Low-strength high-surface-hardness cold-rolled fine-blanked steel strip and manufacturing method and application thereof

Abstract

The invention provides a low-strength and high-surface-hardness cold-rolled fine-punched steel strip as well as a manufacturing method and application of the low-strength and high-surface-hardness cold-rolled fine-punched steel strip. 0.15 to 0.35 percent of Si; 0.40% to 0.60% of Mn; less than or equal to 0.020% of P; less than or equal to 0.020% of S; 0.015%-0.045% of Al, and the balance Fe and inevitable impurities. Compared with the prior art, the cold-rolled fine blanking steel product with matched tensile strength and surface hardness is obtained by accurately controlling hot rolling, cold rolling, annealing and leveling processes, the tensile strength of the cold-rolled fine blanking steel product is 400-500 MPa, the surface hardness HRB is 70-80, and the cold-rolled fine blanking steel product is suitable for fine blanking of the automobile gearbox friction plate with the thickness of 1.5-3.0 mm.

Description

Technical Field

[0001] The invention belongs to the field of cold rolling technology for metal materials, and in particular relates to a low-strength and high-surface-hardness cold-rolled fine-blanking steel strip and a manufacturing method and application thereof. Background Art

[0002] With the development of technology, the design functions of modern automobile transmissions are becoming more and more powerful. The number of parts in the transmission is huge, and the working conditions are relatively harsh. As a result, the friction plates of automobile transmissions have extremely high requirements for material strength, surface hardness, thickness accuracy, etc. Therefore, the friction plates of transmissions are usually produced with high-strength and high-hardness cold-rolled fine-blanking steel products. However, when the raw material thickness is thick (1.5-3.0mm), it is difficult to form high-strength and high-hardness steel in fine-blanking processing, and the dimensional accuracy of parts is poor. Therefore, a cold-rolled fine-blanking steel product with low tensile strength (400-500MPa) and high hardness (HRB 70-80) with excellent comprehensive performance is needed to meet both fine-blanking processing and service performance.

[0003] The patent with publication number CN115478223A published on December 16, 2022, discloses a cold-rolled fine-blanking steel strip and its preparation method, by adding a large amount of C, Mn and other alloying elements Cr, Nb, etc., wherein the C content is 0.80% to 0.90%, the Cr content is 0.10% to 0.20%, and the Nb content is 0.02% to 0.12%. The product made by high-temperature annealing and heat preservation at 680 to 750 ° C for 14 to 28 hours has a yield strength of 400 to 500 MPa, a tensile strength of 600 to 700 MPa, an elongation after fracture (A50) of 20 to 30%, and a hardness (HV5) of 165-180 HV. The hardness and tensile strength of the products produced by this process are relatively high, which is not conducive to fine-blanking processing.

[0004] The Chinese patent with publication number CN108385019A published on August 10, 2018, discloses a cold-rolled fine-blanking steel for a child seat locking device for a car, the composition of which includes: C: 0.50-0.55, Si: 0.15-0.35, Mn: 0.80-1.10, P≤0.015, S≤0.01, Cr: 0.90-1.20, V: 0.10-0.20, Al≤0.035. The surface hardness of the product made by multiple cold rolling and multiple annealing is HRC47-55. This process adds high-cost alloying elements such as Cr and V, and the hardness of the product produced is relatively high.

[0005] The Chinese patent with the publication number CN105200308A published on December 30, 2015 discloses a method for manufacturing fine blanking steel and its adjusting mechanism fine blanking parts, and its chemical composition (wt%): C: 0.12-0.24, Si: ≤0.40, Mn: 0.30-0.80, P≤0.015, S≤0.005, Cr: ≤0.40, Ni≤0.40, Mn≤0.10, Ca: 0.0015-0.0040. The fine blanking steel product produced by hot rolling, pickling, cold rolling, spheroidizing annealing, etc. has a tensile strength of 375-403MPa, which adds high-cost alloy elements such as Cr and Ni, and the product has a low tensile strength.

[0006] A Chinese patent with publication number CN117265424A, published on December 22, 2023, discloses a high-carbon precision-blanking steel strip and a production method thereof, whose chemical composition is (wt%): C: 0.85~1.90%, Si: 0.10~0.50%, 0.85%≤0.8Mn+1.2Cr+1.5Nb≤2.52%, P≤0.018%, S≤0.010%, B: 0.002~0.010%. By controlling the continuous casting, heating, rolling, cooling and slow cooling process parameters, hot-rolled high-carbon precision-blanking steel with high strength is produced.

[0007] The Chinese patent with publication number CN109750227A published on May 14, 2019 discloses a cold-rolled steel plate for double-layer welded pipe with a tensile strength of 400MPa. The composition is C: 0.12-0.18%, Si≤0.05%, Mn: 0.2-0.6%, P≤0.025%, S≤0.020%, Alt: 0.01-0.06%, N≤0.004%, and the rest is Fe and unavoidable inclusion elements. The tensile strength is 400-500MPa, the hardness value is 55-65HR30Tm (equivalent to HRB58-73), the thickness is large and the hardness is low.

[0008] Therefore, it is very necessary to provide a cold-rolled fine-blanking steel product with low cost, low tensile strength, high hardness and excellent comprehensive performance. Summary of the invention

[0009] The purpose of the present invention is to provide a low-strength and high-surface hardness cold-rolled fine-blanking steel strip and a manufacturing method thereof, and to obtain a cold-rolled fine-blanking steel product with matching tensile strength and surface hardness by precisely controlling hot rolling, cold rolling, annealing and flattening processes.

[0010] Another object of the present invention is to provide an application of a low-strength and high-surface-hardness cold-rolled fine-blanking steel strip for use in manufacturing automobile gearbox friction plates.

[0011] The specific technical solutions of the present invention are as follows:

[0012] A low-strength and high-surface-hardness cold-rolled fine-blanking steel strip comprises the following components in percentage by mass:

[0013] C: 0.18~0.23%; Si: 0.15~0.35%; Mn: 0.40%~0.60%; P≤0.020%; S≤0.020%; Al: 0.015%~0.045%, and the rest are Fe and unavoidable impurities.

[0014] The low-strength and high-surface-hardness cold-rolled precision-punching steel strip has a tensile strength of 400-500 MPa and a surface hardness of HRB70-80.

[0015] The thickness of the low-strength and high-surface-hardness cold-rolled precision-punched steel strip is 1.5 to 3.0 mm.

[0016] The structure of the low-strength and high-surface-hardness cold-rolled fine-blanking steel strip is ferrite, pearlite and carbide.

[0017] The present invention provides a method for manufacturing a low-strength and high-surface-hardness cold-rolled fine-blanking steel strip, comprising the following processes:

[0018] Slab heating, hot rolling, cold rolling, annealing and leveling.

[0019] The slab is heated and the slab furnace temperature is controlled at 1180°C to 1220°C. If the furnace temperature is too low, the final rolling temperature cannot be guaranteed. If the furnace temperature is too high, the iron oxide scale is severely pressed into the slab during rolling. If the furnace time is too short, the grains are uneven. If the furnace time is too long, there is a lot of iron oxide scale on the surface.

[0020] In the hot rolling, the final rolling temperature is controlled at 860° C. to 900° C. If the final rolling temperature is too low, rolling in the two-phase region is easy, and if the final rolling temperature is too high, banded structure is easy to be formed.

[0021] In the hot rolling, the convexity C40 is controlled within a range of 0.020 to 0.040 mm, thereby improving the transverse thickness accuracy of the fine blanking steel.

[0022] In the hot rolling, the coiling temperature is controlled at 700° C. to 720° C. If the coiling temperature is too low, a banded structure is easily formed.

[0023] In the cold rolling, the total cold rolling reduction rate is controlled at 50% to 70%. If the reduction rate is lower than this range, the plastic strain ratio of the product is low. If the reduction rate is too high, the rolling stability during the cold rolling process is poor.

[0024] In the annealing, the steel strip is heated to 620-710°C at a speed of 0.5-1°C / min in a full hydrogen bell-type annealing furnace, and then cooled with the furnace after being kept warm for 4-10 hours. If the heating speed is too high, the uniformity of the product structure will be affected, and if the heating speed is too low, the production efficiency will be affected; if the heating temperature is too high, the tensile strength and hardness of the product will be low, and if the heating temperature is too low, the product has not yet completed recrystallization; if the holding time is too short, the banded structure in the steel is not easy to eliminate, and if the holding time is too long, the production efficiency will be affected.

[0025] The flattening elongation of the flattening machine is controlled to be between 3% and 10%. If the flattening elongation is too low, the tensile strength and hardness of the product are low, and if the flattening elongation is too high, the product plate shape is poor.

[0026] The invention provides a low-strength and high-surface-hardness cold-rolled fine-blanking steel strip for use in manufacturing automobile gearbox friction plates.

[0027] In view of the shortcomings of the prior art, the present invention provides a low-strength and high-surface-hardness cold-rolled fine-blanking steel strip and a manufacturing method thereof, and obtains a cold-rolled fine-blanking steel product with matching tensile strength and surface hardness by precisely controlling the hot rolling, cold rolling, annealing and flattening processes. The design ideas of the present invention are as follows:

[0028] The role of the alloying elements in the present invention is mainly based on the following principles:

[0029] C: Carbon is the most economical and effective solid solution element, which can significantly improve the hardness and hardenability of steel. However, excessive carbon can easily lead to excessive strength and hardness of steel. Therefore, the C percentage content in the present invention is controlled within a range of 0.18 to 0.23%;

[0030] Si: Silicon is a solid solution strengthening element that enhances the strength and hardness of steel. It can also improve the liquid fluidity and solidification of steel and improve castability. However, excessive Si will make it difficult to remove the iron oxide scale on the surface of the steel plate, affecting the surface quality of the finished product. Therefore, the Si percentage in the present invention is controlled within a range of 0.15 to 0.35%.

[0031] Mn: Manganese is a solid solution strengthening element, which can enhance the strength of steel and improve the stability of austenite, reduce the phase transition temperature of austenite to ferrite, expand the hot working temperature range, and is conducive to refining the ferrite grain size. However, if the Mn content is too high, the Mn segregation degree of the ingot increases during the continuous casting process to form a banded structure, which affects the mechanical properties and fine blanking properties of the fine blanking steel. Therefore, the Mn percentage content in the present invention is controlled in the range of 0.40-0.60%;

[0032] P: P has a low diffusion rate in ferrite and is prone to segregation and banded structure, which is detrimental to the fine blanking performance of fine blanking steel. Therefore, the P content in the present invention is controlled within a range of ≤0.020%;

[0033] S: Sulfur exists in steel in the form of iron sulfide (FeS), which reduces the ductility and toughness of the steel and causes cracks during forging and rolling. Therefore, the S percentage content in the present invention is controlled within a range of ≤0.020%;

[0034] Al: Al is the main deoxidizer and also has a certain effect on grain refinement. However, excessive Al will reduce the cutting performance. Therefore, the Al percentage content in the present invention is controlled within the range of 0.015% to 0.045%.

[0035] Compared with the prior art, the present invention obtains a cold-rolled fine-blanking steel product with matching tensile strength and surface hardness by precisely controlling the hot rolling, cold rolling, annealing and flattening processes, especially by precisely controlling the annealing temperature, holding time and flattening elongation. The product has a tensile strength of 400-500 MPa and a surface hardness of HRB70-80, and is suitable for fine-blanking of automobile gearbox friction plates with a thickness of 1.5-3.0 mm. DETAILED DESCRIPTION

[0036] 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 below 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.

[0037] Example 1-Example 2

[0038] A low-strength and high-surface-hardness cold-rolled fine-blanking steel strip comprises the following components in mass percentage: as shown in Table 1, the remainder not shown in Table 1 is Fe and unavoidable impurities.

[0039] Comparative Example 1-Comparative Example 4

[0040] A cold-rolled fine-blanking steel strip comprises the following components in mass percentage: as shown in Table 1, the remainder not shown in Table 1 is Fe and unavoidable impurities.

[0041] Table 1 Chemical composition of each embodiment and comparative example, wt%

[0042] C Si Mn P S Al Example 1 0.20 0.25 0.51 0.016 0.005 0.032 Example 2 0.21 0.24 0.48 0.013 0.006 0.041 Comparative Example 1 0.20 0.25 0.51 0.016 0.005 0.032 Comparative Example 2 0.21 0.24 0.48 0.013 0.006 0.041 Comparative Example 3 0.20 0.25 0.51 0.016 0.005 0.032 Comparative Example 4 0.21 0.24 0.48 0.013 0.006 0.041

[0043] The components of Example 1 are the same as those of Comparative Examples 1 and 3. The components of Example 2 are the same as those of Comparative Examples 2 and 4.

[0044] The manufacturing method of the cold-rolled fine-blanking steel strip in the above embodiments and comparative examples comprises the following steps:

[0045] Slab heating, hot rolling, cold rolling, annealing and leveling.

[0046] The slab is heated and the slab exiting furnace temperature is controlled at 1180°C to 1220°C.

[0047] In the hot rolling, the final rolling temperature is controlled at 860°C to 900°C.

[0048] The convexity C40 is controlled at 0.020~0.040mm.

[0049] In the hot rolling, the coiling temperature is controlled at 700°C to 720°C.

[0050] In the cold rolling, the total cold rolling reduction rate is controlled at 50% to 70%.

[0051] For the annealing, the whole coil of steel strip is heated to 620-710° C. at a speed of 0.5-1° C. / min in a full hydrogen bell-type annealing furnace, kept at the temperature for 4-10 hours, and then cooled with the furnace.

[0052] The flattening elongation of the flattening machine is controlled between 3% and 10%.

[0053] The main production process parameters of each embodiment and comparative example and the tensile strength and surface hardness results tested according to GB / T 228.1 and GB / T 230.1 are shown in Table 2.

[0054] Table 2 Main process parameters and performance of each embodiment and comparative example

[0055]

[0056]

[0057] It can be seen from the above examples that the fine-blanking steel produced by the present invention has uniform performance with a thickness of 1.5 to 3.0 mm, a tensile strength of 400 to 500 MPa, and a surface hardness of HRB70 to 80. Its tensile strength and surface hardness are well matched, which is conducive to the fine-blanking forming of thick-gauge cold-rolled fine-blanking steel.

[0058] Comparative Example 1 has low flattening elongation, resulting in low tensile strength and surface hardness. Comparative Example 2 has low flattening elongation and low surface hardness, which cannot meet the requirements of the present invention. Comparative Example 3 has high tensile strength and surface hardness due to low annealing temperature and high flattening elongation. Comparative Example 4 has high tensile strength due to low annealing temperature and short holding time.

[0059] The underlined data above do not meet the requirements of the present invention.

[0060] The description of the above embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A low-strength and high-surface-hardness cold-rolled fine-blanking steel strip, characterized in that: The low-strength and high-surface-hardness cold-rolled fine-blanking steel strip comprises the following components in percentage by mass: C：0.18～0.23％； Si: 0.15%~0.35%; Mn: 0.40%~0.60%; P≤0.020%; S≤0.020%; Al: 0.015%~0.045%, and the rest is Fe and unavoidable impurities.

2. The low-strength and high-surface-hardness cold-rolled fine-blanking steel strip according to claim 1, characterized in that: The low-strength and high-surface-hardness cold-rolled precision-punching steel strip has a tensile strength of 400-500 MPa and a surface hardness of HRB70-80.

3. The low-strength and high-surface-hardness cold-rolled fine-blanking steel strip according to claim 1, characterized in that: The thickness of the low-strength and high-surface-hardness cold-rolled precision-punched steel strip is 1.5 to 3.0 mm.

4. A method for manufacturing a low-strength and high-surface-hardness cold-rolled fine-blanking steel strip according to any one of claims 1 to 3, characterized in that: The manufacturing method comprises the following processes: Slab heating, hot rolling, cold rolling, annealing and leveling.

5. The manufacturing method according to claim 4, characterized in that: In the hot rolling, the final rolling temperature is controlled at 860°C to 900°C.

6. The manufacturing method according to claim 4, characterized in that: In the hot rolling, the convexity C40 is controlled at 0.020-0.040 mm, and the coiling temperature is controlled at 700° C.-720° C.

7. The manufacturing method according to claim 4, characterized in that: In the cold rolling, the total cold rolling reduction rate is controlled at 50% to 70%.

8. The manufacturing method according to claim 4, characterized in that: For the annealing, the whole coil of steel strip is heated to 620-710° C. at a speed of 0.5-1° C. / min in a full hydrogen bell-type annealing furnace, kept at the temperature for 4-10 hours, and then cooled with the furnace.

9. The manufacturing method according to claim 4, characterized in that: The flattening and flattening elongation are controlled between 3% and 10%.

10. An application of the low-strength and high-surface-hardness cold-rolled fine-blanking steel strip according to any one of claims 1 to 3, characterized in that: Used in the manufacture of friction plates for automobile gearboxes.

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