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CN114107798A - 980 MPa-grade bainite high-reaming steel and manufacturing method thereof - Google Patents

980 MPa-grade bainite high-reaming steel and manufacturing method thereof Download PDF

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Publication number
CN114107798A
CN114107798A CN202010897959.5A CN202010897959A CN114107798A CN 114107798 A CN114107798 A CN 114107798A CN 202010897959 A CN202010897959 A CN 202010897959A CN 114107798 A CN114107798 A CN 114107798A
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percent
steel
equal
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content
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CN202010897959.5A
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Chinese (zh)
Inventor
王焕荣
杨峰
张晨
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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Application filed by Baoshan Iron and Steel Co Ltd filed Critical Baoshan Iron and Steel Co Ltd
Priority to CN202010897959.5A priority Critical patent/CN114107798A/en
Priority to US18/043,267 priority patent/US20230313333A1/en
Priority to KR1020237009928A priority patent/KR20230059810A/en
Priority to EP21860563.2A priority patent/EP4206351A4/en
Priority to JP2023513802A priority patent/JP2023539650A/en
Priority to PCT/CN2021/115433 priority patent/WO2022042731A1/en
Publication of CN114107798A publication Critical patent/CN114107798A/en
Pending legal-status Critical Current

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    • 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
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel

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Abstract

A980 MPa bainite high-reaming steel and a manufacturing method thereof comprise the following chemical components in percentage by weight: 0.05 to 0.10 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, more than or equal to 0.1 percent of Mo, 0.01 to 0.05 percent of Ti, 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 0.0030 percent of O, and the balance of Fe and other inevitable impurities. The yield strength of the high-hole-expansion steel is more than or equal to 800MPa, the tensile strength is more than or equal to 980MPa, and the high-hole-expansion steel has good elongation (in the transverse direction A)50Not less than 11 percent) and hole expansion performance (the hole expansion rate is not less than 40 percent), can be applied to parts needing high strength thinning, such as a control arm, an auxiliary frame and the like of a chassis of a passenger car.

Description

980 MPa-grade bainite high-reaming steel and manufacturing method thereof
The technical field is as follows:
the invention relates to the field of high-strength steel, in particular to 980 MPa-grade bainite 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 requirements of parts of many vehicle types in the domestic automobile industry require the use of hot-rolled or pickled plates, such as chassis parts, torsion beams, auxiliary frames of cars, wheel spokes and rims, front and rear axle assemblies, body structural parts, seats, clutches, safety belts, truck box plates, protective nets, automobile girders and other parts of automobiles. Wherein, the proportion of the chassis steel to the total steel used by the car can reach 24 to 34 percent.
The light weight of passenger cars is not only a development trend in the automotive industry, but also a requirement of legal regulations. The fuel consumption is regulated by laws and regulations, the weight of a vehicle body is required to be reduced in a phase-changing manner, and the requirement reflected on materials is high strength, thinning and light weight. High strength subtracts heavy is the inevitable requirement of follow-up new motorcycle type, and this must lead to the fact with the steel grade higher, also must bring the change on the chassis structure: if the parts are more complex, the requirements on material performance, surface and the like and the forming technology are improved, such as hydraulic forming, hot stamping, laser welding and the like, and the performances of high strength, stamping, flanging, resilience, fatigue and the like of the material are further converted.
Compared with the foreign countries, the development of domestic high-strength high-hole-expansion steel has relatively lower strength level and poor performance stability. For example, high-expansion-hole steel used by domestic automobile part enterprises is basically high-strength steel with the tensile strength of below 600MPa, and high-expansion-hole steel with the tensile strength of below 440MPa competes for whitening. High hole expansion steel with 780 MPa-grade tensile strength is gradually used in batch at present, but higher requirements are provided for two important indexes of forming elongation and hole expansion rate. And 980 MPa-grade high-reaming steel is still in a research and development certification stage at present and does not reach a batch use stage. However, 980 h-bore steel with higher strength and higher hole expansion rate is a necessary development trend in the future. In order to better meet the potential future demands of users, 980MPa grade high hole-expanding steel with good hole-expanding performance needs to be developed.
At present, most of the related patent documents are high hole-expanding steel with the grade of 780MPa and below. Few documents are related to 980MPa grade high-hole-expansion steel. Chinese patent CN106119702A discloses 980 MPa-grade hot-rolled high-reaming steel, which is mainly characterized by low-carbon V-Ti microalloying design, wherein the microstructure is granular bainite and a small amount of martensite, and trace Nb and Cr are added. Are very different from the present invention in terms of composition, process, and organization.
As known from the literature, in general, the elongation of a material is in inverse proportion to the hole expansion rate, i.e., the higher the elongation, the lower the hole expansion rate; conversely, the lower the elongation, the higher the hole expansion ratio. It is very difficult to obtain high elongation high hole expansion steel having high strength. Further, the higher the strength of the material, the lower the hole expansion ratio, under the same or similar strengthening mechanisms.
In order to obtain a steel material having good plasticity and burring performance, a better balance between the two is required. Of course, the rate of hole enlargement 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 measurements of the rate of hole enlargement. Generally, a single homogeneous structure is advantageous for achieving higher porosities, while a bi-or multi-phasic structure is generally disadvantageous for increasing the porosities.
Disclosure of Invention
The invention aims to provide 980 MPa-grade bainite high-hole-expansion steel and a manufacturing method thereof, wherein the yield strength of the high-hole-expansion steel is more than or equal to 800MPa, the tensile strength of the high-hole-expansion steel is more than or equal to 980MPa, and the high-hole-expansion steel has good elongation (transverse A)50Not less than 11 percent) and hole expansion performance (the hole expansion rate is not less than 40 percent), can be applied to parts needing high strength thinning, such as a control arm, an auxiliary frame and the like of a chassis of a passenger car.
In order to achieve the purpose, the technical scheme of the invention is as follows:
the invention adopts the lower C content in the component design, which can ensure that the welding performance is excellent when the user uses the welding tool and the obtained martensite structure has good hole expanding performance and impact toughness; on the basis of meeting the requirement that the tensile strength is more than or equal to 980MPa, the lower the carbon content is, the better the carbon content is; designing higher Si content, and obtaining more residual austenite by matching with the process, thereby improving the plasticity of the material; meanwhile, the higher Si content is beneficial to reducing the non-recrystallization temperature of the steel, so that the steel can finish the dynamic recrystallization process within a wider finish rolling temperature range, thereby improving the structural anisotropy of the steel, refining austenite grains and the final bainite lath size, and improving the plasticity and the hole expansion rate.
Specifically, the 980MPa bainite high-reaming steel comprises the following chemical components in percentage by weight: 0.05 to 0.10 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, more than or equal to 0.1 percent of Mo, 0.01 to 0.05 percent of Ti, 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 0.0030 percent of O, and the balance of Fe and other inevitable impurities.
Further, one or more elements of Nb less than or equal to 0.06 percent, V less than or equal to 0.05 percent, Cu less than or equal to 0.5 percent, Ni less than or equal to 0.5 percent and Ca less than or equal to 0.005 percent are also contained; wherein, the content of Nb and V is preferably less than or equal to 0.03 percent respectively; the content of Cu and Ni is preferably less than or equal to 0.3 percent respectively, and the content of Ca is preferably less than or equal to 0.002 percent.
In the composition design of the high hole expansion steel of the invention:
carbon is an essential element in steel and is also one of the important elements in the present invention. Carbon expands the austenite phase region and stabilizes austenite. Carbon, which is an interstitial atom in steel, plays a very important role in increasing the strength of steel, and has the greatest influence on the yield strength and tensile strength of steel. In the invention, because the obtained structure is low-carbon bainite, in order to obtain high-strength steel with tensile strength reaching 980MPa, the content of carbon is required to be ensured to be more than 0.05 percent, otherwise, the content of carbon is less than 0.05 percent, and the tensile strength can not reach 980MPa even if the steel is completely quenched to room temperature; but the carbon content cannot be higher than 0.10%. The content of carbon is too high, the strength of the formed bainite is too high, and more maroland is easy to appear in the structure, which is not good for both elongation and hole expansion rate. Therefore, the carbon content should be controlled to be between 0.05 and 0.10%, and preferably in the range of 0.06 to 0.08%.
Silicon is an essential element in steel and is also one of the important elements in the present invention. The Si content is increased, so that the solid solution strengthening effect is improved, and more importantly, the following two effects are achieved. Firstly, the non-recrystallization temperature of the steel is greatly reduced, and the dynamic recrystallization of the steel can be completed within a wide temperature range. Therefore, in the actual rolling process, the final rolling temperature can be within the final rolling temperature range of 800-920 ℃, so that the difference of the transverse and longitudinal tissues is reduced, the strength and the plasticity are favorably improved, and the good hole expanding rate is favorably obtained; the other important function of Si is to inhibit cementite from precipitating, and a certain amount of residual austenite can be reserved under the condition of a proper rolling process, particularly when a structure mainly comprising bainite is obtained, so that the elongation is improved. This effect of Si must be exhibited when its content reaches 0.5% or more; but the content of Si is not too high, otherwise, the rolling force load is too large in the actual rolling process, and the stable production of products is not facilitated. Therefore, the Si content in the steel is usually controlled to be between 0.5 and 2.0%, and preferably in the range of 0.8 to 1.6%.
Manganese, the most basic element in steel, is also one of the most important elements in the present invention. Mn is an important element for expanding an austenite phase region, and can reduce the critical cooling speed of steel, stabilize austenite, refine grains and delay the transformation from austenite to pearlite. However, in the present invention, because a certain amount of molybdenum is added, the effect of molybdenum on the retardation of ferrite and pearlite and the reduction of the critical cooling rate is much greater than that of manganese. Therefore, the content of manganese in steel can be properly reduced and is generally controlled to be more than 1.0 percent; meanwhile, the Mn content is generally not more than 2.0%, otherwise Mn segregation is likely to occur during steel making, and hot cracking is also likely to occur during slab continuous casting. Therefore, the Mn content in the steel is generally controlled to 1.0 to 2.0%, preferably in the range of 1.4 to 1.8%.
Phosphorus, an impurity element in steel. P is easy to be partially gathered on the grain boundary, and Fe is formed when the content of P in steel is higher (more than or equal to 0.1 percent)2P is precipitated around the crystal grains to reduce the plasticity and toughness of the steel, so the lower the content of the P is, the better the P content is generally controlled within 0.02 percent, and the steelmaking cost is not increased.
Sulfur, an impurity element in steel. S in steel is usually combined with Mn to form MnS inclusions, and particularly when the contents of S and Mn are high, the steel forms more MnS, and 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 structural anisotropy is increased, and the hole expansion performance is not favorable. Therefore, the lower the S content in the steel, the better, considering that the Mn content in the present invention must be at a high level, the S content is strictly controlled in order to reduce the MnS content, and the S content is required to be controlled to be within 0.003%, and preferably to be within 0.0015%.
The role of aluminum in steel is mainly deoxidation and nitrogen fixation. In the presence of strong carbide forming elements such as Ti, etc., Al mainly functions to deoxidize and refine grains. In the invention, Al is taken as a common deoxidizing element and an element for refining grains, and the content of Al is usually controlled to be 0.02-0.08%; the Al content is lower than 0.02 percent, and the effect of refining grains is not achieved; similarly, when the Al content is higher than 0.08%, the effect of refining grains is saturated. Therefore, the Al content in the steel may be controlled to be 0.02 to 0.08%, and preferably 0.02 to 0.05%.
Nitrogen, which is an impurity element in the present invention, is preferably contained in a lower amount. Nitrogen is an unavoidable element in the steel making process. Although the content thereof is small, the formed TiN particles, in combination with a strong carbide forming element such as Ti or the like, have a very adverse effect on the properties of the steel, particularly on the hole expansibility. Because TiN is square, great stress concentration exists between the sharp corner and the substrate, and cracks are easily formed by the stress concentration between the TiN and the substrate in the reaming deformation process, so that the reaming performance of the material is greatly reduced. On the premise of controlling the nitrogen content as much as possible, the lower the content of the element forming strong carbide such as Ti, the better. In the present invention, a trace amount of Ti is added to fix nitrogen, and the adverse effect of TiN is minimized. Therefore, the nitrogen content should be controlled to 0.004% or less, and preferably 0.003% or less.
Titanium is one of important elements in the present invention. Ti plays two main roles in the present invention: firstly, the nitrogen-fixing agent is combined with impurity element N in steel to form TiN, and plays a part of the role of nitrogen fixation; secondly, a certain amount of TiN with fine dispersion is formed in the subsequent welding process of the material, thereby inhibiting the size of austenite grains, refining the structure and improving the low-temperature toughness. Therefore, the Ti content in the steel is controlled in the range of 0.01 to 0.05%, and preferably in the range of 0.01 to 0.03%.
Molybdenum, is one of the important elements in the present invention. The molybdenum is added into the steel, so that the phase transformation of ferrite and pearlite can be greatly delayed, and the bainite structure can be obtained in a medium-high temperature range; meanwhile, the addition of molybdenum can also improve the structural and performance stability of the steel and refine grains. The effect of the molybdenum is beneficial to adjusting various processes in the actual rolling process, such as sectional cooling after finishing the final rolling, air cooling before water cooling and the like. In the invention, two modes of air cooling or direct cooling after rolling are adopted, and in the air cooling process, the addition of molybdenum can ensure that structures such as ferrite, pearlite and the like cannot be formed in the air cooling process; on the other hand, the dynamic recovery of the deformed austenite in the air cooling process is beneficial to improving the uniformity of the structure and the performance and is beneficial to the hole expansion performance. The molybdenum content needs to be more than 0.10% for its ferrite and pearlite formation inhibiting effect. Therefore, the content of molybdenum should be controlled to be 0.10% or more, preferably 0.15% or more.
Chromium is one of the important elements in the present invention. Chromium is not intended to increase the hardenability of steel in the present invention, but is intended to be combined with B, which is advantageous for forming an acicular ferrite structure in the weld heat affected zone after welding, and can greatly improve the low-temperature toughness of the weld heat affected zone. Since the final application parts related to the invention are passenger car chassis products, the low-temperature toughness of the welding heat affected zone is an important index. Besides ensuring that the strength of the welding heat affected zone cannot be reduced too much, the low-temperature toughness of the welding heat affected zone also meets certain requirements. In addition, chromium itself has some resistance to solder softening. Therefore, a small amount of chromium is added to the steel, generally in the range of 0.5% or less, preferably in the range of 0.2 to 0.4%.
Boron mainly has the function of being segregated at austenite grain boundaries in steel and inhibiting the formation of proeutectoid ferrite; boron added to steel can also greatly improve the hardenability of steel. However, in the present invention, the trace amount of boron is added not mainly for the purpose of enhancing hardenability but for the purpose of improving the structure of the weld heat affected zone in combination with chromium to obtain an acicular ferrite structure having good toughness. The addition of boron in the steel is generally controlled below 0.002%, and the preferred range is 0.0005-0.0015%.
Calcium, an added element in the present invention. Calcium can improve the form of sulfides such as MnS, so that elongated sulfides such as MnS and the like are changed into spherical CaS, the inclusion form is favorably improved, the adverse effect of the elongated sulfides on the hole expanding performance is further reduced, but the addition of excessive calcium can increase the amount of calcium oxide, and is adverse to the hole expanding performance. Therefore, the addition amount of calcium in steel grades is usually less than or equal to 0.005%, and the preferable range is less than or equal to 0.002%.
Oxygen, which is an inevitable element in the steel making process, is an essential element in the present invention, and the content of O in steel after deoxidation is generally 30ppm or less, and does not cause significant adverse effects on the properties of the steel sheet. Therefore, the O content in the steel is controlled to be within 30 ppm.
Niobium, is one of the elements that may be added in the present invention. Niobium is similar to titanium and is a strong carbide element in steel, niobium is added into the steel to greatly improve the non-recrystallization temperature of the steel, deformed austenite with higher dislocation density can be obtained in the finish rolling stage, and the final phase change structure can be refined in the subsequent transformation process. However, the addition amount of niobium is not so large that the addition amount of niobium exceeds 0.06%, which tends to form relatively coarse carbonitrides of niobium in the microstructure, consume part of carbon atoms, and reduce the precipitation strengthening effect of carbides. Meanwhile, the niobium content is high, the anisotropy of hot-rolled austenite structures is easily caused, and the anisotropy is transmitted to final structures in the subsequent cooling phase change process, so that the reaming performance is not good. Therefore, the niobium content in the steel is usually controlled to 0.06% or less, and preferably in the range of 0.03% or less.
Vanadium, is an additive element in the present invention. Vanadium, like titanium and niobium, is also a strong carbide former. However, vanadium carbides are low in solid solution or precipitation temperature, and are usually all solid-dissolved in austenite in the finish rolling stage. Only when the temperature is lowered to initiate transformation is vanadium to begin to form in the ferrite. Because the solid solubility of vanadium carbide in ferrite is larger than that of niobium and titanium, the vanadium carbide has larger size in ferrite, is not beneficial to precipitation strengthening and contributes far less to the strength of steel than titanium, but because certain carbon atoms are consumed in the formation of vanadium carbide, the steel strength is not beneficial to improvement. Therefore, the amount of vanadium added to the steel is usually 0.05% or less, preferably 0.03% or less.
Copper, which is an additive element in the present invention. The corrosion resistance of the steel can be improved by adding the copper into the steel, and the corrosion resistance effect is better when the copper and the P element are added together; when the addition amount of Cu exceeds 1%, an epsilon-Cu precipitated phase can be formed under certain conditions, and a strong precipitation strengthening effect is achieved. However, addition of Cu is likely to cause the phenomenon of "Cu embrittlement" during rolling, and in order to fully utilize the effect of Cu on improving corrosion resistance in some applications without causing significant "Cu embrittlement", the content of Cu element is usually controlled to be within 0.5%, preferably within 0.3%.
Nickel, which is an additive element in the present invention. The nickel added into the steel has certain corrosion resistance, but the corrosion resistance effect is weaker than that of copper, the nickel added into the steel has little influence on the tensile property of the steel, but the structure and the precipitated phase of the steel can be refined, and the low-temperature toughness of the steel is greatly improved; meanwhile, in the steel added with copper element, a small amount of nickel is added to inhibit the generation of Cu brittleness. The addition of higher nickel has no significant adverse effect on the properties of the steel itself. If copper and nickel are added simultaneously, not only can the corrosion resistance be improved, but also the structure and precipitated phase of the steel are refined, and the low-temperature toughness is greatly improved. However, both copper and nickel are relatively expensive alloying elements. Therefore, in order to minimize the cost of alloy design, the amount of nickel added is usually 0.5% or less, preferably 0.3% or less.
The invention relates to a method for manufacturing 980MPa bainite high-reaming steel, which comprises the following steps:
1) smelting and casting
Smelting by adopting a converter or an electric furnace, secondarily refining by adopting a vacuum furnace, and then casting into a casting blank or an ingot;
2) the casting blank or the cast ingot is heated again at the temperature of 1100 ℃ and 1200 ℃, and the heat preservation time is 1-2 hours;
3) hot rolling
The initial rolling temperature: at 950-1100 ℃, under 3-5 times of large pressure above 950 ℃ and the accumulated deformation is more than or equal to 50 percent, and the main purpose is to refine austenite grains; then, the intermediate blank is heated to 920 ℃ and 950 ℃, and then the final 3-7 passes of rolling are carried out, and the accumulated deformation is more than or equal to 70 percent; the finishing temperature is 800 ℃ and 920 ℃;
4) cooling down
Firstly, air cooling is carried out for 0-10s to carry out dynamic recovery so as to enable deformed austenite to be more uniform, then the strip steel is cooled to 400-550 ℃ by water at a cooling speed of more than or equal to 10 ℃/s for coiling, and the strip steel is cooled to room temperature after being coiled;
5) acid pickling
The pickling speed of the strip steel is adjusted within the range of 30-100 m/min, the pickling temperature is controlled within the range of 75-85 ℃, the withdrawal and straightening rate is controlled to be less than or equal to 2% so as to reduce the elongation loss of the strip steel, and then the strip steel is rinsed, dried on the surface of the strip steel and coated with oil.
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 the surface of the strip steel is dried and oiled at the temperature of 120-140 ℃.
The innovation points of the invention are as follows:
the invention adopts the lower C content in the component design, which can ensure that the welding performance is excellent when the user uses the welding tool and the obtained martensite structure has good hole expanding performance and impact toughness; on the basis of meeting the requirement that the tensile strength is more than or equal to 980MPa, the lower the carbon content is, the better the carbon content is; designing higher Si content, and obtaining more residual austenite by matching with the process, thereby improving the plasticity of the material; meanwhile, the higher Si content is beneficial to reducing the non-recrystallization temperature of the steel, so that the steel can finish the dynamic recrystallization process within a wider finish rolling temperature range, thereby improving the structural anisotropy of the steel, refining austenite grains and the final bainite lath size, and improving the plasticity and the hole expansion rate.
The structural design adopts a low-carbon bainite design idea, higher silicon is added to inhibit and reduce the formation of cementite, the non-recrystallization temperature is reduced, the final rolling temperature range is expanded, and a bainite structure with fine and uniform grains can be obtained by directly cooling or air cooling for a certain time after rolling and then cooling, and simultaneously contains a small amount of residual austenite or martensite components. The bainite structure gives the steel plate higher strength, and the retained austenite gives the steel plate higher plasticity, and the combination of the bainite structure and the retained austenite enables the steel plate to show good strength, plasticity and hole expansion matching.
In the design of the rolling process, the rhythm of the rolling process is required to be completed as fast as possible in the stages of rough rolling and finish rolling. After finishing rolling, firstly carrying out air cooling for a certain time and then carrying out water cooling or directly carrying out water cooling. The main purpose of air cooling is as follows: because certain manganese and molybdenum are contained in the composition design, the manganese is an element for stabilizing austenite, and the molybdenum greatly delays the transformation of ferrite and pearlite and promotes the transformation of bainite. Therefore, in the short space-time cooling process, the rolled deformed austenite does not undergo phase transformation, i.e., ferrite structure is not formed, but a dynamic recovery process occurs. Dislocation in the austenite grains after dynamic recovery can be greatly reduced, the austenite structure is more uniform, and the bainite structure formed in the subsequent phase transformation process is more uniform. In order to avoid ferrite formation in the continuous cooling process, the water cooling speed of the strip steel is required to be more than or equal to 10 ℃/s.
In order to obtain a single-phase uniform bainite structure, the steel strip needs to be cooled to a bainite transformation temperature range, and in the invention, the bainite transformation temperature range is 400-550 ℃ according to different components. Within the temperature range, as the coiling temperature is reduced, the bainite lath is smaller, the structure is relatively more uniform, the strength is increased, and the plasticity is reduced; conversely, as the coiling temperature increases, lath bainite may be partially transformed into granular bainite in the structure, resulting in a decrease in strength and an increase in plasticity. Theoretical calculation and experiments prove that bainite structures with excellent comprehensive performance can be obtained by cooling the strip steel to the temperature of 400-550 ℃. When the coiling temperature is more than or equal to 550 ℃, a relatively coarse upper bainite can be formed in the structure, and the strength requirement of over 980MPa cannot be met; when the coiling temperature is less than or equal to 400 ℃, the structure can be transformed into martensite. For the above reasons, the coiling temperature needs to be controlled between 400 ℃ and 550 ℃. Based on the innovative components and process design thought, 980MPa grade high-expansion-ratio steel with excellent strength, plasticity and hole expansion performance can be obtained.
The invention has the beneficial effects that:
(1) by adopting a relatively economic component design idea, if precious metal elements are not added or are less added, and an innovative cooling process path is adopted, 980MPa grade high-hole-expansion steel with excellent strength, plasticity, toughness, cold bending and hole expansion performance can be obtained;
(2) the steel coil or the steel plate has excellent matching of strength, plasticity and reaming and flanging performance, the yield strength is more than or equal to 800MPa, the tensile strength is more than or equal to 980MPa, and the steel coil or the steel plate has good elongation (transverse A)50Not less than 11 percent) and hole expansion performance (the hole expansion rate is not less than 40 percent), can be applied to the manufacture of parts needing high-strength thinning and hole expansion flanging, such as automobile chassis, auxiliary frames and the like, and has very wide application prospect.
Drawings
FIG. 1 is a process flow diagram of a method for manufacturing 980MPa grade bainite high-expansion steel according to the present invention;
FIG. 2 is a schematic diagram of a rolling process in the method for manufacturing 980MPa bainite high-expansion steel according to the present invention;
FIG. 3 is a schematic view of a cooling process in the method for manufacturing 980MPa grade bainite high-expansion steel according to the present invention;
FIG. 4 is a typical metallographic structure photograph of 980MPa Bainite high-hole-expansion steel of example 2 according to the present invention;
FIG. 5 is a typical metallographic structure photograph of 980MPa Bainite high-hole-expansion steel according to example 4 of the present invention;
FIG. 6 is a typical metallographic structure photograph of 980MPa Bainite high-hole-expansion steel according to example 6 of the present invention;
FIG. 7 is a typical metallographic structure photograph of 980MPa Bainite high hole-expansion steel of example 8.
Detailed Description
Referring to fig. 1 to 3, the method for manufacturing 980MPa grade bainite high hole expansion steel according to the present invention includes the following steps:
1) smelting and casting
Smelting by adopting a converter or an electric furnace, secondarily refining by adopting a vacuum furnace, and then casting into a casting blank or an ingot;
2) the casting blank or the cast ingot is heated again at the temperature of 1100 ℃ and 1200 ℃, and the heat preservation time is 1-2 hours;
3) hot rolling
The initial rolling temperature: at 950-1100 ℃, under 3-5 times of large pressure above 950 ℃ and the accumulated deformation is more than or equal to 50 percent, then the intermediate billet is heated to 920 ℃ and 950 ℃, and then the final 3-7 times of rolling are carried out and the accumulated deformation is more than or equal to 70 percent; the finishing temperature is 800 ℃ and 920 ℃;
4) cooling down
Firstly, air cooling is carried out for 0-10s to carry out dynamic recovery so as to enable deformed austenite to be more uniform, then the strip steel is cooled to 400-550 ℃ by water at a cooling speed of more than or equal to 10 ℃/s for coiling, and the strip steel is cooled to room temperature after being coiled;
5) acid pickling
The pickling speed of the strip steel is adjusted within the range of 30-100 m/min, the pickling temperature is controlled within the range of 75-85 ℃, the withdrawal and straightening rate is controlled to be less than or equal to 2%, rinsing is carried out within the temperature range of 35-50 ℃, and surface drying and oiling are carried out within the temperature range of 120-140 ℃.
The components of the high hole expansion steel embodiment of the invention are shown in table 1, and table 2 shows the production process parameters of the steel embodiment of the invention, wherein the thickness of a billet in the rolling process is 120 mm; table 3 shows the mechanical properties of the steel sheets of examples of the present invention.
As can be seen from Table 3, 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 between 10 and 13 percent, and the hole expansion rate is more than or equal to 40 percent.
Typical metallographic photographs of examples 2, 4, 6 and 8 are given in figures 4 to 7, respectively. It can be seen that the typical microstructure is bainite and contains small amounts of retained austenite or martensite.
The embodiment shows that the 980MPa high-reaming steel has good matching of strength, plasticity and reaming performance, is particularly suitable for parts such as automobile chassis structures and the like which need high-strength thinning and reaming flanging forming, such as control arms and the like, and can also be used for parts such as wheels and the like which need hole flanging, and has wide application prospect.
Figure BDA0002658983970000111
Figure BDA0002658983970000121
Figure BDA0002658983970000131
Figure BDA0002658983970000141

Claims (16)

1. A980 MPa-grade bainite high-reaming steel comprises the following chemical components in percentage by weight: 0.05 to 0.10 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, more than or equal to 0.1 percent of Mo, 0.01 to 0.05 percent of Ti, 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 0.0030 percent of O, and the balance of Fe and other inevitable impurities.
2. The 980MPa grade bainite high hole expansion steel according to claim 1, further including one or more elements selected from Nb 0.06%, V0.05%, Cu 0.5%, Ni 0.5%, and Ca 0.005%; wherein, the content of Nb and V is preferably less than or equal to 0.03 percent respectively; the content of Cu and Ni is preferably less than or equal to 0.3 percent respectively, and the content of Ca is preferably less than or equal to 0.002 percent.
3. The 980MPa grade bainitic high-bore-expansion steel of claim 1, wherein the C content is 0.06-0.08%.
4. The 980MPa grade bainitic high-bore-expansion steel of claim 1, wherein the Si content is 0.8-1.6%.
5. The 980MPa grade bainitic high-bore-expansion steel of claim 1, wherein the Mn content is 1.4-1.8%.
6. The 980MPa grade bainitic high-bore-expansion steel of claim 1, wherein the S content is controlled to be less than 0.0015%.
7. The 980MPa grade bainitic high-bore-expansion steel of claim 1, wherein the Al content is 0.02-0.05%.
8. The 980MPa grade bainitic high-bore-expansion steel of claim 1, wherein the N content is controlled to 0.003% or less.
9. The 980MPa grade bainitic high-bore-expansion steel of claim 1, wherein the Ti content is 0.01-0.03%.
10. The 980MPa grade bainitic high-reaming steel according to claim 1, wherein the Mo content is 0.15% or more.
11. The 980MPa grade bainitic high-bore-expansion steel of claim 1, wherein the Cr content is 0.2-0.4%.
12. The 980MPa grade bainitic high-bore-expansion steel of claim 1, wherein the B content is 0.0005-0.0015%.
13. The 980MPa grade bainitic high bore expanding steel of claim 1, wherein the microstructure of the high bore expanding steel is bainite + retained austenite.
14. 980MPa grade bainitic high-reaming steel according to claim 1 or 13, characterized in that it has a yield strength of 800MPa or more, a tensile strength of 980MPa or more, and a transverse elongation a50More than or equal to 10 percent, cold bending performance (d is less than or equal to 4a and 180 degrees), and hole expanding rate is more than or equal to 50 percent.
15. The method of manufacturing 980MPa grade bainitic high-expansion steel according to any one of claims 1 to 14, wherein: the method comprises the following steps:
1) smelting and casting
Smelting by a converter or an electric furnace and performing secondary refining by a vacuum furnace according to the components of the alloy according to the claims 1-13, and then casting into a casting blank or an ingot;
2) the casting blank or the cast ingot is heated again at the temperature of 1100 ℃ and 1200 ℃, and the heat preservation time is 1-2 hours;
3) hot rolling
The initial rolling temperature: at 950-1100 ℃, under 3-5 times of large pressure above 950 ℃, the accumulated deformation is more than or equal to 50 percent; then, the intermediate blank is heated to 900 ℃ and 920 ℃, and then the final 3-7 passes of rolling are carried out, and the accumulated deformation is more than or equal to 70 percent; the finishing temperature is 800 ℃ and 920 ℃;
4) cooling down
Firstly, air cooling is carried out for 0-10s, then the strip steel is cooled to 400-550 ℃ by water at the cooling speed of more than or equal to 10 ℃/s for coiling, and the strip steel is cooled to room temperature after coiling;
5) acid pickling
Adjusting the strip steel pickling operation speed within the range of 30-100 m/min, controlling the pickling temperature to be 75-85 ℃, controlling the withdrawal and straightening rate to be less than or equal to 2%, rinsing, drying the surface of the strip steel, and coating oil.
16. The method for manufacturing 980MPa grade bainite high-hole-expansion steel as claimed in claim 15, wherein the pickling in step 5) is followed by rinsing at 35-50 ℃, surface drying at 120-140 ℃ and oiling.
CN202010897959.5A 2020-08-31 2020-08-31 980 MPa-grade bainite high-reaming steel and manufacturing method thereof Pending CN114107798A (en)

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