High-toughness low-carbon-equivalent Q390 steel plate and production method thereof

Abstract

The invention provides a high-toughness low-carbon-equivalent Q390 steel plate and a production method thereof. The high-toughness low-carbon-equivalent Q390 steel plate comprises the following chemical components in percentage by mass: 0.10%-0.15% of C, 0.40%-0.55% of Si, 1.5%-1.8% of Mn, 0.01%-0.02% of Ti, 0.035%-0.045% of Nb, 0.001%-0.0015% of B, less than or equal to 0.02% of P, less than or equal to 0.01% of S, less than or equal to 0.39% of Ceq and the balance of Fe and inevitable impurities. A basic design thought of low C, Nb and Ti microalloying is adopted, a controlled rolling and controlled cooling process and quenching and tempering heat treatment parameters are strictly controlled, the high-strength extra-thick low-temperature high-toughness Q390 with good comprehensive mechanical properties is prepared, the thickness of the steel plate is 20-30 mm, the yield strength of the steel plate is larger than or equal to 385 MPa, the tensile strength is larger than or equal to 510 MPa, the percentage elongation after fracture is larger than or equal to 31%, the 0 DEG C impact absorbing energy KV2 is larger than or equal to 189 J, and the yield ratio is smaller than or equal to 0.75.

Description

Technical Field

[0001] The invention belongs to the field of steel material preparation, and relates to a high-toughness, low-carbon equivalent Q390 steel plate and a production method thereof. Background Art

[0002] Low alloy high strength steel plate Q390C is widely used in engineering machinery, building structures, plant structures and other fields due to its good comprehensive performance. The use environment of this type of steel plate usually requires it to have high strength while still maintaining excellent toughness, especially impact toughness under low temperature conditions. According to Chinese national standards, the impact energy of Q390C steel plate at 0°C must be greater than 27J, which places high demands on the balance of its strength and toughness. Therefore, how to optimize the toughness of steel plates while ensuring high strength has become an important topic in the research and production of steel materials.

[0003] The room temperature and low temperature microstructure of Q390C steel plate is usually composed of ferrite and pearlite. In order to improve the strength while ensuring its toughness, micro-alloying elements such as Nb, V, Ti, etc. are often added to optimize the strength-toughness match. In addition, in the production process of steel plates, precise control of cooling rate and final cooling temperature is also crucial, which can refine the ferrite grains, thereby improving the mechanical properties of the steel plate, especially the toughness under low temperature conditions. However, the balance between high strength and toughness of traditional Q390C steel plates often has certain limitations. With the continuous development of engineering technology, the requirements for the comprehensive improvement of steel are getting higher and higher. Steel plates not only need to withstand higher workloads, but also must maintain sufficient toughness and impact resistance in low temperature environments. To this end, the development of new low-alloy high-strength steels with better performance has become an urgent problem to be solved.

[0004] CN105525210B discloses "a low yield ratio Q390GJ steel plate for construction and its production method", whose chemical composition is C: 0.15~0.17%, Si: 0.30~0.40%, Mn: 1.45~1.55%, P ≤0.015%, S≤0.010%, Nb: 0.03~0.04%, V: 0.03~0.04%, Ti: 0.01~0.02%, Als: 0.020~0.040%, and the rest is Fe and impurities. It has many micro-alloy elements and a high C content, and its elongation and 0℃ impact energy are low.

[0005] CN102330020A discloses a "manufacturing method of high-toughness steel plate with yield strength of 345-390MPa", whose chemical composition is C: 0.08~0.20%, Si: 0.03~0.55%, Mn: 0.8~1.70%, P≤0.015%, S≤0.010%, V: 0.015~0.05%, Nb: 0.01~0.07%, Ti: 0.005~0.05%, N: 0.003~0.012%, and the rest is Fe and unavoidable impurities. The patent adopts molten iron pretreatment, converter smelting, refining, continuous casting, heating, controlled rolling and controlled cooling, normalizing, and water spray cooling, and the process is relatively complicated. Summary of the invention

[0006] The present invention aims to provide a high-toughness low-carbon equivalent Q390 steel plate and a production method thereof, and produce a 20-30mm thick low-yield ratio high-toughness low-C equivalent Q390 steel plate. The steel plate not only has good mechanical properties, but also has a simple production process, is economical and applicable, and is suitable for large-scale industrial applications.

[0007] The technical solution of the present invention: A high-toughness low-carbon equivalent Q390 steel plate, the chemical composition and mass percentage of the steel are C=0.10%~0.15%, Si=0.40%~0.55%, Mn=1.5%~1.8%, Ti=0.01%~0.02%, Nb=0.035%~0.045%, B=0.001%~0.0015%, P≤0.02%, S≤0.01%, Ceq=C+(Mn+Mo) / 10+(Cu+Cr) / 20+Ni / 40≤0.39, the remainder is Fe and unavoidable impurities; the finished steel plate has a thickness of 20~30mm, a yield strength of ≥385MPa, a tensile strength of ≥510MPa, an elongation after fracture of ≥31%, and an impact absorption work of KV at 0℃ 2 ≥189J.

[0008] A method for producing a high-toughness, low-carbon equivalent Q390 steel plate comprises the following process steps: (1) Smelting and casting: After desulfurization treatment, the molten iron is smelted in a converter, LF refined and RH vacuum treated, and then continuously cast into billets. The sulfur content of the molten iron after desulfurization treatment is controlled at ≤0.0021%. The converter adopts the double slag method to control P≤0.013%. LF refining adopts white slag operation for desulfurization and deoxidation. The vacuum degree of RH vacuum treatment is ≤3.0 mbar, and the vacuum holding time is ≥15min. After the vacuum is over, calcium treatment is carried out, and then the static stirring time is ≥12min. Finally, mold casting is used for casting. The thickness of the billet is 150~200mm. After the casting, it is slowly cooled in a windproof place for more than 24h; (2) Heating: Heat the 150~200mm thick die-cast billet in a soaking furnace for 60~80min and the tapping temperature is 1100~1150℃; (3) Controlled rolling: The die casting billet adopts a two-stage rolling process of rough rolling and finishing rolling. The starting rolling temperature of rough rolling is 1040~1060℃, and it is rolled through 4 passes. The reduction rate of the middle two passes is ≥20%, and large reduction rolling is adopted; the cumulative reduction ratio of rough rolling is not less than 2.3, and the thickness of the intermediate billet is 64~75mm; the starting rolling temperature of the finishing rolling stage is 850~900℃, and it is rolled through 4 passes. The reduction rate of each pass is 11%~19%, and the final rolling temperature is ≥730℃; (4) Controlled cooling: The die-cast billet is rolled to a finished product thickness of 20-30 mm. After rolling, it is cooled by air or water. The starting cooling temperature of the water cooling method is 750-800°C, the final cooling temperature is 700-710°C, and then air-cooled to room temperature.

[0009] The thickness of the Q390 grade finished product is 20~30mm, the yield strength of the steel plate is ≥385MPa, the tensile strength is ≥510MPa, the elongation after fracture is ≥31%, and the impact absorption energy at 0℃ is KV 2 ≥214J.

[0010] The mechanism of action of each alloy component used in the present invention is as follows: The C content determines the equilibrium structure of the iron-carbon alloy, which in turn affects the mechanical properties of the material. The carbon mass fraction in the steel, as well as the carbon form and distribution, affect the plasticity of the steel at room temperature. During the thermal deformation of the steel plate, the C element will cause banded segregation, thereby reducing the isotropy of the steel and deteriorating the room temperature plasticity; as the carbon content increases, carbides will aggregate in large quantities in the aggregate, producing coarse precipitates, which will affect the mechanical properties of the material, so the present invention controls the C content range to be 0.10%~0.15%.

[0011] Si is a commonly used deoxidizer, used to remove oxygen from molten steel. 2 The presence of Si in the form of Si can also play a role in solid solution strengthening, but when the mass fraction of Si is high, the toughness and plasticity of the steel will be reduced. Adding an appropriate amount of Si to the steel can also inhibit the growth of carbides, thereby improving the toughness of the steel. However, the higher the Si content, the better. Too much silicon content affects the surface properties of the material and the toughness of the steel. Therefore, the present invention controls the Si content range to be 0.40%~0.55%.

[0012] Mn is also one of the deoxidizers, which can effectively improve the hot working performance and tear resistance of steel. Mn has the effect of delaying the transformation of austenite to ferrite in steel, which is beneficial to refining ferrite and improving strength and toughness. When the content of Mn is low, the above effect is not significant, the strength and toughness of the steel plate are low, etc., and too high will cause segregation of the die casting blank and poor toughness. Therefore, the preferred Mn content range of the present invention is 1.5%~1.8%.

[0013] Nb can effectively prevent the recovery and recrystallization of deformed austenite during the rolling stage, which has the effect of refining grains and can also play a role in precipitation strengthening. The solute drag effect of Nb and the pinning effect of Nb (C, N) on the austenite grain boundary both inhibit the recrystallization of deformed austenite, expand the non-recrystallization range of austenite, and reduce the waiting time for the production of extra-thick plates. Precipitates are formed during cooling or tempering, thereby improving both strength and toughness, and can also improve the corrosion resistance of steel. The present invention controls the Nb content range to be 0.035%~0.045%.

[0014] Ti also has the effect of grain refinement and precipitation strengthening. Studies have shown that TiN can effectively increase the austenite grain coarsening temperature and prevent the tendency of grain coarsening during thermal deformation. Ti has a strong affinity with O, C, N, and S. Ti combines with CN to form stable TiC, Ti (C, N) or TiN particles, which can withstand high temperatures above 1300°C without dissolving, and play a role in pinning austenite grain boundaries and maintaining austenite grain refinement. Ti and S can combine to form Ti4C2S2 or (MnTi)S, reduce the content of MnS inclusions in steel, improve the heterogeneity of the structure, and thus improve the impact toughness of steel. Therefore, the present invention controls the Ti content range to be 0.010%~0.02%.

[0015] B gathers at the austenite grain boundary, effectively inhibiting ferrite nucleation and delaying the ferrite-pearlite transformation, thereby improving the hardenability of steel. Adding extremely low content of B can significantly improve the hardenability of steel. To a large extent, it can replace the addition of alloying elements such as Ni, Cr, and Mo to achieve the same hardenability effect; B can also be used to control phase transformation and microstructure in low-alloy high-strength steel. Therefore, the content range of B controlled by the present invention is 0.001%~0.0015%.

[0016] P and S are unfavorable elements in steel, which seriously affect the strength, toughness and corrosion resistance of steel, and their content needs to be controlled as low as possible. Therefore, the present invention controls the content of P to be below 0.010%, and the content of S to be below 0.004%.

[0017] Low C is one of the key designs to optimize the toughness of Q390 steel plates and improve low-temperature impact toughness. Low C content helps to reduce the precipitation of carbides in the steel, avoids the formation of hard-brittle phases, and thus inhibits the brittle transition of the steel under low temperature conditions. The low C design helps to promote the precipitation of ferrite, optimize the microstructure of the steel, and enhance its ductility and crack resistance. In a low-temperature environment, low C alloying can significantly improve the impact toughness, so that the steel plate can exhibit good plasticity and impact energy absorption under cold conditions. As a microalloying element, the addition of B to steel has a significant strengthening effect. The B element can refine the grains and promote grain boundary strengthening, thereby inhibiting crack propagation and brittle fracture. Specifically, B can form stable borides (such as FeB, Fe 2 B), these compounds are evenly distributed in the steel and play a strengthening role. At the same time, the B element can also inhibit the growth of grains at high temperatures and improve the thermal stability and high temperature strength of the steel.

[0018] Microalloying of Nb and Ti not only improves the strength of the steel plate through the dispersion strengthening mechanism, but also refines the grains and improves its mechanical properties under extreme environments. Especially under low temperature conditions, the microalloying effect of Nb and Ti enhances the toughness and crack growth resistance of Q390 steel plate. The main function of Nb is to promote the precipitation of carbides or nitrides. These fine carbides (such as NbC) act as dispersion strengthening phases in steel, significantly improving the tensile strength and hardness of steel. In addition, Nb can effectively inhibit the growth of grains, especially during high-temperature rolling, which helps to refine the grains of steel, thereby improving the strength and ductility of the steel plate. The addition of Ti can form stable TiC and TiN compounds. These microalloying phases are evenly distributed in the steel, effectively hindering the growth of grains and playing a role in dispersion strengthening the strength. Ti can also combine with impurity elements (such as nitrogen, sulfur, etc.) in steel to form stable compounds, further improving the purification degree and corrosion resistance of steel. Working synergistically with Nb, Ti can improve the toughness of steel, especially under low-temperature impact conditions, and enhance the low-temperature impact energy absorption capacity of the steel plate.

[0019] The beneficial effects of the present invention are as follows: 1) In terms of composition design, the basic design concept of low C, B alloying and Nb+Ti microalloying is adopted to improve the hardenability of the core of the steel plate, refine the microstructure of the steel plate, and ensure that the steel plate has good strength and low-temperature toughness, as well as weldability. 2) In terms of process, the controlled rolling and controlled cooling process parameters are strictly controlled to ensure that the rolling force can penetrate into the core of the billet, while changing the internal organizational state of the steel plate, refining the grains, and ensuring the uniformity and low-temperature toughness of the core organization of the steel plate. 3) In terms of performance, the steel plate produced by the present invention has a yield strength of ≥385MPa, a tensile strength of ≥510MPa, an elongation after fracture of ≥31%, and an impact absorption energy KV2 of ≥189J at 0°C, and can produce steel plates with a thickness specification of 20~30mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a microstructure morphology diagram of the steel plate at the 1 / 2 position in the tempered state in Example 1 of the present invention.

[0021] Figure 2 This is a microstructure morphology diagram of the steel plate at the 1 / 2 position in the tempered state in Example 2 of the present invention. DETAILED DESCRIPTION

[0022] The present invention is further described in detail below in conjunction with specific embodiments. The chemical composition mass percentage of each embodiment is shown in Table 1, and the balance is Fe and unavoidable impurities; the performance indicators of the steel plate are shown in Table 2, and the core structure morphology of the steel plate is shown in the attached figure. Example 1

[0023] A high-toughness low-carbon equivalent Q390 steel plate and a production method thereof, wherein the produced steel plate has a thickness of 20 mm. The process flow includes smelting, continuous casting, heating, controlled rolling, controlled cooling and quenching and tempering processes, and the key process steps are as follows: (1) Smelting and casting: After desulfurization treatment, the molten iron is smelted in a converter, LF refined and RH vacuum treated, and then continuously cast into billets. The sulfur content of the molten iron after desulfurization treatment is controlled at ≤0.0021%. The converter adopts the double slag method to control P≤0.013%. LF refining adopts white slag operation for desulfurization and deoxidation. The vacuum degree of RH vacuum treatment is ≤3.0mbar, and the vacuum holding time is ≥15min. After the vacuum is over, calcium treatment is carried out, and then the static stirring time is ≥12min. Finally, mold casting is used for casting. The thickness of the billet is 150mm. After the casting is completed, it is slowly cooled in a windproof place for more than 24h; (2) Heating: The 150 mm thick die-cast billet is heated in a soaking furnace for 60 min and the tapping temperature is 1100 °C. (3) Controlled rolling: The die casting billet adopts a two-stage rolling process of rough rolling and finishing rolling. The starting rolling temperature of rough rolling is 1050℃. After 4 rolling passes, the reduction rates of the middle two passes are ≥20% and are 23.1% and 20% respectively. Large reduction rolling is adopted; the cumulative reduction ratio of rough rolling is not less than 2.2, and the thickness of the intermediate billet is 64mm. The starting rolling temperature of the finishing rolling stage is 870℃. After 6 rolling passes, the reduction rates of the passes are 18.7%, 19.2%, 19.0%, 17.6%, 17.8%, 11.5%, and the final rolling temperature is 730℃. (4) Controlled cooling: The die-cast billet is rolled to a finished product thickness of 20 mm and then air-cooled to room temperature after rolling. Example 2

[0024] A high-toughness low-carbon equivalent Q390 steel plate and a production method thereof, the thickness of the produced steel plate is 20 mm, the chemical composition and mass percentage thereof are shown in Table 1, and the remainder is Fe and unavoidable impurities. It includes smelting, continuous casting, heating, controlled rolling, controlled cooling and quenching and tempering processes, and the specific process steps are as follows: (1) Smelting and casting: After desulfurization treatment, the molten iron is smelted in a converter, LF refined and RH vacuum treated, and then continuously cast into billets. The sulfur content of the molten iron after desulfurization treatment is controlled at ≤0.0021%. The converter adopts the double slag method to control P≤0.013%. LF refining adopts white slag operation for desulfurization and deoxidation. The vacuum degree of RH vacuum treatment is ≤3.0mbar, and the vacuum holding time is ≥15min. After the vacuum is over, calcium treatment is carried out, and then the static stirring time is ≥12min. Finally, mold casting is used for casting. The thickness of the billet is 150mm. After the casting is completed, it is slowly cooled in a windproof place for more than 24h; (2) Heating: The 150 mm thick die-cast billet is heated in a soaking furnace for 60 min and the tapping temperature is 1100 °C. (3) Controlled rolling: The die casting billet adopts a two-stage rolling process of rough rolling and finishing rolling. The starting rolling temperature of rough rolling is 1050℃. After 4 rolling passes, the reduction rates of the middle two passes are ≥20% and are 23.1% and 20% respectively. Large reduction rolling is adopted; the cumulative reduction ratio of rough rolling is not less than 2.2, and the thickness of the intermediate billet is 64mm. The starting rolling temperature of the finishing rolling stage is 870℃. After 6 rolling passes, the reduction rates of the passes are 18.7%, 19.2%, 19.0%, 17.6%, 17.8%, 11.5%, and the final rolling temperature is 730℃. (4) Controlled cooling: The die-cast billet is rolled to a finished product thickness of 20 mm and then water-cooled to room temperature after rolling. Example 3

[0025] A high-toughness low-carbon equivalent Q390 steel plate and a production method thereof, the thickness of the produced steel plate is 30 mm, the chemical composition and mass percentage thereof are shown in Table 1, and the remainder is Fe and unavoidable impurities. It includes smelting, continuous casting, heating, controlled rolling, controlled cooling and quenching and tempering processes, and the specific process steps are as follows: (1) Smelting and casting: After desulfurization, the molten iron is smelted in a converter, LF refined and RH vacuum treated, and then continuously cast into billets. The sulfur content of the molten iron after desulfurization is controlled at ≤0.0021%. The converter uses the double slag method to control P≤0.013%. LF refining uses white slag operation for desulfurization and deoxidation. The vacuum degree of RH vacuum treatment is ≤3.0mbar, and the vacuum holding time is ≥15min. After the vacuum is over, calcium treatment is carried out, and then the static stirring time is ≥12min. The continuous casting controls the superheat of the tundish to 10~15℃. Finally, mold casting is used for casting, and the billet thickness is 200mm. After the casting is completed, it is slowly cooled in a sheltered place for more than 24h; (2) Heating: The 200 mm thick die-cast billet is heated in a soaking furnace for 60 min and the tapping temperature is 1100 °C. (3) Controlled rolling: The die casting billet adopts a two-stage rolling process of rough rolling and finishing rolling. The starting rolling temperature of rough rolling is 1050℃. After 4 rolling passes, the reduction rates of the middle two passes are ≥20% and are 24.1% and 22.3% respectively. Large reduction rolling is adopted; the cumulative reduction ratio of rough rolling is not less than 2.3, and the thickness of the intermediate billet is 70mm. The starting rolling temperature of the finishing rolling stage is 880℃. After 6 rolling passes, the reduction rates of the passes are 19.7%, 19.2%, 19.1%, 18.6%, 17.5%, and 11.7%, and the final rolling temperature is 810℃; (4) Controlled cooling: The die-cast billet is rolled to a finished product thickness of 30 mm. After rolling, it is cooled by laminar water cooling. The initial cooling temperature is 800°C, the final cooling temperature is 700°C, and then air-cooled to room temperature.

[0026] Table 1 Chemical composition of steel plates in the examples (%) .

[0027] Table 2 Performance indicators of steel plates in the embodiments of the present invention .

Claims

1. A high-toughness, low-carbon equivalent Q390 steel plate, characterized in that: The chemical composition and mass percentage of the steel are C=0.10%~0.15%, Si=0.40%~0.55%, Mn=1.5%~1.8%, Ti=0.01%~0.02%, Nb=0.035%~0.045%, B=0.001%~0.0015%, P≤0.02%, S≤0.01%, Ceq=C+(Mn+Mo) / 10+(Cu+Cr) / 20+Ni / 40≤0.39, and the balance is Fe and unavoidable impurities; The finished steel plate has a thickness of 20~30mm, a yield strength ≥385MPa, a tensile strength ≥510MPa, an elongation after fracture ≥31%, and an impact absorption energy KV2 ≥189J at 0℃.

2. A method for producing high-toughness low-carbon equivalent Q390 steel plate, characterized in that The process steps include: (1) Smelting and casting: After desulfurization treatment, the molten iron is smelted in a converter, LF refined and RH vacuum treated, and then continuously cast into billets. The sulfur content of the molten iron after desulfurization treatment is controlled at ≤0.0021%. The converter adopts the double slag method to control P≤0.013%. LF refining adopts white slag operation for desulfurization and deoxidation. The vacuum degree of RH vacuum treatment is ≤3.0 mbar, and the vacuum holding time is ≥15 min. After the vacuum is over, calcium treatment is carried out, and then the static stirring time is ≥12 min. Finally, mold casting is used for casting. The thickness of the billet is 150~200 mm. After the casting, it is slowly cooled in a wind-proof place for more than 24 hours; (2) Heating: Heat the 150~200mm thick die-cast billet in a soaking furnace for 60~80min and the tapping temperature is 1100~1150℃; (3) Controlled rolling: The die casting billet adopts a two-stage rolling process of rough rolling and finishing rolling. The starting rolling temperature of rough rolling is 1040~1060℃, and it is rolled through 4 passes. The reduction rate of the middle two passes is ≥20%, and large reduction rolling is adopted; the cumulative reduction ratio of rough rolling is not less than 2.3, and the thickness of the intermediate billet is 64~75mm; the starting rolling temperature of the finishing rolling stage is 850~900℃, and it is rolled through 4 passes. The reduction rate of each pass is 11%~19%, and the final rolling temperature is ≥730℃; (4) Controlled cooling: The die-cast billet is rolled to a finished product thickness of 20-30 mm. After rolling, it is cooled by air or water. The starting cooling temperature of the water cooling method is 750-800°C, the final cooling temperature is 700-710°C, and then air-cooled to room temperature.

Images