420mpa grade low cost fire resistant hot rolled h-beam and method of manufacturing the same

Abstract

The application discloses a 420MPa-grade low-cost fire-resistant hot-rolled H-shaped steel and a preparation method thereof, wherein Ti elements are not added in the chemical components of the 420MPa-grade low-cost fire-resistant hot-rolled H-shaped steel, and the controlled rolling process technical parameters are as follows: a heating temperature is 1250-1300 DEG C, a holding time is greater than or equal to 3.5 hours, a starting rolling temperature is 1200-1250 DEG C, a final rolling temperature is 830-850 DEG C, and a cooling interval of a cooling bed is 15-20 mm, so that the 420MPa-grade low-cost fire-resistant hot-rolled H-shaped steel with a yield strength not less than 2 / 3 of a room temperature and not less than 340MPa under the condition of 600 DEG C and 3 hours of holding can be prepared.

Description

Technical Field

[0001] This invention belongs to the field of materials metallurgy technology, specifically relating to a low-cost refractory hot-rolled H-beam of 420MPa grade and its preparation method. Background Technology

[0002] The concept of fire-resistant steel was proposed by Japan in the 1980s.

[0003] For example, patent document CN108220798A (hereinafter referred to as Document 1) discloses a 460MPa grade earthquake-resistant and fire-resistant building steel and its preparation method. Its chemical composition is as follows: C: 0.03-0.08%, Mn: 1.0-1.8%, Si: 0.1-0.5%, Cr: 0.2-0.7%, Mo: 0.1-0.3%, Ti: 0.05-0.12%, V: 0.04-0.12%, Nb: 0.01-0.06%, Al: 0.01-0.05%, P: ≤0.008%, S: ≤0.002%, with the remainder being iron and unavoidable trace amounts of chemical elements. The 460MPa grade earthquake-resistant and fire-resistant building steel produced by this document exhibits excellent fire resistance, meeting the requirement that the yield strength after holding at 600℃ for three hours is not less than 2 / 3 of that at room temperature. Patent document CN103866188A (hereinafter referred to as Document 2) discloses a fire-resistant, corrosion-resistant, and earthquake-resistant building steel with a yield strength of 460MPa and a production method thereof. Its chemical composition by weight percentage is as follows: C: 0.095-0.180%, Si: 0.28-0.55%, Mn: 1.40-1.60%, P: ≤0.008%, S: ≤0.002%, Nb: 0.014-0.045%, Ti: 0.004-0.030%, V: 0.034-0.044%, Mo: 0.09-0.29%, W: 0.06-0.12%, Mg: 0.0080-0.0100%, Sn: 0.08-0.13%, O: ≤0.0016%. The fire resistance of the 460MPa grade fire-resistant, corrosion-resistant, and earthquake-resistant building steel produced in this document is such that its yield strength at 600℃ is not less than 2 / 3 of that at room temperature. Patent document CN102181792A (hereinafter referred to as document 3) discloses a low-cost, high-strength, high-toughness, earthquake-resistant, and fire-resistant steel and its preparation process. Its chemical composition is C 0.05-0.09%, Si 0.10-0.30%, Mn 0.60-1.00%, Mo 0.20-0.40%, Cr <0.10%, Cu <0.10%, Ni <0.10%, Nb 0.02-0.04%, V 0.01-0.04%, Ti 0.01-0.04%, Al 0.02-0.04%, N≤0.006%, P≤0.010%, S≤0.006%, as well as Fe and impurity elements. The fire resistance performance of the low-cost, high-strength, high-toughness, earthquake-resistant fire-resistant steel disclosed in this document meets the following requirement: yield strength at 600℃ ≥ 307MPa.Patent document CN111549297A (hereinafter referred to as Document 4) discloses a method for preparing high-strength, earthquake-resistant, weather-resistant, fire-resistant, low-temperature resistant, and easily weldable H-beams. The chemical composition percentage requirements are as follows: C: 0.07-0.10%, Mn: 1.20-1.30%, Si: 0.20-0.25%, Cr: 0.30-0.40%, Mo: 0.20-0.35%, Nb: 0.02-0.04%, Ti: 0.06-0.08%, V: 0.08-0.11%, Cu: 0.30-0.35%, Ni: 0.50-0.60%, P: <0.010%, S: <0.002%, with the remainder being iron and unavoidable trace amounts of chemical elements. The fire resistance performance of the high-strength, earthquake-resistant, weather-resistant, fire-resistant, low-temperature resistant, and easily weldable H-beams disclosed in this document meets the following requirement: a yield strength of not less than 320 MPa after holding at 600℃ for 3 hours. However, the steel chemical compositions disclosed in the aforementioned documents 1-4 all contain Ti, which increases the production cost of the steel to some extent. Furthermore, the method disclosed in document 1 employs a controlled rolling and cooling process, which is complex and suitable for building steel plates, but not for H-beams. The preparation process in document 3 includes sequential heat preservation, two-stage rolling, and cooling steps, which is more expensive than traditional hot rolling followed by air cooling, and cannot effectively reduce the production cost of the steel. While the high-strength, earthquake-resistant, weather-resistant, fire-resistant, low-temperature resistant, and easily weldable H-beams disclosed in document 4 can achieve a yield strength of not less than 320 MPa after holding at 600℃ for 3 hours, the results of Examples 1 and 2 show that its fire resistance does not meet the requirement that the yield strength after holding at 600℃ for 3 hours should not be less than 2 / 3 of the room temperature, thus failing to meet production needs. Summary of the Invention

[0004] To address the problems existing in the prior art, one aspect of the present invention provides a method for preparing low-cost refractory hot-rolled H-beams of 420MPa grade, comprising a steelmaking process and a rolling process. The steelmaking process includes a converter—ladle refining—VD vacuum treatment—continuous casting of shaped billets. The rolling process includes a walking beam furnace—high-pressure water descaling—rolling—cooling—straightening. In the rolling process, BD initial rolling and CCS finishing rolling are employed, and the following technical parameters are controlled: heating temperature 1250-1300℃, holding time ≥3.5 hours; initial rolling temperature 1200-1250℃; final rolling temperature 830-850℃; cooling bed spacing 15-20mm.

[0005] The chemical composition of the 420MPa grade low-cost fire-resistant hot-rolled H-beam is as follows (by mass percentage): C: 0.07-0.08%, Mn: 1.40-1.50%, Si: 0.20-0.30%, Cr: 0.30-0.40%, Mo: 0.18-0.20%, Nb: 0.02-0.04%, V: 0.08-0.10%, Cu: 0.30-0.40%, Ni: 0.20-0.30%, P≤0.010%, S≤0.005%, with the remainder being iron and unavoidable impurities.

[0006] The fire resistance performance of the 420MPa grade low-cost fire-resistant hot-rolled H-beams meets the following requirements: the yield strength after holding at 600℃ for 3 hours is not less than 2 / 3 of that at room temperature, and not less than 340MPa.

[0007] In some embodiments, the fire resistance of the 420MPa grade low-cost fire-resistant hot-rolled H-beams meets the following requirements: yield strength ≥341MPa after holding at 600℃ for 3 hours.

[0008] In some embodiments, the mechanical properties of the 420MPa grade low-cost fire-resistant hot-rolled H-beams meet the following requirements: yield strength ≥ 460MPa, tensile strength ≥ 645MPa, yield-to-tensile ratio < 0.75, elongation after fracture ≥ 22%, and low-temperature toughness: -20℃ KV2 ≥ 100J.

[0009] In some embodiments, the weathering performance of the 420MPa grade low-cost fire-resistant hot-rolled H-beams meets the following requirement: corrosion resistance factor I ≥ 6.0.

[0010] In some embodiments, the chemical composition of the 420MPa grade low-cost fire-resistant hot-rolled H-beam is as follows by mass percentage: C: 0.07-0.08%, Mn: 1.42-1.45%, Si: 0.22-0.24%, Cr: 0.34-0.35%, Mo: 0.19-0.20%, Nb: 0.03-0.04%, V: 0.08-0.10%, Cu: 0.32-0.34%, Ni: 0.25-0.28%, P≤0.010%, S≤0.005%, with the remainder being iron and unavoidable impurities.

[0011] In some embodiments, the chemical composition of the 420MPa grade low-cost fire-resistant hot-rolled H-beam is as follows by mass percentage: C: 0.07%, Mn: 1.45%, Si: 0.22%, Cr: 0.35%, Mo: 0.20%, Nb: 0.03%, V: 0.08%, Cu: 0.32%, Ni: 0.28%, P: 0.009%, S: 0.005%, with the remainder being iron and unavoidable impurities.

[0012] In some embodiments, the chemical composition of the 420MPa grade low-cost fire-resistant hot-rolled H-beam is as follows by mass percentage: C: 0.07%, Mn: 1.42%, Si: 0.24%, Cr: 0.34%, Mo: 0.19%, Nb: 0.04%, V: 0.10%, Cu: 0.34%, Ni: 0.25%, P: 0.010%, S: 0.003%, with the remainder being iron and unavoidable impurities.

[0013] Another aspect of the present invention provides a low-cost fire-resistant hot-rolled H-beam with a strength of 420MPa, which is obtained by the above-described preparation method.

[0014] This invention, through rational chemical composition design and optimized steel rolling process, enables the production of 420MPa grade fire-resistant hot-rolled H-beams with excellent fire resistance (yield strength at 600℃ for 3 hours not less than 2 / 3 of that at room temperature, and not less than 340MPa) and mechanical properties (yield strength ≥460MPa, tensile strength ≥645MPa, yield ratio <0.75, elongation after fracture ≥22%, low-temperature toughness: -20℃ KV2 ≥100J) without the addition of the chemical element Ti. This reduces production costs and minimizes or eliminates the need for anti-corrosion and fire-retardant coatings, making it a green product with no adverse effects on the environment or personnel. It creates a favorable environment for green development of enterprises, aligns with the national development concepts of low-carbon, green, and environmental protection, and improves the safety of building structures, making it suitable for high-rise buildings and large-span steel structures. Detailed Implementation

[0015] This invention aims to produce a low-cost, 420MPa grade fire-resistant hot-rolled H-beam at low cost. This is mainly achieved through a rational chemical composition design combined with an optimized rolling process. The technical solution is as follows:

[0016] A method for preparing low-cost refractory hot-rolled H-beams with a strength of 420MPa includes a steelmaking process and a rolling process. The steelmaking process includes a converter, ladle refining, VD vacuum treatment, and continuous casting of shaped billets. The rolling process includes a walking beam furnace, high-pressure water descaling, rolling, cooling, and straightening. In the rolling process, BD initial rolling and CCS finishing rolling are employed, and the following technical parameters are controlled: heating temperature 1250-1300℃, holding time ≥3.5 hours; initial rolling temperature 1200-1250℃; final rolling temperature 830-850℃; cooling bed spacing 15-20mm.

[0017] The chemical composition of the 420MPa grade low-cost fire-resistant hot-rolled H-beam is as follows (by mass percentage): C: 0.07-0.08%, Mn: 1.40-1.50%, Si: 0.20-0.30%, Cr: 0.30-0.40%, Mo: 0.18-0.20%, Nb: 0.02-0.04%, V: 0.08-0.10%, Cu: 0.30-0.40%, Ni: 0.20-0.30%, P≤0.010%, S≤0.005%, with the remainder being iron and unavoidable impurities.

[0018] The fire resistance performance of the 420MPa grade low-cost fire-resistant hot-rolled H-beams meets the following requirements: the yield strength after holding at 600℃ for 3 hours is not less than 2 / 3 of that at room temperature, and not less than 340MPa.

[0019] The reasons for limiting the main chemical components of the invention are as follows:

[0020] C: C is the most effective element for improving the strength of steel. Increasing the C content increases the tensile strength and yield strength of the steel, but decreases elongation and impact toughness, reduces corrosion resistance, and causes hardening in the weld heat-affected zone, leading to cold cracking. To ensure good overall performance of the steel plate, the C content of the steel in this invention is designed to be 0.07-0.08%.

[0021] Mn: Mn is an important strengthening and toughening element, and its content is low. With increasing manganese content, the strength of steel is significantly improved, and its processing performance is enhanced, while the ductile-brittle transition temperature remains almost unchanged. However, excessively high manganese content can inhibit ferrite transformation, affecting the yield strength of the steel and hindering the control of the yield-to-tensile ratio. The Mn content of the steel in this invention is designed to be 1.40-1.50%.

[0022] Cu: In addition to having largely the same effects as Ni, Cu precipitates also improve the high-temperature strength and atmospheric corrosion resistance of steel. The Cu content of the steel in this invention is designed to be 0.30-0.40%.

[0023] Ni: Ni is a relatively stable element. Adding Ni can change the self-corrosion potential of steel in a positive direction, increasing the stability of the steel. In addition, the main purpose of adding Ni is to form Cu-Ni alloy with Cu, preventing "copper embrittlement". Taking all factors into consideration, the Ni content should be controlled within the range of 0.20%-0.30%.

[0024] Cr: Cr can improve the strength, hardness, and atmospheric corrosion resistance of steel, and the effect is more significant when added with other alloying elements. Chromium can slow down the decomposition rate of austenite, significantly improve the hardenability of steel, and has a secondary hardening effect, but it also increases the steel's tendency to temper brittleness. However, excessive chromium content will reduce the toughness of the base material and the heat-affected zone. The Cr content of the steel in this invention is designed to be 0.30-0.40%.

[0025] Mo: Mo can improve the uniformity of corrosion and inhibit localized corrosion. Furthermore, Mo is also the most effective element for improving the high-temperature strength of steel plates. Generally, the higher its content, the higher the high-temperature strength; however, Mo is expensive, and excessive amounts can cause a decrease in weldability. The Mo content of the steel in this invention is designed to be 0.18-0.20%.

[0026] Nb: During rolling, Nb dissolved in austenite and deformation-induced precipitation of niobium carbonitride particles significantly increases the non-recrystallization temperature of austenite, refines the austenite grains, and further refines the grains of ferrite and other materials, thus improving strength. Nb dissolved in austenite also improves hardenability; during quenching, precipitated Nb carbide particles or Nb carbides combined with V and Mo to precipitate a second phase, enhancing high-temperature strength. Therefore, the Nb content in the steel of this invention is designed to be 0.02-0.04%.

[0027] Si (Si): Si improves the corrosion resistance of steel and is often added to stainless steel, low-alloy steel, and corrosion-resistant alloys to enhance their corrosion resistance, giving them resistance to chloride stress corrosion cracking, pitting corrosion, hot concentrated nitric acid corrosion, oxidation, and seawater corrosion. Studies have shown that in humid and hot atmospheric environments, Si significantly improves the atmospheric corrosion resistance of carbon steel and low-alloy steel. Furthermore, Si can improve the corrosion resistance of low-alloy steel in seawater splash zones. The Si content of the steel in this invention is designed to be 0.20-0.30%.

[0028] P and S: P and S are impurity elements in steel. P has a certain effect on improving corrosion resistance, but it is an element that easily segregates, causing severe segregation in localized areas of the steel, reducing plasticity and toughness, and is extremely detrimental to low-temperature toughness. S is also prone to segregation and enrichment in steel, and is a detrimental element to corrosion resistance. The steel of this invention strictly controls the sulfur and phosphorus content levels in terms of metallurgical quality, i.e., P ≤ 0.010% and S ≤ 0.005%, to meet the requirements of the steel grade for purity, impact toughness, weldability, and corrosion resistance.

[0029] V: V has a strong bonding ability with C, O, and N, forming extremely stable compounds with them. This refines the grain size and reduces the heat sensitivity and temper brittleness of steel. It can significantly improve the weldability of ordinary low-alloy steels. The V content of the steel in this invention is designed to be 0.08-0.10%.

[0030] In some preferred embodiments, the fire resistance of the 420MPa grade low-cost fire-resistant hot-rolled H-beam meets the following requirements: yield strength ≥341MPa after holding at 600℃ for 3 hours.

[0031] In some preferred embodiments, the mechanical properties of the 420MPa grade low-cost fire-resistant hot-rolled H-beams meet the following requirements: yield strength ≥ 460MPa, tensile strength ≥ 645MPa, yield-to-tensile ratio < 0.75, elongation after fracture ≥ 22%, and low-temperature toughness: -20℃ KV2 ≥ 100J.

[0032] In some preferred embodiments, the weathering performance of the 420MPa grade low-cost fire-resistant hot-rolled H-beams meets the following requirement: corrosion resistance factor I ≥ 6.0.

[0033] In some preferred embodiments, the chemical composition of the 420MPa grade low-cost fire-resistant hot-rolled H-beam is as follows by mass percentage: C: 0.07-0.08%, Mn: 1.42-1.45%, Si: 0.22-0.24%, Cr: 0.34-0.35%, Mo: 0.19-0.20%, Nb: 0.03-0.04%, V: 0.08-0.10%, Cu: 0.32-0.34%, Ni: 0.25-0.28%, P≤0.010%, S≤0.005%, with the remainder being iron and unavoidable impurities.

[0034] In some preferred embodiments, the chemical composition of the 420MPa grade low-cost fire-resistant hot-rolled H-beam is as follows by mass percentage: C: 0.07%, Mn: 1.45%, Si: 0.22%, Cr: 0.35%, Mo: 0.20%, Nb: 0.03%, V: 0.08%, Cu: 0.32%, Ni: 0.28%, P: 0.009%, S: 0.005%, with the remainder being iron and unavoidable impurities.

[0035] In some preferred embodiments, the chemical composition of the 420MPa grade low-cost fire-resistant hot-rolled H-beam is as follows by mass percentage: C: 0.07%, Mn: 1.42%, Si: 0.24%, Cr: 0.34%, Mo: 0.19%, Nb: 0.04%, V: 0.10%, Cu: 0.34%, Ni: 0.25%, P: 0.010%, S: 0.003%, with the remainder being iron and unavoidable impurities.

[0036] The present invention will be described in detail below through specific embodiments. These embodiments are intended to help understand the present invention and are not intended to limit the scope of the present invention.

[0037] Example

[0038] Table 1 shows the chemical composition and weight percentage of hot-rolled H-beams in the various embodiments and comparative examples of the present invention.

[0039] Table 2 shows the heating, rolling, and cooling process parameters for the hot-rolled H-beams in the various embodiments and comparative examples of the present invention.

[0040] Table 3 shows the test results of the mechanical, fire-resistant, and weather-resistant properties of the hot-rolled H-beams in the various embodiments and comparative examples of the present invention.

[0041] The embodiments of the present invention are produced according to the following process steps:

[0042] Steelmaking process: converter — ladle refining — VD vacuum treatment — continuous casting of special-shaped billets, wherein the smelting is carried out in a converter or electric furnace and the casting is carried out in a continuous casting process; specifically, according to the chemical composition requirements shown in Table 1 below, the smelting is carried out in a converter or electric furnace and the casting is carried out in a continuous casting process to obtain a continuously cast billet.

[0043] Steel rolling process: walking beam furnace—high pressure water descaling—BD primary rolling—CCS finishing rolling—cooling—straightening—sawing—inspection—packaging—warehousing. The controlled technical parameters are as follows: after the continuous casting billet or the billet is opened, it is loaded into the heating furnace for heating at a temperature of 1250-1300℃ for a time of ≥3.5 hours. After heating, the initial rolling temperature is 1200-1250℃, and the final rolling temperature is 830-850℃. The cooling bed spacing is 15-20mm, and the temperature is cooled to room temperature.

[0044] Table 1: Chemical composition and weight percentage of the examples and comparative examples

[0045] Example C Si Mn P S Cu Ni Cr Nb V Mo 1 0.07 0.22 1.45 0.009 0.005 0.32 0.28 0.35 0.03 0.08 0.20 2 0.07 0.24 1.42 0.010 0.003 0.34 0.25 0.34 0.04 0.10 0.19 Comparative Example 1 0.07 0.22 1.45 0.009 0.005 0.32 0.28 0.35 0.03 0.08 0.20 Comparative Example 2 0.07 0.22 1.45 0.009 0.005 0.32 0.28 0.35 0.03 0.08 0.20 Comparative Example 3 0.07 0.22 1.45 0.009 0.005 0.32 0.28 0.35 0.03 0.08 0.20

[0046] Table 2: Heating, rolling, and cooling process parameters for the examples and comparative examples

[0047] Example Heating / ℃ Rolling temperature / ℃ Final rolling temperature / ℃ Cooling bed spacing / mm 1 1250 1200 830 15 2 1300 1250 850 20 Comparative Example 1 1200 1150 880 9 Comparative Example 2 1200 1150 810 10 Comparative Example 3 1250 1150 850 8

[0048] Table 3: Test results of mechanical, fire-resistant, and weather-resistant properties of hot-rolled H-beams in each embodiment and comparative example.

[0049]

[0050] As can be seen from the data in Table 3 above, the steel products of the embodiments of the present invention have excellent fire resistance, mechanical properties, and weather resistance. The fire resistance meets the following requirements: yield strength at 600℃ for 3 hours is not less than 340 MPa, preferably ≥341 MPa. The mechanical properties meet the following requirements: yield strength ≥460 MPa, tensile strength ≥645 MPa, yield ratio <0.75, elongation after fracture ≥22%, and low-temperature toughness: -20℃ KV2 ≥100 J. The weather resistance meets the following requirement: corrosion resistance factor I ≥6.0. As can be seen from Comparative Examples 1-3, although they use the same chemical composition design as Example 1, the controlled rolling process parameters are different from those of Example 1. This results in the obtained hot-rolled H-beams exhibiting inferior fire resistance and / or mechanical properties compared to the hot-rolled H-beams prepared in the embodiments of the present invention.

[0051] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of producing a 420 MPa grade low cost fire resistant hot rolled H-beam characterized in that, The preparation method comprises a steelmaking process and a rolling process, wherein the steelmaking process comprises a converter- secondary refining- VD vacuum treatment- an abnormal blank continuous casting; the rolling process comprises a step-by-step heating furnace- high-pressure water descaling- rolling- cooling- straightening; wherein BD rough rolling and CCS finish rolling are adopted in the rolling, and the following technical parameters are controlled: heating temperature 1250-1300 DEG C, holding time greater than or equal to 3.5 hours; rough rolling temperature 1200-1250 DEG C; finish rolling temperature 830-850 DEG C; cooling interval of the cooling bed 15-20 mm; The chemical composition of the 420 MPa grade low-cost fire-resistant hot-rolled H-shaped steel comprises, by mass percent: C: 0.07-0.08%, Mn: 1.40-1.50%, Si: 0.20-0.30%, Cr: 0.30-0.40%, Mo: 0.18-0.20%, Nb: 0.02-0.04%, V: 0.08-0.10%, Cu: 0.30-0.40%, Ni: 0.20-0.30%, P≤0.010%, S≤0.005%, and the balance being iron and inevitable impurities; and no Ti element is added in the chemical composition. The fire resistance of the 420 MPa grade low-cost fire-resistant hot-rolled H-shaped steel satisfies that the yield strength at 600 DEG C for 3 hours is not less than 2 / 3 of the yield strength at room temperature, and is not less than 340 MPa. The mechanical properties of the 420 MPa grade low-cost fire-resistant hot-rolled H-shaped steel satisfy that the yield strength is greater than or equal to 460 MPa, the tensile strength is greater than or equal to 645 MPa, the yield strength ratio is less than 0.75, the elongation after fracture is greater than or equal to 22%, and the low-temperature toughness is greater than or equal to 100 J at -20 DEG C.

2. The production method according to claim 1, characterized by, The fire resistance of the 420 MPa grade low-cost fire-resistant hot-rolled H-shaped steel satisfies that the yield strength at 600 DEG C for 3 hours is greater than or equal to 341 MPa.

3. The production method according to claim 1 or 2, characterized by, The weather resistance of the 420 MPa grade low-cost fire-resistant hot-rolled H-shaped steel satisfies that the corrosion resistance factor I is greater than or equal to 6.

0.

4. The production method according to claim 1 or 2, characterized by, The chemical composition of the 420 MPa grade low-cost fire-resistant hot-rolled H-shaped steel comprises, by mass percent: C: 0.07-0.08%, Mn: 1.40-1.50%, Si: 0.20-0.30%, Cr: 0.30-0.40%, Mo: 0.18-0.20%, Nb: 0.02-0.04%, V: 0.08-0.10%, Cu: 0.30-0.40%, Ni: 0.20-0.30%, P≤0.010%, S≤0.005%, and the balance being iron and inevitable impurities.

5. The preparation method according to claim 4, characterized in that, The chemical composition of the 420 MPa grade low-cost fire-resistant hot-rolled H-shaped steel comprises, by mass percent: C: 0.07-0.08%, Mn: 1.40-1.50%, Si: 0.20-0.30%, Cr: 0.30-0.40%, Mo: 0.18-0.20%, Nb: 0.02-0.04%, V: 0.08-0.10%, Cu: 0.30-0.40%, Ni: 0.20-0.30%, P≤0.010%, S≤0.005%, and the balance being iron and inevitable impurities.

6. The preparation method according to claim 4, characterized in that, The chemical composition of the 420 MPa grade low-cost fire-resistant hot-rolled H-beam steel is as follows in terms of mass percentage: C: 0.07%, Mn: 1.42%, Si: 0.24%, Cr: 0.34%, Mo: 0.19%, Nb: 0.04%, V: 0.10%, Cu: 0.34%, Ni: 0.25%, P: 0.010%, S: 0.003%, and the balance being iron and inevitable impurities.

7. A 420 MPa grade low-cost fire-resistant hot-rolled H-beam steel obtained by the preparation method of any one of claims 1-6.

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