Economical high-plate-shape-quality Q355B steel for engineering mechanical structure and production method of economical high-plate-shape-quality Q355B steel

Abstract

The invention discloses economical high-plate-shape-quality Q355B steel for an engineering mechanical structure and a production method of the economical high-plate-shape-quality Q355B steel, and belongs to the field of low-alloy steel manufacturing, the structural steel comprises the following chemical components in percentage by mass: 0.15-0.19% of C, 0.10-0.25% of Si, 0.90-1.2% of Mn, less than or equal to 0.025% of P, less than or equal to 0.015% of S, 0.20-0.30% of Cr, 0.025-0.045% of Ti, 0.021-0.025% of Alt and the balance of Fe and inevitable impurities, and the sum of the components is 100%. The production method is simple and easy to implement, the strength, hardness and toughness of the steel plate are effectively improved, flatness of the steel plate is guaranteed, and good unevenness is obtained.

Description

Technical Field

[0001] The present invention relates to a steel and a production method thereof, in particular to a Q355B engineering machinery structural steel with economical high plate shape quality and a production method thereof, belonging to the field of low alloy steel manufacturing. Background Art

[0002] With the development of China's engineering machinery industry, higher requirements are put forward for the plate shape quality of engineering machinery structural steel. In addition to having high strength, good plasticity and toughness, cold deformation ability and surface quality, it is particularly required that the steel plate has good flatness.

[0003] The Q355B steel for engineering machinery belongs to the standard product of GB / T 1591-2018, and the main component requirements are C≤0.24%, Si≤0.55%, Mn≤1.60%, Cr≤0.30%, Ti≤0.20%. At present, there has been a low-cost composition design for the production of Q355B in the medium and heavy plate field, such as: Application No. 201910220423.7, Publication No. CN110042315A discloses "a low-cost Q355B structural steel plate and a production method thereof". In the composition design, the C-Mn-Si system is adopted. Specifically, it is required that: C 0.20-0.24%, Mn 0.70-0.90%, Si 0.20-0.40%, P≤0.025%, S≤0.015%, and the rest is Fe and impurities, with a thickness ≤50mm. By ensuring higher C and Si components, the performance of the steel plate meets the standard requirements. At the same time, the simple alloy composition system effectively reduces the manufacturing cost. However, the low Mn component has a weak effect on strength improvement, and a lower reheating temperature must be adopted, making it extremely difficult to control the plate shape quality and unable to meet the manufacturing requirements of high plate shape quality steel plates.

[0004] Application No. 202010372949.X, Publication No. CN111621700A discloses "a production process for low-cost Q355C steel plates". In the composition design, the C-Si-Mn-Alt system is adopted. Specifically, it is required that: C: 0.11%~0.20%, Si: ≤0.20-0.45%, Mn: 1.1%~1.6%, Alt: 0.015%-0.045%, P: ≤0.025%, S: ≤0.015%, and the rest is Fe. Mainly by reducing the C content, improving the impact toughness at 0°C, and using cheap Al to replace Nb and V elements to reduce the production cost, thus manufacturing low-cost Q355C steel plates. However, this composition results in weak performance, and ACC cooling must be adopted, and the finish cooling temperature is controlled to 600°C, which will make it difficult to control the plate shape quality and does not meet the requirements of high plate shape quality.

[0005] Application No. 202011064332.8 and Publication No. CN112267072A disclose "An Economical Thin-Gauge Q355B Steel Plate and Its Preparation Method". This invention adopts a C-Si-Mn-Alt-Nb system, where: C: 0.16 - 0.19%, Si ≤ 0.24%, Mn ≤ 0.95%, Al: 0.020 - 0.045%, Nb: 0.008 - 0.015%, Cr ≤ 0.10%, P ≤ 0.025%, S ≤ 0.008%, N ≤ 0.080%, and the rest is iron and inevitable impurity elements. By mainly reducing the Mn content and adding a small amount of Nb component, a low-Mn micro-Nb design system is formed, which balances the alloy design cost. At the same time, the addition of micro-alloying elements is beneficial to improving the strength and toughness of the steel plate. However, this invention requires a strong cooling rate during the post-rolling cooling process, which will cause serious deformation of the steel plate and the plate shape quality does not meet the manufacturing requirements of high-quality plate-shaped steel. In addition, due to the addition of the precious alloy element Nb, the cost cannot be effectively reduced, and the overall manufacturing cost is still relatively high.

[0006] The 355MPa grade structural steel designed by the above prior art has greatly improved performance compared with ordinary 355MPa grade structural steel. At the same time, the manufacturing cost is effectively reduced, and the market competitiveness is relatively strong, which can meet the usage requirements of general structural steel. However, the above invention cannot meet the usage requirements of construction machinery steel with high plate shape quality. With the rapid development of the construction machinery industry, it is particularly urgent to develop an economical construction machinery steel with high plate shape quality. Summary of the Invention

[0007] The technical problem to be solved by the present invention is, aiming at the above-mentioned disadvantages existing in the prior art, to provide an economical Q355B construction machinery structural steel with high plate shape quality and its production method, which effectively improves the strength, hardness and toughness of the steel plate, ensures the flatness of the steel plate, and obtains good flatness.

[0008] The technical solution of the present invention to solve the above technical problems is: An economical Q355B construction machinery structural steel with high plate shape quality, the chemical composition of the structural steel by mass percentage includes: C: 0.15 - 0.19%, Si: 0.10 - 0.25%, Mn: 0.90 - 1.2%, P ≤ 0.025%, S ≤ 0.015%, Cr: 0.20 - 0.30%, Ti: 0.025 - 0.045%, Alt: 0.021 - 0.025%, and the rest is Fe and inevitable impurities, and the sum of the above components is 100%.

[0009] Further defined in the present invention, in the Q355B engineering machinery structural steel with economical high flatness quality, the chemical composition of the structural steel by mass percentage includes: C: 0.17%, Si: 0.12%, Mn: 1.1%, Alt: 0.025%, P: 0.015%, S: 0.010%, Cr: 0.24%, Ti: 0.032%, and the rest is Fe and inevitable impurities, and the sum of the above components is 100%.

[0010] In the Q355B engineering machinery structural steel with economical high flatness quality, the chemical composition of the structural steel by mass percentage includes: C: 0.17%, Si: 0.20%, Mn: 1.1%, Alt: 0.021%, P: 0.013%, S: 0.010%, Cr: 0.22%, Ti: 0.042%, and the rest is Fe and inevitable impurities, and the sum of the above components is 100%.

[0011] The chemical composition of the structural steel by mass percentage includes: C: 0.19%, Si: 0.18%, Mn: 1.05%, Alt: 0.025%, P: 0.014%, S: 0.010%, Cr: 0.22%, Ti: 0.041%, and the rest is Fe and inevitable impurities, and the sum of the above components is 100%.

[0012] The present invention also designs a production method of Q355B engineering machinery structural steel with economical high flatness quality, which specifically includes the following steps: (1) Prepare a continuous casting billet that meets the requirements according to the designed composition of the engineering machinery steel; (2) Rolling, specifically including a heating process, a rolling process, and a cooling process, where: Heating process: Keep the continuous casting billet at 1200 - 1240 °C for 3 - 4 h; The rolling process adopts a two-stage control process: In the first stage, the starting rolling temperature ≥ 1160 °C, the finishing rolling temperature ≥ 1020 °C, rolling 6 - 9 passes, and the cumulative reduction ≥ 60%; In the second stage, the starting rolling temperature is 900 - 940 °C, the finishing rolling temperature is 800 - 840 °C, rolling 4 - 8 passes, and the cumulative reduction ≥ 75%; Cooling process: During the cooling process, the starting cooling temperature is 740 - 800 °C; For steel plates with a thickness of 30 mm and below, directly air-cool; For steel plates with a thickness of more than 30 mm, adopt weak cooling, cool at 3 °C / s to 700 - 720 °C, and then air-cool to room temperature; Obtain the final required engineering machinery steel plate.

[0013] Further defined in the present invention, in the production method of the Q355B engineering machinery structural steel with economical high flatness quality, in step (1) when preparing the continuous casting billet for steelmaking, the P content in the converter dephosphorized molten steel is controlled at 0.013 - 0.015%, and the S content in the LF desulfurized molten steel is 0.010%.

[0014] In the production method of the Q355B engineering machinery structural steel with economical high flatness quality, in step (2), the heating coefficient of the continuous casting billet in the heating process is 8 - 8.3 min / cm.

[0015] In the production method of the Q355B engineering machinery structural steel with economical high flatness quality, for the produced engineering machinery structural steel, the yield strength of the steel plate is 392 - 445 MPa, the tensile strength is 546 - 582 MPa, the elongation after fracture is 28 - 36%, the impact energy KV2 at 20°C is 185 - 252 J, and the flatness of the steel plate is ≤ 4 mm / 2 m.

[0016] The beneficial effects of the present invention are: The alloy element composition design of the present invention is mainly based on the following mechanism: C: Adding C element can effectively improve the strength of the steel plate. However, with the increase of C content, the low-temperature toughness and welding performance of the steel plate will deteriorate, and at the same time, the center segregation of the continuous casting billet will be aggravated, making it easy to form cracks. But the C content cannot be too low, otherwise the strength of the steel plate will be severely reduced. Therefore, the C content is controlled at 0.15 - 0.19%.

[0017] Mn: Mn is a common alloy element in hot-rolled low alloys. It mainly improves the yield strength and tensile strength of the steel through solid solution strengthening. It can also hinder the growth of austenite grains, thus forming finer grains during the cooling process. This fine grain strengthening effect can improve the strength and toughness of the steel. However, the addition of Mn element will aggravate the center segregation of the continuous casting billet during the continuous casting process, and in severe cases, it will lead to the generation of cracks. Therefore, the Mn content is controlled at 0.90 - 1.2%.

[0018] Si: Si will dissolve into the matrix to improve the strength of the steel. However, Si will hinder the diffusion of carbides and the precipitation of carbides, thereby increasing the carbon concentration of the carbon-rich austenite to improve its stability, promoting the formation of carbon-rich austenite, resulting in a large increase in M-A components and greatly reducing the toughness of the steel. Therefore, the Si content is controlled at 0.10 - 0.25%.

[0019] Cr: The Cr element can effectively improve the strength and hardness of the steel plate. It mainly forms an infinite solid solution with iron, causing solid solution strengthening, lattice distortion, hindering dislocation movement, and enhancing the resistance to slip deformation, thereby improving the strength and hardness of the material. However, if the Cr content is too high, it will seriously affect the welding performance of the steel plate, mainly leading to an increase in the carbon equivalent of the steel and making it easier to generate cracks during welding. Therefore, the Cr content is controlled within 0.20 - 0.30%.

[0020] Ti: Ti can form a large number of Ti(C,N) precipitation phases, which pin the original austenite grain boundaries during the rolling process, hinder the growth of austenite grains, and thus refine the grains, effectively improving the strength and toughness of the steel plate. However, the Ti content should not be too much. During the steelmaking process, Ti is extremely easy to form large-sized TiN particles, which exist as crack sources in the steel plate and seriously deteriorate the low-temperature toughness of the steel plate. Therefore, Ti is controlled within 0.025 - 0.045%.

[0021] Through the alloying design of C - Si - Mn - Cr - Ti in the present invention, the use of precious alloys Nb and V is reduced, and low-cost alloying elements Ti and Cr are adopted. By optimizing the addition of Ti content and the TMCP production process, a large amount of Ti(C,N) precipitates are utilized as nucleation sites to effectively refine the grain size, improve the strength and toughness of the steel. At the same time, through the solid solution strengthening effect of Cr, the strength and hardness of the steel plate are effectively improved. Through the reasonable ratio of low Si and high Cr, while ensuring the strength and toughness of the steel plate, it is ensured that the structural steel for construction machinery only needs air cooling or weak water cooling after rolling, and the deformation of the steel plate is very small, making it fully meet the standard requirements of the construction machinery industry.

[0022] In the optimized two-stage rolling process of the present invention, a large number of Ti(C,N) particles will precipitate, pin the original austenite grain boundaries at the high-temperature stage, refine the grains and thus improve the strength and toughness. After the two-stage rolling process, air cooling or slow cooling is adopted to avoid or reduce the intervention of cooling water, effectively protecting and ensuring the flatness of the steel plate and obtaining good flatness.

[0023] The present invention reasonably designs the alloy and combines it with a specific production process, effectively reducing costs. During production, simple air cooling can be used, and finished products can be obtained without using complex air cooling. The process is simple, and the shape quality of the plate is easy to control, meeting the requirements of structural steel for construction machinery with high shape quality requirements. Specific Embodiments Example 1

[0024] In this embodiment, a Q355B engineering machinery structural steel with economical high flatness quality is provided. The chemical composition of the structural steel by mass percentage includes: C: 0.17%, Si: 0.12%, Mn: 1.1%, Alt: 0.025%, P: 0.015%, S: 0.010%, Cr: 0.24%, Ti: 0.032%, and the rest is Fe and inevitable impurities, and the sum of the above components is 100%.

[0025] The production method of the above-mentioned Q355B engineering machinery structural steel with economical high flatness quality specifically includes the following steps: (1) Prepare a continuous casting billet that meets the requirements according to the designed composition of the engineering machinery steel. When steelmaking, the P content of the converter-dephosphorized molten steel is 0.015%, and the S content of the LF-desulfurized molten steel is 0.010%; (2) Rolling, specifically including a heating process, a rolling process, and a cooling process, where: Heating process: The slab heating coefficient is 8 min / cm, and the continuous casting billet is kept at 1210 °C for 3 h; The rolling process adopts a two-stage control process: In the first stage, the starting rolling temperature is 1165 °C, the finishing rolling temperature is 1025 °C, rolling is carried out for 7 passes, and the cumulative reduction is 60%; In the second stage, the starting rolling temperature is 920 °C, the finishing rolling temperature is 822 °C, rolling is carried out for 7 passes, and the cumulative reduction is 75%; Cooling process: During the cooling process, the starting cooling temperature is 776 °C, air cooling is adopted, and slow cooling is carried out after rolling and stacking; Obtain the final required engineering machinery steel plate. The yield strength of the steel plate is 415 MPa, the tensile strength is 562 MPa, the elongation after fracture is 34%, and the impact energy KV at 20 °C 2 : 205, 211, 232 J, and the flatness of the steel plate is measured at 3 mm / 2 m. Example 2

[0026] In this embodiment, a Q355B engineering machinery structural steel with economical high flatness quality is provided. The chemical composition of the structural steel by mass percentage includes: C: 0.17%, Si: 0.20%, Mn: 1.1%, Alt: 0.021%, P: 0.013%, S: 0.010%, Cr: 0.22%, Ti: 0.042%, and the rest is Fe and inevitable impurities, and the sum of the above components is 100%.

[0027] The production method of the above-mentioned Q355B engineering machinery structural steel with economical high flatness quality specifically includes the following steps: (1) Prepare a continuous casting billet that meets the requirements according to the designed composition of the steel for construction machinery. During steelmaking, the P content of the converter-dephosphorized molten steel is 0.014%, and the S content of the LF-desulfurized molten steel is 0.010%. (2) Rolling, specifically including a heating process, a rolling process, and a cooling process, where: Heating process: The slab heating coefficient is 8 min / cm, and the continuous casting billet is kept at 1215 °C for 4 h. The rolling process adopts a two-stage control process: In the first stage, the starting rolling temperature is 1172 °C, the final rolling temperature is 1035 °C, rolling is carried out for 7 passes, and the cumulative reduction ratio is ≥60%. In the second stage, the starting rolling temperature is 936 °C, the final rolling temperature is 835 °C, rolling is carried out for 7 passes, and the cumulative reduction ratio is 75%. Cooling process: During the cooling process, the starting cooling temperature is 792 °C, air cooling is adopted, and slow cooling is carried out after rolling and stacking. Obtain the final required steel plate for construction machinery. The yield strength of this steel plate is 392 MPa, the tensile strength is 546 MPa, the elongation after fracture is 36%, and the impact energy KV at 20 °C 2 : 185, 202, 175 J, and the flatness of the steel plate is measured at 3 mm / 2 m. Example 3

[0028] In this example, a Q355B structural steel for construction machinery with economical high plate shape quality is provided. The chemical composition of this structural steel by mass percentage includes: C: 0.19%, Si: 0.18%, Mn: 1.05%, Alt: 0.025%, P: 0.014%, S: 0.010%, Cr: 0.22%, Ti: 0.041%, and the rest are Fe and unavoidable impurities, and the sum of the above components is 100%.

[0029] The production method of the above-mentioned Q355B structural steel for construction machinery with economical high plate shape quality specifically includes the following steps: (1) Prepare a continuous casting billet that meets the requirements according to the designed composition of the steel for construction machinery. During steelmaking, the P content of the converter-dephosphorized molten steel is 0.013%, and the S content of the LF-desulfurized molten steel is 0.010%. (2) Rolling, specifically including a heating process, a rolling process, and a cooling process, where: Heating process: The slab heating coefficient is 8.3 min / cm, and the continuous casting billet is kept at 1219 °C for 3.5 h. The rolling process adopts a two-stage control process: In the first stage, the starting rolling temperature is 1162 °C, the final rolling temperature is 1022 °C, rolling is carried out for 7 passes, and the cumulative reduction ratio is 60%. The starting rolling temperature in the second stage is 915 °C, the finishing rolling temperature is 802 °C, and it is rolled for 7 passes with a cumulative reduction of 75%; Cooling process: air cooling, slow cooling by stacking after rolling, and then air cooling to room temperature; The final required steel plate for construction machinery is obtained. The yield strength of this steel plate is 445 MPa, the tensile strength is 582 MPa, the elongation after fracture is 28%, and the impact energy KV at 20 °C 2 is 235, 245, 252 J, and the flatness of the steel plate is measured at 4 mm / 2 m.

[0030] In addition to the above embodiments, the present invention may also have other embodiments. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope required by the present invention.

Claims

1. An economical high-quality Q355B engineering machinery structural steel, characterized by: The chemical composition of the structural steel includes, by mass percentage, C: 0.15-0.19%, Si: 0.10-0.25%, Mn: 0.90-1.2%, P≤0.025%, S≤0.015%, Cr: 0.20-0.30%, Ti: 0.025-0.045%, Alt: 0.021-0.025%, and the rest are Fe and unavoidable impurities. The sum of the above components is 100%.

2. The economical high-quality Q355B engineering machinery structural steel according to claim 1 is characterized by: The chemical composition of the structural steel includes, by mass percentage, C: 0.17%, Si: 0.12%, Mn: 1.1%, Alt: 0.025%, P: 0.015%, S: 0.010%, Cr: 0.24%, Ti: 0.032%, and the rest are Fe and unavoidable impurities. The sum of the above components is 100%.

3. The economical high-quality Q355B engineering machinery structural steel according to claim 1 is characterized by: The chemical composition of the structural steel includes, by mass percentage, C: 0.17%, Si: 0.20%, Mn: 1.1%, Alt: 0.021%, P: 0.013%, S: 0.010%, Cr: 0.22%, Ti: 0.042%, and the rest are Fe and unavoidable impurities. The sum of the above components is 100%.

4. The economical high-quality Q355B engineering machinery structural steel according to claim 1 is characterized by: The chemical composition of the structural steel includes, by mass percentage, C: 0.19%, Si: 0.18%, Mn: 1.05%, Alt: 0.025%, P: 0.014%, S: 0.010%, Cr: 0.22%, Ti: 0.041%, and the rest are Fe and unavoidable impurities. The sum of the above components is 100%.

5. The method for producing economical high-quality Q355B engineering machinery structural steel according to any one of claims 1 to 4, characterized in that: The specific steps include: (1) Prepare continuous casting billets that meet the requirements according to the design composition of engineering machinery steel; (2) Rolling, which specifically includes heating process, rolling process and cooling process, in which: In the heating process, the continuous casting billet is kept at 1200-1240° C. for 3-4 hours; The rolling process adopts a two-stage control process: The first stage rolling temperature is ≥1160℃, the final rolling temperature is ≥1020℃, the rolling is 6-9 times, and the cumulative reduction is ≥60%; The second stage rolling temperature is 900-940℃, the final rolling temperature is 800-840℃, the rolling is 4-8 times, and the cumulative reduction is ≥75%; In the cooling process, the cooling temperature is 740-800°C; Steel plates with specifications of 30mm and below are directly cooled by air; For steel plates with a thickness of 30 mm or more, weak cooling is adopted, cooling to 700-720°C at 3°C / s, and then air cooling to room temperature; Obtain the final required steel plate for construction machinery.

6. The method for producing economical high-quality Q355B engineering machinery structural steel according to claim 5, characterized in that: In the step (1), when preparing the continuous casting billet steelmaking, the P content in the converter desulfurized steel water is controlled to be 0.013-0.015%, and the S content in the LF desulfurized steel water is controlled to be 0.010%.

7. The method for producing economical high-quality Q355B engineering machinery structural steel according to claim 5, characterized in that: The heating coefficient of the continuous casting billet in the heating process in step (2) is 8-8.3 min / cm.

8. The method for producing economical high-quality Q355B engineering machinery structural steel according to claim 5, characterized in that: The steel plates produced for engineering machinery structures have a yield strength of 392-445MPa, a tensile strength of 546-582MPa, an elongation after fracture of 28-36%, an impact energy KV2 of 185-252J at 20°C, and a steel plate unevenness of ≤4mm / 2m.