CN109385570B - High-strength steel plate and manufacturing method thereof - Google Patents

High-strength steel plate and manufacturing method thereof Download PDF

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CN109385570B
CN109385570B CN201811141661.0A CN201811141661A CN109385570B CN 109385570 B CN109385570 B CN 109385570B CN 201811141661 A CN201811141661 A CN 201811141661A CN 109385570 B CN109385570 B CN 109385570B
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steel plate
steel
temperature
strength
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CN109385570A (en
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宋凤明
温东辉
张国民
陆敏
杨阿娜
吴祖国
何晓明
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Baoshan Iron and Steel Co Ltd
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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226Hot rolling
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C33/00Making ferrous alloys
    • C22C33/04Making ferrous alloys by melting
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/002Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/06Ferrous alloys, e.g. steel alloys containing aluminium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/12Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/14Ferrous alloys, e.g. steel alloys containing titanium or zirconium

Abstract

A high-strength steel plate and a manufacturing method thereof are disclosed, wherein the high-strength steel plate comprises the following components in percentage by weight: c: 0.165-0.185%, Si: 0.02 to 0.08%, Mn: 1.15-1.25%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, Al: 0.025 to 0.05%, Mo: 0.1-0.3%, B: 0.001-0.005%, N is less than or equal to 0.004%, Nb: 0.01-0.02%, Ti: 0.01-0.025%, V: 0.045-0.055%, and the balance of Fe and inevitable impurity elements. The yield strength of the steel plate is more than or equal to 1100MPa, the tensile strength is more than or equal to 1300MPa, the elongation is more than or equal to 10 percent, the straightness meets the requirement of less than or equal to 3mm/m, and the cold bending meets the requirements of D being 6a and 90 degrees. The invention adopts simple and economic C-Mn component design, is assisted by a small amount of Mo and microalloy elements such as Nb, V, Ti, B and the like, and realizes high strength and high toughness of steel grades through microalloy fine grain strengthening and phase change strengthening; and the content of noble alloy elements is low, so that the production difficulty and the production cost are reduced.

Description

High-strength steel plate and manufacturing method thereof
Technical Field
The invention relates to the field of low alloy steel manufacturing, in particular to a high-strength steel plate with yield strength exceeding 1100MPa and a manufacturing method thereof, and particularly relates to a hot-rolled heat-treated high-strength steel plate with the thickness specification of 2.5-6mm for engineering machinery.
Background
High-strength steel is widely applied to engineering machinery such as cranes, concrete pump trucks, dump trucks and the like, and the strength of steel plates is also developed from original plain carbon steel to low-alloy steel of 960MPa grade. Along with the increase of the tonnage of equipment, the strength of the high-strength steel is gradually increased to the level of yield strength 1100MPa, even 1300MPa, and higher requirements are provided for the unevenness, welding performance and forming performance of the steel plate.
The high-strength steel plate can be obtained by adopting a controlled cooling mode and controlled rolling and offline heat treatment. The production of the high-strength steel plate by controlling the air cooling mode is limited by the equipment capacity (such as a coiler), the yield strength of the steel plate is usually below 800MPa, the specification is limited, and the problems of uneven performance and structure and poor plate shape exist at the same time, so that the normal use of a user is influenced; the steel plate with higher strength grade can be produced by adopting the controlled rolling and offline heat treatment process, particularly, the hardenability of the steel is improved by adding alloy elements, and a martensite structure with higher strength and hardness can be obtained. The simple martensite structure has a defect of high density between martensite lamellar layers, and is liable to become a crack source, thereby deteriorating impact toughness of the material. Therefore, it is necessary to adopt a specific component system and a production process to obtain a high-strength steel plate with higher strength, good plasticity and toughness.
In recent years, high-strength steel in the field of engineering machinery is disclosed domestically. For example, chinese patent publication No. CN1265709A, "weldable ultra-high strength steel with excellent ultra-low temperature toughness", and chinese patent publication No. CN101481779A, "high-plasticity high-toughness ultra-high strength steel plate and manufacturing method thereof", relate to the yield strength of steel plate is mostly below 1000MPa, even lower; although the steel type related to the Chinese patent publication No. CN1329178A 'delayed fracture-resistant high-strength steel' and the Chinese patent publication No. CN102534423 'high-strength steel plate and manufacturing method thereof' has high strength, the steel type has high alloy content such as C, Cr, Mo, Zr, Ni and the like, on one hand, the cost is high, and meanwhile, the high alloy reduces the weldability, thereby limiting the application.
Further, U.S. Pat. No. US2007095444 "a high-strength steel sheet excellent in formability and a method for producing the same" and japanese patent No. JP2012041611 "a method for producing a high-strength steel sheet stable in performance" refer to steel sheets having a low strength.
U.S. Pat. No. US2014162088 "high strength galvanized sheet with good formability and method for manufacturing the same" steel sheet strength is also only about 900MPa and contains a high C, Si content.
Chinese patent publication No. CN102747303A, CN102337480A discloses 'a high-strength steel plate with yield strength of 1100MPa grade and a manufacturing method thereof' and 'an ultrahigh-strength steel plate with excellent environmental brittleness resistance and fatigue performance and a manufacturing method thereof', the two patents relate to steel plates which are thick plate products, wherein the former contains up to 2.0% of Ni and has higher cost; the latter has Mn content of 2-6% and contains much Si and Cr, which is unfavorable for welding performance.
Chinese patent publication No. CN104498834A discloses 'a component of high-toughness ultrahigh-strength steel and a preparation process thereof', relating to high-strength steel with yield strength of 1600MPa, but containing higher Cr, Ni and W, and being a forged piece with poor toughness, which is not suitable for the field of engineering machinery.
On the component system, the patent steels with the yield strength exceeding 960MPa are designed by adopting higher C-Si-Mn, and simultaneously, more elements such as Cu, Cr, Ni, Mo and even Al are required to be added; higher strength patented steels require high C alloys such as up to 0.45% C and 2-4.5% Cr, Ni, W, etc. Although C has a phase change strengthening effect, excessive C inevitably causes the deterioration of welding performance, and more Si is also unfavorable for the surface, toughness and plasticity and welding performance; and the addition of more precious alloys increases the cost, and simultaneously, the welding and forming performances are inevitably deteriorated, and the using and processing difficulty is greatly improved.
Disclosure of Invention
The invention aims to provide a high-strength steel plate and a manufacturing method thereof, the steel plate has high strength and good toughness, plasticity and straightness, the yield strength is more than or equal to 1100MPa, the tensile strength is more than or equal to 1300MPa, the elongation is more than or equal to 10%, the straightness meets the requirement of less than or equal to 3mm/m, the cold bending meets the requirements of D6 a and 90 degrees, and the steel plate has good forming performance and is suitable for the field of engineering machinery.
In order to achieve the purpose, the technical scheme of the invention is as follows:
the invention adopts Nb, V and Ti microalloy reinforcement on the basis of proper C-Mn, limits the Si content to be not more than 0.08 percent, and simultaneously assists a small amount of alloying elements such as Mo, B and the like, and the manufacturing method realizes high strength and high toughness and low cost by heat treatment phase change reinforcement and microalloy fine grain reinforcement and precipitation reinforcement.
Specifically, the high-strength steel plate comprises the following components in percentage by weight: c: 0.165-0.185%, Si: 0.02 to 0.08%, Mn: 1.15-1.25%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, Al: 0.025 to 0.05%, Mo: 0.1-0.3%, B: 0.001-0.005%, N is less than or equal to 0.004%, Nb: 0.01-0.02%, Ti: 0.01-0.025%, V: 0.045-0.055%, and the balance of Fe and inevitable impurity elements.
In the composition design of the steel of the invention:
c is an element that does not necessarily need to be reduced in the transformation from austenite to martensite, bainite, or other strengthening phases, and can improve the hardenability of steel, so that the steel sheet forms a martensite structure having higher hardness after quenching. The content of C determines the strength grade of the steel plate to a great extent and also directly determines the carbon equivalent of the steel grade, and the carbon equivalent is a key index influencing the strength and the welding performance of the steel. A low C content does not allow sufficient strengthening phase to be formed to ensure strength, while too high a C content is detrimental to plasticity, weldability and toughness. The range is limited to 0.165-0.185% under the condition that the performance requirements are met.
Si is a deoxidizing element, and can raise the Ac3 temperature of the steel sheet and promote the formation of ferrite. Si substitutes Fe atoms in the steel in a replacement mode, so that dislocation movement is hindered, and solid solution strengthening is realized. However, Si has a strength-improving effect smaller than that of C, and increases the work hardening rate at cold working, thereby lowering the toughness and plasticity of steel to some extent. Meanwhile, the excessively high Si promotes the graphitization of C, which is unfavorable for toughness; while being detrimental to surface quality and weldability. Therefore, in the present invention, Si is used only as a deoxidizing element, and the content of Si is limited to 0.02 to 0.08%.
Mn is an element that promotes the formation of austenite. Mn solid-dissolved in austenite can inhibit diffusion type phase transformation (CCT curve is shifted to the right) in the quenching process, thereby improving hardenability and promoting the refinement of martensite structure. Meanwhile, Mn obviously reduces the phase transition temperature of the steel, refines the microstructure of the steel, is an important strengthening and toughening element, can inhibit the formation of reticular cementite and is beneficial to toughness. However, an excessive content of Mn causes segregation, deteriorates the matrix structure and forms large MnS inclusions, thereby deteriorating the weldability and the weld heat-affected zone toughness of the steel sheet. And the higher Mn reduces the Ms point, leads to increase of residual austenite, reduces the yield ratio of the steel plate and increases the temper brittleness of the steel plate. In addition, excessive Mn lowers the thermal conductivity of the steel, lowers the cooling rate, may cause coarse grains, and is disadvantageous in toughness and fatigue properties. So the control is between 1.15 and 1.25 percent.
P is a main corrosion resistant element in the traditional atmospheric corrosion resistant steel, and meanwhile, the P is easy to generate segregation at a crystal boundary, so that the crystal boundary bonding energy and the toughness and plasticity of the steel are reduced; meanwhile, the coexistence of P and Mn can aggravate the temper brittleness of the steel, and the deviated P ensures that the steel plate is easy to generate crystal fracture and reduces the impact toughness of the steel plate. And P is disadvantageous in welding performance. Therefore, in the present invention, P is treated as an impurity element to reduce the content of P in the steel as much as possible, and the content is required to be 0.015% or less in the present invention.
S is easy to form plastic inclusion manganese sulfide with Mn in the solidification process, and is unfavorable for transverse plasticity and toughness; s is easily oxidized to form SO during welding2Gas, leading to weld porosity and porosity defects. Further, S is also a main element which causes hot shortness in hot rolling, and therefore, the lower the S, the better. In the present invention, the content is controlled to be 0.005 or less.
Al is generally added as a deoxidizer in the steel making process. The trace Al forms fine AlN precipitation during steel making, has the function of refining austenite grains in the subsequent cooling process, and improves the toughness of steel. Therefore, it is also used as a N-fixing agent in steel. However, when AIN exists independently in steel as a non-metallic inclusion, continuity of a steel matrix is damaged, and particularly, when AIN with a high Al content is formed in a large amount and in an aggregated distribution, the AIN is more harmful and forms oxides with poor plasticity. And too high Al forms coarse alumina particles, increasing the ferrite brittleness in the steel, decreasing the toughness of the steel. Therefore, the content is controlled to be 0.. 025-0.05%.
B is concentrated in dislocations and defects in the steel, reduces grain boundary energy, inhibits ferrite transformation, and thus has good hardenability, thereby improving steel sheet hardness. The addition of B can replace the addition of noble alloy elements such as Mo and Ni, thereby reducing the cost. In addition, B can improve the low-temperature impact toughness of the steel plate after low-temperature tempering and reduce the ductile-brittle transition temperature. However, too high B causes a decrease in grain boundary strength, and is cleaved by intergranular fracture under stress to form "boron embrittlement", and too high B is not favorable for welding, and therefore, the respective ranges thereof are controlled to be 0.001 to 0.005%.
Nb is a strong nitrogen carbide forming element and can be combined with carbon and nitrogen in steel to form intermediate phases such as NbC, Nb (CN), NbN and the like, and formed fine carbide particles are subjected to nail-rolling of austenite grain boundaries in the austenitizing process, so that the abnormal growth of the austenite grain is inhibited, and the structure is refined. The method has the advantages that the Nb carbonitride is controlled to be separated out in austenite and low-temperature ferrite in the rolling process, and interphase precipitation occurs, so that the method has the functions of precipitation strengthening and precipitation strengthening, and the strength of the steel plate is obviously improved; meanwhile, the carbonitrides are beneficial to improving the toughness of the steel plate after quenching. Nb can also suppress the expansion of the austenite interface and increase the recrystallization temperature of the steel. If Nb is too high, coarse carbonitride particles are formed at the grain boundaries when the Nb content to be formed is high, and the impact toughness deteriorates. Meanwhile, Nb is used as a precious alloy element, and the cost is increased when the addition amount is too high. Therefore, the content is controlled within the range of 0.01 to 0.02 percent.
V is an element that reduces austenite and is the only element that can be precipitated both in the austenite-ferrite transformation process and in the ferrite. Reducing the diffusion rate of C in austenite increases hardenability. The carbonitride of V can be completely dissolved in austenite when the content of N is low, and interphase precipitation and ferrite precipitation strengthening are realized in the austenite-ferrite transformation process; at high N VN is less soluble in austenite and ferrite and therefore precipitates in large amounts during rolling and prevents grain growth and increases tempering stability and thus strength. When the V content is too high, coarse carbonitride particles are likely to be formed, and the impact toughness is deteriorated. Therefore, the content is controlled to be 0.045-0.055%.
Ti is a strong ferrite forming element, the formation temperature of titanium nitride is above 1400 ℃, the titanium nitride is precipitated in high-temperature liquid phase or delta ferrite, and fine precipitates can pin-roll grain boundaries, so that austenite grains are refined, and the welding performance of the steel plate is improved. Ti reduces the temper brittleness of the steel at the temperature of 250-400 ℃, and the addition of Ti and B can obviously reduce the temper brittleness. Ti in Al-containing steel can preferentially bind with N in the steel, and the amount of AlN in the steel can be reduced. Therefore, the range of the components of the limiter is 0.01-0.025%.
Mo narrows the austenite region, and improves hardenability during quenching heat treatment, thereby promoting formation of martensite structure. Carbide type of Mo includes MC, M2C、M23C6And M6C, Mo exists in the steel in the form of carbide and solid solution, and further causes a solid solution strengthening effect. Mo can improve the tempering stability of the steel, slow down the tempering softening phenomenon and inhibit the high-temperature tempering brittleness. When Mo is present together with Cr and Mn, the temper brittleness caused by other elements is reduced, and the low-temperature impact toughness of the steel plate is improved. However, higher Mo is disadvantageous in welding performance and increases cost, so that the content thereof is limited to 0.1 to 0.3%.
Addition of Ca to steel can change the shape of sulfides and inhibit the hot brittleness of S. And when there is an excess of Ti in the steel, it may form titanium sulfide or titanium carbosulfide with the sulfide. The effect of low Ca content is not obvious, the size of Ca (O, S) formed by high Ca content is too large, the brittleness is also increased, the Ca can become the starting point of fracture crack, and the purity of steel is reduced. The addition of Ca is controlled to be 0.001-0.023 percent, and the Ca/S is more than or equal to 0.5-2.0.
N can form nitrides with Al and Ti in steel, and fine precipitates have the function of nail rolling grain boundaries so as to refine austenite grains. Higher N combines with Al in the steel to easily form AIN, thereby significantly increasing the number of nitrides in the steel. When AIN independently exists in steel as a non-metallic inclusion, the continuity of a steel matrix is damaged, and particularly when the AIN is formed in a large quantity and in an aggregation distribution manner when the Al content is high, the damage degree is higher, and oxides with poor plasticity are formed; and higher N tends to concentrate at the defect site, deteriorating low temperature impact toughness. The N content must be controlled to 0.0050% or less. The addition of Ti and N ensures that Ti/N is more than or equal to 3.42, and Ti completely fixes N, so that Nb and V can form enough carbide strengthening.
The strengthening mechanism of steel includes solid solution strengthening, phase transformation strengthening, fine crystal strengthening, precipitation strengthening, dislocation strengthening, and the like. Solid solution strengthening produces a strengthening effect by causing the ferrite lattice to deform in the c-axis direction due to the interstitial atoms being solid-dissolved in the ferrite lattice and a strengthening effect by a stress field affecting dislocations by the substitutional atoms being solid-dissolved in the ferrite lattice. Fine grain strengthening improves the grain boundary area and improves the strength through grain refinement; the precipitation strengthening is to precipitate movable dislocation of nail rolling by carbon nitride and block the dislocation movement to realize the improvement of strength; dislocation strengthening is to form dislocation walls and dislocation bands in crystal grains by improving the dislocation density in steel so as to block the movement of dislocations to generate a strengthening effect; while the phase transformation strengthening increases the strength by forming a phase with higher hardness such as bainite and martensite. The lattice constant of the material is changed during interstitial solid solution strengthening, and the resistance of dislocation motion is obviously increased, so that the strengthening effect is obvious. In the invention, on one hand, the strengthening of the high-strength steel is to add micro-alloy of Nb, V, Ti and the like and combine control rolling to strengthen the fine grains and precipitate of the steel matrix structure, and meanwhile, the phase transformation strengthening effect is formed by subsequent heat treatment.
The invention adopts a C-Mn component system, and the component ranges are strictly limited to 0.165-0.185% and 1.15-1.25%, wherein the addition of C can realize martensite phase transformation strengthening through heat treatment, and Mn has the effects of solid solution strengthening and structure refinement. The lower content does not achieve the strengthening effect, and the higher content is unfavorable for welding, fatigue, toughness and the like. The steel grade of the invention limits Si to a lower component range (0.02-0.08%) to further improve the welding performance; meanwhile, noble alloy elements such as Cr and Ni are not added, the corrosion resistance functions of the two elements are not needed in the high-strength steel, and the solid solution strengthening and structure refining functions of the high-strength steel are respectively replaced by Mn and a small amount of Mo, Nb, V and Ti. The steel of the invention is added with a small amount of microalloy such as Nb, V, Ti and the like, and takes V as the main part (0.045-0.055%) and Nb and Ti (less than 0.025%) as the auxiliary parts. The V is low in cost, the high Ti is unfavorable for impact toughness, the composition can further reduce the manufacturing cost on one hand, and meanwhile, the composition combines control rolling to realize sufficient fine-grain strengthening and precipitation strengthening effects on a steel matrix structure in the hot rolling process.
The comprehensive application of Nb, Ti and V enlarges the temperature range of the unrecrystallized austenite of the steel grade, and creates conditions for fully refining the matrix structure by carrying out multi-pass deformation accumulation on austenite grains in the rolling deformation process. By controlling the deformation and the rolling temperature, the fine grain strengthening and the precipitation strengthening of the microalloy carbonitride are realized. The microalloy elements, C and a small amount of N form a large amount of nano-scale carbonitride in steel, and the carbonitride is precipitated in the austenite and controlled rolling process to play a role in refining a matrix structure. When the Ti content in the steel is less than 0.02 percent, TiN particles with the size of less than 20nm can be formed in austenite, the growth of an austenite structure is well inhibited, and the steel is still quite stable in the whole subsequent processing process (such as heating, hot rolling, welding and the like), so that the hot rolled steel plate has fine intrinsic grain size, and the carbonitride particles continuously play the roles of fine grain strengthening and precipitation strengthening in the subsequent heat treatment process. Particularly, 0.045-0.055% of V is added into the steel, a precipitated phase of the V is mainly VC under the condition of low nitrogen, the strengthening capability of the V is in direct proportion to the content, the V has a good strengthening effect, and the overall strengthening effect is better by combining the grain refining effect of 0.015-0.025% of Nb. V can inhibit the formation of non-polygonal ferrite, is beneficial to improving the martensite content in the quenching process and promotes the phase transformation strengthening; mo (0.1-0.3%) added into steel can raise hardenability and promote phase transformation strengthening effect, and can be combined with Mn to raise low-temp. toughness, and the formed carbide possesses solid-solution strengthening effect.
According to the invention, the high-strength steel plate with the yield strength of more than 1100MPa is prepared by optimizing the component system and fully utilizing the action of each alloy element. The steel grade designed by the components obtains a high-strength martensite structure after controlled rolling and off-line heat treatment, and has high strength and good toughness, plasticity and straightness. The yield strength is more than or equal to 1100MPa, the tensile strength is more than or equal to 1300MPa, the elongation is more than or equal to 10%, the straightness meets the requirement of less than or equal to 3mm/m, and the cold bending meets the requirements of D6 a and 90 degrees, so that the cold-bending forming die has good forming performance and is suitable for the field of engineering machinery.
The method for manufacturing the high-strength steel plate comprises the following steps:
a) smelting and casting
According to the components, firstly, the iron water is deeply subjected to S removal, and the top and the bottom of the converter are subjected to combined blowing, external refining and continuous casting to form a plate blank;
b) slab reheating
The heating temperature is above 1230 ℃, the heat preservation time of the heating section is above 2h, wherein the soaking heat preservation time is not less than 40 min;
c) controlled rolling
The rolling is divided into two stages of rough rolling and finish rolling; rolling with large reduction in the rough rolling stage, controlling pass reduction rate to be more than 18% or pass reduction to be more than 40mm, and simultaneously requiring that the thickness ratio of the intermediate blank to the finished product is more than or equal to 5; and controlling the reduction rate of the last pass of finish rolling to be not less than 16 percent; the start temperature of finish rolling is controlled to be over 980 ℃, and the finish temperature is controlled to be over 850 ℃;
d) controlled cooling, coiling, uncoiling, straightening and cutting plate
The coiling temperature is controlled to be 580 +/-50 ℃; uncoiling and straightening a steel coil cooled to room temperature, and then cutting the steel coil into plates;
e) quenching and tempering treatment
The quenching heating temperature is 890 +/-20 ℃, the heat preservation time is calculated from the center of the steel plate to the temperature, the quenching heat preservation time T1 is 1.5-2 times of the thickness H of the steel plate, T1 is more than or equal to 4min, T1 is unit min; plate thickness, H, unit mm; directly carrying out water quenching on the steel plate after the steel plate is taken out of the furnace to room temperature, and controlling the cooling speed to be more than or equal to 30 ℃/s;
the tempering temperature is 200-240 ℃, the tempering heat preservation time is counted from the center of the steel plate to the temperature, the tempering heat preservation time T2 is 1.5-2.5 times of the thickness H of the steel plate, T2 is more than or equal to 6min, T2 is unit min; the thickness H of the steel plate is unit mm;
f) the quenched and tempered steel sheet is subjected to finishing treatment and delivered.
In the method for producing the steel of the present invention:
heating and preserving heat of a casting blank before rolling, wherein the heating temperature is required to be above 1230 ℃ and the heat preservation time of a heating section is required to be above 2h to be beneficial to full solid solution of microalloy compounds considering that steel contains microalloy elements such as Nb, V, Ti and the like. Wherein the soaking and heat preservation time is not less than 40 min. In addition, the casting blank can be hot-charged into the furnace after the casting is finished, namely the casting blank is directly conveyed to the heating furnace for heating and heat preservation from the casting area through the roller way after the fact that the surface of the casting blank has no quality problem is confirmed, so that the energy consumption can be reduced; if the casting blank can not be hot-charged, the cast blank must be placed in a heat preservation pit for slow cooling, and the heat preservation pit can be removed for air cooling after the temperature is reduced to below 200 ℃.
The rolling is divided into two stages of rough rolling and finish rolling. In order to obtain fine original austenite grain size, a casting blank is rolled by adopting large reduction in a rough rolling stage, and the pass reduction rate is controlled to be more than 18% or the pass reduction is controlled to be more than 40 mm. In order to obtain fine grain size and good plate shape, the thickness ratio of the intermediate blank to the finished product is required to be more than or equal to 5, and the reduction rate of the final pass of finish rolling is controlled to be not less than 16%. The steel billet is deformed enough in the austenite recrystallization temperature range, the microalloy plays a role in retarding the austenite dynamic recrystallization in the deformation process, and the deformed austenite is gradually refined through repeated rolling and recrystallization. In the range from Ar3 to the austenite non-recrystallization temperature, austenite recrystallization is suppressed, strain is accumulated, elongated austenite grains are formed, high-density dislocations are generated in the deformation zone, and microalloy solid solution atoms are induced to precipitate as carbonitrides by deformation.
The invention relates to a method for realizing grain refinement and precipitation strengthening of a matrix of steel mainly by controlling deformation and rolling temperature in the rolling stage. From the continuous transition curve of fig. 1, the a → γ transition point of the steel grade is about 817 ℃, so that a finish rolling temperature of 860 ℃ or higher is recommended. At this temperature, V carbonitride is completely dissolved in solid solution, and Nb carbonitride is also mostly in solid solution, thereby ensuring that a large amount of carbonitride is precipitated in the cooling process after the rolling is completed, and playing a role in refining grains. According to the finish rolling temperature and the temperature drop in the finish rolling process, the finish rolling start temperature is determined to be not lower than 930 ℃, and particularly, the finish rolling start temperature is required to be controlled to be over 980 ℃ in consideration of the temperature drop of the intermediate billet in the finish rolling process. The invention relates to a steel grade having an austenite non-recrystallization temperature of approximately 965 ℃, meaning that the steel strip is in the austenite non-recrystallization temperature range shortly after the start of the finish rolling. In the temperature range from Ar3 to austenite non-recrystallization temperature, austenite recrystallization is inhibited, strain is accumulated, slender austenite grains are formed, high-density dislocation is generated in a deformation zone, microalloy solid solution atoms are precipitated into carbonitride through deformation induction, grains are refined, and fine intrinsic grain size is formed.
Because the subsequent off-line heat treatment is needed, the invention relates to that the stop cooling temperature of the steel grade is controlled to be higher than the martensite phase transformation starting temperature. As shown in FIG. 1, the martensite start transformation temperature of the steel grade related to the invention is about 400 ℃, the coiling temperature is recommended to be controlled within 580 +/-50 ℃ in combination with the equipment capacity and the effective temperature measurement range in the hot rolling process, and if the coiling temperature is too high, the cooling speed is too low, so that the fine intrinsic grain size cannot be obtained by coarsening the crystal grains; the lower the take-up load increases.
And uncoiling and straightening the steel coil cooled to room temperature, then cutting the steel coil into plates, and quenching and tempering the steel plate. The quenching heating temperature directly influences the granularity of the subsequent martensite structure, and further influences the toughness of the steel plate. The austenite grains are easy to coarsen due to the overhigh heating temperature, the martensite structure is coarse after quenching, and the toughness is deteriorated; and the heating temperature is lower, so that austenitizing is insufficient, solid solution of micro-alloy elements is insufficient, a complete martensite structure cannot be obtained after quenching, and the effects of precipitation and fine grain strengthening of the micro-alloy are realized. In order to ensure that the carbonitride formed by Nb, V and Ti is dissolved as much as possible in the heat treatment heating process, the heating temperature is controlled according to the solid solubility formula. For example, Nb, according to the solid solubility formula Lg [ Nb ] [ C ] ═ 2.96-7510/T +0.248[ Mn ], and both V and Ti have similar formulas. And simultaneously, the quenching heating temperature of the steel grade is determined to be 890 +/-20 ℃ by combining the Ac3 temperature point of the steel grade, so that the sufficient solid solution of the carbonitride is ensured, and the martensite phase transformation strengthening, the precipitation strengthening of the microalloy and the fine grain strengthening are realized in the subsequent quenching cooling process.
The heat preservation time has a similar rule to the quenching performance, crystal grains are easy to be large if the time is too long, energy consumption is increased, cost is improved, austenitizing is insufficient if the time is too short, and the hardness and strength after quenching cannot meet the requirements. The heat preservation time is required to be 1.5-2 times (min) of the thickness (mm) of the steel plate from the center of the steel plate to the temperature, but the minimum time is not less than 4 min. Directly water-quenching the steel plate to room temperature after discharging, and controlling the cooling speed to be more than or equal to 30 ℃/s according to the phase change transformation curve shown in figure 1.
The tempering treatment mainly slows down and eliminates the quenching stress and improves the toughness and the toughness. The higher tempering temperature easily causes the strength and hardness of the steel plate to be reduced too much, so that the design requirements cannot be met, and meanwhile, the cost is increased. The tempering process parameters of the steel sheet should be limited. In the invention, the steel plate is tempered in the temperature range of 200-240 ℃, and the tempering heat preservation time is 1.5-2.5 times (min) of the plate thickness (mm) from the center of the steel plate to the beginning of the temperature, but the minimum time is not less than 6 min. And finally, finishing (straightening and trimming) the quenched and tempered steel plate, controlling the unevenness to be less than or equal to 3mm/m, and leaving the factory after the mechanical properties are qualified.
Compared with the prior art, the invention has the advantages that:
in terms of composition, the content of Si in the steel type of Chinese patent publication No. CN102747303A 'a high-strength steel sheet with a yield strength of 1100MPa grade and a manufacturing method thereof' is 0.1-0.5%, and the Si is obviously used as an important additive element, while Si is only used as a deoxidizing element and the content is only 0.02-0.08%. But also simultaneously adding 0.2-0.55% of Cr and 0.2-2.0% of Ni noble elements and requiring more Mo, which is obviously different from the invention. Meanwhile, the thick plate process efficiency is lower.
In Chinese patent publication No. CN102337480A, "ultra-high strength steel plate with excellent environmental brittleness resistance and fatigue performance and manufacturing method thereof", both C, Si and Mn are higher than those of the invention, particularly, Si content is up to 1.5-1.9%, and noble alloy elements Cr and Ni are required to be added;
chinese patent publication No. CN10449883A 'A composition of a high-toughness ultrahigh-strength steel and a production method thereof' has a higher C, Si content, and simultaneously the Cr content and the Ni content are respectively as high as 2.2.35% and 2.5-4.0%, which obviously increases the cost, and simultaneously the V content is more as high as 0.1-0.4%, and contains 2.0-4.5% of W, so that the steel not only has high cost, but also has a complex production process and needs forging processing, and is obviously different from the delivery state of the steel plate of the invention, and the application field is also different.
The invention has the following advantages:
the invention adopts simple and economic C-Mn component design, is assisted by a small amount of Mo and microalloy elements such as Nb, V, Ti, B and the like, and realizes high strength and high toughness of steel grades through microalloy fine grain strengthening and phase change strengthening.
The steel grade has good low-temperature impact toughness, forming performance and flatness, and meets the use requirements of the steel for engineering machinery.
The invention relates to a steel grade with simple production process and low content of noble alloy elements, which reduces the production difficulty and the production cost and is beneficial to the large-scale popularization of the steel grade.
Drawings
FIG. 1 is a schematic diagram of CCT curve (calculation) of steel grade according to the invention.
In the figure, M is martensite, B is bainite, F is ferrite, and P is pearlite.
Detailed Description
The present invention will be further described with reference to the following examples.
The manufacturing process of the embodiment of the invention comprises the following steps: the method comprises the following steps of iron water deep stripping S (ensuring low S content in steel) → converter top and bottom combined blowing (controlling C content) → external refining → continuous casting (mechanical cleaning) → slab reheating → controlled rolling → controlled cooling → coiling → uncoiling → straightening → cutting plate → heat treatment (quenching + tempering) → finishing → delivery.
According to the chemical component requirements of the high-strength steel plate, steel plates with different thickness specifications are prepared. The chemical components are shown in table 1, the heating temperature of the steel billet is 1240 ℃, the finishing temperature is 880 ℃, and the steel billet is cooled to 550 ℃ after rolling and coiled. And cutting the steel coil after straightening, and quenching and tempering the steel plate. The heating temperature is 890 +/-20 ℃, and the tempering temperature is 200-. The specification and properties of the high-strength steel sheet produced in this example are shown in Table 2.
TABLE 1 units wt%
Numbering C Si Mn P S Al N Ti V Nb Mo B Ca
A 0.168 0.021 1.16 0.008 0.0024 0.028 0.0044 0.015 0.045 0.012 0.21 0.0045 0.0014
B 0.17 0.025 1.23 0.012 0.0035 0.034 0.0031 0.022 0.054 0.014 0.13 0.0017 0.0018
C 0.184 0.032 1.24 0.013 0.0036 0.042 0.0038 0.024 0.033 0.018 0.28 0.0038 0.0028
D 0.173 0.074 1.18 0.009 0.003 0.036 0.0033 0.018 0.048 0.012 0.30 0.0014 0.0024
TABLE 2
Figure BDA0001815949660000121
As can be seen from the above table, the yield strength of the steel plate related by the invention is more than 1100MPa, the elongation is more than or equal to 10 percent, the plasticity is excellent, and the low-temperature impact value meets the requirement (full-size sample) that the temperature is minus 40 ℃ and more than or equal to 40J; meanwhile, the steel plate has good flatness and forming performance, and is suitable for the field of engineering machinery.

Claims (3)

1. A high-strength steel plate comprises the following components in percentage by weight: c: 0.165-0.185%, Si: 0.02 to 0.08%, Mn: 1.15-1.25%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, Al: 0.025 to 0.05%, Mo: 0.1-0.3%, B: 0.001-0.005%, N is less than or equal to 0.004%, Nb: 0.01-0.02%, Ti: 0.01-0.025%, V: 0.045-0.055%, and the balance of Fe and inevitable impurity elements; the yield strength of the steel plate is more than or equal to 1100MPa, the tensile strength is more than or equal to 1300MPa, the elongation is more than or equal to 10 percent, the straightness meets the requirement of less than or equal to 3mm/m, and the cold bending meets the requirements of D being 6a and 90 degrees; the thickness of the steel plate is 2.5-6 mm.
2. The method of manufacturing a high-strength steel sheet according to claim 1, comprising the steps of:
a) smelting and casting
The composition of claim 1, wherein the molten iron is first deep-drawn to remove S, and then undergoes top-bottom combined blowing in a converter, external refining and continuous casting to form a slab;
b) slab reheating
The heating temperature is above 1230 ℃, the heat preservation time of the heating section is above 2h, wherein the soaking heat preservation time is not less than 40 min;
c) controlled rolling
The rolling is divided into two stages of rough rolling and finish rolling; rolling with large reduction in the rough rolling stage, controlling pass reduction rate to be more than 18% or pass reduction to be more than 40mm, and simultaneously requiring that the thickness ratio of the intermediate blank to the finished product is more than or equal to 5; and controlling the reduction rate of the last pass of finish rolling to be not less than 16 percent; the start temperature of finish rolling is controlled to be over 980 ℃, and the finish temperature is controlled to be over 850 ℃;
d) controlled cooling, coiling, uncoiling, straightening and cutting plate
The coiling temperature is controlled to be 580 +/-50 ℃; uncoiling and straightening a steel coil cooled to room temperature, and then cutting the steel coil into plates;
e) quenching and tempering treatment
The quenching heating temperature is 890 +/-20 ℃, the heat preservation time is calculated from the center of the steel plate to the temperature, the quenching heat preservation time T1 is 1.5-2 times of the thickness H of the steel plate, T1 is more than or equal to 4min, T1 is unit min; plate thickness, H, unit mm; directly carrying out water quenching on the steel plate after the steel plate is taken out of the furnace to room temperature, and controlling the cooling speed to be more than or equal to 30 ℃/s; the tempering temperature is 200-240 ℃, the tempering heat preservation time is counted from the center of the steel plate to the temperature, the tempering heat preservation time T2 is 1.5-2.5 times of the thickness H of the steel plate, T2 is more than or equal to 6min, T2 is unit min; the thickness H of the steel plate is unit mm;
f) the quenched and tempered steel sheet is subjected to finishing treatment and delivered.
3. The method for manufacturing a high-strength steel sheet according to claim 2, wherein the slab after casting is placed in a holding pit for slow cooling if it cannot be charged into the furnace, and the holding pit is removed and air-cooled after the temperature is lowered to 200 ℃ or lower.
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