600mpa grade hot-rolled steel sheet for automobile parts and method for manufacturing the same

Abstract

The application provides a 600MPa-grade hot-rolled steel plate for automobile parts and a manufacturing method thereof, which comprises the following components: Cr 0.35%-0.45%, Ni 0.40%-0.50%, Mo 0.26%-0.30%, C 0.15%-0.20%, Si 0.20%-0.30%, Mn 0.60%-0.70%, P≤0.012%, S≤0.002%, Alt 0.030%-0.050%, and the rest is Fe and inevitable inclusions, and (C+Mn) / (Cr+Ni+Mo)=0.69-0.71; (Cr+Mo) / Ni=1.47-1.63; compared with the prior art, the application produces the hot-rolled steel plate with excellent performance by reasonably designing smelting components, continuous casting, hot rolling, cooling and coiling processes, and meets the requirements of special steel for parts on strength, toughness, wear resistance, decarburization and uniformity of structure.

Description

Technical Field

[0001] This invention belongs to the field of steel materials technology, specifically relating to a 600MPa grade hot-rolled steel plate for automotive parts and its manufacturing method. Background Technology

[0002] The automotive industry is one of the pillar industries of my country's national economy, playing a vital role in driving economic growth and ensuring employment. Developing the automotive parts industry and its specialized steels is essential for the development of my country's modern automotive industry. The increasing demands for vehicle safety and long service life place ever higher requirements on the quality and performance of specialized steels used in automotive parts, especially for specialized steel products with comprehensive technical properties such as high strength, high toughness, and high fatigue resistance, which are experiencing rapid growth in demand.

[0003] The brake pad is one of the important components of a car. It is a pedal used to limit the power of the car. It is one of the most important control components in driving. It is used very frequently and has a direct impact on driving safety. It places high demands on the strength, toughness, wear resistance, decarburization and uniformity of the raw materials.

[0004] Chinese invention patent CN104213019A, published on December 17, 2014, discloses a 600MPa grade automotive axle housing steel and its production method. Its composition (by weight percentage) is: C: 0.21%–0.26%, Si: 0.51%–0.6%, Mn: 1.1%–1.5%, Al: 0.01%–0.06%, P≤0.02%, S≤0.01%, V: 0.05%–0.06%, N: 0.012%–0.016%, with the remainder being Fe and unavoidable impurities, wherein V:N ≤ 5:1. The patent utilizes precise V and N content design and controlled rolling and cooling process windows to produce 600MPa grade hot-rolled strip steel for automotive axle housings. The steel coils exhibit good strength and toughness, while ensuring the various mechanical properties of the axle housing after hot forming at temperatures above 800℃. However, it does not disclose how to control the decarburized layer depth.

[0005] Chinese invention patent CN110791708A, published on February 14, 2020, discloses a non-quenched and tempered steel for automotive parts and its production process. The disclosed non-quenched and tempered steel contains Ni ≤ 0.20%, Ti: 0.010%–0.025%, Nb: 0.012%–0.025%, and N: 0.013%–0.019%. The process includes converter smelting, LF refining, RH vacuum treatment, continuous casting, and rolling. This patent uses elements such as Ni, Nb, and Ti in its composition, and employs high-temperature heating followed by low-temperature controlled rolling, achieving a tensile strength exceeding 800 MPa. However, its cost is high, and it does not disclose how to control the decarburized layer depth. Summary of the Invention

[0006] The purpose of this invention is to provide a 600MPa grade hot-rolled steel sheet for automotive parts and its manufacturing method. Through the design of reasonable smelting composition, continuous casting, hot rolling, cooling, and coiling processes, a high-performance hot-rolled steel sheet is produced. The thickness of the hot-rolled coil ranges from 6 to 10 mm, and it can be directly used for heat treatment processing of automotive brake pad components.

[0007] The specific technical solution of this invention is as follows:

[0008] A hot-rolled steel sheet for automotive parts with a 600MPa grade comprises the following components by weight percentage:

[0009] Cr: 0.35%–0.45%, Ni: 0.40%–0.50%, Mo: 0.26%–0.30%, C: 0.15%–0.20%, Si: 0.20%–0.30%, Mn: 0.60%–0.70%, P: ≤0.012%, S: ≤0.002%, Alt: 0.030%–0.050%, with the remainder being Fe and unavoidable inclusions.

[0010] The composition of the hot-rolled steel sheet for automotive parts of the 600MPa grade also meets the following requirements:

[0011] (C+Mn) / (Cr+Ni+Mo)=0.69~0.71;

[0012] The composition of the hot-rolled steel sheet for automotive parts of the 600MPa grade also satisfies: (Cr+Mo) / Ni=1.47~1.63;

[0013] In the above formulas, the symbol of each element represents its mass percentage content in the steel grade × 100;

[0014] The microstructure of the 600MPa grade hot-rolled steel sheet for automotive parts is ferrite + pearlite, with pearlite area content accounting for 27% to 32%, average grain size of 5.5 to 6.5 μm, banded structure grade 1.5 to 2.0, and total decarburized layer depth of 10 to 20 μm on one side.

[0015] The non-metallic inclusions in the 600MPa grade hot-rolled steel plate for automotive parts are classified as follows: Class A inclusions have a coarse and fine series of grade 0; Class B inclusions have a coarse and fine series of grade ≤0.5; Class C inclusions have a coarse and fine series of grade 0; and Class D inclusions have a coarse and fine series of grade ≤1.0.

[0016] The 600MPa grade hot-rolled steel sheet for automotive parts has a yield strength of 473-495MPa, a tensile strength of 615-650MPa, an elongation of 23%-27%, and a Brinell hardness of 173-190HBW.

[0017] The present invention provides a method for manufacturing a 600MPa grade hot-rolled steel sheet for automotive parts, comprising continuous casting, heating, rolling, cooling and coiling.

[0018] The continuous casting process employs dynamic light pressure and electromagnetic stirring. After the billet is removed from the production line, it enters a slow cooling pit for 72 hours for slow cooling, which helps reduce center segregation of the billet. The center segregation requirement is to reach level B1.0.

[0019] The heating process involves the slab entering a heating furnace for heating. The homogenization temperature T is controlled between 1180℃ and 1200℃, and the total time in the furnace t is controlled between 160min and 180min. The total time in the furnace t is related to the homogenization temperature T; 6.5 < T / t < 7.5 is necessary to ensure uniform heating, promote austenite homogenization, and ensure the complete dissolution of microalloying elements such as Cr, Ni, and Mo in the austenite. This also avoids excessive coarsening of austenite grains and decarburization of the slab. When calculating T / t, the value before the unit is directly substituted into the formula.

[0020] The rolling process is as follows: the roughing stage is rolled in the austenite recrystallization zone, with the descaling water fully open at the roughing mill inlet and outlet. The finishing mill opening temperature is controlled at 1010-1040℃, and the finishing mill temperature is controlled at 850-870℃. This avoids rolling in the low-temperature dual-phase zone, which would cause excessive mill load. It also avoids mixed crystals and excessively high finishing mill temperature, which would cause austenite grain coarsening.

[0021] The thickness range of hot-rolled coils is 6 to 10 mm.

[0022] The cooling process involves laminar flow cooling of the rolled strip before coiling. The cooling method is front-end laminar flow cooling, where the strip undergoes laminar water cooling immediately after exiting the F7 stand, while all cooling water nozzles in the rear section are closed. The front-end laminar flow cooling rate is 30–35°C / s, with a front-end cooling time of 4–5 seconds, and the rear-end air cooling time is 2–3 seconds. Controlling the coiling temperature of the cooled steel plate at 620–640°C is beneficial for controlling the pearlite ratio, refining ferrite grains, and reducing grain size inhomogeneity.

[0023] The design concept of this invention is as follows:

[0024] C is the most economical strengthening element. Adding C can significantly improve the strength of steel plates. However, too high a C content will increase the degree of center segregation of the steel. The C content should be controlled between 0.15% and 0.20%.

[0025] Si plays a solid solution strengthening role in steel, thereby improving the strength of steel plates. It can also expand the ferrite formation range, which is beneficial to expanding the rolling process window. However, excessive Si content will affect the surface quality of steel. The Si content needs to be controlled between 0.20% and 0.30%.

[0026] Mn is an inexpensive element that can improve the strength of steel plates through solid solution strengthening. However, excessive Mn content can exacerbate the central segregation of steel. Therefore, the Mn content should be controlled between 0.60% and 0.70%.

[0027] P and S are harmful residual elements that easily cause segregation and aggregation. Therefore, the P and S contents are controlled to P≤0.012% and S≤0.002%.

[0028] Cr, Ni, and Mo are elements that can improve the strength of steel. Cr and Mo improve the hardenability of steel and the uniformity of the microstructure of thick steel plates. Ni improves the strength of steel without reducing its plasticity and improves the low-temperature toughness of steel. C, Mn, Cr, Ni, and Mo are all elements that can improve the strength of steel, but too high a content of C and Mn will aggravate the center segregation of steel, and too high a content of Cr, Ni, and Mo will affect the economy of steel. Therefore, while ensuring the excellent comprehensive performance of the steel plate, the economy should be ensured as much as possible. It is specified that (C+Mn) / (Cr+Ni+Mo) = 0.69~0.71, (Cr+Mo) / Ni = 1.47~1.63.

[0029] Compared with existing technologies, this invention produces high-performance hot-rolled steel sheets through the design of reasonable smelting composition, continuous casting, hot rolling, cooling, and coiling processes. The hot-rolled coils range in thickness from 6 to 10 mm and can be directly used for heat treatment processing of automotive brake pad components. Through the design of the C, Mn, Cr, Ni, and Mo formulas, the invention ensures excellent comprehensive performance of the steel sheet while maximizing economic efficiency and reducing costs. The product has a banded microstructure of grade 1.5 to 2.0, with a total decarburized layer depth of 10 to 20 μm on one side. It exhibits a yield strength of 473 to 495 MPa, a tensile strength of 615 to 650 MPa, an elongation of 23% to 27%, and a Brinell hardness of 173 to 190 HBW, demonstrating excellent comprehensive performance and meeting the requirements for strength, toughness, wear resistance, decarburization, and microstructure uniformity of special steels used in automotive parts. Attached Figure Description

[0030] Figure 1 The image shows the microstructure of Embodiment 1 of the present invention under an optical microscope. The microstructure consists of ferrite and pearlite. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Examples 1-5

[0033] A 600MPa grade hot-rolled steel sheet for automotive parts comprises the following composition by mass percentage: as shown in Table 1. The balance not shown in Table 1 is Fe and unavoidable impurities. Composition analysis was performed according to GB / T 4336 "Spark Source Atomic Emission Spectrometry Analysis Method (Conventional Method) for Carbon Steel and Low-Alloy Steel".

[0034] Comparative Examples 1-4

[0035] A 600MPa grade hot-rolled steel sheet for automotive parts comprises the following composition by mass percentage: as shown in Table 1. The balance not shown in Table 1 is Fe and unavoidable impurities. Composition analysis was performed according to GB / T 4336 "Spark Source Atomic Emission Spectrometry Analysis Method (Conventional Method) for Carbon Steel and Low-Alloy Steel".

[0036] Table 1. Chemical composition and content (wt%) of each embodiment and comparative example of the present invention.

[0037]

[0038]

[0039] The manufacturing of 600MPa grade hot-rolled steel plates for automotive parts described in the above embodiments and comparative examples includes hot metal pretreatment, converter smelting, LF furnace refining, continuous casting, heating, rolling, cooling, and coiling. Among them, in continuous casting: dynamic light pressure and electromagnetic stirring are used, and the billet is slowly cooled in a slow cooling pit for 72 hours after it comes off the production line, which helps to reduce center segregation of the billet. The center segregation requirement is to reach grade B1.0.

[0040] Heating: The slab enters the heating furnace for heating. The homogenization temperature T is controlled at 1180℃~1200℃, and the total time in the furnace t is controlled at 160min~180min. The total time in the furnace t is related to the homogenization temperature T. 6.5<T / t<7.5 is necessary to ensure uniform heating, promote the homogenization of austenite and the full solidification of microalloying elements such as Cr, Ni, and Mo in austenite, and at the same time avoid excessive coarsening of austenite grains and decarburization of the slab.

[0041] Rolling: The roughing stage is rolled in the austenite recrystallization zone. The descaling water at the roughing mill inlet and outlet is fully open. The finishing mill opening temperature is controlled at 1010-1040℃, and the finishing mill temperature is controlled at 850-870℃. Rolling in the low-temperature dual-phase zone will avoid excessive mill load. At the same time, mixed crystals should be avoided, and the finishing mill temperature should also be avoided to prevent coarsening of austenite grains due to excessively high temperature.

[0042] After rolling, the strip is cooled by laminar flow cooling before coiling. The cooling method is laminar flow cooling in the front section, and laminar water cooling is performed immediately after the strip exits the F7 stand. All cooling water nozzles in the rear section are closed. The cooling rate is 30-35℃ / s, the front section cooling time is 4-5s, and the rear section air cooling time is 2-3s. The coiling temperature of the cooled steel plate is controlled at 620-640℃, which is beneficial for controlling the pearlite ratio, refining ferrite grains, and reducing grain size inhomogeneity.

[0043] The specific parameters of each embodiment and comparative example are shown in Tables 2 and 3.

[0044] Table 2 Heating process parameters of various embodiments and comparative examples of the present invention

[0045] Sample number Thickness / mm Isotropic temperature T / ℃ Total furnace time t / min T / t Example 1 6.0 1200 165 7.3 Example 2 7.0 1180 180 6.6 Example 3 8.0 1190 160 7.4 Example 4 9.0 1200 170 7.1 Example 5 10.0 1190 175 6.8 Comparative Example 1 8.0 1200 170 7.1 Comparative Example 2 7.0 1240 210 5.9 Comparative Example 3 9.0 1200 170 7.1 Comparative Example 4 10.0 1190 180 6.6

[0046] The main rolling process parameters of the various embodiments and comparative examples of the present invention are shown in Table 3.

[0047] Table 3. Main process parameters of the rolling process in each embodiment and comparative example of the present invention.

[0048]

[0049] The mechanical properties of the various embodiments and comparative examples of the present invention are shown in Table 4, and were tested in accordance with GB / T 228.1.

[0050] Table 4 Mechanical properties of various embodiments and comparative examples of the present invention

[0051] Sample number Thickness / mm Yield strength / MPa Tensile strength / MPa The ratio of yield strength Elongation / % Brinell hardness / HBW Example 1 6.0 489 642 0.76 24 185 Example 2 7.0 491 650 0.76 27 187 Example 3 8.0 482 638 0.76 23 190 Example 4 9.0 495 642 0.77 24 180 Example 5 10.0 473 615 0.77 26 173 Comparative Example 1 8.0 420 567 0.74 29 167 Comparative Example 2 7.0 452 604 0.75 26 170 Comparative Example 3 9.0 430 582 0.74 31 162 Comparative Example 4 10.0 427 591 0.72 27 169

[0052] The inclusion size distribution results of the various embodiments and comparative examples of the present invention are shown in Table 5, and the tests were conducted in accordance with GB / T10561.

[0053] Table 5 Non-metallic inclusions in various embodiments and comparative examples of the present invention

[0054]

[0055] The microstructure and decarburization layer results of the various embodiments and comparative examples of the present invention are shown in Table 6, and were tested in accordance with GB / T 224.

[0056] Table 6. Microstructure and decarburization of various embodiments and comparative examples of the present invention.

[0057]

[0058] In Table 6, F represents ferrite and P represents pearlite.

[0059] In summary, the hot-rolled steel sheet for automotive parts of grade 600MPa, designed and produced according to the chemical composition, steelmaking and hot rolling process provided by this invention, has a yield strength of 473-495MPa, tensile strength of 615-650MPa, elongation of 23%-27%, Brinell hardness of 173-190HBW, and a total decarburized layer depth of 10-20μm on one side. The embodiments of this invention have good comprehensive mechanical properties.

[0060] The Cr, Ni and Mo composition of Comparative Example 1 does not meet the requirements of this invention, resulting in (C+Mn) / (Cr+Ni+Mo) and (Cr+Mo) / Ni also not meeting the requirements of this invention. Even if produced according to the process of this invention, the strength and hardness of the obtained product are low, and the total decarburization layer depth on one side is high.

[0061] The composition of Comparative Example 2 was smelted according to the present invention, but the homogenization temperature and time did not meet the requirements of the present invention, the grain size increased significantly, the total decarburization layer depth on one side reached 60μm, and the product strength and hardness were both low.

[0062] Comparative Example 3 was smelted according to the present invention, but the final rolling and coiling temperatures were high, resulting in an increase in product grain size and a total decarburized layer depth of 50 μm on one side, with both product strength and hardness being low.

[0063] Comparative Example 4 was smelted according to the present invention, but the post-rolling cooling was not controlled according to the parameters of the present invention, resulting in a low cooling rate, long water cooling time, reduced pearlite content, and lower product strength and hardness.

[0064] The data underlined above do not meet the requirements of this invention.

[0065] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any 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 invention should be within the protection scope of the present invention.

Claims

1. A hot-rolled steel sheet for automotive parts with a strength of 600MPa, characterized in that, The 600MPa grade hot-rolled steel sheet for automotive parts comprises the following composition by weight percentage: Cr 0.35%-0.45%, Ni 0.40%-0.50%, Mo 0.26%-0.30%, C 0.15%-0.20%, Si 0.20%-0.30%, Mn 0.60%-0.70%, P≤0.012%, S≤0.002%, Alt 0.030%-0.050%, with the remainder being Fe and unavoidable inclusions; The composition of the hot-rolled steel sheet for automotive parts of the 600MPa grade also satisfies (C+Mn) / (Cr+Ni+Mo)=0.69-0.71; (Cr+Mo) / Ni=1.47-1.63; The microstructure of the hot-rolled steel sheet for automotive parts of the 600MPa grade is ferrite + pearlite, with a pearlite content of 27%-32%, an average grain size of 5.5-6.5μm, a banded structure of grade 1.5-2.0, and a total decarburized layer depth of 10-20μm on one side. The non-metallic inclusions in the 600MPa grade hot-rolled steel plate for automotive parts are classified as follows: Class A inclusions, with both coarse and fine inclusions at grade 0; Class B inclusions, with both coarse and fine inclusions at grade ≤0.5; Class A inclusions, with both coarse and fine inclusions at grade 0; and Class D inclusions, with both coarse and fine inclusions at grade ≤1.

0.

2. The 600MPa grade hot-rolled steel plate for automotive parts according to claim 1, characterized in that, The 600MPa grade hot-rolled steel plate for automotive parts has a yield strength of 473-495MPa, a tensile strength of 615-650MPa, an elongation of 23%-27%, and a Brinell hardness of 173-190HBW.

3. A method for manufacturing the 600MPa grade hot-rolled steel sheet for automotive parts as described in claim 1 or 2, characterized in that, The manufacturing method includes continuous casting, heating, rolling, cooling, and coiling.

4. The manufacturing method according to claim 3, characterized in that, The heating process involves controlling the homogenization temperature T at 1180℃-1200℃ and the total furnace time t at 160min-180min. The total furnace time t is related to the homogenization temperature T, with a ratio of 6.5 < T / t < 7.

5.

5. The manufacturing method according to claim 3, characterized in that, The rolling process involves controlling the initial rolling temperature at 1010-1040℃ and the final rolling temperature at 850-870℃.

6. The manufacturing method according to claim 3, characterized in that, The cooling process employs a front-end laminar flow cooling system with a cooling rate of 30-35℃ / s and a front-end cooling time of 4-5s; the rear-end air cooling time is 2-3s, and the coiling temperature of the cooled steel plate is controlled at 620-640℃.

Images