A method for controlling surface cracks of high carbon alloy steel strip

Abstract

The present application relates to a kind of high carbon alloy steel strip surface crack control method, the production process of high carbon alloy steel strip includes: hot metal smelting, top and bottom combined blowing and alloying of converter, LF furnace refining, continuous casting, heating, rolling, laminar cooling, annealing and coiling;The present application is mainly aimed at the problem that high carbon alloy (such as 60Si2Mn) steel strip is prone to surface crack in hot rolling and cooling process.Optimizing steel strip composition design, rolling process parameters, cooling path and subsequent heat treatment process, significantly reduce the surface crack incidence rate, while ensuring that the mechanical properties of material meet the standard requirements.

Description

Technical Field

[0001] This invention relates to the field of high-carbon alloy steel strip production technology, and in particular to a method for controlling surface cracks in high-carbon alloy steel strip. Background Technology

[0002] 60Si2Mn steel is a widely used silicon-manganese spring steel, mainly used to manufacture flat springs that bear heavy loads or helical springs with a wire diameter of less than 30mm. It is also suitable for manufacturing heat-resistant springs that operate in non-corrosive media at temperatures below 250℃, as well as large and important coiled springs that bear alternating loads and operate under high stress, and automotive shock absorption systems. Due to its high carbon (0.56%–0.64%) and high silicon (1.5%–2.0%) chemical composition, it is prone to surface cracking during hot working (especially in continuous casting and rolling processes).

[0003] With the increasing demands for surface quality in modern industrial production, crack defects (such as M-shaped cracks, black line cracks, and edge peeling) have become a core issue restricting product qualification rates. Especially in recent years, with the improvement of rolling capacity, the maximum thickness of cast billets has increased from 160mm to over 180mm, significantly increasing solidification stress and further exacerbating the crack incidence rate (typically exceeding 5%), severely impacting the yield of downstream processing (such as leveling and quenching).

[0004] At the technological level, traditional continuous casting processes employ high casting speeds (around 1.60 m / min) and forced secondary cooling (specific water volume ≥ 0.73 m³ / min). 3 The method of using a single-stage annealing process ( / h) resulted in an excessively large temperature difference between the core and surface of the cast billet, with the temperature in the straightening zone falling below 900℃, inducing phase transformation stress cracks. Simultaneously, insufficient protective slag properties (e.g., low basicity, high viscosity) led to an excessively thin liquid slag layer (<6mm), exacerbating friction and uneven heat transfer within the crystallizer. Furthermore, the rolling process suffered from insufficient billet heating time (<130min) and a single annealing process (e.g., single-stage annealing), failing to effectively eliminate surface microcracks and decarburization stress, causing residual defects to expand into visible cracks during subsequent processing.

[0005] The Chinese patent application with publication number CN 117816919 A discloses "a method for controlling surface cracks and slag inclusions of high-carbon alloy steel continuously cast slabs", wherein the weight percentage of chemical components of the high-carbon alloy steel is: C: 0.3% to 0.80%, Si: 0.2% to 0.45%, Mn: 0.60% to 1.20%, P≤0.018%, S≤0.010%, Al: 0.01% to 0.08%, Cr≤5.0%, Ni≤1.0%, Mo≤2.2%, V≤0.5%, and the balance is iron and unavoidable impurities. The control method comprises: 1) during continuous casting, the continuously cast slab has a thickness of 200 to 250 mm and a width of 800 to 1600 mm, and a CaO-SiO₂-Al₂O₃-Na₂O-CaF₂-B₂O₃ multi-component slag system is adopted as mold flux; 2) during continuous casting, the casting speed of the continuous caster is 0.3 to 1.5 m / min, and the superheat of molten steel in the tundish is 5 to 30°C; 3) during continuous casting, the vibration parameters of the crystallizer are controlled. The produced high-carbon alloy steel continuously cast slab is directly hot charged to a heating furnace for heating, and the surface of the hot-rolled steel plate obtained after hot rolling has no defects caused by surface cracks and slag inclusions of the slab. However, the contents of C and Si in the high-carbon alloy steel described therein are both lower than those of 60Si2Mn steel, and it only relates to the continuous casting process, so it is different from the present invention.

[0006] The Chinese patent application with publication number CN 114367645 A discloses "a method for reducing surface cracks of alloy steel, alloy steel and a preparation method thereof", which comprises continuous casting and heating steps; in the continuous casting step, the upper surface temperature of the continuous casting billet in the straightening section is controlled to be greater than the lower surface temperature, satisfying 0<ΔT≤60°C, and the lower surface temperature of the continuous casting billet in the straightening section satisfies: 840≤T≤900°C; the heating step at least comprises two heating stages, the time of the first heating stage satisfies 0<t≤15min, the temperature satisfies 1060≤T≤1100°C, and the total heating time is 110 to 160 min. By controlling the straightening temperature of the continuous casting billet, the straightening temperature difference between the upper and lower surfaces and the heating process, the "copper embrittlement" phenomenon of sulfuric acid dew corrosion resistant steel strips is effectively reduced, the finished product has no visible edge cracks at the edge, no V-shaped "copper embrittlement" cracks on the plate surface, and the "copper embrittlement" defect rate is reduced. However, it only relates to the continuous casting and billet heating processes, and the content of C in the alloy steel described therein is 0.06% to 0.09%, and Si is 0.25% to 0.40%, which are far lower than those of the high-carbon alloy steel described in the present invention, so it is different from the present invention. Summary of the Invention

[0007] The present invention provides a method for controlling surface cracks of high-carbon alloy steel strips, which addresses the problem that high-carbon alloy (such as 60Si2Mn) steel strips are prone to surface cracks during hot rolling and cooling. By optimizing the composition design of the steel strip, rolling process parameters, cooling paths and subsequent heat treatment processes, the incidence of surface cracks is significantly reduced, while ensuring that the mechanical properties of the material meet the standard requirements.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A method for controlling surface cracks in high-carbon alloy steel strip, wherein the production process of high-carbon alloy steel strip includes: hot metal smelting, converter top and bottom combined blowing and alloying, LF furnace refining, continuous casting, heating, rolling, laminar flow cooling, annealing and coiling; the specific control process is as follows:

[0010] 1) The converter endpoint is controlled by mass ratio: [C]≥0.08%, [P]≤0.015%, and the tapping temperature is 1600~1640℃; the amount of phosphorus returned to the slag is controlled by mass ratio ≤0.003%;

[0011] 2) During the refining process in the LF furnace, ensure that the argon blowing time is ≥6min before calcium treatment, and control N≤45ppm and Ti≤0.002% at the refining endpoint by mass ratio;

[0012] 3) The continuous casting speed is controlled at 1.45–1.50 m / min, and the secondary cooling zone adopts an ultra-weak cooling process. The cooling water flow rate of the crystallizer is controlled at 128–132 m across the wide face. 3 / h, narrow face 26~30m 3 / h, straightening temperature ≥900℃; temperature difference between inlet and outlet water of the crystallizer ≤9℃, superheat of molten steel controlled at 20~30℃; crystallizer nozzle inclination angle 14°~16°, nozzle immersion depth 80~100mm, nozzle outlet flow velocity controlled at 0.8~1.2m / s;

[0013] 4) The furnace time for heating the billet is ≥150 min, and the furnace temperature is controlled at 646~738℃; the surface milling amount of the billet is ≥0.2 mm;

[0014] 5) Rolling includes roughing and finishing; the initial rolling temperature is 1050–1110℃, and the final rolling temperature is 930–970℃;

[0015] 6) The laminar flow cooling temperature is 710–750℃;

[0016] 7) Two-stage annealing is adopted. The first stage annealing temperature is 500-550℃ and the holding time is 1-2h; the second stage annealing temperature is 300-350℃ and the holding time is 4-6h. A nitrogen-hydrogen mixed protective gas is used during the annealing process, and the volume ratio of H2 in the nitrogen-hydrogen mixed protective gas is 3%-5%, and the dew point is ≤-40℃.

[0017] The high-carbon alloy steel strip is 60Si2Mn steel strip, and its chemical composition by mass percentage is: C: 0.58%–0.62%; Mn: 0.77%–0.83%; Si: 1.65%–1.85%; S≤0.010%; P≤0.015%; Als: 0.015%–0.025%; Cr: 0.15%–0.21%.

[0018] Special protective slag is used in the continuous casting process. The ternary basicity of the protective slag (CaO / SiO2 / AlO3) is 1.05-1.15, the hemispherical point temperature is 1100-1150℃, and the viscosity is 0.15-0.25 Pa·s. The consumption of protective slag is ≥0.30 kg / t steel, and the thickness of the liquid slag layer is ≥8 mm.

[0019] The heated billet and the rough-rolled strip are descaled separately.

[0020] The finished steel strip has a tensile strength ≥1570MPa and an elongation after fracture ≥12%.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] 1) Existing solutions for eliminating surface cracks in alloy steel strips often focus on a single step, including: adding titanium to refine grains through microalloying (e.g., the Chinese patent application CN105803308A, "A Magnesium-Calcium 45MnVS Free-Cutting Non-Temperature Steel and Its Manufacturing Method"), but excessive titanium in the steel can form a brittle phase; low-temperature rolling (e.g., the Chinese patent application CN106191729A, "A Device for Reducing the Formation of Zinc Dross in Continuous Hot-Dip Galvanizing Zinc Pots"), while beneficial for reducing cracks, significantly increases equipment load. Furthermore, slag optimization is often limited to alkalinity adjustment, lacking comprehensive control over the thickness of the liquid slag layer and the heat transfer coefficient. The methods mentioned above have failed to achieve synergy across the entire "steelmaking-continuous casting-rolling" process, resulting in a persistently high crack rate for wide steel strips (600-780mm wide), which is difficult to meet the needs of high-end equipment manufacturing. In contrast, this invention provides a systematic optimization scheme that achieves low-cost and highly adaptable crack control through step-by-step improvements and synergistic effects of composition control, continuous casting steady-state process, protective slag performance enhancement, and rolling annealing.

[0023] 2) This invention mainly addresses the problem of surface cracks easily generated in 60Si2Mn steel strip during hot rolling and cooling. By optimizing the steel strip composition design, rolling process parameters, cooling path and subsequent heat treatment process, the incidence of surface cracks is significantly reduced. Specifically, this includes: adjusting the content of residual elements (such as S, P, O, N) in the steel, optimizing the temperature range between rough rolling and finishing rolling, using gradient cooling technology to suppress stress concentration, and combining a two-stage annealing process to eliminate internal stress.

[0024] 3) This invention can reduce the surface crack rate of 60Si2Mn steel strip from more than 3% in the traditional process to less than 0.5%, of which M-shaped cracks are reduced from more than 5.2% to less than 0.8%, black line cracks are reduced from more than 3.5% to less than 0.5%, and edge peeling defects are reduced from more than 2.1% to less than 0.3%.

[0025] 4) While significantly reducing surface crack defects in the steel strip, the mechanical properties of the material are guaranteed to meet the requirements of GB / T 1222 "Spring Steel" standard, namely tensile strength ≥1570MPa and elongation after fracture ≥12%, thus improving toughness while increasing strength.

[0026] 5) Production costs were reduced, the consumption of protective slag was reduced by about 15%, and the rolling qualification rate was increased from below 92% to above 98%. Detailed Implementation

[0027] The present invention discloses a method for controlling surface cracks in high-carbon alloy steel strip. The production process of high-carbon alloy steel strip includes: hot metal smelting, converter top and bottom combined blowing and alloying, LF furnace refining, continuous casting, heating, rolling, laminar flow cooling, annealing, and coiling; the specific control process is as follows:

[0028] 1) The converter endpoint is controlled by mass ratio: [C]≥0.08%, [P]≤0.015%, and the tapping temperature is 1600~1640℃; the amount of phosphorus returned to the slag is controlled by mass ratio ≤0.003%;

[0029] 2) During the refining process in the LF furnace, ensure that the argon blowing time is ≥6min before calcium treatment, and control N≤45ppm and Ti≤0.002% at the refining endpoint by mass ratio;

[0030] 3) The continuous casting speed is controlled at 1.45–1.50 m / min, and the secondary cooling zone adopts an ultra-weak cooling process. The cooling water flow rate of the crystallizer is controlled at 128–132 m across the wide face. 3 / h, narrow face 26~30m 3 / h, straightening temperature ≥900℃; temperature difference between inlet and outlet water of the crystallizer ≤9℃, superheat of molten steel controlled at 20~30℃; crystallizer nozzle inclination angle 14°~16°, nozzle immersion depth 80~100mm, nozzle outlet flow velocity controlled at 0.8~1.2m / s;

[0031] 4) The furnace time for heating the billet is ≥150 min, and the furnace temperature is controlled at 646~738℃; the surface milling amount of the billet is ≥0.2 mm;

[0032] 5) Rolling includes roughing and finishing; the initial rolling temperature is 1050–1110℃, and the final rolling temperature is 930–970℃;

[0033] 6) The laminar flow cooling temperature is 710–750℃;

[0034] 7) Two-stage annealing is adopted. The first stage annealing temperature is 500-550℃ and the holding time is 1-2h; the second stage annealing temperature is 300-350℃ and the holding time is 4-6h. A nitrogen-hydrogen mixed protective gas is used during the annealing process, and the volume ratio of H2 in the nitrogen-hydrogen mixed protective gas is 3%-5%, and the dew point is ≤-40℃.

[0035] The high-carbon alloy steel strip is 60Si2Mn steel strip, and its chemical composition by mass percentage is: C: 0.58%–0.62%; Mn: 0.77%–0.83%; Si: 1.65%–1.85%; S≤0.010%; P≤0.015%; Als: 0.015%–0.025%; Cr: 0.15%–0.21%.

[0036] Special protective slag is used in the continuous casting process. The ternary basicity of the protective slag (CaO / SiO2 / AlO3) is 1.05-1.15, the hemispherical point temperature is 1100-1150℃, and the viscosity is 0.15-0.25 Pa·s. The consumption of protective slag is ≥0.30 kg / t steel, and the thickness of the liquid slag layer is ≥8 mm.

[0037] The heated billet and the rough-rolled strip are descaled separately.

[0038] The finished steel strip has a tensile strength ≥1570MPa and an elongation after fracture ≥12%.

[0039] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0040]

Example 1

[0041] This embodiment takes the production of 60Si2Mn steel strip (600mm wide × 10mm thick) as an example. The production process includes molten iron smelting → converter top and bottom combined blowing and alloying treatment → LF furnace refining → continuous casting → hot charging → heating → descaling → rough rolling → descaling → finish rolling → laminar flow cooling → coiling; the following production processes are controlled:

[0042] 1. Converter endpoint control: [C] = 0.09% (mass percentage), [P] = 0.010% (mass percentage), tapping temperature 1620℃. Ensure round tapping, use sliding plate tapping with slag blocking, and ensure slag phosphorus return rate is 0.002%.

[0043] 2. LF furnace refining ensures submerged arc effect and reduces nitrogen increase during refining; argon blowing time before calcium treatment is 8 minutes, and S = 0.012%, N = 42ppm, and Ti = 0.0015% by mass ratio;

[0044] 3. The superheat of the molten steel in continuous casting is controlled at 25℃, the casting speed is 1.48m / min, and the secondary cooling water flow rate is 0.65m³ / min. 3 / h, crystallizer inlet inclination angle 15°; crystallizer cooling water flow rate: 130m³ / h (wide face) 3 / h, narrow face 28m 3 / h, tundish liquid level height 816mm, slag layer thickness 30mm, straightening temperature 975℃.

[0045] 4. Use 60Si2Mn special protective slag. The ternary basicity of the protective slag CaO / SiO2 / AlO3 is 1.10, the hemispherical point temperature is 1130℃, the viscosity is 0.20Pa·s, and the consumption of protective slag is 0.32kg / t.

[0046] 5. The furnace time for heating the billet should be controlled at 155 minutes;

[0047] 6. The final rolling temperature is controlled at 943℃, the laminar flow cooling temperature is controlled at 721℃, and the surface milling amount is 0.3mm;

[0048] 7. After rolling, a two-stage annealing process is adopted. The annealing temperature and time are: 530℃×1.5h+330℃×5h. The protective gas is a nitrogen-hydrogen mixture N2-4%H2.

[0049] Finished product test results:

[0050] 1. The status of inclusion control is shown in the table below:

[0051]

[0052] That is, the inclusions of types A, B, C and D in the finished strip steel do not exceed grade 0.5, the inclusions of type Ds are grade 1.0, and the banded structure is grade 0.5.

[0053] 2. Surface crack rate: 0.9%.

[0054] 3. The tensile strength of the finished strip steel is 1585 MPa, and the elongation is 14.1%.

[0055] 4. The low magnification center porosity of the billet is grade 0.5, with no intermediate cracks.

[0056] Conclusion: The 60Si2Mn spring steel strip produced by the control method described in this invention has good surface quality, with no fine black lines or M-shaped cracks. Compared with conventional processes, the surface crack defects are significantly improved.

[0057]

Example 2

[0058] This embodiment takes the production of 45# steel strip (650mm wide × 10mm thick) as an example, and the production process is the same as in Embodiment 1; the following production processes are controlled:

[0059] 1. Converter endpoint control: [C] = 0.09% (mass percentage), [P] = 0.010% (mass percentage), tapping temperature 1640℃. Ensure round tapping, use sliding plate tapping with slag blocking, and ensure phosphorus return in the slag is 0.003%.

[0060] 2. LF furnace refining ensures submerged arc effect and reduces nitrogen increase during refining; argon blowing time before calcium treatment is 8 minutes, and S = 0.010%, N = 38ppm, and Ti = 0.0020% by mass ratio;

[0061] 3. The superheat of the molten steel in continuous casting is controlled at 27℃, the casting speed is 1.50 m / min, and the secondary cooling water flow rate is 0.65 m³ / min. 3 / h, crystallizer inlet inclination angle 15°; crystallizer cooling water flow rate: 130m³ / h (wide face) 3 / h, narrow face 28m 3 / h, tundish liquid level height 816mm, slag layer thickness 30mm, straightening temperature 975℃.

[0062] 4. Use 45# steel special protective slag. The ternary basicity of the protective slag CaO / SiO2 / AlO3 is 1.10, the hemispherical point temperature is 1130℃, the viscosity is 0.20Pa·s, and the consumption of protective slag is 0.32kg / t.

[0063] 5. The furnace time for heating the billet should be controlled at 155 minutes;

[0064] 6. The final rolling temperature is controlled at 943℃, the laminar flow cooling temperature is controlled at 721℃, and the surface milling amount is 0.3mm;

[0065] 7. After rolling, a two-stage annealing process is adopted. The annealing temperature and time are: 540℃×1.5h+340℃×5h. The protective gas is a nitrogen-hydrogen mixture N2-4%H2.

[0066] Finished product test results:

[0067] 1. The status of inclusion control is shown in the table below:

[0068]

[0069] That is, the inclusions of types A, B, C and D in the finished strip steel do not exceed grade 0.5, the inclusions of type Ds are grade 1.0, and the banded structure is grade 0.5.

[0070] 2. Surface crack rate: 0.9%.

[0071] 3. The tensile strength of the finished strip steel is 1580MPa, and the elongation is 15.3%.

[0072] 4. The low magnification center porosity of the billet is grade 0.5, with no intermediate cracks.

[0073] Conclusion: The 45# steel strip produced by the control method described in this invention has good surface quality, with no fine black lines or M-shaped cracks. Compared with conventional processes, the surface crack defects are significantly improved.

[0074] This invention addresses both M-shaped cracks and black line cracks simultaneously through synergistic optimization of the continuous casting steady-state process and protective slag; it significantly improves the surface defect merging rate by extending the billet heating time and precisely controlling rolling process parameters; and it reduces thermal stress and avoids the generation of secondary cracks by employing an ultra-weak cooling process.

[0075] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for controlling surface cracks in high-carbon alloy steel strips, characterized in that, The high-carbon alloy steel strip is 60Si2Mn steel strip, and the chemical composition of the steel, by mass percentage, is: C: 0.58%~0.62%; Mn: 0.77%~0.83%; Si: 1.65%~1.85%; S≤0.010%; P≤0.015%. Als: 0.015%~0.025%; Cr: 0.15%~0.21%; The production process of high-carbon alloy steel strip includes: molten iron smelting, converter top and bottom combined blowing and alloying, LF furnace refining, continuous casting, heating, rolling, laminar flow cooling, annealing and coiling; the specific control process is as follows: 1) The converter's final output should be controlled by mass ratio: [C] ≥ 0.08%, [P] ≤ 0.015%, and the tapping temperature should be 1600~1640℃; the amount of phosphorus returned to the slag should be controlled by mass ratio ≤ 0.003%; 2) During the LF furnace refining process, ensure that the argon blowing time is ≥6 min before calcium treatment, and control N ≤45 ppm and Ti ≤0.002% by mass ratio at the refining endpoint; 3) The continuous casting speed is controlled at 1.45–1.50 m / min, and the secondary cooling zone adopts an ultra-weak cooling process. The cooling water flow rate of the crystallizer is controlled at 128–132 m across the wide face. 3 / h, narrow face 26~30m 3 The straightening temperature is ≥900℃; the temperature difference between the inlet and outlet water of the crystallizer is ≤9℃; the superheat of the molten steel is controlled at 20~30℃; the inclination angle of the crystallizer nozzle is 14°~16°; the immersion depth of the nozzle is 80~100mm; and the outlet flow velocity is controlled at 0.8~1.2m / s. A special protective slag is used during continuous casting. The ternary basicity of the protective slag (CaO / SiO2 / AlO3) is 1.05~1.15, the hemispherical point temperature is 1100~1150℃, and the viscosity is 0.15~0.25Pa·s. The consumption of protective slag is ≥0.30kg / t steel, and the thickness of the liquid slag layer is ≥8mm. 4) The furnace time for heating the billet is ≥150 min, and the furnace temperature is controlled at 646~738℃; the surface milling amount of the billet is ≥0.2 mm; 5) Rolling includes roughing and finishing; the initial rolling temperature is 1050–1110℃, and the final rolling temperature is 930–970℃; 6) The laminar flow cooling temperature is 710–750℃; 7) Two-stage annealing is adopted. The first stage annealing temperature is 500-550℃ and the holding temperature is 1-2h; the second stage annealing temperature is 300-350℃ and the holding temperature is 4-6h. A nitrogen-hydrogen mixed protective gas is used during the annealing process, and the volume ratio of H2 in the nitrogen-hydrogen mixed protective gas is 3%-5%, and the dew point is ≤-40℃. The finished steel strip has a tensile strength ≥1570MPa and an elongation after fracture ≥12%.

2. The method for controlling surface cracks in high-carbon alloy steel strip according to claim 1, characterized in that, The heated billet and the rough-rolled strip are descaled separately.

Images