Method for preparing fine-grain stellite alloy plate through vertical reducing hot rolling

Abstract

The invention provides a method for preparing a fine-grain stellite alloy plate through vertical reducing hot rolling. Compared with a traditional symmetric rolling process, the preparation method adopts a vertical reducing deformation hot rolling process, rolling cracks of the stellite alloy plate are eliminated, additional shear strain is introduced in the thickness direction of the stellite alloy plate through reducing rollers and the linear velocity difference of the reducing rollers, more uniform plastic flow in the material is promoted, and the stellite alloy plate is prepared. And the problem of uneven surface and core deformation in traditional symmetric rolling is relieved, and the crack risk caused by local stress concentration is reduced. And meanwhile, the deformation uniformity in the rolling process is improved, grains of the stellite plate are refined, residual stress is reduced, the surface quality of the plate is improved, rolling force is reduced, passes are reduced, and the service life of the roller is prolonged.

Description

Technical Field

[0001] This invention belongs to the technical field of Stellite high-temperature alloy plates used in major engineering machinery equipment, nuclear power, petrochemical, aerospace and other industries. Specifically, it relates to a method for preparing fine-grained Stellite alloy plates, and more particularly to a method for preparing fine-grained Stellite alloy plates by vertical differential deformation hot rolling. Background Technology

[0002] Stellite alloys are high-performance metallic materials with cobalt as the base material, containing chromium (Cr), tungsten (W), carbon (C), molybdenum (Mo), and small amounts of nickel (Ni), silicon (Si), manganese (Mn), and iron (Fe). Studies have shown that Stellite alloys possess excellent wear resistance and erosion resistance, making them particularly suitable for high-load, high-speed friction environments. Furthermore, Stellite alloys exhibit good resistance to cavitation erosion, mortar abrasion, corrosion resistance, and high-temperature resistance.

[0003] Unlike Ni-based and Fe-based superalloys, Stellite alloys do not employ precipitated phases (γ') coherently bonded to the matrix for strengthening; instead, they rely on carbides and solid-solution elements for reinforcement. Therefore, Stellite alloys have a very high carbon content (around 1 wt%), far exceeding that of ordinary superalloys. Some Stellite alloy grades even have carbide contents exceeding 10 wt%, giving the alloys both excellent mechanical properties and high wear resistance. However, while the high carbon content produces excellent properties, it also leads to high brittleness and low plasticity, making rolling very difficult. Therefore, a suitable rolling process is crucial for the successful preparation of high-performance Stellite alloy sheets. Summary of the Invention

[0004] In view of this, the object of the present invention is to provide a method for preparing fine-grained Stellite alloy sheets by vertically variable diameter hot rolling. This method can improve the performance and surface quality of the sheet, and reduce the risk of cracking.

[0005] To achieve this objective, the present invention adopts the following technical solution:

[0006] This invention provides a method for preparing fine-grained Stellite alloy plates by vertically variable diameter hot rolling, comprising the following steps:

[0007] Stellite alloy slabs are obtained by vertical hot rolling with different diameters;

[0008] The hot rolling is carried out using upper and lower rolls of different diameters;

[0009] The diameter ratio of the lower roll to the upper roll is (1.1 to 1.3):1;

[0010] The rolling speed of the lower roll is 30-40 m / min, and the rolling speed ratio of the upper roll linear speed to the lower roll linear speed is (1.05-1.2):1.

[0011] Preferably, during the vertical differential hot rolling process, the deformation per pass is 5-15%.

[0012] Preferably, the upper and lower rolls are preheated.

[0013] Preferably, the temperature of the preheating treatment to the surface of the upper and lower rolls is 200–400°C.

[0014] Preferably, the Stellite alloy slab is heat-treated before being subjected to vertical differential hot rolling.

[0015] Preferably, the heat treatment temperature is 1130–1190°C, and the holding time is 10–20 min.

[0016] Preferably, the heat penetration rate of the heat treatment is 0.5 to 1.0 mm / min.

[0017] Preferably, after the heat treatment is completed, the interval between the vertical differential hot rolling is less than 10 seconds.

[0018] Preferably, after the vertical differential hot rolling is completed, the obtained Stellite alloy sheet is heated to 400-600°C, held for 5-10 minutes, and then leveled.

[0019] Preferably, the leveling process further includes a step of removing oxide scale from the board.

[0020] Preferably, the thickness of the final Stellite alloy sheet is not higher than 1.2 mm and the surface roughness is not higher than 1 μm.

[0021] Preferably, the final Stellite alloy sheet has a tensile strength > 1000 MPa, elongation ≥ 8%, hardness ≥ 46 HRC, and grain size finer than grade 5.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] This invention addresses the problems of crack sensitivity and poor microstructure uniformity in Stellite alloy sheets by designing a vertically variable diameter deformation hot rolling process. This process improves the rolling uniformity and yield, ensuring smooth hot rolling operations. Compared to traditional symmetrical rolling processes, the proposed vertically variable diameter deformation hot rolling process eliminates rolling cracks in Stellite alloy sheets. By using rolls of different diameters and their varying linear speeds, additional shear strain is introduced along the thickness direction of the Stellite alloy sheet, promoting more uniform plastic flow within the material. This alleviates the problem of uneven deformation between the surface and core in traditional symmetrical rolling, reducing the risk of cracks caused by localized stress concentration. Simultaneously, it improves deformation uniformity during rolling, refines the grain size of the Stellite sheet, reduces residual stress, improves surface quality, resulting in a smoother surface, reduces rolling force, and decreases the number of passes, thus extending roll life.

[0024] The vertical differential deformation hot rolling process provided by this invention offers a three-in-one solution for Stellite alloy plates: a more uniform microstructure, lower crack initiation, and higher production efficiency. This provides broad application prospects for the preparation of high-performance Stellite alloy plates in high-end fields such as major equipment, nuclear power, petrochemicals, and aerospace. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] To address the problems of high rolling difficulty, poor microstructure uniformity, easy cracking, and low yield of high-quality Stellite alloy plates, this invention proposes a process for hot rolling high-quality fine-grained Stellite alloy plates with vertical differential deformation.

[0027] Specifically, the present invention provides a method for hot-rolling high-quality fine-grained Stellite alloy sheet with vertically shaped deformation, comprising the following steps:

[0028] Stellite alloy slabs are obtained by vertical hot rolling with different diameters.

[0029] According to the present invention, a Stellite alloy slab is first provided.

[0030] In this invention, the Stellite alloy slab is first sawn to a size suitable for rolling. Then, a milling machine is used to machine the upper and lower surfaces of the slab, removing surface defects such as pits, oxide scale, inclusions, and folds. In some embodiments of this invention, the Stellite alloy slab can be sawn to a size of 300mm long × 200mm wide × 50mm thick; or 400mm long × 260mm wide × 55mm thick. This invention does not impose any particular limitation on these dimensions; those skilled in the art can design them as needed.

[0031] After the above processing is completed, the Stellite alloy slab is preferably placed in a high-temperature heating furnace and heated at a heat penetration rate of 0.5–1 mm / min and a heating temperature of 1130–1190°C. After the center of the slab is fully heated, it is held at that temperature for 10–20 minutes. Then, it is removed from the furnace and subjected to vertical unequal diameter hot rolling. The purpose of the above heating is to improve the high-temperature plasticity of the material and reduce its deformation resistance.

[0032] The aforementioned heat penetration rate of 0.5 to 1 mm / min can be 0.5 mm / min, 0.6 mm / min, 0.7 mm / min, 0.8 mm / min, 0.9 mm / min, or 1 mm / min, etc.

[0033] The heating temperature of 1130 to 1190°C can be 1130°C, 1140°C, 1150°C, 1160°C, 1170°C, 1180°C, or 1190°C, etc.

[0034] The above-mentioned heat preservation time is 10 to 20 minutes, which can be 10 minutes, 11 minutes, 12 minutes, 13 minutes, 14 minutes, 15 minutes, 16 minutes, 17 minutes, 18 minutes, 19 minutes, or 20 minutes, etc.

[0035] Then, according to the present invention, the heated slab is taken out of the furnace and sent to the rolling mill for vertical differential hot rolling.

[0036] In this invention, the axes of the upper and lower rolls of the rolling mill are both on a vertical plane and parallel to each other. The diameter ratio of the lower roll to the upper roll ranges from (1.1 to 1.3):1, such as 1.1:1, 1.15:1, 1.2:1, 1.25:1, or 1.3:1. In this invention, the roll diameter setting utilizes a smaller roll to apply shear force and a larger roll to focus on compressive force. By dispersing the deformation resistance of the Stellite sheet, not only can the mill load be reduced, but the service life of the rolls can also be extended.

[0037] In this invention, the upper and lower rolls are preferably preheated before hot rolling. In some embodiments of this invention, flame heating is preferably used to uniformly heat the surfaces of the upper and lower rolls to 200–400°C, such as 200°C, 220°C, 250°C, 280°C, 300°C, 320°C, 350°C, 380°C, or 400°C, to avoid excessively rapid local cooling and cracking of the sheet material during rolling, as this would occur when the sheet material comes into contact with the rolls.

[0038] In this invention, the rolling speed range of the lower roll is controlled to be 30–40 m / min, such as 30 m / min, 31 m / min, 32 m / min, 33 m / min, 34 m / min, 35 m / min, 36 m / min, 37 m / min, 38 m / min, 39 m / min, or 40 m / min, etc.; the rolling speed ratio of the upper roll linear speed to the lower roll linear speed is controlled to be (1.05–1.2):1, such as 1. The ratios are 0.05:1, 1.1:1, 1.15:1, or 1.2:1, etc.; the time for the slab to be transferred from the furnace to the rolling mill should be less than 10 seconds to reduce temperature drop and ensure the high-temperature plasticity of the material. Specific times could be 5s, 6s, 7s, 8s, or 9s, etc.; the deformation per pass should be controlled at 5-15% to avoid rolling cracks. Specific values ​​could be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%, etc. After one pass of rolling is completed, the slab is flipped and rolled in the next pass. This process is repeated until the slab thickness reaches the required dimensional specifications.

[0039] It should be noted that if the rolling speed of the lower roll is not within the range of 30 to 40 m / min, and / or the rolling speed ratio of the upper roll linear speed to the lower roll linear speed is not within the range of (1.05 to 1.2):1, and / or the deformation amount per pass is not controlled within the range of 5 to 15%, the mechanical properties of the obtained sheet will decrease or cracking will occur, and it will not meet the usage requirements.

[0040] The present invention employs the above-mentioned vertical differential hot rolling, which has the following advantages:

[0041] (1) Different stress states in the deformation zone. The stress field of differential rolling is a combination of shear stress and compressive stress. The difference in linear velocity between the upper and lower rolls introduces a shear component in the thickness direction of the plate, promoting plastic flow in the core of the Stellite plate and reducing deformation inhomogeneity (strain uniformity is improved by 20-40% compared to traditional symmetrical rolling). At the same time, the rolling stress field forms a higher triaxial compressive stress, which inhibits grain boundary cracking of the hot brittle phase of the alloy (crack rate is reduced by more than 50%). Traditional symmetrical rolling (i.e., same diameter rolling) is a symmetrical stress field (mainly composed of simple compressive stress). Symmetrical deformation leads to a large strain gradient between the surface and core of the plate, which can cause cracks in the core of the alloy plate due to insufficient plasticity.

[0042] (2) The microstructure of the sheet metal is more optimized. The shear strain during the differential rolling process provides additional energy storage, reducing the critical strain for dynamic recrystallization of Stellite alloy (critical strain is reduced by 15-25%), and further refining the grain size. Furthermore, it breaks the traditional rolling texture of sheet metal, reducing anisotropy (lateral elongation is increased by 10-15%). In contrast, the same-diameter rolling process is limited to strong compressive strain, which easily leads to grain elongation along the rolling direction, forming a banded structure, and incomplete dynamic recrystallization, especially for Stellite alloys with high carbide strengthening phase content.

[0043] (3) Higher surface quality and shape accuracy of the sheet metal. The shearing effect generated by the speed difference in differential rolling can break the oxide layer (such as Cr2O3) on the surface of Stellite alloy sheet metal, reduce oxide scale indentation defects (the surface defect density is reduced by 30-50%), and improve surface quality. By adjusting the roll diameter ratio, the thinning of the sheet metal edges can be compensated, and the thickness tolerance can be controlled within ±0.05mm (better than ±0.1mm in same-diameter rolling). Same-diameter rolling is suitable for conventional alloy sheet metal with low deformation resistance and low requirements for microstructure uniformity, or for the roughing stage of thick plates. The differential rolling method proposed in this invention effectively solves the bottleneck technical problems of poor microstructure uniformity and crack sensitivity in the processing of Stellite high-temperature alloy sheet metal in same-diameter rolling, and is an innovative process for the preparation of high-performance Stellite alloy sheet metal.

[0044] In some preferred embodiments of the present invention, after the vertical differential hot rolling is completed, the obtained Stellite alloy sheet is preferably placed in a heating furnace and heated to 400-600°C, such as 400°C, 420°C, 450°C, 480°C, 500°C, 520°C, 550°C, 580°C, or 600°C, and held at that temperature for 5-10 minutes, such as 5 minutes, 6 minutes, 7 minutes, 8 minutes, 9 minutes, or 10 minutes. This is to ensure the high-temperature plasticity of the material and avoid rolling cracking.

[0045] Then, after being taken out of the oven, it is leveled multiple times using a multi-roller leveling machine.

[0046] In some preferred embodiments of the present invention, after the leveling is completed, the oxide scale on the leveled Stellite alloy sheet is preferably removed by alkaline degradation and acid pickling.

[0047] Finally, the microstructure, mechanical properties and dimensional specifications of the rolled Stellite alloy sheets are inspected, and qualified sheets are packaged and put into storage for shipment.

[0048] In summary, in a preferred embodiment of the present invention, the method for preparing Stellite alloy plates includes the following steps:

[0049] (1) Stellite alloy slab machining. Stellite slabs are sawn to the length × width × thickness dimensions suitable for rolling, and then the upper and lower surfaces of the slabs are machined by a milling machine to remove defects such as pits, oxide scale, inclusions, and folds from the clean surface;

[0050] (2) Roll diameter ratio range setting. The diameter ratio of the lower roll to the upper roll is (1.1~1.3):1;

[0051] (3) Roll preheating. Use flame heating to evenly heat the surfaces of the upper and lower rolls to 200-400°C to avoid the plate coming into contact with the rolls during rolling, which could cause localized rapid cooling and cracking of the plate.

[0052] (4) Slab heating. The Stellite alloy slab is placed in a high-temperature heating furnace and heated at a heat penetration rate of 0.5-1 mm / min, with a heating temperature of 1130-1190℃. After the center of the slab is fully heated, it is held at that temperature for 10-20 minutes. Then it is removed from the furnace and rolled.

[0053] (5) Vertical Differential Deformation Hot Rolling. The heated slab is removed from the furnace and fed into a rolling mill. The upper and lower roll axes of the rolling mill are both on a vertical plane and parallel to each other. The rolling speed of the lower roll is controlled within the range of 30–40 m / min, the rolling speed ratio of the upper roll linear speed to the lower roll linear speed is controlled at (1.05–1.2):1, the time for the slab to be transferred from the furnace to the rolling mill is required to be less than 10 seconds, and the deformation per pass is controlled at 5–15%. After one pass of rolling is completed, the slab is flipped and rolled in the next adjacent pass. This process is repeated until the slab thickness reaches the required dimensional specifications.

[0054] (6) Plate leveling. The rolled Stellite alloy plate is placed in a heating furnace, heated to 400-600℃, held for 5-10 minutes, removed from the furnace, and leveled in multiple passes using a multi-roll leveling machine;

[0055] (7) Removal of oxide scale. The oxide scale on the leveled Stellite alloy plates was removed by alkaline chipping and acid pickling;

[0056] (8) Inspection. The rolled Stellite alloy plates are inspected for microstructure, mechanical properties and dimensional specifications. Qualified plates are packaged and put into storage for shipment.

[0057] In this invention, the thickness of the final Stellite alloy sheet obtained by the above method is no more than 1.2 mm, and the surface roughness is no more than 1 μm.

[0058] Tests showed that the final Stellite alloy sheet obtained by the above method has a tensile strength >1000MPa, elongation ≥8%, hardness ≥46HRC, and grain size finer than grade 5.

[0059] To further illustrate the present invention, the following embodiments provide a detailed description. The experimental materials used in the following embodiments of the present invention are all commercially available products.

[0060] Example 1

[0061] (1) Stellite 6K alloy slab machining. Stellite saws slabs with a length of 300mm × width of 200mm × thickness of 50mm. The upper and lower surfaces of the slabs are machined using a milling machine to remove defects such as pits, oxide scale, inclusions, and folds.

[0062] (2) Roll diameter ratio range setting. The diameter ratio of the lower roll to the upper roll is 1.1;

[0063] (3) Roll preheating. Use flame heating to uniformly heat the surfaces of the upper and lower rolls to 200°C to avoid the plate coming into contact with the rolls during rolling, which could cause localized rapid cooling and cracking of the plate.

[0064] (4) Slab heating. The Stellite alloy slab is placed in a high-temperature heating furnace and heated. According to the heat penetration rate of 0.5 mm / min, the heating temperature is 1130℃. After the center of the slab is heated through, it is held at the temperature for 20 minutes before being taken out of the furnace for rolling.

[0065] (5) Vertical Differential Deformation Hot Rolling. The heated slab is removed from the furnace and fed into a rolling mill. The upper and lower roll axes are both on a vertical plane and parallel to each other. The lower roll rolling speed is 40 m / min, the ratio of the upper roll linear speed to the lower roll linear speed is controlled at 1.05, the time for the slab to be transferred from the furnace to the rolling mill is 4 seconds, and the deformation per pass is controlled at 5%. After one pass of rolling is completed, the slab is flipped and rolled in the next adjacent pass. This process is repeated until the slab thickness meets the required product specifications (1.0–5.0 mm).

[0066] (6) Plate leveling. The rolled Stellite alloy plate is placed in a heating furnace, heated to 400°C, held for 5 minutes, removed from the furnace, and leveled in multiple passes using a multi-roll leveling machine;

[0067] (7) Removal of oxide scale. The oxide scale on the leveled Stellite alloy plates was removed by alkaline chipping and acid pickling;

[0068] (8) Inspection. The rolled Stellite alloy plates are inspected for microstructure, mechanical properties and dimensional specifications. Qualified plates are packaged and put into storage for shipment.

[0069] Upon observation, the obtained Stellite alloy sheet has a smooth and flat surface without cracks.

[0070] Comparative Example 1

[0071] This comparative example provides a common rolling process. Compared with Example 1, the only difference is that in step (5), the rolling speed of the lower roll is 30 m / min, and the rolling speed ratio of the upper roll linear speed to the lower roll linear speed is controlled to be 1. The other parameters and steps are the same as in Example 1.

[0072] Upon observation, the surface of the obtained Stellite alloy sheet had multiple cracks, failing to meet the usage requirements.

[0073] The Stellite alloy plates obtained in Example 1 and Comparative Example 1 were tested using the following methods: GB / T228.1 Metallic materials—Tensive testing—Part 1: Test methods at room temperature; GB / T230.1 Metallic materials—Rockwell hardness testing—Part 1: Test methods; GB / T 6394 Metallic materials—Mean average grain size test method.

[0074] The test results are shown in Table 1:

[0075] Table 1

[0076] Group Tensile strength (MPa) Elongation (%) Hardness (HRC) Grain size Example 1 1010 8 43 6 Comparative Example 1 960 4.5 39 4

[0077] Example 2

[0078] (1) Machining of Stellite 6B alloy slab. Stellite slab is sawn to a size suitable for rolling, 400mm long × 260mm wide × 55mm thick. The upper and lower surfaces of the slab are machined by milling to remove defects such as pits, oxide scale, inclusions, and folds.

[0079] (2) Roll diameter ratio range setting. The diameter ratio of the lower roll to the upper roll is 1.3;

[0080] (3) Roll preheating. Use flame heating to evenly heat the surfaces of the upper and lower rolls to 400°C to avoid the plate coming into contact with the rolls during rolling, which could cause localized rapid cooling and cracking of the plate.

[0081] (4) Slab heating. The Stellite alloy slab is placed in a high-temperature heating furnace and heated. According to the heat penetration rate of 0.5 mm / min, the heating temperature is 1190℃. After the center of the slab is heated through, it is held at the temperature for 10 minutes before being taken out of the furnace for rolling.

[0082] (5) Vertical differential deformation hot rolling. The heated slab is removed from the furnace and fed into the rolling mill for rolling. The rolling speed of the lower roll is controlled at 30 m / min, the rolling speed ratio of the upper roll linear speed to the lower roll linear speed is controlled at 1.2, the time for the slab to be transferred from the furnace to the rolling mill is 10 s, and the deformation amount per pass is controlled at 15%. After one pass of rolling is completed, the adjacent next pass is rolled after flipping. The above process is repeated until the slab thickness reaches the required product specifications (1.0~5.0 mm);

[0083] (6) Plate leveling. The rolled Stellite alloy plate is placed in a heating furnace, heated to 600°C, held for 10 minutes, removed from the furnace, and leveled in multiple passes using a multi-roll leveling machine;

[0084] (7) Removal of oxide scale. The oxide scale on the leveled Stellite alloy plates was removed by alkaline chipping and acid pickling;

[0085] (8) Inspection. The rolled Stellite alloy plates are inspected for microstructure, mechanical properties and dimensional specifications. Qualified plates are packaged and put into storage for shipment.

[0086] Comparative Example 2

[0087] This comparative example provides a common rolling process. Compared with Example 2, the only difference is that in step (5), the rolling speed of the lower roll is 25 m / min, and the rolling speed ratio of the upper roll linear speed to the lower roll linear speed is controlled to be 1. The other parameters and steps are the same as in Example 2.

[0088] The Stellite alloy plates obtained in Example 2 and Comparative Example 2 were tested using the methods described above.

[0089] The test results are shown in Table 2:

[0090] Table 2

[0091] Group Tensile strength (MPa) Elongation (%) Hardness (HRC) Grain size Example 2 1005 6 44 6 Comparative Example 2 983 4.0 40 4

[0092] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing fine-grained Stellite alloy plates by vertical differential hot rolling, characterized in that, Includes the following steps: Stellite alloy slabs are obtained by vertical hot rolling with different diameters; The hot rolling is carried out using upper and lower rolls of different diameters; The diameter ratio of the lower roll to the upper roll is (1.1 to 1.3):1; The rolling speed of the lower roll is 30-40 m / min, and the rolling speed ratio of the upper roll linear speed to the lower roll linear speed is (1.05-1.2):

1.

2. The method according to claim 1, characterized in that, During the vertical differential hot rolling process, the deformation per pass is 5-15%.

3. The method according to claim 1 or 2, characterized in that, The upper and lower rolls are preheated. The preheating treatment is carried out to a temperature of 200–400°C on the surfaces of the upper and lower rolls.

4. The method according to any one of claims 1 to 3, characterized in that, The Stellite alloy slab is heat-treated and then subjected to vertical differential hot rolling. The heat treatment temperature is 1130–1190℃, and the holding time is 10–20 min.

5. The method according to claim 4, characterized in that, The heat penetration rate of the heat treatment is 0.5 to 1 mm / min.

6. The method according to claim 4 or 5, characterized in that, After the heat treatment is completed, the interval between vertical unequal diameter hot rolling is less than 10 seconds.

7. The method according to any one of claims 1 to 6, characterized in that, After the vertical differential hot rolling is completed, the resulting Stellite alloy sheet is heated to 400-600℃, held for 5-10 minutes, and then leveled.

8. The method according to any one of claims 1 to 7, characterized in that, The leveling process also includes a step of removing the oxide scale from the board.

9. The method according to any one of claims 1 to 8, characterized in that, The final Stellite alloy sheet has a thickness of no more than 1.2 mm and a surface roughness of no more than 1 μm.

10. The method according to any one of claims 1 to 9, characterized in that, The resulting Stellite alloy sheet has a tensile strength >1000MPa, elongation ≥8%, hardness ≥46HRC, and grain size finer than grade 5.