Steel plate with ultralow yield ratio and excellent ductility and production method thereof

Abstract

The invention discloses a steel plate with ultralow yield ratio and excellent ductility and a production method thereof. The steel plate comprises the following chemical components: 0.03 to 0.07 percent of C, 0.20 to 0.50 percent of Si, 0.40 to 0.70 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.003 percent of S, 20.00 to 35.00 percent of Ni, 20.00 to 27.00 percent of Cr, 0.20 to 0.50 percent of Co, 0.050 to 0.100 percent of Nb, 0.010 to 0.030 percent of Ti, 0.020 to 0.050 percent of V, 0.030 to 0.060 percent of Al, 0.0020 to 0.0035 percent of B and the balance of Fe and other inevitable impurities. The production method comprises the steps of steelmaking, continuous casting, heating rolling and heat treatment. The thickness of the steel plate ranges from 10 mm to 30 mm, and meanwhile the steel plate has the high ductility and the ultralow yield ratio.

Description

Technical Field

[0001] The invention belongs to the technical field of steel plate production, and in particular relates to a steel plate with ultra-low yield ratio and excellent ductility and a production method thereof. Background Art

[0002] With the rapid development of social technology, the use of steel products is becoming more and more extensive, especially stainless steel, which has entered all aspects of human life due to its good corrosion resistance and high temperature performance. At present, stainless steel has occupied a dominant position in the manufacture of kitchenware and tableware, and is widely used in the manufacture of kitchen utensils such as pots and pans, oven trays, and grills, as well as various tableware such as knives, forks, and spoons. Stainless steel is also widely used in the manufacture of various equipment and parts, such as pumps, valves, pipes, pressure vessels, etc. Due to its excellent corrosion resistance, it is also one of the important materials in industries such as chemical, pharmaceutical, and food processing.

[0003] Compared with ordinary steel, stainless steel has a higher content of precious metals such as Ni, Cr, and Mo, and has a higher production cost, so it is generally used in key parts, surfaces, or small parts with complex structures. These special steels require very low yield ratio and high ductility, so it is of great significance to develop special steels with ultra-low yield ratio and excellent ductility. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a steel plate with ultra-low yield ratio and excellent ductility and a production method thereof. The steel plate has a thickness of 10 to 30 mm, and has both high ductility and ultra-low yield ratio, and can be widely used in key components with complex structures that require various deformations.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: A steel plate with ultra-low yield ratio and excellent ductility, wherein the chemical composition and weight percentage are C: 0.03-0.07%, Si: 0.20-0.50%, Mn: 0.40-0.70%, P≤0.015%, S≤0.003%, Ni: 20.00-35.00%, Cr: 20.00-27.00%, Co: 0.20-0.50%, Nb: 0.050-0.100%, Ti: 0.010-0.030%, V: 0.020-0.050%, Al: 0.030-0.060%, B: 0.0020-0.0035%, and the rest are Fe and other inevitable impurities.

[0006] The role and proportion of each element in the steel plate of the present invention are as follows: C: 0.03-0.07%. Carbon has a significant effect on various properties of steel, especially the strength, impact toughness and welding performance of steel. The low carbon content in stainless steel is mainly to prevent carbon from combining with chromium, thereby preventing the appearance of chromium-poor areas at the grain boundaries, which makes the areas non-corrosive, and to prevent the formation of chromium carbides with chromium, which consumes chromium and reduces the effective chromium content of steel, thereby reducing the corrosion resistance of steel.

[0007] Si: 0.20-0.50%. Silicon is the main reducing agent and deoxidizer in the steelmaking process. Silicon can significantly improve the elastic limit, yield point and tensile strength of steel. However, when the silicon content exceeds 0.50%, the toughness of the steel will be reduced, and the welding performance of the steel will be reduced.

[0008] Mn: 0.40~0.70%. Manganese is cheap and is a good deoxidizer and desulfurizer. It can increase the toughness and strength of steel, while improving the hardenability and hot working properties of steel. However, if the manganese content is too high, it will weaken the corrosion resistance of steel, reduce welding performance, and cause serious segregation.

[0009] P≤0.015%. Generally speaking, phosphorus is a harmful element in steel. It increases the cold brittleness of steel, reduces its plasticity, and deteriorates its cold bending performance. Under the condition of controlling the cost reasonably, the phosphorus content should be reduced as much as possible.

[0010] S≤0.003%. Sulfur is also a harmful element in steel. It increases the hot brittleness of steel and reduces the ductility and toughness of steel. The sulfur content in steel should be reduced as much as possible under economic benefits.

[0011] Ni: 20.00~35.00%. Nickel can improve the strength of steel, and also enhance the plasticity and toughness of steel. It is an important alloy element in stainless steel. However, metallic nickel is a scarce resource and its price is relatively high.

[0012] Cr: 20.00~27.00%. Chromium can improve the strength and hardenability of steel plates at the same time. It is also one of the important elements in stainless steel. Therefore, a certain amount of chromium needs to be added, otherwise the steel will rust.

[0013] Co: 0.20-0.50%. The main function of cobalt in steel is to refine the grains, improve thermal strength, increase the coercive force of magnetic steel, and have a good effect on magnetism. It can refine the grains of steel and reduce the tendency of steel to overheat.

[0014] Nb: 0.050-0.100%. Niobium can promote grain refinement of steel microstructure, while improving strength and toughness. Niobium can effectively refine the microstructure by inhibiting austenite recrystallization during controlled rolling, and improve the hardenability of steel through precipitation strengthening, while reducing overheating sensitivity and temper brittleness of steel.

[0015] Ti: 0.010-0.030%. Titanium is a strong carbonitride-forming element. Carbonitride has a high melting point and can hinder the growth of austenite grains during heating. It can also refine the grains and improve the welding performance of steel plates.

[0016] V: 0.020-0.050%. Vanadium is a good deoxidizer in steel. It can refine grains and improve the strength and toughness of steel. The carbide formed by vanadium and carbon can improve the resistance to hydrogen corrosion under high temperature and high pressure.

[0017] Al: 0.030-0.060%. Aluminum is a commonly used deoxidizer in steel. Adding a small amount of aluminum to steel can refine the grains and improve impact toughness. Aluminum also has antioxidant and corrosion resistance. Too high a content will affect the hot working properties, welding properties and cutting properties of steel.

[0018] B: 0.0020~0.0035%. Boron in steel can significantly improve the hardenability of steel, thereby improving the strength and hardness of steel. This is because boron atoms can effectively pin on the austenite grain boundaries, hindering grain growth, thereby refining the grains and increasing the hardness of the material. It can also significantly improve the corrosion resistance of steel, so it is necessary to add an appropriate amount of boron.

[0019] Furthermore, the thickness of the steel plate of the present invention is 10 to 30 mm.

[0020] Furthermore, the steel plate of the present invention has a yield strength of ≥300 MPa, a tensile strength of ≥700 MPa, an elongation of ≥0.40%, and a yield strength ratio of ≤0.5.

[0021] Furthermore, the method for producing the steel plate with ultra-low yield ratio and excellent ductility of the present invention comprises steelmaking, continuous casting, hot rolling and heat treatment steps.

[0022] Preferably, in the steelmaking process of the present invention, qualified molten steel smelted in the primary refining furnace is transferred to the LF furnace for refining. During the LF furnace refining process, argon blowing is ensured to be good throughout the whole process, the white slag is maintained for ≥20 min, and alloys are added in batches after deoxidation to make the alloy content within the required range; the qualified molten steel refined in the LF is hoisted into a VOD furnace for oxygen blowing and decarburization to the requirement, and then vacuum degassing treatment is carried out, the vacuum holding time is ≥20 min, and after the vacuum degassing treatment is completed, soft blowing is performed for 8 to 12 min to promote the floating and removal of non-metallic inclusions.

[0023] Preferably, in the continuous casting process of the present invention, qualified molten steel is cast into wide and thick slabs with a thickness of 200 to 250 mm through a large thick slab continuous casting machine, and the superheat is controlled within the range of 15 to 25° C. during the casting process.

[0024] Preferably, in the heating rolling process of the present invention, the slab is heated in a continuous furnace with a maximum heating temperature of 1200°C, a temperature in the insulation stage of 1160-1180°C, a total heating time ≥15min / cm, a starting rolling temperature of 1000-1100°C during rolling, a reduction rate of 10-15% per pass, a final rolling temperature of 850-950°C, and air cooling to room temperature after rolling.

[0025] Preferably, in the heat treatment process of the present invention, the solution quenching temperature is 900-950°C, the total heating time is 3.0-4.0 min / mm, and after being taken out of the furnace, it is quickly water-cooled to ≤300°C and then air-cooled to room temperature to obtain the steel plate.

[0026] The present invention reduces the generation of internal inclusions in molten steel and improves the purity of molten steel by controlling the LF furnace refining and VOD furnace vacuum treatment processes; then, it continuously casts the molten steel into wide and thick slabs with excellent internal quality, and adopts appropriate rolling technology and solution heat treatment technology to make the steel plates have good comprehensive properties.

[0027] The beneficial effects of adopting the above technical solution are: The steel plate produced by the present invention has a uniform and dense internal structure, high strength, ultra-high ductility and ultra-low yield strength ratio. The steel plate thickness specification is 10 to 30 mm. It can be widely used in special steel materials that require stamping or complex structure, good processing ductility and low yield strength ratio. At the same time, it can replace imports and reduce the import price of similar domestic steel plates. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. The following embodiments are further illustrated. Example 1

[0029] The thickness specification of the steel plate in this embodiment is 15 mm, and its chemical composition and its weight percentage are C: 0.06%, Si: 0.32%, Mn: 0.63%, P: 0.012%, S: 0.002%, Ni: 33.54%, Cr: 24.65%, Co: 0.28%, Nb: 0.065%, Ti: 0.015%, V: 0.037%, Al: 0.060%, B: 0.0035%, and the rest are Fe and other inevitable impurities.

[0030] The production method of the steel plate in this embodiment includes steelmaking, continuous casting, hot rolling, and heat treatment processes. The specific production process steps are as follows: (1) Steelmaking process: The qualified molten steel from the primary furnace is transferred to the LF furnace for refining. During the LF furnace refining process, argon is blown well and the white slag is kept for 27 minutes. After deoxidation, alloys are added in batches to meet the composition requirements. The qualified molten steel from the LF refining is hoisted into the VOD furnace for oxygen blowing and carbon removal to meet the composition requirements, and then vacuum degassing is carried out. The vacuum is maintained for 24 minutes. After the vacuum degassing treatment, soft blowing is carried out for 10 minutes to promote the floating and removal of non-metallic inclusions.

[0031] (2) Continuous casting process: The qualified molten steel is cast into 240mm thick continuous casting billets through a large slab continuous casting machine. During the casting process, the molten steel is superheated to 25°C to ensure that the billets have good internal quality.

[0032] (3) Heating and rolling process: The continuous casting billet is heated in a continuous furnace with a maximum heating temperature of 1200°C, a holding stage temperature of 1180°C, a total heating time of 360 min, a starting rolling temperature of 1040°C, a reduction rate of 13% per pass, a final rolling temperature of 950°C, and air cooling to room temperature after rolling.

[0033] (4) Heat treatment process: solution quenching temperature is 940°C, total heating time is 50 min, after being taken out of the furnace, it is quickly water-cooled to 260°C, and then air-cooled to room temperature to obtain the steel plate.

[0034] The steel plate of this embodiment has a yield strength of 312 MPa, a tensile strength of 721 MPa, an elongation of 0.47, a yield strength ratio of 0.43, and excellent overall performance throughout the entire plate thickness. Example 2

[0035] The thickness specification of the steel plate in this embodiment is 10 mm, and its chemical composition and its weight percentage are C: 0.05%, Si: 0.20%, Mn: 0.70%, P: 0.013%, S: 0.003%, Ni: 20.00%, Cr: 23.36%, Co: 0.20%, Nb: 0.050%, Ti: 0.010%, V: 0.020%, Al: 0.057%, B: 0.0020%, and the rest are Fe and other inevitable impurities.

[0036] The production method of the steel plate in this embodiment includes steelmaking, continuous casting, hot rolling, and heat treatment processes. The specific production process steps are as follows: (1) Steelmaking process: The qualified molten steel from the primary furnace is transferred to the LF furnace for refining. During the LF furnace refining process, argon is blown well and the white slag is kept for 20 minutes. After deoxidation, alloys are added in batches to meet the composition requirements. The qualified molten steel from the LF refining is hoisted into the VOD furnace for oxygen blowing and carbon removal to meet the composition requirements, and then vacuum degassing is carried out. The vacuum is maintained for 20 minutes. After the vacuum degassing treatment is completed, soft blowing is carried out for 8 minutes to promote the floating and removal of non-metallic inclusions.

[0037] (2) Continuous casting process: The qualified molten steel is cast into 200mm thick continuous casting billets through a large slab continuous casting machine. During the casting process, the molten steel is superheated by 15°C to ensure that the billet has good internal quality.

[0038] (3) Heating and rolling process: The continuous casting billet is heated in a continuous furnace with a maximum heating temperature of 1200°C, a holding stage temperature of 1160°C, a total heating time of 320 min, a starting rolling temperature of 1000°C, a reduction rate of 11% per pass, a final rolling temperature of 850°C, and air cooling to room temperature after rolling.

[0039] (4) Heat treatment process: solution quenching temperature is 900°C, total heating time is 35 min, after being taken out of the furnace, it is quickly water-cooled to 300°C, and then air-cooled to room temperature to obtain the steel plate.

[0040] The steel plate of this embodiment has a yield strength of 322 MPa throughout the entire plate thickness, a tensile strength of 700 MPa, an elongation of 0.40, a yield strength ratio of 0.46, and excellent overall performance. Example 3

[0041] The thickness of the steel plate in this embodiment is 30 mm, and its chemical composition and its weight percentage are C: 0.03%, Si: 0.25%, Mn: 0.48%, P: 0.015%, S: 0.002%, Ni: 23.41%, Cr: 22.12%, Co: 0.45%, Nb: 0.084%, Ti: 0.019%, V: 0.032%, Al: 0.045%, B: 0.0027%, and the rest are Fe and other unavoidable impurities.

[0042] The production method of the steel plate in this embodiment includes steelmaking, continuous casting, hot rolling, and heat treatment processes. The specific production process steps are as follows: (1) Steelmaking process: Qualified molten steel from the primary furnace is transferred to the LF furnace for refining. During the LF furnace refining process, argon is blown well and the white slag is kept for 30 minutes. After deoxidation, alloys are added in batches to meet the composition requirements. The qualified molten steel from LF refining is hoisted into the VOD furnace for oxygen blowing and carbon removal to meet the composition requirements, and then vacuum degassing is performed. The vacuum is maintained for 23 minutes. After the vacuum degassing treatment is completed, soft blowing is performed for 11 minutes to promote the floating and removal of non-metallic inclusions.

[0043] (2) Continuous casting process: The qualified molten steel is cast into 230mm thick continuous casting billets through a large slab continuous casting machine. During the casting process, the molten steel is superheated to 23°C to ensure that the billets have good internal quality.

[0044] (3) Heating and rolling process: The continuous casting billet is heated in a continuous furnace with a maximum heating temperature of 1200°C, a holding stage temperature of 1162°C, a total heating time of 350 min, a starting rolling temperature of 1060°C, a reduction rate of 15% per pass, a final rolling temperature of 910°C, and air cooling to room temperature after rolling.

[0045] (4) Heat treatment process: solution quenching temperature is 950°C, total heating time is 120 min, after being taken out of the furnace, it is quickly water-cooled to 290°C, and then air-cooled to room temperature to obtain the steel plate.

[0046] The steel plate of this embodiment has a yield strength of 330 MPa throughout the entire plate thickness, a tensile strength of 718 MPa, an elongation of 0.42, a yield strength ratio of 0.45, and excellent overall performance. Example 4

[0047] The thickness specification of the steel plate in this embodiment is 20 mm, and its chemical composition and its weight percentage are C: 0.05%, Si: 0.45%, Mn: 0.52%, P: 0.014%, S: 0.003%, Ni: 35.00%, Cr: 26.27%, Co: 0.32%, Nb: 0.075%, Ti: 0.030%, V: 0.027%, Al: 0.037%, B: 0.0029%, and the rest are Fe and other unavoidable impurities.

[0048] The production method of the steel plate in this embodiment includes steelmaking, continuous casting, hot rolling, and heat treatment processes. The specific production process steps are as follows: (1) Steelmaking process: Qualified molten steel from the primary furnace is transferred to the LF furnace for refining. During the LF furnace refining process, argon is blown well and the white slag is kept for 25 minutes. After deoxidation, alloys are added in batches to meet the composition requirements. The qualified molten steel from the LF refining is hoisted into the VOD furnace for oxygen blowing and carbon removal to meet the composition requirements, and then vacuum degassing is performed. The vacuum is maintained for 22 minutes. After the vacuum degassing treatment is completed, soft blowing is performed for 12 minutes to promote the floating and removal of non-metallic inclusions.

[0049] (2) Continuous casting process: The qualified molten steel is cast into 220mm thick continuous casting billets through a large slab continuous casting machine. During the casting process, the molten steel is superheated to 19°C to ensure that the billets have good internal quality.

[0050] (3) Heating and rolling process: The continuous casting billet is heated in a continuous furnace with a maximum heating temperature of 1200°C, a holding stage temperature of 1176°C, a total heating time of 360 min, a starting rolling temperature of 1050°C, a reduction rate of 10% per pass, a final rolling temperature of 890°C, and air cooling to room temperature after rolling.

[0051] (4) Heat treatment process: solution quenching temperature is 930°C, total heating time is 70 min, after being taken out of the furnace, it is quickly water-cooled to 280°C, and then air-cooled to room temperature to obtain the steel plate.

[0052] The steel plate of this embodiment has a yield strength of 327 MPa, a tensile strength of 723 MPa, an elongation of 0.45, and a yield strength ratio of 0.45 throughout the entire plate thickness, and has excellent overall performance. Example 5

[0053] The thickness specification of the steel plate in this embodiment is 25 mm, and its chemical composition and its weight percentage are C: 0.04%, Si: 0.50%, Mn: 0.43%, P: 0.010%, S: 0.001%, Ni: 25.62%, Cr: 20.00%, Co: 0.50%, Nb: 0.091%, Ti: 0.023%, V: 0.050%, Al: 0.030%, B: 0.0031%, and the rest are Fe and other inevitable impurities.

[0054] The production method of the steel plate in this embodiment includes steelmaking, continuous casting, hot rolling, and heat treatment processes. The specific production process steps are as follows: (1) Steelmaking process: Qualified molten steel from the primary furnace is transferred to the LF furnace for refining. During the LF furnace refining process, argon is blown well and the white slag is kept for 31 minutes. After deoxidation, alloys are added in batches to meet the composition requirements. The qualified molten steel from the LF refining is hoisted into the VOD furnace for oxygen blowing and carbon removal to meet the composition requirements, and then vacuum degassing is performed. The vacuum is maintained for 21 minutes. After the vacuum degassing treatment is completed, soft blowing is performed for 10 minutes to promote the floating and removal of non-metallic inclusions.

[0055] (2) Continuous casting process: The qualified molten steel is cast into 250mm thick continuous casting billets through a large slab continuous casting machine. During the casting process, the molten steel is superheated to 21°C to ensure that the billets have good internal quality.

[0056] (3) Heating and rolling process: The continuous casting billet is heated in a continuous furnace with a maximum heating temperature of 1200°C, a holding stage temperature of 1173°C, a total heating time of 380 min, a starting rolling temperature of 1080°C, a reduction rate of 12% per pass, a final rolling temperature of 880°C, and air cooling to room temperature after rolling.

[0057] (4) Heat treatment process: solution quenching temperature is 920°C, total heating time is 75 min, after being taken out of the furnace, it is quickly water-cooled to 270°C, and then air-cooled to room temperature to obtain the steel plate.

[0058] The steel plate of this embodiment has a yield strength of 318 MPa, a tensile strength of 728 MPa, an elongation of 0.44, a yield strength ratio of 0.43, and excellent overall performance throughout the entire plate thickness. Example 6

[0059] The thickness specification of the steel plate in this embodiment is 12 mm, and its chemical composition and its weight percentage are C: 0.07%, Si: 0.38%, Mn: 0.57%, P: 0.011%, S: 0.002%, Ni: 28.74%, Cr: 27.00%, Co: 0.37%, Nb: 0.057%, Ti: 0.028%, V: 0.046%, Al: 0.041%, B: 0.0024%, and the rest are Fe and other unavoidable impurities.

[0060] The production method of the steel plate in this embodiment includes steelmaking, continuous casting, hot rolling, and heat treatment processes. The specific production process steps are as follows: (1) Steelmaking process: The qualified molten steel from the primary furnace is transferred to the LF furnace for refining. During the LF furnace refining process, argon is blown well and the white slag is kept for 33 minutes. After deoxidation, alloys are added in batches to meet the composition requirements. The qualified molten steel from the LF refining is hoisted into the VOD furnace for oxygen blowing and carbon removal to meet the composition requirements, and then vacuum degassing is carried out. The vacuum is maintained for 25 minutes. After the vacuum degassing treatment is completed, soft blowing is carried out for 9 minutes to promote the floating and removal of non-metallic inclusions.

[0061] (2) Continuous casting process: The qualified molten steel is cast into a 210 mm thick continuous casting billet through a large slab continuous casting machine. During the casting process, the molten steel is superheated to 17°C to ensure that the billet has good internal quality.

[0062] (3) Heating and rolling process: The continuous casting billet is heated in a continuous furnace with a maximum heating temperature of 1200°C, a holding stage temperature of 1168°C, a total heating time of 320 min, a starting rolling temperature of 1100°C, a reduction rate of 14% per pass, a final rolling temperature of 930°C, and air cooling to room temperature after rolling.

[0063] (4) Heat treatment process: solution quenching temperature is 910°C, total heating time is 40 min, after being taken out of the furnace, it is quickly water-cooled to 240°C, and then air-cooled to room temperature to obtain the steel plate.

[0064] The steel plate of this embodiment has a yield strength of 325 MPa, a tensile strength of 733 MPa, an elongation of 0.46, a yield strength ratio of 0.44, and excellent overall performance throughout the entire plate thickness.

[0065] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A steel plate with ultra-low yield strength ratio and excellent ductility, characterized in that: The chemical composition of the steel plate and its weight percentage are: C: 0.03-0.07%, Si: 0.20-0.50%, Mn: 0.40-0.70%, P≤0.015%, S≤0.003%, Ni: 20.00-35.00%, Cr: 20.00-27.00%, Co: 0.20-0.50%, Nb: 0.050-0.100%, Ti: 0.010-0.030%, V: 0.020-0.050%, Al: 0.030-0.060%, B: 0.0020-0.0035%, and the rest are Fe and other inevitable impurities.

2. The steel plate with ultra-low yield ratio and excellent ductility according to claim 1, characterized in that: The steel plate has a thickness of 10 to 30 mm.

3. The steel plate with ultra-low yield ratio and excellent ductility according to claim 1, characterized in that: The steel plate has a yield strength of ≥300MPa, a tensile strength of ≥700MPa, a yield strength ratio of ≤0.50, and an elongation of ≥0.40%.

4. A method for producing a steel plate with ultra-low yield ratio and excellent ductility according to any one of claims 1 to 3, comprising steelmaking, continuous casting, hot rolling and heat treatment steps.

5. The method for producing a steel plate with ultra-low yield ratio and excellent ductility according to claim 4, characterized in that: In the steelmaking process, qualified molten steel smelted in the primary refining furnace is transferred to the LF furnace for refining. During the LF furnace refining process, argon blowing is ensured to be good throughout the whole process, and the white slag is kept for ≥20 minutes. After deoxidation, various alloys are added in batches to the required range; the qualified molten steel refined in the LF is hoisted into the VOD furnace for oxygen blowing and carbon removal to the component requirements, and then vacuum degassing treatment is carried out, and the vacuum holding time is ≥20 minutes. After the vacuum degassing treatment is completed, soft blowing is carried out for 8-12 minutes to promote the floating and removal of non-metallic inclusions.

6. The method for producing a steel plate with ultra-low yield ratio and excellent ductility according to claim 4, characterized in that: In the continuous casting process, qualified molten steel is cast into wide and thick slabs with a thickness of 200 to 250 mm through a large thick slab continuous casting machine, and the superheat is controlled within the range of 15 to 25° C. during the casting process.

7. The method for producing a steel plate with ultra-low yield ratio and excellent ductility according to claim 4, characterized in that: The heating rolling process heats the slab in a continuous furnace, with a maximum heating temperature of 1200°C, a temperature of 1160-1180°C in the insulation stage, a total heating time of ≥15 min / cm, a starting rolling temperature of 1000-1100°C during rolling, a reduction rate of 10-15% per pass, a final rolling temperature of 850-950°C, and air cooling to room temperature after rolling.

8. The method for producing a steel plate with ultra-low yield ratio and excellent ductility according to claim 4, characterized in that: The heat treatment process has a solution quenching temperature of 900-950°C and a total heating time of 3.0-4.0 min / mm. After being taken out of the furnace, the steel plate is quickly water-cooled to ≤300°C and then air-cooled to room temperature to obtain the steel plate.