980MPa-grade high-reaming steel and production method thereof

Abstract

The invention discloses 980MPa-grade high-reaming steel and a production method thereof, and belongs to the technical field of metallurgy. The high reaming steel comprises the following chemical components in percentage by mass: 0.025 to 0.050 percent of C, less than or equal to 0.05 percent of Si, 1.60 to 1.80 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.003 percent of S, 0.20 to 0.30 percent of Cr, 0.020 to 0.050 percent of Als, 0.100 to 0.120 percent of V, 0.15 to 0.25 percent of Mo, 0.0100 to 0.0120 percent of N, 0.0025 to 0.0050 percent of B, less than or equal to 0.0020 percent of O and the balance of Fe and impurities. The 980MPa-grade high-hole-expansion steel with good mechanical and hole-expansion performance is prepared by adopting the component design of micro-low C and V-N-Mo-B composite microalloying and through the processes of precise heating, front-section rapid cooling and medium-high temperature coiling.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a 980MPa grade high hole expansion steel and a production method thereof. Background Art

[0002] The automobile industry occupies a very important position in my country's economic development. The materials used for passenger car chassis, seats and some body parts are mainly hot-rolled pickling or hot-base galvanized products. The lightweighting of passenger cars, especially new energy vehicles, has become one of the important trends in the development of the automobile industry, and the lightweighting of parts is reflected in the material requirements of "enhancement-thinning-weight reduction". With the advancement and development of forming technology, such as: high pressure forming, hot stamping, laser welding and other technologies, this will inevitably require more complex component design, higher raw material steel grade, and better comprehensive service performance, which in turn puts forward higher requirements for high material strengthening, stamping and flanging performance, rebound, fatigue and coating performance.

[0003] FBDP steel is also called stretch-flangeable (SF) steel or high hole-expansion (HEE) steel. When the bainite content is 10% to 20%, it has very good forming properties, especially stretch flange performance. Compared with HSLA and FMDP steels, the main advantage of FBDP steel is that it improves the shear edge ductility, takes into account the contradiction and balance between strength and ductility, and has good welding, fatigue and hole expansion performance, which has advantages that general materials cannot match; the hardness of the heat-affected zone of FBDP steel after flash welding is higher than that of the base metal, and there will be no softening phenomenon during subsequent processing. At the same time, the steel has good light-fatigue performance. Compared with HSLA steel of the same strength level, FBDP steel has higher strain hardening index (n value) and total elongation, so FBDP steel is more suitable for stamping parts with high requirements and good elongation performance, such as automobile chassis.

[0004] The Chinese invention patent with publication number CN113106337B discloses a high hole expansion steel of 980MPa grade or above and a production method thereof, wherein the chemical composition and its mass percentage are as follows: C: 0.08-0.16%, Si: 0.4-1.0%, Mn: 1.5%-2.5%, S≤0.0035%, P≤0.0080%, Al: 0.1-0.5%, Cr: 0.2-0.5%, Mo: 0.1-0.3%, O≤0.0010%, N≤0.0030%, V: 0.02-0.08%, Nb: 0.02%-0.06%, Ti: 0.02-0.06%, B≤0.0010%, Cu≤0.01%, Sn≤0.05%, H≤0.0004%, and the balance is Fe and unavoidable impurities. The production method includes smelting, continuous casting, heating, rolling, and slow cooling. The invention adopts a segmented cooling process, but the first and second laminar cooling rates are relatively slow, and the final cooling temperature of the first laminar cooling is in the pearlite transformation zone, which easily leads to the transformation of austenite or residual austenite to pearlite, thereby causing the hole expansion performance to deteriorate.

[0005] The Chinese invention patent with publication number CN114107790B discloses a 980MPa grade ultra-low carbon martensitic high hole expansion steel, whose chemical composition weight percentage is: C: 0.03%~0.06%, Si: 0.5%~2.0%, Mn: 1.0%~2.0%, P≤0.02%, S≤0.003%, Al: 0.02%~0.08%, N≤0.004%, Mo: 0.1%~0.5%, Ti: 0.01%~0.05%, O≤0.0030%, and also contains one or more elements of Cr≤0.5%, B≤0.002%, Ca≤0.005%, Nb≤0.06%, V≤0.05%, Cu≤0.5%, Ni≤0.5%, and the rest is Fe and other inevitable impurities. The production method includes smelting, casting, slab heating, hot rolling, cooling, pickling and other processes to obtain low-carbon martensitic structure and ensure the hole expansion performance of the material. The slab heating temperature of the invention is low, but the insulation time is too long, which easily leads to decarburization and burning of the slab surface, and the production efficiency is low and the energy consumption is high.

[0006] The existing production method usually prepares high hole expansion steel above 980MPa by adding more precious alloy elements such as Cr, Nb, Ti, V and Mo. Although ferrite / bainite dual-phase structure can be obtained under the condition of low cooling rate, its production cost is relatively high. In addition, the publication numbers are CN 106119702 B, CN 106636899 B, CN 114107788 B, CN114107794B, CN 114107791 B, CN 114107797 B, CN 106521327 B, CN 114107798 A, CN117305730A and CN 117305731 The composition design of Chinese invention patents A has a high Si content. Once the descaling effect is not good during the rolling stage, "red rust" oxide scale is easily generated on the surface of the strip steel. The surface shows serious color difference after pickling, which is not conducive to direct electrophoretic coating of parts. In addition, the slab heating temperature is low (1100-1200°C), but the insulation time is too long (1-2h), which easily leads to complete decarburization and burning of the slab surface, low production efficiency and high energy consumption. At the same time, the composition design feature of CN 106521327 B is the addition of higher Re, but it is very easy to exist in the form of inclusions during the smelting process, which deteriorates the comprehensive performance of the steel plate. Summary of the invention

[0007] The technical problem to be solved by the present invention is that the composition design and preparation process of the existing high-expansion steel of 980MPa grade and above easily lead to "red rust" iron oxide scale, and there are problems such as high alloy cost, low production efficiency, high energy consumption and poor comprehensive material performance.

[0008] The technical solution adopted by the present invention to solve its technical problem is:

[0009] In a first aspect, the present invention provides a 980MPa grade high hole expansion steel, whose chemical composition is as follows by mass percentage: 0.025≤C≤0.050%, Si≤0.05%, 1.60%≤Mn≤1.80%, P≤0.015%, S≤0.003%, 0.20≤Cr≤0.30%, 0.020%≤Als≤0.050%, 0.100%≤V≤0.120%, 0.15%≤Mo≤0.25%, 0.0100%≤N≤0.0120%, 0.0025%≤B≤0.0050%, O≤0.0020%, and the balance is Fe and unavoidable impurities.

[0010] The microstructure of the high hole expansion steel is granular bainite + acicular ferrite + precipitation phase.

[0011] The R of the above high hole expansion steel eL ≥800MPa, R m ≥980MPa, A 50≥15%, hole expansion rate λ≥50%.

[0012] The thickness specification of the above-mentioned high hole expansion steel is 2.0 to 5.0 mm.

[0013] In a second aspect, the present invention provides a method for producing the above-mentioned 980 MPa grade high hole expansion steel, the production method comprising smelting, heating, rough rolling, finish rolling, laminar cooling and coiling steps, wherein:

[0014] Heating process: The continuous casting slab heating temperature is 1250±15℃, the heating time is 200~300min, and the holding time is ≥30min;

[0015] Laminar cooling process: adopt the front-stage rapid cooling method, the laminar cooling rate is ≥70℃ / s, and the midpoint temperature is 610~640℃;

[0016] Coiling process: The coiling temperature is 540-580°C. After coiling, it is slowly cooled to room temperature in a slow cooling pit.

[0017] In the above production method, the smelting process includes molten iron desulfurization, converter, RH vacuum treatment, LF refining, and slab continuous casting.

[0018] Furthermore, high carbon ferromanganese is used for manganese matching in the converter process without decarburization; decarburization is carried out during RH vacuum treatment; and quantitative and precise carburization is carried out in LF.

[0019] In the above production method, the thickness of the intermediate billet at the rough rolling outlet is controlled to be 35 to 50 mm in the rough rolling process.

[0020] Furthermore, when the intermediate billet thickness is ≤45 mm, the intermediate billet is coiled and descaled by a hot coil box.

[0021] In the above production method, the finishing rolling inlet temperature is controlled to be 1030-1080°C and the final rolling temperature is controlled to be 860-900°C in the finishing rolling process.

[0022] The beneficial effects of the present invention are as follows: the present invention adopts a composition design of micro-low C and VN-Mo-B composite microalloying, and produces a 980MPa grade high hole expansion steel with good mechanical properties and hole expansion performance through a production process of precise heating, rapid cooling in the front section, and medium-high temperature coiling. The obtained steel is widely used in the production of swing arms, longitudinal arms and other parts with flanging and hole expansion requirements, and is widely used in the lightweight design of new energy vehicles and fuel vehicles. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The microstructure of the finished steel of Example 3-1;

[0024] Figure 2 The microstructure of the finished steel of Comparative Example 1-1;

[0025] Figure 3 The microstructure of the finished steel of Comparative Example 2-1;

[0026] Figure 4 The surface morphology of the finished steel of Example 3-1;

[0027] Figure 5 This is the surface morphology of the finished steel of Comparative Example 1-1. DETAILED DESCRIPTION

[0028] The technical solution of the present invention can be specifically implemented in the following manner.

[0029] 980MPa grade high hole expansion steel, the composition is designed as follows: 0.025≤C≤0.050%, Si≤0.05%, 1.60%≤Mn≤1.80%, P≤0.015%, S≤0.003%, 0.20≤Cr≤0.30%, 0.020%≤Als≤0.050%, 0.100%≤V≤0.120%, 0.15%≤Mo≤0.25%, 0.0100%≤N≤0.0120%, 0.0025%≤B≤0.0050%, O≤0.0020%, the balance is Fe and unavoidable impurities.

[0030] The chemical composition design principle of the 980MPa grade high hole expansion steel of the present invention is explained.

[0031] (1) C: It must have a certain content to promote the formation of carbide strengthening phase, thereby ensuring the strength level of the steel strip. When its content is too high, the precipitated carbide particles will increase, which is not conducive to the hole expansion performance of the steel plate; when its content is too low, the strength cannot meet the design requirements. The present invention controls the C content between 0.025% and 0.050%, and forms a nano V (C, N) precipitation phase by adding V and N, fixes free C, reduces the Fe3C content, and achieves the purpose of strengthening ferrite and bainite at the same time.

[0032] (2) Si: It has a solid solution strengthening effect, which can simultaneously improve the strength and elongation of the steel strip and improve the formability of the steel strip. Generally, the silicon content should not be less than 0.30%. However, when the Si content is high, Fe2SiO4 red rust is easily generated on the surface of the steel strip, which is difficult to remove during pickling. In order to improve the surface quality of the steel strip and reduce the difficulty of industrial production, the present invention designs a Si-free composition scheme, that is, Si≤0.05%.

[0033] (3) Mn: It is a solid solution strengthening element. According to the strength requirements of this steel grade, when the content is too low, the strength will not meet the design requirements; when the content is too high, the proportion of pearlite will increase, and high Mn content will easily lead to Mn component segregation in the ingot, which is one of the main reasons for the appearance of banded structure in the steel strip, and the banded structure will cause the hole expansion performance of the steel strip to drop sharply. At the same time, for carbon manganese steel, the interaction between C and Mn has a certain influence on the formation and distribution of pearlite structure. When the content of both is too high, coarse pearlite and banded structure are easily generated. Therefore, under the premise that the strength meets the design requirements, the C content and Mn content are designed to be as low as possible. The Mn content of the present invention is controlled at 1.60% to 1.80%.

[0034] (4) P: As the strongest element for strengthening ferrite, it has strong solid solution strengthening and cold work hardening effects. When used as an alloying element, it can improve the strength and atmospheric corrosion resistance of the steel strip. However, when the content is too high, the cold stamping performance of the steel strip will drop sharply. At the same time, there will be segregation of the components of the continuous casting slab, causing uneven structure, increased temper brittleness, and significantly reduced plasticity and toughness of the steel strip, so that the steel strip will become "cold brittle" and brittle during cold working. The present invention controls the content to be below 0.015%.

[0035] (5) S: It is an impurity element in steel and is easy to form MnS in steel. The amount and shape of sulfides in steel directly affect the hole expansion performance of the steel strip. In particular, strip-shaped sulfide inclusions are prone to cracking during deformation. Therefore, when designing the chemical composition, the S content must be reduced, that is, below 0.003%.

[0036] (6) V: It is a strong carbide-forming element. However, the solid solution or precipitation temperature of vanadium carbides is low, and they are usually all dissolved in austenite during the finishing rolling stage. Only when the temperature drops and the phase transformation begins, vanadium carbides begin to form in ferrite; however, the solid solubility and size of vanadium carbides in ferrite are relatively large, which is not conducive to precipitation strengthening. Studies have shown that when a certain amount of N element is added to steel, a large number of dispersed fine V (C, N) particles are formed, and the strength of the steel is significantly improved through nanoscale V (C, N) precipitation strengthening and grain refinement strengthening, thereby improving or maintaining the good plasticity and toughness of the steel. The V content of the present invention is 0.100% to 0.120%.

[0037] (7) N: Usually a harmful element in steel, it is also an unavoidable element in the steelmaking process. However, in vanadium-containing steel, N can combine with V and C to form dispersed fine V (C, N) particles, achieving nanoscale V (C, N) precipitation strengthening and grain refinement strengthening. The N content of the present invention is 0.0100% to 0.0120%.

[0038] (8) Mo: Mo can greatly delay the phase transformation of ferrite and pearlite in steel, which is beneficial to obtain bainite structure in the medium and high temperature range; at the same time, the addition of molybdenum can also improve the stability of the steel structure and performance and refine the grains. In addition, the dynamic recovery of deformed austenite during air cooling is beneficial to improving the uniformity of structure and performance, which is beneficial to hole expansion performance. Studies have shown that the effect of Mo inhibiting the formation of pearlite requires its content to reach 0.10% or more, while to achieve stable expectations, it needs to reach 0.15% or more. The Mo content range of the present invention is 0.15% to 0.25%.

[0039] (9) Cr: It can delay the transformation of pearlite, significantly improve the hardenability of steel, facilitate the formation of acicular ferrite and granular bainite, and refine the structure, thus achieving a strengthening effect. Too high a Cr content will deteriorate the processing and formability of the material. The principle of selecting the Cr content is to promote the formation of acicular ferrite and granular bainite and avoid the formation of martensite structure which has an adverse effect on elongation. Therefore, the Cr content of the present invention is in the range of 0.20% to 0.30%.

[0040] (10) Al: Its basic function in the present invention is deoxidation during the steelmaking process. The Als content in steel is generally less than 0.010%; at the same time, if the Als content exceeds 0.060%, its grain refinement effect is weakened. The present invention controls the Als content to 0.020% to 0.050%.

[0041] (11) B: Adding B element to steel can greatly improve the hardenability of steel and is conducive to obtaining martensite structure. Considering that the structure expected to be obtained in the hot rolling stage of the present invention is ferrite rather than martensite, the present invention adopts medium-high coiling temperature control after rolling. In order to obtain a higher proportion of needle-shaped quasi-polygonal ferrite structure, a certain amount of B element needs to be added. In addition, in order to avoid the addition of B element to induce the production of large-sized martensite components, the toughness is reduced, and the coiling temperature should not be too low. The B content range of the present invention is 0.0025% to 0.0050%.

[0042] (12) O: As a harmful element in steel, it is also an unavoidable element in the steelmaking process. The present invention achieves ultra-low oxygen content control through double RH vacuum treatment, and the O content is controlled within 0.0020%.

[0043] The production method of 980MPa grade high hole expansion steel includes smelting, heating, rough rolling, finishing rolling, laminar cooling and coiling processes.

[0044] The production method of the 980MPa grade high hole expansion steel of the present invention is described.

[0045] (1) Smelting process: The smelting process includes "hot iron desulfurization → converter → RH vacuum treatment → LF refining → RH vacuum treatment → slab continuous casting" and other processes: high carbon ferromanganese is used for manganese matching in the converter process without decarburization; decarburization is carried out during the first RH vacuum treatment, and then quantitative and precise carbon addition is carried out in LF; low nitrogen sponge titanium must be used for alloying in the second RH process. This technology can use relatively cheap high carbon ferromanganese alloy, reduce the consumption of aluminum deoxidizer, and greatly reduce iron loss and oxygen content in molten steel. The chemical composition of the ingot is controlled according to the chemical composition of 780MPa low-cost nano-precipitation high-hole expansion steel.

[0046] (2) Heating process: To ensure that alloy elements such as Ti, Mn, Cr, and B are fully dissolved, the heating temperature of the continuous casting slab is controlled to be 1250±15℃, and the heating time is 200-300min, of which the holding time is ≥30min.

[0047] (3) Rough rolling process: The thickness of the intermediate billet at the rough rolling outlet is controlled to be 35-50 mm.

[0048] (4) Hot coil box: When the intermediate billet thickness is ≤45mm, the intermediate billet is coiled and scaled.

[0049] (5) Finishing rolling process: The finishing rolling inlet temperature is controlled at 1030-1080°C and the final rolling temperature is controlled at 860-900°C.

[0050] (6) Laminar cooling process: After finishing rolling, the steel strip adopts the front-end rapid cooling method, the laminar cooling rate is ≥70℃ / s, and the midpoint temperature is 610~640℃.

[0051] (7) Coiling temperature: To ensure that there is enough nano-level V (C, N), and under the action of strong hardenability elements Cr and B, the coiling temperature is controlled at 540-580 °C to obtain a stable acicular ferrite structure. The steel is then coiled and slowly cooled to room temperature in a cooling pit.

[0052] The thickness specification of the 980MPa grade high hole expansion steel prepared according to the alloy chemical composition and production method of the present invention is 2.0-5.0mm; its microstructure is granular bainite + acicular ferrite + precipitation phase; mechanical properties: R eL ≥800MPa, R m ≥980MPa, A 50 ≥15%; Hole expansion performance: Hole expansion rate λ≥50%.

[0053] The technical solution and effects of the present invention are further illustrated below through practical examples.

[0054] Example

[0055] In order to further understand the present invention, three groups of Examples 1 to 3 using the chemical composition of the present invention and two groups of Comparative Examples 1 to 2 are provided, and their specific chemical compositions are shown in Table 1, with the remainder being Fe and unavoidable impurities.

[0056] Table 1 Chemical composition (by mass percentage, %)

[0057] project C Si Mn P S Als Cr V Mo N B O Example 1 0.028 0.02 1.79 0.012 0.0012 0.035 0.29 0.104 0.25 0.0103 0.0025 0.0012 Example 2 0.038 0.03 1.72 0.007 0.0019 0.040 0.26 0.112 0.19 0.0110 0.0037 0.0016 Example 3 0.049 0.04 1.65 0.006 0.0028 0.027 0.22 0.119 0.16 0.0118 0.0049 0.0018 Comparative Example 1 0.050 0.70 1.75 0.010 0.0030 0.036 0.03 0.105 0.20 0.1000 0.0028 0.0015 Comparative Example 2 0.040 0.03 1.75 0.007 0.0020 0.035 0.25 0.110 0.20 0.0115 0.0035 0.0012

[0058] The ingot is smelted according to the chemical composition described in Table 1, and then the finished steel is prepared through the ingot heating, rough rolling, finish rolling, laminar cooling and coiling processes. The key process parameters in the preparation method are shown in Table 2.

[0059] Table 2 Process parameters

[0060]

[0061] Figure 1 The microstructure diagram of the finished steel obtained in Example 3-1 is as follows: Figure 1 It can be seen that the microstructure of the finished steel obtained by adopting the chemical composition and production method of the present invention is granular bainite + acicular ferrite + precipitation phase; Figure 4 The surface morphology of the finished steel obtained in Example 3-1 is as follows: Figure 4 It can be seen that no red rust defects appear on the surface of the finished steel obtained in Example 3-1.

[0062] Figure 2 The microstructure diagram of the finished steel obtained in Comparative Example 1-1 is as follows: Figure 2 It can be seen that the microstructure of the finished steel obtained in Comparative Example 1-1 is lath bainite + acicular ferrite + precipitation phase; Figure 5 The surface morphology of the finished steel obtained in Comparative Example 1-1 is as follows: Figure 5 It can be seen that the surface of the finished steel obtained in Example 3-1 has iron oxide scale defects.

[0063] Figure 3 The microstructure diagram of the finished steel obtained in Comparative Example 2-1 is as follows: Figure 3 It can be seen that the microstructure of the finished steel obtained in Comparative Example 2-1 is lath bainite + acicular ferrite + precipitation phase.

[0064] The mechanical properties of the finished steel products obtained in Examples 1 to 3 and Comparative Examples 1 to 2 were tested, and the results are shown in Table 3.

[0065] Table 3 Mechanical properties

[0066]

[0067]

Claims

1. 980MPa grade high hole expansion steel, characterized by: Its chemical composition by mass percentage is: 0.025≤C≤0.050%, Si≤0.05%, 1.60%≤Mn≤1.80%, P≤0.015%, S≤0.003%, 0.20≤Cr≤0.30%, 0.020%≤Als≤0.050%, 0.100%≤V≤0.120%, 0.15%≤Mo≤0.25%, 0.0100%≤N≤0.0120%, 0.0025%≤B≤0.0050%, O≤0.0020%, and the balance is Fe and unavoidable impurities.

2. The 980MPa high hole expansion steel according to claim 1, characterized in that: The microstructure of the high hole expansion steel is granular bainite+acicular ferrite+precipitation phase.

3. The 980MPa grade high hole expansion steel according to claim 1, characterized in that: The R of the high hole expansion steel eL ≥800MPa, R m ≥980MPa, A 50 ≥15%, hole expansion rate λ≥50%.

4. The 980MPa grade high hole expansion steel according to claim 1, characterized in that: The thickness specification of the high hole expansion steel is 2.0 to 5.0 mm.

5. The method for producing 980MPa grade high hole expansion steel according to any one of claims 1 to 4, characterized in that: The production method comprises smelting, heating, rough rolling, finishing rolling, laminar cooling and coiling steps, wherein: Heating process: The continuous casting slab heating temperature is 1250±15℃, the heating time is 200~300min, and the holding time is ≥30min; Laminar cooling process: adopt the front-stage rapid cooling method, the laminar cooling rate is ≥70℃ / s, and the midpoint temperature is 610~640℃; Coiling process: The coiling temperature is 540-580°C. After coiling, it is slowly cooled to room temperature in a slow cooling pit.

6. The method for producing 980MPa grade high hole expansion steel according to claim 5, characterized in that: The smelting process includes molten iron desulfurization, converter, RH vacuum treatment, LF refining, and slab continuous casting.

7. The method for producing 980MPa grade high hole expansion steel according to claim 6, characterized in that: In the converter process, high carbon ferromanganese is used for manganese matching without decarburization; decarburization is carried out during RH vacuum treatment; quantitative and precise carburization is carried out in LF.

8. The method for producing 980MPa grade high hole expansion steel according to claim 5, characterized in that: In the rough rolling process, the thickness of the intermediate billet at the rough rolling outlet is controlled to be 35 to 50 mm.

9. The method for producing 980MPa grade high hole expansion steel according to claim 8, characterized in that: When the intermediate billet thickness is ≤45mm, the intermediate billet is coiled and scaled through a hot coil box.

10. The method for producing 980MPa grade high hole expansion steel according to claim 5, characterized in that: In the finishing rolling process, the finishing rolling inlet temperature is controlled to be 1030-1080°C, and the final rolling temperature is controlled to be 860-900°C.

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