Method for reducing performance fluctuation of titanium microalloyed steel

Abstract

The invention discloses a method for reducing performance fluctuation of titanium microalloyed steel, and belongs to the technical field of production of high-strength hot continuous rolling steel. The method for reducing the performance fluctuation of the titanium microalloyed steel is provided for improving the stability of the performance of the titanium microalloyed steel, the effective titanium content is accurately controlled, the corresponding coiling temperature is matched according to the control intervals of different effective titanium contents, cooperative control over the effective titanium content and the coiling temperature is achieved, and the performance fluctuation of the titanium microalloyed steel is reduced. And further, the stability of the mechanical property of the titanium microalloyed steel is improved. After the titanium microalloyed steel is treated by the method, the fluctuation range of the yield strength and the tensile strength can be accurately controlled within + / -30MPa, the product quality is effectively improved, and the problem that the performance of the existing titanium microalloyed steel is unstable is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of production of high-strength hot-rolled steel, and relates to a method for reducing performance fluctuations of titanium microalloyed steel, and specifically to a method for improving the performance of titanium microalloyed steel based on the stability of coordinated control of effective titanium content and coiling temperature. Background Art

[0002] In recent years, Ti has been widely used in hot-rolled high-strength steels due to its low alloy price and strong precipitation strengthening effect. However, due to its relatively active chemical properties, Ti easily forms liquid inclusions such as TiN and Ti4C2S2 with impurity elements such as N and S in molten steel, resulting in a decrease in the amount of nano-TiC precipitated during subsequent rolling, which in turn leads to a decrease in the mechanical properties of the finished steel. At the same time, because Ti forms TiC precipitates of varying sizes and quantities at each stage of hot rolling, Ti microalloyed steels are highly process-sensitive, meaning that fluctuations in the production process can easily lead to fluctuations in the mechanical properties of the finished steel. Therefore, it is necessary to conduct research on stable control technologies for the mechanical properties of Ti microalloyed steels to improve the product quality of hot-rolled high-strength steels.

[0003] After searching, CN119220905A discloses a method for improving the performance stability of titanium microalloyed steel. By optimizing the chemical composition of titanium microalloyed steel, the control accuracy is: C content ±0.01%, Si content ±0.03%, Mn content ±0.05%, Nb content ±0.005%, Ti content ±0.005%, N≤0.0040%, O≤0.0020%, and optimizing the smelting, slab heating, controlled rolling and cooling, coiling, and slow cooling processes, the performance stability of titanium microalloyed steel is significantly improved, so that the proportion of its tensile strength fluctuation range of ±30MPa is not less than 95%, thereby ensuring the yield of finished products.

[0004] CN119040591A discloses a production method for improving the performance stability of a single high-Ti high-strength steel on a CSP production line, comprising: a soaking stage, wherein the soaking furnace outlet temperature is 1080-1160°C; a controlled rolling stage, carried out on a hot rolling mill, wherein the starting rolling temperature is ≥1030°C, the first three passes have a reduction ratio of ≥50%, the F7 pass has a reduction ratio of 10-20%, and the final rolling temperature is 860-900°C; and a controlled cooling stage, wherein an ultra-fast cooling mode is first adopted, the cooling rate is controlled at 92-130°C / s, and the cooling time is 3-9s; then a low-speed cooling mode is entered, the cooling rate is controlled at 20-60°C / s, and the steel is cooled to 460-540°C for coiling.

[0005] From the above, it can be seen that the existing patents mainly improve the performance stability of Ti microalloyed steel by controlling the composition control accuracy and production process control accuracy, and no control measures for the coordinated control of effective Ti content and coiling temperature are found. Summary of the Invention

[0006] The technical problem to be solved by the present invention is the problem of unstable performance of existing titanium microalloyed steel.

[0007] To achieve the above-mentioned application objectives, the present invention provides a method for reducing the performance fluctuation of titanium microalloyed steel: the raw materials are sequentially subjected to converter smelting, LF refining, RH refining, and continuous casting to form steel ingots, which are then sequentially subjected to slab heating, rough rolling, finish rolling, laminar cooling, coiling, and slow cooling to obtain titanium microalloyed steel with a yield strength and tensile strength fluctuation range of within ±30 MPa; The ladle is washed during converter smelting to ensure that the bottom of the ladle is clean and the molten steel in the upper furnace is low-sulfur steel. The bottom blowing gas supply mode is adopted during smelting, and the argon blowing flow rate is 50~80 m 3 / h; LF refining uses desulfurized slag with a slag optical basicity Λ>0.75, a Mannesmann index MI>0.50, and a final slag (FeO+MnO) content <1.5%; low-nitrogen ferroniobium and low-nitrogen ferrotitanium alloys are added during LF and RH refining; the shroud sealing effect must be ensured during continuous casting, and the argon blowing flow rate for the tundish must be 50-60 L / min; the S content in the finished steel must be controlled to ≤0.005%, and the N content to ≤0.0040%; In the coiling process, when the Ti* content is 0.060~<0.070%, the coiling temperature is 600±15℃; when the Ti* content is 0.070~0.080%, the coiling temperature is 580±15℃; The chemical composition of the titanium microalloyed steel includes, by weight percentage, C 0.05-0.07%, Si 0.05-0.10%, Mn 1.40-1.50%, Nb 0.035-0.045%, P≤0.015%, S≤0.005%, N≤0.0040%, Ti* 0.060-0.080%, and the balance is Fe and unavoidable impurity elements; wherein Ti*=[Ti]-1.5[S]-3.43[N], wherein Ti* is the effective Ti content, i.e., the difference between the total titanium content and the Ti content consumed by impurity elements such as N and S; [Ti], [S], and [N] are the weight percentages of titanium, sulfur, and nitrogen in the steel, respectively; and 1.5 and 3.43 are the ideal chemical ratios of Ti, S, and N in Ti4C2S2 and TiN inclusions, respectively.

[0008] The thickness of the above-mentioned steel ingot is 200~250mm.

[0009] The steel ingot is air-cooled to room temperature and then placed in a slab heating furnace for heating.

[0010] In the above slab heating process, the slab out-of-furnace temperature is controlled to be 1230±15℃, and the slab time in the furnace is 200~300min.

[0011] After the above-mentioned slabs are taken out of the furnace, they are descaled before entering the rough rolling process. After rough rolling, intermediate slabs are obtained, and after finish rolling, finished steel plates are obtained. In the rough rolling process, the deformation amount of a single rough rolling pass is ≥18%; in the finish rolling process, the finish rolling inlet temperature is ≤1050℃, and the finish rolling outlet temperature is 870±15℃.

[0012] Furthermore, the thickness of the finished steel plate is 1.8~10.0mm.

[0013] Furthermore, when the thickness of the finished steel plate is 1.8 to less than 4.0 mm, the thickness of the intermediate billet is 40±1 mm; when the thickness of the finished steel plate is 4.0 to 10.0 mm, the thickness of the intermediate billet is 52±1 mm.

[0014] In the above laminar cooling process, front-stage centralized cooling is adopted, and the cooling rate is 25~35℃ / s.

[0015] After being coiled into steel coils, the steel plates are placed in a slow cooling pit for slow cooling for 70 to 80 hours.

[0016] The titanium microalloyed steel has a yield strength of 660-720 MPa, a tensile strength of 740-800 MPa, and an elongation of ≥16%.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The present invention proposes a method for controlling the fluctuation range of effective Ti content to reduce performance fluctuations in finished steel products, thus avoiding fluctuations in the content of impurity elements such as S and N in the molten steel, which can lead to performance fluctuations in the finished steel. Furthermore, the present invention proposes corresponding coiling temperatures for different effective Ti content control ranges. By synergistically controlling the effective Ti content and coiling temperature, performance stability is improved, ultimately achieving yield strength and tensile strength fluctuations within ±30 MPa, thereby improving product quality. Furthermore, the method for reducing performance fluctuations in Ti microalloyed steels provided by the present invention can be extended to steels of other strength levels. DETAILED DESCRIPTION

[0018] In order to make the technical problems, technical solutions and beneficial effects to be solved in the present application clearer, the present application will be further described in detail below in conjunction with the embodiments. It will be understood by those skilled in the art that the following examples are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. Unless otherwise defined, all scientific and technological terms used herein have the same meanings understood by those of ordinary skill in the art.

[0019] A method for reducing performance fluctuations of titanium microalloyed steel. The titanium microalloyed steel comprises, by weight percentage, the following chemical components: C 0.05-0.07%, Si 0.05-0.10%, Mn 1.40-1.50%, Nb 0.035-0.045%, P≤0.015%, S≤0.005%, N≤0.0040%, Ti* 0.060-0.080%, and the balance being Fe and unavoidable impurity elements.

[0020] The reasons for limiting the main alloying elements in the steel of the present invention are explained below.

[0021] Appropriately reducing the carbon content helps reduce pearlite and cementite content, thereby improving the elongation of the steel. Furthermore, carbon is a component of the TiC second phase, so reducing the fluctuation range of the carbon content helps stabilize and control the TiC precipitation phase. Therefore, the present invention controls the carbon content to 0.05-0.07%.

[0022] Mn plays a role in solid solution strengthening and improving hardenability in steel. Using a higher Mn content promotes the formation of fine-grained ferrite in the steel described herein. Simultaneously, reducing the compositional fluctuation range of the Mn content helps minimize fluctuations in the solid solution strengthening effect. Therefore, the present invention controls the Mn content to 1.40-1.50%.

[0023] Nb plays a role in grain refinement in steel, reducing the compositional fluctuation range of Nb content is conducive to reducing the fluctuation of grain refinement strengthening effect. Therefore, the present invention controls the Nb content to 0.035-0.045%.

[0024] Ti forms TiC precipitates in steel, achieving precipitation strengthening. However, Ti easily reacts with S and N to form Ti4C2S2 and TiN inclusions, which reduce the steel's toughness and ductility while also reducing the TiC precipitate content, ultimately leading to performance fluctuations. Therefore, the present invention limits the S and N contents to ≤0.005% and ≤0.0040%, respectively. Furthermore, it limits fluctuations in the effective Ti content, requiring an effective Ti content Ti* ([Ti]-1.5[S]-3.43[N]) of 0.060-0.080%.

[0025] A method for reducing performance fluctuations of titanium microalloyed steel comprises the following steps: smelting raw materials sequentially through converter smelting, LF refining, RH refining, and continuous casting to form steel ingots; and the steel ingots sequentially through slab heating, rough rolling, finish rolling, laminar cooling, coiling, and slow cooling to obtain titanium microalloyed steel with a yield strength and tensile strength fluctuation range within ±30 MPa.

[0026] The reasons for the production process limitations are explained below in conjunction with the control requirements for the stability of the mechanical properties of steel described in the present invention.

[0027] (1) Coordinated control of effective Ti content and coiling temperature As can be seen from the foregoing, the present invention abandons the idea of ​​separately limiting the composition of Ti, S, and N in conventional technology, and limits the fluctuation of the effective Ti content, that is, it requires that Ti*=[Ti]-1.5[S]-3.43[N] be controlled within the range of 0.060~0.080%.

[0028] At the same time, personalized coiling temperature requirements were made for the effective Ti content Ti* of 0.06~<0.07% and 0.07~0.08% respectively to achieve stable control of mechanical properties. When Ti* is small, 0.06~<0.07%, at this time, due to the high Ti content consumed by impurity elements such as S and N, the Ti content available for forming TiC precipitation is low, so a coiling temperature close to the TiC precipitation nose point temperature (600℃) is adopted, that is, 600±15℃. When Ti* is higher, 0.07~0.08%, at this time, the Ti content consumed by impurity elements such as S and N is low, and the Ti content available for forming TiC precipitation is high, so a coiling temperature slightly deviated from the TiC precipitation nose point temperature (600℃) is adopted, that is, 580±15℃.

[0029] (2) Smelting process In order to control the effective Ti content in steel, it is necessary to stably control impurity elements such as S and N in the molten steel. Desulfurization is mainly completed through slag-liquid reaction in the LF refining process. SO2 in the molten steel is an acidic oxide, and the alkaline oxide in the slag can react with SO2 to achieve the purpose of desulfurization. The following factors need to be controlled for slag: 1) optical alkalinity Λ, specifically refers to the ratio of the mass percentage concentration of alkaline oxides to the mass percentage concentration of acidic oxides. The higher the optical alkalinity Λ value, the better the alkalinity of the slag, and the better the desulfurization effect. The present invention requires that the optical alkalinity Λ value is greater than 0.75; 2) Mannes index MI, specifically refers to the ratio of the mass percentage concentration of CaO to the mass percentage concentration product of SiO2 and Al2O3 in the slag, where CaO is an alkaline oxide, SiO2 is an acidic oxide, and Al2O3 is an amphoteric oxide, but it is an acidic oxide in the alkaline environment of the slag. The higher the Mannes index MI, the more conducive to desulfurization. The present invention requires that the Mannes index MI is greater than 0.50; 3) (FeO+MnO) in the final slag should be controlled at a low level, because when (FeO+MnO) is high, it will cause O in the steel. 2- As the concentration increases, the chemical potential of oxygen increases, which is not conducive to desulfurization. The present invention limits (FeO+MnO) to <1.5%.

[0030] In addition, the present invention also limits the converter bottom blowing argon flow rate and the tundish blowing argon flow rate to 50~80 m 3 / h and 50~60L / min to promote the removal of impurity elements such as S and N and the floating of inclusions.

[0031] (3) Slab heating system Slab heating primarily dissolves alloying elements. When the slab exits the furnace at a low temperature and remains in the furnace for a short time, the dissolved Ti content decreases, leading to reduced properties in the finished steel. When the slab exits the furnace at a high temperature and remains in the furnace for an extended time, the dissolved Ti content becomes excessively high, causing fluctuations in the properties of the finished steel. This also leads to coarsening of the grains in the original austenite and finished product, reducing the material's plasticity. Therefore, the present invention controls the slab exit temperature and furnace time within the range of 1230±15°C and 200-300 minutes.

[0032] (4) Rolling process During roughing, the deformation per pass must be controlled. If the deformation per pass is too low, the center of the billet will not be fully deformed, leading to dynamic recrystallization at the surface. This, however, will prevent dynamic recrystallization from occurring in the center of the billet, causing coarsening of the finished steel structure and, in turn, reducing the strength of the finished steel. Therefore, the present invention controls the deformation per roughing pass to above 18%.

[0033] At the same time, when the finishing rolling entrance temperature is too high, the first few passes of finishing rolling will be rolled in the incomplete recrystallization zone of austenite, resulting in the formation of mixed crystal structure, thereby reducing the strength of the finished steel. Therefore, the present invention limits the finishing rolling entrance temperature to below 1050°C.

[0034] Cumulative deformation during the finishing rolling process promotes flattening of austenite, thereby providing more nucleation cores for subsequent phase transformation and promoting grain refinement. The intermediate bar thickness significantly influences the cumulative deformation during finishing. Therefore, thinner steel gauges use smaller intermediate bar thicknesses, while thicker steel gauges use larger intermediate bar thicknesses to achieve similar cumulative deformation. Therefore, the present invention limits the intermediate bar thickness to: 40 ± 1 mm for finished steel plate thicknesses between 1.8 and < 4.0 mm; and 52 ± 1 mm for finished steel plate thicknesses between 4.0 and 10.0 mm.

[0035] (5) Laminar cooling process The present invention limits the finishing outlet temperature and coiling temperature within the range of ±15°C to reduce the performance fluctuation range of the finished steel. The finishing outlet temperature is limited to 870±15°C. If the finishing outlet temperature fluctuates greatly, it will affect the amount of deformation-induced TiC precipitation during the rolling process; if the coiling temperature fluctuates greatly, it will affect the amount of TiC precipitated due to supersaturation of ferrite. At the same time, the present invention limits the laminar cooling rate to 25~35°C / s. If the cooling rate is too low, it may cause coarse tissue and even form grain boundary cementite structure, thereby reducing the plasticity of the material; if the cooling rate is too high, it may cause the steel plate shape to deteriorate and form wave-shaped defects, thereby causing uneven cooling of the steel plate surface and even water accumulation on the surface, thereby causing fluctuations in the performance of the steel plate. Example

[0036] Table 1 shows the chemical composition of the steels according to the embodiments of the present invention and the comparative examples (in percentage by mass, with the remainder being Fe).

[0037] Table 1 Chemical composition / % According to the chemical composition shown in Table 1, the smelting raw materials are sequentially subjected to converter smelting, LF refining, RH refining, and continuous casting to form steel ingots. The production process parameters of the embodiments of the present invention and the comparative examples are shown in Table 2.

[0038] Table 2 Billet production process The steel ingot obtained by smelting according to the process shown in Table 2 is sequentially subjected to slab heating, rough rolling, finish rolling, laminar cooling, coiling and slow cooling to obtain titanium microalloyed steel. The production process parameters of the embodiment of the present invention and the comparative example are shown in Table 3, and the mechanical properties of the obtained titanium microalloyed steel are shown in Table 4.

[0039] Table 3 Process parameters of the steel of the present invention Table 4 Mechanical properties The titanium microalloyed steels described in Examples 1-4 of the present invention were subjected to hot continuous rolling, laminar cooling, and coiling to obtain finished steel plates. The specific composition and process are shown in Tables 1-3. As shown in Table 4, the mechanical properties of the finished steels obtained in Examples 1-4 all met the requirements, namely, a yield strength of 660-720 MPa, a tensile strength of 740-800 MPa, and an elongation of 16% or greater. Furthermore, the yield strength and tensile strength fluctuation ranges were within ±30 MPa (with an upper and lower limit of 60 MPa), namely, 18 MPa and 25 MPa, respectively.

[0040] As can be seen from Tables 1 to 4, the effective Ti content in the chemical composition of the steel described in Comparative Example 1 is relatively high, at 0.085%, exceeding the required value of 0.060-0.080%. Therefore, more Ti content ultimately forms a TiC precipitate phase, increasing the precipitation strengthening increment, ultimately resulting in higher yield strength and tensile strength of the finished steel, at 731 MPa and 822 MPa, respectively, exceeding the required values ​​of 660-720 MPa and 740-800 MPa.

[0041] During the smelting process of the steel described in Comparative Example 2, the converter bottom and tundish argon flow rates were low, the LF slag optical basicity Λ and Mannes index MI were low, and the LF final rolling (FeO + MnO) was high. This resulted in high S and N content in the chemical composition of the finished steel, at 0.010% and 0.0083%, respectively, exceeding the requirements of ≤0.005% and ≤0.0040%. N and S, respectively, combined with Ti to produce TiN and Ti4C2S2 inclusions, resulting in a low effective Ti content of 0.055%, below the required value of 0.060-0.080%. This, in turn, reduced the Ti content actually available for precipitation strengthening by forming the TiC second phase. This, in turn, led to low yield strength and tensile strength of 644 MPa and 723 MPa, respectively, below the required values ​​of 660-720 MPa and 740-800 MPa.

[0042] The chemical composition of the steels described in Comparative Examples 3 and 4 met the requirements. However, the slabs in Comparative Example 3 exhibited a relatively low furnace temperature and short furnace time of 1206°C and 164 minutes, respectively, below the required values ​​of 1215-1245°C and 200-300 minutes, resulting in a high content of dissolved Ti in the steel. Furthermore, the initial roughing pass deformation was low, at 16%, below the required value of ≥18%, leading to grain coarsening and reduced strength gains from grain refinement. Furthermore, the cooling rate was low, at 23°C / s, below the required value of 25-35°C / s. The coiling temperature was high, at 632°C, above the required value of 585-615°C and significantly deviating from the nose point temperature (600°C) for TiC precipitation. These factors combined to produce a low TiC content, resulting in low yield and tensile strengths of the finished steel, at 652 MPa and 736 MPa, respectively, below the required values ​​of 660-720 MPa and 740-800 MPa.

[0043] In Comparative Example 4, the intermediate billet thickness was too high, at 59 mm, exceeding the required value of 51–53 mm. The coiling temperature was also too high, at 603°C, exceeding the required value of 565–595°C. The higher intermediate billet thickness promoted grain refinement, increasing the strength contribution of grain refinement strengthening. The 603°C coiling temperature resulted in a higher amount of TiC precipitation, which also increased the strength contribution of precipitation strengthening. These factors combined resulted in higher yield strength and tensile strength of the finished steel, at 726 MPa and 803 MPa, respectively, exceeding the required values ​​of 660–720 MPa and 740–800 MPa.

Claims

1. A method for reducing performance fluctuations of titanium microalloyed steel, characterized in that: The method comprises the following steps: smelting raw materials in a converter, LF refining, RH refining, and continuous casting to form steel ingots; the steel ingots in turn undergo slab heating, rough rolling, finish rolling, laminar cooling, coiling, and slow cooling to obtain titanium microalloyed steel with a yield strength and tensile strength fluctuation range of within ±30 MPa; The ladle is washed during converter smelting to ensure that the bottom of the ladle is clean and the molten steel in the upper furnace is low-sulfur steel. The bottom blowing gas supply mode is adopted during smelting, and the argon blowing flow rate is 50~80m 3 / h; LF refining uses desulfurized slag with a slag optical basicity Λ>0.75, a Mannesmann index MI>0.50, and a final slag (FeO+MnO) content <1.5%; low-nitrogen ferroniobium and low-nitrogen ferrotitanium alloys are added during LF and RH refining; the shroud sealing effect must be ensured during continuous casting, and the argon blowing flow rate for the tundish must be 50-60 L / min; the S content in the finished steel must be controlled to ≤0.005%, and the N content to ≤0.0040%; In the coiling process, when the Ti* content in the steel is 0.060~<0.070%, the coiling temperature is 600±15℃; when the Ti* content in the steel is 0.070~0.080%, the coiling temperature is 580±15℃; The chemical composition of the titanium microalloyed steel includes, by weight percentage, C 0.05-0.07%, Si 0.05-0.10%, Mn 1.40-1.50%, Nb 0.035-0.045%, P≤0.015%, S≤0.005%, N≤0.0040%, Ti* 0.060-0.080%, and the balance is Fe and unavoidable impurity elements; wherein Ti*=[Ti]-1.5[S]-3.43[N], Ti* is the effective Ti content, and [Ti], [S], and [N] are the weight percentages of titanium, sulfur, and nitrogen in the steel, respectively.

2. The method for reducing performance fluctuations of titanium microalloyed steel according to claim 1, characterized in that: The thickness of the steel ingot is 200~250mm.

3. The method for reducing performance fluctuation of titanium microalloyed steel according to claim 1, characterized in that: The steel ingot is air-cooled to room temperature and then loaded into the slab heating furnace for heating.

4. The method for reducing performance fluctuation of titanium microalloyed steel according to claim 1, characterized in that: During the slab heating process, the slab out-of-furnace temperature is controlled at 1230±15℃, and the slab time in the furnace is 200~300min.

5. The method for reducing performance fluctuation of titanium microalloyed steel according to claim 1, characterized in that: After the slab is taken out of the furnace, it is descaled before entering the rough rolling process. The intermediate slab is obtained after rough rolling, and the finished steel plate is obtained after finishing rolling. In the rough rolling process, the deformation of a single rough rolling pass is ≥18%; in the finishing rolling process, the finishing rolling inlet temperature is ≤1050℃, and the finishing rolling outlet temperature is 870±15℃.

6. The method for reducing performance fluctuations of titanium microalloyed steel according to claim 5, characterized in that: The thickness of the finished steel plate is 1.8~10.0mm.

7. The method for reducing performance fluctuation of titanium microalloyed steel according to claim 6, characterized in that: When the thickness of the finished steel plate is 1.8~<4.0mm, the thickness of the intermediate billet is 40±1mm; when the thickness of the finished steel plate is 4.0~10.0mm, the thickness of the intermediate billet is 52±1mm.

8. The method for reducing performance fluctuation of titanium microalloyed steel according to claim 1, characterized in that: In the laminar cooling process, front-stage centralized cooling is adopted, and the cooling rate is 25~35℃ / s.

9. The method for reducing performance fluctuation of titanium microalloyed steel according to claim 1, characterized in that: After the steel plate is coiled into a coil, it is placed in a slow cooling pit for 70 to 80 hours of slow cooling.

10. The method for reducing performance fluctuation of titanium microalloyed steel according to claim 1, characterized in that: The titanium microalloyed steel has a yield strength of 660-720 MPa, a tensile strength of 740-800 MPa, and an elongation of ≥16%.

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