Method for improving product elongation based on total iron smelting and continuous casting and rolling production line

Abstract

The invention belongs to the technical field of steel smelting and rolling, and provides a method for improving the elongation of a product based on an all-iron smelting and continuous casting and rolling production line. According to the total-iron smelting and cast-rolling method, harmful element control is carried out based on the total-iron smelting condition, the content of residual elements is controlled through a deep desulfurization process based on the continuous casting and rolling production line, high-pulling-speed and constant-pulling-speed production is achieved based on process control of the continuous casting and rolling production line, and the obtained product is low in impurity content, good in ductility and high in yield. Cost can be effectively reduced, and profit can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel smelting and rolling, and relates to a method for improving the elongation of products based on an all-iron smelting and continuous casting and rolling production line. Background Art

[0002] Disclosing the information of this background art aims to increase the understanding of the overall background of the present invention, and it is not necessarily regarded as an admission or an implication in any form that this information constitutes the prior art already known to those of ordinary skill in the art.

[0003] Currently, the main technologies for obtaining low phosphorus and titanium in all-iron smelting are hot metal pretreatment for dephosphorization, hot metal pretreatment for detitanium, and double slag blowing in the converter. Its main disadvantages are high cost and impact on the normal process production efficiency. Currently, the main means for removing nitrogen in molten steel at home and abroad is through a vacuum furnace. However, if the molten steel is deoxidized, the removal of nitrogen in the molten steel in the vacuum furnace is mainly carried out through the interface between the molten steel surface and the vacuum interface, with a small reaction interface and low efficiency.

[0004] In addition, the continuous casting and rolling production line has a rigid connection between continuous casting and rolling. Under high casting speed conditions, the risks of slag entrainment and cracks during casting are relatively high, and it is impossible to stably achieve high casting speed and constant casting speed. Abnormal casting speed fluctuations directly affect the final rolling temperature, and further affect product performance, such as elongation. Summary of the Invention

[0005] Aiming at the problems that the conventional process methods for removing or controlling trace elements and harmful elements in hot metal are limited, and the non-compliance rate of product elongation is high, the present invention provides a method for improving the elongation of products based on an all-iron smelting and continuous casting and rolling production line.

[0006] To achieve the above object, the present invention adopts the following technical solutions.

[0007] A method for improving the elongation of products based on an all-iron smelting and continuous casting and rolling production line, comprising the following steps: The hot metal is smelted in a converter, subjected to RH vacuum degassing, LF refining, and continuous casting and rolling to obtain a strip steel; The total content of Cu, Cr, and Ni in the hot metal < 0.10%; During the converter smelting process, a slag-forming agent is added before the CO content reaches 40%; When blowing for 3 min - 5 min, the slag system of the furnace slag is controlled as follows: CaO: 35% - 50%, SiO2: 18% - 23%, MgO: 5% - 8%, Al2O3: 1% - 5%, FeO: 15% - 20%, MnO: 5% - 9%, P2O5: 2% - 4%; The softening temperature of the obtained slag system is controlled at 1230°C - 1270°C, the hemispherical temperature is 1280°C - 1300°C, and the flow temperature ≤ 1350°C.

[0008] At the end of blowing, lance control is carried out. When it is a 300T furnace, the lance position is controlled at 1.8m - 1.9m, the lance pressing time is ≥30s, and at the same time, the bottom blowing intensity is increased from 0.07 Nm 3 / (t·min) to not less than 0.15 Nm 3 / (t·min). After the blowing is completed, continue post-stirring for ≥30s, and control the oxidizability of the molten steel at 250ppm - 400ppm; control the slag system of the furnace slag as follows: CaO: 40% - 60%, SiO2: 13% - 18%, MgO: 6% - 8%, Al2O3: 2% - 4%, FeO ≤ 18%, MnO: 4% - 7%, P2O5: 3% - 4%; do not deoxidize during tapping, N ≤ 35ppm.

[0009] The slag formers for converter smelting are limestone, dolomite, mill scale, and kiln slag.

[0010] During the RH vacuum degassing process, the vacuum degree is ≤3 bar, and the terminal C ≤ 0.01%; add aluminum after decarburization is completed to make the Al content in the molten steel 0.03% - 0.05%.

[0011] During the LF refining process, add the slag after casting of low-phosphorus, low-titanium, and low-nitrogen steel grades, and the slag addition amount is 2T - 3T per furnace; For desulfurization slag formation in LF refining, add lime at 10kg / T; deoxidize the molten steel with aluminum wire, deoxidize the slag with calcium carbide and aluminum pellets, control the aluminum content not to exceed 0.06%; control the stirring time not to exceed 15min; then add calcium wire and control the calcium content at 10ppm - 15ppm.

[0012] During the continuous casting and rolling process, the superheat of the tundish is 15℃ - 25℃; the composition of the molten steel in the tundish: C ≤ 0.015%, Si: 0.03% - 0.05%, Al ≤ 0.030%, Mn: 0.1% - 0.15%, P ≤ 0.025%, S ≤ 0.003%, Ti ≤ 0.0030%, N ≤ 0.0045%, Cu + Cr + Ni < 0.10%; in the mold, the transverse flow velocity of the molten steel surface is not greater than 0.5m / s; the specific water ratio for cooling is 2.7L / kg - 3.0L / kg; control the current from -50A to -30A, and the casting speed is 5.0 - 5.5m / min; the finish rolling temperature in hot rolling is set according to the composition: when 10·w(Ti)% + w(P)% ≥ 0.050%, the finish rolling temperature ≥ 855℃; when 10·w(Ti)% + w(P)% < 0.050%, the finish rolling temperature is 845℃ - 855℃ (excluding).

[0013] The composition of the strip steel obtained by the above method is C ≤ 0.015%, Si: 0.03% - 0.05%, Al ≤ 0.030%, Mn: 0.1% - 0.15%, P ≤ 0.025%, S ≤ 0.003%, Ti ≤ 0.0030%, N ≤ 0.0045%, Cu + Cr + Ni < 0.10%, and the balance is Fe and inevitable impurities; the elongation A80 ≥ 27.0%.

[0014] The present invention has the following advantages: The full-iron smelting and continuous casting and rolling method provided by the present invention controls harmful elements under full-iron smelting conditions, controls the content of residual elements by adopting a deep desulfurization process based on the continuous casting and rolling production line, realizes high drawing speed and constant drawing speed production based on the process control of the continuous casting and rolling production line, and the obtained product has low impurity content and good elongation, and can effectively reduce costs and increase profits. Specific embodiments

[0015] The present invention provides a method for improving the elongation of products based on full-iron smelting and a continuous casting and rolling production line, including the following processes: molten iron is smelted by a converter, subjected to RH vacuum degassing, LF refining, and continuous casting and rolling to obtain strip steel; Specifically, the specific steps and control points of each process are as follows: 1. Converter smelting process (1) Since trace elements [Cu], [Cr], and [Ni] in molten iron cannot be removed under converter smelting conditions, it is necessary to select molten iron, and it is required that [Cu] + [Cr] + [Ni] < 0.10% in the molten iron entering the converter.

[0016] (2) Converter pre-control: The converter pre-blowing under full-iron smelting conditions (lance position 2.1m, flow rate 65000 Nm 3 / h) Control. Due to the abundant heat in the converter under full-iron smelting, the tapping temperature is high, ≥1620°C, and the dephosphorization load at the end of the converter is relatively large. Therefore, it is crucial to balance the hot metal temperature through the material mix in the early stage. To improve the dephosphorization efficiency in the early stage of the converter and reduce the residual phosphorus content of elements, the blowing process in the early stage of the converter should refer to the CO curve of converter gas. The slag-making process in the early stage needs to be completed before the CO content in the gas reaches 40%, that is, after the oxidation of silicon and manganese in the hot metal ends and before the intense carbon-oxygen reaction in the hot metal, the slag-making materials such as lime and dolomite should be added. Since no scrap steel is added, the average silicon content in the converter bath is relatively high. To ensure a certain slag basicity, the addition amount of slag-making materials (lime and limestone) is relatively high, increasing the blowing difficulty. To ensure the dephosphorization effect, metal materials such as scale and kiln slag are added in the early stage of blowing (0 - 5 min), so that the slag system of the converter slag at 3 - 5 min of blowing is controlled as follows: CaO: 35% - 50%, SiO2: 18% - 23%, MgO: 5% - 8%, Al2O3: 1% - 5%, FeO: 15% - 20%, MnO: 5% - 9%, P2O5: 2% - 4%; the softening temperature of the obtained slag system is controlled at 1230°C - 1270°C, the hemispherical temperature is 1280°C - 1300°C, and the flowing temperature ≤1350°C to obtain a slag with good fluidity for dephosphorization and de-titanium.

[0017] (3) Control in the later stage of the converter: From a thermodynamic analysis, to control the residual content of [P] and [Ti] elements at a low level, it can be achieved by increasing the oxidability of the molten steel. However, the over-oxidation of the molten steel is often accompanied by a high content of the gas element [N] in the molten steel, and at the same time, the slag foaming is severe, and it is easy to slag down to the ladle during tapping. The following reactions occur between P2O5 and TiO2 in the slag and [Al] in the molten steel: (P2O5) + [Al] → [P] + (Al2O3), (TiO2) + [Al] → [Ti] + (Al2O3), resulting in an increase in the residual elements [P] and [Ti] in the molten steel. To prevent the uncontrollability of elements such as [N], [P], and [Ti] at the end point, gun lowering control is carried out at the end of blowing, the gun position is controlled at 1.8 m - 1.9 m, and the gun lowering time ≥30 s. At the same time, the bottom blowing intensity is increased from about 0.07 Nm 3 / (t·min) to ≥0.15 Nm 3 / (t·min). After the blowing is completed, continue to stir for ≥30 s. The oxidability of the molten steel is controlled at 250 ppm - 400 ppm, and the slag composition is controlled: CaO: 40% - 60%, SiO2: 13% - 18%, MgO: 6% - 8%, Al2O3: 2% - 4%, FeO ≤18%, MnO: 4% - 7%, P2O5: 3% - 4%).

[0018] (4)Deoxidation control: [O] and [S] in the molten steel are active elements in the molten steel, which can reduce the dissolution rate of nitrogen elements in the air into the molten steel. Therefore, non-deoxidation during tapping is beneficial to the control of the nitrogen content in the molten steel, and it can ensure that the nitrogen content in the molten steel is controlled within 35 ppm when leaving the BOF station. In addition, without deoxidizing the molten steel, the oxygen in the molten steel reacts with the carbon in the molten steel in the vacuum chamber, and the reaction produces CO. First, it stirs the molten steel, increasing the kinetics of denitrification; second, the generated CO bubbles also have the effect of removing nitrogen elements. Countless small CO bubbles enlarge the reaction interface for denitrification, and the denitrification effect is good.

[0019] 2. RH process control (1)The vacuum degree ≤ 3 bar, making full use of the carbon-oxygen reaction for deoxidation and decarburization, and the end-point carbon ≤ 0.01%.

[0020] (2)Add aluminum after decarburization is completed. Adding aluminum in the RH furnace can be directly added to the molten steel without passing through the slag layer. Therefore, the recovery rate of aluminum is high, and at the same time, the redox reaction between aluminum and the slag in the furnace is reduced. Therefore, the content of [Al] in the molten steel in the RH process is controlled at 0.03% - 0.05%.

[0021] 3. LF process control (1)In the LF process, the slag folding process is adopted. After slag folding, the slag layer in the furnace is thick, the covering effect on the molten steel is good, and the effect of submerged arc during power supply is good, which can reduce the increase of nitrogen and oxygen in the molten steel during the power supply process; adopting the slag folding process can also increase the contact area between the molten steel and the slag, realizing rapid desulfurization. At the same time, to ensure that the ladle has a certain safe clearance, the slag folding amount is controlled at 2 - 3 t / furnace; the slag used for slag folding is the slag after continuous casting of the same steel grade series such as low-phosphorus, low-titanium, and low-nitrogen steel grades.

[0022] (2)Desulfurization slag-making process: After slag folding, desulfurization slag-making is carried out. To achieve precise titanium control, the lime addition amount in the LF refining furnace > 10 kg / T. Because the TiO2 content in the slag from the converter is relatively high (about 2%), increasing the lime in the refining furnace can dilute the TiO2 content in the slag; the molten steel and slag are deoxidized separately, which can further reduce the transfer of harmful elements in the slag to the molten steel. Among them, the molten steel is deoxidized with aluminum wire, and the slag is deoxidized with calcium carbide and aluminum particles. And the aluminum content in the molten steel during the process is controlled not to exceed ≤ 0.06%. Control the large stirring time for desulfurization, and the large stirring time ≤ 15 min to prevent excessive reduction of P2O5 and TiO2 in the slag by aluminum in the molten steel.

[0023] (3) Calcium treatment process: Calcium treatment is carried out after desulfurization and slag making. Since calcium has a low boiling point (1487℃) and is in gaseous state at the temperature of molten steel, the molten steel will churn severely during the calcium treatment process. Calcium treatment of molten steel will increase oxygen and nitrogen, and strengthen the stirring of molten steel and slag, resulting in the return of titanium and phosphorus in the molten steel, which is not conducive to the cleanliness control of the molten steel. For aluminum-killed molten steel, if calcium treatment is not carried out, the deoxidation products of the molten steel will block the water outlet, so a weak calcium treatment process is adopted. In order to achieve uniform composition of molten steel after calcium treatment, the entire calcium treatment process is completed in two to three times to complete the calcium line input, and the calcium content is controlled at 10ppm-15ppm.

[0024] 4. Continuous casting and rolling process control (1) Control the superheat of the tundish to 15-25℃, and control the content of the tundish as follows: C≤0.015%, Si: 0.03%-0.05%, Al≤0.030%, Mn: 0.1%-0.15%, P≤0.025%, S≤0.003%, Ti≤0.0030%, N≤0.0045%, Cu+Cr+Ni<0.10%.

[0025] (2) Continuous casting speed control: In continuous casting and rolling production lines, the high casting speed directly leads to a higher final rolling temperature. The increase in the pulling speed is mainly restricted by the control of the crystallizer liquid level. After the pulling speed is increased, the crystallizer liquid level control is unstable, and the lateral flow rate of the liquid surface is ≥0.5m / s. In severe cases, it is easy to roll slag and even cause production accidents. Continuous casting and rolling production lines often reduce the flow rate of molten steel in the crystallizer by increasing the current intensity of electromagnetic braking. However, under high pulling speed conditions, the impact depth of molten steel in the crystallizer becomes shallower after the current is increased, and the distance between the left and right reflux and the crystallizer liquid surface is shortened, which is not conducive to the crystallizer liquid level control. Therefore, under high pulling speed conditions, the method of increasing the deep impact depth of molten steel in the crystallizer is used to reduce the shear flow rate of the left and right reflux on the crystallizer liquid surface. Under the conditions of current control of -50A to -30A and pulling speed of 5.0-5.5m / min, the lateral flow rate of the liquid surface can be ≤0.5m / s. In order to ensure the cooling effect of the ingot after drawing, the specific water volume is controlled at 2.7L / kg-3.0L / kg.

[0026] (3) Steel rolling process control: The hot rolling final rolling temperature is set according to the composition. When 10·w(Ti)%+w(P)%≥0.050%, the final rolling temperature is ≥855℃; when 10·w(Ti)%+w(P)%<0.050%, the final rolling temperature is ≥845-855℃.

[0027] The present invention will be further described below in conjunction with examples, but the present invention is not limited by the following examples.

[0028] Example 1 Strip steel production 1. Converter smelting (1) Molten iron selection: In the molten iron, Cu: 0.0030%, Cr: 0.068%, Ni: 0.019%, C: 4.5%, Si: 0.3%, Mn: 0.4%, P: 0.113%, S: 0.022%.

[0029] (2) Early-stage control of the converter: In the converter gas curve, the CO content reaches 30% to complete the early-stage slag formation. The component contents of the early-stage slag are as follows: CaO: 40.5%, SiO2: 19%, MgO: 5%, Al2O3: 3%, FeO: 20%, MnO: 9%, P2O5: 3.5%. The softening temperature of the obtained slag system is controlled at 1250 °C, the hemispherical temperature is 1285 °C, and the flow temperature is 1330 °C.

[0030] (3) Late-stage control of the converter: The lance position in the late stage of the converter is controlled at 1.85 m, the lance is pressed down for 35 s, the bottom blowing intensity is controlled at 0.16 Nm 3 / (t·min). After the blowing is completed, the post-stirring continues for 35 s, the oxidability of the molten steel is controlled at 320 ppm, and the FeO content of the slag is 15%.

[0031] 2. RH control (1) Vacuum degree: 2.9 bar, final carbon: 0.008%.

[0032] (2) Aluminum is added after the decarburization is completed, and the [Al] content of the molten steel is controlled at 0.035%.

[0033] 3. LF control (1) The slag-splitting process is adopted, and 2.5 t of slag is split.

[0034] (2) Desulfurization slag-making process: The lime addition amount in the LF refining furnace is 3.5 t. The molten steel and the slag are deoxidized separately. Among them, the molten steel is deoxidized with aluminum wire, and the slag is deoxidized with calcium carbide and aluminum pellets. And the aluminum content in the molten steel during the process is controlled at a maximum of 0.055%. The large stirring time is 11 min to complete rapid desulfurization.

[0035] (3) Calcium treatment process: Weak calcium treatment process. The calcium wire is added in three times, and the calcium content is controlled at 11 ppm.

[0036] 4. Continuous casting and rolling control (1) Medium-package component control: C: 0.011%, Si: 0.04%, Al: 0.025%, Mn: 0.12%, P: 0.020%, S: 0.002%, Ti: 0.0020%, N: 0.0035%, Cu + Cr + Ni: 0.09%.

[0037] (2) The electromagnetic braking adopts negative current control, the current is revised by -40 A, the casting is carried out at a constant casting speed of 5.3 m / min, the transverse liquid surface flow velocity is 0.3 m / s, and the surface quality of the continuous casting billet is normal.

[0038] (3)Rolling process control: Finish rolling temperature is 845°C - 855°C.

[0039] By adopting this process, the residual elements and harmful elements in molten steel can be stably controlled, high casting and rolling speed can be achieved, the surface quality of the strip steel is good, and the strip steel performance is sampled and tested, with the transverse A80 elongation rate being 29%.

[0040] Example 2 Strip steel production 1. Converter smelting (1)Hot metal selection, in the hot metal, Cu: 0.0130%, Cr: 0.055%, Ni: 0.010%, C: 4.7%, Si: 0.28%, Mn: 0.41%, P: 0.123%, S: 0.025%.

[0041] (2)Converter pre - control: In the converter gas curve, the CO content is 33% to complete the pre - slagging. The component contents of the pre - slag are as follows: CaO: 42.5%, SiO2: 20%, MgO: 5%, Al2O3: 3%, FeO: 18%, MnO: 8%, P2O5: 3.5%. The softening temperature of the obtained slag system is controlled at 1260°C, the hemisphere temperature is 1289°C, and the flowing temperature is 1338°C.

[0042] (3)Converter post - control: The lance position in the later stage of the converter is controlled at 1.81m, the lance is pressed for 39s, the bottom - blowing intensity is controlled at 0.17 Nm 3 / (t·min), continue post - stirring for 32s after the blowing ends, the oxidability of the molten steel is controlled at 335 ppm, and the FeO content in the slag is 16%.

[0043] 2. RH control (1)Vacuum degree is 2.1 bar, final carbon: 0.007%.

[0044] (2)Add aluminum after decarburization ends, and the molten steel composition [Al] is controlled at 0.033%.

[0045] 3. LF control (1)Adopt the slag - folding process, and the slag - folding amount is 2.3t.

[0046] (2)Desulfurization slag - making process: The lime addition amount in the LF refining furnace is 3.8t. The molten steel and the slag are deoxidized separately. Among them, the molten steel is deoxidized with aluminum wire, and the slag is deoxidized with calcium carbide and aluminum particles. And the aluminum content in the molten steel during the process is controlled at a maximum of 0.053%. The large - stirring time is 12min to complete rapid desulfurization.

[0047] (3)Calcium treatment process: Weak calcium treatment process, the calcium wire is added in three times, and the component calcium is controlled at 13 ppm.

[0048] 4. Continuous casting and rolling control (1)Control of the middle package composition: C: 0.014%, Si: 0.041%, Al: 0.024%, Mn: 0.12%, P: 0.025%, S: 0.002%, Ti: 0.0026%, N: 0.0035%, Cu + Cr + Ni: 0.078%.

[0049] (2)Electromagnetic braking is controlled by negative current. The current is revised by -35 A. Casting is carried out at a constant casting speed of 5.2 m / min. The lateral flow rate of the liquid surface is 0.33 m / s, and the surface quality of the billet is normal.

[0050] (3)Rolling process control: The final rolling temperature is 860 °C.

[0051] Adopting this process, the residual elements and harmful elements in the molten steel can be stably controlled, and continuous casting and rolling at high casting speeds can be achieved. The surface quality of the strip steel is good. Samples of the strip steel properties are taken for testing, and the transverse A80 elongation is 30%.

[0052] The above are only the embodiments of the present application and are not used to limit the present application. For those skilled in the art, various changes and modifications can be made to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included within the scope of the claims of the present application.

Claims

1. A method for improving the elongation of products based on an all-iron smelting and continuous casting and rolling production line, characterized in that, It includes the following steps: The molten iron is smelted in a converter, subjected to RH vacuum degassing, LF refining, and continuous casting and rolling to obtain strip steel; The total content of Cu, Cr, and Ni in the molten iron < 0.10%; During the converter smelting process, a slag-forming agent is added before the CO content reaches 40%; when blowing for 3 min - 5 min, the slag system of the furnace slag is controlled as follows: CaO: 35% - 50%, SiO2: 18% - 23%, MgO: 5% - 8%, Al2O3: 1% - 5%, FeO: 15% - 20%, MnO: 5% - 9%, P2O5: 2% - 4%; The FeO of the furnace slag system at the end of blowing ≤ 18%; the oxidation of the molten steel is 250 ppm - 400 ppm, N ≤ 35 ppm; no deoxidation is carried out during tapping; The vacuum degree of RH vacuum degassing ≤ 3 bar, the end-point C of the molten steel ≤ 0.01%, and the Al content is 0.03% - 0.05%; For LF refining, lime is added at 10 kg / T for slag formation; the Al content is controlled not to exceed 0.06%; the stirring time is controlled not to exceed 15 min; the Ca content is controlled to be 10 ppm - 15 ppm; During continuous casting and rolling, the superheat of the tundish is 15°C - 25°C; the transverse flow rate of the molten steel surface in the mold is not greater than 0.5 m / s; the specific water flow for cooling is 2.7 L / kg - 3.0 L / kg.

2. The method according to claim 1, characterized in that At the end of blowing, lance pressure control is carried out, the lance position is controlled at 1.8m - 1.9m, the lance pressure time ≥ 30s, and at the same time the bottom blowing intensity is increased to not less than 0.15 Nm 3 / (t·min). After blowing, continue post-stirring for ≥ 30s.

3. The method according to claim 1, wherein The furnace slag system at the end of blowing is: CaO: 40% - 60%, SiO2: 13% - 18%, MgO: 6% - 8%, Al2O3: 2% - 4%, FeO ≤ 18%, MnO: 4% - 7%, P2O5: 3% - 4%.

4. The method according to claim 1, wherein When blowing for 3 min - 5 min, the softening temperature of the furnace slag system is 1230°C - 1270°C, the hemispherical temperature is 1280°C - 1300°C, and the flow temperature ≤ 1350°C; The slag-forming agents for converter smelting are limestone, dolomite, mill scale, and kiln slag.

5. The method according to claim 1, wherein LF refining adopts the slag-folding process, and the slag is the slag after casting of low-phosphorus, low-titanium, and low-nitrogen steel grades, and the slag addition amount is 2 T - 3 T / furnace; During LF refining, the molten steel is deoxidized with aluminum wire, and the furnace slag is deoxidized with calcium carbide and aluminum pellets.

6. The method according to claim 1, wherein During continuous casting and rolling, the current is controlled from -50 A to -30 A, and the casting speed is 5.0 - 5.5 m / min; The final rolling temperature of hot rolling is set according to the composition: when 10·w(Ti)% + w(P)% ≥ 0.050%, the final rolling temperature ≥ 855°C; when 10·w(Ti)% + w(P)% < 0.050%, the final rolling temperature is 845°C - 855°C (excluding); 7. A high elongation strip steel, characterized in that, The chemical composition by weight percentage is C ≤ 0.015%, Si: 0.03% - 0.05%, Al ≤ 0.030%, Mn: 0.1% - 0.15%, P ≤ 0.025%, S ≤ 0.003%, Ti ≤ 0.0030%, N ≤ 0.0045%, Cu + Cr + Ni < 0.10%, and the balance is Fe and unavoidable impurities; the elongation A80 ≥ 27.0%.