Production method of thick steel plate with low-alloy structure

Abstract

The invention belongs to the technical field of medium-thickness plate production, and relates to a production method of a thick steel plate with a low-alloy structure. The steel plate comprises the following chemical components in percentage by mass: 0.09%-0.12% of C, 0.15%-0.55% of Si, 1.40%-1.70% of Mn, less than or equal to 0.014% of P, less than or equal to 0.005% of S, less than or equal to 0.30% of Cr, less than or equal to 0.25% of Ni, 0.025%-0.035% of Nb, 0.008%-0.020% of Ti, 0.020%-0.050% of Al and the balance of Fe and inevitable impurities. The key production steps include converter smelting, refining, continuous casting, heating, rolling and heat treatment. The problem that the uniformity of the mechanical property in the thickness direction of the medium-thickness plate is poor is solved. The adopted production method combines the advantages of a hot rolling process and a heat treatment process, the produced thick steel plate has excellent low-temperature toughness and good welding performance, and medium-thickness plate production with a wide chemical component range is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of medium and thick plate production, and specifically relates to a production method of a low-alloy structural thick steel plate. Background Art

[0002] There are large differences in the composition and process of steel plates with thickness changes, which is caused by the difference in cooling rate at different thickness positions of the steel plate. For steel plates with smaller thickness, the difference in cooling rate is smaller throughout the thickness, and the cooling rate is also larger. For such steel plates, conventional processes can achieve excellent performance. With the increase of steel plate thickness, especially for medium and thick plates with a thickness of more than 50mm, the insufficient cooling rate of the steel plate and the difference in cooling rate at different thickness positions of the steel plate will have an adverse effect on the performance of the steel plate, such as the difference in strength and toughness at different thickness positions of the steel plate.

[0003] For steel plates delivered in hot-rolled state (including NR, CR, TMCP, etc.), manufacturers can refine grains, increase microstructure and distortion energy in the organization through rough and fine rolling, and promote phase transformation of steel plates with post-rolling cooling to improve the organization homogenization within the thickness range of steel plates, and the core strength can reach more than 420MPa. However, for thicker steel plates, in order to ensure the comprehensive performance of the steel plates, it is necessary to make substantial adjustments to the chemical composition of the steel plates and the rolling cooling process. When the cooling rate inside the steel plates reaches the set target, the cooling rate on the surface of the steel plates is usually too fast, resulting in a poorly matched organization on the surface of the steel plates due to the excessive cooling rate.

[0004] For the steel plates delivered after heat treatment, the austenite structure of the steel plates after heating is close to the equilibrium state (that is, the structural refinement effect of the rolled steel plates has been eliminated), and its structural properties basically depend only on the cooling rate and the hardenability of the material itself. The cooling rate of the core of the thick steel plate is difficult to be further improved due to the limitations of the material properties and the existing heat treatment process, resulting in large differences in the performance of the thick plate within the thickness. Summary of the invention

[0005] The technical problem to be solved by the present invention is to improve the uniformity of mechanical properties in the thickness direction of medium and thick plates, and provide a production method for low-alloy structural thick steel plates, which adopts a new process of controlled rolling and controlled cooling + heat treatment to achieve uniform organization and properties in the thickness direction of medium and thick plates. The produced thick steel plates have excellent low-temperature toughness and good welding performance.

[0006] In order to achieve the above object, the technical solution of the present invention is as follows:

[0007] A method for producing a low-alloy structural thick steel plate, wherein the chemical composition of the steel plate is C: 0.09%-0.12%, Si: 0.15%-0.55%, Mn: 1.40%-1.70%, P≤0.014%, S≤0.005%, Cr≤0.30%, Ni≤0.25%, Nb: 0.025%-0.035%, Ti: 0.008%-0.020%, Al: 0.020%-0.050%, and the rest is Fe and unavoidable impurities.

[0008] Furthermore, the chemical composition of the steel plate is C: 0.10%, Si: 0.21%, Mn: 1.42%, P: 0.011%, S: 0.002%, Cr: 0.04%, Ni: 0.18%, Nb: 0.026%, Ti: 0.014%, Al: 0.030%, and the rest is Fe and unavoidable impurities.

[0009] The functions of each element in the present invention are as follows:

[0010] Carbon: Carbon exists in steel in the form of interstitial solid solution and can also form carbides with other elements. Carbon can greatly increase the strength of steel. Changes in the carbon content in steel will change the content of ferrite and pearlite in the steel, and affect the toughness of the steel and process properties such as welding.

[0011] Silicon: Silicon is one of the deoxidizing elements in the steelmaking process. When it exists in steel as a solid solution element, it can significantly improve the strength of the steel. Silicon also has many effects on carbides in steel. It can not only change the type and morphology of carbides, promote the decomposition and refinement of carbides, but also affect the transformation and distribution of carbides to a certain extent.

[0012] Manganese: Manganese is one of the main alloying elements in steel. It can improve strength through solid solution strengthening. Manganese can lower the critical transformation temperature and refine pearlite, indirectly improving the strength of steel. Manganese can eliminate or weaken the hot brittleness of steel caused by sulfur. Manganese can improve the low-temperature impact toughness of steel. Manganese is a carbide-forming element and affects the microstructure and properties of steel.

[0013] Chromium: Chromium itself has very high hardness. Adding it to alloy steel can significantly increase the surface hardness and wear resistance of the steel. Chromium can combine with the iron in the steel to form a corrosion-resistant Cr2O3 oxide film, thereby protecting the steel from corrosion. Chromium significantly increases the corrosion resistance of steel, making it perform better in humid or corrosive environments.

[0014] Nickel: Nickel can infinitely dissolve with iron, expand the austenite phase region, and is conducive to the formation and stabilization of austenite; nickel can strengthen ferrite and refine and increase pearlite through solid solution strengthening; nickel can improve the low-temperature toughness of steel; nickel can improve the weldability of steel, reduce the precipitation of intermetallic compounds, prevent and reduce their harmful effects; nickel can increase the stacking fault energy of steel, increase the movable dislocations in the deformation process, and thus improve the plasticity and toughness of steel.

[0015] Niobium: As a microalloying element, adding a small amount of niobium will have a significant impact on the properties of steel. Niobium can effectively refine austenite grains during heating and rolling, inhibit austenite recrystallization, and niobium precipitates can improve the mechanical properties of steel. Niobium can reduce the interlamellar spacing of pearlite, reduce polygonal ferrite, refine the structure and improve impact toughness; Niobium has a significant effect on the yield strength of steel.

[0016] Titanium: Titanium is an alloy element that has a strong affinity with nitrogen and carbon, which is beneficial to controlling the surface quality of the slab. Titanium can inhibit the growth of austenite grains during the heating process, and is beneficial to refining austenite grains during the heating and rolling processes. Titanium precipitates can improve the mechanical properties of steel, and titanium can also improve the toughness of steel at low temperatures.

[0017] Aluminum: Aluminum is a strong deoxidizer, mainly added to molten steel as a deoxidizing alloy. At the same time, Al has a strong affinity with N, which can inhibit the harm of nitrogen in steel; Al can refine the structure of steel, thereby improving the strength and toughness of steel.

[0018] Phosphorus: Phosphorus is a harmful element in steel. It will reduce the plasticity and toughness of steel, especially at low temperatures, where it will greatly damage the toughness. Phosphorus will also deteriorate the welding performance of steel.

[0019] Sulfur: Sulfur is a harmful element in steel. It will cause hot brittleness of steel at high temperatures, reduce the plasticity and toughness of steel, and cause internal cracks in steel. Sulfur and manganese produce manganese sulfide. The presence of manganese sulfide in steel will affect the uniformity and consistency of steel and have an adverse effect on performance.

[0020] Furthermore, a method for producing a low alloy structural thick steel plate comprises the following main steps:

[0021] (1) Converter smelting: top and bottom double-blowing converter, control the tapping temperature at 1560-1640°C, converter tapping P ≤ 0.008%, add deoxidizer and alloy for deoxidation and alloying during the tapping process;

[0022] (2) Refining: After the molten steel in the ladle is heated by electricity in the LF furnace, the chemical composition is precisely adjusted. Calcium treatment is carried out in the later stage of refining, and then the ladle enters the RH furnace or VD furnace for vacuum treatment. After the vacuum treatment, the molten steel is left to stand;

[0023] (3) Continuous casting: The tundish superheat is 8-30°C. Dynamic light pressure reduction or heavy pressure reduction technology is used to improve the internal quality of the continuous casting billet. The thickness of the continuous casting billet is 300-450 mm.

[0024] (4) Heating: The slab is heated to 1100-1200°C in a step-beam continuous heating furnace and then taken out of the furnace;

[0025] (5) Rolling: Two-stage controlled rolling is adopted, wherein the rough rolling temperature is ≥950°C, the intermediate billet thickness is ≥the target steel plate thickness + 40 mm, the intermediate billet is water-cooled, the target final rolling temperature of the finishing rolling is 800-840°C, and the steel is directly put into water for accelerated cooling after rolling, and the red-return temperature is 420-460°C;

[0026] (6) Heat treatment: The furnace temperature is heated to 890-930°C. The steel plate is air-cooled and then water-cooled after it comes out of the furnace.

[0027] Preferably, the deoxidizer is an aluminum block or a silicon-manganese deoxidizer, and the alloy is a low-P alloy to avoid P increase in molten steel.

[0028] Furthermore, the precise adjustment of chemical composition in the refining stage refers to adding ferroniobium, ferronickel and ferrotitanium to precisely adjust the chemical composition after the LF furnace is powered on and heated.

[0029] Furthermore, in the refining stage, the molten steel is allowed to stand for 10 to 20 minutes after vacuum treatment.

[0030] Furthermore, the water cooling time during the rolling stage is 50 to 120 seconds.

[0031] Furthermore, the furnace time of the heat treatment stage is calculated according to the following formula:

[0032] T=(0.3~0.5×H)min / mm

[0033] Wherein, T is the time in the furnace and H is the thickness of the low alloy structural steel plate.

[0034] Furthermore, during the heat treatment stage, the steel plate is air-cooled to 700-750°C after being taken out of the furnace and then water-cooled.

[0035] Furthermore, the mechanical properties of the steel plate near the surface, at one quarter and at one half in the thickness direction are yield strength ≥420MPa, tensile strength ≥500MPa, and elongation ≥25%.

[0036] Principle of the invention: The low-alloy structural thick steel plate of the present invention adopts a low-carbon component system, adds an appropriate amount of Si, Mn and a small amount of Ni, and adds a small amount of Nb, Ti, and Al to refine the structure. The steelmaking process strictly limits harmful elements such as P and S in steel grades, and adopts LF and vacuum treatment to further improve the cleanliness of molten steel; adopts a large-thickness continuous casting billet, refines austenite grains through heating temperature control and rough rolling process, promotes the appearance of fine-grained structure after austenite transformation inside the thick steel plate through fine rolling and accelerated cooling process to improve the strength and toughness of the steel plate, and a mixed structure of bainite and ferrite-pearlite will be generated on the surface of the steel plate after rolling. The steel plate surface is re-austenitized and a proper cooling process is used to ensure that the surface structure of the steel plate is transformed into a refined ferrite-pearlite structure, thereby improving the surface strength and toughness. At the same time, the time the steel plate is in the furnace is strictly limited to avoid austenitization in the core of the thick steel plate so as to retain the excellent refined structure in the core of the steel plate; by controlling the heat treatment heating process and the cooling process to control the cooling rate of the critical phase change zone in the thickness of the steel plate, the deterioration of the performance in the transition zone of the steel plate can be avoided.

[0037] Beneficial effects of the present invention: The low alloy structural thick steel plate of the present invention has good mechanical properties and weldability. The production method adopted combines the advantages of hot rolling process and heat treatment process, realizes the production of medium and thick plates with a wide range of chemical composition, solves the problem of poor consistency of structure and performance of thick steel plates in the full thickness range, and effectively improves the toughness and weldability of the steel plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a near-surface cross-sectional metallographic structure diagram of Example 1 of the present invention;

[0039] Figure 2 This is a 1 / 4 cross-section metallographic structure diagram of Example 1 of the present invention;

[0040] Figure 3 This is a 1 / 2 cross-section metallographic structure diagram of Example 1 of the present invention. DETAILED DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in 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. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] Embodiment 1:

[0043] The chemical composition mass percentage of the low-alloy structural thick steel plate described in this embodiment is: C: 0.10%, Si: 0.21%, Mn: 1.42%, P: 0.011%, S: 0.002%, Cr: 0.04%, Ni: 0.18%, Nb: 0.026%, Ti: 0.014%, Al: 0.030%, and the rest is Fe and unavoidable impurities.

[0044] The specific production method steps are as follows:

[0045] (1) Converter smelting: top and bottom double blowing converter, converter tapping temperature 1626°C, converter tapping measurement P = 0.008%, aluminum blocks, silicon manganese deoxidizer, ferrosilicon, silicon manganese, and low carbon ferromanganese are added during the tapping process for alloying.

[0046] (2) Refining: After the LF furnace is powered on and heated, ferroniobium, ferronickel and ferrotitanium are added to the bulk steel to accurately adjust the chemical composition. Calcium treatment is performed in the later stage of refining. After LF refining, the steel enters the RH furnace for vacuum treatment. After the vacuum treatment, the steel is left to stand for 15 minutes.

[0047] (3) Continuous casting: The overheat temperature of the tundish is 9-15°C. Dynamic soft reduction is used to increase the internal density of the slab and reduce internal segregation. The thickness of the continuous casting slab is 300 mm.

[0048] (4) Heating: The slab is heated in a step-beam continuous heating furnace, and the furnace temperature is 1189°C.

[0049] (5) Rolling: Two-stage controlled rolling is adopted, and the finished product specification is 75×3100mm. The rough rolling temperature is 1100-1170℃, the intermediate billet thickness is 150mm, the intermediate billet is water-cooled for 70s, the finishing rolling start temperature is 820℃, the finishing rolling final rolling temperature is 805℃, and the red-returning temperature is 430℃.

[0050] (6) Heat treatment: The furnace temperature of heat treatment is 900℃, the furnace time is 25min, and the steel plate is air-cooled to 730℃ and water-cooled after being taken out of the furnace.

[0051] The metallographic structure of the near-surface cross-section of the low-alloy structural thick steel plate produced by the above method is as follows: Figure 1 As shown, the metallographic structure of 1 / 4 cross section is as follows Figure 2 As shown, the metallographic structure of the 1 / 2 section is as follows Figure 3 shown.

[0052] Embodiment 2:

[0053] The chemical composition mass percentage of the low yield ratio carbon-manganese low-temperature steel plate described in this embodiment is: C: 0.12%, Si: 0.32%, Mn: 1.47%, P: 0.009%, S: 0.0015%, Cr: 0.17%, Ni: 0.02%, Nb: 0.031%, Ti: 0.012%, Al: 0.036%, and the rest is Fe and unavoidable impurities.

[0054] The specific production method steps are as follows:

[0055] (1) Converter smelting: top and bottom double blowing converter, converter tapping temperature 1612°C, converter tapping measurement P = 0.007%, aluminum blocks, silicon manganese deoxidizer, ferrosilicon, silicon manganese, and low carbon ferromanganese are added during the tapping process for alloying.

[0056] (2) Refining: After the LF furnace is powered on and heated, ferroniobium, ferrochromium and ferrotitanium are added to the bulk steel to accurately adjust the chemical composition. Calcium treatment is performed in the later stage of refining. After LF refining, the steel enters the RH furnace for vacuum treatment. After the vacuum treatment, the steel is left to stand for 15 minutes.

[0057] (3) Continuous casting: The tundish superheat temperature is 9-15°C. Dynamic soft reduction is used to increase the internal density of the slab and reduce internal segregation. The thickness of the continuous casting slab is 350 mm.

[0058] (4) Heating: The slab is heated in a step-beam continuous heating furnace, and the furnace temperature is 1147°C.

[0059] (5) Rolling: Two-stage controlled rolling is adopted, and the finished product specification is 90×2600mm. The rough rolling temperature is 1070~1130℃, the intermediate billet thickness is 180mm, the intermediate billet is water-cooled for 90s, the finishing rolling start temperature is 840℃, the finishing rolling final rolling temperature is 795℃, and the red-returning temperature is 420℃.

[0060] (6) Heat treatment: The furnace temperature of heat treatment is 910℃, the furnace time is 30min, and the steel plate is air-cooled to 730℃ and water-cooled after being taken out of the furnace.

[0061] Embodiment 3:

[0062] The chemical composition mass percentage of the low yield ratio carbon-manganese low-temperature steel plate described in this embodiment is: C: 0.12%, Si: 0.43%, Mn: 1.58%, P: 0.007%, S: 0.0015%, Cr: 0.22%, Ni: 0.21%, Nb: 0.031%, Ti: 0.012%, Al: 0.032%, and the rest is Fe and unavoidable impurities.

[0063] The specific production method steps are as follows:

[0064] (1) Converter smelting: top and bottom double blowing converter, converter tapping temperature 1606°C, converter tapping measurement P = 0.006%, aluminum blocks and silicon manganese deoxidizer are added during the tapping process, and ferrosilicon, silicon manganese, and low carbon ferromanganese are added for alloying.

[0065] (2) Refining: After the LF furnace is powered on and heated, ferroniobium, ferrochromium, ferronickel and ferrotitanium are added to the bulk steel to precisely adjust the chemical composition. Calcium treatment is performed in the later stage of refining. After LF refining, the steel enters the RH furnace for vacuum treatment. After the vacuum treatment, the steel is left to stand for 18 minutes.

[0066] (3) Continuous casting: The overheat temperature of the tundish is 8-15°C. Dynamic soft reduction is used to increase the internal density of the slab and reduce internal segregation. The thickness of the continuous casting slab is 450 mm.

[0067] (4) Heating: The slab is heated in a step-type continuous heating furnace, and the furnace temperature is 1137°C.

[0068] (5) Rolling: Two-stage controlled rolling is adopted, and the finished product specification is 120×2200mm. The rough rolling temperature is 1060~1130℃, the intermediate billet thickness is 200mm, the intermediate billet is water-cooled for 90s, the finishing rolling start temperature is 840℃, the finishing rolling final rolling temperature is 800℃, and the red-returning temperature is 430℃.

[0069] (6) Heat treatment: The furnace temperature of heat treatment is 900℃, the furnace time is 40min, and the steel plate is air-cooled to 740℃ and water-cooled after being taken out of the furnace.

[0070] Comparative Example 1:

[0071] The chemical composition mass percentages of the low-alloy structural thick steel plate described in this comparative example are: C: 0.10%, Si: 0.21%, Mn: 1.42%, P: 0.011%, S: 0.002%, Ni: 0.18%, Nb: 0.026%, Ti: 0.014%, Al: 0.030%, and the rest are Fe and unavoidable impurities.

[0072] The specific production method steps are as follows:

[0073] (1) Converter smelting: top and bottom double blowing converter, converter tapping temperature 1626°C, converter tapping measurement P = 0.008%, aluminum blocks, silicon manganese deoxidizer, ferrosilicon, silicon manganese, and low carbon ferromanganese are added during the tapping process for alloying.

[0074] (2) Refining: After the LF furnace is powered on and heated, ferroniobium, ferronickel and ferrotitanium are added to the bulk steel to accurately adjust the chemical composition. Calcium treatment is performed in the later stage of refining. After LF refining, the steel enters the RH furnace for vacuum treatment. After the vacuum treatment, the steel is left to stand for 15 minutes.

[0075] (3) Continuous casting: The overheat temperature of the tundish is 9-15°C. Dynamic soft reduction is used to increase the internal density of the slab and reduce internal segregation. The thickness of the continuous casting slab is 300 mm.

[0076] (4) Heating: The slab is heated in a step-beam continuous heating furnace with a furnace exit temperature of 1170°C.

[0077] (5) Rolling: Two-stage controlled rolling is adopted, and the finished product specification is 75×3100mm. The rough rolling temperature is 1100-1160℃, the intermediate billet thickness is 150mm, the finishing rolling start temperature is 820℃, the finishing rolling final rolling temperature is 794℃, and the red-returning temperature is 440℃.

[0078] (6) Heat treatment: The furnace temperature of heat treatment is 900℃, the furnace time is 130min, and the steel plate is air-cooled after being taken out of the furnace.

[0079] Table 1 Mechanical properties of different positions of the embodiments and comparative examples

[0080]

[0081]

[0082] It can be seen from Table 1 that the method of the present invention can ensure that the mechanical properties of the low-alloy structural thick steel plate near the surface, at one quarter and at one half are uniform and meet the requirements.

[0083] The above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A method for producing a low alloy structural thick steel plate, characterized in that: The chemical composition of the steel plate is as follows: C: 0.09%-0.12%, Si: 0.15%-0.55%, Mn: 1.40%-1.70%, P≤0.014%, S≤0.005%, Cr≤0.30%, Ni≤0.25%, Nb: 0.025%-0.035%, Ti: 0.008%-0.020%, Al: 0.020%-0.050%, and the rest is Fe and unavoidable impurities.

2. The method for producing a low alloy structural thick steel plate according to claim 1, characterized in that: The chemical composition of the steel plate is C: 0.10%, Si: 0.21%, Mn: 1.42%, P: 0.011%, S: 0.002%, Cr: 0.04%, Ni: 0.18%, Nb: 0.026%, Ti: 0.014%, Al: 0.030%, and the rest is Fe and unavoidable impurities.

3. A method for producing a low alloy structural thick steel plate as claimed in claim 1 or 2, characterized in that: The main steps include: (1) Converter smelting: top and bottom double-blowing converter, control the tapping temperature at 1560-1640°C, converter tapping P ≤ 0.008%, add deoxidizer and alloy for deoxidation and alloying during the tapping process; (2) Refining: After the molten steel in the ladle is heated by electricity in the LF furnace, the chemical composition is precisely adjusted. Calcium treatment is carried out in the later stage of refining, and then the ladle enters the RH furnace or VD furnace for vacuum treatment. After the vacuum treatment, the molten steel is left to stand; (3) Continuous casting: The tundish superheat is 8-30°C. Dynamic light pressure reduction or heavy pressure reduction technology is used to improve the internal quality of the continuous casting billet. The thickness of the continuous casting billet is 300-450 mm. (4) Heating: The slab is heated to 1100-1200°C in a step-beam continuous heating furnace and then taken out of the furnace; (5) Rolling: Two-stage controlled rolling is adopted, wherein the rough rolling temperature is ≥950°C, the intermediate billet thickness is ≥the target steel plate thickness + 40 mm, the intermediate billet is water-cooled, the target final rolling temperature of the finishing rolling is 800-840°C, and the steel is directly put into water for accelerated cooling after rolling, and the red-return temperature is 420-460°C; (6) Heat treatment: The furnace temperature is heated to 890-930°C. The steel plate is air-cooled and then water-cooled after it comes out of the furnace.

4. The method for producing a low alloy structural thick steel plate according to claim 3, characterized in that: The deoxidizer is an aluminum block or a silicon-manganese deoxidizer, and the alloy is a low-P alloy to prevent the molten steel from increasing in P.

5. The method for producing a low alloy structural thick steel plate according to claim 3, characterized in that: The precise adjustment of chemical composition in the refining stage refers to adding ferroniobium, ferronickel and ferrotitanium to precisely adjust the chemical composition after the LF furnace is powered on and heated.

6. The method for producing a low alloy structural thick steel plate according to claim 3, characterized in that: After vacuum treatment in the refining stage, the molten steel is left to stand for 10 to 20 minutes.

7. The method for producing a low alloy structural thick steel plate according to claim 3, characterized in that: The water cooling time during the rolling stage is 50 to 120 seconds.

8. The method for producing a low alloy structural thick steel plate according to claim 3, characterized in that: The heat treatment stage in the furnace time is calculated according to the following formula: T=(0.3~0.5×H)min / mm Wherein, T is the time in the furnace and H is the thickness of the low alloy structural steel plate.

9. The method for producing a low alloy structural thick steel plate according to claim 3, characterized in that: In the heat treatment stage, the steel plate is air-cooled to 700-750° C. after being taken out of the furnace and then water-cooled.

10. A method for producing a low alloy structural thick steel plate according to any one of claims 4 to 9, characterized in that: The mechanical properties of the steel plate near the surface, at one quarter and at one half in the thickness direction are yield strength ≥420MPa, tensile strength ≥500MPa, and elongation ≥25%.

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