High-surface low-cost gigabar-level quenched partition steel and production method thereof

Abstract

The invention discloses high-surface low-cost gigabar-grade quenched partition steel and a production method thereof. The quenched partition steel comprises the following chemical components: 0.17-0.25% of C, 1.50-3.00% of Mn, less than or equal to 0.005% of S, less than or equal to 0.020% of P, 1.30-2.00% of Si, 0.010-0.080% of Als, less than or equal to 0.060% of Nb, less than or equal to 0.100% of Ti, less than or equal to 0.100% of Sn and the balance of Fe and unremovable impurities. The production method comprises the procedures of continuous casting, heating, hot rolling, acid pickling, cold rolling, continuous annealing and leveling. Industrial mass production can be achieved through synergistic coupling of alloy elements and a conventional continuous annealing device, the product structure is ferrite, martensite, bainite and retained austenite, the yield strength Rp0.2 ranges from 650 MPa to 950 MPa, the tensile strength Rm ranges from 950 MPa to 1100 MPa, and the percentage elongation after fracture A50 is larger than or equal to 20%.

Description

Technical Field

[0001] The present invention belongs to the field of metallurgical technology, and particularly relates to a GPa-level quenching and partitioning steel with high surface quality and low cost and a production method thereof. Background Art

[0002] Ultra-high strength quenching and partitioning steel obtains higher work hardening performance and elongation level by adding elements such as Si and Mn, optimizing the microstructure, using martensite as the matrix phase, and utilizing the transformation-induced plasticity effect of retained austenite during deformation; this makes QP steel have higher plasticity and formability than ultra-high strength steel of the same level. With the increasing demand for lightweight and energy conservation in the automotive industry, ultra-high strength quenching and partitioning steel has a broader application prospect in structural parts such as automotive skeletons and bumpers.

[0003] The patent application with the publication number CN114381655A provides a high-strength and high-ductility cold-rolled QP steel and its annealing process and manufacturing method. First, the strip is heated from room temperature to 860 - 950°C at a heating rate of 50 - 300°C / s, held for 0 - 100 s, then the strip is cooled to 260 - 320°C at a cooling rate of 50 - 100°C / s, and then the strip is heated to 390 - 430°C at a rate of 10 - 30°C / s and held in this temperature range for 90 - 200 s, finally obtaining a yield strength of 750 - 1100 MPa, a tensile strength of 1000 - 1400 MPa, and an elongation of 17 - 22%. The production process of this technology is complex, requires the production line to have a re-heating function, and the additional cost of secondary partitioning of the strip by re-heating increases the production cost.

[0004] The patent application with the application number 20081011982.5 provides a production method of high-strength cold-rolled TRIP steel, which is strengthened by adding alloying elements such as Nb, V, Ti, Cu, and Ni, and the maximum tensile strength reaches 1000 MPa; however, due to its composition design with only high Si, it is difficult to solve the problem of surface quality.

[0005] In view of the above situation, based on a conventional continuous annealing production line, through composition design and microstructure control, the present invention provides a GPa-level quenching and partitioning steel with high surface quality and low cost, obtaining a ferrite + martensite + bainite + retained austenite microstructure in the finished strip, with good surface quality and low production cost. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a GPa-level quenching and partitioning steel with high surface quality and low cost and a production method thereof, and the provided quenching and partitioning steel has a yield strength Rp 0.2 : 650 - 950 MPa, a tensile strength Rm: 950 - 1100 MPa, an elongation after fracture A 50 ≥20%, with good surface quality and low industrial production cost.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is as follows: A high-surface low-cost GPa-level quenching and partitioning steel, the chemical composition and its mass percentage content are as follows: C: 0.17 - 0.25%, Mn: 1.5 - 3.0%, S ≤ 0.005%, P ≤ 0.020%, Si: 1.3 - 2.0%, Als: 0.01 - 0.08%, Nb: ≤ 0.060%, Ti: ≤ 0.100%, Sn ≤ 0.100%, and the rest is Fe and ineradicable impurities.

[0008] Preferably, for the high-surface low-cost GPa-level quenching and partitioning steel of the present invention, the chemical composition and its mass percentage content are C: 0.18 - 0.22%, Mn: 1.90 - 2.40%, S ≤ 0.003%, P ≤ 0.015%, Si: 1.40 - 1.80%, Als: 0.030 - 0.060%, Nb: 0.020 - 0.050%, Ti: 0.040 - 0.060%, Sn: 0.06 - 0.085%, and the rest is Fe and ineradicable impurities.

[0009] The main reason for adding alloy tin to the strip steel of the present invention is that its chemical properties are stable, it is not easily oxidized, and it has strong corrosion resistance. It has no reaction with water, water vapor, and carbon dioxide at normal temperature. Adding it to steel can significantly inhibit internal oxidation in hot coils, especially intergranular oxidation, which helps to improve the surface quality of products.

[0010] Furthermore, the thickness of the quenching and partitioning steel of the present invention is 0.8 - 2.5 mm.

[0011] Furthermore, for the quenching and partitioning steel of the present invention, the tensile strength Rm: 950 - 1100 MPa, the yield strength Rp0.2: 650 - 950 MPa, and the elongation A 50 ≥ 20%.

[0012] Furthermore, the structure of the quenching and partitioning steel of the present invention is composed of ferrite + martensite + bainite + retained austenite.

[0013] The present invention also provides a production method of a high-surface low-cost GPa-level quenching and partitioning steel, including continuous casting, heating, hot rolling, pickling, cold rolling, continuous annealing, and skin pass rolling processes.

[0014] Furthermore, in the continuous casting process of the present invention, the tundish temperature is 1500 - 1520 °C, and the casting speed is 1.0 - 1.2 m / min.

[0015] Furthermore, in the heating process of the present invention, the heating temperature is 1270 - 1330 °C, and the total heating time is 150 - 240 min.

[0016] Furthermore, in the hot rolling process of the present invention, the finish rolling temperature in the finishing mill is 870 - 900 °C, and the coiling temperature is 580 - 630 °C.

[0017] Furthermore, in the pickling process of the present invention, the acid solution temperature ≥ 80 °C, and the free acid concentration ≥ 50 g / L.

[0018] Furthermore, in the cold rolling process of the present invention, the cold rolling reduction rate ≥ 45%.

[0019] Furthermore, in the continuous annealing process of the present invention, the soaking temperature is 780 - 840 °C, and the holding time is 80 - 160 s; first, it is slowly cooled to 680 - 720 °C, and the slow cooling rate is 10 - 20 °C / s; then it is rapidly cooled to 340 - 380 °C, and the rapid cooling rate is 45 - 75 °C / s. The temperature in the overaging section is 320 - 360 °C, and the final cooling temperature is 50 - 150 °C.

[0020] Furthermore, in the annealing process of the present invention, the dew point in the annealing furnace is -30 - -50 °C.

[0021] One of the key points in the annealing process is the control of the soaking, rapid cooling, and overaging temperatures. When heating in the two-phase region, the formation and growth of austenite are diffusion processes. In Fe-C alloys, the diffusion of carbon will control the growth of austenite. Different heating temperatures result in different austenite contents. Therefore, in the subsequent slow cooling and rapid cooling stages, the volume fractions and compositions of ferrite, martensite, and retained austenite are different. When the strip is cooled at a rate exceeding the critical cooling rate to below the martensite transformation point (Ms point), austenite will transform into martensite. Then, it undergoes a relatively long overaging section, which allows the carbon atoms in the martensite to diffuse into the retained austenite to improve its room temperature stability, and at the same time, the martensite matrix is tempered; therefore, in the present invention, the soaking temperature of the Q&P steel is controlled at 780 - 840 °C. After holding for 80 - 160 s, it is first slowly cooled to 680 - 720 °C, and the slow cooling rate is 10 - 20 °C / s; then it is rapidly cooled to 340 - 380 °C, and the rapid cooling rate is 45 - 75 °C / s. After the rapid cooling section, overaging treatment is carried out, and the overaging temperature range is 320 - 360 °C, and the final cooling temperature is 50 - 150 °C.

[0022] Another key point in the annealing process is the control of the atmosphere in the heating furnace. To ensure a good surface of the strip, the dew point in the furnace needs to be controlled at -30 - -50 °C.

[0023] Furthermore, in the skin pass rolling process of the present invention, a constant elongation rate control mode is adopted. The elongation rate of the skin pass mill is 0.5 - 0.8%, and the rolling force is 5000 - 8000 kN.

[0024] The beneficial effects produced by the technical solution of the present invention are as follows: (1) By adding an appropriate amount of Sn element to inhibit oxidation inside the hot coil and combining with a reasonable pickling system, the surface quality of the strip steel is improved.

[0025] (2) Based on the primary partitioning of the conventional continuous annealing line, the quenching and partitioning steel of 980 Mpa grade is obtained in the present invention, with low cost and easy to organize production.

[0026] (3) The cold-rolled continuous annealing steel provided by the present invention has good mechanical properties, with tensile strength Rm: 950 - 1100 MPa, yield strength Rp 0.2 : 650 - 950 MPa, elongation A 50 ≥ 20%, and the overall formability is relatively good. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is the microstructure diagram of the strip steel in Example 1.

[0028] Figure 2 It is the surface of the strip steel in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0029] The present invention will be further described in detail below in combination with specific embodiments and the drawings.

[0030] Examples 1 - 10

[0031] The chemical compositions of the high-surface and low-cost GPa-grade quenching and partitioning steel in each example are shown in Table 1.

[0032] Table 1 Chemical composition and mass percentage content (%) of the strip steel in each example

[0033] The remaining components in Table 1 are iron and inevitable impurities.

[0034] The production methods of the high-surface and low-cost GPa-grade quenching and partitioning steel in each example include continuous casting, heating, hot rolling, pickling, cold rolling, continuous annealing, and skin pass processes; specifically as follows: (1) Continuous casting process: The tundish temperature is 1500 - 1520 °C, and the casting speed is 1.0 - 1.2 m / min; (2) Heating process: The heating temperature is 1270 - 1330 °C, and the total heating time is 150 - 240 min; (3) Hot rolling process: The finishing rolling temperature of the finishing mill is 870 - 900 °C, and the coiling temperature is 580 - 630 °C; (4) Pickling process: The acid solution temperature ≥ 80 °C, and the free acid concentration ≥ 50 g / L; (4) Cold rolling process: The cold rolling reduction rate ≥ 45%; (5)Continuous annealing process: soaking temperature is 780 - 840 °C, holding time is 80 - 160 s; first slow cooling to 680 - 720 °C, slow cooling rate is 10 - 20 °C / s; then rapid cooling to 340 - 380 °C, rapid cooling rate is 45 - 75 °C / s, overaging section temperature is 320 - 360 °C, final cooling temperature is 50 - 150 °C; dew point in the annealing furnace is -30 - -50 °C (6)Skin pass process: adopting constant elongation control mode, skin pass mill elongation is 0.5 - 0.8%, rolling force is 5000 - 8000 kN.

[0035] The process parameters of continuous casting, heating, hot rolling, pickling and cold rolling of the quenching and partitioning steel in each example are shown in Table 2, and the process parameters of annealing and skin pass are shown in Table 3.

[0036] Table 2 Process parameters of continuous casting, heating, hot rolling, pickling and cold rolling in each example

[0037] Table 3 Process parameters of annealing and skin pass in each example

[0038] The microstructure of the strip steel in Example 1 is shown in Figure 1 , and it can be seen that its microstructure consists of ferrite + martensite + bainite + retained austenite. The strip steel microstructures in the remaining examples are the same as those in Example 1 and will not be shown one by one.

[0039] The surface of the strip steel in Example 1 is shown in Figure 2 , and it can be seen that its surface is smooth and clean without defects such as mill scale. The strip steel surfaces in the remaining examples are basically the same as those in Example 1 and will not be shown one by one.

[0040] The mechanical properties of the high-surface low-cost GPa-level quenching and partitioning steel in each example are shown in Table 4.

[0041] Table 4 Mechanical properties of the strip steel in each example

[0042] As can be seen from Table 4, for the quenching and partitioning steel provided by the present invention, the tensile strength Rm is 950 - 1100 MPa, the yield strength Rp0.2 is 650 - 950 MPa, and the elongation A 50 ≥20%.

[0043] The above examples are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above examples, those of ordinary skill in the art should understand that: the present invention can still be modified or equivalently replaced, and any modification or partial replacement without departing from the spirit and scope of the present invention shall be covered by the scope of the claims of the present invention.

Claims

1. A high-surface low-cost GPa-level quenching and partitioning steel, characterized in that, Its chemical composition and its mass percentage are as follows: C: 0.17 - 0.25%, Mn: 1.50 - 3.00%, S ≤ 0.005%, P ≤ 0.020%, Si: 1.30 - 2.00%, Als: 0.010 - 0.080%, Nb ≤ 0.060%, Ti ≤ 0.100%, Sn ≤ 0.100%, and the rest is Fe and ineradicable impurities.

2. The high-surface low-cost GPa-level quenching and partitioning steel according to claim 1, characterized in that, Its chemical composition and its mass percentage are as follows: C: 0.18 - 0.22%, Mn: 1.90 - 2.40%, S ≤ 0.003%, P ≤ 0.015%, Si: 1.40 - 1.80%, Als: 0.030 - 0.060%, Nb: 0.020 - 0.050%, Ti: 0.040 - 0.060%, Sn: 0.06 - 0.085%, and the rest is Fe and ineradicable impurities.

3. A high surface low-cost GPa-class quenching and partitioning steel according to claim 1, characterized in that, The thickness of the quenching and partitioning steel is 0.8 - 2.5 mm.

4. A high-surface low-cost GPa-class quenching and partitioning steel according to claim 1, characterized in that, The tensile strength Rm of the quenching and partitioning steel is 950 - 1100 MPa, the yield strength Rp0.2 is 650 - 950 MPa, and the elongation A 50 ≥ 20%.

5. A high-surface low-cost GPa-class quenching and partitioning steel according to claim 1, characterized in that, The steel structure of the quenching and partitioning steel consists of ferrite + martensite + bainite + retained austenite.

6. A production method of a high-surface low-cost GPa-level quenching and partitioning steel according to claims 1-5, characterized in that, It includes continuous casting, heating, hot rolling, pickling, cold rolling, continuous annealing and skin pass rolling processes; in the continuous annealing process, the soaking temperature is 780 - 840 °C, and the holding time is 80 - 160 s; first slow cool to 680 - 720 °C, and the slow cooling rate is 10 - 20 °C / s; then fast cool to 340 - 380 °C, and the fast cooling rate is 45 - 75 °C / s, the overaging section temperature is 320 - 360 °C, and the final cooling temperature is 50 - 150 °C; the dew point in the annealing furnace is -30 - -50 °C.

7. The production method of a high-surface low-cost GPa-level quenching and partitioning steel according to claim 6, characterized in that, In the continuous casting process, the tundish temperature is 1500 - 1520 °C, and the casting speed is 1.0 - 1.2 m / min.

8. The production method of a high surface and low cost GPa-level quenching and partitioning steel according to claim 6, characterized in that In the heating process, the heating temperature is 1270 - 1330 °C, and the total heating time is 150 - 240 min.

9. The production method of a high-surface low-cost GPa-level quenching and partitioning steel according to claim 6, characterized in that In the hot rolling process, the finish rolling temperature of finish rolling is 870 - 900 °C, and the coiling temperature is 580 - 630 °C.

10. The production method of a high-surface low-cost GPa-level quenching and partitioning steel according to claim 6, characterized in that, In the pickling process, the acid solution temperature ≥ 80 °C, and the free acid concentration ≥ 50 g / L.

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