500MPa-grade niobium-containing chloride ion corrosion-resistant steel bar and rolling process thereof

Abstract

The invention relates to the field of chloride ion corrosion resistant steel bars, in particular to a 500MPa-grade niobium-containing chloride ion corrosion resistant steel bar which comprises the following components in percentage by weight: 0.02% < = C < = 0.10%, 0.1% < = Si < = 0.8%, 0.6% < = Mn < = 1.6%, 0.2% < = Cu < = 0.6%, 0.03% < = Nb < = 0.12%, 1.0% < = Cr < = 3.0%, 0.025% < = P, 0.015% < = S, 0.5% < = Ni, 0.015% < = N and the balance of Fe and inevitable impurities. The invention further relates to a rolling process of the steel bar, the steel bar is rolled and formed on a bar rolling mill through three stages of rough rolling, intermediate rolling and finish rolling, deformation induction ferrite rolling is conducted in the finish rolling stage, and the rolled steel bar meets the GB / T33953 requirement.

Description

Technical Field

[0001] The invention relates to a chloride ion corrosion resistant steel bar, in particular to a 500MPa grade niobium-containing chloride ion corrosion resistant steel bar. Background Art

[0002] Chloride ion corrosion-resistant steel bars are widely used in coastal houses, ports, cross-sea bridges and other buildings that are easily corroded by seawater, as well as various national key construction projects due to their high strength and good corrosion resistance.

[0003] Chloride ion corrosion-resistant steel bars are mainly improved by adjusting the chemical composition and controlling the post-rolling structure to improve the corrosion resistance of the steel bars while meeting the performance requirements.

[0004] The patent document with Chinese patent publication number CN104846286A discloses corrosion-resistant steel bars smelted from laterite nickel ore resources. This method mainly improves the corrosion resistance of steel by adding a certain amount of rare earth elements, but the addition of rare earth elements is not conducive to the control of steelmaking and continuous casting processes. The patent document with Chinese patent publication number CN103834871A discloses a 500MPa-grade corrosion-resistant Cr-containing steel bar and its rolling process. It mainly improves the corrosion resistance of steel bars by adding a certain amount of Cr elements, but the strength value is not easy to meet the requirements. Summary of the invention

[0005] The technical problem to be solved by the present invention is: how to provide a steel bar with a yield strength greater than 500MPa and good mechanical properties, corrosion resistance, fatigue performance and comprehensive processing performance.

[0006] The technical scheme adopted by the present invention is: a 500MPa grade niobium-containing chloride ion corrosion-resistant steel bar, wherein the steel bar components are, by weight percentage, 0.02%≤C≤0.10%, 0.1%≤Si≤0.8%, 0.6%≤Mn≤1.6%, 0.2%≤Cu≤0.6%, 0.03%≤Nb≤0.12%, 1.0%≤Cr≤3.0%, P≤0.025%, S≤0.015%, Ni≤0.5%, N≤0.015%, and the rest are Fe and inseparable impurities. The steel bar is rolled and formed on a bar rolling mill through three stages of rough rolling, intermediate rolling and finish rolling, and deformation-induced ferrite rolling is performed in the finish rolling stage. After rolling, the steel bar has a yield strength ReL ≥500MPa and a tensile strength Rm≥ 630MPa, yield ratio R°eL / ReL≤1.3, strength yield ratio R°m / R°eL≥1.25, maximum force total elongation Agt≥9.0%, steel bar relative corrosion rate≤70%, in line with GB / T33953 requirements. Deformation induced ferrite rolling is carried out in the finishing stage, which can effectively increase the ferrite content in the rolled material while increasing the strength. Increasing the ferrite content can not only increase the plasticity value of the finished product, but also further improve the corrosion resistance of the steel.

[0007] As a preferred embodiment, the steel bar components, by weight percentage, are 0.06%≤C≤0.07%, 0.45%≤Si≤0.55%, 1.20%≤Mn≤1.35%, 0.3%≤Cu≤0.45%, 0.07%≤Nb≤0.09%, 2.3%≤Cr≤2.6%, P≤0.020%, S≤0.015%, 0.3%≤Ni≤0.4%, N≤0.012%, and the rest are Fe and unavoidable impurities.

[0008] The steel bar is continuously rolled into shape on the bar rolling mill through three stages of rough rolling, intermediate rolling and finish rolling. During rolling, it is heated in a heating furnace at 1200-1260℃, the starting rolling temperature is 1100-1200℃, the finish rolling temperature is 880-930℃, and the upper cooling bed temperature is 850-920℃.

[0009] The temperature of the soaking section of the heating furnace is 1240±20℃, the rough rolling temperature is 1160±20℃, the finishing rolling temperature is 900±20℃, and the upper cooling bed temperature is 880±20℃.

[0010] The effects of each component of the present invention are as follows: C is an important strengthening element in steel. It mainly improves the strength of steel by solid solution strengthening. However, the higher the C content, the higher the strength and hardness of the steel, and the plasticity and toughness will decrease accordingly, and it is not good for the corrosion resistance of the steel. In order to improve the corrosion resistance of steel, it is necessary to add alloying elements such as Nb, V, Cu, and Cr. The addition of these alloying elements not only improves the corrosion resistance, but also improves the strength. Therefore, in the present invention, the C content is selected to be 0.02-0.10% of the total steel quality.

[0011] Si is a strengthening element that improves the strength of steel through solid solution strengthening. At the same time, Si can effectively deoxidize and reduce inclusions in steel. In the present invention, the Si content range is controlled to be 0.1-0.8% of the total steel quality.

[0012] Mn is also an important strengthening element in steel, and has a strong affinity with S, which can reduce the harm of S, but too high Mn will reduce the plasticity of steel. In the present invention, the Mn content is controlled at 0.6-1.6% of the total steel quality.

[0013] P can improve the strength, hardness and corrosion resistance of steel, but it is easy to cause cold brittleness and reduce the low-temperature toughness of steel. In the present invention, the P content is controlled within 0.025% of the total steel quality.

[0014] S is a harmful impurity element. MnS inclusions formed with Mn can easily become the origin of corrosion, seriously reducing the corrosion resistance of steel. Therefore, in the present invention, the S content is controlled within 0.015% of the total steel quality.

[0015] Nb is a strong carbide-forming element, and plays a strengthening role in steel through fine grain strengthening and precipitation strengthening. At the same time, the addition of niobium promotes the precipitation of deformation ferrite, which can improve the strength while also improving toughness, reducing hardness, and improving processing performance. In addition, niobium can also inhibit intergranular corrosion and has good corrosion resistance. Therefore, the Nb content in the present invention is controlled at 0.03-0.12% of the total steel mass.

[0016] Cr is a corrosion-resistant element and can significantly increase the corrosion resistance of steel. However, the Cr element reduces the austenite phase region and increases the bainite formation temperature, which is not conducive to post-rolling cooling control and microstructure control. Therefore, in the present invention, the Cr content is controlled within 1.0-3.0% of the total steel quality.

[0017] Cu is a corrosion-resistant element, which helps to improve the corrosion resistance of steel. However, too much Cu will reduce the plasticity of the steel, and it is easy to crack when bent. In the present invention, the Cu content is controlled at 0.2-0.6% of the total steel quality.

[0018] Ni can improve strength and fatigue performance as well as corrosion resistance. In the present invention, the Ni content is controlled within 0.5% of the total steel quality.

[0019] N has a solid solution strengthening effect and can form nitrides with alloy elements such as Nb and Cr to play a precipitation strengthening role, effectively improving the strength and hardness of the steel. However, too high a N content will reduce the toughness of the steel and increase the notch sensitivity. In the present invention, the N content is controlled within 0.015% of the total steel mass.

[0020] Taking into account the coordination of various chemical components in steel bars, in fact, the steel bar composition of mechanical properties, corrosion resistance, fatigue properties and comprehensive processing properties has been adjusted to: by weight percentage, 0.06%≤C≤0.07%, 0.45%≤Si≤0.55%, 1.20%≤Mn≤1.35%, 0.3%≤Cu≤0.45%, 0.07%≤Nb≤0.09%, 2.3%≤Cr≤2.6%, P≤0.020%, S≤0.015%, 0.3%≤Ni≤0.4%, N≤0.012%, and the rest is Fe and inseparable impurities.

[0021] The beneficial effect of the present invention is that the weight percentage can be fine-tuned within the composition range of the present invention according to different rolling specifications. The steel can be smelted in a converter, electric furnace or other smelting furnace according to conventional processes, and rolled on an ordinary bar mill using a controlled rolling and controlled cooling process, that is, rolling is performed by selecting appropriate heating temperature, starting rolling temperature (rough rolling temperature), finishing rolling temperature and upper cooling bed temperature. Deformation-induced ferrite rolling is performed in the finishing rolling stage, which can effectively increase the ferrite content in the rolled material while improving the strength. Increasing the ferrite content can not only improve the plasticity value of the finished product, but also further improve the corrosion resistance of the steel.

[0022] The hot-rolled steel bar produced by the present invention can meet the following requirements: yield strength ReL≥500MPa, tensile strength Rm≥630MPa, yield ratio R°eL / ReL≤1.3, strength yield ratio R°m / R°eL≥1.25, maximum force total elongation Agt≥9.0%, steel bar relative corrosion rate≤70%, which meets the requirements of GB / T33953.

[0023] The invention increases the strength and toughness of the steel, reduces the hardness, improves the processing performance, and significantly improves the corrosion resistance by adding Nb element to the components and performing deformation-induced ferrite rolling in the finishing rolling stage. DETAILED DESCRIPTION Example 1

[0024] A 500MPa grade niobium-containing chloride ion corrosion-resistant steel bar, wherein the steel bar components, by weight percentage, are 0.06% C, 0.50% Si, 1.25% Mn, 0.384% Cu, 0.085% Nb, 2.520% Cr, 0.015% P, 0.012% S, 0.0348% Ni, 0.011% N, and the remainder is Fe and inseparable impurities.

[0025] This embodiment adopts 120-ton converter for smelting and rolling on Ф350mm bar rolling mill, and the steel bar specification is Ф25mm.

[0026] During rolling, a heating furnace at 1200-1260°C is used for heating, and the temperature of the heating furnace soaking section (heating furnace temperature in Table 2) is 1250°C. The starting rolling temperature is 1030℃, the finishing rolling temperature is 910℃, and the upper cooling bed temperature is 890℃.

[0027] Deformation induced ferrite rolling is carried out in the finishing stage, which can improve the strength and effectively increase the ferrite content in the rolled material. Increasing the ferrite content can not only increase the plasticity value of the finished product, but also further improve the corrosion resistance of the steel.

[0028] The salt spray corrosion test was carried out in a periodic immersion corrosion tester. The test time was 72 hours, the temperature was 45 °C, the humidity was 70%, the corrosive medium was a 2.0% NaCl solution by mass, the cycle was 60 minutes, and the immersion time was 10 minutes. The results are shown in Table 3, which meet the test requirements of YB / T4367.

[0029] Embodiment 2-6 The difference from Example 1 is that, within the scope of the present invention, the chemical composition is slightly adjusted by weight percentage, see Table 1 for details.

[0030] Table 1, chemical element composition (weight percentage).

[0031]

[0032] Another difference from Example 1 is that, within the scope of the present invention, the heating temperature, the start rolling temperature, the finishing rolling temperature and the upper cooling bed temperature are fine-tuned, as shown in Table 2. Table 2, process parameters table.

[0033]

[0034] The same detection method as in Example 1 was used, and the results are shown in Table 3.

[0035] Table 3, mechanical properties and corrosion resistance table.

[0036]

[0037] The results show that Examples 2-6 all meet the test requirements of YB / T4367.

[0038] Comparative Example 1 The chemical composition of Comparative 1 is shown in Table 1. The rolling was performed in the same manner as in Example 1. The process parameters are shown in Table 2. The testing was performed in the same manner as in Example 2. The results are shown in Table 3.

Claims

1. A 500MPa grade niobium-containing chloride ion corrosion resistant steel bar, characterized in that: The components of the steel bar are calculated by weight percentage: 0.02%≤C≤0.10%, 0.1%≤Si≤0.8%, 0.6%≤Mn≤1.6%, 0.2%≤Cu≤0.6%, 0.03%≤Nb≤0.12%, 1.0%≤Cr≤3.0%, P≤0.025%, S≤0.015%, Ni≤0.5%, N≤0.015%, and the rest are Fe and inseparable impurities. The steel bar is rolled and formed on a bar rolling mill through three stages of rough rolling, intermediate rolling and finish rolling, and deformation induced ferrite rolling is performed in the finish rolling stage. After rolling, the steel bar has a yield strength ReL≥500MPa, a tensile strength Rm≥630MPa, a yield ratio R°eL / ReL≤1.3, a strength yield ratio R°m / R°eL≥1.25, a maximum force total elongation Agt≥9.0%, and a relative corrosion rate of the steel bar ≤70%.

2. The 500MPa grade niobium-containing chloride ion corrosion-resistant steel bar according to claim 1, characterized in that: The components of the steel bar are calculated by weight percentage: 0.06%≤C≤0.07%, 0.45%≤Si≤0.55%, 1.20%≤Mn≤1.35%, 0.3%≤Cu≤0.45%, 0.07%≤Nb≤0.09%, 2.3%≤Cr≤2.6%, P≤0.020%, S≤0.015%, 0.3%≤Ni≤0.4%, N≤0.012%, and the rest are Fe and inseparable impurities.

3. A rolling process for the 500MPa grade niobium-containing chloride ion corrosion-resistant steel bar according to claim 1, characterized in that: The steel bar is continuously rolled into shape on the bar rolling mill through three stages of rough rolling, intermediate rolling and finish rolling. During rolling, it is heated in a heating furnace at 1200-1260℃, the starting rolling temperature is 1100-1200℃, the finish rolling temperature is 880-930℃, and the upper cooling bed temperature is 850-920℃.

4. The rolling process of a 500MPa grade niobium-containing chloride ion corrosion-resistant steel bar according to claim 3, characterized in that: The temperature of the soaking section of the heating furnace is 1240±20℃, the rough rolling temperature is 1160±20℃, the finishing rolling temperature is 900±20℃, and the upper cooling bed temperature is 880±20℃.

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