CN101775552B - High-strength high-flexibility low-yield-ratio fire resistant steel and production method thereof - Google Patents

Abstract

The invention provides a high-strength fire-resistant steel with room temperature yield strength greater than 460 MPa, yield strength ratio less than 0.80, 600°C high temperature yield strength greater than 2/3 of the room temperature yield strength (ie ≥ 307 MPa), and good impact toughness. And the weight percentage content is: C: 0.05～0.14, Si: 0.15～0.50, Mn: 0.5～1.2, P≤0.025, S≤0.015, Cu: 0.10-0.40, Ni: 0.10-0.50, Nb: 0.015-0.050 , Ti: 0.005-0.040, V: 0.010-0.050, Cr: 0.10-0.50, Mo: 0.20-0.50, Al: 0.015-0.05, and the balance is Fe and inevitable inclusions. The present invention also provides the manufacturing method of the refractory steel, which includes: adopting vacuum smelting to obtain molten steel, and casting it into a billet; reheating the billet to 1150-1250°C for 1.5-2.5 hours, then controlling rolling to the finished plate thickness, The final rolling temperature is controlled at about 820-950°C. After rolling, the steel plate is air-cooled or quickly water-cooled to 450-650°C and then air-cooled.

Description

A high-strength, high-toughness, low-yield-ratio refractory steel and its manufacturing method

technical field

The invention belongs to the field of low-alloy steel manufacturing, and relates to a high-temperature steel alloy with a room temperature yield strength greater than 460 MPa, a yield strength ratio less than 0.80, a high temperature yield strength of 600°C greater than 2/3 of the room temperature yield strength (ie ≥ 307 MPa), and good impact toughness. Strength refractory steel and method for its production.

Background technique

Ordinary construction steel, such as SN400 and SN490 steel in JIS standard, Q235 and Q345 in Chinese national standard, etc., the yield strength drops rapidly to less than 2/3 of its room temperature yield strength at a high temperature above 350 °C, and almost no at 600 °C. It has bearing capacity and cannot meet the design requirements of modern steel structure high-rise buildings. In order to improve the fire-resistant ability of steel structures, Japanese researchers proposed the concept of fire-resistant steel in the 1980s. By adding a small amount of alloying elements such as chromium (Cr), molybdenum (Mo), and niobium (Nb) to the steel, a fire-resistant steel for construction with a refractory temperature of 600°C has been developed, and the yield strength at 600°C is kept at 2/ of the room temperature value. 3 or more. The prosperity and development of steel structure buildings has promoted the research and development of fire-resistant steel. At present, refractory steel mainly has a yield strength of 235MPa and 345MPa (Japanese 400MPa grade steel and 490MPa grade steel). It mainly ensures the strength at high temperature by adding more Mo, usually Mo content exceeds 0.50%. In recent years, Japan has developed a refractory steel with a yield strength of 440MPa (590MPa grade). The Mo content in the steel has reached 0.7%. The weldability of the steel is very poor and the cost is high, which seriously restricts its application. At present, the research frontiers of refractory steel mainly include:

(1) Increase the yield strength and reduce the yield ratio; (2) Reduce the content of alloying elements.

The patent CN 1524976A "fire-resistant steel and its manufacturing method" applied by Anshan Iron and Steel Company provides high-performance fire-resistant steel with strength levels of 235MPa and 345MPa. It is mainly composed of Cr and Mo alloying elements, supplemented by copper (Cu) and niobium (Nb) , Vanadium (V), Nickel (Ni), Titanium (Ti), Aluminum (Al), Boron (B) and Rare Earth (RE) and other elements are produced by controlled rolling and controlled cooling processes. The refractory steel provided by this patent has a lower strength grade and a higher content of alloying elements. Nippon Steel's patent CN 101111621A "High-strength fire-resistant steel with excellent weldability and gas cutting performance and its manufacturing method" in China provides a high-strength fire-resistant steel with a yield strength of 440MPa or more, which is mainly strengthened by the composite of Mo and Nb The effect is to increase the strength to above 440MPa and reduce the Mo content to below 0.70%. Its manufacturing process is online direct quenching and tempering or offline quenching and tempering process. The refractory steel has high content of alloying elements, Mo content of about 0.5% or even higher, and high yield strength ratio of greater than 0.80. The patent JP 2077523 "Production of buildinglow yield ratio steel having excellent fire resistance and building steelmaterial using same steel" applied by Nippon Steel provides a low yield ratio fire-resistant steel and its manufacturing method. The yield strength of the steel plate at room temperature ranges from 235MPa to 440MPa, Yield strength ratio≤0.80, yield strength at 600°C/yield strength at room temperature≥2/3. The manufacturing method is to heat at 1100-1300°C, finish rolling at 800-1000°C, and then air-cool to room temperature. The refractory steel plate has relatively low yield strength and high content of alloying elements.

Contents of the invention

The object of the present invention is to provide a high-performance fire-resistant steel with high strength, high toughness and low yield ratio and its manufacturing method. This fire-resistant steel has excellent mechanical properties: yield strength ≥ 460Pa, yield strength ratio ≤ 0.80, high temperature yield strength at 600°C is greater than 2/3 room temperature yield strength (≥ 307MPa), and has good impact toughness. The applicable fields are mainly steel structure buildings and other steel structure fields that require fire resistance, such as three-dimensional garages, supermarkets, etc.

The fire-resistant steel provided by the present invention has chemical composition (by weight percentage): C: 0.05-0.14, Si: 0.15-0.50, Mn: 0.5-1.2, P≤0.025, S≤0.015, Cu: 0.10-0.40, Ni : 0.10-0.50, Nb: 0.015-0.050, Ti: 0.005-0.040, V: 0.010-0.050, Cr: 0.10-0.50, Mo: 0.20-0.50, Al: 0.015-0.05, the balance is Fe and unavoidable inclusions .

The content of C in the fire-resistant steel of the invention is controlled at 0.05-0.14%. C is one of the cheap strengthening elements in steel. In order to ensure the high strength of steel, the lower limit of C content is 0.05%. However, too much C content will adversely affect the strength and toughness matching and weldability of the steel, so the upper limit of the C content is 0.14%.

The content of Si in the fire-resistant steel of the invention is controlled at 0.15-0.50%. Si is one of the necessary elements for deoxidation in steelmaking, and Si can improve the strength of steel through solid solution strengthening. However, Si has an adverse effect on weldability, toughness, etc., and the upper limit must be limited.

The content of Mn in the refractory steel of the present invention is controlled at 0.5-1.2%, and Mn is also one of the necessary elements for steelmaking and deoxidation, and Mn is indispensable for ensuring strength, and its lower limit is 0.5%. However, too high Mn content will adversely affect the weldability of the steel, and may make the center segregation more serious, and the upper limit is controlled at 1.2%.

In the fire-resistant steel of the present invention, the P content is ≤0.025%, and the S content is ≤0.015%. S and P are harmful impurity elements in steel. If the content is too high, the toughness of the steel will be deteriorated. Therefore, the content of sulfur and phosphorus in the steel should be reduced as much as possible.

The Cu content in the fire-resistant steel of the invention is controlled at 0.10-0.40%. Cu usually exists in the steel in the form of fine precipitated particles, which produces precipitation strengthening. Cu can not only improve the strength, but also improve the atmospheric corrosion resistance of steel. However, when the Cu content is high, it will cause cracks in the billet heating or hot rolling process, and deteriorate the surface properties of the steel, so the upper limit is controlled at 0.40%.

The content of Ni in the fire-resistant steel of the invention is controlled at 0.10-0.50%. Ni can improve the low-temperature toughness while increasing the strength of the steel. In order to exert the effect of Ni, at least 0.10% is required. At the same time, Ni can reduce the tendency of Cu-containing steel to produce steel surface defects. But Ni is a precious element, so the upper limit of Ni content is controlled at 0.50%.

The content of Nb in the fire-resistant steel of the present invention is controlled to be 0.015-0.05%. Nb is a strong carbonitride forming element, which can increase the strength of steel by precipitating carbonitrides in the cooling process after rolling to produce fine grain strengthening and precipitation strengthening. In addition, the high dissolution temperature of Nb precipitates can increase the strength of steel at 600°C and improve the fire resistance, but too high Nb content may also have adverse effects.

The content of Ti in the fire-resistant steel of the present invention is controlled to be 0.005-0.040%. Ti is a strong carbonitride forming element, and its carbonitride has a high melting point, which hinders the growth of austenite grains during heating. Moreover, the precipitation during the cooling process after rolling will increase the yield strength of the steel, and is beneficial to the high temperature strength of the steel. However, if the Ti content is too high, coarse precipitated particles may appear, and the upper limit is controlled at 0.040%.

The content of V in the fire-resistant steel of the present invention is controlled to be 0.010-0.050%. V is a strong carbonitride forming element, and its strengthening effect is similar to that of Nb, but the effect is relatively small. When the V content is high, the low-temperature toughness of the steel is obviously deteriorated, especially the toughness of the welding heat-affected zone, and the upper limit is controlled at 0.050%.

The content of Cr in the fire-resistant steel of the invention is controlled at 0.10-0.50%. Cr is an element that improves hardenability and can increase the strength and hardness of steel. However, too high Cr content will affect the toughness of steel and cause temper brittleness, and its upper limit is controlled at 0.50%.

The Mo content in the fire-resistant steel of the invention is controlled at 0.20-0.50%. Mo is the most effective element to improve the high temperature strength of steel, the higher the Mo content, the higher the high temperature strength. However, the cost of Mo is relatively high, and the weldability of the steel is reduced, so the Mo content is controlled at a maximum of 0.50%.

The content of Al in the refractory steel of the invention is controlled at 0.015-0.05%. Al is one of the effective deoxidizing elements and can form nitrides to refine grains. Excessive Al content will damage the toughness of the steel, and the toughness obtained by welding heat will also become poor.

In the fire-resistant steel of the present invention, the preferred content ranges of its components are C: 0.06-0.09, Si: 0.25-0.35, Mn: 0.8-1.0, P≤0.025, S≤0.015, Cu: 0.20-0.30, Ni: 0.15-0.35 .

The present invention also provides a production method of the high-strength, high-toughness and low-yield-ratio refractory steel. The basic process includes:

Vacuum smelting is used to obtain molten steel with the composition and content of the present invention, and cast into a billet; after the billet is reheated to 1150-1250 ° C for 1.5-2 hours, it is controlled to roll to the finished plate thickness, and the final rolling temperature is controlled at about 820-950 ℃, after rolling, the steel plate is air-cooled or quickly water-cooled to 450-650 ℃ and then air-cooled.

Fire-resistant steel of the present invention has following characteristics:

1) The more expensive alloy elements have less content and lower cost;

2) The controlled rolling and cooling process is adopted for production, without post-rolling heat treatment. The production process is simple and easy to operate;

3) It has high strength, low yield ratio, good toughness and weldability, and has excellent high-temperature strength, which can meet the requirements of steel structures for high-strength refractory steel with low yield ratio.

The comparison of fire-resistant steel of the present invention and the composition range, process system and mechanical properties of steel grades in the prior art is shown in the following table:

Example

According to the chemical composition requirements of the refractory steel of the present invention, the chemical composition of the obtained steel is smelted in a 500kg vacuum induction furnace, and the chemical composition of the obtained steel is shown in the following table, and the balance is Fe and inevitable inclusions. The molten steel is poured into billets, and then the billets are heated to 1150-1250°C and held for 2 hours. The steel ingots are rolled above 1000°C into 60mm intermediate slabs, and the temperature of the slabs is 850-980°C. For 20mm thick steel plate, the final rolling temperature is controlled at 820-950°C. Air-cool to room temperature directly after steel rolling, or take rapid water cooling to 450-650°C (cooling rate about 10-30°C/s) and then air-cool to room temperature. The specific process parameters and mechanical properties are shown in the table below. The table also lists the chemical composition, processing technology and corresponding mechanical properties of the aforementioned comparative steel grades.

The room temperature tensile strength measurement of the steel plate is carried out in accordance with GB/T 228-2002 "Metallic Materials Tensile Test Method at Room Temperature", the sample size is M16Φ10L120; the high temperature tensile strength measurement of the steel plate is in accordance with GB/T 4338-2006 "Metallic Materials Tensile test method", the sample size is M16Φ10L112, the sample is heated to 600°C and held for 15 minutes, and then the tensile test is started, and the sample is kept at 600°C during the stretching process. The impact toughness measurement of the steel plate is carried out in accordance with GB/T 229-2007 "Metallic Materials Charpy Pendulum Impact Test Method", the sample size is 10*10*55, V-shaped notch.

It can be seen that, by adding less precious metal elements, the present invention can obtain high strength with room temperature yield strength exceeding 460 MPa, yield strength ratio lower than 0.80%, yield strength greater than 307 MPa at 600°C, and excellent toughness by adopting controlled rolling and controlled cooling process. Refractory steel. The fire-resistant steel of the invention can be widely used in steel structure buildings and other steel structure fields requiring fire-resistant performance.

Claims (3)

1. A method for manufacturing fire-resistant steel, the composition and weight percentage of which are: C: 0.05-0.14, Si: 0.15-0.50, Mn: 0.5-1.2, P≤0.025, S≤0.015, Cu: 0.10-0.40 , Ni: 0.10-0.50, Nb: 0.015-0.050, Ti: 0.005-0.040, V: 0.010-0.050, Cr: 0.10-0.50, Mo: 0.20-0.50, Al: 0.015-0.05. the inclusion of; The manufacturing methods of the steel include: Molten steel is obtained by vacuum smelting and cast into billets; after the billets are reheated to 1150-1250°C for 1.5-2.5 hours, they are rolled to the finished plate thickness under control, and the final rolling temperature is controlled at 820-950°C. After rolling, the steel plates are air-cooled or Rapid water cooling to 450~650°C and then air cooling. 2. The method according to claim 1, characterized in that its composition and weight percentage are: C: 0.06～0.09, Si: 0.25～0.35, Mn: 0.8～1.0, P≤0.025, S≤0.015, Cu: 0.20-0.30, Ni: 0.15-0.35, Nb: 0.025-0.035, Ti: 0.015-0.025, V: 0.025-0.045, Cr: 0.25-0.40, Mo: 0.20-0.40, Al: 0.025-0.045, balance For Fe and inevitable inclusions. 3. The refractory steel manufactured by the method according to claim 1 or 2.