阅读说明：本技术 一种1500MPa级冷成形带钢及其生产方法 (1500 MPa-grade cold-formed strip steel and production method thereof ) 是由 刘靖宝 张洪波 杜明山 王朝 陈卓 刘丽萍 李孟星 杨明维 韩世绪 于 2019-10-12 设计创作，主要内容包括：本发明公开了一种1500MPa级冷成形带钢及其生产方法,所述带钢化学成分组成及其质量百分含量为：C：0.25～0.40%,Mn：1.10～1.70%,S≤0.012%,P≤0.020%,Si：0.10～0.30%,Als：0.030～0.050%,Cr：0.20～0.40%,B：0.0010～0.0030%,Ti：0.035～0.050%,V：0.15～0.30%,N≤0.0030%,其余为铁和不可避免的杂质；所述生产方法包括铁水预处理、转炉炼钢、LF炉精炼、RH精炼、连铸、热轧、冷轧、连续退火和平整工序。所得带钢的组织为分布均匀的马奥岛和极少量的铁素体,具有超高强度、优异的焊接和冷成形性能。(The invention discloses 1500MPa grade cold forming strip steel and a production method thereof, wherein the strip steel comprises the following chemical components in percentage by mass: c: 0.25 to 0.40%, Mn: 1.10-1.70%, S is less than or equal to 0.012%, P is less than or equal to 0.020%, and Si: 0.10-0.30%, Als: 0.030-0.050%, Cr: 0.20-0.40%, B: 0.0010-0.0030%, Ti: 0.035-0.050%, V: 0.15-0.30%, N is less than or equal to 0.0030%, and the balance is iron and inevitable impurities; the production method comprises the working procedures of molten iron pretreatment, converter steelmaking, LF furnace refining, RH refining, continuous casting, hot rolling, cold rolling, continuous annealing and leveling. The structure of the obtained strip steel is uniformly distributed Marozland and a very small amount of ferrite, and the strip steel has ultrahigh strength and excellent welding and cold forming performances.)

1. The 1500MPa cold-formed strip steel is characterized by comprising the following chemical components in percentage by mass: c: 0.25 to 0.40%, Mn: 1.10-1.70%, S is less than or equal to 0.012%, P is less than or equal to 0.020%, and Si: 0.10-0.30%, Als: 0.030-0.050%, Cr: 0.20-0.40%, B: 0.0010-0.0030%, Ti: 0.035-0.050%, V: 0.15-0.30%, N is less than or equal to 0.0030%, and the balance is iron and inevitable impurities.

2. The cold-formed steel strip of 1500MPa grade according to claim 1, wherein the thickness of the steel strip is 0.8-2.5 mm; when the thickness t of the strip steel is less than or equal to 1.8mm, the bending angle is 90 degrees and the bending diameter is d in the rolling direction and the vertical rolling direction₀=3t no cracking; when the thickness of the strip steel is more than 1.8mm and t is less than or equal to 2.5mm, the bending angle is 90 degrees and the bending diameter is d in the rolling direction and the vertical rolling direction₀=4t does not crack.

3. A cold-formed steel strip of 1500MPa grade according to claim 1 or 2, wherein the structure of the steel strip is uniformly distributed mohnos and a very small amount of ferrite, and the grain size of the ferrite is less than or equal to 5 μm; the grain size of the Maroza is less than or equal to 6 mu m; the yield strength is 650-900 MPa, the tensile strength is more than or equal to 1500MPa, and the elongation after fracture A₅₀≥5%。

4. The production method of 1500MPa grade cold-formed strip steel according to any one of claims 1 to 3, characterized in that the production method comprises the working procedures of molten iron pretreatment, converter steelmaking, LF furnace refining, RH refining, continuous casting, hot rolling, cold rolling, continuous annealing and leveling; in the RH refining process, vanadium and titanium are added, the mass percent of the vanadium is controlled to be 0.15-0.30%, and the mass percent of the titanium is controlled to be 0.035-0.050%.

5. The production method of 1500MPa grade cold-formed strip steel according to claim 4, wherein the converter steelmaking process is carried out at 1660-1690 ℃, and the oxygen level is as follows: 5-20 ppm; and C, converter end point control: 0.022-0.040%, P: less than or equal to 0.009%; adding medium-carbon ferromanganese, aluminum particles, low-carbon ferrochromium and ferrovanadium in the tapping process, and controlling the contents of all elements in a converter ladle as follows: c: 0.30-0.32%, Mn: 1.20-1.30%, Cr: 0.18 to 0.25%, Als: 0.030-0.050%, V: 0.18-0.20%, and the temperature of the converter ladle is 1620-1640 ℃.

6. The production method of 1500MPa grade cold-formed strip steel according to claim 4, wherein in the continuous casting process, the superheat degree of a tundish is 35-45 ℃, and the continuous casting drawing speed is 1.0-1.2 m/min.

7. A production method of 1500MPa grade cold-formed strip steel according to any one of claims 4 to 6, wherein the hot rolling process is performed at a casting blank heating temperature of 1250 to 1320 ℃, a heating time of 160 to 240min, a finish rolling inlet temperature of 1020 to 1100 ℃, a finish rolling temperature of 860 to 900 ℃, and a coiling temperature of 630 to 670 ℃.

8. A production method of cold-formed strip steel at 1500MPa according to any one of claims 4 to 6, wherein the cold rolling step has a cold rolling reduction of 55 to 70%.

9. A production method of 1500MPa grade cold-formed strip steel according to any one of claims 4 to 6, characterized in that the continuous annealing process, the heating second stage and soaking temperature is 820 to 860 ℃, the soaking time is 50 to 100s, the rapid cooling starting temperature is 660 to 680 ℃, the cooling rate is 80 to 100 ℃/s, and the overaging temperature is 260 to 300 ℃.

10. A production method of cold-formed strip steel with the pressure of 1500MPa according to any one of claims 4 to 6, wherein the flattening process adopts a constant rolling force control mode, and the rolling force is 8000-10000 KN.

Technical Field

The invention belongs to the technical field of metallurgy, and particularly relates to 1500MPa cold-formed strip steel and a production method thereof.

Background

With the improvement of the safety performance requirement of automobiles, automobile host plants increasingly adopt 1500 MPa-level hot forming steel, so that the aim of reducing weight is fulfilled, and the safety of the automobiles can be improved. However, the cost of hot forming steel is too high, which leads to the increase of the cost of automobile parts, and on the basis, the demand of cold forming ultrahigh strength steel in automobile main engine plants is more and more urgent. There is a need to develop a steel grade which has high strength and good formability, consists of ferrite and martensite, has the characteristics of low yield ratio, high work hardening rate, good strength and ductility matching and the like, and is currently favored by various large automobile manufacturers. In order to design cold-forming steels having excellent properties, it is necessary to study the influence of each alloying element on the structural properties of the cold-forming steel. In general cold-forming steel, the types of alloying elements are not many, and the main components are generally: C. mn, Si, Al, Cr, B and small amounts of Nb, V, Ti, Mo, etc., which have different effects on cold forming steel.

Disclosure of Invention

The invention aims to solve the technical problem of providing 1500MPa grade cold forming strip steel; the invention also provides a production method of the 1500MPa grade cold-formed strip steel; the resulting strip product has ultra-high strength and excellent cold formability.

In order to solve the technical problems, the invention adopts the technical scheme that: the 1500MPa grade cold forming strip steel comprises the following chemical components in percentage by mass: c: 0.25 to 0.40%, Mn: 1.1-1.7%, S is less than or equal to 0.012%, P is less than or equal to 0.020%, and Si: 0.10-0.30%, Als: 0.030-0.050%, Cr: 0.20-0.40%, B: 0.0010-0.0030%, Ti: 0.035-0.050%, V: 0.15-0.30%, N is less than or equal to 0.0030%, and the balance is Fe and inevitable impurities.

The design idea of the chemical components of the invention is as follows:

c: is the cheapest solid solution strengthening element and austenite stabilizing element, carbon can stabilize austenite phase by interstitial solid solution strengthening, increase austenite stability, and reduce Ac₃The annealing temperature is reduced, and the energy consumption is reduced; the C atoms play a role in interstitial solid solution strengthening in the Maoer islands, and the strengthening effect on the Maoer islands is far greater than that on the Maoer islands and replacement solid solution strengthening alloy elements, so that the carbon content is controlled to be 0.25-0.40%.

Mn: is an austenite stabilizing element and can effectively improve the hardenability of the austenite island, thereby reducing the cooling rate necessary for obtaining a dual-phase structure in the cooling process after the heating of the two-phase region. Mn can also reduce solid solution C in ferrite, promote C to transfer into austenite, improve austenite hardenability and purify a ferrite matrix, thereby improving the ductility of steel, and the Mn content is controlled to be 1.10-1.70%.

Si: si can enlarge Fe-C phase diagram

A + gamma two-phase region; si solutionized into ferrite can affect the interaction of dislocations, increasing the work hardening rate and uniform elongation at a given strength level. The hardenability of the steel is improved by the combined action of Si, C, Mn and other elements, and more lath martensite islands are produced by quenching. Therefore, the Si content is controlled to be 0.10-0.30%.

V: the addition of V element in the steel grade utilizes the following characteristics of V: (1) the precipitation strengthening and the fine grain strengthening (Hall-Petch formula) of the VC improve the strength and the plasticity; (2) the elements V and C are combined, so that the content of C in the Maoer island is reduced, the appearance of a twin crystal type Maoer island is inhibited, and the plasticity and the toughness are improved; (3) solid-solution V improves hardenability; (4) the V-containing steel contains irreversible hydrogen traps with high binding energy, and the hydrogen brittleness resistance is improved.

The thickness of the strip steel is 0.8-2.5 mm; when the thickness t of the strip steel is less than or equal to 1.8mm, the bending angle is 90 degrees and the bending diameter is d in the rolling direction and the vertical rolling direction₀=3t no cracking; when the thickness of the strip steel is more than 1.8mm and t is less than or equal to 2.5mm, the bending angle is 90 degrees and the bending diameter is d in the rolling direction and the vertical rolling direction₀=4t no cracking; and t is the thickness of the strip steel.

The structure of the strip steel is uniformly distributed Marozland and a very small amount of ferrite, and the grain size of the ferrite is less than or equal to 5 mu m; the grain size of the Maroza is less than or equal to 6 mu m; the yield strength is 650-900 MPa, the tensile strength is more than or equal to 1500MPa, and the elongation after fracture A₅₀≥5%。

The invention also provides a production method of 1500MPa grade cold-formed strip steel, which comprises the working procedures of molten iron pretreatment, converter steelmaking, LF furnace refining, RH refining, continuous casting, hot rolling, cold rolling, continuous annealing and leveling; in the RH refining process, vanadium and titanium are added, the mass percentage of the vanadium and the titanium is adjusted, the mass percentage of the vanadium is controlled to be 0.15-0.30%, and the mass percentage of the titanium is controlled to be 0.035-0.050%.

The converter steelmaking process has the following steps that the temperature is 1660-1690 ℃, and the oxygen level is as follows: 5-20 ppm; and C, converter end point control: 0.022 to 0.040%, P: less than or equal to 0.009%; adding medium-carbon ferromanganese, aluminum particles, low-carbon ferrochromium and ferrovanadium in the tapping process, and controlling the contents of all elements in a converter ladle as follows: c: 0.30-0.32%, Mn: 1.20-1.30%, Cr: 0.18 to 0.25%, Als: 0.030-0.050%, V: 0.18-0.20 percent of Fe and inevitable impurities as the rest, and the temperature of the converter bale is 1620-1640 ℃.

In the continuous casting process, the superheat degree of a tundish is 35-45 ℃, and the continuous casting drawing speed is 1.0-1.2 m/min.

In the hot rolling process, the casting blank heating temperature is 1250-1320 ℃, the heating time is 160-240 min, the finish rolling inlet temperature is 1020-1100 ℃, the finish rolling temperature is 860-900 ℃, and the coiling temperature is 630-670 ℃.

In the cold rolling process, the cold rolling reduction rate is 55-70%.

In the continuous annealing process, the heating second stage and the soaking temperature are 820-860 ℃, the soaking time is 50-100 s, the rapid cooling starting temperature is 660-680 ℃, the cooling rate is 80-100 ℃/s, and the overaging temperature is 260-300 ℃.

The leveling process adopts a constant rolling force control mode, and the rolling force is 8000-10000 KN.

The 1500MPa grade cold-formed strip steel alloy is added with more than 0.15 percent of V and more than 0.25 percent of C, and a precipitated phase VC or (V, Ti, Nb) C after continuous annealing, wherein uniform and fine second phase particles can improve the tensile strength to more than 100MPa, the size of the precipitated particles is 1-20 nm, and the average particle size is 4.5 nm. According to the precipitation strengthening mechanism, the precipitation strengthening reinforcing quantity can reach 240 MPa. The precipitation of VC or (V, Ti) C consumes carbon in austenite, reduces the carbon content of the austenite, and reduces the fraction of twin-crystal martensite islands formed in martensite islands after the phase transformation. Therefore, based on VC precipitation in the steel of the invention, the toughness of the martensite island can be improved, the strength of the martensite island is reduced due to the reduction of the carbon content in the martensite island, but the strength of the material is improved through VC precipitation strengthening and fine grain strengthening of prior austenite grains.

Adopt the produced beneficial effect of above-mentioned technical scheme to lie in: 1. the V element is added in the component design, so that the tensile strength is increased and the plasticity is improved simultaneously through the precipitation strengthening of VC; 2. the invention obtains the strip steel product with ultrahigh strength and excellent quality by the change of the content of the alloy elements and the adjustment of the process parametersDissimilar welding and cold forming properties. 3. The structure of the strip steel is uniformly distributed Marozland and a very small amount of ferrite, and the grain size of the ferrite is less than or equal to 5 mu m; the grain size of the Maroza is less than or equal to 6 mu m; 4. the yield strength of the strip steel is 650-900 MPa, the tensile strength is more than or equal to 1500MPa, and the elongation percentage A after fracture₅₀≥5%。

Detailed Description

The present invention will be described in further detail with reference to specific examples.

The invention relates to a production method of 1500MPa grade cold-formed strip steel, which comprises the working procedures of molten iron pretreatment, converter steelmaking, LF furnace refining, RH refining, continuous casting, hot rolling, cold rolling, continuous annealing and leveling, and the specific process steps are as follows:

(1) a molten iron pretreatment process: reducing S in molten iron to below 30ppm by adopting composite blowing particles Mg;

(2) a steel making process: and (3) adopting a 100-ton top-bottom combined blown converter, and controlling the end point of the converter C: 0.022 to 0.040%, P: less than or equal to 0.009%; the temperature is 1660-1690 ℃, and the oxygen level is as follows: 5-20 ppm, pushing off slag in the tapping process, adopting front and back double-shift slag pushing off mode to ensure the phosphorus content of the finished product, adding medium carbon ferromanganese, aluminum particles, low carbon ferrochromium and ferromolybdenum in the tapping process, and controlling the content of each element in the converter bale as follows: c: 0.30-0.32%, Mn: 1.20-1.30%, Cr: 0.18 to 0.25%, Als: 0.030-0.050%, V: 0.18-0.20% of Fe, and the balance of Fe, wherein the converter bale temperature is 1620-1640 ℃;

(3) an LF refining procedure: slagging by using lime and fluorite, adjusting contents of C, Cr, Mn, V, Als and the like, heating molten steel by using an electrode, wherein the LF outbound temperature is 1630-1665 ℃;

(4) RH refining process: fine adjustment is carried out on Mn, Als, B and Ti, Ti alloying is carried out, the components of molten steel are adjusted to a target range, and the outlet temperature is 1575-1610 ℃;

(5) and (3) continuous casting process: the superheat degree of the tundish is 35-45 ℃, and the continuous casting drawing speed is 1.0-1.2 m/min;

(6) a hot rolling procedure: the heating temperature of the casting blank is 1250-1320 ℃, the heating time is 160-240 min, the inlet temperature of finish rolling is 1020-1100 ℃, the finish rolling temperature is 860-900 ℃, and the coiling temperature is 630-670 ℃;

(7) a cold rolling procedure: the cold rolling reduction rate is 55-70%;

(8) a continuous annealing process: the heating second section and the soaking temperature are 820-860 ℃, the soaking time is 50-100 s, the rapid cooling starting temperature is 660-680 ℃, the cooling rate is 80-100 ℃/s, and the overaging temperature is 260-300 ℃;

(9) leveling: and a constant rolling force control mode is adopted, and the rolling force is 8000-10000 KN.