Ultra-thin low-carbon steel and manufacturing method thereof

Abstract

The present invention provides an ultra-thin low-carbon steel and a manufacturing method thereof. The method utilizes a rational smelting, continuous casting, heating, rolling, cooling, and coiling process, wherein a "Z-type" cooling process is employed. The length L1 of the high-temperature zone at the head of the hot coil is set according to the thickness h of the finished strip, L1 = (100-120) h, where L1 is expressed in meters and h is expressed in millimeters. The coiling temperature CT1 of the high-temperature zone at the head of the hot coil is set to CT + ΔT, where ΔT is 40-60°C. The coiling temperatures at the middle and tail of the hot coil are both set to CT = 600-650°C. This method solves the problem of large fluctuations in the mechanical properties, especially the plastic strain ratio, of ultra-thin low-carbon steel produced through conventional hot rolling processes. The method improves the uniformity of the mechanical properties of the low-carbon steel through coil and reduces the probability of cracking during stamping. Furthermore, the method requires no additional equipment investment and is cost-effective.

Description

Technical Field

[0001] The invention belongs to the field of steel production, and in particular relates to an extremely thin low-carbon steel and a manufacturing method thereof. Background Art

[0002] Due to its excellent formability and cost-effectiveness, low-carbon steel is widely used in home appliances, hardware, food packaging, and other fields. In recent years, with the increasing demand for "hot-to-cold" plate and strip products, the demand for extremely thin strip (≤2.0mm) has continued to increase. According to the production process, extremely thin strip steel can be produced by thin slab continuous casting and rolling processes and traditional hot rolling processes. Considering factors such as the number of inclusions, surface quality, and strength, the traditional hot rolling process is still the mainstream production process. Due to the use of an accelerated rolling process, the strip threading speed is close to the equipment capacity limit, and the rolling speed at the head of the hot coil is lower than that of the middle and tail sections. As a result, the traditional hot rolling process for producing extremely thin low-carbon steel often has a problem of "lower at the head and higher in the middle and tail sections" along the length of the hot coil. The head of the hot coil is 40-60℃ or even higher than the middle or tail sections. This leads to large fluctuations in the mechanical properties of the low-carbon steel hot coil at the beginning, middle, and end. In particular, large fluctuations in the plastic strain ratio directly affect the material's stamping performance and affect downstream users.

[0003] Therefore, attention should be paid to the large fluctuations in the mechanical properties of ultra-thin low-carbon steel coils produced by the traditional hot rolling process, especially the plastic strain ratio.

[0004] The patent with publication number CN110614280A, published on December 27, 2019, discloses a method for online temperature homogenization of hot coils. After hot rolling, the coiled and bundled hot coils are sent to an online mobile enclosed space for insulation and / or heating within 15 minutes. At the same time, they are directly transferred to the cold rolling unit via a steel coil transport chain or a mobile trolley. During the above-mentioned hot coil transportation process, the hot coils are subjected to online insulation treatment in a closed space; the coiling temperature is controlled at 350-650°C, the online insulation time is at least 1 hour, and the hot coils are directly sent to the cold rolling unit. It is to add an online insulation cover equipment after the coiler to improve the temperature uniformity and performance uniformity of the strip in the length and width direction. This method requires new equipment investment and has no significant effect on improving the performance uniformity of fine-grained + solid solution strengthened low-carbon steel. Summary of the Invention

[0005] The present invention aims to provide an ultra-thin low-carbon steel and its manufacturing method. By utilizing a rational smelting, continuous casting, heating, rolling, cooling, and coiling process, the present invention addresses the problem of large fluctuations in mechanical properties, particularly the plastic strain ratio, of ultra-thin low-carbon steel produced through conventional hot rolling processes. This improves the uniformity of the mechanical properties of the low-carbon steel through coiling and reduces the probability of cracking during stamping. Furthermore, the method eliminates the need for additional equipment investment and is cost-effective.

[0006] The specific technical solutions of the present invention are as follows:

[0007] A method for manufacturing ultra-thin low-carbon steel comprises converter smelting, LF furnace refining, continuous casting, heating, rolling, cooling and coiling.

[0008] After converter smelting and LF furnace refining, alloy is added to the converter steel. The alloy auxiliary material of Si is high-purity ferrosilicon with the chemical element B content ≤ 0.0005%. Manganese silicon alloy or ferrosilicon 75B alloy is not used.

[0009] The continuous casting process has a billet thickness of 230 mm;

[0010] The heating process is such that the temperature of the billet out of the furnace is 1150-1200°C, and the temperature at the head 0-2m in the length direction of the billet is 20-40°C higher than the temperature at the tail 0-2m. The billet is kept in the furnace for 150-200 minutes.

[0011] The rolling is carried out by using a 2-stand roughing rolling and a 7-stand finishing hot rolling mill group. The roughing mill group adopts a 3+3 rolling mode. The third pass rolling speed of the R2 stand is ≥4.0 m / s. The thickness H of the intermediate billet after roughing and the thickness h of the finished strip steel must satisfy the relationship H=(20~25)×h; the unit of H is mm, and the unit of h is mm.

[0012] The insulation covers between the rolling, roughing and finishing mills are all put into use;

[0013] In the rolling process, the F2-F6 finishing mills use a roll gap lubrication process, the volume ratio of oil and water in the lubricating liquid is 0.30%-0.60%, the lubricating liquid flow rate is 80-120 ml / min, and the final rolling temperature FDT of the F7 finishing mill is 830-880°C.

[0014] The cooling process involves laminar cooling after finish rolling, using a "Z-type" cooling process. The length of the hot coil head high-temperature zone, L1, is set based on the finished strip thickness, h: L1 = (100-120) × h, where L1 is expressed in meters and h is expressed in millimeters. Substitute the values ​​before the units for direct calculation. The coiling temperature, CT1, of the hot coil head high-temperature zone, L1, is CT + ΔT, where ΔT = 40-60°C. The coiling temperatures at the middle and tail of the hot coil are both CT = 600-650°C, with CT1, CT, and ΔT all expressed in degrees Celsius. After coiling, the hot coil is air-cooled to room temperature. The tail of the hot coil refers to the area within 150 meters of the tail.

[0015] The present invention provides an ultra-thin low-carbon steel, which is manufactured by the above method. The ultra-thin low-carbon steel comprises the following chemical components in weight percentage: C 0.020-0.060%; Si

[0016] 0.010~0.060%; Mn 0.08~0.30%; B≤0.0005%; Als 0.015~0.050%, the rest is Fe and inevitable inclusions.

[0017] The thickness of the ultra-thin low-carbon steel is ≤2.0 mm;

[0018] The ultra-thin low-carbon steel hot coil has an average yield strength of 265-270 MPa at the head, middle and tail, an average tensile strength of 366-375 MPa, an average hardness of 59-61 HRB, and an average plastic strain ratio r=0.99-1.01.

[0019] The yield strength fluctuation range of the ultra-thin low-carbon steel hot-rolled coil at the head, middle, and tail is ≤10 MPa, the tensile strength fluctuation range is ≤10 MPa, the hardness fluctuation range is ≤5 HRB, and the plastic strain ratio r fluctuation range is ≤0.05. The head refers to the first 6m of the hot-rolled coil, the tail refers to the last 6m of the hot-rolled coil, and the middle refers to the center length of the hot-rolled coil.

[0020] The inventors have found that the existing technologies mainly improve the uniformity of the mechanical properties of steel strips through the following methods: (1) studying how to improve the uniformity of the performance of medium and thick plates; (2) improving the uniformity of the performance of steel strips in the longitudinal direction by optimizing the continuous annealing process; (3) improving the uniformity of the performance in the width direction of the steel strip, and not in the longitudinal direction; (4) improving the uniformity of the performance in the longitudinal direction of the steel strip through segmented head and tail shielding or U-shaped cooling process; (5) adding online heat preservation cover equipment. None of the above methods are suitable for improving the uniformity of the mechanical properties of ultra-thin low-carbon steel coils. Therefore, the present invention provides a method for improving the uniformity of the mechanical properties of ultra-thin low-carbon steel coils without increasing equipment investment.

[0021] The design ideas of the present invention are as follows: The present invention limits the category of alloy auxiliary materials of Si, mainly because manganese silicon alloy or ferrosilicon 75B alloy contains a certain amount of residual alloying element B, and the content of B element is relatively high, up to 0.0015%. As a typical grain boundary segregation element, B element will significantly inhibit the nucleation and growth of proeutectoid ferrite recrystallization, refine the grain size of the material, and improve the strength of the material. At the same time, it will also reduce the content of {111} texture components and reduce the plastic strain ratio of the material, which is not conducive to the deep drawing performance of the material. Therefore, in order to avoid the influence of B on the strength and deep drawing performance of the material, the alloy auxiliary material category of Si adopts high-purity ferrosilicon, and its chemical element B content is ≤0.0005%.

[0022] In the heating process design, the design of the slab discharge temperature and furnace time is primarily based on the need to control the surface oxide scale of the strip and improve its surface quality. To reduce the temperature drop at the head of the hot coil and improve the uniformity of the finishing temperature and mechanical properties at the head and middle of the hot coil, the slab heating process is designed with temperature compensation, with the head of the slab being 20-40°C higher than the tail along the length of the slab.

[0023] In the rolling process design of the present invention, the rough rolling adopts a high-temperature fast rolling strategy, and the rolling speed of the R2 last pass is controlled to be ≥4.0m / s; the rough rolling R1 and R2 stands select the "3+3" mode for rolling passes. Compared with the "3+5" mode, it can significantly reduce the residence time of the intermediate billet in the roughing mill by 16-20s and reduce the temperature drop of the intermediate billet by 15-20°C. The full use of the insulation covers between the roughing and finishing mills is conducive to reducing the temperature drop of the intermediate billet and improving the subsequent temperature difference between the head and tail of the hot coil. The thickness H of the intermediate billet and the thickness h of the hot coil product must meet the relationship: H = (20-25) × h. Properly increasing the thickness of the intermediate billet is conducive to reducing the temperature drop of the intermediate billet and increasing the temperature of the intermediate billet. If the intermediate billet thickness is too thin, it is easy to cause excessive temperature drop at the head and tail of the intermediate billet and a larger temperature difference at the head and tail of the hot coil, which is not conducive to improving the uniformity of the hot coil through-coiling performance; if the intermediate billet thickness is too thick, the rolling pressure of the finishing mill is too high, which is not conducive to the plate shape control of the hot rolled product. The primary purpose of implementing the lubricated rolling process on the F2-F6 finishing stands is to reduce the friction coefficient between the rolls and the workpiece surface, mitigate the shear strain and unfavorable {110}(1l0) texture caused by friction, and control the consistency of the texture across the thickness of the workpiece, thereby facilitating the formation of a {111} texture and improving the deep-drawing performance of the material. A finishing temperature <830°C can easily lead to: On the one hand, the rolling speed of the finishing stand F7 is too slow, which in turn increases the difference in finishing temperature between the head and tail of the hot coil, hindering the uniformity of the hot coil's through-coil performance; on the other hand, a finishing temperature that is too low can easily lead to mixed crystal problems, which in turn is detrimental to reducing material strength and improving the plastic strain ratio.

[0024] The present invention adopts a "Z-type" cooling process in its cooling process design. The coiling temperature at the head of the hot coil is 40-60°C higher than that at the middle or tail. The main purpose is to provide temperature compensation. The high coiling temperature allows for sufficient ferrite recovery or static recrystallization, thereby allowing the ferrite grains to fully grow. This helps reduce the degree of ferrite grain refinement at the head of the hot coil caused by the low final rolling temperature. This can further reduce the performance difference between the head and the middle or tail of the hot coil, and improve the uniformity of the hot coil's through-coil performance. The length L1 of the hot coil head's high-temperature zone is set according to the thickness h of the finished strip. The thinner the finished strip, the longer the hot coil head's high-temperature zone length. The two must satisfy the relationship L1 = (100-120)h, where L1 is in meters and h is in millimeters.

[0025] Compared with the existing technology, the present invention solves the problem of large fluctuations in the mechanical properties of ultra-thin low-carbon steel coils produced by traditional hot rolling processes, especially the plastic strain ratio, through reasonable smelting, continuous casting, heating, rolling, cooling, and coiling processes. It improves the uniformity of the mechanical properties of low-carbon steel coils and reduces the probability of cracking during stamping. Furthermore, there is no need to increase equipment investment, and the cost is low. The ultra-thin low-carbon steel produced by the present invention has a hot-rolled yield strength of 265-270 MPa, a tensile strength of 366-375 MPa, a hardness of 59-61 HRB, and a plastic strain ratio r=0.99-1.01. Compared with conventional processes, the ultra-low carbon steel hot-rolled coils produced by the present invention have an average yield strength reduced by 7.31-9.26%, while the average tensile strength and hardness are basically the same, and the average r is increased by 6.42-10.26%. The reduction in the yield strength of the material and the increase in the r value are both conducive to improving the deep drawing performance of the material. The ultra-thin low-carbon steel produced by the present invention has a yield strength fluctuation range of ≤10MPa at the head, middle and tail of the hot coil, a tensile strength fluctuation range of ≤10MPa, a hardness fluctuation range of ≤5HRB, and a plastic strain ratio r fluctuation range of ≤0.05. Compared with conventional processes, the uniformity of the hot coil's through-coiling strength, hardness and plastic strain ratio is significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of the "Z-type" cooling process adopted in the present invention. DETAILED DESCRIPTION

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0028] Example 1-Example 3

[0029] An extremely thin gauge low carbon steel comprises the following chemical components in weight percentage: as shown in Table 1, wherein the balance not shown in Table 1 is Fe and unavoidable inclusions.

[0030] Comparative Example 1-Comparative Example 6

[0031] An extremely thin gauge low carbon steel comprises the following chemical components in weight percentage: as shown in Table 1, wherein the balance not shown in Table 1 is Fe and unavoidable inclusions.

[0032] Table 1 Chemical composition of the embodiments and comparative examples (mass percentage, wt%)

[0033] serial number C Si Mn Als B Example 1 0.035 0.041 0.10 0.030 0.0002 Example 2 0.025 0.015 0.18 0.018 0.0004 Example 3 0.055 0.036 0.25 0.038 0.0001 Comparative Example 1 0.036 0.028 0.10 0.030 0.0012 Comparative Example 2 0.031 0.019 0.11 0.028 0.0003 Comparative Example 3 0.038 0.028 0.10 0.026 0.0004 Comparative Example 4 0.042 0.025 0.12 0.025 0.0003 Comparative Example 5 0.035 0.020 0.10 0.031 0.0003 Comparative Example 6 0.030 0.026 0.11 0.025 0.0008

[0034] The manufacturing method of the ultra-thin low-carbon steel of the above embodiments and comparative examples includes converter smelting, LF furnace refining, continuous casting, heating, rolling, cooling, and coiling.

[0035] According to the composition in Table 1, after converter smelting and LF furnace refining, alloy is added to the converter steel. The alloy auxiliary material type of Si is high-purity ferrosilicon, and its chemical element B content is ≤0.0005%. Manganese silicon alloy or ferrosilicon 75B alloy is not used.

[0036] The continuous casting process has a billet thickness of 230 mm;

[0037] The heating process is such that the temperature of the billet out of the furnace is 1150-1200°C, and the temperature at the head 0-2m in the longitudinal direction of the billet is 20-40°C higher than the temperature at the tail 0-2m. The billet is kept in the furnace for 150-200 minutes.

[0038] The rolling is carried out by using a 2-stand rough rolling and a 7-stand finishing hot rolling mill group. The rough rolling mill group adopts a 3+3 rolling mode, the R2 third pass rolling speed is ≥4.0m / s, and the intermediate billet thickness H after rough rolling and the finished strip thickness h must satisfy the relationship H=(20~25)×h; the unit of H is mm, and the unit of h is mm.

[0039] The insulation covers between the rolling, roughing and finishing mills are all put into use;

[0040] In the rolling process, the F2-F6 finishing mills use a roll gap lubrication process, the volume ratio of oil and water in the lubricating liquid (oil-water mixture concentration) is 0.30%-0.60%, the lubricating liquid flow rate is 80-120 ml / min, and the final rolling temperature FDT of the F7 finishing mill is 830-880°C.

[0041] The cooling is carried out by laminar cooling after finish rolling, and a "Z-type" cooling process is adopted. The length L1 of the high-temperature zone at the head of the hot coil is set according to the thickness h of the finished strip steel, L1 = (100-120) × h, the unit of L1 is m, the unit of h is mm, the coiling temperature CT1 of the high-temperature zone L1 at the head of the hot coil is CT + ΔT, ΔT = 40-60°C, the coiling temperature of the middle and tail of the hot coil is CT = 600-650°C, and the units of CT1, CT and ΔT are all °C; the tail of the hot coil refers to the 150m within the tail of the hot coil, and the hot coil is air-cooled to room temperature after coiling.

[0042] The hot rolling temperature parameters of each embodiment and comparative example are shown in Table 2, and the lubrication and cooling processes are shown in Table 3.

[0043] Table 2 Hot rolling temperature parameters of the embodiments and comparative examples

[0044]

[0045]

[0046] Table 3 Lubrication process and cooling process parameters of the embodiment and comparative example

[0047]

[0048] The mechanical properties and fluctuation ranges of the embodiments and comparative examples are shown in Table 4, and the average mechanical properties of the head, middle and tail of the hot coil are shown in Table 5.

[0049] The mechanical properties and their fluctuation range were assessed as follows: Three specimens were taken from the hot-rolled strip at 6m in the longitudinal direction, 6m in the middle, and 6m in the longitudinal direction, corresponding to a quarter of the strip width. Two longitudinal tensile specimens and one Rockwell hardness specimen were cut from each specimen. Room-temperature tensile testing was performed according to GB / T 228.1 to determine the yield strength, tensile strength, and plastic strain ratio. The arithmetic mean of the test data from the two tensile specimens was used as the yield strength, tensile strength, and r-value test values. The Rockwell hardness (HRB) index was determined according to GB / T 230.1. Among them, the fluctuation amplitude of the yield strength of the hot coil = Max (yield strength of the tensile specimens at the head, middle and tail) - Min (yield strength of the tensile specimens at the head, middle and tail), the fluctuation amplitude of the tensile strength = Max (tensile strength of the tensile specimens at the head, middle and tail) - Min (tensile strength of the tensile specimens at the head, middle and tail), the average yield strength is the arithmetic mean of the yield strengths corresponding to the tensile specimens at the head, middle and tail positions, the average tensile strength is the arithmetic mean of the tensile strengths corresponding to the tensile specimens at the head, middle and tail positions, the average hardness is the arithmetic mean of the hardnesses corresponding to the tensile specimens at the head, middle and tail positions, and the average r is the arithmetic mean of the r corresponding to the tensile specimens at the head, middle and tail positions.

[0050] Table 4 Mechanical properties and fluctuation range of examples and comparative examples

[0051]

[0052] Table 5 Average mechanical properties of the head, middle and tail of the hot roll in the embodiments and comparative examples

[0053]

[0054] Comparative Example 1 is produced according to the process of the present invention, but the content of chemical element B is 0.0012%. B will significantly inhibit the nucleation and growth of proeutectoid ferrite recrystallization, refine its grain size, and lead to increased strength of the material and reduced plastic strain ratio.

[0055] In comparative example 2, the temperature difference between the head and tail of the slab is -10°C, and the rough rolling adopts the "3+5" mode, which results in the final rolling temperature of the head of the hot coil being 37-46°C lower than that of the middle or tail. The ferrite grain size at the head of the hot coil is small and the strength is relatively high, resulting in worse performance uniformity in the length direction of the hot coil.

[0056] In Comparative Example 3, the intermediate billet thickness is 34 mm, H / h < 20. The thin thickness causes the intermediate billet temperature to drop too quickly, and the FDT < 830°C, resulting in a low hot coil finishing temperature, high material strength, and low plastic strain ratio.

[0057] In comparative example 4, the temperature difference between the head and tail of the slab is -15°C, and the cooling adopts the "conventional mode" without the "Z-type cooling". As a result, the final rolling temperature of the head of the hot coil is 34-44°C lower than that of the middle or tail. The ferrite grain size of the head of the hot coil is small and the strength is high, and the performance uniformity of the hot coil in the longitudinal direction is worse.

[0058] The composition of Comparative Example 5 meets the requirements of the present invention, but the lubrication process is not adopted, resulting in severe shear strain and uneven deformation caused by friction between the roller and the steel plate surface, and the {110}(1l0) texture orientation density increases, which in turn reduces the plastic strain ratio of the material.

[0059] In comparative example 6, the rough rolling adopts the "3+5" mode, which will increase the residence time of the intermediate billet in the rough rolling unit and aggravate the temperature drop of the intermediate billet by 15 to 20°C compared with the "3+3" mode. In addition, the temperature difference between the head and tail of the slab is 2°C, and the cooling does not adopt the "Z-type" cooling mode, resulting in the overall final rolling temperature of the hot coil being low. The final rolling temperature of the head of the hot coil is still lower than that of the middle or tail, and the problem of large performance fluctuations in the length direction of the hot coil still exists.

[0060] The underlined data do not meet the requirements of the present invention.

[0061] The above embodiments are described to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for producing ultra-thin low-carbon steel, comprising converter smelting, LF furnace refining, continuous casting, heating, rolling, cooling and coiling, characterized in that: The cooling process is as follows: the length L1 of the high-temperature zone at the head of the hot coil is set according to the thickness h of the finished strip steel, L1 = (100-120) × h, where L1 is in m and h is in mm; the coiling temperature CT1 of the high-temperature zone L1 at the head of the hot coil is CT + ΔT, where ΔT is 40-60°C; the coiling temperatures of the middle and tail of the hot coil are both CT = 600-650°C; and the hot coil is air-cooled to room temperature after coiling; The heating process is such that the temperature of the billet out of the furnace is between 1150°C and 1200°C, and the temperature at the head 0-2m in the longitudinal direction of the billet is 20-40°C higher than the temperature at the tail 0-2m in the longitudinal direction of the billet, and the billet is kept in the furnace for 150-200 minutes; The rough rolling mill adopts a 3+3 rolling mode, the third pass rolling speed of R2 is ≥4.0m / s, and the thickness H of the intermediate billet after rough rolling and the thickness h of the finished strip steel must satisfy the relationship H=(20~25)×h; In the rolling process, the F2-F6 finishing mills use a roll gap lubrication process, the oil-water mixture concentration is 0.30%-0.60%, the lubricating liquid flow rate is 80-120 ml / min, and the final rolling temperature FDT of the F7 finishing mill is 830-880°C; The ultra-thin low-carbon steel comprises the following chemical components in weight percentage: C 0.020-0.060%; Si 0.010-0.060%; Mn 0.08-0.30%; B ≤0.0005%; Als 0.015-0.050%, with the remainder being Fe and unavoidable inclusions.

2. The manufacturing method according to claim 1, characterized in that The heat preservation covers between the rolling, rough rolling and finishing rolling mills are all put into use.

3. An ultra-thin low-carbon steel manufactured by the manufacturing method according to any one of claims 1-2, characterized in that: The ultra-thin low-carbon steel comprises the following chemical components in weight percentage: C 0.020-0.060%; Si 0.010-0.060%; Mn 0.08-0.30%; B ≤0.0005%; Als 0.015-0.050%, with the remainder being Fe and unavoidable inclusions.

4. The ultra-thin low-carbon steel according to claim 3, characterized in that: The thickness of the ultra-thin low-carbon steel is ≤2.0 mm.

5. The ultra-thin low carbon steel according to claim 3 or 4, characterized in that: The ultra-thin low-carbon steel hot-rolled coil has a yield strength of 265-270 MPa, a tensile strength of 366-375 MPa, a hardness of 59-61 HRB, and a plastic strain ratio r=0.99-1.

01.

6. The ultra-thin low-carbon steel according to any one of claims 3 or 4, characterized in that: The yield strength fluctuation range of the head, middle and tail of the ultra-thin low-carbon steel hot coil is ≤10MPa, the tensile strength fluctuation range is ≤10MPa, the hardness fluctuation range is ≤5HRB, and the plastic strain ratio r fluctuation range is ≤0.05.

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