature range is 1130°C. The heating time of the post-forging heating is positively correlated with the thickness of the forging. For forgings with a thickness of every 200mm, heat for 1 - 1.2h.

[0050] (3) The third heat: Forging heating, the forging temperature is 1100°C. After that, upset and roll and punch the forging into shape, with the upset ratio > 2.2, and cool the forging by water cooling.

[0051] Step S4: Heat treatment of the forging, including the following steps:

[0052] (1) Post-forging hydrogen-expansion annealing treatment: Cool the forging in the furnace to 500°C - 600°C, then immediately heat it to 940°C, and then air-cool it to 550°C - 600°C and immediately put it into the furnace to heat to 670°C for heat preservation. The heat preservation time is positively correlated with the thickness of the forging. For each 1 mm thickness of the forging, heat preservation is carried out for 2.3 - 2.5 min.

[0053] (2) Conduct mechanical rough machining on the forging.

[0054] (3) Quenching and tempering treatment: Heat the forging to the austenitizing temperature range of 905°C, then carry out heat preservation. The heat preservation time is positively correlated with the thickness of the forging. For each 1 mm thickness of the forging, heat preservation is carried out for 1.5 - 1.7 min. After the heat preservation is completed, take the forging out of the furnace and water-cool it. The water temperature for water-cooling is less than 40°C. After the cooling is completed, temper the forging. The tempering temperature is 650°C, and then carry out tempering heat preservation. The tempering heat preservation time is positively correlated with the thickness of the forging. For each 1 mm thickness of the forging, heat preservation is carried out for 2.3 - 2.5 min. After the heat preservation is completed, take the forging out of the furnace and cool it in still air.

[0055] Example 2:

[0056] The steps different from Example 1 are as follows:

[0057] Step S3, forging, includes three heating passes:

[0058] (1) The first heating pass, forging heating, the forging temperature is 1140°C. Then, elongate and upset the forging in sequence. The total elongation ratio > 2.2; the total upset ratio > 2.2, and the final forging temperature ≥ 850°C. Then, put the forging into the forging heating furnace for post-forging heating. The heating temperature range is 1150°C. The heating time for post-forging heating is positively correlated with the thickness of the forging. For each 200 mm thickness of the forging, heat it for 1 - 1.2 h.

[0059] (2) The second heating pass, forging heating, the forging temperature is 1140°C. Then, elongate, upset, and punch the forging into shape in sequence. The elongation ratio > 2.2, the upset ratio > 2.2, and the final forging temperature ≥ 850°C. Then, put the forging into the forging heating furnace for post-forging heating. The heating temperature range is 1130°C. The heating time for post-forging heating is positively correlated with the thickness of the forging. For each 200 mm thickness of the forging, heat it for 1 - 1.2 h.

[0060] (3) The third heating pass, forging heating, the forging temperature is 1120°C. Then, upset and roll the forging into shape by punching. The upset ratio > 2.2, and cool the forging by water-cooling.

[0061] Step S4, forging heat treatment, includes the following steps:

[0062] (3) Tempering treatment: The forging is heated to the austenitizing temperature range of 910 °C, then held for a period of time. The holding time is positively correlated with the thickness of the forging. For each 1 mm thickness of the forging, it is held for 1.5 - 1.7 min. After the holding is completed, the forging is taken out of the furnace and water-cooled. The water temperature for water-cooling is less than 40 °C. After the cooling is completed, the forging is tempered. The tempering temperature is 660 °C, and then tempering holding is carried out. The tempering holding time is positively correlated with the thickness of the forging. For each 1 mm thickness of the forging, it is held for 2.3 - 2.5 min. After the holding is completed, the forging is taken out of the furnace and cooled in still air.

[0063] Example 3:

[0064] The steps different from Example 1 are as follows:

[0065] (1) In the first heat treatment, forging heating is carried out at a forging temperature of 1150 °C. Then the forging is successively drawn out and upset. The total drawing ratio > 2.2; the total upset ratio > 2.2, and the final forging temperature ≥ 850 °C. Then the forging is put into a forging heating furnace for post-forging heating. The heating temperature range is 1150 °C. The heating time for post-forging heating is positively correlated with the thickness of the forging. For each 200 mm thickness of the forging, it is heated for 1 - 1.2 h.

[0066] (2) In the second heat treatment, forging heating is carried out at a forging temperature of 1150 °C. Then the forging is successively drawn out, upset, and punched into shape. The drawing ratio > 2.2, the upset ratio > 2.2, and the final forging temperature ≥ 850 °C. Then the forging is put into a forging heating furnace for post-forging heating. The heating temperature range is 1130 °C. The heating time for post-forging heating is positively correlated with the thickness of the forging. For each 200 mm thickness of the forging, it is heated for 1 - 1.2 h.

[0067] (3) In the third heat treatment, forging heating is carried out at a forging temperature of 1130 °C. Then the forging is upset and rolled and punched into shape. The upset ratio > 2.2, and the forging is cooled by water-cooling.

[0068] Step S4, heat treatment of the forging, includes the following steps:

[0069] (3) Tempering treatment: The forging is heated to the austenitizing temperature range of 920 °C, then held for a period of time. The holding time is positively correlated with the thickness of the forging. For each 1 mm thickness of the forging, it is held for 1.5 - 1.7 min. After the holding is completed, the forging is taken out of the furnace and water-cooled. The water temperature for water-cooling is less than 40 °C. After the cooling is completed, the forging is tempered. The tempering temperature is 670 °C, and then tempering holding is carried out. The tempering holding time is positively correlated with the thickness of the forging. For each 1 mm thickness of the forging, it is held for 2.3 - 2.5 min. After the holding is completed, the forging is taken out of the furnace and cooled in still air.

[0070] Comprehensive performance testing of the forging:

[0071] A. Comprehensive mechanical experiment of the forging:

[0072] 1. Experimental group: Randomly select two forgings from Examples 1 - 3, and denote them as Experimental Group 1, Experimental Group 2, and Experimental Group 3 for experiments respectively.

[0073] 2. The experimental results are shown in Table 1.

[0074]

[0075] Table 1

[0076] Conclusion:

[0077] The tensile strength and yield strength of the alloy are increased by 9 - 15%, the elongation rate is increased by about 50%, and the reduction of area is increased by about 80%. Under the low - temperature condition of - 20°C, it still has good impact toughness. Macroscopically, it shows that the effect of fine - grain strengthening is significant.

[0078] B. Metallographic inspection of forgings:

[0079] Figure 2 This is the metallographic structure of Experimental Group 1. The grain size is 7.5 levels. The metallographic structure is fine and uniform, without defects such as porosity and fracture.

[0080] Figure 3 This is the metallographic structure of Experimental Group 2. The grain size is 8 levels. The metallographic structure is fine and uniform, without defects such as porosity and fracture.

[0081] Figure 4 This is the metallographic structure of Experimental Group 3. The grain size is 7.5 levels. The metallographic structure is fine and uniform, without defects such as porosity and fracture.

[0082] The technical features of the above - mentioned embodiments can be combined arbitrarily. For the sake of concise description, not all possible combinations of the technical features in the above - mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0083] The above - mentioned embodiments only represent several implementation manners of the present invention. Their descriptions are relatively specific and detailed, but they should not be understood as limiting the scope of the invention patent. It should be pointed out that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the invention patent should be subject to the appended claims.

Claims

1. A forging and heat treatment process for a 925A marine pressure-resistant hull steel forging, characterized in that, It includes the following steps: Step S1: Melting the raw materials according to the designed alloy components; Step S2: Forging preheating treatment, including the following steps:

1. Loading the furnace at a low temperature; 2. Heating stage, with the temperature range of 400 - 850 °C; 3. Insulating in stages: the first-stage insulation temperature of the blank is 850 - 870 °C, and the second-stage insulation temperature of the blank is 1140 - 1180 °C; Step S3: Forging, including three heats: (1) The first heat: Forging heating, with the forging temperature of 1130 - 1150 °C. After that, the forging is drawn out and upset in sequence, and then the forging is put into the forging heating furnace for post-forging heating, with the heating temperature range of 1150 - 1170 °C; (2) The second heat: Forging heating, with the forging temperature of 1130 - 1150 °C. After that, the forging is drawn out, upset, and punched into shape in sequence, and then the forging is put into the forging heating furnace for post-forging heating, with the heating temperature range of 1130 - 1150 °C; (3) The third heat: Forging heating, with the forging temperature of 1100 - 1130 °C. After that, the forging is upset, rolled round, and punched into shape, and the forging is cooled; Step S4: Heat treatment of the forging, including the following steps: (1) Post-forging hydrogen-expansion annealing treatment: Cooling the forging in the furnace to 500 °C - 600 °C, then immediately heating to 940 °C, then air-cooling to 550 °C - 600 °C and immediately putting it into the furnace to heat to 670 °C for insulation; (2) Rough machining the forging mechanically; (3) Quenching and tempering treatment: Heating the forging to the austenitizing temperature range of 900 - 920 °C, then insulating. After the insulation ends, taking the forging out of the furnace and water-cooling it. After the cooling ends, tempering the forging, with the tempering temperature of 650 - 670 °C, then carrying out tempering insulation. After the insulation ends, taking the forging out of the furnace and cooling it in still air; In step S1, the weight percentage of each chemical element in the alloy is: C: 0.13 - 0.18%, Mn: 0.3 - 0.6%, P: ≤0.020%, S ≤0.015%, Si: 0.17 - 0.37%, Cr: 0.9 - 1.2%, Ni: 2.6 - 3.0%, Cu: ≤0.25%; Mo: 0.20 - 0.27%, V: 0.03 - 0.08%, and the balance is Fe and impurities; In the low-temperature furnace loading in step S2, the furnace loading temperature of the blank ≤500 °C. In the staged insulation in step S2, the insulation time of each insulation stage is positively correlated with the thickness of the forging, and for every 200 mm thickness of the forging, the insulation time is 1.2 - 1.5 h; In the first heat of step S3, the total drawing ratio > 2.2; the total upset ratio > 2.2, the final forging temperature ≥850 °C, and the heating time of the post-forging heating is positively correlated with the thickness of the forging. For every 200 mm thickness of the forging, the heating time is 1 - 1.2 h; In the second heat of step S3, the drawing ratio > 2.2, the upset ratio > 2.2, the final forging temperature ≥850 °C, and the heating time of the post-forging heating is positively correlated with the thickness of the forging. For every 200 mm thickness of the forging, the heating time is 1 - 1.2 h; In the third heat of step S3, the upset ratio > 2.2, and the forging is cooled by water-cooling; In the post-forging hydrogen-expansion annealing treatment in step S4, the holding time is positively correlated with the thickness of the forging, and for each 1 mm thickness of the forging, the holding time is 2.3 - 2.5 min.

2. A forging and heat treatment process for a 925A marine pressure-resistant hull steel forging according to claim 1, characterized in that: In the quenching and tempering process in step S4, the holding time is positively correlated with the thickness of the forging. For each 1 mm thickness of the forging, the holding time is 1.5 - 1.7 min, and the water temperature for water cooling is less than 40 °C; the tempering holding time is positively correlated with the thickness of the forging, and for each 1 mm thickness of the forging, the holding time is 2.3 - 2.5 min.

Images