Cold-rolled high-strength steel plate with tensile strength of 620-680 MPa for automobile and production method of cold-rolled high-strength steel plate

Abstract

The invention discloses an automobile cold-rolled high-strength steel plate with the tensile strength of 620-680 MPa and a production method of the automobile cold-rolled high-strength steel plate. The cold-rolled high-strength steel plate comprises the following chemical components in percentage by weight: 0.05-0.07% of C; less than or equal to 0.50% of Si; 0.7%-1.1% of Mn; 0.025% to 0.045% of Al; ti is less than or equal to 0.080%; c + Si / 30 + Mn / 20 is less than or equal to 0.13, and Nb / Ti is less than or equal to 0.6; the low-C component design lower than the peritectic reaction zone range is adopted, the reasonable alloy element proportion is added, the hot rolling and continuous annealing process is controlled, the strengthening effect of microalloy elements is fully played, and the yield strength Rp0.2 of the produced cold-rolled high-strength steel is larger than or equal to 460 MPa, the tensile strength Rm is 620-680 MPa, and the ductility A80 is larger than or equal to 20%.

Description

Technical Field

[0001] The invention belongs to the technical field of cold-rolled automobile steel, and specifically relates to a cold-rolled high-strength automobile steel plate with a tensile strength of 620-680 MPa and a production method thereof. Background Art

[0002] With the development of the automobile industry, energy saving and consumption reduction, increased safety and reliability, and improved vehicle body lightweighting have become the common pursuit of users. In order to achieve lightweight automobile bodies and improve fuel consumption, low-alloy high-strength steel is increasingly used in vehicle body manufacturing, especially in the production of some safety parts and structural parts. At present, the high-strength cold-rolled low-alloy high-strength steel with a tensile strength of more than 600MPa in the market is costly and difficult to control mechanical properties, which limits its further promotion and application.

[0003] Chinese patent CN116815070A discloses a low-cost Ti microalloyed 600MPa grade hot-rolled steel strip and its preparation method and application. The chemical composition weight percentage of the hot-rolled steel strip is C: ≤0.10%; Si: 0.10-0.20%; Mn: ≤0.80; P: ≤0.015%; S: ≤0.008%; Ti: 0.07-0.09%; Als: 0.015-0.035%. The hot-rolled steel strip in this patent is a hot-rolled product designed with a single Ti component, and its cost is relatively low, but it is used to manufacture photovoltaic brackets and is not suitable for stamping complex automotive parts.

[0004] Chinese patent CN 114540707 A discloses a 590MPa grade cold-rolled high-strength steel and its production method. The chemical composition weight percentage of the hot-dip galvanized steel sheet is: C: 0.07% to 0.14%, Si: 0.40% to 0.70%, Mn: 1.50% to 2.00%, P: ≤0.012%, S: ≤0.008%, Al: 0.25 to 0.50%, N: ≤0.004%, the rest is Fe and unavoidable impurities; the rest is iron and other impurities. This patent adopts Nb microalloy strengthening, which has a relatively high cost, and the organization is a complex phase organization of ferrite + bainite + martensite + residual austenite.

[0005] Chinese patent CN 116103573 A discloses a hot-rolled steel strip for automobile structure suitable for direct hot charging production and its preparation method, which is calculated by mass percentage: C: 0.05% to 0.09%; Si: 0.05% to 0.15%; Mn: 0.40% to 0.60%; S: 0 to 0.010%; P: 0 to 0.025%; Alt: 0.010% to 0.050%; Nb+Ti+V: 0 to 0.050%; impurities: 0 to 0.5%, and the balance is Fe. The C content range of this patent is within the steelmaking peritectic range, which cannot completely avoid the corner cracking of the ingot, and the edge quality of the strip cannot be guaranteed under the hot charging process. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a cold-rolled high-strength steel plate for automobiles with a tensile strength of 620-680 MPa and a production method thereof. A low C component is added with a reasonable micro-alloy element design to obtain a cold-rolled high-strength steel for automobiles with a tensile strength of 620-680 MPa, which has a low production cost, stable performance and good stamping performance.

[0007] The technical solution adopted by the present invention is as follows:

[0008] A cold-rolled high-strength steel plate for automobiles with a tensile strength of 620-680 MPa, comprising the following chemical components in weight percentage: C 0.05-0.07%; Si≤0.50%; Mn 0.7-1.1%; P≤0.030%; S≤0.020%; Al 0.025-0.045%; Ti≤0.080%; Nb≤0.030%, the remainder being Fe and unavoidable impurities, wherein C+Si / 30+Mn / 20≤0.13, Nb / Ti≤0.6.

[0009] Furthermore, in the cold-rolled high-strength steel plate, Si is 0.20-0.50%, Ti is 0.05-0.080%, and Nb is 0.02-0.030%.

[0010] The metallographic structure of the cold-rolled high-strength steel plate is ferrite, pearlite and dispersed Nb-Ti precipitates.

[0011] The volume fraction of ferrite is 70-80%, the volume fraction of pearlite is 19-29%, and the average grain size is ≤4.5μm.

[0012] The yield strength Rp of the cold rolled high strength steel plate 0.2 ≥460MPa, tensile strength Rm 620~680MPa, elongation A 80 ≥20%.

[0013] The production method of the cold-rolled high-strength steel plate for automobiles with a tensile strength of 620-680 MPa comprises the following steps: smelting, continuous casting, hot rolling, coiling, pickling and cold rolling, continuous annealing, aging treatment, and leveling;

[0014] In the hot rolling step, the cast slab is hot-charged into a furnace;

[0015] In the coiling step, a U-shaped coiling mode is adopted to control the temperature difference between the coiling temperature at the head and tail of the strip and the temperature in the middle section Δt=100°C; the temperature change rate of the head 50m and the tail 100m is v=2°C / s, that is, from the moment of coiling, starting from the head of the strip, the temperature drops by 2°C every second for 50m, and then the coiling temperature is kept unchanged. When coiling to the remaining 100m at the tail, the temperature of the strip rises by 2°C every second until the coiling is completed. The U-shaped coiling method can reduce the strength difference between the head, tail and middle of the steel coil, and improve the overall strength consistency of the steel coil.

[0016] In the continuous annealing step, different annealing temperatures are used for different strip thicknesses. When the strip thickness is 0.6mm≤<1.2mm, the annealing temperature is controlled at 785±10℃; when the strip thickness is 1.2≤<1.8mm, the annealing temperature is controlled at 780±10℃; when the strip thickness is 1.8≤≤2.5mm, the annealing temperature is controlled at 775±10℃. This reduces the fluctuation of mechanical properties caused by the fluctuation of strip thickness.

[0017] In the hot rolling step, the ingot is fed into a heating furnace at a temperature above 400°C; and the final rolling temperature is 880-890°C.

[0018] In the coiling step, the coiling temperature of the middle portion of the strip is 490-560°C.

[0019] In the pickling and cold rolling step, the cold rolling reduction rate is 55-70%.

[0020] In the continuous annealing step, the annealing speed is 100-150 mm / min; after annealing and heat preservation, the steel is cooled to 660-680°C at an average cooling rate of 10-15°C / s, and then cooled to 350-370°C at an average cooling rate of 30-40°C / s, and then an aging treatment is performed.

[0021] In the over-aging treatment step, the over-aging treatment condition is to keep the temperature at 350-370° C. for 5-15 minutes.

[0022] In the flattening step, the flattening elongation is 0.8-1.0%.

[0023] The cold-rolled high-strength steel plate for automobiles with a tensile strength of 620-680 MPa provided by the present invention adopts a low C component design below the range of the peritectic reaction zone, adds an appropriate amount of Si element to effectively purify ferrite, controls the added Nb content to be below 0.030% to ensure the fine grain strengthening effect of micro-alloy elements, adds an appropriate amount of Ti element to give full play to its precipitation strengthening effect, and improves the product strength.

[0024] In the production method of the cold-rolled high-strength steel plate for automobiles with a tensile strength of 620-680 MPa provided by the present invention, a reasonable chemical composition ratio is used to achieve a casting blank without corner cracks, and such a casting blank can be directly hot-rolled without corner cleaning, thereby improving production efficiency. The hot rolling, coiling, cold rolling, and continuous annealing process system ensure good uniformity of the organization, and the microstructure thereof is characterized by ferrite + fine pearlite + dispersed Nb-Ti precipitates, wherein the volume fraction of ferrite is 70-80%, the volume fraction of pearlite is 19-29%, and the average grain size is ≤4.5 μm.

[0025] The product of the present invention mainly adopts the composition design concept of Ti element precipitation strengthening as the main and Nb fine grain strengthening as the auxiliary. Compared with the design concept of Nb element as the main, the alloy cost is lower under the condition of consistent strengthening effect, and the contradiction of large strength fluctuation caused by high Ti element strengthening is effectively solved through the segmented hot rolling coiling temperature and the graded regulation of annealing temperature. Its chemical composition satisfies C+Si / 30+Mn / 20<0.13, Nb / Ti≤0.6, the precipitation ratio of Ti element reaches more than 90%, of which the proportion of precipitated particles below 18nm accounts for more than 35% of the total precipitation amount, and (Ti, Nb) C particles are mostly spherical and ellipsoidal and dispersed in the matrix structure.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The present invention adopts a low C component design below the range of the peritectic reaction zone, adds a reasonable ratio of alloy elements, fully exerts the strengthening effect of micro-alloy elements by controlling the steelmaking, hot rolling and continuous annealing processes, optimizes the ingot hot rolling production process, and invents an economical cold-rolled high-strength steel for automobiles with a tensile strength of 620-680MPa.

[0028] The present invention adopts hot-charging rolling of cast billets, which has low production cost and high production efficiency. Compared with the traditional steelmaking hot rolling process, the production cycle is shortened, the manufacturing cost is reduced, and the production efficiency is significantly improved. The mechanical properties are stable, the strength is high and the stamping forming performance is good. The cold-rolled high-strength steel produced has a yield strength Rp 0.2 ≥460MPa, tensile strength Rm620~680MPa, elongation A 80 ≥20%, 0a cold bending performance 180° qualified. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is the metallographic structure diagram of the steel plate in Example 1;

[0030] Figure 2 is a scanning electron microscope image of the steel plate in Example 1;

[0031] Figure 3 The morphology of (Ti, Nb)C microalloy precipitates and the distribution of precipitated particles of different sizes in the steel plate in Example 2;

[0032] Figure 4 This is a picture of the ingot in Example 1, and the edge quality is good;

[0033] Figure 5 This is a picture of the ingot in Comparative Example 2, in which cracks appeared. DETAILED DESCRIPTION

[0034] The invention provides a cold-rolled high-strength steel plate for automobiles with a tensile strength of 620-680 MPa, wherein the chemical composition and weight percentage are as follows: C 0.05-0.07%, Si 0.20-0.50%, Mn 0.7-1.1%, P≤0.030%, S≤0.020%, Al 0.025-0.045%, Ti 0.05-0.080%, Nb 0.02-0.030%, and the balance is Fe and unavoidable impurities, wherein C+Si / 30+Mn / 20≤0.13, Nb / Ti≤0.6.

[0035] The production method of the cold-rolled high-strength steel plate for automobiles with a tensile strength of 620-680 MPa comprises the following steps: smelting, continuous casting, hot rolling, coiling, pickling and cold rolling, continuous annealing, over-aging treatment, and leveling;

[0036] The smelting steps specifically include: molten iron pretreatment - converter smelting - alloy fine-tuning station - RH furnace refining;

[0037] In the hot rolling step, the ingot is fed into the heating furnace at a temperature above 400°C; the final rolling temperature is 880-890°C; in the coiling step, a U-shaped coiling mode is adopted to control the temperature difference between the coiling temperature at the head and tail of the strip and the temperature of the middle section Δt=100°C, and the temperature change rate v=2°C / s at the head 50m and the tail 100m; the coiling temperature of the middle part of the strip is 490-560°C;

[0038] In the pickling and cold rolling step, the cold rolling reduction rate is 55-70%;

[0039] In the continuous annealing step, different annealing temperatures are used for different strip thicknesses. When the strip thickness is 0.6mm≤<1.2mm, the annealing temperature is controlled at 785±10℃; when the strip thickness is 1.2≤<1.8mm, the annealing temperature is controlled at 780±10℃; when the strip thickness is 1.8≤≤2.5mm, the annealing temperature is controlled at 775±10℃. The annealing speed is 100-150mm / min; after annealing and heat preservation, the strip is cooled to 660-680℃ at an average cooling rate of 10-15℃ / s, and then cooled to 350-370℃ at an average cooling rate of 30-40℃ / s, and then over-aging treatment is performed.

[0040] In the over-aging treatment step, the over-aging treatment condition is to keep the temperature at 350-370° C. for 5-15 minutes.

[0041] In the flattening step, the flattening elongation is 0.8-1.0%.

[0042] The present invention is described in detail below with reference to the embodiments.

[0043] Example 1

[0044] A cold-rolled high-strength steel for automobiles with a tensile strength of 620 MPa, the chemical composition mass percentage (%) of which is C: 0.069; Si: 0.301; Mn: 0.901; P 0.020; S 0.0032; Al 0.0375; Ti: 0.069; Nb: 0.025, and the balance is Fe and unavoidable impurities.

[0045] The method for preparing the cold-rolled high-strength steel for automobiles with a tensile strength of 620 MPa comprises the following steps:

[0046] (1) Smelting process;

[0047] (2) Hot rolling and coiling of ingots or slabs: The ingots obtained by continuous casting after smelting do not need to be cooled to room temperature. They are sent to a heating furnace at above 400°C and heated at 1230°C. The hot rolling unit performs 7 passes of rolling to a thickness of 1.28 mm. The starting rolling temperature is 1120°C and the final rolling temperature is 881°C. A U-shaped coiling mode is adopted, and the temperature difference between the coiling temperature at the head and tail of the strip and the middle section is controlled to be Δt=100°C. The temperature change rate of the head 50m and the tail 100m is v=2°C / s, and the coiling temperature of the middle section is 526°C.

[0048] (3) Pickling and cold rolling: The strip is pickled in a hydrochloric acid tank to remove the surface oxide scale, and then cold rolled or cold rolled with a pickling reduction rate of 70% to a target thickness of 0.9 mm;

[0049] (4) Continuous annealing: Continuous annealing is performed, the steel coil is heated to 785°C, the strip speed is 120 mm / min, and then slowly cooled to 670°C at an average cooling rate of 12°C / s, and then cooled to 360°C at an average cooling rate of 35°C / s, and an over-aging treatment is performed at 360°C for 10 min;

[0050] (5) Flattening: Flattening elongation is 1.0%.

[0051] The chemical compositions of the cold-rolled high-strength steels in Examples 2 to 3 and Comparative Examples are shown in Table 1. In Table 1, CE=C+Si / 30+Mn / 20.

[0052] Table 1

[0053]

[0054] The production process parameters of the cold-rolled high-strength steel in Examples 2 to 3 and the comparative examples that are different from those in Example 1 are shown in Table 2, and the other process parameters are the same as those in Example 1.

[0055] Table 2

[0056] Finishing temperature Middle coiling temperature Annealing temperature Flat elongation / % Thickness / mm Example 1 881 526 785 1.0 0.9 Example 2 883 522 785 1.0 1.0 Example 3 883 519 780 0.8 1.5 Comparative Example 1 882 521 780 0.8 1.5 Comparative Example 2 881 529 780 0.6 1.5 Comparative Example 3 881 530 780 0.6 1.5

[0057] The performance test results of the cold-rolled high-strength steel in each embodiment and comparative example are shown in Table 3.

[0058] Table 3

[0059]

[0060] Comparative Example 4 in Table 3 is the same as Example 1 in other aspects, except that a different winding method is adopted, which is non-U-shaped winding, and the yield strength of the head and tail is 18 to 24 MPa higher than that of the middle part, and the through-winding performance stability is not as good as the solution of the present invention.

[0061] From the above, it can be seen that the yield strength Rp of the cold-rolled high-strength steel sheet produced by the present invention is 0.2 ≥460MPa, tensile strength Rm 620~680MPa, elongation A 80 ≥20%, and the performance uniformity of the whole roll is good.

[0062] In Comparative Example 1, Nb element is used as the main precipitation strengthening component design, and its strength level and product quality can achieve the effect of the solution of the present invention. However, due to the high amount of Nb added, its alloy cost is significantly higher than that of the solution of the present invention, which is not conducive to the market competitiveness of the product.

[0063] In Comparative Examples 2 and 3, due to the high C content, the peritectic reaction zone was not avoided, resulting in cracks in the ingot.

[0064] The detailed description of a cold-rolled high-strength steel plate for automobiles with a tensile strength of 620-680 MPa and its production method with reference to the above-mentioned embodiment is illustrative rather than restrictive, and several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A cold-rolled high-strength steel sheet for automobiles with a tensile strength of 620-680 MPa, characterized in that: The cold-rolled high-strength steel plate includes the following chemical components in weight percentage: C 0.05-0.07%; Si≤0.50%; Mn0.7-1.1%; P≤0.030%; S≤0.020%; Al 0.025-0.045%; Ti≤0.080%; Nb≤0.030%, and the balance is Fe and unavoidable impurities, wherein C+Si / 30+Mn / 20≤0.13, Nb / Ti≤0.

6.

2. The cold-rolled high-strength steel sheet for automobiles with a tensile strength of 620-680 MPa according to claim 1, characterized in that: The cold-rolled high-strength steel plate contains 0.20-0.50% Si, 0.05-0.080% Ti, and 0.02-0.030% Nb.

3. The cold-rolled high-strength steel sheet for automobiles with a tensile strength of 620-680 MPa according to claim 1, characterized in that: The metallographic structure of the cold-rolled high-strength steel plate is ferrite, pearlite and dispersed Nb-Ti precipitates.

4. The cold-rolled high-strength steel sheet for automobiles with a tensile strength of 620-680 MPa according to claim 2, characterized in that: The volume fraction of ferrite is 70-80%, the volume fraction of pearlite is 19-29%, and the average grain size is ≤4.5μm.

5. The cold-rolled high-strength steel sheet for automobiles with a tensile strength of 620-680 MPa according to claim 1, characterized in that: The yield strength Rp of the cold rolled high strength steel plate 0.2 ≥460MPa, tensile strength Rm620~680MPa, elongation A 80 ≥20%.

6. The method for producing a cold-rolled high-strength steel sheet for automobiles with a tensile strength of 620-680 MPa as claimed in any one of claims 1 to 5, characterized in that: The production method comprises the following steps: smelting, continuous casting, hot rolling, coiling, pickling and cold rolling, continuous annealing, over-aging treatment, and leveling; In the hot rolling step, the cast slab is hot-charged into a furnace; In the coiling step, a U-shaped coiling mode is adopted to control the temperature difference between the coiling temperature at the head and tail of the strip and the temperature in the middle section Δt=100°C; In the continuous annealing step, different annealing temperatures are used for different strip thicknesses. When the strip thickness is 0.6mm≤<1.2mm, the annealing temperature is controlled at 785±10℃; when the strip thickness is 1.2mm≤<1.8mm, the annealing temperature is controlled at 780±10℃; when the strip thickness is 1.8mm≤≤2.5mm, the annealing temperature is controlled at 775±10℃.

7. The production method according to claim 6, characterized in that: In the hot rolling step, the ingot is fed into a heating furnace at a temperature above 400°C; the final rolling temperature is 880-890°C; and the coiling temperature of the middle portion of the strip is 490-560°C.

8. The production method according to claim 6, characterized in that: In the pickling and cold rolling step, the cold rolling reduction rate is 55-70%.

9. The production method according to claim 6, characterized in that: In the continuous annealing step, the annealing speed is 100-150 mm / min; after annealing and heat preservation, the steel is cooled to 660-680°C at an average cooling rate of 10-15°C / s, and then cooled to 350-370°C at an average cooling rate of 30-40°C / s, and then an aging treatment is performed.

10. The production method according to claim 6, characterized in that: In the over-aging treatment step, the over-aging treatment condition is 350-370° C. for 5-15 minutes.

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