A narrow hardenability high cleanliness sulfur-containing Cr-Mn-Ti gear steel smelting and rolling production method

Abstract

The application discloses a narrow quenching and high-purity sulfur-containing Cr-Mn-Ti gear steel smelting and rolling production method, which comprises the following steps: molten iron pretreatment, LF refining, continuous casting, rolling, and controlled cooling; and the chemical components in percentage by mass are as follows: C: 0.19-0.22%, Si: 0.21-0.28%, Mn: 1.00-1.05%, P: ≤0.020%, S: 0.015-0.030%, Cr: 1.20-1.25%, Ti: 0.06-0.09%, Al: 0.010-0.050%, and the balance is Fe and impurities. The gear steel has the following advantages: quenching width ≤5HRC, good cleanliness, non-metallic inclusion A / B / C / D ≤1.0 grade, casting blank center porosity ≤1.0 grade, segregation ≤1.0 grade, rolled material grain size ≥8 grade, and tensile strength fluctuation ≤30MPa.

Description

Technical Field

[0001] This invention belongs to the field of metal smelting and rolling technology, and particularly relates to a method for smelting and rolling high-purity sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability. Background Technology

[0002] To address the requirements of narrow hardenability (hardenability bandwidth ≤ 5HRC, compositional fluctuation of Mn ± 0.03%, Cr ± 0.02%) and high cleanliness ([H] ≤ 2ppm, [N] ≤ 40ppm, non-metallic inclusions A / B / C / D class ≤ 1.0) for 20CrMnTiH gear steel, precise control of the entire "smelting-continuous casting-rolling" process achieves uniform steel composition, low and controllable inclusion morphology, and stable microstructure and properties. This invention patent relates to a narrow hardenability, high cleanliness sulfur-containing Cr-Mn-Ti gear steel suitable for heavy-duty gears in automobiles, construction machinery, etc., aiming to solve the problems of large hardenability fluctuation, high inclusion content, and poor machinability of traditional gear steels. Summary of the Invention

[0003] The purpose of this invention is to provide a method for smelting and rolling high-purity sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability, where the hardenability bandwidth (J9 / J15) is ≤5HRC. Purity: [O] ≤15ppm, [H] ≤2ppm, [N] ≤50ppm, non-metallic inclusions A / B / C / D class ≤1.0 grade (GB / T 10561), large inclusions (≥20μm) ≤0.5 per 100g. Microstructure and properties: Central porosity of the cast billet ≤1.0 grade, segregation ≤1.0 grade; grain size of the rolled material ≥8 grade, tensile strength fluctuation ≤30MPa.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] This invention discloses a method for smelting and rolling high-purity sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability, comprising:

[0006] Hot metal pretreatment requires deep dephosphorization and desulfurization to reduce initial harmful elements P and S, thus reducing the burden on subsequent refining. KR mechanical stirring desulfurization is employed, using CaO-CaF2 as the desulfurizing agent at a rate of 8-12 kg / t of iron, with a stirring time of 15-20 min, controlling [S] ≤ 0.005%. Pre-dephosphorization is carried out before the converter using oxidizing slag, controlling [P] ≤ 0.010% to prevent phosphorus redissolution during subsequent refining. Converter steelmaking requires low-oxygen, low-slag molten steel control, the purpose of which is... To reduce steel oxidation and slag contamination, a foundation for pure molten steel is laid for refining; the endpoint adopts high carbon content + endpoint control, with an endpoint carbon content of 0.15-0.25% and a temperature of 1600-1650℃, to reduce refractory material erosion; slag-blocking during tapping uses pneumatic slag-blocking plugs + electromagnetic induction slag blocking, with slag carryover during tapping ≤5kg / t, preventing FeO and P2O5 in the slag from contaminating the molten steel; during tapping deoxidation, Si-Mn alloy pre-deoxidation is added with the steel stream to inhibit premature formation of Al2O3 inclusions;

[0007] LF refining precisely adjusts alloy composition, performs deep deoxidation, and removes inclusions by creating reducing slag. Composition fine-tuning utilizes online direct-reading spectroscopy analysis to control the hardenability elements Mn, Cr, and Ti in real time, employing a "batch addition + uniform stirring" strategy. For composite deoxidation, Al (0.5-0.8 kg / t) is added first for strong deoxidation, followed by weak deoxidation with Si-Mn alloy, controlling Al content to 0.025-0.035%. The reducing slag system requires a binary basicity of CaO / SiO2 = 5-7, and MgO... 8-12%, FeO+MnO≤0.5%, promotes the transformation of inclusions into low-melting-point composite inclusions and their flotation, refining time 40-60min, ensuring inclusion flotation rate ≥90%; Calcium treatment: feed Ca-Si wire, addition amount 0.2-0.4kg / t, control [Ca] 0.0015-0.0030%, Ca / S=1.2-2.0, transform MnS plastic inclusions into spherical CaS or CaO-Al2O3-CaS composite inclusions; VD vacuum degassing and deep purification of inclusions aim to reduce [H] and [N], promote the aggregation and flotation of small inclusions; vacuum degree ≤67Pa, holding time 20-30min, stirring intensity 0.10-0.15kW / t, argon flow rate 0.5-1.0L / min·t; control [H] ≤2ppm, [N] ≤40ppm, steel liquid circulation ≥3 times;

[0008] To avoid center segregation, porosity, and subcutaneous inclusions, and to ensure the uniformity of the billet composition, continuous casting requires low superheat casting, with the molten steel in the tundish superheated at 20-30℃ and the liquid level fluctuation in the crystallizer ≤±3mm; the electromagnetic stirring crystallizer (EMS) + solidification end EMS, with strengths of 0.3-0.5T and 0.2-0.4T respectively, breaks up columnar crystals, promotes an equiaxed crystal ratio ≥50%, and reduces center segregation; secondary cooling and slow cooling: adopts a "weak cooling" system, with a specific water volume of 0.8-1.2L / kg, controlled in stages: 0.6-0.8L / kg in the upper stage and 1.0-1.2L / kg in the lower stage. After the billet exits the crystallizer, it is pile-cooled for ≥24 hours, and the temperature is ≥600℃ before being placed in the slow cooling pit to eliminate hydrogen-induced cracks;

[0009] Rolling aims to refine the microstructure and homogenize the properties. Temperature-controlled rolling refines the grains, improves microstructure homogeneity, and indirectly stabilizes hardenability. Heating: The soaking temperature is 1180-1220℃ to ensure uniform austenitization and avoid overheating. Rough rolling deformation is ≥50%, pass reduction is 15-25%, finish rolling starting temperature is 900-950℃, finishing rolling temperature is 820-860℃, and cumulative deformation is ≥80%. Controlled cooling: Laminar flow cooling rate is 1-3℃ / s, and final cooling temperature is 650-700℃ to obtain a uniform pearlite + ferrite microstructure.

[0010] The chemical composition of the gear steel by mass percentage is as follows: C: 0.19-0.22%, Si: 0.21-0.28%, Mn: 1.00-1.05%, P: ≤0.020%, S: 0.015-0.030%, Cr: 1.20-1.25%, Ti: 0.06-0.09%, Al: 0.010-0.050%, with the balance being Fe and unavoidable impurity elements.

[0011] Furthermore, the main components of the desulfurizing agent in terms of percentage content are: CaO 80-85%, CaF2 10-15%.

[0012] Furthermore, in the "batch addition + uniform mixing" strategy: the mixing power is 0.05-0.08 kW / t, and the time is 5-8 min / batch.

[0013] Furthermore, an immersion-type inlet with a flow control device is used to prevent slag entrapment.

[0014] Furthermore, the compositional fluctuations are: Mn ± 0.03%, Cr ± 0.02%, Ti ± 0.01%.

[0015] Furthermore, the hardenability bandwidth J9 / J15≤5HRC; cleanliness: [O]≤15ppm, [H]≤2ppm, [N]≤50ppm, non-metallic inclusions A / B / C / D class≤1.0 grade, large inclusions (≥20μm)≤0.5 pieces / 100g.

[0016] Furthermore, the microstructure and properties are as follows: the central porosity of the billet is ≤1.0 grade, and the segregation is ≤1.0 grade; the grain size of the rolled material is ≥8 grade, and the tensile strength fluctuation is ≤30MPa.

[0017] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0018] Hardenability bandwidth (J9 / J15) ≤ 5HRC. Cleanliness: [O] ≤ 15ppm, [H] ≤ 2ppm, [N] ≤ 50ppm, non-metallic inclusions A / B / C / D class ≤ 1.0 grade (GB / T 10561), large inclusions (≥20μm) ≤ 0.5 pieces / 100g. Microstructure and properties: central porosity of billet ≤ 1.0 grade, segregation ≤ 1.0 grade; grain size of rolled material ≥ 8 grade, tensile strength fluctuation ≤ 30MPa. Attached Figure Description

[0019] Figure 1 This refers to the band-like tissue of Example 1;

[0020] Figure 2 This is the sulfur content distribution in Example 1. Detailed Implementation

[0021] A high-purity, sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability is characterized by the following chemical composition (Wt%): C: 0.19–0.22%, Si: 0.21–0.28%, Mn: 1.00–1.05%, P: ≤0.020%, S: 0.015–0.030%, Cr: 1.20–1.25%, Ti: 0.06–0.09%, Al: 0.010–0.050%, with the balance being Fe and unavoidable impurity elements. Composition fluctuations: Mn ±0.03%, Cr ±0.02%, Ti ±0.01%, hardenability bandwidth (J9 / J15) ≤5HRC. Cleanliness: [O] ≤ 15 ppm, [H] ≤ 2 ppm, [N] ≤ 50 ppm, non-metallic inclusions A / B / C / D class ≤ 1.0 grade (GB / T 10561), large inclusions (≥20μm) ≤ 0.5 pieces / 100g. Microstructure and properties: Porosity at the center of the cast billet ≤ 1.0 grade, segregation ≤ 1.0 grade; grain size of rolled material ≥ 8 grade, tensile strength fluctuation ≤ 30MPa.

[0022] The production steps for high-purity, sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability mainly include: Hot metal pretreatment requires deep dephosphorization and desulfurization to reduce initial harmful elements (P, S) and decrease the burden on subsequent refining. KR mechanical stirring desulfurization is used, with CaO-CaF2 (CaO 80-85%, CaF2 10-15%) as the desulfurizing agent, added at 8-12 kg / t of iron, and stirred for 15-20 min, controlling [S] ≤ 0.005%. Pre-dephosphorization is performed before the converter, using oxidizing slag (CaO + Fe2O3), controlling [P] ≤ 0.010% (target ≤ 0.008%) to prevent phosphorus redissolution during subsequent refining. Converter steelmaking requires low-oxygen, low-slag molten steel control to reduce molten steel oxidation and slag contamination, laying a foundation for pure molten steel for refining. The endpoint is controlled using high carbon content and precise carbon control, with an endpoint carbon content of 0.15-0.25% (to avoid over-oxidation) and a temperature of 1600-1650℃ (adjusted according to the steel grade; the lower limit is used for medium carbon gear steel) to reduce refractory material erosion. Slag-blocking during tapping employs a pneumatic slag plug and electromagnetic induction slag blocking, ensuring that the amount of slag carried in during tapping is ≤5kg / t to prevent FeO (≤1.0%) and P2O5 from contaminating the molten steel. During tapping, a Si-Mn alloy (Si 0.2-0.3%, Mn 0.5-0.7%) is added with the steel stream for pre-deoxidation to inhibit premature Al2O3 inclusion formation.

[0023] LF refining precisely adjusts the alloy composition (key to narrow hardenability), performs deep deoxidation, and removes inclusions by creating reducing slag. Composition fine-tuning is achieved through online direct-reading spectroscopy analysis, real-time control of hardenability elements such as Mn (±0.03%), Cr (±0.02%), and Ti (±0.01%), employing a "batch addition + uniform stirring" strategy (stirring power 0.05-0.08 kW / t, time 5-8 min / batch). For composite deoxidation, Al (0.5-0.8 kg / t) is added first for strong deoxidation, followed by Si-Mn alloy for weak deoxidation, controlling Als (acid-soluble aluminum) at 0.025-0.035% (to avoid hard Al2O3 inclusions or insufficient deoxidation). The reducing slag system requires a binary basicity of CaO / SiO2 = 5-7, MgO 8-12%, and FeO+MnO ≤ 0.5% to promote the conversion and flotation of inclusions (such as SiO2 and MnO) into low-melting-point composite inclusions (CaO-Al2O3-SiO2). The refining time is 40-60 minutes, ensuring an inclusion flotation rate ≥ 90%. Calcium treatment: Feed Ca-Si wire (Ca 28-32%) at a rate of 0.2-0.4 kg / t, controlling [Ca] at 0.0015-0.0030% and Ca / S = 1.2-2.0, converting MnS plastic inclusions into spherical CaS or CaO-Al2O3-CaS composite inclusions (size ≤ 5 μm). Vacuum degassing (VD) and deep purification of inclusions aim to reduce [H] and [N], promoting the aggregation and flotation of small inclusions. Vacuum degree ≤67Pa, holding time 20-30min, stirring intensity 0.10-0.15kW / t (argon flow rate 0.5-1.0L / min·t), sulfur wire is fed after VD. Control [H] ≤2ppm, [N] ≤40ppm, and molten steel circulation times ≥3 times (to ensure uniform degassing of the entire furnace of molten steel).

[0024] To avoid center segregation, porosity, and subcutaneous inclusions, and to ensure the uniformity of the billet composition, continuous casting requires low superheat casting. The superheat of the molten steel in the tundish is 20-30℃ (using an immersion nozzle + flow control device to avoid slag entrapment), and the liquid level fluctuation in the crystallizer is ≤±3mm. An electromagnetic stirring crystallizer (EMS) with a strength of 0.3-0.5T + a solidification end EMS (0.2-0.4T) breaks up columnar crystals, promotes an equiaxed crystal ratio ≥50%, and reduces center segregation (C segregation index ≤1.05). Secondary cooling and slow cooling: a "weak cooling" regime is adopted, with a specific water volume of 0.8-1.2L / kg (segmented control: 0.6-0.8 in the upper section, 1.0-1.2 in the lower section). After exiting the crystallizer, the billet is pile-cooled for ≥24 hours (temperature ≥600℃ before being placed in the slow cooling pit) to eliminate hydrogen-induced cracking.

[0025] Rolling aims to refine the microstructure and homogenize the properties. Temperature-controlled rolling refines the grains, improves microstructure homogeneity, and indirectly stabilizes hardenability. Heating: Soaking temperature 1180-1220℃ (holding time 1.5-2.0h / cm to ensure uniform austenitization), avoiding overheating (grain size ≤50μm). Rough rolling deformation ≥50% (pass reduction 15-25%), finishing rolling initial temperature 900-950℃, finishing rolling temperature 820-860℃ (rolling in the non-recrystallized austenite zone), cumulative deformation ≥80%; Controlled cooling: laminar cooling rate 1-3℃ / s (adjusted according to steel grade, upper limit for Mo-containing steels), final cooling temperature 650-700℃, obtaining a uniform pearlite + ferrite microstructure (grain size ≥8).

[0026] Example 1

[0027] Typical composition: C 0.20%, Si 0.28%, Mn 1.03%, Cr 1.20%, Ti 0.07%, Al 0.020%, S 0.025%, P 0.012%, [O] 12ppm, [N] 45ppm, [H] 1.5ppm, with the remainder being Fe and impurities. Precise composition and high purity are achieved through full-process control of "smelting-refining-continuous casting-rolling". Smelting: Converter smelting is used, with the final C controlled at 0.05~0.10%, P≤0.015%, and tapping temperature 1600~1650℃. LF refining: Si-Mn alloy and Cr ferroalloy are added to adjust the composition; lime is used to create reducing slag; Ti ferroalloy is added (added in-stream to avoid Ti oxidation loss), and S is finely adjusted using MnS alloy (controlling the S / Mn ratio to 0.01~0.03 to ensure MnS morphology). Vacuum degassing: VD vacuum treatment (vacuum degree ≤67Pa, holding for 20~30min) removes H and N, reducing O content to ≤15ppm. Continuous casting: Full-process protective casting (long nozzle + submerged nozzle + mold flux), continuous casting billet cross-section Φ150~300mm, casting speed 0.8~1.2m / min, secondary cooling uses weak cooling (specific water 0.6~0.8L / kg) to avoid internal cracks in the billet. Rolling: Control the rolling process to refine grains, initial rolling temperature 1100~1150℃, final rolling temperature 850~900℃ (avoiding the austenite coarsening zone), post-rolling air cooling or stack cooling to ensure the rolled material microstructure is uniform ferrite + pearlite. Heat treatment: After rolling, isothermal normalizing is carried out (heated to 920~950℃, held for 2~3h, and air-cooled to 650~680℃ for 1~2h) to refine the microstructure (grain size ≤ 8 grade), make the hardness uniform (180~220HBW), and make the banded microstructure ≤ 2.0 grade.

[0028] Sampling was used to test performance. Hardenability Jominy end-quench curve (20℃ water quenching): hardness fluctuation ΔHRC ≤ 5 (hardness range 44~49HRC at 1.5~30mm from the water-cooled end); non-metallic inclusion rating (GB / T 10561): Class A (sulfides) 2.0 grade, Class B (oxides) 0.5 grade, Class C (silicates) 0.5 grade, Class D (spherical oxides) 0.5 grade; large inclusions (≥20μm) ≤ 0.5 per 100g. Rolled material grain size grade 8. Mechanical properties: tensile strength in normalized state 820MPa, 846MPa, 822MPa, tensile strength fluctuation 26MPa. Yield strength 550MPa, elongation after fracture 22%, impact energy (-20℃) 65J, 76J, 69J. Sulfur content was stably controlled, such as... Figure 2 As shown, the sulfur content range is controlled within 0.006%, which improves milling efficiency by 15-20% and extends tool life by 10-15% compared with similar steels without sulfur. This method is applicable to the production of gear steels with high fatigue performance requirements, such as automotive transmission gears and wind turbine gears, and can improve gear contact fatigue life by 15-20%.

[0029] Table 1

[0030]

[0031] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for smelting and rolling high-purity sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability, characterized in that, include: Hot metal pretreatment requires deep dephosphorization and desulfurization to reduce initial harmful elements P and S, thus reducing the burden on subsequent refining. KR mechanical stirring desulfurization is employed, using CaO-CaF2 as the desulfurizing agent at a rate of 8-12 kg / t of iron, with a stirring time of 15-20 min, controlling [S] ≤ 0.005%. Pre-dephosphorization is carried out before the converter using oxidizing slag, controlling [P] ≤ 0.010% to prevent phosphorus redissolution during subsequent refining. Converter steelmaking requires low-oxygen, low-slag molten steel control, the purpose of which is... To reduce steel oxidation and slag contamination, a foundation for pure molten steel is laid for refining; the endpoint adopts high carbon content + endpoint control, with an endpoint carbon content of 0.15-0.25% and a temperature of 1600-1650℃, to reduce refractory material erosion; slag-blocking during tapping uses pneumatic slag-blocking plugs + electromagnetic induction slag blocking, with slag carryover during tapping ≤5kg / t, preventing FeO and P2O5 in the slag from contaminating the molten steel; during tapping deoxidation, Si-Mn alloy pre-deoxidation is added with the steel stream to inhibit premature formation of Al2O3 inclusions; LF refining precisely adjusts alloy composition, performs deep deoxidation, and removes inclusions by creating reducing slag. Composition fine-tuning is achieved through online direct-reading spectroscopy analysis, real-time control of hardenability elements such as Mn, Cr, and Ti, employing a "batch addition + uniform stirring" strategy. For composite deoxidation, Al (0.5-0.8 kg / t) is added first for strong deoxidation, followed by weak deoxidation with Si-Mn alloy, controlling Al content to 0.025-0.035%. The reducing slag system requires a binary basicity of CaO / SiO2 = 5-7, and MgO... 8-12%, FeO+MnO≤0.5%, promotes the transformation of inclusions into low-melting-point composite inclusions and their flotation, refining time 40-60min, ensuring inclusion flotation rate ≥90%; Calcium treatment: feed Ca-Si wire, addition amount 0.2-0.4kg / t, control [Ca] 0.0015-0.0030%, Ca / S=1.2-2.0, transform MnS plastic inclusions into spherical CaS or CaO-Al2O3-CaS composite inclusions; VD vacuum degassing and deep purification of inclusions aim to reduce [H] and [N], promote the aggregation and flotation of small inclusions; vacuum degree ≤67Pa, holding time 20-30min, stirring intensity 0.10-0.15kW / t, argon flow rate 0.5-1.0L / min·t; control [H] ≤2ppm, [N] ≤40ppm, steel liquid circulation ≥3 times; To avoid center segregation, porosity, and subcutaneous inclusions, and to ensure the uniformity of the billet composition, continuous casting requires low superheat casting, with the molten steel in the tundish superheated at 20-30℃, and the liquid level fluctuation in the crystallizer ≤±3mm; the electromagnetic stirring crystallizer (EMS) + solidification end EMS, with strengths of 0.3-0.5T and 0.2-0.4T respectively, breaks up columnar crystals, promotes an equiaxed crystal ratio ≥50%, and reduces center segregation; secondary cooling and slow cooling: adopts a "weak cooling" system, with a specific water volume of 0.8-1.2L / kg, controlled in stages: 0.6-0.8L / kg in the upper stage and 1.0-1.2L / kg in the lower stage. After the billet exits the crystallizer, it is pile-cooled for ≥24 hours, and the temperature is ≥600℃ before being placed in the slow cooling pit to eliminate hydrogen-induced cracks; Rolling aims to refine the microstructure and homogenize the properties. Temperature-controlled rolling refines the grains, improves the microstructure uniformity, and indirectly stabilizes hardenability. Heating: Homogenization temperature 1180-1220℃ to ensure uniform austenitization and avoid overheating; rough rolling deformation ≥50%, pass reduction 15-25%, finish rolling start temperature 900-950℃, finish rolling temperature 820-860℃, cumulative deformation ≥80%; Controlled cooling: laminar cooling rate 1-3℃ / s, final cooling temperature 650-700℃ to obtain a uniform pearlite + ferrite microstructure; The chemical composition of the gear steel by mass percentage is as follows: C: 0.19-0.22%, Si: 0.21-0.28%, Mn: 1.00-1.05%, P: ≤0.020%, S: 0.015-0.030%, Cr: 1.20-1.25%, Ti: 0.06-0.09%, Al: 0.010-0.050%, with the balance being Fe and unavoidable impurity elements.

2. The method for smelting and rolling high-purity sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability according to claim 1, characterized in that, The main components of the desulfurizing agent are: CaO 80-85%, CaF2 10-15%.

3. The method for smelting and rolling high-purity sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability according to claim 1, characterized in that, In the "batch addition + uniform mixing" strategy: the mixing power is 0.05-0.08kW / t, and the time is 5-8min / batch.

4. The method for smelting and rolling high-purity sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability according to claim 1, characterized in that, An immersion-type inlet and flow control device are used to prevent slag entrapment.

5. The method for smelting and rolling high-purity sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability according to claim 1, characterized in that, Compositional fluctuations: Mn±0.03%, Cr±0.02%, Ti±0.01%.

6. The method for smelting and rolling high-purity sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability according to claim 1, characterized in that, Hardenability bandwidth J9 / J15≤5HRC; Cleanliness: [O]≤15ppm, [H]≤2ppm, [N]≤50ppm, Non-metallic inclusions A / B / C / D class≤1.0 grade, Large inclusions≤0.5 pieces / 100g.

7. The method for smelting and rolling high-purity sulfur-containing Cr-Mn-Ti gear steel with narrow hardenability according to claim 1, characterized in that, Microstructure and properties: Porosity at the center of the billet ≤ 1.0 grade, segregation ≤ 1.0 grade; grain size of rolled material ≥ 8 grade, tensile strength fluctuation ≤ 30 MPa.

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