Production method of cover-annealing phosphorus-adding high-strength interstitial-free steel cold-rolled sheet

Abstract

The invention discloses a production method of a cover-annealing and phosphorus-adding high-strength interstitial-free steel cold-rolled sheet. The production method comprises the following steps: step 1, preparing a steel billet; secondly, the steel billet is rolled into a plate with the thickness ranging from 2.3 mm to 6.0 mm through hot rolling; thirdly, the plate obtained through hot rolling is subjected to cold rolling, and a plate with the thickness ranging from 0.35 mm to 1.2 mm is obtained; fourthly, the cold-rolled plate is subjected to cover annealing operation; and fifthly, after cover annealing is completed, the plate is subjected to flattening operation and then rolled, and a finished product is obtained. According to the method, no B element is added in the steel billet refining process, meanwhile, the content of P element in the steel is reduced, the content of Mn element in the steel is increased, the cold rolling reduction rate is increased, and the problem that the annealing temperature is increased due to segregation of B atoms at the grain boundary is solved; meanwhile, the problem that low-temperature annealing grain recrystallization driving force is insufficient is solved; the problem of cold brittleness caused by segregation at the grain boundary due to high content of P element in the steel can be solved; and the problem that the strength of reduced P content cannot meet the standard requirement is solved.

Description

Technical Field

[0001] The invention belongs to the field of phosphorus-added high-strength interstitial-free steel production, in particular to a method for producing a cold-rolled phosphorus-added high-strength interstitial-free steel plate. Background Art

[0002] Phosphorus-added high-strength interstitial-free steel is currently mainly used in automobile manufacturing, such as the manufacture of automobile panels and structural parts. The main characteristics of phosphorus-added high-strength interstitial-free steel are high strength and good cold forming performance, as well as good impact resistance and fatigue resistance. The addition of phosphorus can refine the grains, improve the strength and toughness of the steel, and delay the recrystallization process of the steel. This type of steel usually has a high tensile strength and can meet the demand for high-strength materials in heavy vehicles, engineering machinery, building structures and other fields.

[0003] At present, most manufacturers use continuous annealing process to produce phosphorus-added high-strength interstitial steel. The composition design of interstitial steel uses the solid solution strengthening of Mn and P elements to improve its strength. The solubility of P element in ferrite is limited and it is very easy to segregate at the grain boundary, which leads to the weakening of the solid solution strengthening effect of P element in steel and the reduction of low-temperature plasticity of steel. Therefore, for interstitial steel with high phosphorus content, trace B element is often added to reduce the segregation problem of P element at the grain boundary. Compared with the continuous annealing production line, the cold rolling hood annealing production mode shows higher flexibility in scheduling, can easily meet the needs of different specifications and batches of products, reduce production costs, improve production efficiency, and provide strong support for enterprises to cope with market fluctuations. The initial investment of hood annealing equipment is more economical, and it can be expanded using the existing cold rolling line infrastructure, which helps small and medium-sized enterprises maintain low operating costs. At the same time, by optimizing the annealing process and composition system, hood annealing can also achieve complete annealing of high-strength steel at low temperature, ensuring product performance. In addition, low temperature annealing also reduces furnace lining erosion, extends service life, reduces maintenance costs, and complies with the green manufacturing trend.

[0004] However, the hood annealing process is different from the continuous annealing process. The hood annealing furnace usually adopts a double-layer structure of an inner hood and an outer hood, and the inner hood is connected with a protective gas to prevent the material from oxidation or corrosion at high temperatures. This structure limits the air circulation in the furnace, making the heating and cooling process relatively slow, so the annealing temperature is also relatively low. The continuous annealing furnace usually has a more efficient heating and cooling system, which can quickly heat up and control temperature fluctuations, so it is more suitable for high-temperature annealing. However, since the B element in the existing phosphorus-added high-strength interstitial atomic steel will increase the annealing temperature of ultra-low carbon steel, it is difficult to produce phosphorus-added high-strength steel with quality that meets customer requirements under the extreme process of the hood annealing furnace, and there is a problem of low pass rate. Summary of the invention

[0005] The invention provides a method for producing a annealed phosphorus-added high-strength interstitial-free cold-rolled steel sheet, which is used to solve the defects in the prior art.

[0006] The present invention is achieved through the following technical solutions:

[0007] A method for producing a annealed phosphorized high-strength interstitial-free cold-rolled steel sheet comprises the following steps:

[0008] Step 1: preparing a steel billet, the alloy composition of which is: C≤0.0030%; N≤0.0030%, Si 0.10-0.30%, Mn 0.50-0.80%, P 0.050-0.070%, S≤0.0010%; Alt 0.020-0.060%, Ti0.035-0.070%, and the balance is Fe;

[0009] Step 2: The steel billet is hot rolled into a plate with a thickness of 2.3-6.0 mm;

[0010] Step 3: cold rolling the hot-rolled sheet to obtain a sheet with a thickness of 0.35-1.2 mm;

[0011] Step 4: After cold rolling, the plate is subjected to a cover annealing operation;

[0012] Step 5: After the annealing is completed, the plate is leveled and rolled to obtain the finished product.

[0013] In the above-mentioned method for producing a cold-rolled sheet of high-strength interstitial-free steel after annealing and phosphorization, the number of hot rolling passes in step 2 is 6.

[0014] In the above-mentioned method for producing a annealed phosphorus-added high-strength interstitial-free cold-rolled steel sheet, in step 2, the hot rolling starting temperature is 1100-1150°C, the hot rolling finishing temperature is 890-930°C, and the coiling temperature after hot rolling is 700-740°C.

[0015] In the above-mentioned method for producing a cold-rolled sheet of high-strength interstitial-free steel after annealing and phosphorization, the number of cold rolling passes in step three is 5.

[0016] In the above method for producing a cold-rolled sheet of annealed and phosphorized high-strength interstitial-free steel, the reduction ratio of the cold rolling in step three is 80-88%.

[0017] In the above-mentioned method for producing a cold-rolled sheet of high-strength interstitial-free steel after annealing and phosphorus addition, the annealing temperature of the annealing operation in step 4 is 720-750° C., and the annealing time is greater than 15 hours.

[0018] In the above method for producing a annealed and phosphorized high-strength interstitial-free cold-rolled steel sheet, the flattening elongation of the flattening operation in step five is 0.7-1.3%.

[0019] The advantages of the present invention are as follows: in the present invention, no B element is added in the smelting of the steel billet, while the P element content in the steel is reduced, the Mn element content in the steel is increased, and the cold rolling reduction rate is increased, thereby solving the problem that B atoms are segregated at the grain boundary and the annealing temperature is increased; at the same time, solving the problem that the driving force for recrystallization of low-temperature annealing grains is insufficient; solving the problem that the high P element content in the steel is prone to segregation at the grain boundary and causing cold brittleness; and solving the problem that the strength cannot meet the standard requirements when the P content is reduced; the method of the present invention can use a bell-type annealing furnace for stable production and can obtain phosphorus-added high-strength interstitial atom-free steel with higher performance. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0021] Example 1

[0022] Step 1: preparing a steel billet, the alloy composition of which is: C 0.0015%; N 0.0021%, Si 0.17%, Mn 0.51%, P 0.062%, S 0.0007%; Alt 0.047%, Ti 0.051%, and the balance is Fe;

[0023] Step 2: The steel billet is heated to 1100-1130°C in a heating furnace, and then hot-rolled for 6 passes. The final rolling temperature is controlled at 890-900°C, and the coiling temperature after hot rolling is controlled at 700-720°C to obtain a plate with a thickness of 2.9 mm.

[0024] Step 3: After the hot-rolled plate is coiled, it is cooled to room temperature and then cold-rolled for 5 times to obtain a plate with a thickness of 0.35 mm;

[0025] Step 4: After cold rolling, the plate is subjected to a cover annealing operation, the annealing temperature is 720°C, and the annealing time is 16 hours;

[0026] Step 5: After the annealing is completed, the plate is leveled, and the leveling elongation is 0.95%. After the leveling is completed, the plate is rolled to obtain the finished product.

[0027] Example 2

[0028] Step 1: preparing a steel billet, the alloy composition of which is: C 0.0022%; N 0.0017%, Si 0.27%, Mn 0.56%, P 0.068%, S 0.0008%; Alt 0.056%, Ti 0.046%, and the balance is Fe;

[0029] Step 2: The steel billet is heated to 1110-1140°C in a heating furnace, and then hot-rolled for 6 passes. The final rolling temperature is controlled at 900-920°C, and the coiling temperature after hot rolling is controlled at 700-730°C to obtain a plate with a thickness of 3.5 mm.

[0030] Step 3: After the hot-rolled plate is coiled, it is cooled to room temperature and then cold-rolled for 5 times to obtain a plate with a thickness of 0.5 mm;

[0031] Step 4: After cold rolling, the plate is subjected to a cover annealing operation, the annealing temperature is 720°C, and the annealing time is 15.5h;

[0032] Step 5: After the annealing is completed, the plate is leveled, and the leveling elongation is 1.1%. After the leveling is completed, the plate is rolled to obtain the finished product.

[0033] Example 3

[0034] Step 1: preparing a steel billet, the alloy composition of which is: C 0.0014%; N 0.0023%, Si 0.24%, Mn 0.62%, P 0.053%, S 0.0003%; Alt 0.052%, Ti 0.063%, and the balance is Fe;

[0035] Step 2: The steel billet is heated to 1120-1150°C in a heating furnace, and then hot-rolled for 6 passes. The final rolling temperature is controlled at 900-930°C, and the coiling temperature after hot rolling is controlled at 720-740°C to obtain a plate with a thickness of 4.5 mm.

[0036] Step 3: After the hot-rolled plate is coiled, it is cooled to room temperature and then cold-rolled for 5 times to obtain a plate with a thickness of 0.7 mm;

[0037] Step 4: After cold rolling, the plate is subjected to a cover annealing operation, the annealing temperature is 730°C, and the annealing time is 15.8h;

[0038] Step 5: After the annealing is completed, the plate is leveled, and the leveling elongation is 0.86%. After the leveling is completed, the plate is rolled to obtain the finished product.

[0039] Example 4

[0040] Step 1: preparing a steel billet, the alloy composition of which is: C 0.0027%; N 0.0019%, Si 0.14%, Mn 0.67%, P 0.061%, S 0.0006%; Alt 0.029%, Ti 0.058%, and the balance is Fe;

[0041] Step 2: The steel billet is heated to 1130-1150°C in a heating furnace, and then hot-rolled for 6 passes. The final rolling temperature is controlled at 920-930°C, and the coiling temperature after hot rolling is controlled at 720-735°C to obtain a plate with a thickness of 5.0 mm.

[0042] Step 3: After the hot-rolled plate is coiled, it is cooled to room temperature and then cold-rolled for 5 times to obtain a plate with a thickness of 0.8 mm;

[0043] Step 4: After cold rolling, the plate is subjected to a cover annealing operation, the annealing temperature is 740°C, and the annealing time is 16.4h;

[0044] Step 5: After the annealing is completed, the plate is leveled, and the leveling elongation is 0.83%. After the leveling is completed, the plate is rolled to obtain the finished product.

[0045] Example 5

[0046] Step 1: preparing a steel billet, the alloy composition of which is: C 0.0021%; N 0.0016%, Si 0.29%, Mn 0.72%, P 0.059%, S 0.0005%; Alt 0.036%, Ti 0.058%, and the balance is Fe;

[0047] Step 2: The steel billet is heated to 1100-1130°C in a heating furnace, and then hot-rolled for 6 passes. The final rolling temperature is controlled at 890-920°C, and the coiling temperature after hot rolling is controlled at 700-720°C to obtain a plate with a thickness of 5.3 mm.

[0048] Step 3: After the hot-rolled plate is coiled, it is cooled to room temperature and then cold-rolled for 5 times to obtain a plate with a thickness of 1.0 mm;

[0049] Step 4: After cold rolling, the plate is subjected to a cover annealing operation, the annealing temperature is 740°C, and the annealing time is 15.5h;

[0050] Step 5: After the annealing is completed, the plate is leveled, and the leveling elongation is 0.7-1.3%. After the leveling is completed, the plate is rolled to obtain the finished product.

[0051] Example 6

[0052] Step 1: preparing a steel billet, the alloy composition of which is: C 0.0019%; N 0.0017%, Si 0.21%, Mn 0.77%, P 0.051%, S 0.0004%; Alt 0.044%, Ti 0.052%, and the balance is Fe;

[0053] Step 2: The steel billet is heated to 1120-1140°C in a heating furnace, and then hot-rolled for 6 passes. The final rolling temperature is controlled at 910-920°C, and the coiling temperature after hot rolling is controlled at 710-730°C to obtain a plate with a thickness of 6.0 mm.

[0054] Step 3: After the hot-rolled plate is coiled, it is cooled to room temperature and then cold-rolled for 5 times to obtain a plate with a thickness of 1.2 mm;

[0055] Step 4: After cold rolling, the plate is subjected to a cover annealing operation, the annealing temperature is 750°C, and the annealing time is 16 hours;

[0056] Step 5: After the annealing is completed, the plate is leveled, and the leveling elongation is 1.23%. After the leveling is completed, the plate is rolled to obtain the finished product.

[0057] The performance of the phosphorus-added high-strength interstitial-free steel cold-rolled sheets prepared in Examples 1-6 was tested, and the results are shown in Table 1.

[0058] Group Thickness(mm) Reduction rate (%) Yield strength(MPa) Tensile strength(MPa) Example 1 0.35 87.9％ 231 461 Example 2 0.5 85.7％ 219 453 Example 3 0.7 84.4％ 243 479 Example 4 0.8 84％ 237 466 Example 5 1.0 81.1％ 241 483 Example 6 1.2 80％ 226 468

[0059] Table 1

[0060] It can be seen from the data in Table 1 that the yield strength of the phosphorus-added high-strength interstitial-free steel cold-rolled plates prepared in Examples 1-6 of the present invention are all above 210 MPa, and the tensile strength is all above 450 MPa, and the mechanical properties are relatively excellent, thereby proving that the present invention can use the hood annealing process to stably produce phosphorus-added high-strength interstitial-free steel cold-rolled plates with relatively excellent performance, and can effectively use the hood annealing process to apply to the production of phosphorus-added high-strength interstitial-free steel cold-rolled plates, thereby reducing production costs, improving production efficiency, and ensuring that product performance is qualified.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for producing a cold-rolled steel sheet with high strength and free of interstitial atoms after annealing and phosphorization, characterized in that: The steps include: Step 1: preparing a steel billet, the alloy composition of which is: C≤0.0030%; N≤0.0030%, Si 0.10-0.30%, Mn0.50-0.80%, P 0.050-0.070%, S≤0.0010%; Alt 0.020-0.060%, Ti0.035-0.070%, and the balance is Fe; Step 2: The steel billet is hot rolled into a plate with a thickness of 2.3-6.0 mm; Step 3: cold rolling the hot-rolled sheet to obtain a sheet with a thickness of 0.35-1.2 mm; Step 4: After cold rolling, the plate is subjected to a cover annealing operation; Step 5: After the annealing is completed, the plate is leveled and rolled to obtain the finished product.

2. The method for producing a annealed phosphorized high-strength interstitial-free cold-rolled steel sheet according to claim 1, characterized in that: The number of hot rolling passes in step 2 is 6.

3. The method for producing a annealed phosphorized high-strength interstitial-free cold-rolled steel sheet according to claim 1, characterized in that: In the step 2, the hot rolling starting temperature is 1100-1150°C, the hot rolling final temperature is 890-930°C, and the coiling temperature after hot rolling is 700-740°C.

4. The method for producing a annealed phosphorized high-strength interstitial-free cold-rolled steel sheet according to claim 1, characterized in that: The number of cold rolling passes in step 3 is 5.

5. The method for producing a annealed phosphorized high-strength interstitial-free cold-rolled steel sheet according to claim 1, characterized in that: The cold rolling reduction rate in step 3 is 80-88%.

6. The method for producing a annealed phosphorized high-strength interstitial-free cold-rolled steel sheet according to claim 1, characterized in that: The annealing temperature of the cover annealing operation in step 4 is 720-750° C., and the annealing time is greater than 15 hours.

7. The method for producing a annealed phosphorized high-strength interstitial-free cold-rolled steel sheet according to claim 1, characterized in that: The flattening elongation of the flattening operation in step 5 is 0.7-1.3%.