A low-cost, high-strength, high-uniformity, extra-thick steel plate and its preparation method

Abstract

The embodiments of the present invention relate to a low-cost, high-strength, high-uniformity, extra-thick steel plate and a preparation method thereof, belonging to the technical field of alloy steel processing. The preparation method comprises the following steps: obtaining a steel ingot by smelting and casting; wherein, in terms of mass percentage, the chemical composition of the steel ingot comprises: C: 0.35-0.40%, Si: 0.10-0.35%, Mn: 0.80-1.10%, Cr: 2.80-3.50%, Ni: 0.80-1.20%, Mo: 0.20-0.40%, Al: 0.015-0.035%, B: 0.0005-0.0050%, P≤0.015%, S≤0.005%, and the remainder is Fe and unavoidable impurity elements; after homogenizing heat treatment of the steel ingot, performing upsetting forging at least three times, wherein the upsetting forging includes at least one gradient temperature forging, to obtain a steel plate; performing quenching and tempering heat treatment on the steel plate to obtain a steel plate with a thickness of 200 mm to 400 mm. The present invention can obtain an extra-thick steel plate with high strength and low-temperature toughness through the method, and can improve the uniformity and consistency of the structure and performance of the extra-thick steel plate.

Description

Technical Field

[0001] The present invention relates to the technical field of alloy steel, and in particular to a low-cost, high-strength, high-uniformity, extra-thick steel plate and a preparation method thereof. Background Art

[0002] Extra-thick and extra-thick steel plates are a type of high-strength, wear-resistant and corrosion-resistant metal material commonly used in various industrial and construction fields. Their excellent performance and reliability make them one of the ideal materials for various heavy equipment and structures.

[0003] With the gradual improvement of equipment large-scale and construction integration in recent years, the structural dimensions and comprehensive mechanical performance requirements of components in various fields have also been continuously improved, thus greatly increasing the high strength, toughness and high uniformity requirements for ultra-thick and extra-thick steel plates. Among them, the maximum thickness of the steel plate is required to reach 400mm, the strength is above 800MPa, and it has excellent low-temperature toughness. However, when the thickness of the steel plate reaches more than 150mm, the uniformity of the internal structure of the steel plate is difficult to control and has a great impact on its performance. The current production process is difficult to achieve the high strength, toughness and high uniformity requirements of extra-thick steel plates. Summary of the invention

[0004] In view of the above analysis, the embodiments of the present invention aim to provide a low-cost, high-strength, high-uniformity extra-thick steel plate and a preparation method thereof, so as to solve at least one of the problems that the uniformity of the internal structure of the extra-thick steel plate is difficult to control and the existing process is difficult to achieve high strength, toughness and high uniformity of the extra-thick steel plate.

[0005] The embodiment of the present invention provides a method for preparing a low-cost, high-strength, high-uniformity and extra-thick steel plate, comprising:

[0006] A steel ingot is obtained by smelting and casting; the chemical composition of the steel ingot includes, by mass percentage, C: 0.35-0.40%, Si: 0.10-0.35%, Mn: 0.80-1.10%, Cr: 2.80-3.50%, Ni: 0.80-1.20%, Mo: 0.20-0.40%, Al: 0.015-0.035%, B: 0.0005-0.0050%, P≤0.015%, S≤0.005%, and the balance is Fe and unavoidable impurity elements;

[0007] After the steel ingot is subjected to homogenization heat treatment, it is subjected to upsetting and drawing forging for at least three times, wherein the upsetting and drawing forging includes at least one gradient temperature forging, to obtain a steel plate;

[0008] The steel plate is subjected to tempering heat treatment to obtain a steel plate with a thickness of 200 to 400 mm.

[0009] Furthermore, when the steel ingot is forged, three upsetting forgings are performed; the total deformation compression ratio of the three upsetting forgings is not less than 6, and the steel ingot is upset to less than 1 / 2 of the height of the original ingot body during each upsetting.

[0010] Furthermore, in the three-stage upsetting forging:

[0011] The steel ingot is heated to 1185-1215°C, kept at this temperature for more than 4 hours, and then forged once, with the reduction rate controlled at 30-40% during the forging;

[0012] After the flattened billet is forged once, it is returned to the furnace and heated to 1185-1215°C, then taken out and allowed to stand for heating. When the billet surface temperature drops to 1000°C, it is forged twice, and the reduction ratio is controlled at 35-45% during the second forging.

[0013] After the secondary forging, the flattened billet is returned to the furnace and heated to 1185-1215°C and kept at this temperature for 1-1.5 hours, and then directly subjected to the tertiary forging, during which the reduction ratio is controlled at 25-30%.

[0014] Furthermore, after the surface temperature of the forged steel plate is cooled to 450-500° C., normalizing and hydrogen annealing are performed.

[0015] Furthermore, the tempering heat treatment includes quenching treatment and tempering treatment; in the quenching treatment, the steel plate is heated to 880-900° C. and kept warm for more than 2 hours; and then the steel plate is cooled.

[0016] Furthermore, when cooling the steel plate, high-pressure spray oil quenching is adopted, and the spray pressure of the quenching oil is controlled to be not less than 0.6 MPa, so that the surface temperature of the steel plate is cooled to below 80°C.

[0017] Furthermore, during the tempering treatment, the tempering temperature is 550 to 650° C., and the tempering time is more than 4 hours.

[0018] Further, the tempering temperature T is determined according to the thickness D of the steel plate, wherein T=560+0.4(D-200)±10°C.

[0019] On the other hand, an embodiment of the present invention further provides a low-cost, high-strength, and high-uniformity extra-thick steel plate, which is prepared by the method of the above embodiment, and the thickness of the extra-thick steel plate is 200 to 400 mm.

[0020] Furthermore, when the thickness D of the steel plate is 200≤D<300mm, the hardness of the steel plate is 235~255BHN, and the low temperature toughness at -40℃ is 75~92J; when the thickness D of the steel plate is 300≤D<400mm, the hardness of the steel plate is 223~241BHN, and the low temperature toughness at -40℃ is 83~100J; the difference in diameter of any Brinell hardness indentation of the steel plate is not greater than 0.15mm, the fluctuation of the low temperature toughness at -40℃ is ≤20J, and the tensile strength reaches more than 800MPa.

[0021] The present invention can achieve at least one of the following beneficial effects:

[0022] 1. Aiming at the high hardenability requirements and high strength and toughness requirements of 200-400mm specification steel plates, the present invention designs a high-cleanliness medium-carbon, low-nickel Cr-Ni-Mo high-strength and toughness extra-thick steel plate. The hardenability of the steel is guaranteed by controlling the contents of C, Mn, Cr and B; P≤0.015%, S≤0.005%, Mn≤1.10% are strictly controlled, and a small amount of Ni is added to optimize the toughness, thereby ensuring the high toughness of the steel plate; 0.35-0.40% C is used as a solid solution strengthening design, and 0.20-0.40% Mo is used for precipitation and fine grain strengthening design to ensure the high strength of the steel plate. At the same time, through repeated upsetting and drawing forging and at least one gradient temperature forging, the uniformity of the structure and consistency of the performance of the extra-thick steel plate are achieved.

[0023] 2. The present invention optimizes the forging process, preferentially improves the forging compression ratio of the core, allows the core to fully undergo thermal deformation, and ensures the uniformity and consistency of the structure of the extra-thick steel plate during the forging process.

[0024] 3. The present invention optimizes the quenching and tempering heat treatment process to ensure that the extra-thick steel plate does not have quenching cracks, achieves high toughness through sufficient tempering, and achieves consistency in the structure and uniformity in performance of the extra-thick steel plate.

[0025] 4. The steel of the present invention has the following advantages: (1) the thickness specification can reach 200 to 400 mm; (2) the tensile strength is ≥800 MPa; (3) the hardness uniformity of the ultra-thick steel plate is high, and the hardness fluctuation range is ≤20BHN; (4) the ultra-thick steel plate achieves high low-temperature toughness under low Ni element conditions, and the fluctuation along the thickness direction is small, and the -40°C low-temperature toughness (KV2) is ≥75J.

[0026] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The drawings are only for the purpose of illustrating particular embodiments and are not to be construed as limiting the invention.

[0028] Figure 1 This is an optical microstructure diagram of a 200 mm thick steel plate in Example 1 of the present invention, wherein (a) is the surface of the steel plate and (b) is the core of the steel plate.

[0029] Figure 2 This is an optical microstructure diagram of a 260 mm thick steel plate in Example 1 of the present invention, wherein (a) is the surface of the steel plate and (b) is the core of the steel plate.

[0030] Figure 3 This is an optical microstructure diagram of a 340 mm thick steel plate in Example 2 of the present invention, wherein (a) is the surface of the steel plate and (b) is the core of the steel plate.

[0031] Figure 4 This is an optical microstructure diagram of a 400 mm thick steel plate in Example 2 of the present invention, wherein (a) is the surface of the steel plate and (b) is the core of the steel plate.

[0032] Figure 5 This is an optical microstructure diagram of a 200 mm thick steel plate in Example 3 of the present invention, wherein (a) is the surface of the steel plate and (b) is the core of the steel plate.

[0033] Figure 6 This is an optical microstructure diagram of a 400 mm thick steel plate in Example 3 of the present invention, wherein (a) is the surface of the steel plate and (b) is the core of the steel plate. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the present invention clearer, exemplary embodiments of the present invention will be described below in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. For the sake of clarity and conciseness, not all features of the actual implementation are described in the specification.

[0035] An embodiment of the present invention provides a method for preparing a high-strength, high-uniformity and extra-thick steel plate, comprising the following steps:

[0036] Through smelting and casting, steel ingots are obtained;

[0037] After the steel ingot is subjected to homogenization heat treatment, it is subjected to upsetting and drawing forging for at least three times, wherein the upsetting and drawing forging includes at least one gradient temperature forging, to obtain a steel plate;

[0038] The steel plate is subjected to quenching and tempering heat treatment to obtain a steel plate with a thickness of 200 mm to 400 mm.

[0039] Among them, the chemical composition of the steel ingot includes, by mass percentage: C: 0.35-0.40%, Si: 0.10-0.35%, Mn: 0.80-1.10%, Cr: 2.80-3.50%, Ni: 0.80-1.20%, Mo: 0.20-0.40%, Al: 0.015-0.035%, B: 0.0005-0.0050%, P≤0.015%, S≤0.005%, and the remainder is Fe and unavoidable impurity elements.

[0040] The functions of the above elements are as follows:

[0041] As the most effective solid solution strengthening element in steel, C element can ensure that the steel plate obtains the required high strength and high toughness after quenching and high temperature tempering. If the C content is lower than 0.35%, it is difficult to ensure the hardenability of ultra-thick and extra-thick steel plates, and it is also difficult to obtain sufficient hardness and strength after high temperature tempering; if the C content exceeds 0.40%, it is difficult to ensure the toughness of the steel plate. Therefore, the optimal C content range in the steel plate is 0.35-0.40%, for example, it can be 0.35%, 0.36%, 0.37%, 0.38%, 0.39%, 0.40%, etc.

[0042] The Si element is used as a deoxidizer in steel. If the Si content is higher than 0.35%, it is easy to cause more silicate inclusions to form in the steel plate and affect the toughness. If the Si content is lower than 0.10%, the deoxidation effect is insufficient. Therefore, the optimal Si content in the ultra-thick steel plate is controlled at 0.10-0.35%, for example, it can be 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, etc.

[0043] The Mn element is used as a deoxidizer and an element to improve hardenability. For extra-thick steel plates, when the Mn content is lower than 0.80%, the hardenability improvement effect is not significant; when the Mn content is too high, there is a tendency to coarsen the steel grains, increase the temper brittleness sensitivity of the steel, and too high Mn content can easily aggravate the segregation of the ingot casting process of the extra-thick specification steel plate, which is not conducive to the control of the performance consistency of the extra-thick steel plate. Therefore, the optimal content range of Mn in the extra-thick steel plate is 0.80-1.10%, for example, it can be 0.80%, 0.85%, 0.90%, 0.95%, 1.00%, 1.05%, 1.10%, etc.

[0044] P is a harmful element in steel. It can form micro segregation when the molten steel solidifies, and when heated to the austenitizing temperature, P tends to segregate at the grain boundary, which significantly increases the brittleness of the steel. If the P content is higher than 0.015%, the harmful effect is obvious. If the P content is lower than 0.015%, it is difficult to control and the smelting cost increases significantly. Therefore, the P content in ultra-thick steel plates should be controlled below 0.015%.

[0045] S is a harmful element in steel. It will form MnS inclusions with Mn and segregate at grain boundaries, which will deteriorate the plasticity and toughness of steel. From the performance point of view, the lower the S content, the better, but too low S content requirements will increase smelting costs. Considering the high toughness requirements of extra-thick steel plates, the S content should be controlled below 0.005%.

[0046] Cr can effectively improve the hardenability of steel to ensure that the required hardness and strength are obtained on the cross section of the extra-thick steel plate. For extra-thick steel plates of 200-400 mm, in order to ensure sufficient hardenability, the Cr content should not be less than 2.80%, but a Cr content higher than 3.50% will aggravate the segregation of the die-cast ingot, thereby reducing the toughness of the steel to a certain extent. Therefore, the optimal Cr content range in the ultra-thick steel plate is 2.80-3.50%, for example, it can be 2.80%, 2.90%, 3.00%, 3.10%, 3.20%, 3.30%, 3.40%, 3.50%, etc.

[0047] For Ni element, a certain content of Ni can reduce the resistance of dislocation movement in steel. With the reduction of dislocation movement resistance, the stress in steel will relax. A small amount of Ni element can be used to change the dislocation and substructure of the matrix structure, thereby improving the toughness of the steel. However, Ni is a precious metal element, and excessive addition will significantly increase the cost of the material. Therefore, the present invention controls the Ni content in the steel to be in the range of 0.80% to 1.20%. For example, the Ni content can be 0.80%, 0.90%, 1.00%, 1.10%, 1.20%, etc.

[0048] The combined use of Mo elements with Cr, Mn, etc. can further improve the hardenability of steel. At the same time, Mo can prevent harmful elements such as P from segregating at grain boundaries, thereby improving toughness. However, when the Mo content is lower than 0.20%, the toughness improvement effect is insufficient, and when it exceeds 0.40%, the cost will increase significantly. Therefore, the optimal content range of Mo in extra-thick steel plates is 0.20-0.40%. For example, the Mo content can be 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, etc.

[0049] Al can be used as a deoxidizing element in steel and can form AlN to refine grains. If the Al content is lower than 0.015%, the grain refinement effect is not obvious; if it is higher than 0.035%, large-sized oxide inclusions are easily formed. Therefore, the optimal content range of Al in extra-thick steel plates is 0.015-0.035%, for example, it can be 0.015%, 0.020%, 0.025%, 0.030%, 0.035%, etc.

[0050] The B element can significantly improve the hardenability of steel, and can also purify the grain boundaries to improve toughness. When the B content is less than 0.0005%, the effect of improving hardenability and toughness is insufficient; when the B content exceeds 0.0050%, the effect is saturated, and it is possible to form a precipitation phase and affect toughness. Therefore, the B element can be added to the extra-thick steel plate, and its optimal content range is 0.0005-0.0050%, for example, it can be 0.0005%, 0.0010%, 0.0020%, 0.0030%, 0.0040%, 0.0050%, etc.

[0051] Aiming at the high hardenability requirement and high strength and toughness requirement of the specification of 200-400mm, the present invention designs a high-cleanliness medium-carbon, low-nickel Cr-Ni-Mo high-strength and toughness extra-thick steel plate; by controlling the contents of C, Mn, Cr, Ni and B elements, it is ensured that the extra-thick steel plate has good hardenability; to ensure high toughness, P≤0.015%, S≤0.005%, Mn≤1.10% are strictly controlled, and a small amount of Ni is added to optimize the toughness; by adding a small amount of Ni, Mo and other elements, the cost of the steel plate is reduced under the premise of ensuring the high strength and toughness of the extra-thick steel plate.

[0052] At the same time, the present invention achieves uniformity of the structure and consistency of the performance of the extra-thick steel plate through repeated upsetting and drawing forging and at least one gradient temperature forging.

[0053] In some embodiments, high-purity molten steel can be obtained by electric arc furnace or converter smelting, refining outside the furnace, and vacuum degassing; and then a steel ingot of a desired ingot shape can be obtained by mold casting.

[0054] Among them, in the selection of die casting ingot type, in order to obtain extra-thick steel plates with a maximum thickness of up to 400mm, steel ingots of more than 20t are selected.

[0055] In some embodiments, when casting the steel ingot, under vacuum conditions, the superheat of the molten steel is controlled to be 15-25° C. to minimize component segregation when the molten steel solidifies.

[0056] In some embodiments, when forging the steel ingot, three upsetting forgings are performed, wherein the total deformation compression ratio of the three upsetting forgings is not less than 6, and the steel ingot is upset to less than 1 / 2 of the height of the original ingot body during each upsetting.

[0057] The biggest problems of large-size ingots of extra-thick steel plates are component segregation and deterioration of core properties. The former is caused by selective crystallization during casting, and the latter is related to both the solidification process and the deformation process during hot working. Therefore, in addition to high-cleanliness smelting and low-superheat casting, the present invention can effectively solve the metallurgical quality problem through the forging process of repeated upsetting and drawing.

[0058] The present invention ensures the overall drawing deformation by controlling the total compression ratio of three upsetting and drawing forgings to be ≥6, and ensures the overall upsetting deformation by controlling the deformation during each upsetting, thereby ensuring the uniformity and consistency of the structure of the extra-thick steel plate during the forging process.

[0059] In some embodiments, during the three-stage upsetting and drawing forging process, the steel ingot is first heated to 1185-1215°C, kept warm for more than 4 hours, and then forged once. The main purpose of the primary forging is to break the casting dendrite structure, preliminarily optimize the high-temperature thermoplasticity of the steel billet, and avoid serious problems such as cracking and breaking of the steel billet during the subsequent high-pressure forging process. In addition, the opening forging temperature of the primary forging is ≥1090°C, and the final forging temperature is ≥850°C.

[0060] Among them, the reduction rate is controlled at 30-40% during the first forging, so that the surface structure of the original ingot can be fully recrystallized. However, when the reduction rate is too high and exceeds 40%, it is easy to cause the steel billet that has not completed full recrystallization to crack due to insufficient high-temperature thermoplasticity.

[0061] In some embodiments, after the first forging is completed, the flattened billet after the first forging is returned to the furnace and heated to 1185-1215°C, then taken out to wait for temperature, and when the billet surface temperature drops to 1000°C, the second forging is performed. When the billet surface temperature drops to 1000°C, the billet surface high temperature strength is about 30MPa; at this time, the core temperature is still 1185-1215°C, and the core high temperature strength is about 10MPa, thereby forming a gradient of deformation resistance in the thickness direction, so that the core structure with higher temperature and significantly lower strength is fully deformed and recrystallized.

[0062] Among them, during the secondary forging, the reduction rate is controlled at 35-45%, and the final forging temperature is ≥850°C, so that the core of the billet can be fully thermally deformed and the structure is recrystallized, which is beneficial to improving the uniformity of the steel plate structure.

[0063] In some embodiments, after the secondary forging is completed, the flattened billet after the secondary forging is returned to the furnace and heated to 1185-1215°C and kept at this temperature for 1-1.5 hours before the tertiary forging is directly performed. The start forging temperature of the tertiary forging is ≥1090°C, and the final forging temperature is ≥850°C.

[0064] Among them, the reduction rate is controlled at 25-30% during the third forging, so that the billet can be fully hot deformed as a whole again, and the recrystallization and refinement of the structure can be achieved again, thereby improving the structural uniformity and toughness of the steel plate.

[0065] Since it is difficult to deform the core structure of steel plates with a thickness of more than 200 mm, insufficient forging deformation can easily lead to problems such as non-recrystallization or mixed crystals and coarse crystals, thereby greatly reducing the strength and toughness of the core of the steel plate. The present invention optimizes the forging process and adopts a gradient temperature forging method in the secondary forging process to preferentially increase the forging compression ratio of the core, thereby improving the uniformity and consistency of the structure of the extra-thick steel plate.

[0066] In some embodiments, after the surface temperature of the steel plate after three forgings is cooled to 450-500°C, normalizing and hydrogen annealing are performed to further refine the grain size of the structure, prevent white spot hydrogen embrittlement in the forged steel structure, and reduce the hardness.

[0067] In some embodiments, the tempering heat treatment of the steel plate includes: quenching treatment and tempering treatment. In the quenching treatment, the steel plate is heated to 880-900° C. and kept at this temperature for more than 2 hours; then the steel plate is cooled.

[0068] In some embodiments, when cooling the steel plate, high-pressure spray oil quenching is used, and the spray pressure of the quenching oil is controlled to be not less than 0.6 MPa, and the surface temperature of the steel plate is cooled to below 80° C. Specifically, when high-pressure spray oil quenching is used, high-pressure spray circulating quenching oil pool cooling is used, and the spray pressure of the oil nozzle is controlled to be ≥ 0.6 MPa.

[0069] The heat treatment process directly determines the structure and mechanical properties of the extra-thick steel plate. For the extra-thick steel plate, the traditional water quenching process is difficult and cannot guarantee that the extra-thick steel plate will not have quenching cracks. Therefore, the present invention adopts high-pressure spray oil quenching.

[0070] Considering that there is a large temperature gradient between the core and the surface of the extra-thick steel plate during quenching, the final cooling temperature of the core quenching must be controlled below 150°C. Therefore, the present invention controls the surface temperature of the steel plate to be cooled to below 80°C.

[0071] In some embodiments, during the tempering treatment, the tempering temperature is 550-650°C and the tempering time is more than 4 hours, so that the quenched martensite of the extra-thick steel plate is fully transformed and the supersaturated carbide is uniformly precipitated to achieve high toughness and uniformity of the structure and performance of the extra-thick steel plate. After tempering, air cooling is performed.

[0072] In some embodiments, the specific tempering temperature T is determined according to the thickness D of the steel plate, wherein T=560+0.4(D-200)±10°C.

[0073] By adopting the above preparation method, by optimizing the chemical composition of the steel ingot, the forging process and the tempering heat treatment process, a high-strength, high-toughness, high-uniformity extra-thick steel plate can be obtained, and its specific properties are as follows: for steel plates with a thickness D of 200≤D<300mm, the hardness range is 235~255BHN, and the -40℃ low-temperature toughness (KV2) range is 75~92J; for steel plates with a thickness D of 300≤D<400mm, the hardness range is 223~241BHN, and the -40℃ low-temperature toughness (KV2) range is 83~100J; the difference in the diameter of any Brinell hardness indentation of a single steel plate is not greater than 0.15mm, the fluctuation of the -40℃ low-temperature toughness is ≤20J, and the tensile strength reaches more than 800MPa.

[0074] The present invention aims at the development demand of low-cost, high-performance, high-homogenization ultra-thick steel plates with a thickness of more than 200 mm, and ensures the stability of the steel plates by controlling the three main factors of metallurgical quality, structural uniformity and hardness consistency. Firstly, the present invention reduces the hardness fluctuation of high-strength and tough ultra-thick steel plates by means of chemical composition optimization, high-cleanliness smelting and dynamic adjustment of heat treatment process; and realizes low cost by low Ni, Mo and other expensive elements; secondly, the metallurgical quality is guaranteed by low superheat casting, ingot optimization and repeated upsetting forging process; and the overall structural uniformity of ultra-thick steel plates is achieved by gradient temperature upsetting forging.

[0075] The embodiment of the present invention further provides an extra-thick steel plate with low cost, high strength and high uniformity, which is prepared by the method of the above embodiment, and the thickness of the extra-thick steel plate is 200-400 mm.

[0076] In some embodiments, when the thickness D of the steel plate is 200≤D<300mm, the hardness of the steel plate is 235~255BHN, and the low temperature toughness at -40℃ is 75~92J; when the thickness D of the steel plate is 300≤D<400mm, the hardness of the steel plate is 223~241BHN, and the low temperature toughness at -40℃ is 83~100J; the difference in diameter of any Brinell hardness indentation of the steel plate is not greater than 0.15mm, the fluctuation of the low temperature toughness at -40℃ is ≤20J, and the tensile strength reaches above 800MPa.

[0077] Compared with the prior art, the steel plate of the present invention has the following advantages: (1) the thickness specification can reach 200 to 400 mm; (2) the tensile strength is ≥800 MPa; (3) the ultra-thick steel plate has high hardness uniformity, and the hardness fluctuation range is ≤20 BHN; (4) the ultra-thick steel plate achieves high low-temperature toughness under low Ni element conditions, and the fluctuation along the thickness direction is small, and the -40°C low-temperature toughness (KV2) is ≥75 J.

[0078] The ultra-thick steel plate and the preparation method thereof of the present invention are further described below through specific embodiments.

[0079] Examples 1 to 3

[0080] The preparation process of extra thick steel plate is as follows:

[0081] Molten steel was smelted by using three 50-ton electric furnaces + refining outside the furnace + vacuum degassing, the three furnaces were respectively Example 1, Example 2 and Example 3, and their chemical compositions are shown in Table 1. Two 20-ton octagonal steel ingots were cast in each furnace, for a total of six steel ingots.

[0082] Table 1 Chemical composition of the steel plates of the present invention and various embodiments (mass percentage)

[0083]

[0084]

[0085] The forging and heat treatment processes are as follows:

[0086] Three upsetting processes were adopted to forge steel plates with thicknesses of 200 mm, 260 mm, 340 mm and 400 mm, respectively. The thicknesses of the steel plates in Examples 1 to 3 are specifically shown in Table 2.

[0087] Among them, the steel ingot is heated to 1200±15℃ by hot transport and kept warm for more than 4 hours, and then the first forging is carried out, the reduction rate of the first forging is controlled at 30-40%, the start forging temperature is ≥1090℃, and the final forging temperature is ≥850℃; the flattened billet of the first forging is returned to the furnace and heated to 1200±15℃, and then taken out from the heating furnace and kept warm. When the surface temperature drops to 1000℃, the second forging is started, the reduction rate of the second forging is controlled at 35-45%, and the final forging temperature is ≥850℃; the flattened billet of the second forging is returned to the furnace and heated to 1200±15℃, and kept warm for 1-1.5 hours, and then directly started the third forging after being taken out from the heating furnace, the start forging temperature is ≥1090℃, the final forging temperature is ≥850℃, and the reduction rate is controlled at 25-30%. And ensure that the forging compression ratio is ≥6, and the upsetting must be below 1 / 2 of the original ingot body during upsetting.

[0088] After forging, the steel plate is cooled to 450-500℃ and then put into the furnace for hot normalizing + hydrogen annealing.

[0089] The specific process parameters during the forging process are shown in Table 2.

[0090] Table 2 Forging process parameters of steel plates in various embodiments

[0091]

[0092]

[0093] The six steel plates were divided into blocks for quenching and tempering heat treatment, and the quenching temperature was 880-900℃. After keeping the temperature for more than 2 hours, high-pressure spray oil quenching was carried out, and the spray pressure was ≥0.6MPa; when the surface temperature of the steel plate dropped below 80℃, tempering treatment was carried out; among them, the tempering temperatures of 200mm, 260mm, 340mm and 400mm steel plates were 560℃, 585℃, 620℃ and 640℃ respectively, and air cooling was carried out after tempering. The specific process parameters during the modulation heat treatment process are shown in Table 3.

[0094] Table 3 Quenching and tempering heat treatment process parameters of steel plates in various examples

[0095]

[0096] Comparative Example 1

[0097] The difference between this comparative example and Example 1 is that during the forging process, an upsetting process is performed, the forging compression ratio is 6, and the steel plate is forged into a thickness of 200 mm.

[0098] Comparative Example 2

[0099] The difference between this comparative example and Example 1 is that only a steel plate with a thickness of 200 mm is prepared; during the quenching and tempering heat treatment, a traditional oil cooling method is adopted, that is, the steel plate is immersed in quenching oil; and the tempering temperature of the steel plate is 500° C., and the tempering time is 3 hours.

[0100] The above examples were sampled for mechanical property testing and analysis. Figure 1 to Figure 6 They are metallographic images of the surface and core structures of each extra-thick steel plate in Examples 1 to 3. Obviously, since the chemical composition of the present invention has high hardenability and a suitable quenching cooling medium is selected, the steel plates in Examples 1 to 3 are all high-temperature tempered troostite uniform structures.

[0101] The hardness, -40°C low temperature impact value (KV2) and tensile strength of the steel plates of each embodiment and comparative example were tested. The test results are shown in Table 4, Table 5 and Table 6.

[0102] Table 4 shows the hardness distribution of the extra-thick steel plates of each embodiment and comparative example along the thickness direction. As shown in Table 4, the hardness range of the steel plates with thickness of 200mm and 260mm in embodiment 1 is 235-255BHN, and the hardness range of the steel plates with thickness of 340mm and 400mm in embodiment 2 is 223-241BHN; the hardness range of the steel plates with thickness of 200mm in embodiment 3 is 235-255BHN, and the hardness range of the steel plates with thickness of 400mm in embodiment 4 is 223-241BHN; and the hardness fluctuation range of the extra-thick steel plates is ≤20BHN. The hardness fluctuation range of comparative examples 1 and 2 is about 40BHN, which is greater than 20BHN.

[0103] Table 4 Distribution of hardness (BHN) of steel plates of various embodiments and comparative examples along the thickness direction

[0104]

[0105]

[0106] Table 5 shows the -40℃ low temperature impact value (KV2) of the extra thick steel plates of each embodiment and comparative example along the thickness direction. As shown in Table 5, the -40℃ low temperature toughness range of the steel plates with a thickness of 200mm and 260mm in Example 1 is 75~92J; the -40℃ low temperature toughness range of the steel plates with a thickness of 340mm and 400mm in Example 2 is 83~100J; the -40℃ low temperature toughness range of the steel plates with a thickness of 200mm in Example 3 is 75~92J, and the -40℃ low temperature toughness range of the steel plates with a thickness of 400mm in Example 3 is 83~100J. It can be seen that the extra thick steel plates of each embodiment have high low temperature impact toughness, and the fluctuation along the thickness direction is small; while the low temperature toughness of comparative examples 1 and 2 is significantly reduced, and the difference between the surface and the core of the steel plates is large.

[0107] Table 5 Distribution of -40℃ impact toughness (KV2,J) of each steel plate along the thickness direction

[0108]

[0109] Table 6 shows the measured values ​​of the tensile strength of the extra thick steel plates of each embodiment and comparative example. As shown in Table 6, the tensile strength of each extra thick steel plate is greater than 800 MPa, while the tensile strength of comparative examples 1 and 2 is less than 800 MPa.

[0110] Table 6 Actual measured values ​​of tensile strength of steel plates in various embodiments and comparative examples

[0111]

[0112]

[0113] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing a low-cost, high-strength, high-uniformity, extra-thick steel plate, characterized in that: include: Through smelting and casting, steel ingots are obtained; In terms of mass percentage, the chemical composition of the steel ingot includes: C: 0.35~0.40%、Si:0.10~0.35%、Mn:0.80~1.10%、Cr:2.80~3.50%、Ni:0.80~1.20%、Mo:0.20~0.40%、Al:0.015~0.035%、B:0.0005~0.0050%、P≤0.015%、S≤0.005%、the balance is Fe and unavoidable impurity elements; After homogenizing the steel ingot, the steel ingot is subjected to upsetting and forging for at least three times, wherein the upsetting and forging includes at least one gradient temperature forging, to obtain a steel plate; The steel plate is subjected to a tempering heat treatment to obtain a steel plate with a thickness of 200 to 400 mm.

2. The method according to claim 1, characterized in that When forging the steel ingot, three upsetting forgings are performed; the total deformation compression ratio of the three upsetting forgings is not less than 6, and the steel ingot is upset to less than 1 / 2 of the height of the original ingot body during each upsetting.

3. The method according to claim 2, characterized in that In the three upsetting and drawing forging processes, The steel ingot is heated to 1185-1215° C., kept at this temperature for more than 4 hours, and then forged once, wherein the reduction rate is controlled at 30-40% during the forging once; After the flattened billet after the primary forging is returned to the furnace and heated to 1185-1215°C, it is taken out and kept warm. When the surface temperature of the billet drops to 1000°C, secondary forging is performed, and the reduction rate is controlled to be 35-45% during the secondary forging; After the secondary forging, the flattened billet is returned to the furnace and heated to 1185-1215° C. and kept at this temperature for 1-1.5 hours, and then directly subjected to the third forging, wherein the reduction ratio is controlled to be 25-30% during the third forging.

4. The method according to claim 3, characterized in that After the surface temperature of the forged steel plate is cooled to 450-500° C., normalizing and hydrogen annealing are performed.

5. The method according to claim 1, characterized in that The quenching and tempering heat treatment includes quenching treatment and tempering treatment; In the quenching treatment, the steel plate is heated to 880-900° C. and kept at this temperature for more than 2 hours; and then the steel plate is cooled.

6. The method according to claim 5, characterized in that When cooling the steel plate, high-pressure spray oil quenching is adopted, and the spray pressure of the quenching oil is controlled to be not less than 0.6 MPa, so that the surface temperature of the steel plate is cooled to below 80°C.

7. The method according to claim 5, characterized in that During the tempering treatment, the tempering temperature is 550-650° C. and the tempering time is more than 4 hours.

8. The method according to claim 7, characterized in that The tempering temperature T is determined according to the thickness D of the steel plate, wherein T=560+0.4(D-200)±10°C.

9. A low-cost, high-strength, high-uniformity extra-thick steel plate, characterized in that: The extra thick steel plate is prepared by the method according to any one of claims 1 to 8, and the thickness of the extra thick steel plate is 200 to 400 mm.

10. The steel plate according to claim 9, characterized in that: When the thickness D of the steel plate is 200≤D<300mm, the hardness of the steel plate is 235-255BHN, and the low temperature toughness at -40°C is 75-92J; When the thickness D of the steel plate is 300≤D<400mm, the hardness of the steel plate is 223-241BHN, and the low temperature toughness at -40°C is 83-100J; The difference in diameter of any Brinell hardness indentation of the steel plate is no more than 0.15 mm, the fluctuation of low temperature toughness at -40°C is ≤20 J, and the tensile strength reaches more than 800 MPa.

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