Ultralow-carbon high-strength high-hole-expansion-rate steel and preparation method thereof

Abstract

The invention belongs to the technical field of steel smelting, and particularly relates to ultralow-carbon high-strength high-hole-expansion-rate steel and a preparation method thereof. The steel comprises the following components in percentage by weight: 0.02-0.04% of C, less than or equal to 0.2% of Si, 1.2-2.0% of Mn, less than or equal to 0.02% of P, less than or equal to 0.003% of S, 0.02-0.06% of Al, less than or equal to 0.0045% of N, 0.02-0.04% of Nb, 0.07-0.15% of Ti, more than or equal to 3.7% of Ti / C and the balance of Fe and other inevitable impurities. The proportion of Ti / C is controlled to be larger than or equal to 3.7, the coiling temperature is controlled to enable most of C to be separated out in the form of TiC, uneven deformation caused by uneven structures due to C segregation is avoided, and finally the high-quality steel with the tensile strength larger than or equal to 780 MPa, the ductility larger than or equal to 18% and the hole expansion rate larger than or equal to 65% is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel smelting, and particularly relates to an ultra-low carbon high-strength high hole-expansion rate steel and a preparation method thereof. Background Art

[0002] When producing automobile chassis parts, in order to reduce stamping processes, it is required that the material has high strength, high plasticity and high hole-expansion rate at the same time. For existing high hole-expansion rate steels, especially 780 MPa grade high hole-expansion rate steels, the design idea of hot-rolled bainite + precipitation strengthening is mostly adopted, and the process route is medium-temperature coiling. With high alloy content, poor temperature control accuracy and tissue uniformity, the indexes such as hole-expansion rate fluctuate greatly, and stamping cracking is likely to occur at the user end. The present invention provides an ultra-low carbon high-strength high hole-expansion rate steel to solve the problem of low hole-expansion rate caused by uneven tissue.

[0003] Patent CN117305731A, a high-strength high hole-expansion rate steel and a manufacturing method thereof, points out a high-strength high hole-expansion rate steel and a manufacturing method thereof, which adopt a low-carbon high-titanium high-vanadium composition design and add precious alloys such as Mo. Without adding silicon element, its component weight percentage is: C 0.01-0.10%, Si≤0.2%, Mn 0.5-2.0%, P≤0.02%, S≤0.003%, Al 0.01-0.08%, N≤0.004%, Ti 0.05-0.2%, V 0.10-0.50%, O≤0.003%, Mo≤0.5%, and the balance contains Fe and other inevitable impurities; cooling adopts segmented cooling. After finish rolling, the steel plate is water-cooled to 600-750°C at a cooling rate of ≥30°C / s, air-cooled for 1-10 seconds, and then cooled to 400-550°C at a cooling rate of ≥10°C / s for coiling, and then cooled to room temperature at a cooling rate of ≤20°C / h. Finally, steels with different strength levels and hole-expansion rates are obtained by different V contents. When V is 0.10-0.20%, the high hole-expansion rate steel has a tensile strength of 590 MPa grade and a hole-expansion rate of ≥70%; when V is 0.20-0.35%, the high hole-expansion rate steel has a tensile strength of 780 MPa grade and a hole-expansion rate of ≥50%; when V is 0.35-0.50%, the high hole-expansion rate steel has a tensile strength of 980 MPa grade and a hole-expansion rate of ≥30%. This method needs to add precious alloys and adopts segmented cooling, which requires a high level of automation of on-site process equipment.

[0004] Patent CN116287991A, "A 780MPa Grade Hot-Rolled High-Expansion-Hole Steel with Excellent Weldability and Its Manufacturing Method", points out a high-strength and high-expansion-hole steel with excellent weldability and its manufacturing method. By adopting the micro-alloy design of adding Mo, Cr, B, and Nb+Ti, its component weight percentages are as follows: C: 0.05% - 0.1%, Si: 0% - 0.4%, Mn: 1.2% - 1.6%, Mo: 0.1% - 0.2%, Nb: 0.02% - 0.04%, Ti: 0.1% - 0.2%, Cr: 0.1% - 0.4%; B: 0.0005% - 0.015%, and the balance is Fe and inevitable impurities. It is necessary to strictly control the coiling temperature. When the coiling temperature ≥ 580°C, the precipitation strengthening of bainite structure increases, resulting in too large a strength difference between ferrite and bainite and a reduction in the expansion-hole rate. When the coiling temperature ≤ 450°C, the structure will transform into martensite, leading to too high strength and a reduction in elongation and expansion-hole rate. Finally, a steel with a yield strength ≥ 660MPa, a tensile strength ≥ 780MPa, an elongation ≥ 15%, and an expansion-hole rate ≥ 35% is obtained, showing excellent matching of strength, plasticity, and expansion-hole property, and is particularly suitable for parts such as control arms that require high-strength thinning and expansion-hole flanging forming in automotive chassis structures. This method requires the addition of precious alloys and has a low expansion-hole rate. Summary of the Invention

[0005] The purpose of the present invention is to provide a super-low-carbon high-strength and high-expansion-hole rate steel and its preparation method to solve the problems existing in the prior art.

[0006] The technical solution adopted by the present invention to solve its technical problems is:

[0007] A super-low-carbon high-strength and high-expansion-hole rate steel, with component weight percentages as follows: C 0.02 - 0.04%, Si ≤ 0.2%, Mn 1.2 - 2.0%, P ≤ 0.02%, S ≤ 0.003%, Al 0.02 - 0.06%, N ≤ 0.0045%, Nb: 0.02% - 0.04%, Ti 0.07 - 0.15%, the ratio of Ti / C ≥ 3.7, and the balance contains Fe and other inevitable impurities.

[0008] Furthermore, the tensile strength of the high-expansion-hole rate steel ≥ 780MPa, the elongation ≥ 18%, and the expansion-hole rate ≥ 65%.

[0009] A preparation method of a super-low-carbon high-strength and high-expansion-hole rate steel, including the following preparation processes: hot metal pretreatment → converter → RH refining → LF refining → continuous casting → rough rolling → induction heating furnace → finish rolling → laminar cooling → coiling → slow cooling and storage.

[0010] Furthermore, the rough rolling exit temperature ≥ 1000°C.

[0011] Furthermore, the induction heating furnace exit temperature is 1150 ± 50°C.

[0012] Furthermore, the finish rolling temperature is 820 - 880 °C.

[0013] Furthermore, the cooling rate of laminar cooling is ≥ 30 °C / s.

[0014] Furthermore, the coiling temperature is controlled at 550 - 650 °C.

[0015] Furthermore, after coiling, it is quickly stored in the warehouse and taken out after slow cooling for ≥ 48 h.

[0016] The present invention has the following beneficial effects:

[0017] By controlling the ratio of Ti / C ≥ 3.7 and controlling the coiling temperature, the vast majority of C precipitates in the form of TiC, avoiding the uneven organization and deformation caused by the segregation of C, and finally obtaining high-quality steel with a tensile strength ≥ 780 MPa, an elongation rate ≥ 18%, and an expansion rate ≥ 65%. Description of the Drawings

[0018] Figure 1 It is a physical diagram of the ultra-low carbon high-strength and high-expansion rate steel obtained by the present invention. Specific Embodiments

[0019] In order to make the objectives, technical solutions and advantages of the present invention clearer, the following further elaborates on the present invention in detail with reference to specific embodiments and the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0020] Example 1:

[0021] As Figure 1 shown, taking the ultra-low carbon high-strength and high-expansion rate steel with a specification of 3.5 mm as an example, its component weight percentages are: C 0.036%, Si 0.15%, Mn 1.60%, P 0.015%, S ≤ 0.002%, Al 0.03%, N 0.0035%, Nb 0.025%, Ti 0.131%. Its preparation process is: hot metal pretreatment → converter → RH refining → LF refining → continuous casting → rough rolling → induction heating furnace → finish rolling → laminar cooling → coiling → slow cooling storage. Among them: the rough rolling outlet temperature is 1050 °C, the finish rolling temperature is 840 °C, rapid cooling after rolling, the cooling rate > 50 °C / s, the coiling temperature is 610 °C, and it is taken out after slow cooling for ≥ 48 h. The detected properties are: yield 718 MPa, tensile 797 MPa, elongation rate 18%, and expansion rate 71%.

[0022] Carbon is a basic element in steel and also one of the important elements in the present invention. Carbon expands the austenite phase region and stabilizes austenite. As an interstitial atom in steel, carbon plays a very important role in increasing the strength of steel. When it combines with strong carbide-forming elements to form carbides during precipitation, it plays a role in precipitation strengthening. Carbon is the element that contributes the most to the strength of steel. In the present invention, in order to control the segregation of C, all C precipitates as TiC precipitation phase, and the carbon content in the present invention is controlled at 0.02 - 0.04%;

[0023] Silicon is a basic element in steel. In order to meet the requirements of high strength, high plasticity and high hole expansion rate put forward by users, more silicon is usually added in the composition design. However, the composition design with high silicon leads to a reduction in the surface quality of the steel plate and there are more red scale defects. In the present invention, in order to ensure good surface quality, the content of silicon should be strictly controlled in the composition design. The silicon content in the steel of the present invention is controlled within 0.2%;

[0024] Manganese is also one of the most basic elements in steel and one of the most important elements in the present invention. As is well known, Mn is an important element that expands the austenite phase region. It can reduce the critical quenching speed of steel, stabilize austenite, refine grains and delay the transformation of austenite to pearlite. In the present invention, in addition to playing a role in solid solution strengthening, Mn and Cr can also reduce the phase transformation temperature, refine the size of TiC precipitation, and are controlled at 1.2 - 2.0%;

[0025] Titanium is an important element in the present invention. When titanium is added to steel, on the one hand, it can combine with N to form TiN at high temperature, playing a role in nitrogen fixation; effectively preventing the growth of austenite grains in the high temperature zone; especially during welding, it can effectively prevent the growth of grains in the heat affected zone and the molten zone, refine the structure, and the precipitates can also pin dislocations to delay the propagation of crack sources at the weld and significantly improve the strength and toughness of the material. Titanium is the main strengthening element of this patent. The Ti content is controlled at 0.07 - 0.15%, and the ratio of Ti / C is controlled ≥ 3.7.

[0026] Example 2:

[0027] Taking the ultra-low carbon high strength and high hole expansion rate steel with a specification of 3.0 mm as an example, its component weight percentages are: C 0.033%, Si 0.18%, Mn 1.62%, P 0.010%, S ≤ 0.001%, Al 0.03%, N 0.0032%, Nb 0.020%, Ti 0.125%. The rough rolling exit temperature is 1080 °C, the finish rolling temperature is 830 °C, rapid cooling is carried out after rolling, the cooling rate > 50 °C / s, the coiling temperature is 600 °C, and it is slowly cooled for ≥ 48 h before leaving the warehouse. The detected properties are: yield 726 MPa, tensile 785 MPa, elongation 20%, hole expansion rate 71%.

[0028] Example 3:

[0029] Taking the extra-low-carbon high-strength and high hole-expansion ratio steel with a specification of 4.0 mm as an example, its component weight percentages are as follows: C 0.025%, Si 0.15%, Mn 1.80%, P 0.015%, S ≤ 0.002%, Al 0.03%, N 0.0025%, Nb 0.03%, Ti 0.125%. The rough rolling exit temperature is 1030 °C, the finish rolling temperature is 820 °C, rapid cooling is carried out after rolling, the cooling rate > 50 °C / s, the coiling temperature is 590 °C, and it is slowly cooled for ≥ 48 h before leaving the warehouse. The detected properties are: yield strength 734 MPa, tensile strength 820 MPa, elongation 22%, and hole-expansion ratio 85%.

[0030] The working principle of the present invention is as follows:

[0031] By controlling the ratio of Ti / C ≥ 3.7 and controlling the coiling temperature to precipitate most of the C in the form of TiC, avoiding the uneven distribution of C causing uneven organization and further uneven deformation, and finally obtaining high-quality steel with a tensile strength ≥ 780 MPa, an elongation ≥ 18%, and a hole-expansion ratio ≥ 65%. The obtained steel is ultra-low-carbon and titanium-strengthened. On the one hand, the precipitation of TiC improves its strength, and on the other hand, by controlling the Ti / C ratio, no or very few interstitial atoms C are obtained, making the organization uniform and enabling the steel to have both high strength and high hole-expansion property.

[0032] The above embodiments only describe the preferred embodiments of the present invention, and do not limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various modifications and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope of the present invention.

[0033] The technologies, shapes, and structures not described in detail in the present invention are all well-known technologies.

Claims

1. An ultra-low carbon, high-strength and high-hole expansion rate steel, characterized in that: The weight percentage of the components is: C 0.02-0.04%, Si≤0.2%, Mn1.2-2.0%, P≤0.02%, S≤0.003%, Al0.02-0.06%, N≤0.0045%, Nb: 0.02%-0.04%, Ti0.07-0.15%, Ti / C ratio≥3.7, and the balance includes Fe and other inevitable impurities.

2. The ultra-low carbon, high-strength, high-hole expansion rate steel according to claim 1, characterized in that: The high hole expansion rate steel has a tensile strength of ≥780MPa, an elongation of ≥18%, and a hole expansion rate of ≥65%.

3. A method for preparing an ultra-low carbon, high-strength and high-hole expansion rate steel according to any one of claims 1-2, characterized in that: The preparation process includes the following: molten iron pretreatment → converter → RH refining → LF refining → continuous casting → rough rolling → induction heating furnace → finishing rolling → layer cooling → coiling → slow cooling storage.

4. The method for preparing ultra-low carbon high-strength high hole expansion rate steel according to claim 3, characterized in that: The rough rolling outlet temperature is ≥1000°C.

5. The method for preparing ultra-low carbon high-strength high hole expansion rate steel according to claim 3, characterized in that: The outlet temperature of the induction heating furnace is 1150±50°C.

6. The method for preparing ultra-low carbon high-strength high hole expansion rate steel according to claim 3, characterized in that: The finishing rolling temperature is 820-880°C.

7. The method for preparing ultra-low carbon high-strength high hole expansion rate steel according to claim 3, characterized in that: The cooling rate of the layer cooling is ≥30°C / s.

8. The method for preparing ultra-low carbon high-strength high hole expansion rate steel according to claim 3, characterized in that: The coiling temperature is controlled at 550-650°C.

9. The method for preparing ultra-low carbon high-strength high hole expansion rate steel according to claim 3, characterized in that: After coiling, the product is quickly put into storage and slowly cooled for ≥48h before being shipped out.

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