180 Mpa-grade low-yield-point anti-seismic building steel and production method thereof

Abstract

The invention discloses 180 Mpa grade low-yield-point anti-seismic building steel. The 180 Mpa grade low-yield-point anti-seismic building steel comprises the following components in percentage by weight: not more than 0.06% of C, not more than 0.01% of Si, not more than 0.60% of Mn, not more than 0.05% of Cu, not more than 0.008% of N, not more than 0.005% of P, not more than 0.003% of S, not more than 0.025% of Alt and the balance of Fe. The production method comprises the steps of molten iron pretreatment; smelting in a converter; refining in an LF furnace; performing RH vacuum treatment; nitrogen blowing is carried out for alloying; casting to form a blank; heating the casting blank; rough rolling; finish rolling; cooling is performed; performing coiling; and carrying out subsequent processes such as uncoiling and straightening. As low carbon, low silicon and low manganese are adopted, a small amount of copper is added, and a nitrogen blowing process is adopted, the yield strength of the steel at room temperature is larger than or equal to 180 MPa, the tensile strength is 300-360 MPa, the elongation A is larger than or equal to 35%, the elastic modulus at-40 DEG C is larger than or equal to 200 GMPa, the low-temperature impact toughness at-40 DEG C is larger than or equal to 45 J, the liquidus temperature of molten steel is effectively reduced, the steel strength is improved, the production process is short, technological parameters are easy to control, and the production cost is low. The method is good in economy and suitable for large-scale production.

Description

Technical Field

[0001] The present invention relates to a steel for construction and a production method thereof, and more particularly to a steel for seismic-resistant construction with a yield strength of 180 Mpa and a production method thereof, which is particularly suitable for construction steel with a thickness of 4-22 mm for high-rise buildings. Background Art

[0002] To reduce the disasters brought by earthquakes to human life, property and economy, some scholars have proposed a technology of energy-dissipating braces, namely buckling-restrained braces. Traditional steel braces for construction are prone to buckling instability when compressed, while buckling-restrained braces will not experience buckling instability when reaching the yield point whether in compression or tension. This is because buckling-restrained braces absorb seismic energy through the plastic strain of steel, thus protecting the main structure and improving the seismic performance of the structure. The core material of buckling-restrained braces usually adopts Q235 steel.

[0003] Some people have studied the restoring force characteristics of buckling-restrained braces made of low-yield-point steel (f y = 100 MPa), and found that using a steel grade with a lower yield strength as the core material can significantly increase the cumulative plastic deformation capacity and equivalent viscous damping ratio of buckling-restrained braces, and can exert its good plastic deformation and energy-dissipating performance under rare earthquake actions.

[0004] After preliminary retrieval:

[0005] The patent document with the Chinese patent application number 202410087085.5 discloses "A 6-20 mm thick low yield strength and high toughness Q235KZE seismic steel plate and its manufacturing method". The mass percentage of its chemical composition is: C 0.07-0.10%, Si 0.20-0.30%, Mn 1.00-1.10%, P≤0.015%, S≤0.003%, Alt 0.020-0.050%, Ca 0.0005-0.0020%, and the rest is Fe and inevitable impurities, CEV≤0.30% (CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15). Through processes such as KR hot metal pretreatment - BOF smelting - LF refining - RH vacuum degassing - Ca treatment - wide slab continuous casting - hot charging of billets - heating - rolling, a 6-20 mm thick low yield strength and high toughness Q235KZE seismic steel plate is produced from a 150 mm thick continuous casting billet. The product has the characteristics of low alloy cost, small strength fluctuation, low yield ratio, high elongation, excellent -40°C low temperature impact toughness, etc. The yield strength is 270-320 MPa, the tensile strength is 430-470 MPa, ReH / Rm≤0.80, the elongation is >40%, and the transverse and longitudinal -40°C Charpy V-notch impact energy (standard specimen) ≥120 J. Although the composition design of this document is simple and the production process flow is mature, it can obtain construction steel with a yield strength above 270 MPa, but it can only be used as a conventional construction steel plate and cannot be used as a special buckling-restrained brace core material for high-rise buildings.

[0006] The document with the Chinese patent application number 201110160102.6 discloses "100 MPa high-performance construction structure soft steel and its manufacturing method". The chemical composition of the steel is: C: 0.001-0.0045%, Si:≤0.05%, Mn: 0.009-0.048%, Ti: 0.011-0.045%, Als: 0.004-0.070%, and the balance is Fe and inevitable impurities. Its manufacturing method includes smelting, continuous casting, billet heating and two-stage controlled rolling. The characteristics are that the billet heating temperature is 1100-1280°C, the heating time is 60-110 s / m; the finish rolling temperature of rough rolling is 950-1080°C; the start rolling temperature of finish rolling is 900-1050°C, the finish rolling temperature is 820-960°C, the accumulated deformation is greater than 45%, and the rolled steel plate is cooled naturally after rolling. Although the chemical composition of the steel in this document is simple and the cost is relatively low; the yield strength fluctuation range of the steel plate is 80-120 MPa, and the -20°C longitudinal low temperature toughness is greater than 80 J, but the low temperature ductility and plasticity of the steel type in this document cannot meet higher requirements.

[0007] The literature with the Chinese patent application number 201210433526.x discloses "A Preparation Method of a Low Yield Ratio Seismic Steel Plate". Its chemical composition is calculated by weight percentage and consists of the following components: C: 0.12 - 0.16, Si: 0.40 - 0.60, Mn: 1.30 - 1.50, Nb: 0.10 - 0.20, Ni: 0.60 - 0.80, Cu: 0.15 - 0.25, Cr: 0.4 - 0.6, B: 0.003 - 0.005, Ti: 0.03 - 0.06, Al: 0.025 - 0.045%, P ≤ 0.010, S ≤ 0.015, and the balance is Fe and inevitable impurities; the preparation method includes the following steps: melting and casting, two-stage hot rolling, air cooling between passes, and the cumulative reduction ratio is greater than 70%; after rolling, the rolled steel plate is rapidly cooled to 300°C at a cooling rate of 15 - 25°C / S, and then air cooled. The steel composition design in this literature is relatively complex, and the yield strength is above 580 MPa. Summary of the Invention

[0008] The present invention aims to overcome the deficiencies of the existing technology and provides a 180 MPa grade low yield point seismic building steel with a yield strength ≥ 180 MPa at room temperature, a tensile strength of 300 - 360 MPa, an elongation rate ≥ 35%, an elastic modulus ≥ 200 GPa at -40°C, and a low temperature impact toughness ≥ 40 J at -40°C, that is, a low yield point and high elongation rate 180 Mpa grade low yield point seismic building steel and its production method.

[0009] Measures to achieve the above objectives:

[0010] A 180 MPa grade low yield point seismic building steel, its composition and weight percentage content are: C: not exceeding 0.06%, Si: not exceeding 0.01%, Mn: not exceeding 0.60%, Cu: not exceeding 0.05%, N: not exceeding 0.008%, P ≤ 0.005%, S ≤ 0.003%, Alt ≤ 0.025%, and the balance is Fe and inevitable inclusions.

[0011] Preferably: the weight percentage content of C is 0.028 - 0.056%.

[0012] Preferably: the weight percentage content of Si is 0.005 - 0.009%.

[0013] Preferably: the weight percentage content of Mn is 0.31 - 0.55%.

[0014] Preferably: the weight percentage content of Cu is 0.02 - 0.045%.

[0015] Preferably: the weight percentage content of N is 0.005 - 0.008%.

[0016] Preferably, the weight percentage content of Alt is 0.015 - 0.022%.

[0017] A production method of a 180Mpa - grade low yield - strength earthquake - resistant steel for construction, the steps are as follows:

[0018] 1) Carry out hot metal pretreatment, control the hot metal temperature not lower than 1250 °C, and after desulfurization of the hot metal, control S < 0.005%;

[0019] 2) Converter smelting. During this period: during tapping, add quicklime at 53 - 65 Kg / ton of steel and add fluorite at 24 - 35 Kg /

[0020] ton of steel for preliminary refining; then add ferromanganese at 3.91 - 8.23 Kg / ton of steel for preliminary deoxidation, and control the oxygen content in the molten steel after preliminary deoxidation not exceeding 30 PPm;

[0021] 3) Carry out LF furnace refining. During this period:

[0022] Blow argon throughout the refining process and keep the furnace in a reducing atmosphere;

[0023] Carry out deoxidation and alloying. Start adding ferromanganese for deoxidation when tapping reaches 1 / 3, add copper plates for alloying, and finish adding before tapping reaches 2 / 3; control the tapping temperature not lower than 1650 °C, and control the tapping time at 2.5 - 6 min; the oxygen content in the molten steel after deoxidation does not exceed 20 PPm;

[0024] 4) Carry out RH vacuum treatment and control the treatment time not lower than 15 min;

[0025] 5) After vacuum treatment, carry out alloying by blowing nitrogen. When blowing nitrogen, the temperature of the molten steel is 1580 - 1600 °C;

[0026] 6) Cast into billets. During this period: control the superheat of the tundish molten steel at 20 - 35 °C; use a long - nozzle brick and argon sealing for casting, control the casting speed at 1.13 - 1.28 m / min; control the cooling intensity in the secondary cooling section at 0.70 -

[0027] 0.78 L / min; control the straightening temperature of the billet not lower than 950 °C;

[0028] 7) Heat the billet: control the furnace inlet temperature > 100 °C; control the temperature in the heating section at 1200 - 1260 °C, and the heating rate is 9 - 15 min / cm;

[0029] 8) Carry out rough rolling. During this period: control the rough rolling starting temperature at 1000 - 1060 °C, the reduction rate of the first rough rolling pass is not lower than 15%, and the thickness of the steel plate after rough rolling is 3.5 - 10.5 times the finished product thickness;

[0030] 9) Perform finish rolling, during which: the finish rolling is carried out in the non-recrystallized austenite region, i.e., below 900 °C, and the cumulative reduction ratio of finish rolling is controlled to be greater than 70%, the reduction ratio of the last pass is not less than 30%, and the finish rolling final rolling temperature is 830 - 870 °C;

[0031] 10) Perform cooling, and cool to the coiling temperature at a cooling rate of 35 - 45 °C / s;

[0032] 11) Perform coiling, and control the coiling temperature at 570 - 630 °C;

[0033] 12) Perform post - processes such as uncoiling and straightening.

[0034] Functions and mechanisms of main elements and processes in the present invention

[0035] C, carbon element is one of the indispensable elements in steel materials. Through solid - solution strengthening, it will increase the yield strength and decrease the elongation. After comprehensive consideration, its range is set not to exceed 0.06%, preferably 0.028 - 0.056%. However, its content cannot be zero, because carbon is one of the essential strength elements in steel; if it is higher than 0.06%, it will lead to an increase in the welding crack sensitivity coefficient of construction steel, affecting the welding performance of structural parts and being unfavorable for atmospheric corrosion resistance.

[0036] Si, silicon element not only plays a deoxidizing role but also has a promoting effect on the strength of steel and can increase the strain hardening rate of steel. Therefore, the content of silicon element in steel is preferably controlled within a range not exceeding 0.01%, preferably 0.005 - 0.009%. However, its content cannot be zero, because silicon is one of the essential strength elements in steel and also the most basic deoxidizer; if it is higher than 0.01%, it will affect the plasticity and ductility of the steel in the present invention.

[0037] Mn, manganese element plays a role in solid - solution strengthening and can increase the hardness and strength of steel with little influence on the ductility of steel. After comprehensive consideration, its range is controlled not to exceed 0.60%, preferably 0.31 - 0.55%. However, its content cannot be zero, as manganese is one of the essential strength elements in steel, and also the most basic deoxidizer and desulfurizer; if it is higher than 0.60%, more cementite will be formed, which is unfavorable for the low - temperature toughness of the steel in the present invention.

[0038] Cu, due to the low content of carbon and manganese elements in the steel of the present invention, copper element is also one of the austenite - forming elements. In order to ensure the low - temperature toughness of the steel, a certain amount of Cu element needs to be added. Combining with the controlled rolling and controlled cooling process, the precipitation strengthening effect of copper in the matrix can be fully exerted. After comprehensive consideration, its range is set not to exceed 0.05%, preferably 0.02 - 0.045%. However, its content cannot be zero, otherwise it is difficult to achieve the purpose of the invention point; if it is higher than 0.05%, the cost will increase.

[0039] N. Nitrogen is mainly blown into the steel in gaseous form. When austenite transforms into ferrite, it mainly precipitates CuN or Cu(CN) from the steel, inhibiting the growth of austenite grains and playing a role in refining ferrite grains. After comprehensive consideration, its range is set not to exceed 0.008%, preferably 0.005 - 0.008%. However, its content cannot be zero. If it is zero, it cannot combine with copper elements in the steel of the present invention to form precipitates; if it is higher than 0.008%, other nitrides will be formed, increasing the risk of low-temperature brittleness of the steel of the present invention.

[0040] Alt. Aluminum is a deoxidizer in the steel, which fixes the free nitrogen in the steel. Its maximum solubility in austenite is about 0.6%. After dissolving in austenite, it only slightly increases the hardenability, can increase the grain coarsening temperature of the steel, reduce the brittle transition temperature of the steel, prevent strain and temperature aging, and improve the impact toughness of the steel. After comprehensive consideration, its range is set not to exceed 0.025%, preferably 0.015 - 0.022%. However, its content cannot be zero; if it is higher than 0.025%, aluminum-containing inclusions will be formed, which is not conducive to the uniformity of the structure and increases the risk of fracture during the application of the steel of the present invention.

[0041] P, S and other impurity elements: P and S are harmful impurity elements in the steel. Although P can greatly improve the strength, it is easy to form segregation in the steel, reducing the toughness and welding performance of the steel. S is easy to form plastic sulfides, making the steel highly anisotropic and deteriorating the impact toughness and processing performance of the steel. Therefore, the contents of P, S and other impurity elements in the steel should be strictly controlled.

[0042] Reasons for setting the main processes:

[0043] The reason why the present invention controls the molten iron temperature not to be lower than 1250°C and controls S < 0.005% after the molten iron is pre-desulfurized is that by pre-treating the molten iron and strictly controlling the sulfur content in the molten iron, the desulfurization burden of the iron-making and steel-making furnaces can be reduced, the purity of the molten steel can be guaranteed, and the requirements for impurity elements in the steel of the present invention can be met.

[0044] The reason why the present invention controls the oxygen content in the molten steel after pre-deoxidation not to exceed 30 ppm is that in the steel-making (oxidation) process, excessive oxygen is absorbed. If not removed, it will reduce the performance of the steel and affect its use after forming. At the same time, if the oxygen content is too high, it will affect the content and morphology of inclusions in the subsequent processes, which is not conducive to the requirement of the purity of the original structure of the continuous casting billet.

[0045] The reason why the molten steel temperature is controlled at 1580 - 1600 °C during nitrogen blowing in alloying in the present invention is that under a certain pressure, the solubility of a gas in a solvent generally decreases with the increase of temperature. Therefore, theoretically speaking, the lower the molten steel temperature, the higher the content of N in the dissolved state in the steel. However, in actual production, if the molten steel temperature is too low, the slag layer is likely to form a plate, which is not conducive to the subsequent power supply operation.

[0046] For deoxidation and alloying, ferromanganese is added for deoxidation when the tapping reaches 1 / 3, and copper plates are added for alloying, which should be added before the tapping reaches 2 / 3; the tapping temperature is controlled not to be lower than 1650 °C, and the tapping time is controlled at 2.5 - 6 min; the oxygen content in the molten steel after deoxidation does not exceed 20 PPm, because appropriate control of the tapping time can make the added alloy melt fully and be more evenly distributed, ensuring the stability of the alloy absorption rate.

[0047] The reason why the superheat of the tundish molten steel is controlled at 20 - 35 °C in the present invention; the casting is carried out using a long nozzle brick and argon sealing, the casting speed is controlled at 1.13 - 1.28 m / min; the cooling intensity in the secondary cooling section is controlled at 0.70 - 0.78 L / min; the straightening temperature of the continuous casting billet is controlled not to be lower than 950 °C, because the casting speed directly affects the solidification speed and internal quality of the molten steel during continuous casting billet casting. On the premise of ensuring a good continuous casting billet structure, the casting speed can be appropriately increased. Since the core of the continuous casting billet coming out of the mold is still liquid, in order to make the continuous casting billet completely solidify before entering the straightening point or before cutting, it is necessary to further cool the continuous casting billet in the secondary cooling zone. Excessive cooling intensity will cause transverse and longitudinal cracks on the surface of the continuous casting billet. Since the temperature range of 700 - 900 °C is the brittle temperature zone of the continuous casting billet, cracks are likely to occur during straightening within this range, and straightening must be carried out above 900 °C.

[0048] The reason why the furnace charging temperature is controlled > 100 °C in the present invention; the temperature in the heating section is controlled at 1200 - 1260 °C, and the heating rate is 9 - 15 min / cm, because effectively utilizing the heat of the continuous casting billet itself can reduce the energy consumption of the heating furnace. The continuous casting billet is heat-insulated and soaked in the heating furnace to ensure the heat-insulating time and the tapping temperature, and reduce the temperature difference between the head and tail of the continuous casting billet.

[0049] The reason why the rough rolling starting temperature is controlled at 1000 - 1060 °C in the present invention, the reduction ratio of the first rough rolling pass is not less than 15%, and the thickness of the steel plate after rough rolling is 3.5 - 10.5 times the finished product thickness, because during rough rolling, the sizing of the billet in terms of length, width dimensions and surface shape is carried out, and at the same time, large elongation of the billet is carried out to reach the thickness of the intermediate billet.

[0050] The reason why the present invention controls the cumulative reduction rate in finish rolling to be greater than 70%, the reduction rate in the last pass to be not less than 30%, and the finish rolling temperature in finish rolling to be 830 - 870°C is that appropriate finish rolling temperature and pass reduction rate can fully refine the austenite grain size and plate shape accuracy, and obtain the target size and good comprehensive properties.

[0051] The reason why the present invention controls the cooling to the coiling temperature of 570 - 630°C at a cooling rate of 35 - 45°C / s is that appropriate cooling rate combined with appropriate coiling temperature can effectively ensure that the grain sizes of ferrite and pearlite structures in the steel plate meet the requirements, and ensure the required performance requirements such as yield strength.

[0052] Compared with the prior art, due to the use of low carbon, low silicon, low manganese in the present invention, adding a small amount of copper and adopting the nitrogen blowing process, the yield strength of the steel at room temperature is ≥180 MPa, the tensile strength is 300 - 360 MPa, the elongation A ≥ 35%, the elastic modulus at -40°C ≥ 200 GPa, the low-temperature impact toughness at -40°C ≥ 45 J. It also effectively reduces the liquidus temperature of the molten steel, improves the strength of the steel, and has a short production process, easy control of process parameters, good economy, is suitable for large-scale production, and can expand the application range of this steel type. Brief Description of the Drawings

[0053] Figure 1 It is a picture of the typical metallographic structure of ferrite and pearlite for the embodiment of the present invention. Detailed Description of the Invention

[0054] The present invention will be described in detail below:

[0055] Table 1 is a list of chemical compositions of each embodiment and comparative example of the present invention;

[0056] Table 2 is a list of main process values of each embodiment and comparative example of the present invention;

[0057] Table 3 is a list of performance detections of each embodiment and comparative example of the present invention.

[0058] Each embodiment of the present invention is produced according to the following steps:

[0059] 1) Carry out hot metal pretreatment, control the hot metal temperature to be not less than 1250°C, and control S < 0.005% after the hot metal is pre-desulfurized;

[0060] 2) Converter smelting. During this period: during the tapping process, add quicklime at 53 - 65 Kg / ton of steel and add fluorite at 24 - 35 Kg /

[0061] ton of steel for pre-refining; then add ferromanganese at 3.91 - 8.23 Kg / ton of steel for pre-deoxidation, and control the oxygen content in the molten steel after pre-deoxidation not to exceed 30 PPm;

[0062] 3) Carry out LF furnace refining, during which:

[0063] Argon is blown throughout the refining process, and a reducing atmosphere is maintained in the furnace;

[0064] Deoxidation and alloying are carried out. Ferromanganese is added for deoxidation when 1 / 3 of the molten steel is tapped, and copper plates are added for alloying. It should be added up before 2 / 3 of the molten steel is tapped. The tapping temperature is controlled not to be lower than 1650 °C, and the tapping time is controlled within

[0065] 2.5 - 6 min; the oxygen content in the molten steel after deoxidation does not exceed 20 PPm;

[0066] 4) Carry out RH vacuum treatment, and control the treatment time not to be lower than 15 min;

[0067] 5) After vacuum treatment, alloying is carried out by blowing nitrogen. The temperature of the molten steel during nitrogen blowing is 1580 - 1600 °C;

[0068] 6) Cast into slabs, during which: control the superheat of the tundish molten steel at 20 - 35 °C; the casting is carried out using a long nozzle brick and argon sealing, control the casting speed at 1.13 - 1.28 m / min; control the cooling intensity in the secondary cooling section at 0.70 - 0.78 L / min; control the straightening temperature of the slab not to be lower than 950 °C;

[0069] 7) Heat the slab: control the furnace inlet temperature > 100 °C; the temperature in the heating section is controlled at 1200 - 1260 °C, and the heating rate is 9 - 15 min / cm;

[0070] 8) Carry out rough rolling, during which: control the rough rolling starting temperature at 1000 - 1060 °C, the reduction ratio of the first rough rolling pass is not lower than

[0071] 15%, and the thickness of the steel plate after rough rolling is 3.5 - 10.5 times the finished product thickness;

[0072] 9) Carry out finish rolling, during which: finish rolling is carried out in the non-recrystallization zone of austenite, i.e., below 900 °C, and control the cumulative

[0073] reduction ratio of finish rolling to be greater than 70%, the reduction ratio of the last pass is not lower than 30%, and the finish rolling final rolling temperature is 830 - 870 °C;

[0074] 10) Carry out cooling, and cool to the coiling temperature at a cooling rate of 35 - 45 °C / s;

[0075] 11) Carry out coiling, and control the coiling temperature at 570 - 630 °C;

[0076] 12) Carry out post-processes such as uncoiling and straightening.

[0077] Table 1 Value list of each example and comparative example of the present invention (wt%)

[0078]

[0079] Table 2 List of main process parameter values for each embodiment and comparative example of the present invention

[0080]

[0081] Continued Table 2

[0082]

[0083]

[0084] Table 3 Performance detection list of each embodiment and comparative example of the present invention

[0085]

[0086] Note: In Table 3: 1) Tensile property test conditions: At room temperature, a transverse specimen is taken at 1 / 4 of the steel plate thickness, and the yield strength ReL, tensile strength Rm, and elongation A of the steel plate are measured according to GB / T 228.1 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature".

[0087] 2) Low-temperature impact toughness test: At an ambient temperature of -40°C, a longitudinal specimen is taken at 1 / 4 of the steel plate thickness, and the test is carried out according to GB / T 229 "Metallic materials - Charpy pendulum impact test method" to obtain the average KV2 impact value of the standard specimen at -40°C.

[0088] 3) Elastic modulus test: At an ambient temperature of -40°C, a transverse specimen is taken at 1 / 4 of the steel plate thickness, and the test is carried out according to GB / T 22315 "Test method for elastic modulus and Poisson's ratio of metallic materials". The dynamic method is adopted to obtain the elastic modulus of the specimen.

[0089] It can be seen from Table 3 that the low-carbon, low-silicon and low-manganese construction steel of the present invention realizes the purpose of low yield point, high elongation after fracture, and good ductility and plasticity at -40°C through the controlled rolling and accelerated cooling processes, meets the requirement of good plastic deformation ability of construction steel in low-temperature cold environments, and fully meets the use requirements of high-rise building structural members at the same time.

[0090] The above embodiments are only the best examples and do not limit the implementation modes of the present invention.

Claims

1. A 180Mpa - grade low yield - strength earthquake - resistant steel for construction, with its composition and weight percentage content as follows: C: not exceeding 0.06%, Si: not exceeding 0.01%, Mn: not exceeding 0.60%, Cu: not exceeding 0.05%, N: not exceeding 0.008%, P ≤ 0.005%, S ≤ 0.003%, Alt ≤ 0.025%, and the balance is Fe and inevitable inclusions.

2. The 180Mpa - grade earthquake - resistant building steel with low yield - strength point as claimed in claim 1, wherein: The weight percentage content of C is in the range of 0.028 - 0.056%.

3. An earthquake-resistant building steel with a yield strength ratio of 180 Mpa as described in claim 1, characterized in that: The weight percentage content of Si is in the range of 0.005 - 0.009%.

4. The 180Mpa grade low yield-strength earthquake-resistant steel for construction according to claim 1, characterized in that: The weight percentage content of Mn is in the range of 0.31 - 0.55%.

5. An earthquake-resistant building steel with a yield strength ratio of 180 Mpa as described in claim 1, characterized in that: The weight percentage content of Cu is in the range of 0.02 - 0.045%.

6. The 180 Mpa grade low yield point earthquake-resistant building steel according to claim 1, characterized in that: The weight percentage content of N is in the range of 0.005 - 0.008%.

7. An earthquake-resistant building steel with a yield strength ratio of 180 MPa as described in claim 1, characterized in that: The weight percentage content of Alt is in the range of 0.015 - 0.022%.

8. A production method of the 180Mpa - grade low yield - strength earthquake - resistant steel for construction as claimed in claim 1, the steps are as follows: 1) Conduct hot metal pretreatment, control the hot metal temperature not lower than 1250°C, and after pre - desulfurization of the hot metal, control S < 0.005%; 2) Converter smelting, during which: during tapping, add quicklime at 53 - 65 Kg / ton of steel and add fluorite at 24 - 35 Kg / ton of steel for pre - refining; then add ferromanganese at 3.91 - 8.23 Kg / ton of steel for pre - deoxidation, and control the oxygen content in the molten steel after pre - deoxidation not exceeding 30 PPm; 3) Conduct LF furnace refining, during which: Argon is blown throughout the refining process, and keep the furnace in a reducing atmosphere; Conduct deoxidation and alloying. Start adding ferromanganese for deoxidation when the tapping reaches 1 / 3, add copper plates for alloying, and finish adding before the tapping reaches 2 / 3; control the tapping temperature not lower than 1650°C, and control the tapping time at 2.5 - 6 min; the oxygen content in the molten steel after deoxidation does not exceed 20 PPm; 4) Conduct RH vacuum treatment, and control the treatment time not lower than 15 min; 5) After vacuum treatment, use nitrogen blowing for alloying, and the temperature of the molten steel during nitrogen blowing is 1580 - 1600°C; 6) Cast into slabs, during which: control the superheat of the tundish molten steel at 20 - 35°C; use long - nozzle bricks and argon sealing for casting, control the casting speed at 1.13 - 1.28 m / min; control the cooling intensity in the secondary cooling section at 0.70 - 0.78 L / min; control the straightening temperature of the slab not lower than 950°C; 7) Heat the slab: control the furnace inlet temperature > 100°C; control the temperature in the heating section at 1200 - 1260°C, and the heating rate is 9 - 15 min / cm; 8) Conduct rough rolling, during which: control the rough rolling starting temperature at 1000 - 1060°C, the reduction ratio of the first rough rolling pass is not lower than 15%, and the thickness of the steel plate after rough rolling is 3.5 - 10.5 times the finished product thickness; 9) Conduct finish rolling, during which: finish rolling is carried out in the non - recrystallization zone of austenite, i.e., below 900°C, control the cumulative reduction ratio of finish rolling to be greater than 70%, the reduction ratio of the last pass is not lower than 30%, and the finish rolling final rolling temperature is 830 - 870°C; 10) Cooling is carried out, and it is cooled to the coiling temperature at a cooling rate of 35 - 45 °C / s; 11) Coiling is carried out, and the coiling temperature is controlled at 570 - 630 °C; 12) Post - processes such as uncoiling and straightening are carried out.

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