Corrosion-resistant low-welding-crack-sensitivity high-strength steel Q620CF with thickness of 150-250 mm and production method thereof

Abstract

The invention discloses 150-250mm corrosion-resistant low-welding crack sensitivity high-strength steel Q620CF and a production method thereof, and the corrosion-resistant low-welding crack sensitivity high-strength steel Q620CF is prepared from the following chemical components in percentage by mass (unit, wt%): 0.05-0.08% of C, 0.2-0.3% of Si, 0.6-1.0% of Mn, 0.010-0.015% of P, less than or equal to 0.003% of S, 0.020-0.030% of Nb, 0.020-0.030% of V, 0.015-0.030% of Als, 0.50-0.60% of Cr, 0.40-0.50% of Ni, 0.30-0.35% of Mo, 0.30-0.35% of Cu, 0.0014-0.0020% of B and the balance of Fe. The method comprises the steps of smelting, water-cooling mold casting, cast ingot heating, hot rolling, repeated cooling, slow cooling, quenching, tempering and air cooling. The maximum thickness of the obtained steel plate reaches 250 mm, the low welding crack sensitivity index Pcm of the steel plate is 0.17-0.22%, meanwhile, the corrosion resistance coefficient I of the steel plate is 6.5-7.0, and the corrosion resistance problem of the large-thickness low welding crack sensitivity high-strength steel Q620CF in the severe use environment is solved.

Description

Technical Field

[0001] The invention relates to the technical field of die casting plate production, and in particular to 150-250mm corrosion-resistant low welding crack sensitivity high-strength steel Q620CF and a production method thereof. Background Art

[0002] Q620CF is a high-strength steel with low welding crack sensitivity, which has high strength, high hardness, good plasticity and toughness. It has excellent welding performance, small deformation of steel and low residual stress during welding, which is conducive to improving welding quality and structural safety. In recent years, with the development of large-scale hydropower, bridges, ships, marine engineering, etc., the research and development of this type of high-strength steel with low welding crack sensitivity has gradually expanded to larger thickness. At present, the market for steel plates with higher strength and low welding crack sensitivity with a thickness of more than 150mm is relatively rare, and its performance has always been a difficult point to control. In addition, this type of steel plate is generally used in a harsh environment. If the steel plate also has certain corrosion resistance, the production equipment will be safer, more reliable and have a longer service life. However, Q620CF produced by existing technology cannot take into account the special thickness, low welding crack sensitivity and corrosion resistance.

[0003] Using the I coefficient (corrosion resistance index) to determine the corrosion resistance of steel plates is a common and effective method. The I value is a comprehensive indicator that takes into account the oxidation resistance, corrosion resistance and wear resistance of steel plates in a specific environment. Generally, when the I value reaches a certain value (such as 6.0 or higher), the steel plate is considered to have good corrosion resistance. The higher the I value, the better the corrosion resistance of the steel plate, and it can maintain stability and service life for a longer time in a more severe environment.

[0004] For example, Chinese patent CN202211291567X discloses "a 690MPa grade hydrogen-induced delayed fracture-resistant low welding crack sensitivity quenched and tempered steel and a manufacturing method thereof", whose chemical composition (unit, wt%): C: 0.065-0.09, Mn: 1.00-1.20, Ni: 0.50-0.65, Mo: 0.20-0.30, V: 0.3-0.4, Pcm: ≤020; the invention adopts a low C content composition design, a special rolling controlled cooling mode and a specific quenching and tempering process to obtain a 18-50mm thick 690MPa grade hydrogen-induced delayed fracture-resistant low welding crack sensitivity quenched and tempered steel, whose yield strength is ≥550MPa, tensile strength is 690-820MPa, elongation is ≥16%, and -50℃ low-temperature impact is ≥100J. Although the steel has a welding crack sensitivity index Pcm: ≤0.20 and good welding performance, its thickness is only 18 to 50 mm, and its corrosion resistance coefficient is I: 2.1 to 2.7, which means it does not have corrosion resistance.

[0005] Another example is Chinese patent CN2014107536791, which discloses "an 800MPa grade low welding crack sensitivity quenched and tempered hydropower steel plate", whose chemical composition (unit, wt%) is: C: 0.09-0.14, Mn: 0.7-1.45, Ni: 0.30-0.50, V: 0.035-0.060, Cr: 0.25-0.60, Mo: 0.25-0.55, Cu: 0.15 ~0.30, B:0.0008~0.002; its preparation method: smelting and casting into ingot according to the set composition, heating and rolling in two stages, and tempering heat treatment after rapid cooling to obtain steel plate properties: yield strength ≥690MPa, tensile strength 780~930MPa, elongation after fracture ≥17%, -40℃ impact energy absorption ≥47J, welding cold crack sensitivity index Pcm=0.24, but its thickness is only 10~80mm.

[0006] In view of this, the present invention is proposed. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a 150-250mm corrosion-resistant low welding crack sensitivity high-strength steel Q620CF, the crack sensitivity coefficient Pcm of the steel plate is ≤0.22% and the corrosion resistance coefficient I is ≥6.5, the steel plate has a yield strength of 711-756MPa, a tensile strength of 786-810MPa, an elongation of 18-21%, and an impact energy of 155-220J at -60°C. While ensuring the large thickness and low welding crack sensitivity Pcm index, it also meets the corrosion resistance I coefficient requirements, solving the corrosion resistance problem of large thickness and low crack sensitivity steel plates in harsh use environments.

[0008] Another object of the present invention is to provide a method for producing 150-250mm corrosion-resistant, low-welding crack-sensitivity high-strength steel Q620CF.

[0009] To achieve the above object, the technical solution adopted by the present invention is as follows: a 150-250mm corrosion-resistant low welding crack sensitivity high-strength steel Q620CF, the steel adopts the following mass percentage chemical composition (unit, wt%): C: 0.05-0.08, Si: 0.2-0.3, Mn: 0.6-1.0, P: 0.010-0.015, S≤0.003, Nb: 0.020-0.0 30. V: 0.020~0.030, Als: 0.015~0.030, Cr: 0.50~0.60, Ni: 0.40~0.50, Mo: 0.30~0.35, Cu: 0.30~0.35, B: 0.0014~0.0020, others are Fe and residual elements. The crack sensitivity coefficient Pcm: 0.17~0.22%, corrosion resistance coefficient I: 6.5~7.0.

[0010] The dosage and function of each element in the present invention are described as follows:

[0011] Carbon: Carbon is the most important strengthening element in the present invention, which can improve the yield and tensile strength of the steel plate. However, as the carbon content increases, the toughness of the steel plate decreases. Too high a carbon content not only affects the impact performance of the steel plate, but is also not conducive to welding safety. The carbon content also has an important influence on the mechanical properties and corrosion resistance of the steel. If the carbon content is too high, the corrosion resistance and welding performance will be reduced. According to the application scope of the forming processing of this type of steel, it is necessary to ensure that the steel plate has good low welding crack sensitivity and good corrosion resistance. If the carbon content exceeds 0.08%, it will have a greater impact on the welding performance and low-temperature toughness of the steel plate; if the carbon content is less than 0.05%, the strength of the steel plate cannot be guaranteed. Therefore, the carbon element is controlled at 0.05-0.08%.

[0012] Chromium: Chromium is one of the important strengthening and corrosion resistance elements in the present invention. It is a medium carbide forming element. Among all kinds of carbides, chromium carbide is the smallest one. It can be evenly distributed in the steel body, so it has high strength, hardness, yield point and high wear resistance. Chromium can also improve corrosion resistance, and its improvement effect will decrease with the increase of carbon content. According to the application and performance requirements of this steel grade, if the chromium content is higher than 0.6%, the Pcm low welding crack sensitivity index will be too high, affecting the welding performance of the steel plate; if the chromium content is lower than 0.5%, the solid solution strengthening effect will be reduced, the strength of the material may not be met, and the corrosion resistance coefficient I will also decrease sharply, resulting in reduced corrosion resistance of the steel plate. Therefore, the chromium element is controlled at 0.5-0.6%.

[0013] Nickel: Nickel is one of the important elements of strength, toughness and corrosion resistance in the present invention. Nickel has high strength, high toughness, good hardenability and corrosion resistance. On the one hand, it strongly improves the strength of steel, and on the other hand, it always keeps the toughness of steel at an extremely high level. According to the application and performance requirements of this steel grade, if the nickel content is lower than 0.4%, the strength and low-temperature toughness of the thick steel plate of the present invention are difficult to guarantee, and the corrosion resistance coefficient does not meet the requirements; if the nickel content is higher than 0.5%, it will have an adverse effect on the welding performance of the steel plate. In addition, nickel is a relatively expensive alloy element. High nickel content will increase the production cost of steel, which not only puts economic pressure on production enterprises, but also may affect the market competitiveness of products. Therefore, the nickel element is controlled at 0.4-0.5%.

[0014] Molybdenum: Molybdenum is one of the important strengthening and corrosion-resistant elements in the present invention, which can greatly improve the hardenability of the steel plate and has a certain corrosion resistance. Combined with chromium and nickel, it can greatly improve the hardenability, refine the grains, improve the toughness, and make forging processing easier. According to the application and performance requirements of this steel grade, if the molybdenum content is lower than 0.3%, the hardenability of the thick steel plate of the present invention will be greatly reduced, making the strength performance of the steel plate unable to be guaranteed; if the molybdenum content is higher than 0.35%, it will reduce the welding performance of the steel plate, and too high a content will make the high-temperature tempered water-cooled steel brittle and affect the impact toughness. Therefore, the molybdenum element is controlled at 0.30-0.35%.

[0015] Copper: Copper is one of the important anti-corrosion elements in the present invention. Copper can improve the stability of austenite in steel, so it can improve hardenability and hardenability, and has the effect of strengthening ferrite. Adding about 0.20% copper to low alloy steel, especially when used in combination with phosphorus, can improve the steel's resistance to atmospheric corrosion. According to the application scope of forming processing of this steel type, if the copper content exceeds 0.35%, it is easy to cause the steel plate to produce "copper brittleness", which will deteriorate the processing performance of the material, increase the processing difficulty and cost; it will also cause the mechanical properties of the material such as hardness, strength and toughness to drop significantly, affecting the service life and safety of the product; if the copper content is lower than 0.30%, the corrosion resistance of the material will be significantly reduced; therefore, the copper element is controlled at 0.30-0.35%.

[0016] Boron: Boron is the most important hardenability element in the present invention. Adding a trace amount of boron (0.0005-0.005%) to steel can significantly improve the hardenability of the steel, and at this time, it has no effect or very little effect on other properties. This can replace nickel, chromium, and molybdenum to a certain extent. According to the application scope of forming and processing of this type of steel, because the steel plate is thick and has poor hardenability, boron is added to improve its hardenability to ensure that an ideal bainite structure can be obtained after quenching. However, when the boron content exceeds 0.005%, the hardenability becomes worse, which is easy to cause brittleness and affect the strength and impact toughness of the steel plate. Therefore, the boron element is controlled at 0.0014-0.0020%.

[0017] Phosphorus: Phosphorus is one of the important corrosion-resistant elements in the present invention, because phosphorus exists in the form of phosphide in steel, and phosphide will form a brittle phase at the grain boundary. If the content is too high, the low-temperature toughness of the steel will be significantly reduced, and brittle fracture will occur easily; but phosphorus can be used as a beneficial alloy element, and it can work together with copper to promote the formation of the inner rust layer, which is beneficial to enhance the corrosion resistance of the steel. In the present invention, it is mainly used to combine with copper to improve the corrosion resistance of the steel plate, so the phosphorus content is controlled at 0.010-0.015%.

[0018] The production method of the above 150-250mm corrosion-resistant low welding crack sensitivity high-strength steel Q620CF comprises the following steps:

[0019] 1) Using converter + LF refining + VD vacuum degassing smelting process, control the P content at 0.010-0.015%, H content ≤1.2PPm, N content ≤50PPm;

[0020] 2) Use water-cooled mold casting, control the pouring temperature at 1550-1555°C, pour the main body for 18-21 minutes, and pour the riser for 5-8 minutes;

[0021] 3) heating the steel ingot at a holding temperature of 1230-1250°C and a total heating time of 18-22 hours;

[0022] 4) After the steel ingot is dephosphorized, hot rolling begins, with the starting rolling temperature at 1000-1150°C, the pass reduction at 50-55mm, and the final rolling temperature at 950-1000°C. Large reduction is beneficial to crushing the coarse grains that may be produced during the heating process, thereby achieving the purpose of grain refinement.

[0023] 5) After rolling, the steel plate is quickly cooled multiple times using ACC. The red-return temperature is controlled at 530-580℃. Rapid and repeated cooling after hot rolling can prevent the broken grains from growing back and achieve the purpose of grain refinement.

[0024] 6) The steel plates are stacked and slowly cooled, and the stacking temperature is controlled to be ≥450°C, and the slow cooling time is 24 to 48 hours; stacking and slow cooling can reduce the cooling rate and suppress the risk of cracks;

[0025] 7) After slow cooling, quenching is performed, the quenching temperature is 920-940°C, the holding time is 1.8-2.0 min / mm, and the water cooling is 15-35 min to room temperature. The water temperature is controlled by a refrigerator during the quenching process to be less than 16°C, and the cooling rate is 1-2°C / S. It should be noted that the hardenability of the extra-thick plate is relatively poor. Studies have found that the 150-250mm thick Q620CF steel plate needs to be cooled to 556-456°C at a cooling rate of 0.05-40°C / S to obtain the required bainite structure;

[0026] 8) Tempering air cooling, put the steel plate into the tempering furnace, heat up quickly with high power, the tempering temperature is 470-530℃, the insulation coefficient is 4.0min / mm, and air cools to room temperature. The nickel, copper and other alloys added to the steel composition lead to temper brittleness, and the tempering brittleness range is about 400-450℃. The tempering temperature is set at 470-530℃. By rapidly heating up, the brittle area is quickly crossed. When cooling, the air is cooled, and the air cooling rate is about 0.06-0.09℃ / S. Therefore, the use of air cooling to increase the cooling rate can avoid and reduce the occurrence of temper brittleness. It avoids the traditional tempering air cooling rate of only 0.01-0.02℃ / S, so it can effectively avoid the temper brittleness range, ensure that tempering can effectively avoid the temper brittleness zone, and at the same time, the steel plate obtains a low-carbon bainite tempering structure to improve the toughness and stability of the steel.

[0027] After testing, the metallographic structure of the steel plate obtained according to this scheme is: low carbon bainite + ferrite + a small amount of pearlite, and the grain size is level 9.

[0028] Compared with the prior art, the 150-250mm thick Q620CF high-strength steel produced by the present invention not only ensures the large thickness and low welding crack sensitivity Pcm index, but also meets the corrosion resistance I coefficient requirement, solving the corrosion resistance problem of large thickness and low crack sensitivity steel plates in harsh use environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a schematic diagram of the metallographic structure of the steel plate obtained in the present invention at a magnification of 100x at 1 / 4 thickness.

[0030] Figure 2 This is a schematic diagram of the metallographic structure of the steel plate obtained in the present invention at a magnification of 200x at 1 / 4 thickness.

[0031] Figure 3 This is a schematic diagram of the metallographic structure of the steel plate obtained in the present invention at a magnification of 500x at 1 / 4 thickness.

[0032] Figure 4 This is a schematic diagram of the metallographic structure at another position of the steel plate obtained in the present invention at 1 / 4 thickness with a magnification of 500x. DETAILED DESCRIPTION

[0033] In order to make the objectives, technical solutions and advantages of the present invention more clear, a detailed description will be given below in conjunction with specific embodiments.

[0034] Table 1 is a list of chemical compositions of various embodiments and comparative examples of the present invention

[0035] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention.

[0036] Table 3 is a table of performance test results of various embodiments and comparative examples of the present invention.

[0037] The embodiment of the present invention is produced according to the following steps:

[0038] 1) Using converter + LF refining + VD vacuum degassing smelting process, control the P content at 0.010-0.015%, H content ≤1.2PPm, N content ≤50PPm;

[0039] 2) Use water-cooled mold for pouring, control the pouring temperature at 1550-1555℃, pour the main body for 18-21min, and pour the riser for 5-8min;

[0040] 3) heating the steel ingot at a holding temperature of 1230-1250°C and a total heating time of 18-22 hours;

[0041] 4) After the steel ingot is dephosphorized, hot rolling begins, with the starting rolling temperature at 1000-1150°C, the pass reduction at 50-55mm, and the final rolling temperature at 950-1000°C;

[0042] 5) After rolling, the steel plate is quickly cooled by ACC for multiple times, and the red-return temperature is controlled at 530-580℃;

[0043] 6) The steel plates are stacked and slowly cooled, and the stacking temperature is controlled to be ≥450°C. The slow cooling time is 24 to 48 hours to fully diffuse the hydrogen contained in the steel plates and reduce stress;

[0044] 7) After slow cooling, quenching is performed, the quenching temperature is 920-940°C, the holding time is 1.8-2.0 min / mm, and water cooling is performed for 15-35 min to room temperature. During the quenching process, a refrigerator is used to control the water temperature to <16°C, and the cooling rate is 1-2°C / S;

[0045] 8) Tempering and air cooling: put the steel plate into the tempering furnace, heat up quickly with high power, the tempering temperature is 470-530℃, the insulation coefficient is 4.0min / mm, and air cool to room temperature.

[0046] Table 1 Chemical composition of the embodiments of the present invention and comparative examples (wt%)

[0047]

[0048]

[0049] Table 2 Main process parameters of the embodiments of the present invention and comparative examples

[0050]

[0051] Table 3 Performance test results of the embodiments of the present invention and the comparative examples

[0052]

[0053] In summary, the steel plate obtained by this scheme has a thickness of 150-250 mm, a crack sensitivity coefficient Pcm≤0.22%, and a corrosion resistance coefficient I≥6.5, a steel plate yield strength of 711-756 MPa, a tensile strength of 786-810 MPa, an elongation of 18-21%, and an impact energy of 155-220 J at -60°C. It can take into account the requirements of extra thickness, low welding crack sensitivity and corrosion resistance, and is more suitable for use in harsh environments.

Claims

1. A 150-250mm corrosion-resistant, low welding crack sensitivity high-strength steel Q620CF, characterized in that: The steel is composed of the following chemical components in mass percentage (unit, wt%): C: 0.05-0.08, Si: 0.2-0.3, Mn: 0.6-1.0, P: 0.010-0.015, S≤0.003, Nb: 0.020-0.030, V: 0.020-0.030, Als: 0.015-0.030, Cr: 0.50-0.60, Ni: 0.40-0.50, Mo: 0.30-0.35, Cu: 0.30-0.35, B: 0.0014-0.0020, and the rest are Fe and residual elements, and its crack sensitivity coefficient Pcm: 0.17-0.22%, and corrosion resistance coefficient I: 6.5-7.0; The metallographic structure of the steel is: low carbon bainite + ferrite + a small amount of pearlite, and the grain size is grade 9.

2. The method for producing 150-250 mm corrosion-resistant, low welding crack sensitivity, high-strength steel Q620CF according to claim 1, characterized in that The steps include: 1) Using converter + LF refining + VD vacuum degassing smelting process, control the P content at 0.010-0.015%, H content ≤1.2PPm, N content ≤50PPm; 2) Use water-cooled mold casting, control the pouring temperature at 1550-1555°C, pour the main body for 18-21 minutes, and pour the riser for 5-8 minutes; 3) heating the steel ingot at a holding temperature of 1230-1250°C and a total heating time of 18-22 hours; 4) After the steel ingot is dephosphorized, hot rolling begins, with the starting rolling temperature at 1000-1150°C, the pass reduction at 50-55mm, and the final rolling temperature at 950-1000°C; 5) After rolling, the steel plate is quickly cooled by ACC for multiple times, and the red-return temperature is controlled at 530-580℃; 6) The steel plates are stacked and slowly cooled, the stacking temperature is controlled to be ≥450°C, and the stacking cooling time is 24 to 48 hours; 7) After slow cooling, quenching is performed, the quenching temperature is 920-940°C, the holding time is 1.8-2.0 min / mm, and water cooling is performed for 15-35 min to room temperature. During the quenching process, a refrigerator is used to control the water temperature to <16°C, and the cooling rate is 1-2°C / S; 8) Tempering and air cooling: put the steel plate into the tempering furnace, heat up quickly with high power, the tempering temperature is 470-530℃, the insulation time is 4.0min / mm, and then air-cooled to room temperature.

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