450MPa cold-rolled enamel steel and preparation method thereof

Abstract

The invention discloses 450MPa cold-rolled enamel steel and a preparation method thereof. The 450MPa cold-rolled enamel steel comprises the following components in percentage by weight: 0.04-0.08% of C, less than or equal to 0.01-0.05% of Si, 0.5-1.0% of Mn, less than or equal to 0.025% of P, less than or equal to 0.005% of S, 0.03-0.07% of Ti, 0.02-0.04% of Nb, 0.02-0.07% of Als, less than or equal to 0.007% of N and the balance of Fe and inevitable impurities. The enamel steel is composed of ferrite and carbide, the product yield strength Rp0.2 is larger than or equal to 450 MPa, the tensile strength Rm is larger than or equal to 530 MPa, the percentage elongation after fracture A is larger than or equal to 26%, and the hydrogen permeation time is about 8 min.

Description

Technical Field

[0001] The invention belongs to the technical field of steel production and manufacturing, and specifically relates to a 450MPa cold-rolled enameled steel and a preparation method thereof. Background Art

[0002] Enamel refers to the process of coating steel with enamel. After firing, a dense inorganic non-metallic protective layer is formed on the surface of the steel plate, which makes the steel corrosion-resistant. The steel plate used is enameled steel. The enamel layer can resist the damage to the steel surface by oxygen, water, pollutants, etc. The cold-rolled steel plates currently developed and used are usually of low strength and are not suitable for some high-strength use scenarios. In addition, high-strength enameled steel is mainly hot-rolled.

[0003] The patent application number CN201310548310.2 discloses a hot-rolled thin-plate enameled steel with a yield strength of 330 MPa and a manufacturing method. The thickness of the enameled steel is 1.0-2.5 mm, and the chemical composition is C: 0.02-0.07%, Si: 0.01-0.03Mn: 0.10-0.50%, P≤0.020%, S≤0.010%, Ti: 0.04-0.10%, Als: 0.02-0.06%, N≤0.008%, and the rest is Fe and unavoidable inclusions, and Ti / C=1.0-1.5; the manufacturing method is a CSP process, which includes the following fully continuous production steps in sequence: molten iron desulfurization → composite blowing at the top and bottom of the converter → vacuum treatment → continuous casting into slabs → soaking in a soaking furnace → hot rolling controlled rolling and controlled cooling → coiling into steel coils. The hot-rolled thin plate enameled steel of the present invention has the advantages of small rolling load and good plate shape control, low production cost and high efficiency, thereby ensuring that the enameled steel has good anti-scale explosion performance, high strength and good deep drawing processing performance.

[0004] The invention of the patent with application number CN201611205672.1 discloses a hot-rolled enameled steel plate and a preparation method thereof; the chemical composition of the steel plate is by weight percentage C: 0.061-0.089%, Si: 0.11-0.20%, Mn: 0.60-0.70%, P<0.015%, S<0.005%, Alt: 0.020-0.050%, Ti: 0.1-0.2%, N<0.006%, and the rest is iron and unavoidable impurities; wherein, by mass ratio, Ti:C=1.1-3.3; method: 1) molten iron is subjected to Pretreatment, converter steelmaking and refining to obtain molten iron for casting; 2) continuous casting of molten iron to obtain casting; 3) heating the casting and controlling the tapping temperature; 4) performing multiple rough rolling and finish rolling to obtain finished rolled plate; 5) coiling at a certain temperature after laminar cooling to obtain a finished product; the steel plate prepared by the present invention has a yield strength of 550-620MPa, a tensile strength of 630-730MPa, and an elongation after fracture of 22-30%; the hot-rolled enameled steel plate after enameling has a yield strength of 350-410MPa, a tensile strength of 430-530MPa, an elongation after fracture of 33.5-37%, and an enamel adhesion of Class A.

[0005] The patent with application number CN201710064842.7 discloses a hot-rolled enameled steel plate for subway decoration and its manufacturing method, with the chemical composition mass percentage as follows: C: 0.05-0.09%, Si: 0.10-0.20%, Mn: 0.65-0.95%, P≤0.012%, S≤0.015%, Ti: 0.05-0.10%, Als: 0.015-0.045%, Cr: 0.10-0.20%. The casting slab is heated at 1180-1250°C, descaled by high-pressure water after heating, and roughly rolled into an intermediate slab; the finishing rolling temperature is set at 980-1100°C, and the final rolling temperature is set at 880-950°C; after laminar cooling, it is coiled in the temperature range of 580-650°C. The invention can improve the enamel adhesion between the steel plate and the porcelain glaze, and ensure that the steel plate does not scale during the enameling process. The produced hot-rolled enameled steel plate has good comprehensive performance, a yield strength of more than 390MPa, and an elongation after fracture of more than 40%. It is suitable for rolling forming, bending forming, and stamping forming. The steel plate can be stably produced by a continuous casting-hot rolling method.

[0006] The high-strength enameled steels in the above three patents are all hot-rolled products, which are mainly achieved through controlled cooling and controlled rolling phase transformation strengthening to achieve high-strength enameled steel plates. The strength levels are all lower than 400Mpa. At the same time, the surface accuracy and performance stability of hot-rolled products are somewhat different from those of cold-rolled products. However, the current cold-rolled enameled steel plates have low strength, and the surface quality of hot-rolled steel plates is not suitable for some application scenarios with high surface requirements, and the thickness of hot-rolled plates does not meet the requirements for thin material use scenarios. Summary of the invention

[0007] The purpose of the present invention is to provide a 450MPa cold-rolled enameled steel with good anti-scale burst performance and a preparation method thereof in view of the technical defects of the above-mentioned existing cold-rolled enameled steel having a low strength level of components.

[0008] To achieve the above-mentioned purpose, the present invention provides a 450MPa cold-rolled enameled steel, wherein the steel components include, by weight percentage: C: 0.04-0.08%, Si≤0.01-0.05%, Mn: 0.5-1.0%, P≤0.025%, S≤0.005%, Ti: 0.03-0.07%, Nb: 0.02-0.04%, Als: 0.02-0.07%, N≤0.007%, and the rest is Fe and unavoidable inclusions.

[0009] Furthermore, the Ti content is 0.04-0.055%.

[0010] Furthermore, the Nb content is 0.033-0.037%.

[0011] Furthermore, the C content is 0.047-0.069%.

[0012] Furthermore, the steel has a thickness of 0.4 to 1.8 mm.

[0013] A method for preparing the 450MPa cold-rolled enameled steel as described above is also provided, comprising molten iron smelting → converter steelmaking → LF refining → continuous casting → hot rolling → laminar cooling → coiling → pickling → cold rolling → cleaning → continuous annealing → flattening → finished product; characterized in that: the heating temperature of the hot-rolled slab is 1140-1210°C, the rough rolling temperature is 1050-1080°C, the final rolling end temperature is 880-920°C, and the coiling temperature is 660-700°C.

[0014] Furthermore, the total reduction ratio of the cold rolling is 60-75%.

[0015] Furthermore, in the continuous annealing process, a continuous annealing furnace is used, which includes steel coil heating, soaking and heat preservation, slow cooling, rapid cooling and aging in sequence; wherein, the heating rate of the steel coil heating is 2-15°C / s; the soaking and heat preservation temperature is 780-820°C, and the insulation time is 40-160s; the slow cooling temperature is 660-740°C, and the slow cooling rate is 5-20°C / s; the rapid cooling end temperature is 380-440°C, and the rapid cooling rate is 30-80°C / s; the aging temperature is 380-430°C, and the aging time is 150-500s; and the protective atmosphere is nitrogen + hydrogen.

[0016] Furthermore, the flattened compression elongation is 0.6-1.6%.

[0017] Furthermore, when the steel thickness is ≤1.0 mm, the flattening elongation is 0.6-1.0%; when the steel thickness is >1.0 mm, the flattening elongation is 1.0-1.6%.

[0018] Yield strength Rp of the enameled steel produced by the present invention 0.2 ≥450MPa, tensile strength R m ≥530MPa, elongation after break ≥26%, hydrogen penetration time is about 8min.

[0019] Functions and mechanisms of the main elements and processes in the present invention:

[0020] C: Carbon is the most economical strengthening element. The yield point and tensile strength of steel increase with the increase of carbon content, but too high C content will affect the welding performance of the material. At the same time, appropriate C can form fine precipitate phases with second phase forming elements such as Ti and Nb in steel, playing the role of precipitation strengthening and hydrogen trap. Therefore, the C content in steel is selected to be 0.04-0.08%.

[0021] Si: Si can be dissolved in ferrite and austenite to improve the hardness and strength of steel. Too much Si can easily form striped iron oxide scale during hot rolling, which is difficult to remove. Therefore, the Si content in the present invention is 0.01-0.05%.

[0022] Mn: Mn is a solid solution strengthening element. Mn can infinitely solid-solve with Fe to improve the strength of steel, but too high Mn will reduce the plasticity and welding performance of steel, so the Mn content in the present invention is 0.5-1.0%.

[0023] Als: Al is a commonly used deoxidizer in steel, which can refine grains and play a role in nitrogen fixation. However, too high an Al content will increase Al-based inclusions in the steel. Therefore, the Al content in the present invention is 0.02-0.07%.

[0024] P: P is usually an impurity element in steel. Although it can improve the strength and hardness of steel, it is easy to cause segregation and phosphorus embrittlement. It is usually controlled at a low level, so the P content is ≤0.025%.

[0025] S: S is a harmful element in steel, which reduces the ductility and toughness of steel. It should be controlled at a low level, but controlling it too low will increase smelting time and production costs. Considering all factors, it should be controlled below 0.005%.

[0026] Ti: Ti is a strong carbide-forming element, which can form TiC with C in steel to play a role in precipitation strengthening, refine the material grains, and TiC also serves as a hydrogen absorption trap. The addition of Ti causes the steel to precipitate a second phase during the continuous casting and cooling stages. When the Ti content is low, TiN and Ti4C2S2 are mainly precipitated during the continuous casting cooling stage. There is not enough Ti to form TiC in the subsequent cooling process. Therefore, a certain Ti content must be ensured to effectively precipitate TiC. Therefore, the Ti content in the present invention is 0.03-0.07%.

[0027] Nb: The function of Nb is similar to that of Ti. It can form a fine second phase with C to hinder the migration of grain boundaries and the growth of grains. However, too high Nb increases the production cost. Therefore, the content of Nb in the present invention is 0.02-0.04%. The composite addition of Nb and Ti can form a finer composite precipitate phase in the steel, further increase the hydrogen absorption traps, and improve the anti-scale explosion performance of the enameled steel.

[0028] N: The increase of N content significantly improves the strength of steel, but significantly reduces the plasticity and toughness, and aggravates the cold brittleness. Therefore, in the present invention, the N content is controlled to be ≤0.006%.

[0029] Both Ti and Nb are strong carbonitride-forming elements. When Ti and Nb are added to steel, carbonitrides dissolve during the hot rolling process and precipitate during the subsequent rolling cooling process, affecting the microstructure and properties of the hot-rolled material. Nb is added to refine the microstructure and improve the toughness of the steel. The precipitation of Ti in the ferrite improves the strength of the steel. The composite addition of Ti and Nb achieves fine grain strengthening and precipitation strengthening. At the same time, the size of the Nb and Ti composite precipitate phase (Nb, Ti) C is smaller, which effectively increases the hydrogen absorption trap of the steel.

[0030] The main process reasons in the present invention are analyzed as follows:

[0031] LF refining is used as an out-of-furnace refining, which can quickly form slag for desulfurization and deoxidation, and the lower S content improves the toughness of the product.

[0032] The higher final rolling temperature of hot rolling keeps the rolling temperature in the austenite phase region, avoiding force fluctuation and uneven organization caused by rolling in the two-phase region. The hot rolling coiling temperature is 660-700℃. Coiling is conducive to the refinement of hot rolling organization grains. The grain size of the intermediate organization will be inherited to the subsequent process, affecting the grain size of the final product organization. Therefore, the coiling temperature is selected to be 660-700℃.

[0033] A high cold rolling reduction rate is used in the cold rolling process, so that the hot rolled structure can be broken and refined. Therefore, the total cold rolling reduction rate in the present invention is 60-75%.

[0034] It is produced in a continuous annealing furnace. Since the enameled steel plate is designed to have a high strength, the strip is operated under tension during the continuous annealing process, and the continuous annealing furnace uses nitrogen + hydrogen as the protective atmosphere. Nitrogen is an inert gas and hydrogen is reducing, which can ensure that the strip obtains a better plate shape and surface state during continuous annealing, which is beneficial to the uniformity of the organization in the width and length directions of the steel coil, eliminate internal stress, and obtain uniform internal organization and properties.

[0035] After annealing, the strip is flattened, and a flattening elongation of 0.6 to 1.6% is used to ensure that the strip obtains a corresponding yield strength, and finally obtains enameled steel with good surface and dimensional accuracy.

[0036] The product of the present invention adopts the addition of appropriate alloy elements and hot rolling controlled cold rolling process, and the continuous annealing production line with high production efficiency to achieve the production of enameled steel products with a yield strength of 450MPa.

[0037] Hot rolling soaking temperature: TiN and Ti4C2S2 are precipitated during the continuous casting of the steel billet. The solid solution temperature of TiN is 1200-1350℃, while the solid solution temperature of Ti4C2S2 is relatively low. The Ti after solid solution re-precipitates to form a second phase during the rolling and coiling process, which plays a role in grain refinement and precipitation strengthening. A higher heating temperature should be used to ensure sufficient solid solution of Ti compounds and produce sufficient strengthening effect. However, too high heating temperature can easily lead to coarse austenite grains and reduce the strength of the steel plate. Therefore, the soaking temperature is 1180-1250℃.

[0038] Hot rolling coiling temperature: The temperature range for the precipitation of microalloy carbonitride in ferrite is mainly 600-700℃. At this time, a large number of second phases are precipitated on the ferrite matrix, and the grain refinement and precipitation strengthening effects are significant. Therefore, coiling is carried out in the temperature range of 660-700℃ to ensure the precipitation of Ti and Nb second phases, while avoiding too low coiling temperature to cause excessive strength of hot rolled materials, affecting the subsequent cold rolled plate shape and reducing the load of the cold rolling mill.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows: the enamel steel of the present invention is composed of ferrite and carbide, and the yield strength Rp of the product is 0.2 ≥450MPa, tensile strength R m ≥530MPa, elongation after break A≥26%, hydrogen penetration time is about 8min (European standard). BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a typical organizational diagram of the enamel tank of the present invention. DETAILED DESCRIPTION

[0041] The present invention is described in detail below with reference to specific embodiments.

[0042] Table 1 is a list of values ​​of various embodiments and comparative examples of the present invention;

[0043] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0044] Table 3 is a table of performance test results of various embodiments of the present invention and comparative examples.

[0045] Each embodiment of the present invention is produced according to the following steps:

[0046] Main process flow: molten iron smelting → converter steelmaking → LF refining → continuous casting → hot rolling → laminar cooling → coiling → pickling → cold rolling → cleaning → continuous annealing → flattening → finished product.

[0047] Table 1 Chemical composition list of various embodiments and comparative examples of the present invention (wt%)

[0048] Serial number C Si Mn P S Als Ti Nb N Example 1 0.058 0.035 0.58 0.016 0.0008 0.041 0.053 0.037 0.005 Example 2 0.047 0.044 0.86 0.018 0.0009 0.039 0.038 0.036 0.004 Example 3 0.056 0.035 0.79 0.019 0.0017 0.044 0.051 0.035 0.003 Example 4 0.069 0.038 0.91 0.016 0.0015 0.042 0.043 0.033 0.003 Example 5 0.068 0.037 0.82 0.015 0.0014 0.050 0.049 0.038 0.004 Comparative Example 0.007 0.001 0.13 0.015 0.03 0.053 0.082 / 0.007

[0049] Table 2 List of hot rolling process parameters of various embodiments of the present invention and comparative examples

[0050] Serial number Hot rolled thickness mm Billet heating temperature ℃ Rough rolling temperature℃ Finishing rolling temperature℃ Coiling temperature℃ Example 1 1.4 1151.8 1055 894.8 663.7 Example 2 2.2 1185.6 1073 912.7 673.3 Example 3 3.2 1192.9 1067 896.9 691.8 Example 4 4.2 1189.4 1074 908.2 697.7 Example 5 4.6 1201.3 1071 895.3 689.7 Comparative Example 5.5 1230.4 1040 923.4 710.5

[0051] Table 3 List of continuous annealing process parameters of various embodiments of the present invention and comparative examples

[0052]

[0053] Table 4 List of mechanical properties test results of various embodiments of the present invention and comparative examples

[0054]

[0055] Table 5 Hydrogen permeation test results of the present invention

[0056] Steel Type Hydrogen penetration time test (European standard) 450Mpa grade cold rolled enameled steel 8.5min

[0057] It can be seen from Examples 1 to 5 in Tables 4 and 5 that the performances all reach the target, and at the same time, the performances of steel plates with different finished thicknesses remain stable; the hydrogen permeability is good. Figure 1 The above embodiments are only the best examples and are not intended to limit the implementation of the present invention.

Claims

1. A 450MPa cold-rolled enameled steel, Features: The steel components include by weight percentage: C: 0.04-0.08%, Si≤0.01-0.05%, Mn: 0.5-1.0%, P≤0.025%, S≤0.005%, Ti: 0.03-0.07%, Nb: 0.02-0.04%, Als: 0.02-0.07%, N≤0.007%, and the rest is Fe and unavoidable inclusions.

2. According to claim 1, the 450MPa cold-rolled enameled steel, Features: The content of Ti is 0.04-0.055%.

3. According to claim 1, the 450MPa cold-rolled enameled steel, Features: The Nb content is 0.033-0.037%.

4. According to claim 1, the 450MPa cold-rolled enameled steel, Features: The C content is 0.047-0.069%.

5. According to claim 1, the 450MPa cold-rolled enameled steel, Features: The steel thickness is 0.4-1.8 mm.

6. A method for preparing the 450MPa cold-rolled enameled steel according to any one of claims 1 to 5, the method comprising molten iron smelting → converter steelmaking → LF refining → continuous casting → hot rolling → laminar cooling → coiling → pickling → cold rolling → cleaning → continuous annealing → leveling → finished product; Features: The heating temperature of the hot rolled slab is 1140-1210°C, the rough rolling temperature is 1050-1080°C, the final rolling end temperature is 880-920°C, and the coiling temperature is 660-700°C.

7. The method for preparing 450MPa cold-rolled enameled steel according to claim 6, Features: The total reduction ratio of the cold rolling is 60-75%.

8. The method for preparing 450MPa cold-rolled enameled steel according to claim 6, Features: In the continuous annealing process, a continuous annealing furnace is used, which includes coil heating, soaking and heat preservation, slow cooling, rapid cooling and aging in sequence; wherein, the heating rate of the coil heating is 2-15°C / s; the soaking and heat preservation temperature is 780-820°C, and the heat preservation time is 40-160s; the slow cooling temperature is 660-740°C, and the slow cooling rate is 5-20°C / s; the rapid cooling end temperature is 380-440°C, and the rapid cooling rate is 30-80°C / s; the aging temperature is 380-430°C, and the aging time is 150-500s; and the protective atmosphere is nitrogen + hydrogen.

9. The method for preparing 450MPa cold-rolled enameled steel according to claim 6, Features: The flattened compression elongation is 0.6-1.6%.

10. The method for preparing 450MPa cold-rolled enameled steel according to claim 9, Features: When the steel thickness is ≤1.0 mm, the flattening elongation is 0.6-1.0%; when the steel thickness is greater than 1.0 mm, the flattening elongation is 1.0-1.6%.

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