Ferrite-based HSLA steel with yield strength of 850 MPa or above and preparation method of ferrite-based HSLA steel

Abstract

The invention provides ferrite-based HSLA steel with the yield strength being 850 MPa or above and a preparation method of the ferrite-based HSLA steel. The HSLA steel comprises 0.02%-0.10% of C, 0.01%-0.40% of Si, 0.80%-2.50% of Mn, 0.030%-0.13% of Nb, 0.06%-0.15% of Ti, smaller than or equal to 0.2% of V, 0.01%-0.20% of Al, smaller than or equal to 0.010% of S, smaller than or equal to 0.05% of P, smaller than or equal to 0.003% of N and the balance Fe and inevitable impurities. The preparation method comprises the steps of smelting, continuous casting, heating, hot rolling, cooling, coiling, cold rolling and cover annealing. According to the method, the cooperation of hot rolling and annealing processes is fully utilized, precipitates of the HSLA steel are fine and uniform, the local forming performance of a material is extremely good, and the comprehensive technical effects of saturated solid solution carbon and microalloying component design, low-temperature coiling, low-temperature annealing and ultra-long-time annealing are achieved.

Description

Technical Field

[0001] This invention relates to the field of steel processing technology, and more specifically, to a ferritic HSLA steel with a yield strength of 850 MPa or higher and its preparation method. Background Technology

[0002] The current production process for high-strength low-alloy steel generally follows this procedure: smelting → continuous casting → heating → hot rolling → cooling → coiling → cold rolling → continuous annealing. Existing HSLA steels with a yield strength above 850 MPa employ a comprehensive technical solution of Nb, V, and Ti microalloying + controlled hot rolling + high-temperature coiling (550~650℃) + cold rolling + continuous annealing [high temperature (generally above 700℃), short-time annealing].

[0003] The material composition system of existing HSLA steels with a yield strength of 850MPa or higher generally adopts Nb, V, and Ti microalloying technology to refine the ferrite grain size, refine precipitates, and control grain refinement and precipitation strengthening. Current integrated technology solutions for producing HSLA steel in continuous annealing lines have an annealing time of only 3-5 minutes, requiring heating to the HSLA steel recrystallization temperature, which is usually above 700℃. Supersaturated Nb and V easily precipitate, resulting in relatively coarse precipitates and significant recrystallization and growth in the microstructure. Therefore, the material composition design of HSLA steel requires relatively higher carbon atoms and controlled Nb / V / Ti atomic ratios to prevent the precipitation of coarse precipitates. However, excessively high carbon content can also lead to cementite precipitation, thereby reducing the steel plate's hole expansion performance and ultimate bending performance.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by providing a ferritic HSLA steel with a yield strength of over 850 MPa and its preparation method. This invention fully utilizes the synergy of hot rolling and annealing processes, employing a "low temperature + long time" annealing process using a bell-type annealing method. The resulting HSLA steel has fine and uniform precipitates and excellent local forming properties, achieving a comprehensive technical effect of controlling "saturated solid solution carbon + microalloying composition design" - "low temperature coiling" - "low temperature annealing + ultra-long time annealing".

[0006] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: A ferritic HSLA steel with a yield strength of 850 MPa or higher, wherein the chemical composition of the HSLA steel comprises: C 0.02~0.10wt%, Si 0.01~0.40wt%, Mn 0.80~2.50wt%, Nb 0.030~0.13wt%, Ti 0.06~0.15wt%, V≤0.2wt%, Al 0.01~0.20wt%, S≤0.010wt%, P≤0.05wt%, N≤0.003wt%, with the balance being Fe and unavoidable impurities.

[0007] Furthermore, the HSLA steel also contains at least one of the following: Cr≤0.40wt%, Mo≤0.60wt%, Cu≤0.30wt%, and B≤0.005wt%.

[0008] Furthermore, the mass fractions of Nb, V, Ti, Cr, Mo and C satisfy: W (C%) ≤ 0.06wt%, where W (C%) = C - 0.12Nb - 0.21V - 0.25Ti - 0.018Cr - 0.025Mo - 0.003wt%.

[0009] Furthermore, based on the area ratio of metallographic structures, the microstructure of this HSLA steel includes: ferrite content ≥92%, bainite content ≤8%, martensite content ≤2%, totaling 100%.

[0010] Furthermore, the ferrite is disc-shaped, with a short-side grain size ≤3.0μm and a long-side grain size ≤8μm.

[0011] Furthermore, the bainite grain size is ≤1.5μm.

[0012] Furthermore, the martensite grain size is ≤1.5μm.

[0013] Furthermore, the microstructure precipitates of this HSLA steel are at least one of NbC, TiN, TiC, VC, MoC, Mo2C, and Cr23C6, and contain composite precipitates of Nb, V, and Ti.

[0014] Furthermore, the precipitates in this HSLA steel are located at the original austenite grain boundaries, ferrite or bainite grain boundaries and subgrain boundaries, with precipitates smaller than 10 nm accounting for more than 80%.

[0015] Furthermore, the mechanical properties of this HSLA steel meet the following requirements: yield strength ≥ 850 MPa, tensile strength ≥ 980 MPa, uniform elongation ≥ 8%, hole expansion rate ≥ 60%, and ultimate three-point bending (d = 0.4 mm) ≥ 95°.

[0016] The preparation method of the above-mentioned ferritic HSLA steel with a yield strength of 850 MPa or above includes smelting → continuous casting → heating → hot rolling → cooling → coiling → cold rolling → bell annealing.

[0017] Furthermore, the heating temperature is ≥1120℃.

[0018] Furthermore, in the hot rolling process, the roughing temperature is ≥1100℃, the finishing rolling start temperature is ≤1050℃, and the finishing rolling finish temperature is ≥890℃.

[0019] Furthermore, the winding temperature is 300~550℃.

[0020] Furthermore, the total cold rolling reduction rate is ≥20%.

[0021] Furthermore, the annealing temperature is 400~700℃, and the annealing time is 8~72h.

[0022] Furthermore, after annealing, allow it to cool naturally or by air to room temperature.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The present invention provides a method for preparing ferritic HSLA steel with a yield strength of 850 MPa or higher. It makes full use of the synergy of hot rolling and annealing processes and adopts a "low temperature + long time" annealing process with a bell-type annealing. Compared with the existing "high temperature + short time" continuous annealing process, it is more conducive to the formation of finer and more dispersed precipitates. The supersaturated solid solution microalloying elements such as Nb, V, Ti, Mo, and Cr in the coiling stage form precipitates of less than 10 nm with the carbon dissolved in the ferrite at low temperature.

[0024] 2. The preparation method of HSLA steel of the present invention uses a bell-type annealing with a temperature range of 400~700℃. The solid carbon content in ferrite is controlled by carbide precipitation, and the precipitation of cementite in ferrite, bainite and martensite is suppressed, thus ensuring that HSLA steel has extremely high local forming performance.

[0025] 3. The preparation method of HSLA steel of the present invention involves coiling in a temperature range of 300~550℃. The hot-rolled coil has a relatively large amount of solid solution of Nb, V, Ti, etc., and the precipitates in the hot rolling stage are also finer. At the same time, during the subsequent long-term low-temperature annealing, the supersaturated Nb, V, Ti can be fully precipitated, and the precipitates are fine and uniform.

[0026] 4. The ferritic HSLA steel of the present invention with a yield strength of 850MPa or above has fine and uniform precipitates, with the proportion of precipitates below 10nm reaching up to 92%, achieving the comprehensive technical effect of controlling "saturated solid solution carbon + microalloying composition design" - "low temperature coiling" - "low temperature annealing + ultra-long time annealing".

[0027] 5. The microstructure and precipitate distribution of the HSLA steel of the present invention can prevent material softening during the uniform deformation stage; the local forming performance of the HSLA steel of the present invention is excellent (superior to existing HSLA steel): uniform elongation ≥8%, hole expansion rate ≥60%, and ultimate three-point bending (d=0.4mm) ≥95°.

[0028] 6. The preparation process of HSLA steel in this invention adopts a bell-type annealing production method. Since the investment in bell-type annealing production lines is low, most steel mills at home and abroad have this type of production line, resulting in lower product production costs. Attached Figure Description

[0029] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0030] Figure 1 This is a comparison diagram of the preparation process of the HSLA steel of this invention and existing high-strength low-alloy steel; Figure 2 This is a distribution diagram of HSLA ferrite precipitates from the present invention; Figure 3 This is the yield strength-uniform elongation diagram of the HSLA steel of this invention. Detailed Implementation

[0031] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. 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. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0032] A ferritic HSLA steel with a yield strength of 850 MPa or higher, wherein the chemical composition of the HSLA steel comprises: C 0.02~0.10wt%, Si 0.01~0.40wt%, Mn 0.80~2.50wt%, Nb 0.030~0.13wt%, Ti 0.06~0.15wt%, V≤0.2wt%, Al 0.01~0.20wt%, S≤0.010wt%, P≤0.05wt%, N≤0.003wt%, with the balance being Fe and unavoidable impurities.

[0033] Preferably, the HSLA steel also contains at least one of Cr≤0.40wt%, Mo≤0.60wt%, Cu≤0.30wt%, and B≤0.005wt%.

[0034] Preferably, the mass fractions of Nb, V, Ti, Cr, Mo, and C satisfy: W(C%) ≤ 0.06wt%, where the empirical formula is W(C%) = C - 0.12Nb - 0.21V - 0.25Ti - 0.018Cr - 0.025Mo - 0.003wt%. W(C%) is used to limit the carbon content dissolved in ferrite.

[0035] Preferably, based on the area ratio of the metallographic structure (i.e., the volume content ratio), the microstructure of the HSLA steel includes: ferrite content ≥92%, bainite content ≤8%, martensite content ≤2%, and the total is 100%.

[0036] Preferably, the ferrite is in the form of a disc, with a short-side grain size ≤3.0μm and a long-side grain size ≤8μm.

[0037] Preferably, the bainite grain size is ≤1.5μm.

[0038] Preferably, the martensite grain size is ≤1.5μm.

[0039] Preferably, the HSLA steel has at least one of NbC, TiN, TiC, VC, MoC, Mo2C, and Cr23C6 as its microstructure precipitates, and contains composite precipitates of Nb, V, and Ti.

[0040] Preferably, the precipitates in the HSLA steel are mainly located at the original austenite grain boundaries, ferrite or bainite grain boundaries and subgrain boundaries, with precipitates smaller than 10 nm accounting for more than 80%.

[0041] The precipitates produced by this invention are fine and uniform, with precipitates smaller than 10 nm accounting for more than 80%, reaching a maximum of 92%. This achieves the comprehensive technical effect of controlling "saturated solid solution carbon + micro-alloying composition design" - "low temperature winding" - "low temperature annealing + ultra-long time annealing".

[0042] Preferably, the mechanical properties of the HSLA steel meet the following requirements: yield strength ≥ 850 MPa, tensile strength ≥ 980 MPa, uniform elongation ≥ 8%, expansion rate ≥ 60%, and ultimate three-point bending (d = 0.4 mm) ≥ 95°. The HSLA steel material of this invention exhibits excellent local forming performance (superior to existing HSLA steels), and its microstructure and precipitate distribution prevent material softening during the uniform deformation stage.

[0043] The preparation method of the above-mentioned ferritic HSLA steel with a yield strength of 850 MPa or above, such as Figure 1 As shown, the process includes smelting → continuous casting → heating → hot rolling → cooling → coiling → cold rolling → annealing in a bell.

[0044] Preferably, the heating temperature is ≥1120℃.

[0045] Preferably, in the hot rolling process, the roughing temperature is ≥1100℃, the finishing rolling start temperature is ≤1050℃, and the finishing rolling finish temperature is ≥890℃.

[0046] Preferably, the winding temperature is 300~550℃ (including but not limited to 300℃, 350℃, 400℃, 450℃, 500℃, and 550℃).

[0047] This invention makes full use of the synergy of hot rolling and annealing processes, and rolls the coils in a temperature range of 300~550℃. The hot-rolled coils have a relatively large amount of solid solution of Nb, V, Ti, etc., and the precipitates in the hot rolling stage are also finer. At the same time, during the subsequent long-term low-temperature annealing, the supersaturated Nb, V, Ti can be fully precipitated, and the precipitates are fine and uniform.

[0048] Preferably, the total cold rolling reduction rate is ≥20%.

[0049] Preferably, the annealing temperature is 400~700℃ (including but not limited to 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃), and the annealing time is 8~72h (including but not limited to 8h, 16h, 24h, 32h, 40h, 48h, 56h, 64h, 72h).

[0050] This invention employs a "low temperature + long time" annealing process using a bell-type annealing method, which is more conducive to the formation of finer and more dispersed precipitates compared to the existing "high temperature + short time" continuous annealing process. This process allows the supersaturated solid solution microalloying elements such as Nb, V, Ti, Mo, and Cr in the coiling stage to form precipitates smaller than 10 nm with the carbon dissolved in the ferrite at low temperatures.

[0051] The bell-type annealing of this invention uses a temperature range of 400~700℃. It controls the solid carbon content in ferrite by precipitating carbides, inhibits the precipitation of cementite in ferrite, bainite and martensite, and ensures that HSLA steel has extremely high local forming performance.

[0052] Preferably, the annealed material is allowed to cool naturally or by air to room temperature.

[0053] Examples 1-10 and Comparative Example 1 Examples 1-10 of this invention describe ferritic HSLA steel with a yield strength of 850 MPa or higher and its preparation method. The preparation method is as follows: smelting → continuous casting → heating → hot rolling → cooling → coiling → cold rolling → bell annealing to prepare steel plates or coils.

[0054] Comparative Example 1 is a DP980 steel and its preparation method. The preparation method is as follows: smelting → continuous casting → heating → hot rolling → cooling → coiling → cold rolling → continuous annealing to prepare steel plates or steel coils.

[0055] (1) The chemical composition of HSLA steel in Examples 1 to 10 and DP980 steel in Comparative Example 1 is shown in Table 1.

[0056] Table 1 Chemical composition (wt%) of HSLA steel in Examples 1-10 and DP980 steel in Comparative Example 1

[0057] Note: In Table 1, the balance is Fe and unavoidable impurities (including N); the steel grade of Comparative Example 1 is DP980.

[0058] (2) The process parameters for heating, hot rolling and coiling of Examples 1 to 10 and Comparative Example 1 are shown in Table 2.

[0059] Table 2. Process parameters for heating, hot rolling, and coiling in Examples 1-10 and Comparative Example 1

[0060] (3) The cold rolling, annealing and cooling process conditions of Examples 1 to 10 and Comparative Example 1 are shown in Table 3.

[0061] Table 3 Cold rolling, annealing, and cooling process conditions of Examples 1-10 and Comparative Example 1

[0062] (4) The mechanical properties of HSLA steel in Examples 1 to 10 and DP980 steel in Comparative Example 1 are shown in Table 4.

[0063] Table 4 Mechanical properties of HSLA steel in Examples 1-10 and DP980 steel in Comparative Example 1

[0064] (5) The microstructure of HSLA steel in Examples 1 to 10 and DP980 steel in Comparative Example 1 is shown in Table 5.

[0065] Table 5. Microstructure of HSLA steel in Examples 1-10 and DP980 steel in Comparative Example 1

[0066] As can be seen from the table above, the "low temperature + long time" annealing process of the bell-type annealing adopted in Examples 1 to 10 of the present invention is more conducive to the formation of finer and more dispersed precipitates than the "high temperature + short time" continuous annealing process of the traditional process (Comparative Example 1). The supersaturated solid solution microalloying elements such as Nb, V, Ti, Mo, and Cr in the coiling stage form precipitates of less than 10 nm with the carbon dissolved in the ferrite at low temperature.

[0067] The precipitates in Examples 1-10 of this invention are fine and uniform, with the proportion of precipitates below 10nm reaching up to 92%, achieving the comprehensive technical effect of controlling "saturated solid solution carbon + micro-alloying composition design" - "low temperature winding" - "low temperature annealing + ultra-long time annealing".

[0068] In embodiments 1-10 of this invention, the bell-type annealing temperature is in the range of 400-700℃. The solid carbon content in ferrite is controlled by the precipitation of carbides, and the precipitation of cementite in ferrite, bainite and martensite is suppressed, thus ensuring that HSLA steel has extremely high local forming performance.

[0069] The microstructure and precipitate distribution of the HSLA steel in Examples 1-10 of this invention can prevent material softening during the uniform deformation stage.

[0070] The HSLA steel materials in Examples 1-10 of this invention have excellent local forming properties: uniform elongation ≥8%, hole expansion rate ≥60%, and ultimate three-point bending (d=0.4mm) ≥95°.

[0071] Regarding the selection of coiling temperature, the traditional process (Comparative Example 1) selects a relatively high temperature of 550~650℃ for coiling, resulting in a large amount of precipitates during the coiling stage. Cold rolling followed by continuous annealing allows for sufficient precipitate formation in a short time. This invention, however, selects a temperature range of 300~550℃ for coiling. Hot-rolled coils have a relatively high solid solution content of Nb, V, Ti, etc., and the precipitates during the hot rolling stage are also finer. At the same time, during the subsequent long-term (more than 8 hours) low-temperature annealing (400~700℃), the supersaturated Nb, V, and Ti can be fully precipitated, resulting in fine and uniform precipitates.

[0072] like Figures 2-3As shown, the HSLA steel ferrite microstructure after hot rolling, low-temperature coiling, and low-temperature long-term bell annealing of the present invention exhibits a large number of fine Nb, V, and Ti precipitates (see [reference]). Figure 2 Most of the precipitates are less than 10 nm in size. The mechanical properties of the HSLA steel were tested using uniaxial tensile testing. Based on a fine-grained and uniformly small precipitate microstructure, the HSLA steel of this invention exhibits uniform deformation and no softening during the work hardening stage after tensile yielding. Furthermore, compared to low-strength grade steel B (yield strength 500 MPa) and low-strength grade steel C (yield strength 380 MPa), the uniform elongation of the HSLA steel of this invention changes less with increasing strength (see [reference]). Figure 3 It is less prone to cracking during molding.

Claims

1. A ferritic HSLA steel with a yield strength of 850 MPa or higher, characterized in that, The chemical composition of this HSLA steel includes: C 0.02~0.10wt%, Si 0.01~0.40wt%, Mn 0.80~2.50wt%, Nb 0.030~0.13wt%, Ti 0.06~0.15wt%, V≤0.2wt%, Al 0.01~0.20wt%, S≤0.010wt%, P≤0.05wt%, N≤0.003wt%, with the balance being Fe and unavoidable impurities.

2. The ferritic HSLA steel with a yield strength of 850 MPa or higher according to claim 1, characterized in that, The HSLA steel also contains at least one of the following: Cr≤0.40wt%, Mo≤0.60wt%, Cu≤0.30wt%, and B≤0.005wt%.

3. The ferritic HSLA steel with a yield strength of 850 MPa or higher according to claim 2, characterized in that, The mass fractions of Nb, V, Ti, Cr, Mo and C satisfy: W (C%) ≤ 0.06 wt%, where W (C%) = C - 0.12Nb - 0.21V - 0.25Ti - 0.018Cr - 0.025Mo - 0.003 wt%.

4. The ferritic HSLA steel with a yield strength of 850 MPa or higher according to claim 1, characterized in that, Based on the area ratio of metallographic structures, the microstructure of this HSLA steel includes: ferrite content ≥92%, bainite content ≤8%, martensite content ≤2%, and a total of 100%.

5. The ferritic HSLA steel with a yield strength of 850 MPa or higher according to claim 4, characterized in that, Includes at least one of the following technical features: (1) The ferrite is in the shape of a disc, with a short-side grain size ≤3.0μm and a long-side grain size ≤8μm; (2) The bainite grain size is ≤1.5μm; (3) The martensite grain size is ≤1.5μm.

6. The ferritic HSLA steel with a yield strength of 850 MPa or higher according to claim 4, characterized in that, The HSLA steel has at least one of the following precipitates: NbC, TiN, TiC, VC, MoC, Mo2C, and Cr23C6, and contains composite precipitates of Nb, V, and Ti.

7. The ferritic HSLA steel with a yield strength of 850 MPa or higher according to claim 6, characterized in that, The precipitates in this HSLA steel are located at the original austenite grain boundaries, ferrite or bainite grain boundaries and subgrain boundaries, with precipitates smaller than 10 nm accounting for more than 80%.

8. The ferritic HSLA steel with a yield strength of 850 MPa or higher according to claim 7, characterized in that, The mechanical properties of this HSLA steel meet the following requirements: yield strength ≥ 850 MPa, tensile strength ≥ 980 MPa, uniform elongation ≥ 8%, hole expansion rate ≥ 60%, and ultimate three-point bending d = 0.4 mm ≥ 95°.

9. The method for preparing ferritic HSLA steel with a yield strength of 850 MPa or higher as described in any one of claims 1 to 8, characterized in that, The process includes smelting → continuous casting → heating → hot rolling → cooling → coiling → cold rolling → annealing.

10. The method for preparing ferritic HSLA steel with a yield strength of 850 MPa or higher according to claim 9, characterized in that, Includes at least one of the following technical features: (1) Heating temperature ≥ 1120℃; (2) In the hot rolling process, the roughing temperature is ≥1100℃, the finishing rolling start temperature is ≤1050℃, and the finishing rolling finish temperature is ≥890℃; (3) The winding temperature is 300~550℃; (4) Total cold rolling reduction ≥ 20%; (5) The annealing temperature of the bell is 400~700℃ and the annealing time is 8~72h; (6) After annealing, allow it to cool naturally or air-cool to room temperature.

Images