High-strength high-corrosion-resistance economical austenitic stainless steel for automobile fuel tank

Abstract

The invention discloses high-strength and high-corrosion-resistance economical austenitic stainless steel for an automobile fuel tank. The high-strength and high-corrosion-resistance economical austenitic stainless steel comprises the following chemical components in percentage by weight: 0.05-0.12% of C, 0.2-1.2% of Si, 5.0-10.0% of Mn, 17.0-21.0% of Cr, 1.5-3.5% of Ni, 0.8-2.5% of Cu, less than or equal to 0.045% of P, less than or equal to 0.0050% of S, 0.15-0.25% of N and the balance of Fe. 0.005% to 0.20% of Nb; ti: 0.005% to 0.20%; b: 0.0005% to 0.0080% of the total weight of the Nb + Ti is greater than or equal to 0.015% and less than or equal to 0.3%, and the balance is Fe and inevitable impurities. Through reasonable component design, the steel keeps high corrosion resistance, the pitting potential reaches up to 450 mv or above, and the pitting potential is far higher than that of 304 austenitic stainless steel with the nickel content being 8%.

Description

Technical Field

[0001] The invention relates to a steel smelting process, in particular to a high-strength, high-corrosion-resistant, economical austenitic stainless steel for automobile fuel tanks. Background Art

[0002] As a critical automotive safety component, lightweighting fuel tanks is a crucial component of vehicle lightweighting. Fuel tanks are typically made of either composite plastic or metal. Each has its own advantages and disadvantages depending on vehicle design, operating environment, and user material preferences. However, with the introduction of stringent environmental regulations and advancements in steel manufacturing processes, the advantages of steel over plastic are becoming increasingly apparent. Improved fuel quality is placing higher demands on metal materials used in fuel tanks: high strength, high corrosion resistance, and ease of forming. Hot-dip galvanized steel sheets, such as DC01, are widely used in domestic carbon steel fuel tanks. The thickness ranges from 1.5mm to 1.2mm, depending on the tank type. With a yield strength of 150 MPa and an elongation of approximately 30%, these sheets can meet complex forming and welding requirements. As fuel tank capacity and safety requirements increase, thicker materials are often required. Therefore, carbon steel fuel tanks also face challenges in lightweighting and environmentally friendly upgrades.

[0003] Compared to carbon steel, stainless steel possesses unparalleled corrosion resistance and can reduce the need for the galvanizing process required to improve corrosion resistance, thereby reducing environmental pollution. 304 stainless steel is the most common material used in stainless steel fuel tanks. As we all know, 304 stainless steel contains over 8% nickel, a key austenite-forming element in austenitic stainless steel. However, due to its high price, nickel is also one of the elements that increases material costs. Furthermore, the yield strength of 304 stainless steel is typically around 250 MPa, making it a minor contribution to vehicle lightweighting. In addition to 304, ferritic stainless steel is also available in the stainless steel fuel tank market, but it lacks advantages in terms of complex forming or achieving lightweighting through high strength.

[0004] Patent publication number CN104878316A discloses a high-strength, high-toughness, high-nitrogen, low-austenitic stainless steel. Its composition design includes a nitrogen content of 0.5-0.95% and a manganese content of 13-19%. Although this stainless steel exhibits high strength, its high nitrogen content makes its production extremely difficult, particularly through traditional continuous casting processes. Furthermore, its excessive strength hinders forming.

[0005] Patent authorization announcement number CN102337481B discloses a molybdenum-containing, nickel-saving austenitic stainless steel with excellent corrosion resistance. Its composition is designed to contain 0.4-0.8% Mo, 16.5-18% Cr, and 1.5-3.5% Ni. The pitting potential is generally below 320mV, making it unsuitable for applications such as automotive fuel tanks, which require high corrosion resistance.

[0006] Patent publication number CN106133177A discloses a high-strength and high-ductility austenitic high-manganese stainless steel. Its composition is designed to contain a manganese content of 14-26% and a nickel content of less than 0.8%. This stainless steel supposedly utilizes the TWIP effect to achieve high strength and ductility. However, the high nitrogen content, reaching 4000ppm, makes smelting difficult, and excessively high manganese content also hinders forming.

[0007] Patent publication number CN101289729A discloses a nickel-free metastable austenitic stainless steel with the TRIP effect. It has a chromium content of 15-16.5%, a yield strength of 390-1520 MPa, and a tensile strength of 745-1740 MPa. This stainless steel is achieved through high-pressure work hardening, and its elongation, even in the solid solution state, is only 20%, making it unsuitable for use in high-formability industries.

[0008] The nickel-saving austenitic stainless steel disclosed in the above existing patents increases its strength either through high-N alloy strengthening or through direct deformation strengthening. The former is difficult to produce and difficult to implement with traditional continuous casting processes. Furthermore, the welding process for high-N stainless steel is also difficult to compensate for the reduced corrosion resistance of the high-N stainless steel welds due to nitrogen escape. The latter, because the material undergoes deformation strengthening, it increases internal stress in the material and weakens its processing properties. Therefore, neither of these can meet the overall requirements of current automotive fuel tanks for high strength, high corrosion resistance, and easy formability. In view of this, there is an urgent need to develop a highly corrosion-resistant, low-cost, high-strength, and easily formable austenitic stainless steel to provide a cost-effective alternative for current automotive fuel tank materials. Summary of the Invention

[0009] The object of the present invention is to provide a high-strength, high-corrosion-resistant, economical austenitic stainless steel for automobile fuel tanks.

[0010] The technical solution for achieving the object of the present invention is: a high-strength, high-corrosion-resistant, economical austenitic stainless steel for automobile fuel tanks, the chemical composition of which is as follows by weight: C: 0.05-0.12%, Si: 0.2-1.2%, Mn: 5.0-10.0%, Cr: 17.0-21.0%, Ni: 1.5-3.5%, Cu: 0.8-2.5%, P: ≤0.045%, S: ≤0.0050%, N: 0.15-0.25%; Nb: 0.005-0.20%; Ti: 0.005-0.20%; B: 0.0005-0.0080%; Se: 0.001-0.008%, wherein 0.015%≤Nb+Ti≤0.3%, and the remainder is Fe and unavoidable impurities.

[0011] The effects of the various elements and the interactions between the elements in the composition design of the economical high-strength austenitic stainless steel of the present invention are as follows: Cr: Chromium is a ferrite-forming element and is also an essential element for stainless steel to maintain corrosion resistance. At least 10.5% of chromium in the steel can ensure the corrosion resistance of the stainless steel. The nickel-saving austenitic stainless steel of the present invention has high corrosion resistance. At the same time, other elements are combined to ensure full austenitization of the steel, so the chromium content is designed to be 17.0-21.0%.

[0012] Ni: Nickel is an austenite-forming element and a common element in stainless steel. The present invention relates to a nickel-saving austenitic stainless steel. From a cost perspective, the nickel content should not be too high. However, nickel-free austenitic stainless steel has poor low-temperature toughness, so it is not appropriate to completely replace the nickel content with nitrogen and manganese. Taking cost and performance into consideration, the nickel content is designed to be 1.5-3.5%.

[0013] Cu: Copper is a weak austenite-forming element. In austenitic stainless steel, copper as an alloying element significantly reduces the cold work hardening phenomenon of steel and improves cold working formability. However, copper also reduces the hot working performance of steel, forming edge cracks in the steel plate during hot rolling. Especially when the nickel content is low, the edge cracks are particularly obvious, affecting subsequent processing and yield rate. Therefore, the copper content should not be too high. If the copper content is too low, it will not be able to play its role in improving cold forming processability. In addition, in order to meet the requirements of complex forming of fuel tanks, the copper content is designed to be 0.8-2.5% in the present invention.

[0014] Si: Silicon is used in the steel of the present invention mainly as a deoxidizer to remove a certain amount of oxygen in the steel and maintain the cleanliness of the steel. Since silicon is a strong ferrite-forming element, excessively high silicon content will form high-temperature ferrite in the steel, deteriorating the performance of austenitic stainless steel. Therefore, the silicon content is only controlled at 0.2-1.2%.

[0015] Mn: Manganese has a relatively weak austenitizing ability. In nickel-saving austenitic stainless steel, in addition to N, a certain amount of manganese needs to be added to obtain a fully austenitized structure. It can also help to dissolve a certain amount of N. However, if the manganese content is too high, the austenitizing effect will no longer be enhanced with the increase of the content. At the same time, too high manganese content will also deteriorate the corrosion resistance and processability of the steel and increase the cost. Too low manganese content is not conducive to the solid solution of N and is also prone to N precipitation during the welding process. In addition, manganese combines with S in steel to form MnS inclusions, which deteriorate the mechanical properties and pitting corrosion resistance of the steel. Therefore, the manganese content is controlled at 5.0-12.0%.

[0016] C: Carbon is a strong austenite-forming element. A higher C content increases the strength of steel, but too much C can also damage the toughness and plasticity of steel. It can also precipitate Cr carbides at the grain boundaries of stainless steel, affecting the corrosion resistance of the steel. Therefore, the C content is controlled between 0.05% and 0.12%.

[0017] Nitrogen: Nitrogen's ability to inhibit ferrite formation is 25-30 times greater than nickel's. It is also a strong austenite-forming element. Therefore, in austenitic stainless steels, nitrogen is often used to partially replace the precious nickel metal to balance the phase structure and achieve low cost. Furthermore, an appropriate amount of nitrogen improves the steel's strength while minimizing its ductility and toughness, while also enhancing the stainless steel's corrosion resistance. However, nitrogen's solubility in stainless steel has certain limits. At atmospheric pressure, excessive nitrogen content can cause nitrogen to precipitate from the molten steel during continuous casting, leading to bubbles in the steel and deteriorating the quality of the cast ingot. Therefore, considering the steel's cost, performance, and production challenges, the nitrogen content is controlled between 0.15% and 0.25%.

[0018] Nb / Ti: Niobium and titanium are stable elements with strong affinity for carbon and nitrogen. Adding Nb / Ti to steel converts chromium carbon and nitrogen compounds into niobium and titanium carbon and nitrogen compounds, improving the steel's resistance to intergranular corrosion and, in particular, its weldability. In this invention, the niobium and titanium contents are designed to be below 0.2%. When added in combination, 0.15% ≤ Nb + Ti ≤ 0.3%.

[0019] B: In order to improve the thermoplasticity of steel during hot rolling, a small amount of B can be added, preferably 0.001 to 0.008%.

[0020] P and S: Phosphorus and sulfur are both inclusion elements in steel grades. Therefore, their contents should be kept as low as possible according to production capacity. In particular, the steel involved in the present invention contains a certain amount of Mn. In order to avoid the formation of excessive MnS inclusions, which may lead to a decrease in pitting corrosion performance, the P content should be controlled below 0.045% and the S content should be controlled below 0.005% as much as possible.

[0021] Se: High corrosion resistance is a crucial characteristic for stainless steel in fuel tanks, especially in harsh climates and low-quality gasoline (which contains high chloride ion concentrations). Selenium is commonly used in steelmaking to improve the machinability of carbon steel, stainless steel, and copper. However, the inventors discovered that the steel of the present invention contains a high Mn content, along with S, an unavoidable component of the smelting process. This leads to the formation of MnS inclusions, particularly in chloride ion environments. These MnS inclusions reduce the corrosion resistance of manganese-containing stainless steels, forming pitting corrosion pits. By adding a certain amount of Se, these MnS inclusions can be converted into Mn(S,Se) inclusions, which are less soluble in corrosive environments than MnS inclusions. This prevents the stainless steel's passivation film from being destroyed in these corrosive environments, thereby maintaining the steel's corrosion resistance.

[0022] Furthermore, the composition of the steel coils is preferably C: 0.06-0.1%, Si: 0.3-0.5%, Mn: 6.0-10.0%, Cr: 18.0-20.0%, Ni: 2.0-3.0%, Cu: 1.5-2.5%, N: 0.19-0.23%, B: 0.001-0.008%, Nb: 0.05-0.15%, Ti: 0.05-0.15%, and Se: 0.002-0.006%. Within this preferred narrow range, the resulting steel coils exhibit more stable performance and a high yield, avoiding fluctuations in coil performance caused by the composition deviating from either the upper or lower limits of the aforementioned wide range (although this is unlikely).

[0023] The present invention maintains high corrosion resistance through rational composition design, with a pitting potential exceeding 450mV, far exceeding that of 304 austenitic stainless steel with an 8% nickel content. By replacing some nickel with a certain amount of nitrogen and manganese, the cost is significantly reduced compared to using chromium-nickel austenite. Adding trace amounts of elements such as Nb, Ti, B, and Se further enhances corrosion resistance and strength. Furthermore, the rational composition design of the present invention avoids the large amount of martensite formed during the work-hardening process of conventional nickel-saving austenitic stainless steel, thereby avoiding the significant decrease in plasticity and subsequent delayed cracking caused by the large amount of martensite generated during deformation during use. This allows the present invention to maintain high strength while maintaining high plasticity. In particular, the addition of a certain amount of Se to the steel effectively prevents pitting corrosion caused by MnS inclusions, resulting in a low-cost, corrosion-resistant, high-strength austenitic stainless steel containing nitrogen that is easily mass-produced through continuous casting. DETAILED DESCRIPTION

[0024] The following is a detailed description of a preferred embodiment of the preparation process of a high-strength, high-corrosion-resistant, economical austenitic stainless steel for automobile fuel tanks of the present invention: Example 1

[0025] Disclosed is an economical high-strength austenitic stainless steel, the chemical composition of which is as follows by weight: C: 0.058%, Si: 0.3%, Mn: 6.5%, Cr: 18.5%, Ni: 3.2%, Cu: 1.2%, P: 0.04%, S: 0.005%, N: 0.2%, Nb: 0.15%, Ti: 0.15%, B: 0.003%, Se: 0.007%, and the remainder is Fe and unavoidable impurities.

[0026] The process flow for producing economical, high-strength austenitic stainless steel includes the following conventional steps: electric furnace - converter (AOD) - LF (ladle treatment furnace) - continuous casting - hot rolling - solution treatment and pickling - cold rolling - annealing and pickling. The hot rolling heating temperature for continuously cast slabs with a thickness of 180-200 mm is controlled at 1100-1250°C, preferably 1180-1220°C, and the hot-rolled plate has a thickness of 3-5 mm. The solution treatment temperature is 1000-1150°C, and the finished product has a thickness of 0.5-2 mm.

[0027] Examples 2 to 7 Examples 2-7 follow the same process as Example 1, differing in the weight percentages of the chemical components and the actual heat treatment temperatures, all of which fall within the above-required ranges. The weight percentages of the chemical components for each example are shown in Table 1, which also provides the standard composition of 304 austenitic stainless steel as a comparative example.

[0028] Table 1 Chemical composition of Examples and Comparative Examples (wt.%)

[0029] Table 2 shows the heat treatment temperatures, thickness specifications, and corresponding mechanical properties and pitting potentials of Examples 1 to 7 of the present invention. The mechanical property tests of the materials were conducted according to the test methods of GB / T 228.1 "Metallic Materials - Tensile Tests - Part 1".

[0030]

[0031] It can be seen from the performance test results in Table 2 that the mechanical properties of the austenitic stainless steel obtained in the embodiment of the present invention are significantly better than those of 304 austenitic stainless steel under room temperature conditions. Its pitting potential is above 450mV, the solid solution yield strength reaches 420MPa and above, the tensile strength reaches 750Mpa and above, and the elongation reaches more than 50%, which can meet the requirements of automobile fuel tanks for lightweight, low cost, high corrosion resistance, and easy forming. Comparative Example 1 and Comparative Example 2 do not contain Se, and the pitting potential is below 420mV, which shows the contribution of Se element to corrosion resistance. The present invention can convert MnS inclusions into Mn(S,Se) inclusions by adding a certain amount of Se. Compared with MnS inclusions, this type of inclusion is not easy to dissolve in a corrosive environment, thereby resisting the destruction of the stainless steel passivation film in a corrosive environment and ensuring the corrosion resistance of the steel.

[0032] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent process transformations made using the contents of the present invention specification, or any direct or indirect application in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A high-strength, high-corrosion-resistant, economical austenitic stainless steel for automobile fuel tanks, characterized by: The weight percentages of its chemical composition are as follows: C: 0.05-0.12%, Si: 0.2-1.2%, Mn: 5.0-10.0%, Cr: 17.0-21.0%, Ni: 1.5-3.5%, Cu: 0.8-2.5%, P: ≤0.045%, S: ≤0.0050%, N: 0.15-0.25%, Nb: 0.005-0.20%, Ti: 0.005-0.20%, B: 0.0005-0.0080%; Se: 0.001~0.008%, of which 0.015%≤Nb+Ti≤0.3%, and the rest are Fe and unavoidable impurities.

2. The high-strength, high-corrosion-resistant, economical austenitic stainless steel for automobile fuel tanks according to claim 1, characterized in that: C: 0.06~0.1%, Si: 0.3~0.5%, Mn: 6.0~10.0%, Cr: 18.0~20.0%, Ni: 2.0~3.0%, Cu: 1.5~2.5%, N: 0.19~0.23%; B: 0.001~0.008%, Nb: 0.05~0.15%, Ti: 0.05~0.15%, Se: 0.002~0.006%.

Images