600MPa-grade hot-rolled steel plate for automobile parts and manufacturing method of 600MPa-grade hot-rolled steel plate

Abstract

The invention provides a 600MPa-grade hot-rolled steel plate for automobile parts and a manufacturing method of the 600MPa-grade hot-rolled steel plate. The 600MPa-grade hot-rolled steel plate comprises the following components in percentage by weight: 0.35%-0.45% of Cr, 0.40%-0.50% of Ni, 0.26%-0.30% of Mo, 0.15%-0.20% of C, 0.20%-0.30% of Si, 0.60%-0.70% of Mn, less than or equal to 0.012% of P, less than or equal to 0.002% of S, 0.030%-0.050% of Alt and the balance of Fe and inevitable impurities, and (C + Mn) / (Cr + Ni + Mo) is 0.69-0.71. (Cr + Mo) / Ni is equal to 1.47 to 1.63; compared with the prior art, the hot-rolled steel plate with excellent performance is produced through reasonable design of smelting components, continuous casting, hot rolling, cooling and coiling processes, and the requirements of special steel for parts on strength, toughness, wear resistance, decarburization and structure uniformity are met.

Description

Technical Field

[0001] The invention belongs to the technical field of steel materials, and in particular relates to a 600MPa-grade hot-rolled steel plate for automobile parts and a manufacturing method thereof. Background Art

[0002] Automobile is one of the pillar industries of my country's national economy, and plays a very important role in boosting the national economy and ensuring employment. The development of automobile parts industry and special steel for its use is necessary for the development of my country's modern automobile industry. Automobile safety and long life continuously put forward higher requirements on the quality and performance of special steel for its parts, especially the demand for special steel products with comprehensive technical properties such as high strength, high toughness and high fatigue resistance is growing rapidly.

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

[0004] The Chinese invention patent with the publication number CN104213019A published on December 17, 2014 discloses a 600MPa grade automobile bridge housing steel and its production method, whose components are (by weight percentage): C: 0.21% to 0.26%, Si: 0.51% to 0.6%, Mn: 1.1% to 1.5%, Al: 0.01% to 0.06%, P≤0.02%, S≤0.01%, V: 0.05% to 0.06%, N: 0.012% to 0.016%, and the rest are Fe and unavoidable impurities, wherein V:N≤5:1. It designs accurate V and N content and controlled rolling and controlled cooling process window to produce 600MPa grade automobile bridge housing hot-rolled strip steel, the steel coil has good strength and toughness, and can ensure various mechanical performance indicators of the bridge housing after hot forming at above 800℃. However, it does not disclose how to control the depth of the decarburized layer.

[0005] The Chinese invention patent with the publication number CN110791708A published on February 14, 2020, discloses a non-quenched and tempered steel for automotive parts and its production process. The non-quenched and tempered steel disclosed in the patent has Ni≤0.20%, Ti: 0.010%~0.025%, Nb: 0.012%~0.025%, N: 0.013%~0.019%, and the process includes converter smelting, LF refining, RH vacuum treatment, continuous casting and rolling. This patent uses Ni, Nb, Ti and other elements in composition, and the process uses high temperature heating + low temperature controlled rolling, and the tensile strength reaches more than 800MPa, but its cost is relatively high, and it does not disclose how to control the depth of the decarburized layer. Summary of the invention

[0006] The purpose of the present invention is to provide a 600MPa grade hot-rolled steel plate for automobile parts and a manufacturing method thereof, and to produce a hot-rolled steel plate with excellent performance through reasonable design of smelting composition, continuous casting, hot rolling, cooling and coiling process. The hot-rolled coil has a thickness range of 6 to 10 mm and is subsequently directly used for heat treatment processing of automobile brake plate parts.

[0007] The specific technical solutions of the present invention are as follows:

[0008] A 600MPa grade hot-rolled steel plate for automobile parts, comprising the following components in percentage by mass:

[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%, the rest is Fe and unavoidable inclusions.

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

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

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

[0013] In the above formula, 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 plate for automobile parts is ferrite+pearlite, the pearlite area content accounts for 27% to 32%, the average grain size is 5.5 to 6.5μm, the banded structure is 1.5 to 2.0 grades, and the total decarburization layer depth on one side is 10 to 20μm.

[0015] The non-metallic inclusions in the 600MPa grade hot-rolled steel plate for automobile parts are as follows: Class A inclusions are both level 0 for both coarse and fine series, Class B inclusions are both level ≤0.5 for both coarse and fine series, Class C inclusions are both level 0 for both coarse and fine series, and Class D inclusions are both level ≤1.0 for both coarse and fine series.

[0016] The 600MPa-grade hot-rolled steel plate for automobile 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 invention provides a method for manufacturing a 600MPa-grade hot-rolled steel plate for automobile parts, which comprises continuous casting, heating, rolling, cooling and coiling.

[0018] The continuous casting: dynamic light pressure reduction and electromagnetic stirring are used, and the billet is put into a slow cooling pit for slow cooling for 72 hours after it comes off the line, which is beneficial to reducing the center segregation of the billet, and the center segregation is required to reach B1.0 level.

[0019] The heating: the slab enters the heating furnace for heating, the soaking temperature T is controlled at 1180℃~1200℃, the total time in the furnace t is controlled at 160min~180min, and the total time in the furnace t is controlled to have a certain relationship with the soaking temperature T, 6.5<T / t<7.5, in order to ensure uniform heating, promote austenite homogenization and sufficient solid solution of micro-alloy elements such as Cr, Ni, Mo in austenite, and also avoid excessive coarsening of austenite grains and decarburization of the slab. When calculating T / t, directly substitute the value before the unit into the formula for calculation.

[0020] The rolling process is as follows: the rough rolling stage is carried out in the austenite recrystallization zone, the rough rolling inlet and outlet are fully opened for descaling, the finishing rolling start temperature is controlled at 1010-1040°C, and the final rolling temperature is controlled at 850-870°C to avoid excessive mill load caused by rolling in the low-temperature dual-phase zone, to avoid mixed crystals, and to avoid coarsening of austenite grains caused by excessive final rolling temperature.

[0021] The thickness of hot rolled coil ranges from 6 to 10 mm.

[0022] The cooling is that the strip steel is coiled after laminar cooling after rolling, and the cooling method is front-stage laminar cooling. The strip steel is immediately laminar water-cooled after exiting the F7 rack, and all cooling water nozzles in the rear-stage area are closed. The cooling rate of the front-stage laminar cooling is 30-35°C / s, the front-stage cooling time is 4-5s, and the rear-stage air cooling time is 2-3s. The coiling temperature of the cooled steel plate is controlled at 620-640°C, which is conducive to controlling the pearlite ratio, refining the ferrite grains and reducing the unevenness of the grain size.

[0023] The design ideas of the present invention are as follows:

[0024] C is the most economical strengthening element. Adding C can significantly improve the strength of the steel plate, but too high a C content will increase the degree of central segregation of the steel. The C content is controlled within 0.15% to 0.20%.

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

[0026] Mn is a cheap element, which can improve the strength of the steel plate through solid solution strengthening. However, too high a content of Mn tends to increase the degree of central segregation of the steel. The Mn content is controlled within a range of 0.60% to 0.70%.

[0027] P and S elements are harmful residual elements that easily cause segregation and aggregation, so 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 elements improve the hardenability of steel and improve the uniformity of the structure of thick steel plates. Ni element 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 content of C and Mn elements will increase the degree of central segregation of steel, and too high content of Cr, Ni, and Mo elements will affect the economy of steel. Therefore, under the condition of ensuring that the steel plate has excellent comprehensive performance, the economy is guaranteed as much as possible, and it is stipulated that (C+Mn) / (Cr+Ni+Mo)=0.69~0.71, (Cr+Mo) / Ni=1.47~1.63.

[0029] Compared with the prior art, the present invention produces hot-rolled steel plates with excellent performance through the design of reasonable smelting composition, continuous casting, hot rolling, cooling and coiling process. The thickness of the hot-rolled coil ranges from 6 to 10 mm, and is subsequently directly used for heat treatment processing of automobile brake plate parts. Through the design of C, Mn, Cr, Ni, and Mo formulas, the economy is guaranteed as much as possible and the cost is reduced under the condition that the steel plate has excellent comprehensive performance. The product has a banded structure of 1.5 to 2.0 grades, and a total decarburization layer depth of 10 to 20 μm on one side. The yield strength is 473 to 495 MPa, the tensile strength is 615 to 650 MPa, the elongation is 23% to 27%, and the Brinell hardness is 173 to 190 HBW. It has excellent comprehensive performance and meets the requirements of special steel for parts for strength, toughness, wear resistance, decarburization, and uniformity of organization. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The microstructure morphology of Example 1 of the present invention under an optical microscope is ferrite+pearlite. DETAILED DESCRIPTION

[0031] 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.

[0032] Example 1-Example 5

[0033] A 600MPa grade hot-rolled steel plate for automobile parts includes the following mass percentage components: as shown in Table 1, the remainder not shown in Table 1 is Fe and unavoidable impurities. The component detection is carried out according to GB / T 4336 "Spark Source Atomic Emission Spectrometric Analysis Method for Carbon Steel and Medium and Low Alloy Steel (Conventional Method)".

[0034] Comparative Example 1-Comparative Example 4

[0035] A 600MPa grade hot-rolled steel plate for automobile parts includes the following mass percentage components: as shown in Table 1, the remainder not shown in Table 1 is Fe and unavoidable impurities. The component detection is carried out according to GB / T 4336 "Spark Source Atomic Emission Spectrometric Analysis Method for Carbon Steel and Medium and Low Alloy Steel (Conventional Method)".

[0036] Table 1 Chemical composition and content (wt%) of various embodiments and comparative examples of the present invention

[0037]

[0038]

[0039] The manufacture of 600MPa grade hot-rolled steel plates for automotive parts described in the above embodiments and comparative examples includes molten iron pretreatment, converter smelting, LF furnace refining, continuous casting, heating, rolling, cooling and coiling, wherein, in continuous casting, dynamic soft reduction and electromagnetic stirring are used, and the ingot is slowly cooled in a slow cooling pit for 72 hours after it comes off the line, which is beneficial to reducing the center segregation of the ingot, and the center segregation is required to reach level B1.0.

[0040] Heating: The slab enters the heating furnace for heating, the absorptive temperature T is controlled at 1180℃~1200℃, the total time in the furnace t is controlled at 160min~180min, and the total time in the furnace t is controlled to have a certain relationship with the absorptive temperature T, 6.5<T / t<7.5, in order to ensure uniform heating, promote austenite homogenization and sufficient solid solution of micro-alloy elements such as Cr, Ni, Mo in austenite, and also avoid excessive coarsening of austenite grains and decarburization of the slab.

[0041] Rolling: The rough rolling stage is carried out in the austenite recrystallization zone, the rough rolling entrance and exit are fully open for descaling, the finishing rolling temperature is controlled at 1010-1040°C, and the final rolling temperature is controlled at 850-870°C to avoid excessive load on the rolling mill caused by rolling in the low-temperature dual-phase zone, and to avoid mixed crystals. It is also necessary to avoid excessive final rolling temperature that leads to coarsening of austenite grains.

[0042] After rolling, the strip is coiled after laminar cooling. The cooling method is front-stage laminar cooling. After the strip exits the F7 rack, it is immediately laminar water-cooled, and all cooling water nozzles in the rear section are closed. The cooling rate is 30-35℃ / s, the front-stage cooling time is 4-5s, and the rear-stage air cooling time is 2-3s. The coiling temperature of the cooled steel plate is controlled at 620-640℃, which is conducive to controlling the pearlite ratio, refining the ferrite grains and reducing the unevenness of the grain size.

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

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

[0045] Sample No. Thickness / mm Ambient 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 embodiments of the present invention and the comparative examples are shown in Table 3.

[0047] Table 3 Main process parameters of rolling process of various embodiments of the present invention and comparative examples

[0048]

[0049] The mechanical properties of the embodiments of the present invention and the comparative examples are shown in Table 4, and the tests were performed according to GB / T 228.1.

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

[0051] Sample No. Thickness / mm Yield strength / MPa Tensile strength / MPa Yield-to-strength ratio 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 embodiments of the present invention and the comparative examples are shown in Table 5, and the test was performed in accordance with GB / T10561.

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

[0054]

[0055] The microstructures and decarburized layer results of the embodiments of the present invention and the comparative examples are shown in Table 6, and the tests were performed in accordance with GB / T 224.

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

[0057]

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

[0059] In summary, according to the chemical composition, steelmaking and hot rolling process provided by the present invention, a 600MPa grade hot-rolled steel plate for automotive parts is designed and produced, with a yield strength of 473-495MPa, a tensile strength of 615-650MPa, an elongation of 23%-27%, a Brinell hardness of 173-190HBW, and a single-sided total decarburization layer depth of 10-20μm. The embodiment of the present invention has good comprehensive mechanical properties.

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

[0061] The components of Comparative Example 2 were smelted according to the present invention, but the soaking temperature and time did not meet the requirements of the present invention, the grain size increased significantly, the total single-sided decarburization layer depth reached 60 μm, and the product strength and hardness were relatively low.

[0062] The components of Comparative Example 3 were smelted according to the present invention, but the final rolling and coiling temperatures were high, resulting in an increase in the grain size of the product, a total single-sided decarburization layer depth of 50 μm, and low product strength and hardness.

[0063] The components of Comparative Example 4 were smelted according to the present invention, but the cooling after rolling was not controlled according to the parameters of the present invention, the cooling rate was low, the water cooling time was long, the proportion of pearlite was reduced, and the product strength and hardness were low.

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

[0065] 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 600MPa grade hot-rolled steel plate for automobile parts, characterized in that: The 600MPa grade hot-rolled steel sheet for automobile parts comprises the following components in percentage by mass: Cr0.35%~0.45%, Ni0.40%~0.50%, Mo0.26%~0.30%, C0.15%~0.20%, Si0.20%~0.30%, Mn0.60%~0.70%, P≤0.012%, S≤0.002%, Alt0.030%~0.050%, and the rest are Fe and unavoidable inclusions.

2. The 600MPa grade hot-rolled steel sheet for automobile parts according to claim 1, characterized in that: The composition of the 600 MPa grade hot-rolled steel sheet for automobile parts also satisfies (C+Mn) / (Cr+Ni+Mo)=0.69-0.

71.

3. The 600MPa grade hot-rolled steel sheet for automobile parts according to claim 1 or 2, characterized in that: The composition of the 600 MPa grade hot-rolled steel sheet for automobile parts also satisfies (Cr+Mo) / Ni=1.47-1.

63.

4. The 600 MPa grade hot-rolled steel sheet for automobile parts according to claim 1, characterized in that: The microstructure of the 600MPa grade hot-rolled steel plate for automobile parts is ferrite + pearlite, the pearlite content accounts for 27% to 32%, the average grain size is 5.5 to 6.5 μm, the banded structure is 1.5 to 2.0, and the total decarburization layer depth on one side is 10 to 20 μm; The non-metallic inclusions in the 600MPa grade hot-rolled steel plate for automobile parts are as follows: Class A inclusions, both coarse and fine are level 0; Class B inclusions, both coarse and fine are level ≤0.5; Class A inclusions, both coarse and fine are level 0; and Class D inclusions, both coarse and fine are level ≤1.

0.

5. The 600 MPa grade hot-rolled steel sheet for automobile parts according to claim 1 or 5, characterized in that: The 600MPa-grade hot-rolled steel plate for automobile 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.

6. A method for manufacturing the 600 MPa grade hot-rolled steel sheet for automobile parts according to any one of claims 1 to 5, characterized in that: The manufacturing method includes continuous casting, heating, rolling, cooling and coiling.

7. The manufacturing method according to claim 6, characterized in that: In the heating, the soaking temperature T is controlled at 1180° C. to 1200° C., the total time in the furnace t is controlled at 160 min to 180 min, and the total time in the furnace t is controlled to have a certain relationship with the soaking temperature T, 6.5<T / t<7.

5.

8. The manufacturing method according to claim 6, characterized in that: In the rolling, the start temperature of the finishing rolling is controlled at 1010-1040°C, and the final rolling temperature is controlled at 850-870°C.

9. The manufacturing method according to claim 6, characterized in that: The cooling adopts front-stage laminar cooling, with a cooling rate of 30-35°C / s, a front-stage cooling time of 4-5s, and a rear-stage air cooling time of 2-3s, and the coiling temperature of the cooled steel plate is controlled at 620-640°C.

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