Low-yield-ratio hot rolled steel for automobile parts and manufacturing method of low-yield-ratio hot rolled steel

Abstract

The invention provides low-yield-ratio hot rolled steel for automobile parts and a manufacturing method of the low-yield-ratio hot rolled steel. The low-yield-ratio hot rolled steel comprises the following components: 0.30%-0.40% of Cr, 0.40%-0.50% of Ni, 0.26%-0.30% of Mo, 0.17%-0.22% 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, 0.72%-0.78% of (C + Mn) / (Cr + Ni + Mo) and 1.28%-1.50% of (Cr + Mo) / Ni. The balance is Fe and inevitable inclusions; through reasonable design of smelting components, continuous casting, hot rolling, cooling and coiling processes, the hot rolled steel coil with excellent performance is produced, the thickness is 3-6 mm, the yield ratio is 0.66-0.69, and the hot rolled steel coil is favorably used for follow-up process machining such as acid pickling, cold rolling and annealing.

Description

Technical Field

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

[0002] The development of the automobile parts industry and the special steel used in it is a necessity for the development of my country's modern automobile industry. The safety and long life of automobiles continue to put forward higher requirements on the quality and performance of special steels used in their parts, especially the rapid growth in demand for special steel products with comprehensive technical properties such as high strength, high toughness, and high fatigue resistance.

[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 automobile 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. For hot-rolled steel coils that need to undergo cold rolling, the rolling capacity of the cold rolling unit is limited. In order to facilitate cold rolling, while ensuring the tensile strength of the material, the yield strength of the material is expected to be as low as possible, which requires the material to have a lower yield strength ratio.

[0004] The Chinese invention patent with the publication number CN103556048A published on February 5, 2014 discloses a low yield ratio, high strength dual-phase steel plate for automobiles and a production method, the chemical composition of which is: C: 0.1% to 0.40%, Mn: 3.0% to 8.0%, P ≤ 0.020%, S ≤ 0.02%, on this basis, one or more of the following elements can be added: Ni: 0.1% to 3.0%, Cr: 0.2% to 3.0%, Mo: 0.1%% ~0.8%,Si:0.3%~2.3%,Cu:0.5%~2.0%,B:0.0005-0.005%,Nb:0.02%~0.10%,N:0.002%~0.25%,Ti:0.05%~0.25%,V:0.02%~0.25%,Al:0.015%~0.060wt%、RE(rare earth):0.002%~0.005%、Ca:0.005%~0.03%。For the duplex steel with tensile strength of 980MPa, the yield strength ratio does not exceed 0.55, and the elongation is greater than 22%; for the duplex steel with 1180MPa, the yield strength ratio does not exceed 0.55, and the elongation is greater than 17%. However, its composition is complex, and rare earth needs to be added, which is costly.

[0005] The Chinese invention patent with publication number CN110484827A published on November 22, 2019 discloses a low yield ratio hot-rolled pickled steel plate with a tensile strength of 600 MPa, whose chemical composition weight percentage is: C: 0.06%~0.08%, Mn: 0.8%~1.2%, Si: 0~0.10%, P: 0~0.020%, S: 0~0.006%, Alt: 0.015%~0.050%, N: 0~0.005%, Ti: 0.03%~0.05%, the remainder is Fe and unavoidable impurity elements, the yield ratio is below 0.80, but the yield ratio is still above 0.74, which is not conducive to subsequent cold rolling. Summary of the invention

[0006] The purpose of the present invention is to provide a hot-rolled steel for low yield ratio automotive parts and a manufacturing method thereof, and to produce a hot-rolled steel coil with excellent performance through reasonable design of smelting composition, continuous casting, hot rolling, cooling and coiling process. The hot-rolled steel coil has a thickness range of 3 to 6 mm and is subsequently used for pickling, cold rolling, annealing and other processes.

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

[0008] A hot-rolled steel for automobile parts with a low yield ratio, comprising the following chemical components in percentage by weight:

[0009] Cr: 0.30% ~ 0.40%, Ni: 0.40% ~ 0.50%, Mo: 0.26% ~ 0.30%, C: 0.17% ~ 0.22%, 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 hot-rolled steel for automobile parts with low yield strength ratio also meets the following requirements:

[0011] (C+Mn) / (Cr+Ni+Mo)=0.72~0.78;

[0012] The composition of the hot-rolled steel for automobile parts with low yield ratio also satisfies: (Cr+Mo) / Ni=1.28-1.50;

[0014] The microstructure of the hot-rolled steel for low yield ratio automobile parts is ferrite+pearlite, the pearlite area content accounts for 26% to 29%, the average grain size is 7.8 to 8.6 μm, the banded structure is 1.5 to 2.0 grades, the total decarburization layer depth on one side is 10 to 20 μm, and the iron oxide scale depth is 6 to 8 μm.

[0015] The non-metallic inclusions in the hot-rolled steel for low yield ratio automotive parts are as follows: Class A inclusions, both coarse and fine are at level 0; Class B inclusions, both coarse and fine are at level ≤1.0; Class C inclusions, both coarse and fine are at level 0; and Class D inclusions, both coarse and fine are at level ≤1.5.

[0016] The hot-rolled steel for automobile parts with low yield ratio has a yield strength of 388-403 MPa, a tensile strength of 570-592 MPa, a yield ratio of 0.66-0.69, and an elongation of 30%-35%.

[0017] The invention provides a method for manufacturing hot-rolled steel for automobile parts with a low yield ratio, which comprises continuous casting, heating, rolling, cooling and coiling.

[0018] The continuous casting uses dynamic light pressure reduction and electromagnetic stirring. After the ingot is off the line, it enters a slow cooling pit for slow cooling for 72 hours, which is beneficial to reducing the center segregation of the ingot. The center segregation is required to reach level B1.0.

[0019] Heating: The slab enters the heating furnace for heating, and the soaking 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 controlled to have a certain relationship with the soaking temperature T. 6.5≤T / t≤7.5 can ensure uniform heating, promote austenite homogenization and fully dissolve micro-alloy elements such as Cr, Ni, and Mo in austenite, and also avoid excessive coarsening of austenite grains and decarburization of the slab. When calculating T / t, just directly substitute the value before the unit into the formula for calculation.

[0020] The rolling process includes: rolling in the austenite recrystallization zone in the rough rolling stage, fully opening the descaling water at the entrance and exit of the rough rolling to ensure that the iron oxide scale is completely removed, controlling the start rolling temperature of the finishing rolling at 1010-1040°C, and controlling the final rolling temperature at 870-890°C to control the ferrite grain size of the steel coil, avoiding mixed crystals caused by too low a final rolling temperature, and also avoiding coarsening of austenite grains caused by too high a final rolling temperature.

[0021] The cooling is that the rolled strip is coiled after laminar cooling, the cooling method is rear-stage sparse laminar cooling, the strip is air-cooled after exiting the F7 rack, all cooling water nozzles are closed, and laminar water cooling is performed in the rear-stage area. The air cooling time in the front stage is 2 to 3 seconds, the cooling speed of the rear stage water cooling is 30 to 35°C / s, and the water cooling time in the rear stage is 4 to 5 seconds. The coiling temperature of the cooled steel coil is 680 to 700°C, which is conducive to controlling the pearlite ratio, the appropriate ferrite grain size and reducing the unevenness of the grain size.

[0022] The design idea of ​​the present invention is as follows

[0023] C is the most economical strengthening element. Adding C can significantly improve the strength of the steel coil and appropriately reduce the yield ratio. However, too high C content will increase the degree of central segregation of the steel. The C content is controlled at 0.17% to 0.22%.

[0024] Si plays a role of solid solution strengthening in steel, thereby improving the strength of the steel coil. 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 coil. The Si content needs to be controlled at 0.20% to 0.30%.

[0025] Mn is a cheap element, which can improve the strength of the steel coil through solid solution strengthening. However, too high a content of Mn tends to increase the degree of central segregation of the steel coil. The Mn content is controlled within a range of 0.60% to 0.70%.

[0026] 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%.

[0027] 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 coils. 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. Too low content of C and Mn elements is not conducive to obtaining low yield ratio performance, but too high content will increase the degree of central segregation of steel. Too high content of Cr, Ni, and Mo elements affects the economy of steel, and too high content of Cr and Mo affects the grain size, which is not conducive to obtaining low yield ratio performance. Therefore, under the condition of ensuring that the steel coil has excellent comprehensive performance, try to ensure economy, and stipulate (C+Mn) / (Cr+Ni+Mo)=0.72~0.78, (Cr+Mo) / Ni=1.28~1.50.

[0028] Compared with the prior art, the present invention produces hot-rolled steel coils with excellent performance through reasonable design of smelting, continuous casting, hot rolling, cooling and coiling processes. The product microstructure is ferrite + pearlite, the pearlite area content accounts for 26% to 29%, the average grain size is 7.8 to 8.6 μm, the banded structure is 1.5 to 2.0 grades, the total decarburization layer depth on one side is 10 to 20 μm, and the scale depth is 6 to 8 μm. The product yield strength is 388 to 403 MPa, the tensile strength is 570 to 592 MPa, the yield strength ratio is 0.66 to 0.69, the elongation is 30% to 35%, and the hot-rolled steel coil thickness ranges from 3 to 6 mm. The low yield strength ratio is conducive to subsequent processing such as pickling, cold rolling, and annealing. And there is no need to add rare earth, etc., and the cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1The microstructure morphology of Example 1 of the present invention under an optical microscope is ferrite+pearlite. DETAILED DESCRIPTION

[0030] 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.

[0031] Example 1-Example 5

[0032] A hot-rolled steel for automobile parts with a low yield ratio comprises the following chemical components in mass percentage: as shown in Table 1, the remainder not shown in Table 1 is Fe and unavoidable impurities.

[0033] Comparative Example 1-Comparative Example 4

[0034] A hot-rolled steel for automobile parts with a low yield ratio comprises the following chemical components in mass percentage: as shown in Table 1, the remainder not shown in Table 1 is Fe and unavoidable impurities.

[0035] The composition test 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 method for manufacturing the hot-rolled steel for automobile parts with low yield ratio described in the above embodiments and comparative examples comprises molten iron pretreatment, converter smelting, LF furnace refining, continuous casting, heating, rolling, cooling and coiling;

[0040] The continuous casting uses dynamic light pressure reduction and electromagnetic stirring. After the ingot is off the line, it enters a slow cooling pit for slow cooling for 72 hours, which is beneficial to reducing the center segregation of the ingot. The center segregation is required to reach level B1.0.

[0041] Heating: The slab enters the heating furnace for heating, and the soaking 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 controlled to have a certain relationship with the soaking temperature T. 6.5≤T / t≤7.5 can ensure uniform heating, promote austenite homogenization and fully dissolve micro-alloy elements such as Cr, Ni, and Mo in austenite, and also avoid excessive coarsening of austenite grains and decarburization of the slab. When calculating T / t, just directly substitute the value before the unit into the formula for calculation.

[0042] The rolling process: the rough rolling stage is carried out in the austenite recrystallization zone, the rough rolling inlet and outlet descaling water are fully opened to ensure that the iron oxide scale is completely removed, the finishing rolling start temperature is controlled at 1010-1040°C, the final rolling temperature is controlled at 870-890°C, the ferrite grain size of the steel coil is controlled, and the final rolling temperature is too low to cause mixed crystals, and the final rolling temperature is too high to cause coarsening of austenite grains. The thickness of the hot rolled steel coil ranges from 3 to 6 mm.

[0043] The cooling is that the rolled strip is coiled after laminar cooling, the cooling method is rear-stage sparse laminar cooling, the strip is air-cooled after exiting the F7 rack, all cooling water nozzles are closed, and laminar water cooling is performed in the rear-stage area. The air cooling time in the front stage is 2 to 3 seconds, the cooling speed of the rear stage water cooling is 30 to 35°C / s, and the water cooling time in the rear stage is 4 to 5 seconds. The coiling temperature of the cooled steel coil is 680 to 700°C, which is conducive to controlling the pearlite ratio, the appropriate ferrite grain size and reducing the unevenness of the grain size.

[0044] The main rolling process parameters of the embodiments of the present invention and the comparative examples are shown in Table 2.

[0045] Table 2 Heating process parameters of various embodiments of the present invention and comparative examples

[0046] Sample No. Thickness / mm Soaking temperature T / ℃ Total furnace time t / min T / t Example 1 3.0 1200 175 6.9 Example 2 3.5 1200 180 6.7 Example 3 4.0 1190 165 7.2 Example 4 5.0 1200 160 7.5 Example 5 6.0 1180 175 6.7 Comparative Example 1 3.0 1200 180 6.7 Comparative Example 2 4.0 1240 220 5.6 Comparative Example 3 5.0 1190 165 7.2 Comparative Example 4 6.0 1180 180 6.6

[0047] The main rolling process parameters of the embodiments of the present invention and the comparative examples are shown in Table 3.

[0048] Table 3 Main process parameters of rolling process of various embodiments of the present invention and comparative examples

[0049]

[0050]

[0051] 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.

[0052] Table 4 Mechanical properties of various embodiments of the present invention and comparative examples

[0053] Sample No. Thickness / mm Yield strength / MPa Tensile strength / MPa Yield-to-strength ratio Elongation / % Example 1 3.0 403 592 0.68 32 Example 2 3.5 388 588 0.66 30 Example 3 4.0 395 573 0.69 35 Example 4 5.0 390 582 0.67 32 Example 5 6.0 388 570 0.68 34 Comparative Example 1 3.0 484 660 0.73 32 Comparative Example 2 4.0 398 586 0.68 33 Comparative Example 3 5.0 473 615 0.77 26 Comparative Example 4 6.0 413 582 0.71 28

[0054] 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.

[0055] Table 5 Non-metallic inclusions in various embodiments of the present invention and comparative examples

[0056]

[0057]

[0058] The microstructures and decarburized layer results of the embodiments of the present invention and the comparative examples are shown in Table 6. The test was carried out according to GB / T 224.

[0059] Table 6 Microstructure and decarburization of various embodiments of the present invention and comparative examples

[0060]

[0061] In Table 6, F represents ferrite and P represents pearlite.

[0062] In summary, according to the chemical composition, steelmaking and hot rolling process provided by the present invention, a hot-rolled steel coil for automobile parts with a low yield ratio is produced, with a yield strength of 388-403 MPa, a tensile strength of 570-592 MPa, a yield ratio of 0.66-0.69, an elongation of 30%-35%, a total decarburization layer depth of 10-20 μm on a single side, and an oxide scale thickness of 6-8 μm. The embodiment of the present invention has good comprehensive performance and can be used for subsequent pickling, cold rolling, annealing and other process processing.

[0063] The components of C, 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 obtained product has a higher pearlite content, higher strength and higher yield strength ratio.

[0064] The components of Comparative Example 2 were smelted according to the present invention, but (C+Mn) / (Cr+Ni+Mo) and (Cr+Mo) / Ni did not meet the requirements of the present invention, and the soaking temperature and time did not meet the requirements of the present invention, resulting in a total decarburization layer depth of 60 μm on one side and a thick iron oxide scale.

[0065] The composition of Comparative Example 3 meets the requirements of the present invention, but (C+Mn) / (Cr+Ni+Mo) and (Cr+Mo) / Ni do not meet the requirements of the present invention, and the final rolling is low, the cooling rate is fast, and the coiling temperature is low, resulting in a small grain size and a relatively high yield strength of the product.

[0066] The components of Comparative Example 4 are smelted according to the components of the present invention, but (C+Mn) / (Cr+Ni+Mo) and (Cr+Mo) / Ni do not meet the requirements of the present invention, the cooling after rolling is not controlled according to the parameters of the present invention, the proportion of pearlite increases, the grain size is small, and the product yield strength is relatively high.

[0067] The underlined data above do not meet the requirements of the present invention.

[0068] 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 hot-rolled steel for automobile parts with low yield ratio, characterized in that: The hot-rolled steel for automobile parts with low yield ratio comprises the following chemical compositions in mass percentage: C: 0.17% ~ 0.22%, Si: 0.20% ~ 0.30%, Mn: 0.60% ~ 0.70%, P: ≤ 0.012%, S: ≤ 0.002%, Alt: 0.030% ~ 0.050%, Cr: 0.30% ~ 0.40%, Ni: 0.40% ~ 0.50%, Mo: 0.26% ~ 0.30%, the rest is Fe and unavoidable inclusions.

2. The hot-rolled steel for automobile parts with low yield ratio according to claim 1, characterized in that: The composition of the hot-rolled steel for automobile parts with low yield ratio also satisfies: (C+Mn) / (Cr+Ni+Mo)=0.72-0.

78.

3. The hot-rolled steel for automobile parts with low yield ratio according to claim 1 or 2, characterized in that: The composition of the hot-rolled steel for automobile parts with low yield ratio also satisfies: (Cr+Mo) / Ni=1.28-1.

50.

4. The hot-rolled steel for automobile parts with low yield ratio according to any one of claims 1 to 3, characterized in that: The microstructure of the hot-rolled steel for low yield ratio automobile parts is ferrite+pearlite, the pearlite content accounts for 26% to 29%, the average grain size is 7.8 to 8.6 μm, the banded structure is 1.5 to 2.0 grades, the total decarburization layer depth on one side is 10 to 20 μm, and the iron oxide scale depth is 6 to 8 μm.

5. The hot-rolled steel for automobile parts with low yield ratio according to any one of claims 1 to 4, characterized in that: The hot-rolled steel for automobile parts with low yield ratio has a yield strength of 388-403 MPa, a tensile strength of 570-592 MPa, a yield ratio of 0.66-0.69, and an elongation of 30%-35%.

6. A method for producing hot-rolled steel for automobile parts with low yield ratio 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: 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 6.5≤T / t≤7.

5.

8. The manufacturing method according to claim 6, characterized in that: The rolling process includes: the start rolling temperature of the finishing rolling is controlled at 1010-1040°C, and the final rolling temperature is controlled at 870-890°C.

9. The manufacturing method according to claim 6, characterized in that: The cooling is that the rolled strip is coiled after laminar cooling, the cooling method is rear-stage sparse laminar cooling, the strip is air-cooled after leaving the F7 rack for 2 to 3 seconds, all cooling water nozzles are closed, laminar water cooling is performed in the rear area, the cooling rate is 30 to 35°C / s, the rear-stage water cooling time is 4 to 5 seconds, and the coiling temperature of the cooled steel coil is 680 to 700°C.

Images