CN114107792A

CN114107792A - 780 MPa-grade high-surface ultrahigh-hole-expansion steel and manufacturing method thereof

Info

Publication number: CN114107792A
Application number: CN202010896459.XA
Authority: CN
Inventors: 王焕荣; 杨峰; 张晨; 华骏山; 刘运华; 倪亚平; 王明
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: 2022-03-01
Anticipated expiration: 2040-08-31
Also published as: CN114107792B; WO2022042733A1

Abstract

A780 MPa grade high surface ultra-high hole expansion steel and a manufacturing method thereof are disclosed, which comprises the following chemical components by weight percent: 0.03-0.08% of C, less than or equal to 0.2% of Si, 0.5-2.0% of Mn, less than or equal to 0.02% of P, less than or equal to 0.003% of S, 0.01-0.08% of Al, less than or equal to 0.004% of N, 0.05-0.20% of Ti, 0.1-0.5% of Mo, less than or equal to 0.005% of Mg, less than or equal to 0.0030% of O, and the balance of Fe and other inevitable impurities. The surface quality of the steel plate/coil obtained by the invention is good, the red iron sheet on the surface of the strip steel is avoided, and the surface quality of the acid-washing high-strength steel is improved; the yield strength is more than or equal to 700MPa, the tensile strength is more than or equal to 780MPa, the elongation A50 is more than or equal to 17%, the hole expansion rate is more than or equal to 80%, the good matching of high surface quality, high strength, high plasticity and ultrahigh hole expansion rate is realized, and the method can be applied to parts of a chassis of a passenger vehicle, such as a control arm, an auxiliary frame and the like, which need high strength thinning.

Description

780 MPa-grade high-surface ultrahigh-hole-expansion steel and manufacturing method thereof

Technical Field

The invention belongs to the field of high-strength steel, and particularly relates to 780MPa grade high-surface ultrahigh-hole-expansion steel and a manufacturing method thereof.

Background

Many parts in passenger cars, particularly chassis and body parts, are commonly pickled products. 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 be follow-up new motorcycle type's inevitable requirement, this must lead to the fact with the steel grade higher, also must bring the change in the chassis structure simultaneously: if the parts are more complex, higher requirements are put forward on the material performance, the surface and the forming technology such as hydroforming, hot stamping, laser welding and the like, and the requirements are further converted into the performances of high strength, stamping, flanging, resilience, fatigue and the like of the material.

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 batches at home at present, but higher requirements are provided for important indexes in two forming processes of elongation and hole expansion rate. With the gradual gliding of domestic passenger car sales volume, the automobile industry has appeared the flex point, and the competition is increasingly violent. In order to further reduce the process cost, passenger car enterprises further improve the requirements on the performance of materials. For example, in the production of a structural member such as an automobile chassis control arm, in order to reduce the number of punching steps and cost, it is required to further improve the hole expansion ratio index while achieving high strength and high plasticity. If the hole expansion rate index is required to be further improved to be more than or equal to 70 percent from the current index of more than or equal to 50 percent on the basis of the existing 780 MPa-grade high hole expansion steel. At present, 780MPa grade high-bore-expansion steel mostly adopts a design idea of a high-silicon component system, the structure mainly comprises bainite, and meanwhile, certain precipitation strengthening is performed. The surface of the acid-washed strip steel has obvious red iron sheet, the hole expansion rate is basically between 50 and 65 percent, and the elongation rate of a bainite structure is low, so that the performance requirement of higher hole expansion rate provided by a user cannot be met.

780 MPa-grade acid-washing high-reaming steel relates to a plurality of patents. For example, the Chinese patent CN103602895A relates to a low-carbon Nb-Ti microalloyed high-hole-expansion steel, the component design characteristics of the low-carbon high-silicon Nb-Ti microalloyed high-hole-expansion steel are that the hole-expansion rate guarantee value is more than or equal to 50 percent, the high-silicon component design usually brings red iron sheet on the surface of a steel plate, the coiling temperature interval required for forming bainite is about 500 ℃, the control difficulty of the full-length temperature of the steel coil is large, and the full-length performance fluctuation is easy to cause; the patent CN105821301A relates to 800 MPa-level hot-rolled high-strength high-hole-expansion steel, the component design characteristic is low-carbon high-silicon Nb-Ti microalloying, the Ti content reaches a very high degree, which is 0.15-0.18%, in the actual production process, the component design idea not only has the defects of red iron sheet and the like on the surface of the strip steel, but also is easy to form coarse TiN due to ultrahigh Ti content, and is very unfavorable for the stability of the hole expansion rate; patent application

Chinese patent CN108570604A relates to 780 MPa-grade hot-rolled acid-washed high-reaming steel, which has the composition design characteristics of low carbon, high aluminum and high chromium, and adopts a three-section type cooling process in process design. Although the surface of the strip steel has no red iron sheet, the design of high aluminum is easy to cause the blockage of a casting nozzle in the actual production process, the process is complex, the control difficulty of the three-section type cooling process is high, and the hole expanding rate is not high. Compared with the application patent, the above patents all have the problems of red iron sheet, difficulty in steel making, great difficulty in controlling the full-length temperature uniformity of the strip steel and the like.

In order to meet the requirements of users on higher surface quality, better performance stability, better strength, plasticity, ultrahigh hole expansibility matching and the like, subversive changes need to be made on the existing pickling 780MPa grade high hole expansibility steel.

As is well known, in general, 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. This rule is also found in practice by the skilled person of tower steels. 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. On the other hand, in order to obtain good matching of strength, plasticity and hole expansibility, the addition of more silicon elements seems to be indispensable for high-strength high-plasticity high-hole-expansibility steel, but the component design of high silicon brings poor steel plate surface, namely, the red iron sheet defect formed in the hot rolling link is difficult to remove completely in the subsequent pickling process, so that the striped red iron sheet appears on the surface of the pickled high-strength steel, and the surface quality is seriously influenced.

Disclosure of Invention

The invention aims to provide 780MPa grade high-surface ultrahigh-hole-expansion steel and a manufacturing method thereof, the obtained steel plate/coil has good surface quality, red iron sheet on the surface of strip steel is avoided, and the surface quality of acid-washing high-strength steel is improved; the yield strength is more than or equal to 700MPa, the tensile strength is more than or equal to 780MPa, the elongation A50 is more than or equal to 17%, the hole expansion rate is more than or equal to 80%, the good matching of high surface quality, high strength, high plasticity and ultrahigh hole expansion rate is realized, and the method can be applied to parts of a chassis of a passenger vehicle, such as a control arm, an auxiliary frame and the like, which need high strength thinning.

In order to achieve the purpose, the technical scheme of the invention is as follows:

a780 MPa grade high surface ultra-high hole expansion steel comprises the following chemical components in percentage by weight:

C 0.03～0.08％，

Si≤0.2％，

Mn 0.5～2.0％，

P≤0.02％，

S≤0.003％，

Al 0.01～0.08％，

N≤0.004％，

Ti 0.05～0.20％，

Mo 0.1～0.5％，

Mg≤0.005％，

O≤0.0030％，

the balance of Fe and other inevitable impurities.

Furthermore, one or more of 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%, Cr less than or equal to 0.5%, and B less than or equal to 0.001% can be added, wherein the content of Nb and V is preferably less than or equal to 0.03%, the content of Cu, Ni and Cr is preferably less than or equal to 0.3%, the content of B is preferably less than or equal to 0.0005%, and the content of Ca is preferably less than or equal to 0.002%.

The microstructure of the ultrahigh reaming steel is irregular ferrite, nano carbide in ferrite grains and a very small amount of pearlite.

The yield strength of the ultrahigh hole expansion steel is more than or equal to 700MPa, the tensile strength is more than or equal to 780MPa, the elongation A50 is more than or equal to 17 percent, and the hole expansion rate is more than or equal to 80 percent.

In the composition design of the ultrahigh reaming steel, the steel comprises the following components:

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 all-ferrite, the carbon content is required to be ensured to be more than 0.03 percent in order to obtain the high-strength steel with the tensile strength reaching 780MPa level. The carbon content is below 0.03 percent, and the tensile strength of the ferrite type tissue is difficult to reach 780 MPa; however, the carbon content cannot be higher than 0.08%. The carbon content is too high, and a pearlite structure is easily formed during hot rolling and coiling, which is disadvantageous in hole expansion performance. Therefore, the carbon content should be controlled to be between 0.03 and 0.08%, and preferably in the range of 0.04 to 0.06%.

Silicon, which is an essential element in steel. It has been mentioned above that in order to meet the requirements of high strength, high plasticity and ultra-high hole expansion rate proposed by users, more silicon is usually added in the composition design, but the high silicon composition design brings about the reduction of the surface quality of the steel plate and more red iron sheet defects. In the present invention, the silicon content should be strictly controlled in the composition design in order to ensure that good surface quality is obtained. In other words, silicon is an impurity element in the present invention, and it is considered that silicon and manganese are required to deoxidize in actual steel making, and it is difficult to completely avoid the addition of silicon. According to the statistical data of a large number of actual production, when the silicon content is below 0.2%, the surface red iron sheet defect can be avoided during hot rolling, and usually below 0.15%, the red iron sheet can be ensured not to appear. Therefore, the silicon content in the steel should be controlled to be within 0.2%, and preferably within 0.15%.

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 the austenite phase region, and can stabilize austenite, refine grains, and delay transformation of austenite to pearlite. In the invention, in order to ensure the strength and the grain refining effect of the steel plate, the content of Mn is generally controlled to be more than 0.5 percent; meanwhile, the Mn content should not exceed 2.0% in general, 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 be 0.5 to 2.0%, preferably in the range of 1.0 to 1.6%.

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, the more strict the S content must be 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 0.01-0.08%; the Al content is lower than 0.01 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 0.01 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 a sharp corner and the matrix, and a crack initiation source is easily formed by 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. Because the invention adopts the high titanium design on the component system, the adverse effect on hole expansion caused by TiN is reduced as much as possible. 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, the nano-scale carbide which is dispersed evenly and tiny is formed in the ferrite in the coiling phase change process, and the strength, the plasticity and the hole expansion rate are improved. When the content of titanium is less than 0.05%, no obvious precipitation strengthening effect is achieved; when the content of titanium is more than 0.20%, coarse TiN easily causes poor impact toughness of the steel sheet. Therefore, the content of titanium in the steel is controlled to be between 0.05 and 0.20 percent, and the preferable range is between 0.07 and 0.11 percent;

molybdenum, is one of the important elements in the present invention. The molybdenum is added into the steel, so that the phase change of ferrite and pearlite can be greatly delayed, and the irregular ferrite structure can be obtained. Molybdenum and titanium are added into the steel at the same time, and the formed nano-scale titanium carbide molybdenum precipitated phase has the function of resisting high-temperature coarsening, so that coarsening can be prevented from occurring for a long time after coiling, and the strength is prevented from being reduced. Meanwhile, molybdenum has strong welding softening resistance. Because the main purpose of the invention is to obtain ferrite plus nano precipitated structure, the addition of a certain amount of molybdenum can effectively reduce the welding softening degree. Therefore, the content of molybdenum should be controlled between 0.1-0.5%, preferably in the range of 0.15-0.45%.

Magnesium is one of the important elements in the present invention. The magnesium is added into the steel to preferentially form dispersed fine MgO in the steel-making stage, the fine MgO can be used as nucleation points of TiN, and the nucleation points of TiN can be effectively increased and the size of TiN can be reduced in the subsequent continuous casting process. Because TiN has great influence on the hole expansion ratio of the final steel plate, the hole expansion ratio is easy to be unstable. Therefore, the Mg content in the steel is controlled to be within 0.005%.

Oxygen, which is an inevitable element in the steel making process, is generally below 30ppm in the steel after deoxidation for the present invention, and does not cause significant adverse effects on the steel sheet properties. Therefore, the O content in the steel is controlled to be within 30 ppm.

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.

Chromium is an element that may be added in the present invention. The chromium is added into the steel to improve the strength of the steel mainly by means of solid solution strengthening or structure refining and the like. Because the structure of the invention is fine bainitic ferrite and nano precipitated carbide, and a large amount of dispersed fine nano carbide is strongly nailed and dislocated, the yield strength ratio of the steel to the tensile strength, namely the yield ratio, is higher and usually reaches more than 0.90. The yield strength of the steel can be properly reduced by adding a small amount of chromium element, so that the yield ratio is reduced. In addition, the addition of a small amount of chromium can also play a role in improving the corrosion resistance, and the addition amount of chromium is usually less than or equal to 0.5 percent, and the preferable range is less than or equal to 0.3 percent.

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 structure can be refined in the subsequent phase transformation. 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 start the phase transformation does vanadium start 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, contributes far less to the strength of steel than titanium carbide or titanium molybdenum carbide, but consumes certain carbon atoms due to the formation of vanadium carbide, and is not beneficial to the improvement of the strength of steel. Therefore, the amount of vanadium added to the steel is usually 0.05% or less, preferably 0.03% or less.

Boron, an element that can be added in the present invention. Boron can greatly improve the hardenability of steel and is beneficial to obtaining a martensite structure. Considering that the structure desired to be obtained in the hot rolling stage of the present invention is ferrite rather than martensite, the content of boron element in steel needs to be strictly controlled to prevent the formation of martensite due to the excessive addition of boron element. In addition, boron element added into steel can form irregular ferrite structure and even martensite component, which is unfavorable for low temperature impact toughness of steel. Therefore, the amount of boron added to the steel is usually controlled to 0.001% or less, preferably 0.0005% or less.

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%.

The invention relates to a manufacturing method of 780MPa grade high-surface ultrahigh hole-expanding 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) the casting blank or the cast ingot is heated again, the heating temperature is more than or equal to 1230 ℃, and the heat preservation time is 1-2 hours;

3) hot rolling

The initial rolling temperature is 1050-1150 ℃, the accumulated deformation is more than or equal to 50% under the condition of 3-5 times of rough rolling and large pressure above 1050 ℃, then the air cooling or water cooling is carried out on the intermediate billet to 950-;

4) controlled cooling

Cooling the steel plate to 550-650 ℃ at a cooling speed of more than or equal to 10 ℃/s, and slowly cooling to room temperature after coiling;

5) acid pickling

The pickling speed of the strip steel is adjusted within the range of 30-140 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 3%, rinsing is carried out within the temperature range of 35-50 ℃ to ensure the surface quality of the strip steel, and surface drying and oiling are carried out within the temperature range of 120-140 ℃.

The manufacturing method of the ultrahigh reaming steel comprises the following steps:

heating temperature of casting blank (ingot): at the temperature of more than or equal to 1230 ℃, the heat preservation time is as follows: 1-2 hours, rolling temperature: 1050-1150 ℃, under the conditions of 3-5 times of rough rolling and high pressure above 1050 ℃, the accumulated deformation is more than or equal to 50 percent, and the main purpose is to refine austenite grains and retain more solid-solution titanium.

In the hot rolling stage, ferrite with uniform and fine structure and a ferrite in-vivo nano precipitated phase are formed by high-temperature coiling.

In the rough rolling and finish rolling stages, the rolling rhythm should be completed as fast as possible, and more solid solution titanium is ensured to be in austenite. After the high-temperature finish rolling is finished, the strip steel is cooled to 550-650 ℃ on line at a cooling speed of more than or equal to 10 ℃/s so as to obtain ferrite and a nano precipitated structure.

In the pickling process, the nonuniformity of thermal stress and structural stress formed in the steel coil in the high-temperature coiling phase change process is fully released in the pickling straightening and withdrawal process, and the structural uniformity is further improved.

In the invention, the design concept of no Si is adopted in the aspect of component design, so that on one hand, the formation of red iron sheets on the surface of the strip steel can be avoided, and the stable control of key process parameters in the actual production process is facilitated; on the other hand, ferrite and nanometer precipitated tissues with uniform performance can be obtained by adopting the conventional process of high-temperature coiling, which is beneficial to the stability of the overall length performance of the strip steel. In the steelmaking process, a Mg deoxidation mode is adopted, dispersed fine MgO is preferentially formed in molten steel, more nucleation particles are created for the formation of TiN in the subsequent continuous casting process, TiN particles can be effectively refined, and the hole expansion rate stability is improved. Relatively high Ti and a small amount of Mo are added, so that the nano precipitated carbide is prevented from being obviously coarsened after high-temperature coiling, and the strength of the steel is reduced. The phase change stress of the high-temperature coiled strip steel is not high as much as possible, but the phenomenon of nonuniform internal stress also exists, and the internal stress is reduced and homogenized after the pickling and multiple stretch bending processes, so that the improvement of the hole expanding rate is facilitated under the condition of less plastic loss.

The invention has the beneficial effects that:

adopts a unique component design idea and is matched with a corresponding process,

the invention adopts a low-cost component design idea and innovative controlled rolling and pickling processes, firstly obtains ferrite type high-strength steel with uniform structure and performance on the existing hot continuous rolling production line, and then obtains more uniform distribution of internal stress in the ferrite structure through the pickling process. The nano-scale carbide which is evenly and finely dispersed in the ferrite endows the steel plate with high strength and high plasticity, and simultaneously, the steel plate has ultrahigh hole expansion rate due to good structure and even internal stress distribution.

The yield strength of the steel plate/coil is more than or equal to 700MPa, the tensile strength is more than or equal to 780MPa, the thickness is 1.8-6.0mm, the surface quality is high, the elongation is good (the transverse A50 is more than or equal to 17%) and the ultrahigh hole expansion performance is good (the hole expansion rate is more than or equal to 80%), the excellent strength, plasticity and hole expansion performance are matched, the strength, plasticity and the ultrahigh hole expansion rate are excellent, the steel plate/coil can be applied to the manufacturing of complex parts such as automobile chassis, auxiliary frames and the like which need high-strength thinning, hole expansion flanging and the like, and the application prospect is very wide.

The invention has simple process and is suitable for mass production.

Drawings

FIG. 1 is a process flow diagram of a manufacturing method of 780MPa grade high-surface ultra-high hole-expanding steel according to the present invention;

FIG. 2 is a schematic view of a rolling process in the 780MPa grade high-surface ultra-high hole-expanding steel manufacturing method of the present invention;

FIG. 3 is a schematic view of a cooling process in the 780MPa grade high-surface ultra-high hole-expanding steel manufacturing method of the present invention;

FIG. 4 is a typical metallographic photograph of a 780MPa grade high surface ultra-high hole expansion steel of example 1 according to the present invention;

FIG. 5 is a typical metallographic photograph of a 780MPa grade high surface ultra-high hole expansion steel of example 3 according to the present invention;

FIG. 6 is a typical metallographic photograph of 780MPa grade high surface ultra-high hole expansion steel of example 7 according to the present invention.

Detailed Description

The invention is further illustrated by the following examples and figures.

Referring to fig. 1 to 3, the method for manufacturing 780MPa grade high-surface ultra-high hole expansion steel of the present invention comprises the following steps:

1) smelting and casting

3) hot rolling

4) controlled cooling

5) acid pickling

The pickling speed of the strip steel is adjusted within the range of 30-140 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 3%, 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 ultra-high reaming steel embodiment of the invention are shown in table 1, and tables 2 and 3 show the production process parameters of the steel embodiment of the invention, wherein the thickness of a billet in a rolling process is 230 mm; table 4 shows the mechanical properties of the steel sheets of examples of the present invention.

Typical metallographic photographs of the coiling of examples 1, 3 and 7 at different temperatures are shown in fig. 4, 5 and 6, respectively, and it is clear from the photographs that the structure obtained by the composition system designed by the present invention at high temperature coiling is uniform and fine total ferrite. In the subsequent pickling process, the stress and the internal stress of the ferrite structure are further improved, and the strength, the plasticity and the hole expansion performance are improved.

As can be seen from Table 4, the yield strength of the steel coil is more than or equal to 700MPa, the tensile strength is more than or equal to 780MPa, the elongation A50 is more than or equal to 17 percent, and the hole expansion rate is more than or equal to 80 percent. The 780MPa high-strength steel has high strength, high plasticity and excellent matching of ultrahigh hole expansion rate, is particularly suitable for manufacturing parts such as control arms and the like which need high-strength thinning and hole expansion flanging forming, such as automobile chassis structures and the like, can also be used for complex parts such as wheels and the like which need hole expansion, and has wide application prospect.

Claims

1. A780 MPa grade high surface ultra-high hole expansion steel comprises the following chemical components in percentage by weight:

C 0.03～0.08％，

Si≤0.2％，

Mn 0.5～2.0％，

P≤0.02％，

S≤0.003％，

Al 0.01～0.08％，

N≤0.004％，

Ti 0.05～0.20％，

Mo 0.1～0.5％，

Mg≤0.005％，

O≤0.0030％，

the balance of Fe and other inevitable impurities.

2. The 780MPa grade high-surface ultra-high hole expansion steel as claimed in claim 1, wherein one or more of Nb 0.06%, V0.05%, Cu 0.5%, Ni 0.5%, Cr 0.5%, B0.001%, and Ca 0.005% may be added, wherein Nb and V are preferably 0.03% or less, Cu, Ni, and Cr are preferably 0.3% or less, B is preferably 0.0005% or less, and Ca is preferably 0.002% or less.

3. The 780MPa grade high surface ultra-high hole expansion steel of claim 1, wherein the C content is between 0.04-0.06%.

4. The 780MPa grade high surface ultra-high hole expansion steel of claim 1, wherein the Si content is within 0.15%.

5. The 780MPa grade high surface ultra-high hole expansion steel of claim 1, wherein the Mn content is 1.0-1.6%.

6. The 780MPa grade high-surface ultra-high hole expansion steel of claim 1, wherein the S content is controlled to 0.0015%.

7. The 780MPa grade high-surface ultra-high hole expansion steel of claim 1, wherein the Al content is controlled to 0.02-0.05%.

8. The 780MPa grade high-surface ultra-high hole expansion steel of claim 1, wherein the N content is controlled to be 0.003% or less.

9. The 780MPa grade high-surface ultra-high hole expansion steel according to claim 1, wherein the Ti content is controlled to 0.07-0.11%.

10. The 780MPa grade high-surface ultra-high hole expansion steel of claim 1, wherein the Mo content is 0.15-0.45%.

11. The 780MPa grade high surface ultra-high hole expansion steel of claim 1, wherein the microstructure of the ultra-high hole expansion steel is irregular ferrite, ferrite intra-grain nano-carbides, and a very small amount of pearlite.

12. The 780MPa grade high-surface ultra-high hole expansion steel according to claim 1 or 11, wherein the yield strength of the ultra-high hole expansion steel is not less than 700MPa, the tensile strength is not less than 780MPa, the elongation A50 is not less than 17%, and the hole expansion ratio is not less than 80%.

13. The method for manufacturing 780MPa grade high-surface ultra-high hole expansion steel according to any one of claims 1 to 12, comprising the steps of:

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 of claims 1-10, and then casting into a casting blank or an ingot;

3) hot rolling

4) controlled cooling

5) acid pickling

Adjusting the strip steel pickling operation speed within the range of 30-140 m/min, controlling the pickling temperature to be 75-85 ℃, controlling the withdrawal and straightening rate to be less than or equal to 3%, rinsing, drying the surface of the strip steel, and coating oil.

14. The method as claimed in claim 13, wherein the 780MPa grade high-surface ultra-high hole expansion steel is prepared by rinsing at 35-50 ℃ after acid pickling in step 5), and drying and oiling at 120-140 ℃.