CN115679223A - A new type of high yield ratio cold-rolled DH980 steel and its preparation method - Google Patents

Abstract

The invention provides a new type of high yield ratio cold-rolled DH980 steel and a preparation method thereof. The composition of the steel is as follows by weight percentage: C: 0.10%-0.25%, Mn: 1.50%-2.50%, Si: 0.20% ～1.80%, Al: 0.02%～1.0%, P≤0.01%, S≤0.01%, N≤0.005%, Ti: 0.010%～0.150%, Ca: 0.005%～0.150%, Mg: 0.002%～0.150% , and meet C/Ti: 2~10; Ca+Mg: 0.05~0.20%; the balance is Fe and unavoidable impurities; the preparation methods include converter smelting, medium and thin slab continuous casting and rolling, pickling and cold rolling, continuous Annealing and finishing; the yield strength of DH980 steel produced by the present invention is ≥700MPa, the tensile strength is 980～1100MPa, the elongation after breaking A50 is ≥20%, the yield ratio is ≥0.70, and the hole expansion rate is ≥30%. Cost and requirements for excellent ductility and formability.

Description

A new type of high yield ratio cold-rolled DH980 steel and its preparation method

technical field

The invention belongs to the field of metal materials, and in particular relates to a short-flow, low-cost novel high-yield ratio cold-rolled DH980 steel and a preparation method thereof.

Background technique

In the automotive industry, higher requirements have been put forward for body weight reduction, emission restrictions, and safety standards. In order to better serve users, the automotive industry has more and more demands for parts with high formability. Traditional dual-phase steels are difficult to meet the requirements of high-drawability complex punching cups, and TRIP steels are widely used due to high alloy content and expensive production costs. DH steel is the English abbreviation of Dual Phase Steels with Improved Formability, which was first mentioned in the VDA239-100 cold-formed steel plate standard issued by the German Association of Automobile Manufacturers in 2016. DH steel has good formability due to the introduction of a certain amount of retained austenite, which can overcome the shortcomings of DP steel and TRIP steel in the above application process, and thus make it have significant advantages in the future steel application market.

Patent document CN113403551A discloses a high yield ratio anti-hydrogen embrittlement cold-rolled DH980 steel plate and its preparation method. Its main chemical components are: 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%～0.60%; P≤0.01%, S≤0.01%, N≤0.005%, Nb: 0.01%～0.1%, Ti: 0.01% to 0.1%, the balance being iron and unavoidable impurities. This invention produces cold-rolled DH steel with a strength level of 980MPa, which has excellent plasticity and formability. However, the extensive use of precious metals such as Cr, Mo, and Nb will significantly increase the alloy cost of the product of this invention, and the traditional cold rolling-continuous annealing process is adopted. , the production process is very complicated and cumbersome.

Patent document CN112048681B discloses a 980MPa grade high-formability cold-rolled DH steel and its preparation method. Its main chemical components are: C: 0.16%-0.23%, Mn: 1.8%-2.8%, Si: 0.3%-1.5% , Al: 0.02% ~ 1.2%, Mo: 0.02% ~ 0.50%, Cr: 0.02% ~ 0.70%, P≤0.01%, S≤0.01%, Nb≤0.05%, Ti≤0.04%, the balance is iron and unavoidable impurities. This invention produces cold-rolled DH steel with a strength level of 980MPa, which has excellent plasticity and formability. However, the extensive use of precious metals such as Cr, Mo, and Nb will significantly increase the alloy cost of the product of this invention, and the traditional cold rolling-continuous annealing process is adopted. , the production process is very complicated and cumbersome.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned problems and deficiencies and provide a new high-yield ratio cold-rolled DH980 steel and a new type of high-yield ratio cold-rolled DH980 steel that can reduce the cost of the alloy and shorten the production process in the traditional production line while meeting the basic performance indicators of the product. The preparation method provides a reliable technical solution for steel companies and automobile manufacturers in reducing costs and increasing efficiency.

The purpose of the invention is achieved in this way:

A new type of cold-rolled DH980 steel with high yield strength ratio. The composition of the steel is as follows by weight percentage: C: 0.10%-0.25%, Mn: 1.50%-2.50%, Si: 0.20%-1.80%, Al: 0.02% ~1.0%, P≤0.01%, S≤0.01%, N≤0.005%, Ti: 0.010%～0.150%, Ca: 0.005%～0.150%, Mg: 0.002%～0.150%, and satisfy C/Ti:2 ～10; Ca+Mg: 0.05～0.20%; the balance is Fe and unavoidable impurities.

Further, the yield strength of the DH980 steel is ≥700MPa, the tensile strength is 980-1100MPa, the elongation after fracture of A50 is ≥20%, the yield strength ratio is ≥0.70, and the hole expansion rate is ≥30%; it meets the low cost and excellent elongation and forming of automobiles performance requirements.

Further, the microstructure of the DH980 steel is ferrite + martensite + retained austenite; calculated as follows by volume percentage, 20% to 50% of ferrite, 40% to 70% of martensite, and retained austenite Body 4% to 15%.

The composition design reason of the present invention is as follows:

C: Carbon element ensures the strength requirements of steel through solid solution strengthening, and sufficient carbon element helps to stabilize austenite, thereby improving the formability of steel. If the C element content is too low, the mechanical properties of the steel in the present invention cannot be obtained; if the content is too high, the steel will be brittle and there is a risk of delayed fracture. Therefore, in the present invention, the content of element C is controlled to be 0.10%-0.25%.

Mn: Manganese is an austenite-stabilizing element in steel, which can expand the austenite phase region and reduce the critical quenching speed of steel. At the same time, it can also refine the grains and contribute to solid solution strengthening to increase strength. If the Mn element content is too low, the supercooled austenite will not be stable enough, which will reduce the plasticity and toughness of the steel plate; if the Mn element content is too high, the weldability of the steel plate will deteriorate, and the production cost will increase, which is not conducive to industrial production. Therefore, in the present invention, the content of Mn element is controlled to be 1.50%-2.50%.

Si: Silicon has a certain solid solution strengthening effect in ferrite to ensure that the steel has sufficient strength. At the same time, Si can also inhibit the decomposition of retained austenite and the precipitation of carbides, and reduce inclusions in steel. If the content of Si element is too low, the effect of strengthening cannot be achieved; if the content of Si element is too high, the surface quality and welding performance of the steel plate will be reduced. Therefore, in the present invention, the content of Si element is controlled to be 0.20%-1.80%.

Al: Aluminum element contributes to the deoxidation of molten steel. It can also inhibit the decomposition of retained austenite and the precipitation of carbides, and accelerate the transformation of bainite to improve the coordinated deformation ability. Excessive Al element content will not only increase the production cost, but also lead to difficulties in continuous casting production. Therefore, in the present invention, the content of the Al element is controlled within a range of 0.02% to 1.00%.

P: P element is a harmful element in steel, which seriously reduces the plasticity and deformation properties of steel. The lower the content, the better. Considering the cost, the content of P element is controlled at P≤0.01% in the present invention.

S: S element is a harmful element in steel, which seriously affects the formability of steel, and the lower the content, the better. Considering the cost, the content of S element is controlled at S≤0.01% in the present invention.

N: N element is a harmful element in steel, which seriously affects the comprehensive performance of steel, and the lower the content, the better. Considering the cost, the content of N element is controlled to N≤0.005% in the present invention.

Ti: The micro-alloying element Ti improves the overall performance of the material through fine-grain strengthening and precipitation strengthening. In order to control production costs, the content of Ti element is controlled at 0.010%-0.150% in the present invention; and C/Ti: 2-10 .

Ca: A small amount of calcium is added because the price of Ca is low. Adding it to the master alloy can be used as a deoxidizer and inoculant, which plays a role in microalloying and can significantly refine the grain, improve the plasticity and welding performance of steel, etc. Comprehensive performance; at the same time, Ca element has a good desulfurization effect, can change the composition, quantity and shape of non-metallic inclusions, can improve the resistance to hydrogen-induced cracking and lamellar tearing, and can prolong the service life of parts. In order to control the production cost, the content of Ca element is controlled at 0.005%-0.150% in the present invention.

Mg: Magnesium is a good deoxidizer, desulfurizer and spheroidizer in steel. Mg can reduce the number of inclusions in steel, reduce their size, distribute them evenly, and improve their shape. Trace amounts of magnesium can improve the size and distribution of carbides in DH steel, and promote fine and uniform carbide particles. In order to control the production cost, the content of Mg element is controlled at 0.002%-0.150% in the present invention, Ca and Mg are used together in the present invention, and Ca+Mg: 0.05-0.20% is satisfied.

The second technical solution of the present invention is to provide a new method for preparing cold-rolled DH980 steel with a high yield ratio, which includes the following steps: converter smelting, continuous casting and rolling of medium and thin slabs, pickling and cold rolling, continuous annealing, skin skinning;

Converter smelting: smelting through a converter to obtain molten steel that meets the following composition requirements in terms of mass percentage, C: 0.10% to 0.25%, Mn: 1.50% to 2.50%, Si: 0.20% to 1.80%, Al: 0.02% ~1.0%, P≤0.01%, S≤0.01%, N≤0.005%, Ti: 0.010%～0.150%, Ca: 0.005%～0.150%, Mg: 0.002%～0.150%, and satisfy C/Ti:2 ～10; Ca+Mg: 0.05～0.20%; the balance is Fe and unavoidable impurities, and the molten steel temperature is between 1600～1700℃.

Continuous casting and rolling of medium and thin slabs: the casting temperature is 1530-1580°C, the casting speed of the casting machine is 0.8-5.5m/min, and the thickness of the continuous casting slab is between 60-115mm. The slab is pulled out and cut into the furnace directly, the heating temperature is 1100-1300°C, the holding time is 30-120min, the starting rolling temperature is 1000-1150°C, the final rolling temperature is 860-900°C, the coiling temperature is Between 500 and 600°C. Preferably, the thickness of the hot-rolled steel plate is 2.0-4.5 mm, and the microstructure of the hot-rolled product consists of 20%-50% (volume ratio) ferrite, 30%-60% (volume ratio) pearlite, 10%-30% (Volume ratio) Bainite, the rest is composed of a small amount of cementite; the sum is 100%.

Pickling and cold rolling: Before cold rolling, the hot-rolled steel coils use acid solution to remove the oxide scale on the surface, and the cold rolling reduction rate is 40% to 70%. If the reduction rate is too high, the deformation resistance will be too large, making it difficult to roll to the target thickness; if the reduction rate is too low, the elongation of the cold-rolled steel sheet will decrease.

Continuous annealing: the belt speed is controlled at 60-180m/min, the soaking section temperature is 800-900°C, the soaking time is 10-600s, the slow cooling outlet temperature is 650-700°C, and the rapid cooling rate is greater than 25°C/s. The outlet temperature of the quick cooling is 300～450℃,

Skin-passing: without aging treatment, it is directly air-cooled into skin-passing. The skin-passing process adopts rolling force control. The rolling force is controlled at 2000-4000kN, and the rolling tension is 1000-2000kN. The microstructure of the cold-rolled continuous annealing product is 20%-50% (volume ratio) ferrite, 40%-70% (volume ratio) martensite and 4%-15% (volume ratio) retained austenite. The heating temperature of the furnace in the soaking section is 800-900°C. If the temperature in the soaking section is too high, the ductility of the steel will be reduced due to the complete austenitization and the insufficient proportion of ferrite; if the temperature of the annealing furnace is too low, the final An excessively high proportion of the soft phase ferrite in the material will significantly reduce the strength of the material. The soaking time is 10-600s. If the soaking time is too long, the grains of the steel plate will be coarse, and the annealing time is too short. The steel plate will not complete the annealing and recrystallization process in a hurry, resulting in a decrease in the elongation of the steel plate.

Through the above method, the yield strength of the new high-yield-ratio cold-rolled DH980 steel is ≥700MPa, the tensile strength is 980-1100MPa, the elongation after breaking A50 is ≥20%, the yield ratio is ≥0.70, and the hole expansion rate is ≥30%.

The beneficial effects of the present invention are:

(1) The new high-yield ratio cold-rolled DH980 steel produced by the present invention is designed by optimizing the alloy composition, with C, Mn, and Si as the main elements, without adding precious metal elements such as Cr, Mo, Nb, and V, and the alloy cost is very low .

(2) The novel high-yield-ratio cold-rolled DH980 steel produced by the present invention adopts the production process of converter smelting-medium-thin slab continuous casting and rolling-pickling cold rolling-continuous annealing-skin skinning, which can be done on the traditional production line To realize the industrial production of DH980 steel for automobiles, no new production equipment needs to be added; The aging process is saved in the continuous annealing process, the DH steel process is significantly shortened and the production is stable, and the manufacturing cost of the product is greatly reduced.

(3) The novel high-yield ratio cold-rolled DH980 steel prepared by the present invention has yield strength ≥ 700MPa, tensile strength 980-1100MPa, A50 elongation after breaking ≥ 20%, yield ratio ≥ 0.70; hole expansion rate ≥ 30 %, while ensuring the short process and low cost of this product, it also takes into account the advantages of excellent plasticity and hole expansion performance.

(4) The new high-yield-ratio cold-rolled DH980 steel produced by the present invention adds a certain proportion of retained austenite on the basis of traditional cold-rolled dual-phase steel, and under the effect of transformation-induced plasticity (TRIP), realizes It is characterized by high ductility and high formability.

(5) The microstructure of the novel high-yield ratio cold-rolled DH980 steel hot-rolled steel sheet produced by the present invention is as follows by volume percentage: 20% to 50% ferrite, 30% to 60% pearlite, and 10% to 30% shellfish The rest is composed of a small amount of cementite; the microstructure of the steel plate after cold rolling and continuous annealing is calculated by volume percentage as follows: 20% to 50% ferrite, 40% to 70% martensite and 4% to 15% residual austenitic.

Description of drawings

Figure 1 is a microstructure diagram of Example 1 of the present invention.

Fig. 2 is a typical engineering stress-strain curve diagram of Embodiment 1 of the present invention.

Detailed ways

Below by embodiment the present invention will be further described.

In the embodiment of the present invention, according to the component ratio of the technical solution, converter smelting, continuous casting and rolling of medium and thin slabs, pickling and cold rolling, continuous annealing and skin pass are carried out;

Continuous casting and rolling of medium and thin slabs: the casting temperature is 1530-1580°C, the casting speed of the casting machine is 0.8-5.5m/min, and the thickness of the continuous casting slab is between 60-115mm; The heating temperature is 1100-1300°C, the holding time is 30-120min, the rolling start temperature is 1000-1150°C, the final rolling temperature is 860-900°C, and the coiling temperature is 500-600°C;

Pickling and cold rolling: the cold rolling reduction rate is 40% to 70%;

Continuous annealing: the belt speed is controlled at 60-180m/min, the soaking section temperature is 800-900°C, the soaking time is 10-600s, the slow cooling outlet temperature is 650-700°C, and the rapid cooling rate is greater than 25°C/s. The outlet temperature of quick cooling is 300～450℃;

Skin pass: The rolling force during the pass pass is controlled at 2000-4000kN, and the rolling tension is 1000-2000kN.

Further; the temperature of molten steel in the converter is 1600-1700°C.

Further; the thickness specification of the hot-rolled coil after continuous casting and rolling of the medium-thin slab is 2.0-4.5mm, and the microstructure of the hot-rolled state is ferrite + pearlite + bainite + a small amount of cementite; the volume percentage is as follows, 20% to 50% of ferrite, 30% to 60% of pearlite, 10% to 30% of bainite, and the rest is a small amount of cementite.

The composition of the steel of the embodiment of the present invention is shown in Table 1. The main technological parameters of the steel continuous casting and rolling in the embodiment of the present invention are shown in Table 2. The main process parameters of the cold rolling and annealing of the steel of the embodiment of the present invention are shown in Table 3. The performance of the steel of the embodiment of the present invention is shown in Table 4. The steel structure of the embodiment of the present invention is shown in Table 5.

Table 1 Composition (wt %) of the embodiment steel of the present invention

Note: C/Ti has no unit

Table 2 The main process parameters of steel continuous casting and rolling in the embodiment of the present invention

Table 3 The main technological parameters of cold rolling and annealing of the steel of the embodiment of the present invention

Table 4 Performance and structure of the steel of the embodiment of the present invention

Table 5 steel structure of embodiment of the present invention (hot rolling, cold rolling)

As can be seen from the above, the novel high yield ratio cold-rolled DH980 steel prepared by the present invention has yield strength ≥ 700MPa, tensile strength 980-1100MPa, A50 elongation after fracture ≥ 20%, yield ratio ≥ 0.70; hole expansion rate ≥ 30%, while ensuring the short process and low cost of this product, it also takes into account the advantages of excellent plasticity and hole expansion performance.

In order to describe the present invention, the present invention has been properly and fully described through the examples above. The above embodiments are only used to illustrate the present invention, rather than to limit the present invention. In the case of the spirit and scope of the invention, various changes and modifications can also be made, and any modifications, equivalent replacements, improvements, etc., should be included in the protection scope of the present invention, and the patent protection scope of the present invention should be determined by Claims limited.

Claims (6)

1. A novel high-yield-ratio cold-rolled DH980 steel is characterized in that the steel comprises the following components in percentage by weight: c: 0.10-0.25%, mn:1.50% -2.50%, si: 0.20-1.80%, al: 0.02-1.0%, P is less than or equal to 0.01%, S is less than or equal to 0.01%, N is less than or equal to 0.005%, ti: 0.010-0.150%, ca: 0.005-0.150%, mg:0.002% -0.150% and meets the requirement of 2-10% of C/Ti; ca + Mg:0.05 to 0.20 percent; the balance being Fe and unavoidable impurities.

2. The novel high yield ratio cold-rolled DH980 steel as claimed in claim 1, wherein the yield strength of the DH980 steel is 700MPa or more, the tensile strength is 980-1100MPa, the A50 elongation after fracture is 20% or more, and the yield ratio is 0.70 or more; the hole expanding rate is more than or equal to 30 percent.

3. The novel high yield ratio cold rolled DH980 steel as claimed in claim 1, wherein the microstructure of the DH980 steel is ferrite + martensite + retained austenite; the percentage by volume is as follows, ferrite 20% -50%, martensite 40% -70%, retained austenite 4% -15%.

4. A preparation method of a novel high-yield-ratio cold-rolled DH980 steel as claimed in any one of claims 1 to 3, comprising converter smelting, medium and thin slab continuous casting and rolling, acid pickling and cold rolling, continuous annealing and finishing; the method is characterized in that:

continuous casting and rolling of medium and thin slabs: the casting temperature is 1530-1580 ℃, the casting machine drawing speed is 0.8-5.5 m/min, and the thickness of the continuous casting billet is 60-115 mm; directly feeding the casting blank into a furnace after pulling out and cutting, wherein the heating temperature is 1100-1300 ℃, the heat preservation time is 30-120 min, the initial rolling temperature is 1000-1150 ℃, the final rolling temperature is 860-900 ℃, and the coiling temperature is 500-600 ℃;

acid pickling and cold rolling: the cold rolling reduction rate is 40-70%;

and (3) continuous annealing: the belt speed is controlled at 60-180 m/min, the temperature of a soaking section is 800-900 ℃, the soaking time is 10-600 s, the temperature of a slow cooling outlet is 650-700 ℃, the fast cooling rate is more than 25 ℃/s, and the temperature of a fast cooling outlet is 300-450 ℃;

finishing: the rolling force in the finishing process is controlled to be 2000-4000 kN, and the rolling tension is controlled to be 1000-2000 kN.

5. The preparation method of the novel high yield ratio cold rolled DH980 steel as claimed in claim 4, wherein the method comprises the following steps: the temperature of the molten steel in the converter is 1600-1700 ℃.

6. The preparation method of the novel high yield ratio cold rolled DH980 steel as claimed in claim 4, wherein the method comprises the following steps: after the medium and thin slab is continuously cast and rolled, the thickness specification of a hot-rolled coil is 2.0-4.5 mm, and the hot-rolled microstructure is ferrite, pearlite, bainite and a small amount of cementite; the steel comprises, by volume, 20-50% of ferrite, 30-60% of pearlite, 10-30% of bainite, and the balance of a small amount of cementite.

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