1. A high-yield-ratio hydrogen embrittlement-resistant cold-rolled DH980 steel plate is characterized in that the steel plate comprises the following components by weight percent: c: 0.16-0.23%, Mn: 1.8% -2.5%, Si: 0.4% -1.2%, Al: 0.30-0.90%, Cr: 0.10% -0.50%, Mo: 0.10 to 0.60 percent; p is less than or equal to 0.01 percent, S is less than or equal to 0.01 percent, N is less than or equal to 0.005 percent, Nb: 0.01 to 0.1 percent of Ti: 0.01 to 0.1 percent, and the balance of Fe and inevitable impurities.

2. The high yield ratio hydrogen embrittlement resistant cold rolled DH980 steel sheet as claimed in claim 1, wherein Mo: Ti is (8-13) to 1.

3. The high yield ratio hydrogen embrittlement resistant cold rolled DH980 steel plate of claim 1, wherein the microstructure of the steel plate comprises ferrite + martensite + retained austenite + bainite; wherein the steel plate microstructure comprises the following components in percentage by volume: 10 to 35 percent of ferrite, 40 to 70 percent of martensite, 3 to 12 percent of residual austenite and 3 to 15 percent of bainite; the retained austenite is blocky, the grain size is between 0.1um and 0.6um, and the retained austenite is distributed at the interface of martensite/ferrite and in the ferrite.

4. The high yield ratio hydrogen embrittlement resistant cold rolled DH980 steel plate as claimed in claim 1, wherein the steel plate has a yield strength of 720-900 MPa, a tensile strength of 980-1100 MPa, a elongation after fracture perpendicular to rolling direction A80 of not less than 15%, a yield ratio of not less than 0.70, a hole expansion ratio of not less than 30%, a longitudinal 180 ° cold bending minimum bend radius of not more than 1.0t, and t is the thickness of the steel plate.

5. A method for preparing a high yield ratio hydrogen embrittlement resistant cold rolled DH980 steel sheet as claimed in any one of claims 1 to 4, comprising smelting, medium thin slab continuous casting, hot continuous rolling, pickling cold rolling, continuous annealing, finishing; the method is characterized in that:

(1) smelting: the temperature of the molten steel in the converter is 1630-1720 ℃;

(2) continuous casting of medium and thin slabs: the casting temperature is 1580-1630 ℃, and the thickness of the continuous casting billet is 120-180 mm;

(3) hot continuous rolling: the charging temperature of a casting blank is 300-600 ℃, the heating temperature is 1150-1280 ℃, the initial rolling temperature is 1000-1130 ℃, the final rolling temperature is more than 880 ℃, the coiling temperature is 500-650 ℃, and the thickness of a hot rolled coil is 2-4 mm;

(4) acid pickling and cold rolling: the cold rolling reduction rate is 40-80%;

(5) and (3) continuous annealing: the preheating temperature is controlled to be 200-500 ℃, the annealing temperature is 760-860 ℃, the annealing time is 10-600 s, and the slow cooling temperature is 660-740 ℃; then, fast cooling, wherein the fast cooling rate is more than 20 ℃/s, the fast cooling temperature is 350-450 ℃, the overaging temperature is 330-450 ℃, and the overaging time is 30-3600 s;

(6) finishing: the polishing elongation in the polishing process is 0.5-1.2%.

6. The method for preparing the high yield ratio hydrogen embrittlement resistant cold rolled DH980 steel plate as claimed in claim 5, wherein in the step (3) of hot continuous rolling, the microstructure of the hot rolled steel plate comprises ferrite, pearlite, bainite and a small amount of cementite; wherein the steel plate microstructure comprises the following components in percentage by volume: ferrite: 40% -70%, pearlite: 20% -40%, bainite: 5% -20%, cementite: 1% -5%; the grain size of the hot rolled steel plate is above grade 7.0.

Background

In recent years, with the higher and higher requirements of the automotive industry on the formability index of materials, the traditional dual-phase steel has difficulty in meeting the requirements of complex stamping parts on high ductility, and the TRIP steel has limited the wide use due to the expensive production cost caused by high alloy content. The existing DH steel introduces a certain amount of residual austenite into the traditional dual-phase steel, so that the material shows excellent forming performance through the TRIP effect, the defects of DP steel and TRIP steel in the application process can be obviously overcome, and the DH steel is just one of the research hotspots in the field of automobile steel development at present. However, severe hydrogen embrittlement (hydrogen-induced delayed fracture) occurs during the service life of the ultra-high strength steel parts, and the hydrogen-induced delayed fracture sensitivity of the ultra-high strength steel is significantly increased with the increase of the strength grade. Since this phenomenon seriously affects the normal service of parts, the delayed fracture phenomenon of ultra-high strength steel is of great concern to manufacturers and users. Based on the current research situation, the problems of poor formability and hydrogen embrittlement of the ultrahigh-strength automobile steel are urgently needed to be solved, so the invention aims to develop a novel 1.0 GPa-grade ultrahigh-strength automobile steel with high yield ratio and hydrogen embrittlement resistance.

Patent document CN 107058869B discloses a 980MPa grade cold-rolled dual-phase steel with ultra-low yield ratio and a manufacturing method thereof, and the main chemical components are as follows: c: 0.13 to 0.18%, Si: 0.3-0.6%, Mn: 1.7-2.4%, Als: 0.03-0.06%, Nb: 0-0.05%, Cr: 0.3-0.5%, and the balance of Fe and inevitable impurities. The low-C Al-free basic component system is adopted in the dual-phase steel component design, and the cold-rolled dual-phase steel with the strength level of 980MPa is produced by adopting a cold-rolling-continuous annealing production process, the yield ratio of the product is less than 0.5, the plasticity is poor, and the forming problems such as stamping cracking and the like and the service problems such as hydrogen-induced delayed fracture and the like are easily generated in the actual application process.

Patent document CN 107058895A discloses 1000MPa grade hot-galvanized dual-phase steel and a preparation method thereof, and the main chemical components of the steel are as follows: c: 0.07 to 0.15%, Si: 0.1-0.4%, Mn: 1.5-2.5%, P: less than or equal to 0.01 percent, S: less than or equal to 0.01 percent, Al: 0.4-0.8%, Cr: 0.4-0.8%, Mo: 0.2-0.5%, Ti: 0.02-0.04%, V: less than or equal to 0.01 percent, B: less than or equal to 0.005 percent, and the balance of Fe and other inevitable impurities. According to the invention, a noble metal element Mo is added in the component design, and a cold rolling-hot galvanizing production process is adopted to produce the hot galvanizing dual-phase steel with the strength level of 1000MPa, but the product has poor ductility, the elongation after fracture is only about 13%, the requirements of complex parts and parts such as stamping are difficult to meet in the practical application process, and the problem of hydrogen embrittlement is easy to occur.

Disclosure of Invention

Aiming at the problems in the prior art, the invention aims to develop an economical cold-rolled DH980 steel plate with high yield ratio and hydrogen-induced delayed fracture resistance and a preparation method thereof, which can meet the production conditions of the traditional production line and control the alloy cost, and the cold-rolled DH980 steel plate with high yield ratio manufactured by the invention has excellent hole expansion performance, extensibility and hydrogen embrittlement resistance.

The purpose of the invention is realized as follows:

a high-yield-ratio hydrogen embrittlement-resistant cold-rolled DH980 steel plate comprises the following components in percentage by weight: c: 0.16-0.23%, Mn: 1.8% -2.5%, Si: 0.4% -1.2%, Al: 0.30-0.90%, Cr: 0.10% -0.50%, Mo: 0.10 to 0.60 percent; p is less than or equal to 0.01 percent, S is less than or equal to 0.01 percent, N is less than or equal to 0.005 percent, Nb: 0.01 to 0.1 percent of Ti: 0.01 to 0.1 percent, and the balance of Fe and inevitable impurities.

Further, in the steel sheet, Mo and Ti are (8-13) and 1

The microstructure of the steel plate comprises ferrite, martensite, residual austenite and bainite; wherein the steel plate microstructure comprises the following components in percentage by volume: 10 to 35 percent of ferrite, 40 to 70 percent of martensite, 3 to 12 percent of retained austenite and 3 to 15 percent of bainite structure. The residual austenite in the steel plate is blocky, the grain size is 0.1-0.6 um, and the residual austenite is mainly distributed at the martensite/ferrite interface and in the ferrite.

The yield strength of the steel plate is 720-900 MPa, the tensile strength is 980-1100 MPa, the elongation after fracture of A80 vertical to the rolling direction (transverse direction) is more than or equal to 15%, the yield ratio is more than or equal to 0.70, and the hole expansion rate is more than or equal to 30%; the minimum bending core radius of longitudinal 180-degree cold bending is less than or equal to 1.0t, and t is the thickness of the steel plate; adopting 180-degree U-shaped bending forming preset stress, and soaking in 0.5mol/L HCl solution for 14 days without delayed fracture; meets the requirements of high strength, high plasticity, high forming performance and excellent delayed fracture resistance of automobiles.

The reason for the alloy design of the present invention is as follows:

c: the carbon element guarantees the strength requirement of the steel through solid solution strengthening, and meanwhile, the sufficient amount of the carbon element is beneficial to obtaining a stable residual austenite structure at room temperature, so that the yield ratio and the forming performance of the steel are improved. The content of the element C is too low, so that the mechanical property of the steel in the invention can not be obtained; the steel is embrittled due to the excessively high content, the martensite is harder, and the risk of hydrogen embrittlement of the material is greatly improved. Therefore, the content of the C element is controlled to be 0.16-0.23 percent in the invention.

Mn: the manganese element is an austenite stabilizing element in the steel, can expand an austenite phase region, reduce the critical quenching speed of the steel, and simultaneously can refine grains, thereby being beneficial to solid solution strengthening to obviously improve the strength and yield ratio of the material. The content of Mn element is too low, the super-cooled austenite is not stable enough, and the plasticity, the toughness and other processing performances of the steel plate are reduced; the excessively high content of the Mn element causes deterioration in the weldability of the steel sheet, and increases in the production cost, which is not favorable for industrial production. Therefore, the content of the Mn element is controlled to be 1.8-2.5 percent in the invention.

Si: the silicon element has a certain solid solution strengthening effect in ferrite, so that the steel has enough strength and yield ratio, and meanwhile, the Si can inhibit the decomposition of residual austenite and the precipitation of carbide, and reduce the inclusion in the steel. The Si element content is too low to play a role in strengthening; too high content of Si element may degrade the surface quality and weldability of the steel sheet. Therefore, the content of the Si element is controlled to be 0.4-1.2%.

Al: the aluminum element contributes to deoxidation of the molten steel. It is also possible to suppress decomposition of residual austenite and precipitation of carbide, and to accelerate bainite transformation to improve the ability to coordinate transformation. Too high content of Al element not only increases production cost, but also causes difficulties in continuous casting production, etc. Therefore, the content of the Al element is controlled within the range of 0.30-0.90 percent in the invention.

Cr: the chromium element can increase the hardenability of the steel to ensure the strength and yield ratio of the steel and stabilize the retained austenite to improve the ductility and formability of the material, and the lower content of the Cr element affects the hardenability of the steel, and the higher content of the Cr element increases the production cost. Therefore, the content of Cr element is controlled within the range of 0.10-0.50% in the present invention.

Mo: the molybdenum element is a strengthening element in the steel, is beneficial to stabilizing the retained austenite, has obvious effect on improving the hardenability of the steel, can form a large amount of TiMoC precipitates by matching Mo element with Ti, is beneficial to ensuring that the hydrogen diffused in the steel is in dispersion distribution, reduces the aggregation of the hydrogen diffused, and can take high strength and hydrogen embrittlement resistance into consideration. The invention controls the content range of Mo element at 0.10% -0.60%; and the mass percentage ratio of Mo to Ti is controlled between 8:1 and 13: 1.

P: the P element is a harmful element in steel, and the plasticity and the hydrogen embrittlement resistance of the steel are seriously reduced; the lower the content, the better. In the invention, the content of the P element is controlled to be less than or equal to 0.01 percent in consideration of the cost.

S: the S element is a harmful element in steel, seriously affects the formability of steel, and the lower the content, the better the formability. In the invention, the content of the S element is controlled to be less than or equal to 0.01 percent in consideration of the cost.

N: the N element is easy to react with Ti to separate out TiN large particles, and the TiN large particles serve as crack sources in the deformation process and are unfavorable for resisting hydrogen brittleness, so that the content of the N element in the steel needs to be strictly controlled. The invention controls the content of N to be less than or equal to 0.005 percent.

Nb and Ti: the microalloying elements Nb and Ti improve the strength and the yield ratio of the material through fine grain strengthening, and simultaneously can pin diffused hydrogen atoms while ensuring the strength through precipitation strengthening, reduce the aggregation of hydrogen and further improve the delayed fracture resistance of the material; thus, the contents of Nb and Ti are controlled as follows: 0.01 to 0.10 percent.

The invention also provides a preparation method of the high-yield-ratio hydrogen embrittlement-resistant cold-rolled DH980 steel plate, which comprises the following steps: smelting, continuous casting of medium and thin slabs, hot continuous rolling, acid pickling and cold rolling, continuous annealing and finishing. The method comprises the following specific steps:

(1) smelting: smelting by a converter to obtain molten steel meeting the following component requirements in percentage by mass, C: 0.16-0.23%, Mn: 1.8% -2.5%, Si: 0.4% -1.2%, Al: 0.30-0.90%, Cr: 0.10% -0.50%, Mo: 0.10 to 0.60 percent; p is less than or equal to 0.01 percent, S is less than or equal to 0.01 percent, N is less than or equal to 0.005 percent, Nb: 0.01 to 0.1 percent of Ti: 0.01 to 0.1 percent of Fe and inevitable impurities as the rest, and the temperature of the molten steel is 1630 to 1720 ℃.

(2) Continuous casting of medium and thin slabs: the casting temperature is 1580-1630 ℃, and the thickness of the continuous casting billet is 120-180 mm.

(3) Hot continuous rolling: the charging temperature of the casting blank is 300-600 ℃, the heating temperature is 1150-1280 ℃, the initial rolling temperature is 1000-1130 ℃, the final rolling temperature is more than 880 ℃, and the coiling temperature is 500-650 ℃. The thickness of the hot-rolled coil is 2-4 mm, and the microstructure of the hot-rolled steel plate comprises ferrite, pearlite, bainite and a small amount of cementite; wherein the steel plate microstructure comprises the following components in percentage by volume: ferrite: 40% -70%, pearlite: 20% -40%, bainite: 5% -20%, cementite: 1% -5%; the grain size of the hot rolled steel plate is above grade 7.0.

(4) Acid pickling and cold rolling: the iron scale on the surface of the steel coil is removed by acid liquor before cold rolling, and the cold rolling reduction rate is 40-80%. The rolling reduction is too high, so that the deformation resistance is too high, and the rolling is difficult to reach the target thickness; the reduction ratio is too low, resulting in a decrease in the elongation of the cold-rolled steel sheet.

(5) And (3) continuous annealing: the preheating temperature is controlled to be 200-500 ℃, the heating temperature is 760-860 ℃, the annealing time is 10-600 s, and the slow cooling temperature is 660-740 ℃; then rapidly cooling, wherein the rapid cooling rate is more than 20 ℃/s, the rapid cooling temperature is 350-450 ℃, the overaging temperature is 330-450 ℃, and the overaging time is 30-3600 s;

the annealing temperature of the critical zone is 760-860 ℃, and if the annealing temperature is too high, the ductility of the steel is reduced due to complete austenitizing and insufficient ferrite proportion; if the annealing temperature is too low, the proportion of soft phase ferrite in the final material is too high, which may significantly reduce the strength of the material. The annealing time is 10-600 s, if the annealing time is too long, the crystal grains of the steel plate are coarse, the annealing time is too short, and the steel plate does not finish the annealing and recrystallization processes quickly, so that the elongation of the steel plate is reduced.

(6) Finishing: the finishing elongation in the finishing process is controlled within the range of 0.5-1.2%.

The microstructure of the cold-rolled continuous annealing product comprises ferrite, martensite, residual austenite and bainite; wherein the steel plate microstructure comprises the following components in percentage by volume: 10 to 35 percent of ferrite, 40 to 70 percent of martensite, 3 to 12 percent of residual austenite and 3 to 15 percent of bainite; meanwhile, the retained austenite in the product is blocky, the grain size is between 0.1um and 0.6um, and the retained austenite is mainly distributed at the martensite/ferrite interface and in the ferrite.

The cold-rolled continuous annealed DH980 steel plate can be obtained by the method: the yield strength is 720-900 MPa, the tensile strength is 980-1100 MPa, the elongation after fracture of A80 vertical to the rolling direction (transverse direction) is more than or equal to 15%, the yield ratio is more than or equal to 0.70, and the hole expansion rate is more than or equal to 30%; the minimum bending core radius of longitudinal 180-degree cold bending is less than or equal to 1.0t, and t is the thickness of the steel plate; adopting 180-degree U-shaped bending forming preset stress, and soaking in 0.5mol/L HCl solution for 14 days without delayed fracture; meets the requirements of high strength, high plasticity, high forming performance and excellent delayed fracture resistance of automobiles.

The invention has the beneficial effects that:

(1) the invention adopts the production process of converter smelting, medium and thin slab continuous casting, hot continuous rolling, acid pickling cold rolling and continuous annealing, can realize the industrial production of the DH980 cold-rolled product with high yield ratio and hydrogen embrittlement resistance on the traditional cold-rolled dual-phase steel production line, and has the advantages of low production cost, no need of adding new production equipment and stable production process.

(2) The cold-rolled DH980 steel plate produced by the invention introduces the residual austenite and the bainite on the basis of the traditional cold-rolled dual-phase steel, realizes that the ductility of the material is obviously improved in the forming process of parts under the coupling action of the phase transformation induced plasticity (TRIP) effect and the coordinated deformation of the martensite/lower bainite mixed structure, and simultaneously shows better collision energy-absorbing effect.

(3) The yield strength of the cold-rolled DH steel sheet strip produced by the invention is 720-900 MPa, the tensile strength is 980-1100 MPa, the elongation after breakage of A80 vertical to the rolling direction (transverse direction) is more than or equal to 15%, the yield ratio is more than or equal to 0.70, and the hole expansion rate is more than or equal to 30%; the minimum bending core radius of longitudinal 180-degree cold bending is less than or equal to 1.0 t; the pre-set stress is formed by adopting 180-degree U-shaped bending, and the steel pipe is soaked in 0.5mol/L HCl solution for 14 days without delayed fracture.

(4) The structure of the finished steel plate is as follows by volume percentage: 10 to 35 percent of ferrite, 40 to 70 percent of martensite, 3 to 12 percent of residual austenite and 3 to 15 percent of bainite structure.

(5) The retained austenite in the product of the invention is blocky, the grain size is between 0.1um and 0.6um, and the retained austenite is mainly distributed at the martensite/ferrite interface and in the ferrite.

Drawings

FIG. 1 is a metallographic microstructure of a steel sheet of example 1;

FIG. 2 is an engineering stress-strain curve of example 1.

Detailed Description

The following examples are intended to illustrate the invention in detail, and are intended to be a general description of the invention, and not to limit the invention.

According to the component proportion of the technical scheme, the embodiment of the invention carries out smelting, medium and thin slab continuous casting, hot continuous rolling, acid pickling and cold rolling, continuous annealing and finishing.

(1) Smelting: the temperature of the molten steel in the converter is 1630-1720 ℃;

(2) continuous casting of medium and thin slabs: the casting temperature is 1580-1630 ℃, and the thickness of the continuous casting billet is 120-180 mm;

(3) hot continuous rolling: the charging temperature of a casting blank is 300-600 ℃, the heating temperature is 1150-1280 ℃, the initial rolling temperature is 1000-1130 ℃, the final rolling temperature is more than 880 ℃, the coiling temperature is 500-650 ℃, and the thickness of a hot rolled coil is 2-4 mm;

(4) acid pickling and cold rolling: the cold rolling reduction rate is 40-80%;

(5) and (3) continuous annealing: the preheating temperature is controlled to be 200-500 ℃, the annealing temperature is 760-860 ℃, the annealing time is 10-600 s, and the slow cooling temperature is 660-740 ℃; then, fast cooling, wherein the fast cooling rate is more than 20 ℃/s, the fast cooling temperature is 350-450 ℃, the overaging temperature is 330-450 ℃, and the overaging time is 30-3600 s;

(6) finishing: the polishing elongation in the polishing process is 0.5-1.2%.

Further, the method comprises the following steps of; the microstructure of the hot rolled steel plate comprises ferrite, pearlite, bainite and a small amount of cementite; wherein the steel plate microstructure comprises the following components in percentage by volume: ferrite: 40% -70%, pearlite: 20% -40%, bainite: 5% -20%, cementite: 1% -5%; the grain size of the hot rolled steel plate is above grade 7.0.

The compositions of the steels of the examples of the invention are shown in table 1. The main process parameters of the steel continuous casting and hot rolling of the embodiment of the invention are shown in Table 2. The main process parameters of the cold rolling and annealing of the steel of the embodiment of the invention are shown in the table 3. The steel structure of the inventive example is shown in Table 4. The comprehensive properties of the steels of the examples of the invention are shown in Table 5.

TABLE 1 composition in wt% of steels of examples of the invention

TABLE 2 Main Process parameters for continuous casting and Hot Rolling of steels according to examples of the invention

TABLE 3 Main Process parameters for the Cold-Rolling annealing of the steels according to the examples of the invention

TABLE 4 Steel structures of examples of the invention

TABLE 5 comprehensive properties of steels of examples of the invention

Remarking: 180 degree cold bending minimum bend radius test: the sampling direction is longitudinal direction, and t is the thickness of the steel plate; the hydrogen embrittlement resistance (delayed fracture resistance) performance evaluation adopts U-shaped bending soaking evaluation, the 180-degree cold bending radius is 5mm, each group of 5 parallel samples are placed in 0.5mol/L HCl solution to be soaked for 14 days, if no fracture occurs, the sample is judged to have no delayed fracture risk, and O is marked; if fracture occurs, the specimen is judged to have delayed fracture risk and marked x.

As can be seen from the above examples, the yield strength of the cold-rolled continuous annealing steel plate produced by the method is 720-900 MPa, the tensile strength is 980-1100 MPa, the elongation after breakage of A80 vertical to the rolling direction (transverse direction) is more than or equal to 15%, the yield ratio is more than or equal to 0.70, and the hole expansion rate is more than or equal to 30%; the minimum bending core radius of longitudinal 180-degree cold bending is less than or equal to 1.0 t; adopting 180-degree U-shaped bending forming preset stress, and soaking in 0.5mol/L HCl solution for 14 days without delayed fracture; meets the requirements of high strength, high plasticity, high forming performance and excellent delayed fracture resistance of automobiles.

In order to express the present invention, the above embodiments are properly and fully described by way of examples, and the above embodiments are only used for illustrating the present invention and not for limiting the present invention, and those skilled in the relevant art can make various changes and modifications without departing from the spirit and scope of the present invention, and any modifications, equivalent substitutions, improvements, etc. made by the persons skilled in the relevant art should be included in the protection scope of the present invention, and the protection scope of the present invention should be defined by the claims.