CN114107790B - 980 MPa-grade ultralow-carbon martensitic high-reaming steel and manufacturing method thereof - Google Patents

980 MPa-grade ultralow-carbon martensitic high-reaming steel and manufacturing method thereof Download PDF

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CN114107790B
CN114107790B CN202010896455.1A CN202010896455A CN114107790B CN 114107790 B CN114107790 B CN 114107790B CN 202010896455 A CN202010896455 A CN 202010896455A CN 114107790 B CN114107790 B CN 114107790B
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steel
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CN114107790A (en
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王焕荣
杨峰
张晨
柏明卓
杨阿娜
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Baoshan Iron and Steel Co Ltd
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Baoshan Iron and Steel Co Ltd
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Priority to CN202010896455.1A priority Critical patent/CN114107790B/en
Priority to JP2023513798A priority patent/JP2023539649A/en
Priority to EP21860562.4A priority patent/EP4206350A4/en
Priority to PCT/CN2021/115431 priority patent/WO2022042730A1/en
Priority to KR1020237009927A priority patent/KR20230061413A/en
Priority to US18/043,217 priority patent/US20230313332A1/en
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/04—Making ferrous alloys by melting
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Abstract

A980 MPa grade ultra-low carbon martensite high-reaming steel and a manufacturing method thereof, the chemical components in percentage by weight are as follows: 0.03 to 0.06 percent of C, 0.5 to 2.0 percent of Si, 1.0 to 2.0 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.003 percent of S, 0.02 to 0.08 percent of Al, less than or equal to 0.004 percent of N, 0.1 to 0.5 percent of Mo, 0.01 to 0.05 percent of Ti, less than or equal to 0.0030 percent of O, and the balance of Fe and other unavoidable impurities. The yield strength of the high-reaming steel is more than or equal to 800MPa, the tensile strength is more than or equal to 980MPa, and the elongation is transverse A 50 More than or equal to 8 percent, cold bending performance (d is less than or equal to 4a and 180 degrees), and hole expansion rate is more than or equal to 50 percent, and can be applied to the parts of the chassis of the passenger car, such as a control arm, an auxiliary frame and the like, which need high strength thinning.

Description

980 MPa-grade ultralow-carbon martensitic high-reaming steel and manufacturing method thereof
Technical Field
The invention belongs to the field of high-strength steel, and particularly relates to 980 MPa-grade ultralow-carbon martensitic high-reaming steel and a manufacturing method thereof.
Background
With the development of national economy, the production of automobiles is greatly increased, and the use amount of plates is continuously increased. The original design of parts of various automobile types in the domestic automobile industry requires hot-rolled or pickled plates, such as parts of chassis parts, torsion beams, auxiliary frames, wheel spokes and rims of automobiles, front and rear axle assemblies, automobile body structural parts, seats, clutches, safety belts, truck box plates, protective nets, automobile girders and the like. Wherein, the steel for the chassis accounts for 24-34% of the total steel of the car.
The weight reduction of passenger cars is not only a development trend of the automobile industry, but also a requirement of laws and regulations. The law and regulation prescribes oil consumption, and the actual requirement is that the weight of a vehicle body is reduced in a phase-change manner, and the requirement reflected on materials is that the vehicle body is high-strength, thin and light. High strength and weight reduction are the necessary requirements of the subsequent new vehicle, which tend to lead to higher steel grade, and the chassis structure also has the necessary change: if the parts are more complex, the requirements on material performance, surface and the like and the forming technology are improved, such as hydroforming, hot stamping, laser welding and the like, so that the performances of high strength, stamping, flanging, rebound, fatigue and the like of the materials are converted.
Compared with overseas, the development of the domestic high-strength high-reaming steel has relatively low strength level and poor performance stability. For example, the high-hole-enlarging steel used by domestic automobile spare part enterprises is basically high-strength steel with the tensile strength below 600MPa and high-hole-enlarging steel with the grade below 440MPa competing for white-heat. High-hole-expansion steel with tensile strength of 780MPa grade is gradually used in batches at present, but high requirements are also put forward on important indexes of elongation and hole expansion rate for two forming. The 980 MPa-level high-reaming steel is still in the research and development authentication stage at present, and does not reach the batch use stage yet. But 980 high-hole-enlarging steel with higher strength and higher hole-enlarging rate is a necessary development trend in the future. In order to better meet the future potential needs of users, 980 MPa-grade high-reaming steel with good reaming performance needs to be developed.
Most of the related patent documents are high-reaming steel with the grade of 780MPa or below. There are few documents concerning 980 MPa-grade high-hole-enlarging steels. Chinese patent CN106119702a discloses 980 MPa-grade hot rolled high-hole-enlarging steel, its composition design is mainly characterized by low-carbon V-Ti microalloying design, microstructure is granular bainite and a small amount of martensite, and at the same time trace Nb and Cr are added. There are significant differences from the present invention in terms of composition, process, and organization.
It is known from the literature that in normal cases, the elongation of a material is inversely related to the hole expansion ratio, i.e. the higher the elongation, the lower the hole expansion ratio; conversely, the lower the elongation, the higher the hole expansion ratio. It is very difficult to obtain a high elongation high reaming steel with high strength. In addition, the higher the strength of the material, the lower the hole expansion ratio under the same or similar strengthening mechanism.
In order to obtain a steel product with good plasticity and reaming and flanging properties, a better balance between the two is required. Of course, the hole expansion ratio of a material is closely related to many factors, the most important of which include uniformity of the structure, inclusion and segregation control levels, different structure types, and measurement of hole expansion ratio. Generally, a single uniform tissue is advantageous for achieving higher hole expansion rates, while a dual or multi-phase tissue is generally disadvantageous for increased hole expansion rates.
Disclosure of Invention
The invention aims to provide 980 MPa-grade ultralow-carbon martensitic high-reaming steel and a manufacturing method thereof, wherein the yield strength of the high-reaming steel is more than or equal to 800MPa, the tensile strength is more than or equal to 980MPa, and the elongation is transverse A 50 More than or equal to 8 percent, cold bending performance (d is less than or equal to 4a and 180 degrees), and hole expansion rate is more than or equal to 50 percent, and can be applied to the parts of the chassis of the passenger car, such as a control arm, an auxiliary frame and the like, which need high strength thinning.
In order to achieve the above purpose, the technical scheme of the invention is as follows:
the component design of the invention adopts lower C content, which can ensure that a user has excellent weldability when in use and ensures that the obtained martensitic structure has good hole expansibility and impact toughness; the method has the advantages that the method is designed to have higher Si content, and is matched with the process to obtain more residual austenite, so that the plasticity of the material is improved; meanwhile, the higher Si content is beneficial to reducing the unrecrystallized temperature of the steel, so that the steel can finish the dynamic recrystallization process in a wider finishing temperature range, thereby refining austenite grains and the final martensite grain size and improving the plasticity and the hole expansion rate.
Specifically, the 980 MPa-grade ultra-low carbon martensitic high-reaming steel comprises the following chemical components in percentage by weight: 0.03 to 0.06 percent of C, 0.5 to 2.0 percent of Si, 1.0 to 2.0 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.003 percent of S, 0.02 to 0.08 percent of Al, less than or equal to 0.004 percent of N, 0.1 to 0.5 percent of Mo, 0.01 to 0.05 percent of Ti, less than or equal to 0.0030 percent of O, and the balance of Fe and other unavoidable impurities.
Further, the alloy also comprises one or more elements of less than or equal to 0.5 percent of Cr, less than or equal to 0.002 percent of B, less than or equal to 0.005 percent of Ca, less than or equal to 0.06 percent of Nb, less than or equal to 0.05 percent of V, less than or equal to 0.5 percent of Cu and less than or equal to 0.5 percent of Ni; wherein the Cr content is preferably 0.2-0.4%, and the B content is preferably 0.0005-0.0015%; the Ca content is preferably less than or equal to 0.002%; the Nb and V contents are preferably less than or equal to 0.03 percent respectively; the Cu and Ni contents are preferably less than or equal to 0.3 percent respectively.
In the composition design of the high-reaming steel disclosed by the invention, the following components are adopted:
carbon, which is a basic element in steel, is also one of the important elements in the present invention. Carbon expands the austenite phase region, stabilizing austenite. Carbon plays a very important role in improving the strength of steel as a interstitial atom in steel, and has the greatest influence on the yield strength and tensile strength of steel. In the invention, because the structure to be obtained is low-carbon or ultra-low-carbon martensite, in order to obtain high-strength steel with tensile strength up to 980MPa, the carbon content must be ensured to be more than 0.03%, otherwise, the carbon content is less than 0.03%, and the tensile strength of the steel is less than 980MPa even if the steel is completely quenched to room temperature; but the carbon content cannot be higher than 0.06%. The carbon content is too high, the strength of the formed low-carbon martensite is too high, and the elongation and the hole expansion rate are low. Therefore, the carbon content should be controlled between 0.03-0.06%, preferably in the range of 0.04-0.055%.
Silicon, which is a basic element in steel, is one of the important elements in the present invention. The Si content is increased, not only the solid solution strengthening effect is improved, but also the following two functions are achieved. Firstly, the unrecrystallized temperature of the steel is greatly reduced, so that the steel can be dynamically recrystallized in a very low temperature range. In this way, in the actual rolling process, rolling can be performed in a relatively wide finishing temperature range, such as 800-900 ℃, so that the anisotropy of the structure can be greatly improved, and the final anisotropy of the martensitic structure is reduced, thereby being beneficial to improving the strength and the plasticity and simultaneously being beneficial to obtaining a good hole expansion rate; another important role of Si is to suppress cementite precipitation, and to retain a certain amount of retained austenite under appropriate rolling process conditions, particularly when a structure mainly composed of martensite is obtained, which is advantageous in improving elongation. It is well known that the elongation of martensite is usually the lowest at the same strength level, while retaining a certain amount of stable retained austenite is an important one of them in order to increase the elongation of martensite. This effect of Si generally begins to manifest when its content reaches more than 0.5%; however, the Si content should not be too high, otherwise the rolling force load is too large in the actual rolling process, which is not beneficial to the stable production of the product. Therefore, the Si content in the steel is generally controlled to be between 0.5 and 2.0%, preferably in the range of 0.8 to 1.4%.
Manganese, the most basic element in steel, is one of the most important elements in the present invention. Mn is known to be an important element for enlarging the austenite phase region, and can reduce the critical quenching speed of steel, stabilize austenite, refine grains, and retard the transformation of austenite to pearlite. In the invention, in order to ensure the strength of the steel plate and to stabilize the retained austenite, the Mn content should be generally controlled to be more than 1.0%; meanwhile, the Mn content is not more than 2.0% generally, otherwise Mn segregation is easy to occur during steelmaking, and hot cracking is easy to occur during slab continuous casting. Therefore, the Mn content in the steel is generally controlled to be 1.0-2.0%, preferably in the range of 1.4-1.8%;
phosphorus is an impurity element in steel. P is easily biased to grain boundary, and Fe is formed when the content of P in steel is higher (more than or equal to 0.1 percent) 2 P is precipitated around the grains, and the plasticity and toughness of the steel are reduced, so that the lower the content is, the better the content is generally controlled within 0.02 percent, and the steelmaking cost is not increased.
Sulfur is an impurity element in steel. S in steel is usually combined with Mn to form MnS inclusion, more MnS is formed in the steel especially when the contents of S and Mn are high, the MnS has certain plasticity, and the MnS deforms along the rolling direction in the subsequent rolling process, so that the transverse plasticity of the steel is reduced, the tissue anisotropy is increased, and the reaming performance is unfavorable. Therefore, the lower the S content in the steel, the better, and in view of the fact that the Mn content in the present invention must be at a higher level, the S content is strictly controlled to be within 0.003%, preferably within a range of 0.0015% or less, in order to reduce the MnS content.
The role of aluminum in steel is mainly deoxidation and nitrogen fixation. In the presence of strong carbide forming elements such as Ti, nb, V, etc., the primary role of Al is to deoxidize and refine the grains. In the invention, al is used as a common deoxidizing element and an element for refining grains, and the content of the Al is controlled to be between 0.02 and 0.08 percent; al content is less than 0.02%, and the effect of refining grains is not achieved; also, when the Al content is higher than 0.08%, the effect of refining the crystal grains is saturated. Therefore, the Al content in the steel is controlled to be 0.02-0.08%, preferably 0.02-0.05%.
Nitrogen, which is an impurity element in the present invention, is preferably contained in a lower amount. Nitrogen is an inevitable element in the steelmaking process. Although the content thereof is small, the formed TiN particles have a very adverse effect on the properties of steel, particularly on the reaming properties, in combination with strong carbide forming elements such as Ti and the like. Because TiN is square, a large stress concentration exists between the sharp angle and the matrix, and cracks are easily formed due to the stress concentration between the TiN and the matrix in the reaming deformation process, so that the reaming performance of the material is greatly reduced. The lower the content of the strong carbide forming element such as Ti is, the better the nitrogen content is controlled as much as possible. In the invention, a trace amount of Ti is added to fix nitrogen, so that adverse effects caused by TiN are reduced as much as possible. Therefore, the nitrogen content should be controlled to be 0.004% or less, preferably to be 0.003% or less.
Titanium is one of the important elements in the present invention. Ti plays two main roles in the present invention: firstly, the titanium alloy is combined with impurity element N in steel to form TiN, which plays a part of role of nitrogen fixation; secondly, a certain amount of dispersed and fine TiN is formed in the subsequent welding process of the material, the size of austenite grains is restrained, the structure is thinned and the low-temperature toughness is improved. Therefore, the Ti content in the steel is controlled to be in the range of 0.01 to 0.05%, preferably in the range of 0.01 to 0.03%.
Molybdenum is one of the important elements in the present invention. Molybdenum addition to steel can greatly delay ferrite and pearlite transformation. The effect of the molybdenum is beneficial to the adjustment of various processes in the actual rolling process, such as sectional cooling after finishing rolling, air cooling and water cooling, and the like. In the invention, a process of air cooling and then water cooling or direct water cooling after rolling is adopted, the addition of molybdenum can ensure that ferrite or pearlite and other structures are not formed in the air cooling process, and meanwhile, deformed austenite can be dynamically recovered in the air cooling process, thereby being beneficial to improving the uniformity of the structure; molybdenum has very strong weld softening resistance. Since the main purpose of the invention is to obtain a structure of single low-carbon martensite and a small amount of residual austenite, the low-carbon martensite is easy to soften after welding, and the addition of a certain amount of molybdenum can effectively reduce the welding softening degree. Therefore, the content of molybdenum should be controlled between 0.1 and 0.5%, preferably in the range of 0.15 to 0.35%.
Chromium is one of the additive elements in the present invention. The addition of a small amount of chromium element is not used for improving the hardenability of steel, but is combined with B, so that the acicular ferrite structure is formed in a welding heat affected zone after welding, and the low-temperature toughness of the welding heat affected zone can be greatly improved. Because the final application part related to the invention is a chassis product of a passenger car, the low-temperature toughness of a welding heat affected zone is an important index. In addition to ensuring that the strength of the weld heat affected zone does not decrease too much, the low temperature toughness of the weld heat affected zone also meets certain requirements. In addition, chromium itself has a certain resistance to weld softening. Therefore, the addition amount of chromium element in the steel is generally less than or equal to 0.5 percent, and the preferable range is 0.2 to 0.4 percent;
boron is one of the additive elements in the present invention. Boron in steel mainly gathers at the original austenite grain boundary, and inhibits the formation of proeutectoid ferrite; boron addition to steel can also greatly improve the hardenability of the steel. However, in the present invention, the main purpose of adding the trace boron element is not to improve hardenability, but to improve the weld heat affected zone structure by combining with chromium, and to obtain a needle-like ferrite structure having excellent low-temperature toughness. The boron addition to the steel is generally controlled to be less than 0.002%, preferably in the range of 0.0005-0.0015%.
Calcium is an additive element in the present invention. Calcium can improve the forms of sulfides such as MnS, so that long-strip-shaped sulfides such as MnS are changed into spherical CaS, the forms of inclusions are improved, and further the adverse effect of long-strip-shaped sulfides on reaming performance is reduced, but the addition of excessive calcium can increase the quantity of calcium oxide, and is adverse to reaming performance. Therefore, the addition amount of the steel grade calcium is usually not more than 0.005%, preferably in the range of not more than 0.002%.
Oxygen is an unavoidable element in the steelmaking process, and for the invention, the content of O in the steel can generally reach below 30ppm after deoxidization, and the performance of the steel plate is not obviously adversely affected. Therefore, the O content in the steel may be controlled to 30ppm or less.
Niobium is one of the additive elements of the present invention. Niobium is similar to titanium and is a strong carbide element in steel, the unrecrystallized temperature of the steel can be greatly increased by adding the niobium into the steel, deformed austenite with higher dislocation density can be obtained in the finish rolling stage, and the final phase transformation structure can be refined in the subsequent transformation process. However, the amount of niobium added is not too large, and on the one hand, the amount of niobium added exceeds 0.06%, so that coarse niobium carbonitride is easily formed in the structure, part of carbon atoms are consumed, and the precipitation strengthening effect of carbide is reduced. Meanwhile, the niobium content is high, anisotropy of a hot rolled austenitic structure is easy to cause, and the hot rolled austenitic structure is inherited to a final structure in a subsequent cooling phase transformation process, so that the reaming performance is not good. Therefore, the niobium content in the steel is usually controlled to be 0.06% or less, preferably in the range of 0.03% or less.
Vanadium is an additive element in the present invention. Vanadium, like titanium, niobium, is also a strong carbide forming element. However, the vanadium carbide has a low solution or precipitation temperature, and is usually entirely dissolved in austenite in the finish rolling stage. Vanadium starts to form in ferrite only when the temperature decrease starts to change phase. The solid solubility of vanadium carbide in ferrite is larger than that of niobium and titanium, so that the size formed by the vanadium carbide in ferrite is larger, the precipitation strengthening is not facilitated, the strength contribution to steel is far smaller than that of titanium, but the formation of vanadium carbide also consumes a certain carbon atom, and the strength improvement of steel is not facilitated. Therefore, the addition amount of vanadium in steel is usually not more than 0.05%, and the preferable range is not more than 0.03%.
Copper is an additive element in the invention. Copper is added into the steel to improve the corrosion resistance of the steel, and when the copper and the P element are added together, the corrosion resistance effect is better; when the Cu addition amount exceeds 1%, an epsilon-Cu precipitated phase can be formed under certain conditions, and a stronger precipitation strengthening effect is achieved. However, cu is added to easily form a "Cu embrittlement" phenomenon during rolling, and in order to make full use of the corrosion resistance improving effect of Cu in some applications, the Cu element content is usually controlled to be within 0.5%, preferably within 0.3%, without causing a significant "Cu embrittlement" phenomenon.
Nickel is an additive element in the invention. The nickel added into the steel has certain corrosion resistance, but the corrosion resistance effect is weaker than that of copper, and the nickel added into the steel has little influence on the tensile property of the steel, but can refine the structure and the precipitated phase of the steel, so that the low-temperature toughness of the steel is greatly improved; meanwhile, in the steel added with copper element, the occurrence of Cu embrittlement can be restrained by adding a small amount of nickel. The addition of higher nickel has no significant adverse effect on the properties of the steel itself. If copper and nickel are added at the same time, not only the corrosion resistance can be improved, but also the structure and the precipitated phase of the steel are refined, and the low-temperature toughness is greatly improved. But since copper and nickel are both relatively noble alloying elements. Therefore, in order to reduce the alloy cost as much as possible, the addition amount of nickel is usually not more than 0.5%, and the preferable range is not more than 0.3%.
The invention relates to a 980 MPa-grade ultra-low carbon martensitic high-reaming steel manufacturing method, which comprises the following steps:
1) Smelting and casting
Smelting the components by adopting a converter or an electric furnace, secondarily refining by adopting a vacuum furnace, and casting into a casting blank or an ingot;
2) Reheating the casting blank or the cast ingot, wherein the heating temperature is 1100-1200 ℃, and the heat preservation time is 1-2 hours;
3) Hot rolling
Start rolling temperature: 950-1100 ℃, under 3-5 times of high pressure above 950 ℃ and accumulated deformation not less than 50%, and the main purpose is to refine austenite grains; then the intermediate blank is heated to 920-950 ℃, and then is rolled for the last 3-5 passes, and the accumulated deformation is more than or equal to 70%; the final rolling temperature is 800-920 ℃;
4) Cooling
Firstly, performing air cooling for 0-10s to perform dynamic recovery and dynamic recrystallization, then cooling the strip steel to a certain temperature below the Ms point (between room temperature and the Ms point) at a cooling speed of more than or equal to 50 ℃/s, coiling, and cooling to room temperature after coiling;
5) Acid washing
The strip steel pickling operation speed can be adjusted within the interval of 30-100 m/min, the pickling temperature is controlled between 75-85 ℃, the withdrawal and straightening rate is controlled to be less than or equal to 2%, so that the strip steel elongation loss is reduced, and then the strip steel is rinsed, dried and oiled.
Preferably, after the acid washing in the step 5), rinsing is carried out at the temperature of 35-50 ℃ to ensure the surface quality of the strip steel, and surface drying and oiling are carried out at the temperature of 120-140 ℃.
The innovation point of the invention is that:
the component design of the invention adopts lower C content, which can ensure that a user has excellent weldability when in use and ensures that the obtained martensitic structure has good hole expansibility and impact toughness; the method has the advantages that the method is designed to have higher Si content, and is matched with the process to obtain more residual austenite, so that the plasticity of the material is improved; meanwhile, the higher Si content is beneficial to reducing the unrecrystallized temperature of the steel, so that the steel can finish the dynamic recrystallization process in a wider finishing temperature range, thereby refining austenite grains and the final martensite grain size and improving the plasticity and the hole expansion rate.
The composition design adopts a low-carbon martensite design thought, higher silicon is added to inhibit and reduce cementite formation, meanwhile, the unrecrystallized temperature is reduced, rolling and air cooling after rolling are carried out in a relatively wide final rolling temperature range, original austenite grains with fine and uniform grains and equiaxed grains can be obtained, and finally martensite and residual austenite structures with uniform structures are obtained. The retained austenite endows the steel plate with higher plasticity and cold bending property, martensite endows the steel plate with high strength, and uniform and fine structure endows the steel plate with higher reaming property and low-temperature toughness.
In the design of the rolling process, the rhythm of the rolling process should be completed as fast as possible in the rough rolling and finish rolling stages. After finishing the finish rolling, air cooling is performed for a certain time. The main purpose of air cooling is as follows: manganese is an element that stabilizes austenite due to its high content in composition design, and molybdenum greatly delays ferrite and pearlite transformation. Therefore, during air cooling for a certain period of time, the rolled deformed austenite does not undergo transformation, i.e., does not form ferrite structure, but rather undergoes dynamic recrystallization and relaxation. The deformed austenite is dynamically recrystallized to form austenite with a uniform structure and nearly equiaxial shape, dislocation in austenite grains can be greatly reduced after relaxation, and martensite with a uniform structure can be obtained in the subsequent water-cooling quenching process by combining the austenite with the relaxed austenite. In order to obtain a martensitic structure, the water cooling speed is greater than the critical cooling speed of low-carbon martensite, and in the invention, the critical cooling speed of the martensite is 30-50 ℃/s according to the difference of components and processes, and in order to ensure that the martensite can be obtained by all component designs, the water cooling speed of the strip steel is more than or equal to 50 ℃/s.
Since the microstructure according to the present invention is low-carbon or ultra-low-carbon martensite, the strip steel is cooled to a temperature not higher than the martensitic transformation start point Ms at a cooling rate higher than the critical cooling rate after finishing rolling. The cooling stop temperatures are different, and the content of the residual austenite is different at room temperature. There is generally an optimum quench stop temperature range which varies from one alloy composition to another, typically between 150 and 350 ℃. In order to obtain high-strength steel with good plasticity and hole expansibility, the strip steel needs to be quenched to a certain temperature range below an Ms point, and according to theoretical calculation and practical test verification, the strip steel is quenched to a range less than or equal to 400 ℃ to obtain a structure with excellent comprehensive performance. When the quenching temperature is more than or equal to 400 ℃, the number of the residual austenite is large, but a bainite structure appears in the structure, and the strength requirement of 980MPa or more cannot be met. For the above reasons, the winding temperature needs to be controlled to be 400 ℃ or lower. Based on the innovative components and the technological design thought, 980 MPa-grade ultra-low carbon martensitic high-reaming steel with excellent strength, plasticity, toughness, cold bending and reaming performances can be obtained.
The invention has the beneficial effects that:
(1) The 980 MPa-grade high-reaming steel with excellent strength, plasticity, toughness, cold bending and reaming performance can be obtained by adopting a relatively economical component design thought and adopting an innovative cooling process path;
(2) The steel coil or the steel plate has excellent strength, plasticity and toughness matching, simultaneously has good cold bending performance and reaming and flanging performance, has yield strength of more than or equal to 800MPa, tensile strength of more than or equal to 980MPa, and has good elongation (transverse A 50 Not less than 8%), cold bending performance (d is not more than 4a,180 DEG), reaming performance (reaming rate is not less than 50%), can be applied to manufacturing parts such as automobile chassis, auxiliary frames and the like which need high-strength thinning and reaming flanging, and has very wide application prospect.
Drawings
FIG. 1 is a process flow chart of a 980 MPa-grade ultra-low carbon martensitic high-reaming steel manufacturing method;
FIG. 2 is a schematic drawing of a rolling process in the 980 MPa-grade ultra-low carbon martensitic high-reaming steel manufacturing method;
fig. 3 is a schematic diagram of a cooling process in the 980 MPa-grade ultra-low carbon martensitic high-reaming steel manufacturing method.
Detailed Description
Referring to fig. 1 to 3, the method for manufacturing 980 MPa-grade ultra-low carbon martensitic high-reaming steel comprises the following steps:
1) Smelting and casting
Smelting the components by adopting a converter or an electric furnace, secondarily refining by adopting a vacuum furnace, and casting into a casting blank or an ingot;
2) Reheating the casting blank or the cast ingot, wherein the heating temperature is 1100-1200 ℃, and the heat preservation time is 1-2 hours;
3) Hot rolling
Start rolling temperature: 950-1100 ℃, 3-5 passes of high reduction at 950 ℃ and accumulated deformation more than or equal to 50%, then the intermediate blank is heated to 920-950 ℃, and then the final 3-5 passes of rolling are carried out and accumulated deformation more than or equal to 70%; the final rolling temperature is 800-920 ℃;
4) Cooling
Firstly, performing air cooling for 0-10s to perform dynamic recovery and dynamic recrystallization, then cooling the strip steel to a certain temperature below the Ms point (between room temperature and the Ms point) at a cooling speed of more than or equal to 50 ℃/s, coiling, and cooling to room temperature after coiling;
5) Acid washing
The strip steel pickling operation speed can be adjusted within the interval of 30-100 m/min, the pickling temperature is controlled between 75-85 ℃, the withdrawal and straightening rate is controlled to be less than or equal to 2%, rinsing is carried out within the temperature interval of 35-50 ℃, and the strip steel surface is dried and oiled within the temperature interval of 120-140 ℃.
The components of the high reaming steel embodiment of the invention are shown in table 1, and table 2 and table 3 are production process parameters of the steel embodiment of the invention, wherein the thickness of a steel billet in the rolling process is 120mm; table 4 shows the mechanical properties of the steel sheet according to the example of the present invention.
As can be seen from Table 4, the yield strength of the steel coil is more than or equal to 800MPa, the tensile strength is more than or equal to 980MPa, the elongation is usually 8-13%, the impact power is stable, the low-temperature impact power at-40 ℃ is stable at 150-180J, the residual austenite content is changed along with the coiling temperature, the total content is changed between 2-5%, and the hole expansion rate is more than or equal to 50%.
From the embodiment, the 980MPa high-strength steel has good strength, plasticity, toughness and reaming performance matching, is particularly suitable for parts such as control arms and the like which need high-strength thinning and reaming flanging forming such as automobile chassis structures and the like, can also be used for parts such as wheels and the like which need hole flanging, and has wide application prospect.
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Claims (13)

1. A980 MPa grade ultra-low carbon martensite high-reaming steel comprises the following chemical components in percentage by weight: 0.03-0.06% of C, 0.53-2.0% of Si, 1.0-2.0% of Mn, less than or equal to 0.02% of P, less than or equal to 0.003% of S, 0.02-0.08% of Al, less than or equal to 0.004% of N, 0.1-0.5% of Mo, 0.01-0.05% of Ti, less than or equal to 0.0030% of O, and the balance of Fe and other unavoidable impurities;
the microstructure of the high-reaming steel is ultra-low carbon martensite and retained austenite, and the retained austenite accounts for 2-5%;
the yield strength of the high-reaming steel is more than or equal to 800MPa, the tensile strength is more than or equal to 980MPa, and the elongation is transverse A 50 The cold bending performance d is not less than 8 percent and 180 degrees is not more than 4a, and the hole expansion rate is not less than 50 percent; and is obtained by a process comprising:
1) Smelting and casting
Smelting the components by adopting a converter or an electric furnace, and casting into a casting blank or an ingot after secondary refining by a vacuum furnace;
2) The casting blank or the cast ingot is heated again, the heating temperature is 1100-1200 ℃, and the heat preservation time is 1-2 hours;
3) Hot rolling
Start rolling temperature: 950-1100 ℃, 3-5 times of high pressure at 950 ℃ and accumulated deformation more than or equal to 50%; then the intermediate blank is heated to 900-920 ℃, and then is rolled for the last 3-5 passes, and the accumulated deformation is more than or equal to 70%; the final rolling temperature is 800-920 ℃;
4) Cooling
Firstly performing air cooling for 4-10s, then cooling the strip steel to a room temperature-Ms point at a cooling speed of more than or equal to 50 ℃/s, coiling, and cooling to the room temperature after coiling;
5) Acid washing
The strip steel pickling operation speed is regulated within the interval of 30-100 m/min, the pickling temperature is controlled to be 75-85 ℃, the withdrawal and straightening rate is controlled to be less than or equal to 2%, and then the strip steel is rinsed, dried on the surface of the strip steel and oiled.
2. The 980 MPa-grade ultra-low carbon martensitic high-hole-enlarging steel of claim 1, further comprising one or more elements of Cr less than or equal to 0.5%, B less than or equal to 0.002%, ca less than or equal to 0.005%, nb less than or equal to 0.06%, V less than or equal to 0.05%, cu less than or equal to 0.5%, ni less than or equal to 0.5%.
3. The 980 MPa-grade ultra-low carbon martensitic high-reaming steel according to claim 2, wherein said Cr content is 0.2-0.4%, said B content is 0.0005-0.0015%, and said Ca content is not more than 0.002%; the Nb and V contents are respectively less than or equal to 0.03 percent; the Cu and Ni contents are respectively less than or equal to 0.3 percent.
4. 980 MPa-grade ultra-low carbon martensitic high-reamed steel as claimed in claim 1, wherein said C content is 0.04-0.055%.
5. 980 MPa-grade ultra-low carbon martensitic high-reamed steel as claimed in claim 1, wherein the Si content is 0.8-1.4%.
6. 980 MPa-grade ultra-low carbon martensitic high-reamed steel as claimed in claim 1, wherein the Mn content is 1.4-1.8%.
7. The 980 MPa-grade ultra-low carbon martensitic high-reaming steel of claim 1, wherein said S content is controlled below 0.0015%.
8. 980 MPa-grade ultra-low carbon martensitic high-reaming steel according to claim 1, characterized in that said Al content is comprised between 0.02 and 0.05%.
9. The 980 MPa-grade ultra-low carbon martensitic high-reaming steel of claim 1, wherein said N content is controlled below 0.003%.
10. 980 MPa-grade ultra-low carbon martensitic high-reaming steel according to claim 1, characterized in that said Ti content is comprised between 0.01 and 0.03%.
11. 980 MPa-grade ultra-low carbon martensitic high-reaming steel according to claim 1, characterized in that said Mo content is between 0.15 and 0.35%.
12. The method for manufacturing 980 MPa-grade ultra-low carbon martensitic high-reaming steel according to any one of claims 1 to 11, characterized by: the method comprises the following steps:
1) Smelting and casting
Smelting the components according to any one of claims 1-11 by adopting a converter or an electric furnace, and casting into a casting blank or an ingot after secondary refining by a vacuum furnace;
2) The casting blank or the cast ingot is heated again, the heating temperature is 1100-1200 ℃, and the heat preservation time is 1-2 hours;
3) Hot rolling
Start rolling temperature: 950-1100 ℃, 3-5 times of high pressure at 950 ℃ and accumulated deformation more than or equal to 50%; then the intermediate blank is heated to 900-920 ℃, and then is rolled for the last 3-5 passes, and the accumulated deformation is more than or equal to 70%; the final rolling temperature is 800-920 ℃;
4) Cooling
Firstly performing air cooling for 4-10s, then cooling the strip steel to a room temperature-Ms point at a cooling speed of more than or equal to 50 ℃/s, coiling, and cooling to the room temperature after coiling;
5) Acid washing
The strip steel pickling operation speed is regulated within the interval of 30-100 m/min, the pickling temperature is controlled to be 75-85 ℃, the withdrawal and straightening rate is controlled to be less than or equal to 2%, and then the strip steel is rinsed, dried on the surface of the strip steel and oiled.
13. The method for manufacturing 980 MPa-grade ultra-low carbon martensitic high-reaming steel as claimed in claim 12, wherein step 5) is performed with rinsing at a temperature ranging from 35 ℃ to 50 ℃ and surface drying and oiling at a temperature ranging from 120 ℃ to 140 ℃.
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EP21860562.4A EP4206350A4 (en) 2020-08-31 2021-08-30 High-strength low-carbon martensitic high hole expansion steel and manufacturing method therefor
PCT/CN2021/115431 WO2022042730A1 (en) 2020-08-31 2021-08-30 High-strength low-carbon martensitic high hole expansion steel and manufacturing method therefor
KR1020237009927A KR20230061413A (en) 2020-08-31 2021-08-30 High-strength low-carbon martensitic steel with high hole expandability and manufacturing method thereof
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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN105506494A (en) * 2014-09-26 2016-04-20 宝山钢铁股份有限公司 High-toughness hot-rolled high-strength steel with yield strength being 800 MPa and manufacturing method of high-toughness hot-rolled high-strength steel
CN110475889A (en) * 2017-03-31 2019-11-19 日本制铁株式会社 Hot rolled steel plate and steel forged part and its manufacturing method
CA3110823A1 (en) * 2018-09-20 2020-03-26 Arcelormittal Hot rolled steel sheet with high hole expansion ratio and manufacturing process thereof

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN105506494A (en) * 2014-09-26 2016-04-20 宝山钢铁股份有限公司 High-toughness hot-rolled high-strength steel with yield strength being 800 MPa and manufacturing method of high-toughness hot-rolled high-strength steel
CN110475889A (en) * 2017-03-31 2019-11-19 日本制铁株式会社 Hot rolled steel plate and steel forged part and its manufacturing method
CA3110823A1 (en) * 2018-09-20 2020-03-26 Arcelormittal Hot rolled steel sheet with high hole expansion ratio and manufacturing process thereof

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