420MPa-grade super-thick high-low-temperature toughness ocean engineering steel and heat treatment method thereof

Abstract

The invention discloses 420MPa-grade super-thick high-low-temperature toughness ocean engineering steel and a heat treatment method thereof, and belongs to the technical field of steel and iron materials. The heat treatment method of the 420 MPa-grade super-thick high-low-temperature-toughness ocean engineering steel comprises the steps that the content of important alloy elements such as Mn, Ni, Cr and Mo in the steel is regulated and controlled according to the thickness requirement of the ocean engineering steel, and the production of the super-thick steel plate with the low yield ratio and the high-low-temperature toughness is achieved by combining the high-temperature rolling-quenching-two-phase region tempering-high-temperature tempering heat treatment technology. And a product with high yield and excellent strength and low-temperature toughness is obtained. The yield strength of the prepared 420 MPa-grade super-thick high-low-temperature-toughness steel for ocean engineering ranges from 420 MPa to 525 MPa, the tensile strength ranges from 520 MPa to 630 MPa, the thickness ranges from 60 mm to 180 mm, the yield ratio is low (smaller than or equal to 0.85), the technology is integrated, the percentage elongation after fracture is larger than or equal to 22%, and the low-temperature impact toughness is excellent (the impact absorbing energy of the core at the temperature of-40 DEG C is larger than or equal to 240 J).

Description

Technical Field

[0001] The present invention belongs to the field of development and manufacture of steel materials, and in particular relates to a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel and a heat treatment method thereof, and belongs to the field of development and manufacture of high toughness marine engineering steel. Background Art

[0002] In recent years, with the acceleration of marine resource development, the field of marine engineering has developed rapidly, especially in the construction of marine oil and gas development platforms. The required steel must have high strength, high toughness, fatigue resistance, lamellar tearing resistance, good weldability and machinability to ensure safe and reliable service performance. With the development of marine equipment towards deep sea, large scale and polar regions, the requirements for marine engineering steel are also increasing, especially in terms of high strength and toughness and large thickness. Therefore, structural steel with high strength, easy welding, low yield ratio and excellent low temperature toughness shows broad application prospects in marine engineering. In the process of developing and producing extra-thick marine engineering steel, ensuring the uniformity of strength and toughness in the full thickness direction has become an important requirement for marine steel products.

[0003] At present, the yield strength level of conventional ultra-high strength marine engineering steel ranges from 360MPa to 960MPa, and TMCP or modulation process is used to form a microstructure of pearlite or tempered martensite. The current production of ultra-high strength marine engineering steel has the following shortcomings: (1) As the yield strength of the steel plate with tempered martensite as the characteristic structure increases, the yield strength ratio continues to approach 1, which seriously affects the formability and safety of the material; (2) The low-temperature toughness of marine engineering steel with tempered martensite and pearlite as the characteristic structure is insufficient, and it cannot meet the service requirements of marine engineering projects in extremely cold areas. Therefore, under the premise of meeting the strength requirements, low-temperature toughness is particularly important for marine engineering steel. In addition, due to the large thickness specifications of ultra-thick steel plates for marine engineering, the cooling rate of the core is slow, which often makes it difficult to quench, thereby forming a large number of large-grained M / A structures. This phenomenon makes the low-temperature toughness of the core unstable, seriously affecting the overall performance of the steel plate, and it is difficult to meet the predetermined requirements. Therefore, ensuring the uniformity of the structure from the surface to the core of the steel plate, improving the hardenability and core performance, and achieving an ideal match between strength and toughness have become the technical difficulties and core issues that need to be solved in the research and development of such large and thick plates.

[0004] China invention patent application publication CN104674117A discloses a 420MPa grade marine engineering steel plate and its manufacturing method. The alloy composition of the patent contains C: 0.04-0.09wt%, Si: 0.2-0.5wt%, Mn: 1.35-1.6wt%, P: ≤0.015wt%, S ≤0.005wt%, Als: 0.02-0.055wt%, Nb: 0.03-0.06wt%, V: 0.025-0.06wt%, Ti: 0.008-0.02wt%, Cu: 0.2-0.4wt%, Ni: 0.2-0.4wt%, Cr: 0.1-0.3wt%, Nb+V+Ti≤0.12%, and the balance Fe and unavoidable impurities. The rolling + stacking slow cooling process is adopted. The thickness is not enough for current use, and the 80mm thick steel plate can obtain -40℃ low temperature toughness ≥220J. Its low temperature impact toughness is low.

[0005] The Chinese invention patent application CN111088417A discloses a preparation method of normalized EH36, whose components by mass percentage are: C: 0.14% to 0.15%, Mn: 1.2% to 1.5%, Si: 0.1 to 0.25%, Al: 0.015 to 0.03%, Ti: 0.008 to 0.015%, V: 0.05 to 0.08%, N: 0.011 to 0.014%, B: 0.0005 to 0.001%, P: ≤0.005wt%, S ≤0.005wt%, and the rest are Fe and unavoidable impurities. It adopts normalizing process to treat thick plate marine engineering steel, and the low temperature impact toughness is only -40℃≥120 to 150J under the condition of meeting the carbon equivalent. It still cannot meet the use requirements of marine engineering projects in extremely cold areas.

[0006] China invention patent application publication CN116103566A discloses a thick gauge steel for railway bogies with good low temperature toughness and a preparation method thereof, wherein the composition by mass percentage is: C: 0.06-0.10%, Si: 0.15-0.30%, Mn: 1.2-1.3%, P≤0.015%, S≤0.003%, Al: 0.015-0.035%, N≤0.0040%, O≤0.0030%, H≤0.00015%, and the rest is Fe and unavoidable impurities. By adopting the controlled rolling and controlled cooling method, the low temperature impact toughness is -40℃≥255J, the yield strength is 330MPa-405MPa, and the tensile strength is 430MPa-495MPa. Its strength is low, and it is difficult to meet the strict requirements of high strength for marine engineering steel.

[0007] In summary, low-temperature impact toughness and strength are in a trade-off relationship. How to achieve the preparation of high-strength and high-low-temperature impact toughness steel for marine engineering has become the focus of steel research in this field. Summary of the invention

[0008] In order to solve the problems existing in the prior art, the present invention provides a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel and a heat treatment method thereof. According to the thickness requirements of the marine engineering steel, the content of important alloying elements such as Mn, Ni, Cr, and Mo in the steel is regulated, and high-temperature rolling is performed by hot rolling, and combined with the heat treatment process of quenching, two-phase tempering and high-temperature tempering, finally obtaining a high-toughness marine engineering steel with a yield strength of 420MPa to 525MPa, a tensile strength of 520MPa to 630MPa, a yield strength ratio of ≤0.85, an elongation after fracture of ≥22%, and a low-temperature impact performance of -40℃≥240J℃.

[0009] In order to achieve the above object, the technical solution adopted by the present invention is:

[0010] The invention discloses a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel, which contains the following chemical components and their corresponding mass percentages: C: 0.08%-0.15%, Si: 0.2%-0.3%, Mn: 1.0%-2.2%, Al: 0.03%-0.04%, Cr: 0.1%-0.4%, Ni: 0.26%-1.5%, Mo: 0.01%-0.6%, Nb: 0.01%-0.1%, V: 0.03%-0.08%, Ti: 0.01%-0.04%, Cu: 0.2%-0.4%, B: 0.0003%-0.005%, S: ≤0.0023%, P: ≤0.012%, and the balance is Fe and unavoidable impurities.

[0011] The thickness of 420MPa grade marine engineering steel is 60mm to 180mm. In order to achieve the target performance requirements, the relationship between the mass percentage of the main alloying elements and the thickness must be met: Mn% = 0.008×thickness + (0.6 to 0.7); Ni% = 0.009×thickness-(0.22 to 0.28); Cr% = 0.00167×thickness + (0.044 to 0.104); Mo% = 0.0035×thickness-(0.133 to 0.233). Among them, the thickness unit is mm; other alloying elements will also increase with the increase of thickness.

[0012] The yield strength of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel is maintained at 420MPa-525MPa, the tensile strength is 520MPa-630MPa, the low temperature impact toughness is -40°C≥240J, the impact elongation is ≥22%, and the yield strength ratio is ≤0.85.

[0013] The matrix microstructure of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel is a dual-phase structure consisting of tempered martensite and residual austenite, and there is an MC type precipitation phase in the precipitation phase.

[0014] The heat treatment method of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel of the present invention is carried out according to the following steps:

[0015] Step 1: Smelting and Casting

[0016] According to the thickness of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel, the chemical composition contained in it is determined, and the corresponding raw materials are weighed and added into a vacuum induction furnace, and inert gas is introduced to isolate the raw materials from the atmosphere. The vacuum induction furnace is smelted at a temperature of 1550-1650°C and cast into ingots;

[0017] Step 2: Rolling

[0018] The ingot is first kept at a temperature of 1150-1250°C for 2±0.5 hours to homogenize the structure, and then goes through 10-16 pressing processes. The hot rolling process includes two stages: rough rolling and finish rolling. The starting rolling temperature of the rough rolling is 1250-1200°C, the final rolling temperature is 1100-1000°C, and the rolling passes are 5-10 times; the starting rolling temperature of the finish rolling is 1000-900°C, the final rolling temperature is 750-820°C, and the rolling passes are 5-8 times. The rolled slab is water-cooled and cooled to 600-650°C to obtain the rolled plate.

[0019] Step 3: Quenching heat treatment

[0020] The rolled slab is subjected to quenching heat treatment to obtain a quenched plate, and then cooled to room temperature by water cooling to obtain a quenched plate;

[0021] Step 4: Two-phase zone tempering heat treatment

[0022] The quenched plate is subjected to two-phase tempering heat treatment, the quenched plate is heated to 720±10°C and kept warm, and air-cooled to room temperature to obtain a two-phase tempered plate;

[0023] Step 5: High temperature tempering heat treatment

[0024] The obtained two-phase zone tempered plate is kept at 620±20℃ for 2-4min / mm, and then air-cooled to room temperature to obtain the final 420MPa grade ultra-thick high and low temperature toughness marine engineering steel.

[0025] In the step 1, the equipment used for smelting is a vacuum induction furnace, and the temperature in the furnace is 1550° C. to 1650° C.; the thickness of the ingot is preferably 400 mm to 500 mm.

[0026] In the step 2, the thickness of the slab obtained after rolling is preferably 60 to 180 mm.

[0027] In the step three, the quenching heat treatment is performed, the quenching heating temperature is 930±10°C, preferably 930±5°C, the holding time is 0.5-1 min / mm, the cooling method is water cooling, and the heating rate is controlled to be ≤6.25°C / min.

[0028] In the step 4, the two-phase region tempering heat treatment, the tempering heating temperature is 720±10°C, the holding time is 1.5-3min / mm+20min, the cooling method is air cooling, and the heating rate is controlled to be ≤8.25°C / min;

[0029] In the step 5, the high temperature tempering heat treatment is performed, the tempering heating temperature is 620±20°C, the holding time is 2-4 min / mm, the cooling method is air cooling, and the heating rate is controlled to be ≤8.25°C / min;

[0030] In the step 2, the hot rolling process used includes two stages: rough rolling and finish rolling. The first stage is high-temperature rolling with a higher temperature and a larger pass reduction, ranging from 30% to 50%, which can promote recrystallization and refine austenite. The second stage is low-temperature rolling with a lower temperature and a smaller pass reduction, ranging from 3% to 8%, which mainly introduces defects and promotes nucleation.

[0031] In the step 4, the two-phase tempering heat treatment is used, and the holding time is 20min+1.5-3min / mm. The two-phase tempering process can precipitate retained austenite and precipitate phase in the steel. The increase of retained austenite is an important factor affecting the reduction of yield strength ratio and the improvement of toughness. The precipitation phase also plays an important role in improving the yield strength and tensile strength.

[0032] The microstructure of the finished steel plate is tempered martensite + residual austenite, and there is an MC type precipitation phase in the precipitation phase. The action mechanism of each alloy component in the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel prepared by the present invention is as follows, wherein the percentage symbol % represents the mass percentage:

[0033] C: Carbon is conducive to the formation of austenite, can improve the stability of austenite, and play a role in solid solution strengthening and precipitation strengthening, but too much carbon content will have a negative impact on the ductility and toughness of steel, especially weldability, and seriously affect the mechanical properties of the obtained high and low temperature toughness marine engineering steel. Therefore, the C content of the present invention is controlled within 0.08% to 0.15%.

[0034] Si: Si in the solid solution state has an enhancing effect on the strength and hardness of steel and is one of the main elements that provide strength. At the same time, the addition of Si can inhibit the formation of bainite, inhibit the formation and coarsening of cementite, and improve toughness. However, if the content is too high, it will have a negative impact on strength and toughness. Therefore, the Si content of the present invention is controlled within 0.2% to 0.3%.

[0035] Mn: Manganese has a great influence on the stacking fault energy and the stability of austenite, can improve the stacking fault energy and the stability of austenite, play a role in solid solution strengthening, increase its strength, and reduce the phase transition temperature of austenite, reducing the transformation rate of austenite to martensite. Therefore, the Mn content of the present invention is controlled within 1.0% to 2.2%.

[0036] Al: Aluminum can effectively inhibit the precipitation of cementite, promote the enrichment of carbon in austenite, and effectively enhance the stability of residual austenite. Therefore, the Al content of the present invention is controlled within 0.03% to 0.04%.

[0037] Cr: Adding Cr can improve the strength and hardenability of steel. Its solid solution strengthening can make up for the strength reduced by carbon content. Its presence can enhance the tempering stability of steel. It can also form carbides with carbon to increase its strength and wear resistance. The Cr content of the present invention is controlled within 0.1% to 0.4%.

[0038] Ni: has a solid solution strengthening effect and is the main alloying element for stabilizing austenite. It has the effect of lowering the Ar3 point and shifting the CCT curve to the right, and can form a fine martensitic structure, thereby improving the strength and toughness of steel, especially low-temperature toughness. Ni also affects the lateral slip of dislocations and reduces the ductile-brittle transition temperature of steel. Therefore, the Ni content in the present invention is controlled within a range of 0.26% to 1.5%.

[0039] Mo: an element that improves hardenability, expands the γ phase region, increases the stability of austenite, plays an important role in controlling the phase transformation structure, and can effectively improve the strength of the material; reduces the phase transformation temperature, reduces the critical cooling rate of bainite transformation, and can effectively improve the stability of the strength and toughness properties in the thickness direction of the steel plate. Appropriate Mo elements can overcome the brittleness of steel during modulation treatment, thereby improving the impact toughness of steel. Therefore, the Mo content of the present invention is controlled within 0.01% to 0.6%.

[0040] Nb: a strong carbide-forming element, which can refine the phase transformation structure, improve toughness, increase the nucleation rate of precipitates, increase the number of precipitates, reduce the size of precipitates, and increase the strength of steel. Therefore, the Nb content of the present invention is controlled within 0.01% to 0.1%.

[0041] V: effectively refines the grain size of steel, and at the same time significantly improves the strength of steel by forming a nano-scale second phase and interacting with dislocations. Therefore, the V content of the present invention is controlled within 0.03% to 0.08%.

[0042] Ti: It has two main functions in low-carbon low-alloy steel. It combines with N and C to form stable nitrides and carbides, which prevent the growth of austenite grains at high temperatures and help improve the toughness of the coarse-grained zone in the welding heat-affected zone. When the Ti content is high, fine and dispersed TiC precipitates are formed in the ferrite, producing precipitation strengthening. Therefore, the Ti content of the present invention is controlled within 0.01% to 0.04%.

[0043] Cu: Cu will dissolve in steel to cause lattice distortion, increase the resistance to dislocation movement, and play a role in solid solution strengthening; or form Cu-rich precipitation phase to enhance the strength and hardness of steel. In addition, during welding, Cu can improve the welding quality of steel plates by reducing the crack sensitivity of the heat-affected zone. However, too high a Cu content will increase the hot brittleness of steel, reduce welding performance and cold working performance, affect surface quality, and weaken corrosion resistance under certain conditions. Therefore, the Cu content of the present invention is controlled at 0.2% to 0.4%.

[0044] B: is the most significant element for improving hardenability, but too high a content will have a negative impact on weldability and low-temperature toughness. Therefore, the B content of the present invention is controlled within 0.0003% to 0.005%.

[0045] P: is an element that has an adverse effect on low-temperature toughness and ductility. It may segregate in the center of the slab and aggregate at the grain boundaries, thereby damaging the low-temperature toughness. Therefore, in the present invention, P is controlled to be no more than 0.012%.

[0046] S: is an element that has an adverse effect on low-temperature toughness and ductility. It can form sulfide inclusions and become a crack source. Therefore, in the present invention, S is controlled to be no more than 0.0023%.

[0047] Compared with the prior art, the 420MPa grade ultra-thick high-low temperature toughness marine engineering steel and the heat treatment method thereof of the present invention have the following characteristics and beneficial effects:

[0048] 1) The present invention ensures the weldability of the steel plate through low C content. The Mn and Mo that increase with thickness can significantly improve the hardenability of the steel, reduce the difference between the surface and core structures, and ensure the uniformity of the structure in the thickness direction. At the same time, the precipitation strengthening effect of Mo helps to improve the strength distribution of the core, and also has a positive effect on corrosion resistance and welding. Ni that increases with thickness, as an austenite stabilizing element, helps to maintain a relatively uniform austenite phase in ultra-thick steel and improve the toughness of the material. Cr that increases with thickness can regulate the solid solution of alloy elements and the precipitation behavior of carbides, help to evenly distribute the alloy precipitation phase in the core, reduce organizational segregation and uneven performance distribution, improve organizational uniformity, and prevent local performance weakening. In addition, the added microalloying elements Nb, V, and Ti can form nano-sized precipitation phase MC-type carbides to improve strength, and can also provide effective nucleation sites, refine grain size, and further optimize organizational properties; controlling the S and P contents can reduce the influence of inclusions on low-temperature toughness.

[0049] 2) The present invention adopts a heat treatment process of rolling + quenching + two-phase tempering + high-temperature tempering. The tempered martensite structure obtained by the critical two-phase tempering process can ensure high strength; at the same time, the appropriate amount of residual austenite toughness structure generated will also trigger the TRIP effect, thereby significantly improving the low-temperature toughness of the material and reducing the material yield ratio. The carbide precipitation phase precipitated after high-temperature tempering can further significantly improve the strength of the steel through precipitation strengthening. Due to the dispersed distribution of the second phase particles, the growth of the grain size after tempering is effectively suppressed, and the difference in grain size between the surface and the core is small. Through the coordinated optimization of chemical composition regulation and preparation process, high-performance steel for marine engineering with excellent performance was successfully prepared.

[0050] 3) The yield strength of the steel of the present invention is ≥420MPa, the tensile strength is ≥520MPa, the low yield strength ratio is ≤0.85, the elongation after fracture is ≥22%, and the low-temperature toughness of the core at -40°C is ≥240J, thereby achieving the simultaneous improvement of low-temperature impact toughness and strength.

[0051] 4) The present invention optimizes the heat treatment process, adds a critical two-phase region tempering treatment between the modulation processes, increases the content of austenite structure in the steel, improves the precipitation and distribution of carbides, and greatly enhances its low-temperature toughness compared with hot-rolled steel and traditional modulation steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 Figure 1 The thermal processing diagram of 420MPa grade ultra-thick high and low temperature toughness marine engineering steel of Examples 1 to 3 of the present invention;

[0053] Figure 2 This is the XRD spectrum of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel in implementation case 1.

[0054] Figure 3 This is a metallographic diagram of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel at 1 / 2 of Example 1 of the present invention;

[0055] Figure 4 This is a metallographic diagram of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel at 1 / 2 of Example 2 of the present invention;

[0056] Figure 5 This is a metallographic diagram of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel at 1 / 2 of Example 3 of the present invention;

[0057] Figure 6 The metallographic diagram of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel at 1 / 2 of the comparative example 1 of the present invention;

[0058] Figure 7 The metallographic diagram of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel at 1 / 2 of the comparative example 2 of the present invention;

[0059] Figure 8 This is the metallographic diagram at 1 / 2 of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel of Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0060] The present invention is further described in detail below in conjunction with embodiments.

[0061] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described in combination with the embodiments of the present invention. Obviously, the described embodiments are partial embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0062] The chemical composition of the 420 MPa grade ultra-thick high and low temperature toughness marine engineering steel prepared in the embodiments of the present invention and the comparative examples is shown in Table 1, the heat treatment process is shown in Table 2, and the mechanical properties are shown in Table 3.

[0063] Table 1 Chemical composition of marine engineering steel in the embodiments of the present invention and the comparative examples (in mass percentage %)

[0064]

[0065]

[0066] Example 1

[0067] A heat treatment method of a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel in this embodiment is carried out according to the following steps:

[0068] 1) Add the chemical composition of 420MPa grade ultra-thick high and low temperature toughness marine engineering steel into the vacuum induction furnace, heat the melting furnace to a temperature range of 1550℃~1650℃, and then forge the ingot into a slab. Then cast the smelted molten steel into an ingot.

[0069] 2) The ingot was kept at 1150°C for 2 hours to homogenize the structure, and then the ingot was forged into a slab with a thickness of 200 mm.

[0070] 3) The slab is hot rolled to obtain a hot rolled plate, the hot rolling passes are 12 times, the thickness of the rolled plate is 60 mm, and the plate is cooled to room temperature by air cooling. The hot rolling start temperature is 1120°C, and the final rolling temperature is 800°C.

[0071] 4) Perform quenching heat treatment, heat the hot-rolled plate to 930° C. and keep it at this temperature for 60 min, and then water-cool it to room temperature to obtain a quenched plate.

[0072] 5) The quenched plate is subjected to tempering treatment at 720°C for 200 minutes and then air-cooled, i.e., two-phase zone tempering heat treatment, to obtain a two-phase zone tempered steel plate.

[0073] 6) The two-phase tempered steel plate is subjected to a tempering treatment at 620°C for 240 minutes and then air-cooled, i.e., a high-temperature tempering heat treatment, to obtain a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel.

[0074] Example 2

[0075] A heat treatment method of a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel in this embodiment is carried out according to the following steps:

[0076] 1) Add the steel into the vacuum induction furnace according to its chemical composition, heat the melting furnace to a temperature range of 1550℃~1650℃, and then forge the ingot into a slab. Then cast the smelted molten steel into an ingot.

[0077] 2) The ingot was kept at 1150°C for 2 hours to homogenize the structure, and then the ingot was forged into a slab with a thickness of 400 mm.

[0078] 3) The slab is hot rolled to obtain a hot rolled plate, the hot rolling passes are 14 times, the thickness of the plate after rolling is 120 mm, and the plate is cooled to room temperature by air cooling. The hot rolling start temperature is 1100°C, and the final rolling temperature is 790°C.

[0079] 4) Perform quenching heat treatment, heat the hot-rolled plate to 930° C. and keep it at this temperature for 120 min, and water-cool it to room temperature to obtain a quenched plate.

[0080] 5) The quenched plate is subjected to tempering treatment at 720°C for 260 minutes and then air-cooled, i.e., two-phase zone tempering heat treatment, to obtain a two-phase zone tempered steel plate.

[0081] 6) The two-phase tempered steel plate is subjected to a tempering treatment at 620°C for 360 minutes and then air-cooled, i.e., a high-temperature tempering heat treatment, to obtain a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel.

[0082] Example 3

[0083] A heat treatment method of a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel in this embodiment is carried out according to the following steps:

[0084] 1) Add the steel into the vacuum induction furnace according to its chemical composition, heat the melting furnace to a temperature range of 1550℃~1650℃, and then forge the ingot into a slab. Then cast the smelted molten steel into an ingot.

[0085] 2) The ingot was kept at 1150°C for 2 hours to homogenize the structure, and then the ingot was forged into a slab with a thickness of 500 mm.

[0086] 3) The slab is hot rolled to obtain a hot rolled plate, the hot rolling passes are 16, the thickness of the plate after rolling is 180 mm, and the plate is cooled to room temperature by air cooling. The hot rolling start temperature is 1100°C, and the final rolling temperature is 790°C.

[0087] 4) Perform quenching heat treatment, heat the hot-rolled plate to 930° C. and keep it at this temperature for 140 min, and water-cool it to room temperature to obtain a quenched plate.

[0088] 5) The quenched plate is subjected to a tempering treatment at 720°C for 300 minutes and then air-cooled, i.e., a two-phase zone tempering heat treatment, to obtain a two-phase zone tempered steel plate.

[0089] 6) The two-phase zone tempered steel plate is subjected to a tempering treatment at 620°C for 400 minutes and then air-cooled, i.e., a high-temperature tempering heat treatment, to obtain a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel.

[0090] Comparative Example 1

[0091] The heat treatment method of a 420MPa grade high toughness marine engineering steel in this comparative example is carried out according to the following steps:

[0092] 1) Add the steel into the vacuum induction furnace according to its chemical composition, heat the melting furnace to a temperature range of 1550℃~1650℃, and then forge the ingot into a slab. Then cast the smelted molten steel into an ingot.

[0093] 2) The ingot was kept at 1150°C for 2 hours to homogenize the structure, and then the ingot was forged into a slab with a thickness of 200 mm.

[0094] 3) The slab is hot rolled to obtain a hot rolled plate, the hot rolling passes are 14 times, the thickness of the plate after rolling is 60 mm, and it is cooled to room temperature by air cooling. The hot rolling start temperature is 1120°C, and the final rolling temperature is 800°C.

[0095] The final comparative example 1 obtained is the sample after rolling.

[0096] Comparative Example 2

[0097] A heat treatment method of a 420MPa grade high toughness marine engineering steel in this embodiment is carried out according to the following steps:

[0098] 1) Add the steel into the vacuum induction furnace according to its chemical composition, heat the melting furnace to a temperature range of 1550℃~1650℃, and then forge the ingot into a slab. Then cast the smelted molten steel into an ingot.

[0099] 2) The ingot was kept at 1150°C for 2 hours to homogenize the structure, and then the ingot was forged into a slab with a thickness of 200 mm.

[0100] 3) The slab is hot rolled to obtain a hot rolled plate, the hot rolling passes are 14 times, the thickness of the plate after rolling is 60 mm, and it is cooled to room temperature by air cooling. The hot rolling start temperature is 1120°C, and the final rolling temperature is 800°C.

[0101] 4) Perform quenching heat treatment, heat the hot-rolled plate to 930° C. and keep it at this temperature for 60 min, and then water-cool it to room temperature to obtain a quenched plate.

[0102] 5) The quenched plate is subjected to tempering treatment at 620°C for 240 minutes and then air-cooled, i.e., high-temperature tempering heat treatment, to obtain a high-temperature tempered steel sample.

[0103] Comparative Case 3

[0104] A heat treatment method of a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel in this embodiment is carried out according to the following steps:

[0105] 1) Add the steel into the vacuum induction furnace according to its chemical composition, heat the melting furnace to a temperature range of 1550℃~1650℃, and then forge the ingot into a slab. Then cast the smelted molten steel into an ingot.

[0106] 2) The ingot was kept at 1150°C for 2 hours to homogenize the structure, and then the ingot was forged into a slab with a thickness of 500 mm.

[0107] 3) The slab is hot rolled to obtain a hot rolled plate, the hot rolling passes are 16, the thickness of the plate after rolling is 180 mm, and the plate is cooled to room temperature by air cooling. The hot rolling start temperature is 1100°C, and the final rolling temperature is 790°C.

[0108] 4) Perform quenching heat treatment, heat the hot-rolled plate to 930° C. and keep it at this temperature for 140 min, and water-cool it to room temperature to obtain a quenched plate.

[0109] 5) The quenched plate is subjected to tempering treatment at 720°C for 300 minutes and then air-cooled, i.e., two-phase zone tempering heat treatment, to obtain a two-phase zone tempered steel plate.

[0110] 6) The two-phase zone tempered steel plate is subjected to a tempering treatment at 620°C for 400 minutes and then air-cooled, i.e., a high-temperature tempering heat treatment, to obtain a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel.

[0111] Table 2 Heat treatment process of marine engineering steel in the embodiments of the present invention and the comparative examples

[0112]

[0113] Figure 1 This is a thermal processing diagram of 420MPa grade ultra-thick high and low temperature toughness marine engineering steel of Examples 1 to 3 of the present invention, Figure 2 This is the XRD spectrum of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel in implementation case 1. Figure 3 to Figure 5 The microstructures of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steels prepared in Examples 1 to 3 are shown respectively. Figure 6-Figure 8 The microstructure morphologies of the marine engineering steels prepared in Control Example 1, Control Example 2, and Control Example 3 are respectively.

[0114] Depend on Figure 2 It can be seen that the marine engineering steel treated by the heat treatment process provided by the present invention contains a certain amount of stable residual austenite (γ-Fe) tough phase, and can produce TRIP effect during the plastic deformation process of the material, thereby increasing the plasticity and toughness of the material. Figure 3 to Figure 8 It can be seen that the marine engineering steel treated by the heat treatment process provided by the present invention contains finer grains and precipitates tiny granular carbides, which greatly improves the mechanical properties of the material.

[0115] The marine engineering steel prepared in Examples 1 to 3 and Comparative Examples 1 to 2 was subjected to relevant performance tests. According to GB / T228-2002 "Room Temperature Tensile Test Method for Metallic Materials", the heat-treated steel plate was processed into a standard tensile specimen, and the tensile speed was fixed at 2 mm / min. The mechanical properties of the high-toughness marine engineering steel prepared in Examples 1 to 3 and Comparative Examples 1 to 2 were tested using an MTS-ZBC2452-B pendulum impact tester and an AG-XPLUS100KN electronic universal testing machine, respectively, and the results are listed in Table 3.

[0116] Table 3 Mechanical properties of marine engineering steel in the embodiments of the present invention and the comparative examples

[0117]

[0118] The above data show that the mechanical properties of the marine engineering steel obtained without heat treatment are poor when the scheme of the present invention is compared with the control example 1 without heat treatment, indicating that heat treatment can improve the mechanical properties of marine engineering steel. The scheme of the present invention is compared with the control case 2 that only performs quenching and high-temperature tempering heat treatment, which shows that the technical scheme of the present invention combining high-temperature rolling-online quenching-two-phase tempering-high-temperature tempering heat treatment process can achieve the production of ultra-thick steel plates with low yield ratio and high toughness, and obtain steel plates with high yield rate, stable strength and low-temperature toughness, especially outstanding performance in impact absorption energy. By comparing the implementation case 3 and the control example 3, it can be found that the relationship between the mass percentage of the main alloying elements and the thickness is not satisfied: Mn% = 0.008 × thickness + (0.6 ~ 0.7); Ni% = 0.009 × thickness - (0.22-0.28); Cr% = 0.00167 × thickness + (0.044 ~ 0.104); Mo% = 0.0035 × thickness - (0.133 ~ 0.233). Among them, when the thickness unit is mm, its impact absorption energy is reduced, indicating that only by regulating the main alloying elements can the material hardenability be increased and the core structure uniformity be improved, thereby obtaining ideal mechanical properties.

[0119] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A heat treatment method for 420MPa grade ultra-thick high and low temperature toughness marine engineering steel, characterized by: (1) Smelting and casting According to the thickness of 420MPa grade ultra-thick high and low temperature toughness marine engineering steel, the required chemical composition is determined, the corresponding raw materials are melted in an inert gas atmosphere, and ingots are obtained after die casting; (2) Rolling The ingot is heated and kept warm, then rolled into a slab, hot-rolled to a thickness of 60 mm to 180 mm, and cooled by water cooling to obtain a rolled slab; (3) Heat treatment The rolled slab is subjected to a multi-step heat treatment, firstly subjected to a quenching treatment to obtain a quenched steel; the quenched steel is subjected to a two-phase zone tempering treatment to obtain a two-phase zone tempered steel; finally, the two-phase zone tempered steel is subjected to a high-temperature tempering treatment to obtain a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel.

2. The heat treatment method of 420MPa grade ultra-thick high and low temperature toughness marine engineering steel according to claim 1, characterized in that: The chemical composition and their corresponding mass percentages of 420MPa grade ultra-thick high and low temperature toughness marine engineering steel are: C: 0.08%~0.15%, Si: 0.2%~0.3%, Mn: 1.0%~2.2%, Al: 0.03%~0.04%, Cr: 0.1%~0.4%, Ni: 0.26%~1.5%, Mo: 0.01%~0.6%, Nb: 0.01%~0.1%, V: 0.03%~0.08%, Ti: 0.01%~0.04%, Cu: 0.2%~0.4%, B: 0.0003%~0.005%, S: ≤0.0023%, P: ≤0.012%, and the balance is Fe and unavoidable impurities.

3. A heat treatment method for a 420MPa grade ultra-thick high and low temperature toughness marine engineering steel as claimed in claim 1, characterized in that: The thickness of 420MPa grade marine engineering steel is 60mm~180mm. In order to achieve the target performance requirements, the relationship between the mass percentage of the main alloying elements and the thickness must be met: Mn%=0.008×thickness+(0.6~0.7); Ni%=0.009×thickness-(0.22~0.28); Cr%=0.00167×thickness+(0.044~0.104); Mo%=0.0035×thickness-(0.133~0.233), where the unit of thickness is mm.

4. The heat treatment method of 420MPa grade ultra-thick high and low temperature toughness marine engineering steel according to claim 1, characterized in that: In step (1), the smelting temperature is 1550-1650°C.

5. The heat treatment method of 420MPa grade ultra-thick high and low temperature toughness marine engineering steel according to claim 1, characterized in that: In step (2), the heating and heat preservation process is as follows: heating the ingot to a temperature range of 1150-1250°C and keeping it warm for 2±0.5 hours; the rolling process adopted is 10-16 pressing processes, including two stages of rough rolling and finish rolling, the starting rolling temperature of the rough rolling is 1250-1200°C, the final rolling temperature is 1100-1000°C, and the rolling passes are 5-10 times; the starting rolling temperature of the finish rolling is 1000-900°C, the final rolling temperature is 750-820°C, and the rolling passes are 5-8 times, and the rolled slab is water-cooled to 600-650°C to obtain a rolled plate.

6. The heat treatment method of 420MPa grade ultra-thick high and low temperature toughness marine engineering steel according to claim 1, characterized in that: In step (3), the quenching heat treatment process parameters are: the quenching and holding temperature is 920-940°C, the holding time is 0.5-1 min / mm, the quenching heat treatment process cooling method is water cooling; the heating rate from the quenching and holding temperature to the quenching and holding temperature is ≤6.25°C / min.

7. The heat treatment method of 420MPa grade ultra-thick high and low temperature toughness marine engineering steel according to claim 1, characterized in that: In step (3), the two-phase zone tempering process parameters are: the two-phase zone tempering insulation temperature is 710-730°C, the insulation time is (20min+1.5-3min / mm), the two-phase zone tempering heat treatment process cooling method is air cooling; the heating rate to the two-phase zone tempering insulation temperature is ≤8.25°C / min.

8. The heat treatment method of 420MPa grade ultra-thick high and low temperature toughness marine engineering steel according to claim 1, characterized in that: In step (3), the high temperature tempering process parameters are: the high temperature tempering insulation temperature is 600-640°C, the insulation time is 2-4 min / mm, the high temperature tempering heat treatment process cooling method is air cooling; the heating rate to the insulation temperature of the high temperature tempering treatment is ≤8.25°C / min.

9. A 420MPa grade ultra-thick high and low temperature toughness marine engineering steel, characterized by: The steel is prepared by the heat treatment method according to any one of claims 1 to 8; the yield strength of the 420MPa grade marine engineering steel is maintained at 420-525MPa, the tensile strength is 520-630MPa, the yield strength ratio is ≤0.85, the impact elongation is ≥22%, and the low-temperature impact toughness of the core at -40°C is ≥240J.

10. The 420MPa grade ultra-thick high and low temperature toughness marine engineering steel according to claim 9, characterized in that: The microstructure of the 420MPa grade ultra-thick high and low temperature toughness marine engineering steel is a dual-phase structure consisting of tempered martensite + retained austenite, and there is an MC type precipitation phase in the precipitation phase.

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