Cold-rolled ultrahigh-strength steel plate with high burr sensitivity coefficient and preparation method of cold-rolled ultrahigh-strength steel plate

Abstract

The invention discloses a cold-rolled ultrahigh-strength steel plate with a high burr sensitivity coefficient and a preparation method of the cold-rolled ultrahigh-strength steel plate. The cold-rolled ultrahigh-strength steel plate comprises, by mass, 0.15%-0.22% of C, 1.0%-1.8% of Si, 1.8%-2.6% of Mn, 3.2% < = (Si + Mn) < = 3.8%, less than or equal to 0.010% of P, less than or equal to 0.005% of S, 0.01%-0.12% of Als, 0.10%-0.50% of Cr, 0.02%-0.06% of Nb, 0.01%-0.04% of Ti, less than or equal to 0.0045% of N and the balance iron and inevitable impurities. Through design of alloy components and collaborative optimization of the process, the continuous annealing key process is set to control the regulation and control phase proportion, in the actual application process, the high burr sensitivity is achieved, the punching and flanging performance is improved, and batch production and application of products are facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of materials and metallurgy, and more specifically, to a cold-rolled ultra-high-strength steel plate with a high burr sensitivity coefficient and a preparation method thereof. Background Art

[0002] As people pay more attention to automobile lightweighting and collision safety, and with the increasing prominence of energy shortages and environmental pollution, high-strength automobile steel is an important way to achieve automobile lightweighting and improve safety performance. Advanced high-strength steel is a new type of steel that has emerged in recent years. Bainitic steel, which has bainite and retained austenite as the matrix structure and second phase respectively, has attracted much attention. It avoids the problem of soft-hard phase hardness difference in dual-phase steel and can improve the ductility and local formability of steel.

[0003] When the existing 1180MPa-level advanced ultra-high strength steel (such as Q&P, DP) is used to manufacture parts with complex forming and high requirements for forming performance, local necking, cracking or even fracture is prone to occur during the hole expansion and flanging process. Through the study of the cracking phenomenon during hole expansion, it is found that it is mainly related to the hardness difference between the soft and hard phases in the ultra-high strength steel structure. The ability to coordinate deformation with each other during deformation is poor, and microcracks are easily generated at the interface, resulting in poor extended flange performance and cracking. In the application process of 1180MPa ultra-high strength steel, it was also found that after punching, the direction of the burr is also one of the main factors affecting the hole expansion and flanging performance. Under the same equipment conditions, the sensitivity of the burr after punching is inevitably related to the structure of the ultra-high strength steel.

[0004] The Chinese patent with publication number CN 110724877 A discloses a 1180MPa grade high plasticity bainite complex phase steel plate for automobiles and its preparation method, the chemical composition of which is as follows by weight percentage: C: 0.03-0.20, Si: 0.05-1.50%, Mn: 0.5-2.5%, P≤0.05%, S≤0.05%, Nb: 0.01-0.04%, Ti: 0.01-0.04%, V: 0.01-0.15%, Al: 0.015-1.5%, and the replaceable components contain as follows by weight percentage: Cr: 0-2%, Mo: 0-0.5%, Cu: 0.05-0.5%, and the balance is Fe and unavoidable impurities. The range of Si and Mn content added in this patent is very wide, and the low Si and low Mn state is not enough to achieve a 1180MPa grade strength product, and the key annealing process system for regulating the phase ratio is not clearly defined.

[0005] The Chinese patent with publication number CN 109778062 A discloses a cold-rolled complex phase steel with a tensile strength of 1200 MPa and a preparation method thereof, wherein the chemical composition by weight percentage is: C: 0.1-0.15%, Si: 0.1-0.5%, Mn: 1.5-2.6%, Cr: 0.4-0.7%, Mo: 0.2-0.5%, Nb: 0.02-0.05%, Ti: 0.02-0.05%, P≤0.02%, S≤0.015%, and the remainder is iron and other unavoidable impurity elements; and the following conditions are satisfied at the same time: C+(Si+Mn) / 6+(Cr+Mo+V) / 5≤0.8. The patent adopts Nb-Ti microalloying to prepare a cold-rolled complex phase steel with a tensile strength of 1200 MPa. The plasticity index of the product is low, and the punching burr sensitivity and hole expansion and flanging performance that are concerned by automobile OEMs during application are not mentioned, so its application effect cannot be evaluated. Furthermore, the existing technologies do not involve the analysis of the influence of hole expansion and flanging burrs during the application process of automobile OEMs.

[0006] Therefore, it is urgent to develop a cold-rolled ultra-high strength steel plate with good burr sensitivity to guide the application issues in the hole expansion and flanging forming process of the main engine plant. Summary of the invention

[0007] The purpose of the present invention is to overcome the above-mentioned defects of the prior art and to provide a cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient and a preparation method thereof. By controlling and regulating the phase ratio through the key process of continuous annealing, the steel plate has a higher burr sensitivity in actual application, improves the punching and flanging performance, and is conducive to mass production and application of the product.

[0008] To achieve the above object, the technical solution of the present invention is as follows:

[0009] A cold-rolled ultra-high-strength steel plate with a high burr sensitivity coefficient comprises the following components in mass percentage: C: 0.15% to 0.22%, Si: 1.0% to 1.8%, Mn: 1.8% to 2.6%, 3.2%≤(Si+Mn)≤3.8%, P≤0.010%, S≤0.005%, Als: 0.01% to 0.12%, Cr: 0.10% to 0.50%, Nb: 0.02% to 0.06%, Ti: 0.01% to 0.04%, N≤0.0045%, and the balance is iron and unavoidable impurities.

[0010] The present invention also discloses a method for preparing the cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient as described above, comprising the following steps: a smelting process, a continuous casting process, a hot rolling process, a pickling cold rolling process, and a continuous annealing process;

[0011] In the smelting process, scrap steel and molten iron are used as furnace charge;

[0012] In the continuous casting process, the molten steel after smelting is continuously cast into slabs;

[0013] In the hot rolling process, the slabs produced in the continuous casting process are hot rolled to obtain hot rolled coils;

[0014] In the pickling cold rolling process, the hot rolled coil treated by the hot rolling process is pickled to remove the oxide layer on the surface, and then cold rolled to obtain a cold rolled thin strip steel coil;

[0015] In the continuous annealing process, the cold-rolled thin strip steel coil is continuously annealed, the soaking temperature is 860°C to 900°C, and the over-aging temperature is 370°C to 390°C, so as to obtain the cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient.

[0016] Implementing the embodiments of the present invention will have the following beneficial effects:

[0017] 1. The chemical composition of the cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient of the present invention mainly includes C, Si and Mn as main elements, and the sum of Si content and Mn content is controlled at 2.8% to 3.60%, while playing the role of inhibiting cementite precipitation and stabilizing austenite. Compared with other ultra-high strength steel products for automobiles with a strength grade ≥1180MPa, the design of alloy composition and the coordinated optimization of process form a cold-rolled steel plate with a higher strength grade and takes into account both plasticity and formability.

[0018] 2. In the preparation method of the present invention, the continuous annealing is carried out through three-stage temperature control of the over-aging stage, without the need to adopt the technical means of quenching and re-partitioning, which can meet the needs of the traditional acid rolling-continuous annealing production line on a large scale, and has the advantages of low production cost and stable process.

[0019] 3. The microstructure of the cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient prepared by the present invention is composed of 80% to 90% bainite, 2% to 8% martensite and 8% to 12% retained austenite, and the ratio of bainite to retained austenite is between 6.5 and 11.5. The high proportion of bainite can achieve a steel plate strength level of 1180MPa while taking into account high yield, high elongation and high forming characteristics. The ratio of bainite to retained austenite is directly related to the punching burr sensitivity. As the bainite / austenite ratio increases, the burr sensitivity coefficient increases, so that the burr sensitivity of the cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient prepared is ≥0.1, and at the same time, it meets the use of automotive structural parts (subframes, B-pillars, front longitudinal beams) with high strength plasticity and hole expansion and flanging performance requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the SEM microstructure of the cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient according to Example 1 of the present invention. DETAILED DESCRIPTION

[0021] The present invention is further described below in conjunction with specific embodiments, but the present invention is not limited in any way.

[0022] 1. Chemical composition and mechanical properties

[0023] The invention discloses a cold-rolled ultra-high-strength steel plate with a high burr sensitivity coefficient, comprising the following components in mass percentage: C: 0.15%-0.22%, Si: 1.0%-1.8%, Mn: 1.8%-2.6%, 3.2%≤(Si+Mn)≤3.8%, P≤0.010%, S≤0.005%, Als: 0.01%-0.12%, Cr: 0.10%-0.50%, Nb: 0.02%-0.06%, Ti: 0.01%-0.04%, N≤0.0045%, and the balance is iron and unavoidable impurities.

[0024] In a specific embodiment, the burr sensitivity coefficient of the cold-rolled ultra-high strength steel plate is ≥ 0.1. The burr sensitivity coefficient η is calculated as follows:

[0025]

[0026] Among them, λ 0，0 Indicates the hole expansion rate with burrs pointing upward and penetrating cracks, λ 1，0 It indicates the hole expansion rate with the burr facing downward and penetrating the crack, and the hole expansion rate is measured in accordance with the provisions of T\CSAE 332-2023 group standard.

[0027] In a specific embodiment, the microstructure of the cold-rolled ultra-high strength steel plate is composed of 80% to 90% by mass of bainite, 2% to 8% by mass of martensite, and 8% to 12% by mass of retained austenite; the ratio of bainite to retained austenite is 6.5 to 11.5.

[0028] In a specific embodiment, the yield strength of the cold-rolled ultra-high strength steel plate is 830 MPa to 900 MPa.

[0029] In a specific embodiment, the tensile strength of the cold-rolled ultra-high strength steel plate is 1180 MPa to 1250 MPa.

[0030] In a specific embodiment, the elongation A50 value of the cold-rolled ultra-high strength steel plate is 14% to 18%.

[0031] In a specific embodiment, the hole expansion ratio of the cold-rolled ultra-high strength steel plate is 30% to 40%.

[0032] 2. Production technology

[0033] The present invention also discloses a method for preparing a cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient as in any embodiment of the present invention, comprising the following steps: a smelting process, a continuous casting process, a hot rolling process, a pickling cold rolling process, and a continuous annealing process.

[0034] (1) In the smelting process, scrap steel and molten iron are used as furnace charge.

[0035] (2) In the continuous casting process, the molten steel after smelting is continuously cast into slabs.

[0036] (3) In the hot rolling process, the slab produced in the continuous casting process is hot rolled to obtain a hot rolled coil.

[0037] (4) In the pickling cold rolling process, the hot rolled coil after the hot rolling process is pickled to remove the surface oxide layer, and then cold rolled to obtain a cold rolled thin strip steel coil.

[0038] (5) In the continuous annealing process, the cold-rolled thin strip steel coil is continuously annealed, the soaking temperature is 860° C. to 900° C., and the overaging temperature is 370° C. to 390° C., so as to obtain a cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient.

[0039] In a specific embodiment, in the continuous annealing process, the temperature of the over-aging section is controlled in stages, the temperature of the over-aging section 1 is 380°C to 390°C, the temperature of the over-aging section 2 is 380°C to 390°C, and the temperature of the over-aging section 3 is 370°C to 380°C; the unit speed is 40m / min to 80m / min.

[0040] In a specific embodiment, in the hot rolling process, the slabs produced in the continuous casting process are heated to 1180°C to 1280°C, and the heated slabs are sequentially subjected to high-pressure dephosphorization, rough rolling, and finish rolling, and are coiled after controlled cooling; wherein the finishing rolling temperature is 850°C to 950°C; and the coiling temperature is 600°C to 680°C.

[0041] In a specific embodiment, in the pickling cold rolling process, the cold rolling reduction is 40% to 60%.

[0042] The following are specific embodiments

[0043] Examples 1-2 and Comparative Example 1

[0044] The chemical compositions of the cold-rolled ultra-high strength steel sheets with high burr sensitivity coefficients of Examples 1-2 of the present invention and Comparative Example 1 are shown in Table 1.

[0045] Table 1 Chemical composition of cold-rolled ultra-high strength steel sheets with high burr sensitivity coefficient (%)

[0046] C Si Mn Si+Mn P S Als Cr Ti Nb N Example 1 0.190 1.3 2.25 3.55 0.008 0.002 0.045 0.25 0.026 0.040 0.0032 Example 2 0.202 1.35 2.29 3.64 0.008 0.003 0.030 0.32 0.015 0.035 0.0030 Comparative Example 1 0.172 1.1 2.21 3.31 0.008 0.002 0.045 0.43 0.033 0.055 0.0031

[0047] The preparation methods of Examples 1-2 and Comparative Example 1 of the present invention are as follows:

[0048] The molten steel after smelting treatment is continuously cast into slabs. The slabs are heated, descaled, rough rolled, finished rolled, laminar cooled and coiled to obtain hot rolled coils. The hot rolling process of Examples 1-2 of the present invention and Comparative Example 1 is shown in Table 2.

[0049] Table 2 Hot rolling process parameters of cold-rolled ultra-high strength steel plates with high burr sensitivity coefficient

[0050]

[0051]

[0052] Pickling and cold rolling process: After the hot-rolled coil is pickled to remove the iron oxide scale on the surface, it is cold-rolled into cold-rolled thin strip steel, wherein the thickness of the strip steel in Example 1, Example 2, and Comparative Example 1 are 1.6 mm, 1.8 mm, and 1.2 mm, respectively, and the cold rolling reduction rates are 46.7%, 48.5%, and 52.0%, respectively. Continuous annealing process: The cold-rolled thin strip is processed by a continuous annealing process to form the desired product. The strip is first slowly heated to the soaking temperature (complete austenitization temperature) and kept for a period of time to be fully austenitic, then slowly cooled to the slow cooling end temperature to decompose part of the supercooled austenite, forming a small amount of oriented epitaxial ferrite and "carbon-enriching" the remaining austenite. Thereafter, the strip is rapidly cooled to transform the austenite into martensite and bainite, and the austenite to bainite transformation occurs in the over-aging stage. Finally, it is cooled to room temperature, and the strip shape is adjusted by a skin pass mill to increase the yield strength. The continuous annealing process parameters of Examples 1-2 of the present invention and Comparative Example 1 are shown in Table 3.

[0053] Table 3 Main process parameters of continuous annealing of cold-rolled ultra-high strength steel plates with high burr sensitivity coefficient

[0054]

[0055] Test Case

[0056] 1. Comparison of the microstructures of the cold-rolled ultra-high strength steel sheets with high burr sensitivity coefficients of Examples 1-2 and Comparative Example 1 is shown in Table 4. A typical scanning electron microscope photograph of the cold-rolled ultra-high strength steel sheet with high burr sensitivity coefficient prepared in Example 1 is shown in Table 4. Figure 1 shown.

[0057] Table 4 Microstructure ratio of cold-rolled ultra-high strength steel sheets with high burr sensitivity coefficient, %

[0058] bainite Martensite Retained austenite Example 1 85 5 10 Example 2 86 4 12 Comparative Example 1 74 9 17

[0059] 2. According to GB / T228-2010 "Room Temperature Tensile Test Method for Metallic Materials" and GB / T24524-2021 "Hole Expansion Test Method for Metallic Materials Sheets and Strips", the performance of the cold-rolled ultra-high strength steel plates with high burr sensitivity coefficients of Examples 1-2 and Comparative Example 1 was tested, and the mechanical properties are shown in Table 5 below:

[0060] Table 5 Mechanical properties of cold-rolled ultra-high strength steel plates with high burr sensitivity coefficient

[0061]

[0062]

[0063] In summary, compared with Comparative Example 1, the present invention designs the optimal component system, and matches the smelting, continuous casting, hot rolling, pickling cold rolling, and continuous annealing processes with the alloy composition through the three-stage temperature control of the over-aging stage. The organizational characteristics of the cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient are composed of fine and regular carbide-free bainite lath bundles and residual austenite distributed between the lath bundles on the bainite matrix. At the same time, there is also a very small amount of tempered martensite structure, and it has good comprehensive mechanical properties. At the same time, it plays a role in inhibiting the precipitation of cementite and stabilizing austenite. The design of alloy composition and the coordinated optimization of process form a cold-rolled steel plate with higher strength grade and take into account plasticity and formability, with higher burr sensitivity, improved punching and flanging performance, and conducive to mass production and application of products.

[0064] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient, characterized in that: The following components are included in mass percentage: C: 0.15% ~ 0.22%, Si: 1.0% ~ 1.8%, Mn: 1.8% ~ 2.6%, 3.2% ≤ (Si + Mn) ≤ 3.8%, P ≤ 0.010%, S ≤ 0.005%, Als: 0.01% ~ 0.12%, Cr: 0.10% ~ 0.50%, Nb: 0.02% ~ 0.06%, Ti: 0.01% ~ 0.04%, N ≤ 0.0045%, and the balance is iron and unavoidable impurities.

2. The cold-rolled ultra-high strength steel plate with high burr sensitivity coefficient according to claim 1, characterized in that: The burr sensitivity coefficient of the cold-rolled ultra-high strength steel plate is ≥0.

1.

3. The cold-rolled ultra-high strength steel plate with high burr sensitivity coefficient according to claim 1, characterized in that: The microstructure of the cold-rolled ultra-high strength steel plate consists of 80% to 90% by mass of bainite, 2% to 8% by mass of martensite and 8% to 12% by mass of retained austenite; the ratio of the bainite to the retained austenite is 6.5 to 11.

5.

4. The cold-rolled ultra-high strength steel plate with high burr sensitivity coefficient according to claim 1, characterized in that: The yield strength of the cold-rolled ultra-high strength steel plate is 830MPa to 900MPa; The tensile strength of the cold-rolled ultra-high strength steel plate is 1180MPa to 1250MPa; The elongation A50 value of the cold-rolled ultra-high strength steel plate is 14% to 18%; The hole expansion rate of the cold-rolled ultra-high strength steel plate is 30% to 40%.

5. A method for preparing a cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient as claimed in any one of claims 1 to 4, characterized in that: The following steps are involved: Smelting process, continuous casting process, hot rolling process, pickling and cold rolling process, continuous annealing process; In the smelting process, scrap steel and molten iron are used as furnace charge; In the continuous casting process, the molten steel after smelting is continuously cast into slabs; In the hot rolling process, the slabs produced in the continuous casting process are hot rolled to obtain hot rolled coils; In the pickling cold rolling process, the hot rolled coil treated by the hot rolling process is pickled to remove the oxide layer on the surface, and then cold rolled to obtain a cold rolled thin strip steel coil; In the continuous annealing process, the cold-rolled thin strip steel coil is continuously annealed, the soaking temperature is 860°C to 900°C, and the over-aging temperature is 370°C to 390°C, so as to obtain the cold-rolled ultra-high strength steel plate with a high burr sensitivity coefficient.

6. The preparation method according to claim 5, characterized in that: In the continuous annealing process, the temperature of the over-aging section is controlled in stages, the temperature of over-aging section 1 is 380°C to 390°C, the temperature of over-aging section 2 is 380°C to 390°C, and the temperature of over-aging section 3 is 370°C to 380°C; the unit speed is 40m / min to 80m / min.

7. The preparation method according to claim 5, characterized in that: In the hot rolling process, the slabs produced in the continuous casting process are heated to 1180°C to 1280°C, and the heated slabs are sequentially subjected to high-pressure dephosphorization, rough rolling, and finish rolling, and are coiled after controlled cooling; wherein the finishing rolling temperature is 850°C to 950°C; and the coiling temperature is 600°C to 680°C.

8. The preparation method according to claim 5, characterized in that: In the pickling cold rolling process, the cold rolling reduction is 40% to 60%.

Images