A low-strength high-surface-hardness cold-rolled fine-blanking steel strip, a manufacturing method and applications thereof

Abstract

The application provides a low-strength high-surface-hardness cold-rolled fine-blanking steel strip and a manufacturing method and application thereof, and the composition is as follows: 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 inevitable impurities. Compared with the prior art, the application precisely controls the hot-rolling, cold-rolling, annealing and flattening processes, and obtains a cold-rolled fine-blanking steel product with matched tensile strength and surface hardness, the tensile strength of which is 400-500 MPa, and the surface hardness is HRB70-80, and the product is suitable for fine-blanking of automobile gearbox friction plate with a thickness of 1.5-3.0 mm.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of metal material cold rolling process, and particularly relates to a low-strength high-surface-hardness cold-rolled fine-blanking steel strip and a manufacturing method and application thereof. BACKGROUND

[0002] With the development of technology, the design function of modern automobile transmission is becoming more and more powerful, the number of parts in the transmission is large, and the working condition is relatively harsh, which leads to high requirements on the material strength, surface hardness and thickness precision of the automobile transmission friction plate. Therefore, the high-strength high-hardness cold-rolled fine-blanking steel product is usually used to produce the transmission friction plate. However, when the thickness of the raw material is relatively thick (1.5-3.0 mm), it is difficult to form the high-strength high-hardness steel by fine blanking, and the size precision of the part is poor. Therefore, a cold-rolled fine-blanking steel product with low tensile strength (400-500 MPa) and high hardness (HRB 70-80) and excellent comprehensive performance is needed to meet the fine blanking forming and service performance at the same time.

[0003] The patent with the publication number CN115478223A disclosed on December 16, 2022 discloses a cold-rolled fine-blanking steel strip and a preparation method thereof. A large amount of C, Mn and other alloy elements such as Cr and Nb are added, wherein the content of C is 0.80%-0.90%, the content of Cr is 0.10%-0.20%, and the content of Nb is 0.02%-0.12%. The product prepared by high-temperature annealing at 680-750℃ for 14-28h has a yield strength of 400-500MPa, a tensile strength of 600-700MPa, an elongation after fracture (A50) of 20-30%, and a hardness (HV5) of 165-180HV. The hardness and tensile strength of the product produced by the process are relatively high, which is not conducive to fine blanking forming.

[0004] The Chinese patent with the publication number CN108385019A disclosed on August 10, 2018 discloses a cold-rolled fine-blanking steel for an automobile child seat locking device, which comprises the following components: 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, and Al≤0.035. The surface hardness of the product prepared by multiple cold rolling and multiple annealing is HRC 47-55. The process adds alloy elements such as Cr and V with high cost, and the hardness of the product produced is relatively high.

[0005] The Chinese patent with publication number CN105200308A published on December 30, 2015 discloses a fine blanking steel material and a manufacturing method of fine blanking parts of the adjusting mechanism, the chemical composition (wt%) of which is 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 and the like has a tensile strength of 375-403 MPa, and the product has a low tensile strength due to the addition of high-cost alloy elements such as Cr and Ni.

[0006] The Chinese patent with publication number CN117265424A published on December 22, 2023 discloses a high-carbon fine blanking steel strip and a production method thereof, the chemical composition of which 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%, and the hot-rolled high-carbon fine blanking steel is produced by controlling the continuous casting, heating, rolling, cooling and slow cooling process parameters.

[0007] The Chinese patent with publication number CN109750227A published on May 14, 2019 discloses a cold-rolled steel sheet for a 400 MPa-grade double-layer welded pipe, the composition of which 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 inevitable impurity elements, the tensile strength of which is 400-500 MPa, and the hardness value is 55-65 HR30Tm (equivalent to HRB58-73), and the thickness is large and the hardness is low.

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

[0009] The purpose of the present application is to provide a low-strength 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 matched tensile strength and surface hardness by precisely controlling the hot rolling, cold rolling, annealing and flattening processes.

[0010] Another purpose of the present application is to provide an application of a low-strength high-surface-hardness cold-rolled fine blanking steel strip to the manufacture of automobile transmission friction plates.

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

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

[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%, the rest being Fe and inevitable impurities.

[0014] The tensile strength of the low-strength high-surface-hardness cold-rolled fine-blanking steel strip is 400-500 MPa, and the surface hardness is HRB70-80.

[0015] The thickness of the low-strength high-surface-hardness cold-rolled fine-blanking steel strip is 1.5-3.0 mm.

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

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

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

[0019] The slab heating is controlled to have a discharge temperature of 1180-1220 DEG C; if the discharge temperature is too low, the finish rolling temperature cannot be guaranteed; if the discharge temperature is too high, the iron oxide skin is pressed in seriously during rolling; if the holding time in the furnace is too short, the grains are uneven; and if the holding time in the furnace is too long, the surface iron oxide skin is too much.

[0020] The hot rolling is controlled to have a finish rolling temperature of 860-900 DEG C; if the finish rolling temperature is too low, the two-phase region is easily rolled; and if the finish rolling temperature is too high, the banded structure is easily formed.

[0021] The hot rolling is controlled to have a crown C40 of 0.020-0.040 mm, so as to improve the transverse thickness precision of the fine-blanking steel.

[0022] The hot rolling is controlled to have a coiling temperature of 700-720 DEG C; if the coiling temperature is too low, the banded structure is easily formed.

[0023] The cold rolling is controlled to have a total reduction of 50%-70%; if the reduction is lower than the range, the plastic strain ratio of the product is low; and if the reduction is too high, the rolling stability is poor during the cold rolling.

[0024] The annealing is that the whole volume of the steel strip is heated to 620-710 DEG C at a speed of 0.5-1 DEG C / min in a hydrogen atmosphere cover annealing furnace, and then cooled in the furnace after holding for 4-10 h; if the heating speed is too high, the uniformity of the product structure is affected, and if the heating speed is too low, the production efficiency is affected; if the heating temperature is too high, the tensile strength and hardness of the product are low, and if the heating temperature is too low, the product has not 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 is affected.

[0025] The skin pass is that the skin pass elongation of the skin pass machine is controlled to be between 3% and 10%; if the skin pass elongation is too low, the tensile strength and hardness of the product are low, and if the skin pass elongation is too high, the product has poor flatness.

[0026] The application of the low-strength high-surface-hardness cold-rolled fine-blanking steel strip provided by the application is used for manufacturing automobile gearbox friction plates.

[0027] In view of the defects of the prior art, the application provides a low-strength high-surface-hardness cold-rolled fine-blanking steel strip and a manufacturing method thereof, and the cold-rolled fine-blanking steel product with matched tensile strength and surface hardness is obtained by accurately controlling the hot rolling, cold rolling, annealing and skin pass processes.

[0028] The functions of the alloy elements in the application are 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 the steel, but excessive carbon can easily lead to excessively high strength and hardness of the steel, therefore, the percentage content of C in the application is controlled in the range of 0.18-0.23%;

[0030] Si: Silicon is a solid solution strengthening element, which can enhance the strength and hardness of the steel, and can improve the liquid flowability and solidification of the steel, and improve the castability, but excessive Si can lead to difficulty in removing the iron oxide scale on the surface of the steel plate, and affect the surface quality of the finished product, therefore, the percentage content of Si in the application is controlled in the range of 0.15-0.35%;

[0031] Mn: Manganese is a solid solution strengthening element, which can enhance the strength of the steel and improve the austenite stability, reduce the phase transition temperature of the austenite into ferrite, and expand the hot working temperature range, and is beneficial to the refinement of the ferrite grain size, but excessive Mn can increase the Mn segregation degree in the continuous casting process, form banded structure, and affect the mechanical properties and fine-blanking performance of the fine-blanking steel, therefore, the percentage content of Mn in the application is controlled in the range of 0.40-0.60%;

[0032] P: P has a small diffusion speed in ferrite, and is easy to form segregation and form banded structure, which is not conducive to the fine-blanking performance of the fine-blanking steel, therefore, the content of P in the application is controlled in the range of ≤0.020%;

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

[0034] Al: Al is used as a main deoxidizer, and aluminum also has a certain effect on grain refinement, but excessive Al will reduce the cutting performance, therefore the content of Al in the present application is controlled in the range of 0.015% to 0.045%.

[0035] Compared with the prior art, the present application obtains a cold-rolled fine-blanking steel product with matched 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, so that the tensile strength is 400-500 MPa, and the surface hardness is HRB70-80, which is suitable for the fine-blanking of automobile gearbox friction plates with a thickness of 1.5-3.0 mm. DETAILED DESCRIPTION

[0036] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0037] Embodiment 1-Embodiment 2

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

[0039] Comparative Example 1-Comparative Example 4

[0040] A cold-rolled fine-blanking steel strip, comprising the following mass percentage components: as shown in Table 1, the balance of Table 1 not shown 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 compositions of Embodiment 1 and Comparative Example 1 and Comparative Example 3 are the same. The compositions of Embodiment 2 and Comparative Example 2 and Comparative Example 4 are the same.

[0044] The manufacturing method of the cold-rolled fine-blanking steel strip of each embodiment and comparative example above comprises the following steps:

[0045] The slab is heated, hot-rolled, cold-rolled, annealed and leveled.

[0046] The slab is heated, and the slab discharge temperature is controlled at 1180-1220℃.

[0047] The hot-rolling is controlled, and the finish rolling temperature is controlled at 860-900℃.

[0048] The crown C40 is controlled at 0.020-0.040mm.

[0049] The hot-rolling is controlled, and the coiling temperature is controlled at 700-720℃.

[0050] The cold-rolling is controlled, and the total cold-rolling reduction is controlled at 50-70%.

[0051] The annealing is controlled, and the steel strip is heated to 620-710℃ at a rate of 0.5-1℃ / min in a full-hydrogen bell-type annealing furnace, and then cooled after holding for 4-10h.

[0052] The leveling is controlled, and the leveling elongation is controlled at 3-10%.

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

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

[0055]

[0056]

[0057] As can be seen from the above examples, the fine blanking steel produced according to the method has uniform performance, the tensile strength is 400-500MPa, the surface hardness is HRB70-80, the tensile strength and surface hardness have good matching, and the thick specification cold-rolled fine blanking steel is beneficial to fine blanking forming.

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

[0059] The underlined data above does not meet the requirements of the present application.

[0060] The above description of the embodiments is made for the purpose of enabling a person of ordinary skill in the art to understand and use the application. It is obvious that those skilled in the art can easily make various modifications to the embodiments and apply the general principles described herein to other embodiments without creative labor. Therefore, the present application 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 application without departing from the scope of the present application should be within the scope of the present application.

Claims

1. A low-strength, high-surface-hardness cold-rolled precision-stamped steel strip, characterized in that, The low-strength, high-surface-hardness cold-rolled precision-stamped steel strip comprises the following components by weight percentage: 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%, with the remainder being Fe and unavoidable impurities; The manufacturing method of the low-strength, high-surface-hardness cold-rolled precision-stamped steel strip includes the following processes: Slab heating, hot rolling, cold rolling, annealing, and leveling; The hot rolling process involves controlling the coiling temperature at 700℃~720℃. The annealing process involves heating the entire coil of steel strip in a full-hydrogen bell-type annealing furnace to 620-710°C at a rate of 0.5-1°C / min, holding it at that temperature for 4-10 hours, and then cooling it with the furnace. The smoothing process involves controlling the smoothing elongation rate to be between 3% and 10%. The low-strength, high-surface-hardness cold-rolled precision stamping steel strip has a tensile strength of 400-500 MPa and a surface hardness of HRB70-80. The microstructure of the low-strength, high-surface-hardness cold-rolled precision stamping steel strip is ferrite, pearlite, and carbides.

2. The low-strength, high-surface-hardness cold-rolled precision-stamped steel strip according to claim 1, characterized in that, The thickness of the low-strength, high-surface-hardness cold-rolled precision stamping steel strip is 1.5~3.0mm.

3. A method for manufacturing low-strength, high-surface-hardness cold-rolled precision-stamped steel strip as described in claim 1 or 2, characterized in that, The manufacturing method includes the following processes: Slab heating, hot rolling, cold rolling, annealing, and leveling.

4. The manufacturing method according to claim 3, characterized in that, The hot rolling process involves controlling the final rolling temperature to be between 860℃ and 900℃.

5. The manufacturing method according to claim 3, characterized in that, The crown of the hot-rolled material is controlled between 0.020 and 0.040 mm.

6. The manufacturing method according to claim 3, characterized in that, The total reduction rate of the cold rolling process is controlled at 50% to 70%.

7. The manufacturing method according to claim 3, characterized in that, The annealing process involves heating the entire coil of steel strip in a full-hydrogen bell-type annealing furnace to 620-710°C at a rate of 0.5-1°C / min, holding it at that temperature for 4-10 hours, and then cooling it with the furnace.

8. The manufacturing method according to claim 3, characterized in that, The smoothing process involves controlling the smoothing elongation rate to be between 3% and 10%.

9. An application of the low-strength, high-surface-hardness cold-rolled precision-stamped steel strip as described in claim 1 or 2, characterized in that, Used in the manufacture of friction plates for automotive transmissions.

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