AU2020103572A4 - Ultra-fine grained high-strength steel plate with 1100 mpa-grade yield strength and production method thereof - Google Patents

Abstract

The present invention discloses an ultra-fine grain high-strength steel plate with yield strength of 1,1OOMPa, which relates to the field of iron and steel metallurgy. Its chemical components and weight percentage are as follows: C: 0.15-0.22%, Si: 0.10-0.30%, Mn: 0.80-1.60%, Cr: 0.20-0.70%, Mo: 0.10-0.60%, 5 Nb: 0.020-0.050%, V: 0.020-0.060%, Tis<0.008%, B: 0.0010-0.0030%, Al: 0.02-0.06%, P<0.005 %, Ss0.002%, Os0.0025%, Ns<0.0040%, H<0.00015%, with the balance being Fe and unavoidable impurities and with carbon equivalent Ceq=C+Mn/6+(Cr+Mo+V)/ 5+Ni/15s0.55%`. The present invention is featured in economical components and simple process. The grain refinement effect is used o to make the steel plate have excellent comprehensive mechanical properties and produce good economic and social benefits.

Description

ULTRA-FINE GRAINED HIGH-STRENGTH STEEL PLATE WITH 1100 MPA-GRADE YIELD STRENGTH AND PRODUCTION METHOD THEREOF

Technical Field The present invention relates to the field of iron and steel metallurgy, in particular to an ultra-fine grain high-strength steel plate with yield strength of 1,1OOMPa and a manufacturing method thereof. Background Art In recent years, due to the constraints of environmental protection, energy and cost issues, the construction machinery and mining machinery industries have been developing towards large-scale and lightweight trend, which requires construction machinery and equipment can reduce its own weight without reducing or even increasing its load capacity. Therefore, it is necessary to apply more and higher level structural steel plates. High-strength structural steel plates are mainly used to manufacture structural parts such as crane booms, construction machinery beams and dump truck bodies, etc. The total annual output of cranes in China is about 10 million tons, and the weight of the boom accounts for 15-20% of the total weight of the crane, and its annual consumption of structural steel plates is about 1.5 million tons. Previously, domestic steel plants mainly produced steel plates with yield strength below 960 MPa, and higher-level structural steel plates mainly relied on import. High-strength structural steel plates have very strict requirements on steel quality due to their harsh use environment and stress conditions. The production difficulties are as follows: (1) Ultra-high strength and good ductility and toughness at the same time. Normally, the strength of steel plates increases while the plasticity and toughness decreases. How to increase the strength and plasticity and toughness at the same time is the key to manufacturing high-strength structural steel. (2) How to control the shape of thin steel plates. During quenching, due to the superposition of structural stress and thermal stress, it is easy to produce waves at the edge and core of the steel plates so that it cannot meet the requirements of the plate shape. (3) How to increase the effective hardening depth of thick steel plates. The thicker the steel plates, the greater the difference between the quenching effects on the surface and in the core, which is likely to cause uneven performance in the thickness direction of the steel plates. (4) How to improve the welding performance of steel plates. Structural parts are usually formed by bending and welding. Under normal circumstances, improving the strength by increasing the content of C and alloying elements will significantly deteriorate the welding performance. (5) How to improve the cold bending performance of steel plates. Under normal circumstances, the higher the strength of the steel plates, the worse the plastic deformation ability, and it is difficult to meet the bending requirements of structural parts. (6) How to realize lower production cost. The strength and plasticity can be improved by adding a large amount of alloying elements, but the excessively high production cost is unacceptable for enterprises. Therefore, it is necessary to realize the toughness requirements of steel plates from the structure design and process control. At present, there is no domestic enterprise that can supply high-strength structural steel plates with yield strength of 1,100MPa in bulk, mainly relying on import. The Chinese patent CN102747303B introduces a high-strength steel plate with yield strength of 1,100MPa and its manufacturing method. This invention improves the toughness of steel by adding 0.60%-2.00% of Ni. However, Ni is very expensive, which increases production cost. The Chinese patent CN104513936A discloses a quenched and tempered high-strength steel with yield strength of 1,100 MPa and its production method. It also adds 0.30-1.50% Ni, and adds a certain amount of Ti and Ca, and it reduces the harm of plasticity and toughness of N and S by controlling Ti/N and Ca/S, so as to realize the matching of the strength and toughness of high-strength steel, and the composition design is relatively complicated. The Chinese patent CN100372962C introduces an online quenching + tempering method to produce ultra-high-strength steel plates with yield strength of 1,100 MPa or more. The toughness is improved by adding 0.20-1.20% and 0-0.5%

Cu. In addition, the steel plates that are quenched online are not reheated for austenitization so that the internal stress of the steel plates is relatively large, and cracks are prone to occur during the subsequent cutting and cold bending, and thus there are certain limitations in practical applications. The Chinese patent CN106191673A introduces a steel plate with excellent cold bending performance and yield strength of greater than 1,100 MPa and its preparation method. The central defect of the steel plate is reduced by two straightening processes to control the shape of the steel plate and the dynamic soft reduction process. The process is complicated, with high requirements for the equipment, and it is not conductive to the promotion o and application of this technology. To sum up, at present the existing manufacturing technologies related to ultra-high-strength steel plates with yield strength of1,1OOMPa mainly include: (1) improving the toughness of ultra-high-strength steel by adding a large amount of precious metal Ni or Ni+Cu, which is less economical; (2) reducing N and S by adding Ti and Ca to control Ti/N and Ca/S so as to improve plasticity and toughness, which is featured in complicated composition design; (3) achieving ultra-high strength of a martensite through online quenching and low-temperature tempering; and (4) controlling the flatness of ultra-high-strength steel through two straightening processes, namely warm straightening and strong cold straightening, wherein the process load has high requirements on the equipment. Summary of the Invention In view of the above-mentioned technical problems, the present invention overcomes the shortcomings of the prior art, and provides an ultra-fine grain high-strength steel plate with yield strength of1,1OOMPa. Through simple chemical composition design, the content of alloying elements is reduced to facilitate the steel enterprises to realize easy-to-implement process. It has ultra-high strength and good low-temperature toughness and plastic deformation ability. In order to solve the above-mentioned technical problems, the present invention provides an ultra-fine grain high-strength steel plate with yield strength of 1,1OOMPa, wherein its chemical components and weight percentage are as follows: C: 0.15-0.22%, Si: 0.10-0.30%, Mn: 0.80-1.60%, Cr: 0.20-0.70%, Mo: 0.10-0.60%, Nb: 0.020-0.050%, V: 0.020-0.060%, Ti0.008%, B: 0.0010-0.0030%, Al: 0.02-0.06%, P<0.005 %, S<0.002%, 0<0.0025%, Ns0.0040%, H<0.00015%, with the balance being Fe and unavoidable impurities and with carbon equivalent Ceq=C+Mn/6+(Cr+Mo+V)/ 5+Ni/15s0.55%0.

Technical effects: The present invention reduces a large amount of alloying elements, does not add precious metal elements such as Ni and Cu, which saves production cost. It reduces the carbon equivalent Ceq to less than 0.55%, which o improves the welding performance of the steel plate. The steel plate has yield strength of >1OOMPa, with good toughness and plastic deformation ability, especially with very good low-temperature toughness in the condition of not adding Ni.

The technical solution further limited by the present invention is as follows:

Further, the thickness of the steel plate is 5-30 mm.

Another object of the present invention is to provide a method for producing an ultra-fine grain high-strength steel plate with yield strength of1,1OOMPa, which comprises the following steps:

smelting: smelting with an electric furnace or a converter according to the above-mentioned chemical components, refining with an LF, and finally performing vacuum degassing treatment through a VD or RH furnace;

continuous casting: the designed liquidus temperature is 1,510°C, the pouring temperature is 1,515-1,530°C, and a casting billet is placed into a heat preservation pit or pile with the heat preservation cover for more than 24 hours after off-line for hydrogen expansion treatment;

heating: heating the casting billet to 1,150-1,200°C, and keeping the temperature for 1-1.5 min/mm when the temperature of the core part of the casting billet reaches; rolling: a two-stage controlled rolling process is adopted, wherein first, the casting billet is discharged from the furnace and then dephosphorized by high-pressure water before entering the recrystallization zone for rough rolling; the initial rough rolling temperature is 1,100-1,150°C, and the third percentage pass reduction after rough rolling is>20%, the thickness of the rough-rolled steel plate to be heated is >2.2 H, and H is the final rolled thickness of the steel plate; the finish rolling temperature is 850-950°C, the total reduction in the finishing stage is >70%, the percentage pass reduction is >15%, and the reduction is increased in the non-recrystallization zone; cooling: after rolling, the steel plate is cooled to 600°C at a cooling rate above 30°C/s, and then is air-cooled to room temperature; quenching: the initial austenite transformation temperature Ac3 of the steel is 814 0C, and it is rapidly heated to 840-860 0 C at a heating rate above 50°C/min; after the furnace temperature reaches, keeping the temperature for 1-1.5 min/mm, and then quickly water cooling to room temperature by using a quenching machine heat treatment device; and tempering: heating the quenched steel plate to 200-240 0 C, keeping the temperature for 2-3 min/mm after the furnace temperature reaches, and air cooling to room temperature.

In the method for producing an ultra-fine grain high-strength steel plate with yield strength of 1,1OOMPa, argon protection pouring is applied throughout the continuous casting process.

In the method for producing an ultra-fine grain high-strength steel plate with yield strength of 1,1OOMPa, electromagnetic stirring is applied during the continuous casting, and the electromagnetic stirring parameters are 320 A and 6 Hz.

Technical effects: The above-mentioned process provided by the present invention develops an ultra-fine grain high-strength steel plate with yield strength of greater than 1,1OOMPa and with good plasticity and low-temperature toughness by rationally designing chemical components and combining rolling, cooling process and heat treatment process control measures. Refining grains is the only strengthening method that can improve the strength, plasticity and toughness. The present invention achieves the toughness requirements of the steel plate by refining grains, and it reduces the amount of alloying elements such as Ni, Mn, and Cr added, which reduces the carbon equivalent of the steel plate and improves the welding performance on the one hand, and on the other hand the cost is saved and the competitiveness of the product is improved.

Compared with the prior art, the beneficial effects of the present invention are as follows:

(1) In the present invention, the two-stage control rolling and ultra-fast cooling measures after rolling are used to refine the precipitated particles of (Nb, V) C to less than 10 mm, making it dispersed and precipitate, which significantly improves its pinning grain boundary and prevents the growth of grains;

(2) In the present invention, by increasing the rolling deformation in the non-recrystallization zone, the austenite grain boundary area is increased and the width of the deformed austenite is reduced, thereby increasing the resistance to the lateral growth of grains in the nucleation position of the reheated austenite and the austenite, which plays a role in refining austenite grains.

(3) In the present invention, the rapid heating process is used to heat the steel plate to the quenching temperature, and the quenching temperature is reduced to 840-860°C according to the initial austenite transformation temperature to avoid the growth of austenite grains, so as to obtain austenite grains with an average grain size of less than 10 pm;

(4) In the present invention, 200-240°C low temperature tempering is used to eliminate the internal stress of quenching and a lath martensite with a high dislocation density is retained so as to achieve good strength and toughness;

(5) In the present invention, the microstructure of the ultra-fine grain high-strength steel plate is a tempered lath martensite with a high dislocation density, the original austenite grain size is <10 pm, and the microscopic grain size level reaches 11. The mechanical properties of the steel plate meet the following: tensile strength: 1,250 MPa, elongation: >12%, low-temperature Charpy impact energy at -40°C: >60J, unevenness: <3 mm/m, with good cold bending and welding performance.

Brief Description of the Attached Drawings

Fig. 1 is a topography photo of the optical microstructure of a steel plate in o Embodiment 1 (500x);

Fig. 2 is a topography photo of the scanning electron microscope microstructure of a steel plate in Embodiment 2 (500x);

Fig. 3 is the topography and size of precipitated particles after ultra-fast cooling of the steel plate in Embodiment 3 after rolling;

Fig. 4 is a photo of the microscopic grain size of a steel plate in Embodiment 3 (200x); Fig. 5 is a photo of the microscopic grain size of a steel plate in Embodiment 4 (200x). Embodiments An ultra-fine grain high-strength steel plate with yield strength of1,100MPa provided by the present embodiment, wherein its chemical components and weight percentage are as follows: C: 0.15-0.22%, Si: 0.10-0.30%, Mn: 0.80-1.60%, Cr: 0.20-0.70%, Mo: 0.10-0.60%, Nb: 0.020-0.050%, V: 0.020-0.060%, Tis<0.008%, B: 0.0010-0.0030%, Al: 0.02-0.06%, P<0.005 %, S<0.002%, <O0.0025%, Ns0.0040%, Hs0.00015%, with the balance being Fe and unavoidable impurities and with carbon equivalent Ceq=C+Mn/6+(Cr+Mo+V)/ 5+Ni/15s0.55%.

The design principles of chemical components in the present invention are as follows:

C: The content of C directly determines the strength level and welding performance of the steel plate. C is dissolved in solid in the octahedral or tetrahedral gaps of the ferrite lattice, causing serious lattice distortion and interacting with dislocations to strongly hinder the movement of the dislocations, thereby increasing the strength. However, if the content of C is too high, it will result in greater degree of lattice distortion and increase in shear resistance, which significantly reduces the plasticity and toughness of the steel. In addition, during welding, the heat-affected zone is prone to cold cracks, which reduces the performance of welded joints. Therefore, in the present invention, the content of C is controlled to 0.15-0.22%.

Si: In steel, it mainly replaced Fe atoms in the ferrite crystal lattice by substitution, which plays a role of solid solution strengthening. In addition, Si can reduce the diffusion ability of C atoms and prevent the formation of carbides during tempering, thereby improving the tempering resistance of steel. However, when the content of Si is too high, the surface quality of the steel plate will be significantly deteriorated. Thus, the content of Si is controlled to 0.10-0.30%.

Mn: Mn can inhibit the diffusion-type phase transformation process, improve the hardenability of steel, and reduce the harm of S in the steel. In addition, it can also play a solid solution strengthening effect. However, when the content of Mn is too high, it is easy to cause center segregation of the casting billet, deteriorate the internal structure of steel plates, and reduce the toughness and welding performance of the steel plate. Therefore, the content of Mn is controlled to 0.80-1.60%.

Cr: It is a solid solution strengthening element, which can inhibit the formation of polygonal ferrite and pearlite, and promote the transformation of bainite and martensite, thereby increasing the strength. However, when the content of Cr is too high, Cr carbides are easily formed, which reduces the toughness of the steel plate and is not conducive to the weldability of the steel plate. Therefore, the content of Cr is controlled to 0.20-0.70%.

Ni: It is an element that improves hardenability and can directly improve the toughness of steel. It is a precious metal and is expensive. This element is not added in the present invention so as to reduce production cost and improve the competitiveness of the steel.

Mo: It is an element that improves hardenability and promotes the formation of a martensite during quenching. In addition, Mo can also play a role in refining grains, which is beneficial to obtaining a refined martensite structure. However, excessively high content of Mo will deteriorate the welding performance of steel. Therefore, in the present invention, the content of Mo is controlled to 0.10-0.45% in order to obtain the matching of steel plate strength and toughness and welding performance.

Nb: In the present invention, the role of Nb is very important. The elemental atoms of Nb and the carbides of Nb can significantly pin the austenite grain boundaries and prevent the growth of grains. In addition, the solute drag effect of Nb can increase the recrystallization temperature of austenite, which is beneficial to realizing the rolling of austenite in the non-recrystallization zone, increasing the dislocation density and providing condition for obtaining refined reheated austenite grains.

V: V is a strong carbide forming element. The nano-level V (C, N) dispersed in the steel matrix can play a role of precipitation strengthening and improve the strength of the steel plate. In addition, during heating, it can pin the grain boundaries and prevent the growth of austenite grains so as to play a role in grain refinement. However, when the content of V is too high, the size of the precipitated particles becomes larger, which is detrimental to the toughness. Therefore, the content of V is controlled to 0.020-0.060%.

Ti: Ti has a strong affinity with C and N. Ti and N tend to form coarse TiN during the solidification of molten steel. TiN is hard and brittle, and is not easy to deform, resulting in the inability to coordinate deformation with the matrix during rolling deformation so as to result in microcracks and significantly reduce the toughness and cold bending performance of steel. Thus, the element Ti is not allowed to be added in the present invention.

B: It is an element which is the most effective to improve the hardenability and promote to obtain a refined martensite structure during the cooling of the steel plate, thereby increasing the strength. However, when the content of B is too high, it will be enriched at the grain boundaries, which reduces the binding energy of the grain boundaries, thereby reducing the toughness of steel plates. Thus, the content of B is controlled to 0.0010-0.0030%.

Al: On the one hand, Al plays a deoxidizing effect in steel and purifies the molten steel; and on the other hand, it is also used to fix N in steel to avoid the o formation of coarse TiN to deteriorate the toughness of steel. It can also protect B, and avoid the precipitation of BN in the grain boundaries so as to ensure the role of B to improve the hardenability. In addition, the fine AlN can also inhibit the growth of austenite grains during the subsequent cooling and play a role in refining the grains, and thus the content of Al is controlled to 0.02-0.06%.

N: It is a harmful gas element and can form hard and brittle TiN with Ti, which significantly deteriorates the toughness of steel. It can also form BN with B and enrich at the grain boundaries so as to reduce the cohesive energy of the grain boundaries. It can also form AlN with Al, which can refine the austenite grains and play the role of fine grain strengthening. Thus, the content of N is controlled to be <0.0040%

P: Segregation of P at the grain boundaries will reduce the binding energy of the grain boundaries. Under the action of external impact force, it is prone to fracture along the grain, which is the main reason for the first type of temper brittleness. When P and Mn coexist, it will aggravate the temper brittleness of the steel and significantly deteriorate the toughness of the steel plate. In addition, P will deteriorate the welding performance of steel, and thus the content of P is strictly controlled to be <0.005%.

S: During the solidification of molten steel, S will segregate and form sulfide inclusions to reduce the low-temperature toughness and cold bending performance of steel plates. During welding, the formation of sulfides is also prone to thermal cracks, and the SO 2 gas generated by oxidation of S is prone to pores in the weld metal, which reduces the performance of the welded joints. Therefore, the content of S is strictly controlled to be <0.002%.

0: It is a harmful gas element, with high content and many inclusions, reducing the shape, toughness and cold bending performance of steel plates, and thus the content of 0 is strictly controlled to be <0.0025%.

H: It is a harmful gas element. H atoms tend to accumulate in dislocations in o steel, causing local H partial pressure to be too high, forming micro-cracks and thus resulting in H brittleness and reducing the plasticity and toughness of steel plates, which seriously harms the performance of steel plates. In addition, too high content of H is the main reason for the formation of delayed cutting cracks in ultra-high-strength steel plates, and thus the content of H is strictly controlled to be <0.00015%.

The production process of the present invention is as follows: steelmaking in a converter or an electric furnace - refining in an LF--vacuum degassing treatment in a VD or RH furnace->continuous casting->casting billet hydrogen expansion treatment --heating--rolling--cooling--quenching--tempering. The core of the present invention is to realize the strong toughness and plastic deformation requirements of ultra-high-strength steel through ultra-refined grains. Therefore, there are measures to refine grains in each stage of rolling and heat treatment. The specific process flow is as follows:

smelting: smelting with a 150-ton converter or electric furnace according to the above-mentioned chemical components, refining with an LF, and finally performing vacuum degassing treatment through a VD or RH furnace;

continuous casting: casting the molten steel into a 150-220 mm casting billet, wherein calculated according to the chemical components, the liquidus temperature of the steel to be 1,5100 C, the pouring temperature is the liquidus temperature plus 5-20 0C, namely 1,515-1,530°C; the casting speed is controlled to 1-1.35 m/min; in order to prevent the molten steel from oxidizing, argon protection pouring is performed in the whole process; in order to reduce the looseness and segregation of the center of the casting billet, electromagnetic stirring is performed during continuous casting, wherein the electromagnetic stirring parameters are 320 A and 6 Hz; a casting billet is placed into a heat preservation pit or pile with the heat preservation cover for more than 24 hours after off-line for hydrogen expansion treatment; heating: putting the continuous casting billet into a walking-type heating furnace and heating to 1,150-1,200 0C, and keeping the temperature for 1-1.5 min/mm when the temperature of the core part of the casting billet reaches so as to homogenize the chemical composition in the austenite; rolling: a two-stage controlled rolling process is adopted, wherein first, the casting billet is discharged from the furnace and then dephosphorized by high-pressure water before entering the recrystallization zone for rough rolling; the initial rough rolling temperature is 1,100-1,150 0C, and the third percentage pass reduction after rough rolling is >20% so that the austenite is fully recrystallized and refined to avoid abnormally coarse grains; the thickness of the rough-rolled steel plate to be heated is >2.2 H, and H is the final rolled thickness of the steel plate; the finish rolling temperature is 850-950°C, the total reduction in the finishing stage is >70%, the percentage pass reduction is >15%, and the reduction is increased in the non-recrystallization zone so that the austenite grains are elongated, the grain boundary area of the austenite is fully increased, and that the width of the austenite grains is reduced; cooling: performing on-line ultra-fast cooling on the rolled steel plate, cooling to 600 0C at a cooling rate of 30°C/s or more, then air cooling to room temperature, quickly passing through the ferrite transformation zone, and suppressing the precipitation of (Nb, V) C in the high-temperature ferrite so as to make it precipitate in a uniform nucleation method during the subsequent cooling; at this time, the precipitation driving force is large, the critical nucleation size is small and the growth rate of precipitated particles is slow, and (Nb, V) C with a size of below 10 mm can be obtained, which can significantly pin and reheat the austenite boundary and prevent the growth of grains; quenching: according to the expansion method, the initial austenite transformation temperature Ac3 of the steel is 814°C, and in order to refine the reheated austenite grains, the rolled and cooled steel plate is rapidly heated to o 840-860°C at a heating rate above 50°C/min; after the furnace temperature reaches, keeping the temperature for 1-1.5 min/mm, and then quickly water cooling to room temperature by using a quenching machine heat treatment device; and tempering: heating the quenched steel plate to 200-240°C, keeping the temperature for 2-3 min/mm after the furnace temperature reaches, and air cooling to room temperature to eliminate the quenching internal stress of steel plates.

According to the chemical components as shown in Table 1, the ultra-fine grain high-strength steel plates in Embodiments 1 to 4 are manufactured. The heat-treated steel plate is subjected to transverse tensile and longitudinal impact tests. The specific components and process parameters in Embodiments 1to 4 are shown in Tables 1 to 3, and the properties of the steel plates prepared are shown in Table 4.

Table 1 Chemical Components (wt.%) of the Ultra-Fine Grain High-Strength Steel Plates in Embodiments 1 to 4

Embodiments C Si Mn P S Cr Mo Nb V B Al 0 N H Ceq 1 0.15 0.30 1.25 0.005 0.001 0.20 0.55 0.04 0.05 0.0020 0.055 0.0012 0.0035 0.0010 0.54

2 0.17 0.25 1.1 0.006 0.001 0.30 0.45 0.04 0.04 0.0018 0.050 0.0015 0.0038 0.0012 0.53

3 0.18 0.20 0.9 0.005 0.001 0.40 0.40 0.03 0.04 0.0016 0.055 0.0010 0.0036 0.0011 0.52 4 0.22 0.15 0.7 0.006 0.001 0.50 0.30 0.03 0.03 0.0015 0.052 0.0013 0.0035 0.0010 0.52

Table 2 Rolling and Cooling Process Control of the Ultra-Fine Grain High-Strength Steel Plates in Embodiments 1 to 4 Ultra-fast Initial rough Initial finish Cooling rate Heating Holding cooling final Embodiments Thickness/mm rolling rolling after temperature/°C time/min cooling temperature/°C temperature/°C rolling°C/s temperature/°C 1 6 1180 160 1130 950 30 610 2 8 1180 180 1130 950 30 600 3 20 1200 200 1150 910 30 580

4 30 1200 200 1150 900 30 570

Table 3 Heat Treatment Process Control of the Ultra-Fine Grain High-Strength Steel Plates in Embodiments 1 to 4

Heating Quenching holding Tempering Tempering holding Embodiments Thickness/mm Heating speed °C/s temperature/°C time/min temperature/°C time/°C

1 6 5 840 10 200 20

2 8 5 840 15 210 25

3 20 5 850 25 220 50

4 30 5 860 30 230 75

Table 4 Tensile and Impact Properties of the Ultra-Fine GrainH igh-Strength Steel Plates in Embodiments 1 to 4 Transverse tensile Tensile strength Extensibility/ Longitudinal impact/J Impact specimen Embodiments Thickness/mm Yield strength dimension/mm Mpa MPa 1 6 1187 1396 12.5 -40°C 42 46 45 5x10x55

2 8 1181 1347 12 -40°C 45 51 48 5x10x55

3 20 1183 1376 12.5 -40°C 78 72 75 10x10x55

4 30 1140 1423 12 -40°C 96 90 84 10x10x55

Note: When the thickness of the steel plate is 6 mm and 8 mm, the semi-impact specimens shall be adopted according to the national standard.

As shown in Fig. 1 and 2, the microstructure of the steel is a tempered lath martensite, as shown in Fig. 3 to 5, and the average grain size of the original austenite is less than 10 pm. The mechanical properties of the steel plate meet the following: tensile strength: 1,250 MPa, elongation: >12%, low-temperature Charpy impact energy at -40°C >60J, unevenness: <3 mm/m, with good cold bending and welding performance. From the point of view of ultra-fine structure, the present invention realizes the strength of the steel plate while ensuring the plasticity and toughness through a refined lath martensite structure by optimizing the composition design, controlling the rolling, cooling process and heat treatment process. The steel plate produced by the present invention has been successfully applied to the equipment of construction machinery enterprises in place of import, and has produced good economic and social benefits.

In addition to the above-described embodiments, the present invention may include other embodiments. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope of the present o invention.

Claims (5)

1. An ultra-fine grain high-strength steel plate with yield strength of 1,100MPa, characterized in that its chemical components and weight percentage are as follows: C: 0.15-0.22%, Si: 0.10-0.30%, Mn: 0.80-1.60%, Cr: 0.20-0.70%, Mo: 0.10-0.60%, Nb: 0.020-0.050%, V: 0.020-0.060%, Ti<0.008%, B: 0.0010-0.0030%, Al: 0.02-0.06%, P<0.005 %, S<0.002%, Os0.0025%, Ns0.0040%, H<0.00015%, with the balance being Fe and unavoidable impurities and with carbon equivalent Ceq=C+Mn/6+(Cr+Mo+V)/ 5+Ni/15s0.55%o.

2. The ultra-fine grain high-strength steel plate with yield strength of 1,1OOMPa according to claim 1, characterized in that the thickness of the steel plate is 5-30 mm.

3. A method for producing the ultra-fine grain high-strength steel plate with yield strength of 1,1OOMPa according to claims 1 to 2, characterized in that it comprises the following steps:

smelting: smelting with an electric furnace or a converter according to the above-mentioned chemical components, refining with an LF, and finally performing vacuum degassing treatment through a VD or RH furnace;

continuous casting: The designed liquidus temperature is 1,510°C, the pouring temperature is 1,515-1,530°C, and a casting billet is placed into a heat preservation pit or pile with the heat preservation cover for more than 24 hours after off-line for hydrogen expansion treatment;

heating: heating the casting billet to 1,150-1,200°C, and keeping the temperature for 1-1.5 min/mm when the temperature of the core part of the casting billet reaches;

rolling: a two-stage controlled rolling process is adopted, wherein first, the casting billet is discharged from the furnace and then dephosphorized by high-pressure water before entering the recrystallization zone for rough rolling; the initial rough rolling temperature is 1,100-1,150°C, and the third percentage pass reduction after rough rolling is >20%, the thickness of the rough-rolled steel plate to be heated is >2.2 H, and H is the final rolled thickness of the steel plate; the finish rolling temperature is 850-950°C, the total reduction in the finishing stage is >70%, the percentage pass reduction is >15%, and the reduction is increased in the non-recrystallization zone; cooling: after rolling, the steel plate is cooled to 600°C at a cooling rate above 30°C/s, and then is air-cooled to room temperature; quenching: The initial austenite transformation temperature Ac3 of the steel o is 814°C, and it is rapidly heated to 840-860°C at a heating rate above 50°C/min; after the furnace temperature reaches, keeping the temperature for 1-1.5 min/mm, and then quickly water cooling to room temperature by using a quenching machine heat treatment device; and tempering: heating the quenched steel plate to 200-240°C, keeping the temperature for 2-3 min/mm after the furnace temperature reaches, and air cooling to room temperature.

4. The method for producing an ultra-fine grain high-strength steel plate with yield strength of 1,1OOMPa according to claim 3, characterized in that argon protection pouring is applied throughout the continuous casting process.

5. The method for producing an ultra-fine grain high-strength steel plate with yield

strength of 1,1OOMPa according to claim 3, characterized in that electromagnetic

stirring is applied during the continuous casting, and the electromagnetic stirring

parameters are 320 A and 6 Hz.

Fig. 1

Fig. 2

Fig. 3

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