CN114107788B

CN114107788B - 980 MPa-grade tempered martensite type high-reaming steel and manufacturing method thereof

Info

Publication number: CN114107788B
Application number: CN202010896408.7A
Authority: CN
Inventors: 王焕荣; 杨峰; 张晨; 杨阿娜; 柏明卓
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2020-08-31
Filing date: 2020-08-31
Publication date: 2023-04-11
Anticipated expiration: 2040-08-31
Also published as: CN114107788A

Abstract

A980 MPa-level tempered martensite high-reaming steel and a manufacturing method thereof are disclosed, wherein the steel comprises the following chemical components in percentage by weight: 0.06 to 0.10 percent of C, 0.8 to 2.0 percent of Si, 1.5 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 inevitable impurities. The high-hole-expansion steel has excellent ultrahigh strength, plasticity and toughness matching, and good hole-expansion flanging performance, the yield strength is more than or equal to 800MPa, the tensile strength is more than or equal to 980MPa, and the elongation (transverse A) ₅₀ Not less than 10 percent), impact toughness 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 wide application prospect.

Description

980 MPa-grade tempered martensite type 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 tempered martensite type high-reaming steel and a manufacturing method thereof.

Background

Along with the development of national economy, the production of automobiles is also 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, and 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 lightening. 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-hole-expansion steel used in domestic automobile part enterprises is basically high-strength steel with the tensile strength of below 600MPa, and high-hole-expansion steel with the grade 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. To better meet the potential needs of users, it is necessary to develop 980MPa grade high hole expansion steel with higher strength grade.

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 in that the component design is low-carbon V-Ti microalloying design, the microstructure is granular bainite and a small amount of martensite, and trace Nb and Cr are added. A large number of researches indicate that the elongation rate of the material is in inverse proportion to the hole expansion rate, namely the higher the elongation rate is, the lower the hole expansion rate is; conversely, the lower the elongation, the higher the hole expansion ratio. Also, the higher the strength of the material, the lower the hole expansion ratio. In order to obtain a steel material having good plasticity and hole-enlarging and flanging performance, the relationship between the two needs to be better balanced. A single homogeneous structure is advantageous for achieving higher porosities, whereas a bi-or multiphase structure is generally disadvantageous for increasing the porosities.

Disclosure of Invention

The invention aims to provide 980 MPa-grade tempered martensite high-reaming steel and a manufacturing method thereof, and the 980 MPa-grade tempered martensite high-reaming steel can obtain yield strength of more than or equal to 800MPa, tensile strength of more than or equal to 980MPa and elongation (transverse A) ₅₀ High-strength high-hole-expansion steel with hole expansion rate of more than or equal to 10 percent) and hole expansion rate of more than or equal to 40 percent, and can be applied to chassis parts of passenger cars, such as control arms and pairsThe frame and other parts needing high strength and thinning.

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 steel, and the obtained tempered martensite structure has good hole expanding performance and impact toughness; the higher Si content is designed to be matched with the process to obtain more retained austenite, so that the plasticity of the material is improved; meanwhile, in the process of cover retreat, the aims of improving the uniformity of the structure and improving the plasticity and the hole expansion rate can be fulfilled by eliminating part of quenching stress.

Specifically, the 980 MPa-grade tempered martensite high-reaming steel comprises the following chemical components in percentage by weight: 0.06 to 0.10 percent of C, 0.8 to 2.0 percent of Si, 1.5 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 inevitable 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 content of Cu and Ni is preferably less than or equal to 0.3 percent respectively; the content of Nb and V is preferably less than or equal to 0.03 percent respectively; the content of B is preferably 0.0005-0.0015%, the content of Ca is preferably 0.002% or less, and the content of Cr is preferably 0.2-0.4%.

In the component design of the 980 MPa-grade tempered martensite type high-reaming steel, the components are as follows:

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 single-phase tempered martensite, in order to obtain high-strength steel with the final tensile strength of 980MPa, the content of carbon is required to be ensured to be more than 0.06%, and high-reaming steel with the tensile strength of more than or equal to 980MPa is obtained through medium-temperature tempering; however, the carbon content should not be more than 0.10%. The carbon content is too high, the strength of the formed low-carbon martensite is too high, and the carbide formed in the structure after the covering and withdrawing is too much, so that the elongation and the hole expansibility are not favorable. Therefore, the carbon content should be controlled to be between 0.06-0.10%, and preferably in the range of 0.07-0.09%.

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 in a very low temperature range. Therefore, in the actual rolling process, the rolling can be carried out at a relatively low finish rolling temperature, for example, the rolling can be carried out at the finish rolling temperature range of 800-850 ℃, so that the grain size of austenite can be greatly reduced, the size of a final martensite lath is reduced, the strength and the plasticity are favorably improved, and meanwhile, the good hole expansion rate is favorably obtained; the other important function of Si is to inhibit cementite precipitation, and a certain amount of retained austenite can be reserved under proper rolling process conditions, especially when a structure mainly comprising martensite is obtained, so that the elongation is favorably improved. It is known that the elongation of martensite is usually the lowest under the same strength class conditions, and that retention of a certain amount of stable retained austenite is an important measure in order to increase the elongation of martensite. This effect of Si must be exhibited when its content reaches 0.8% 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 0.8 and 2.0%, and preferably in the range of 1.0 to 1.4%.

Manganese, the most basic element in steel, is also one of the most important elements in the present invention. It is known that Mn is an important element for expanding the austenite phase region, and can reduce the critical quenching rate of steel, stabilize austenite, refine grains, and delay transformation of austenite to pearlite. In the invention, in order to ensure the strength of the steel plate and stabilize the retained austenite, the content of Mn is generally controlled to be more than 1.5%; 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 1.5 to 2.0%, preferably in the range of 1.6 to 1.9%.

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) ₂ P 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, more MnS is formed in the steel, 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 anisotropy of the structure 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 must be at a high level in the present invention, and the S content is strictly controlled in order to reduce the MnS content, the S content is required to be controlled to be within 0.003%, and the preferable range is 0.0015% or less.

Aluminum, its role in steel is mainly deoxidation and nitrogen fixation. In the presence of strong carbide forming elements such as Ti, nb, V, etc., al mainly functions to deoxidize and refine grains. In the invention, al is used 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 more than 0.08%, the effect of refining the crystal 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 finely dispersed TiN is formed in the subsequent welding process of the material, the size of austenite grains is inhibited, the structure is refined, and the low-temperature toughness is improved. 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 addition of molybdenum to the steel can greatly retard ferrite and pearlite transformation. 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, a process of air cooling and then water cooling or direct water cooling after rolling is adopted, the addition of molybdenum can ensure that structures such as ferrite or pearlite and the like cannot be formed in the air cooling process, and simultaneously, deformed austenite can be dynamically restored in the air cooling process, thereby being beneficial to improving the uniformity of the structures; molybdenum has strong resistance to solder softening. Because the invention mainly aims to obtain the 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 softening degree of welding. Therefore, the content of molybdenum should be controlled between 0.1-0.5%, preferably in the range of 0.15-0.35%.

Chromium is one of the elements that can be added in the present invention. The addition of a small amount of chromium element is not used for improving the hardenability of the steel, but is combined with B, so that an 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. Since the final application parts related by 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, the addition amount of the chromium element in the steel is less than or equal to 0.5 percent, and the preferable range is 0.2 to 0.4 percent.

Boron is one of the elements that can be added in the present invention. Boron mainly has the function of being segregated at the original austenite grain boundary in the 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 additive 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 improvement of the form of inclusions is facilitated, and 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, 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 favorable. 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 start the phase transformation does vanadium start to form in the ferrite. Because the solid solubility of vanadium carbide in ferrite is greater than that of niobium and titanium, the vanadium carbide has a larger size in ferrite, which is not beneficial to precipitation strengthening, and contributes far less to the strength of steel than titanium, but because the vanadium carbide also consumes a certain amount of carbon atoms, the vanadium carbide is not beneficial to improving the strength of steel. 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 the copper element, a small amount of nickel is added, so that the generation of 'Cu brittleness' can be inhibited. The addition of higher nickel has no significant adverse effect on the properties of the steel itself. If copper and nickel are added simultaneously, the corrosion resistance can be improved, the structure and precipitated phases 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 manufacturing method of 980MPa tempered martensite high-reaming steel, which comprises the following steps:

1) Smelting and casting

Smelting the components by a converter or an electric furnace, secondarily refining the components by a vacuum furnace, and then casting the components into a casting blank or an ingot;

2) Heating the casting blank or the cast ingot at 1100-1200 ℃ for 1-2 hours;

3) Hot rolling

Initial rolling temperature: at 950-1100 deg.c, 3-5 times of great pressure over 950 deg.c and accumulated deformation not less than 50%; then rolling for 3-7 passes and the accumulated deformation is more than or equal to 70 percent; the finishing temperature is 800-950 ℃;

4) Cooling

Firstly, air cooling for 0-10s, then cooling the strip steel to room temperature at a cooling speed of more than or equal to 30 ℃/s, and then coiling;

5) Annealing

Cover annealing is adopted, the heating speed is more than or equal to 20 ℃/h, the cover annealing temperature is 300-500 ℃, and the cover annealing time is 12-48h; cooling the steel coil to below 300 ℃ at a cooling speed of less than or equal to 50 ℃/h, and discharging;

6) 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 percent 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 oiled.

Preferably, the acid washing in the step 6), the rinsing is carried out at the temperature of 35-50 ℃, 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 lower C content in the component design, which can ensure excellent weldability when used by users and ensure that the obtained tempered martensite structure has good hole expansibility and impact toughness; the higher Si content is designed to be matched with the process to obtain more retained austenite, so that the plasticity of the material is improved; meanwhile, in the process of cover withdrawing, the aims of improving the uniformity of the structure and improving the plasticity and the hole expansion rate can be fulfilled by eliminating part of quenching stress.

In the aspect of structure design, a low-carbon tempered martensite design idea is adopted, higher silicon is added to inhibit and reduce cementite formation, the non-recrystallization temperature is reduced, hot rolling and on-line quenching after rolling are carried out in a relatively wider finish rolling temperature range, original austenite grains with fine and uniform grains can be obtained, and the low-carbon martensite with uniform structure is finally obtained; the higher manganese in the composition design can stabilize austenite, and the molybdenum can remarkably delay ferrite and pearlite transformation. Because the hot-rolled low-carbon martensite has higher strength, the strength can be accurately regulated and controlled within a required range after different process covers are removed, the residual austenite endows the steel plate with higher plasticity and cold bending performance, and the tempered martensite endows the steel plate with high strength, uniform and fine structure and higher hole expansion performance and low-temperature toughness.

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, air cooling is firstly carried out for a certain time. The main purposes of air cooling are as follows: because of the higher manganese and molybdenum content in the composition design, the manganese is an element for stabilizing austenite, and the molybdenum greatly delays ferrite and pearlite phase transformation. Therefore, during the air-cooling for a certain period of time, the rolled deformed austenite does not undergo phase transformation, i.e., ferrite structure, but dynamic recrystallization and relaxation processes. Dynamic recrystallization of the deformed austenite can form nearly equiaxial austenite with uniform structure, dislocation in austenite grains can be greatly reduced after relaxation, and the two can be combined to obtain martensite with uniform structure in the subsequent water-cooling quenching process; the on-line quenching to room temperature can also be directly carried out after the final rolling. In order to obtain a martensite structure, the water cooling speed is higher than the critical cooling speed of the low-carbon martensite, and in the invention, the water cooling speed of the strip steel is required to be more than or equal to 30 ℃/s in order to ensure that the martensite can be obtained by all component designs.

Because the microstructure involved in the invention is low-carbon tempered martensite, the steel strip can be cooled to room temperature at a cooling speed higher than the critical cooling speed after finishing rolling, and the cover annealing temperature and time are controlled within a certain range in the subsequent cover annealing process, so that the ultrahigh-strength hole expanding steel with uniform properties such as strength, plasticity, hole expansion and the like can be obtained. The cover retreating temperature and the cover retreating time are in an inverse relation, namely the lower the cover retreating temperature is, the longer the cover retreating time is; conversely, the higher the hood retreat temperature, the shorter the hood retreat time. If the cover annealing temperature is lower than 300 ℃, the strength is higher, the hole expansion rate is lower and can not reach more than 40%; if the cover annealing temperature is higher than 500 ℃, the strength is difficult to meet the requirement of being more than or equal to 980 MPa. Therefore, the annealing temperature is selected to be 300-500 ℃.

The invention has the beneficial effects that:

(1) By adopting a relatively economic component design idea and simultaneously adopting an innovative cooling and cover annealing process path, 980 MPa-grade tempered martensite high-hole-expansion steel with excellent strength, plasticity, toughness and hole-expansion performance can be obtained;

(2) The steel coil or the steel plate has excellent ultrahigh strength, plasticity and toughness matching, good hole expansion and flanging performance, yield strength of more than or equal to 800MPa, tensile strength of more than or equal to 980MPa, thickness of 2-6mm, and good elongation (transverse A) of the hot-rolled or acid-pickled high hole expansion steel ₅₀ Not less than 10%), impact toughness and hole expansion performance (hole expansion rate not less than 40%), can be applied to manufacturing of parts requiring high-strength thinning and hole expansion flanging such as automobile chassis and auxiliary frames, and has wide application prospect.

Drawings

FIG. 1 is a process flow diagram of a manufacturing method of 980 MPa-grade tempered martensite high-reaming steel according to the present invention;

FIG. 2 is a schematic view of a rolling process in the manufacturing method of 980 MPa-grade tempered martensite-type high-reaming steel according to the present invention;

FIG. 3 is a schematic view of a cooling process in the manufacturing method of the 980 MPa-grade tempered martensite high-reaming steel according to the invention;

FIG. 4 is a schematic diagram of a cover annealing process in the manufacturing method of 980 MPa-grade tempered martensite high-reaming steel.

Detailed Description

Referring to fig. 1 to 4, the method for manufacturing 980 MPa-grade tempered martensite type high-hole-expansion steel according to the present invention 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, the heating temperature is 1100-1200 ℃, and the heat preservation time is 1-2 hours;

3) Hot rolling

The initial rolling temperature: at 950-1100 deg.c, 3-5 times of great pressure over 950 deg.c and accumulated deformation not less than 50%; then rolling for 3-7 passes and the accumulated deformation is more than or equal to 70 percent; the finishing temperature is 800-950 ℃;

4) Cooling

Firstly, air cooling is carried out for 0-10s, and then the strip steel is cooled to room temperature by water at a cooling speed of more than or equal to 30 ℃/s and then coiled;

5) Annealing of

6) Acid pickling

The pickling speed of the strip steel is adjusted within the range 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%, 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, tables 2 and 3 are production process parameters of the steel embodiment of the invention, wherein the thickness of a billet in a rolling process is 120mm, and table 4 is mechanical property of the steel plate of the embodiment of the invention.

As can be seen from Table 4, the yield strength of the steel coil in the embodiment of the high hole expansion steel is more than or equal to 800MPa, the tensile strength is more than or equal to 980MPa, the elongation is usually between 11 and 13 percent, the impact energy is stable, the impact energy at the low temperature of-40 ℃ is stable at 70 to 100J, and the hole expansion rate is more than or equal to 40 percent.

The embodiment shows that the 980MPa high-strength steel has good matching of strength, plasticity, toughness and hole expansion performance, is particularly suitable for parts such as automobile chassis structures and the like which need high-strength thinning and hole expansion 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 expansion, and has wide application prospect.

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Claims

1. A980 MPa-grade tempered martensite high-reaming steel comprises the following chemical components in percentage by weight: 0.06 to 0.10 percent of C, 0.8 to 2.0 percent of Si, 1.5 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,

o is less than or equal to 0.0030 percent, and the balance is Fe and other inevitable impurities;

the microstructure of the high hole expansion steel is tempered martensite;

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, the elongation is 11-13%, the low-temperature impact energy at minus 40 ℃ is stabilized at 70-100J, and the hole expansion rate is more than or equal to 40%;

the high hole expansion steel is obtained by the following method, comprising the following steps of:

1) Smelting and casting

3) Hot rolling

4) Cooling down

Firstly, air cooling is carried out for 2-10s, and then the strip steel is cooled to room temperature by water at a cooling speed of more than or equal to 30 ℃/s and then coiled;

5) Annealing

Cover annealing is adopted, the heating speed is more than or equal to 20 ℃/h, the cover annealing temperature is 300-500 ℃, and the cover annealing time is 12-48h; cooling the steel plate to below 300 ℃ at a cooling speed of less than or equal to 50 ℃/h, and discharging;

6) 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%, and then rinsing, drying the surface of the strip steel and oiling are carried out.

2. The 980 MPa-grade tempered martensitic high-hole-expansion steel as claimed in claim 1, further comprising Cr 0.5% or less, B0.002% or less, nb 0.06% or less, V0.05% or less, cu 0.5% or less,

ni is less than or equal to 0.5 percent, and Ca is less than or equal to 0.005 percent.

3. The 980 MPa-grade tempered martensite-type high-reaming steel according to claim 2, wherein the contents of Cu and Ni are respectively equal to or less than 0.3%; the content of Nb and V is respectively less than or equal to 0.03 percent; the content of B is 0.0005-0.0015%, the content of Ca is less than or equal to 0.002%, and the content of Cr is 0.2-0.4%.

4. The 980MPa grade tempered martensitic high bore steel of claim 1, wherein the C content is 0.07-0.09%.

5. The 980MPa grade tempered martensitic high bore steel of claim 1, wherein the Si content is 1.0-1.4%.

6. The 980MPa grade tempered martensitic high bore steel of claim 1, wherein the Mn content is 1.6-1.9%.

7. The 980MPa grade tempered martensitic high bore steel of claim 1, wherein the S content is controlled to be below 0.0015%.

8. The 980MPa grade tempered martensitic high bore steel of claim 1, wherein the Al content is 0.02-0.05%.

9. The 980MPa grade tempered martensitic high hole expansion steel of claim 1, wherein the N content is controlled to 0.003% or less.

10. The 980MPa grade tempered martensitic high bore steel of claim 1, wherein the Ti content is 0.01-0.03%.

11. The 980MPa grade tempered martensitic high bore steel of claim 1, wherein the Mo content is 0.15-0.35%.

12. The method for manufacturing 980MPa grade tempered martensitic high reaming steel according to any of the claims 1 to 11, characterized by comprising the steps of:

1) Smelting and casting

Smelting by adopting a converter or an electric furnace according to the components, secondarily refining by adopting a vacuum furnace, and then casting into a casting blank or a cast ingot;

3) Hot rolling

4) Cooling down

5) Annealing

6) Acid pickling

13. The method for manufacturing 980 MPa-grade tempered martensite-type high-reaming steel according to claim 12, wherein the pickling in step 6) is performed, rinsing is performed at a temperature range of 35-50 ℃, and the surface of the strip steel is dried and oiled at a temperature range of 120-140 ℃.