A 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel and its manufacturing method

Abstract

The invention discloses a 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel, the chemical composition of which is as follows by weight: C: ≤0.003%, Si: ≤0.03%, Mn: 0.35-0.65%, P: 0.03-0.075%, S≤0.010%, Als: 0.020-0.045%, Nb: 0.008-0.015%, Ti: 0.02-0.03%, B: 0.0004-0.0010%, N≤0.0035%, and the remainder is Fe and unavoidable impurities; The elements also meet the following restrictions: calculated according to the corresponding weight percentage of each element, 10C+3N+2S≤Ti+2Nb≤12C+3N+2S, 60B≤P≤80B; the present invention adds P element to IF steel, and through composition and process optimization control, obtains a P-containing hot-dip galvanized iron alloy high-strength IF steel with a yield strength of 180-320 MPa, a tensile strength of 340-440 MPa, an elongation of 35-46%, a value (average plastic strain ratio) ≥1.8, high surface quality, high formability, and good cold brittleness.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgy, and in particular relates to a 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel and a manufacturing method thereof. Background Art

[0002] Hot-dip galvanized iron alloy coated steel sheet (GA steel sheet) offers superior corrosion resistance, paintability, and weldability compared to hot-dip pure zinc sheet (GI), and is widely used in Japanese sedans. Traditionally, Japanese sedan exterior panels have primarily used IF steel, but with the trend toward lightweight vehicles, high-strength steels are gradually replacing traditional IF steel. Currently, automotive manufacturers primarily use BH steel to replace traditional IF steel. However, BH steel's inferior deep-drawing properties and aging issues limit its application. Therefore, to address this challenge, high-strength IF steel containing phosphorus (P) has emerged.

[0003] P-containing high-strength IF steel is a traditional IF steel with the strengthening element P added. It not only improves the strength, but also retains the characteristics of interstitial-free atom steel and has excellent stamping performance. At the same time, the interstitial-free atoms in the steel also solve the aging problem of BH steel, so it is widely used in automotive exterior panels. However, the P element will segregate at the grain boundaries, seriously hindering the zinc-iron alloy reaction, making the zinc-iron alloy reaction difficult. In addition, the segregation of the P element will also weaken the grain boundaries, affecting the secondary processing brittleness of the steel plate and causing the cold brittleness temperature to rise. At the same time, the addition of alloying elements will reduce the formability of the steel plate. How to improve the strength while ensuring the formability has become a technical problem that needs to be solved urgently.

[0004] Patent CN201710163920.9 "A 340MPa grade high-strength IF steel and its production method" obtains a 340MPa grade high-strength IF steel through reasonable composition design and matching corresponding hot rolling, pickling and continuous annealing processes. The steel has a tensile strength ≥340MPa, a yield strength of 180-230MPa, and an elongation A80 ≥36%. However, the steel has a high alloy content, high cost, and a low yield strength range. It also does not mention solving the problem of secondary processing brittleness.

[0005] Patent CN201310688973.4 discloses "a 340MPa grade hot-dip galvanized iron alloy high-strength IF steel", which adds Si as a strengthening element and has obvious differences in hot rolling and cold rolling processes. Like CN201710163920.9, its yield strength range is relatively low at 100MPa, and there is no mention of solving the secondary processing brittleness problem. Summary of the Invention

[0006] In order to solve the above problems, the present invention provides a 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel and its manufacturing method, with a yield strength of 180-320MPa, a tensile strength of 340-440MPa, and an elongation of 35-46%. The value (average plastic strain ratio) is ≥1.8; by matching the composition and process, not only the surface quality of the steel plate is guaranteed, but also excellent forming performance is achieved. The secondary brittle transition temperature can reach below -50°C, which well meets the use requirements of automobile outer panels.

[0007] In order to achieve the above purpose, the technical solutions adopted are as follows:

[0008] A 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel, the chemical composition of which is as follows by weight percentage:

[0009] C: ≤0.003%, Si: ≤0.03%, Mn: 0.35-0.65%, P: 0.03-0.075%, S≤0.010%, Als: 0.020-0.045%, Nb: 0.008-0.015%, Ti: 0.02-0.03%, B: 0.0004-0.0010%, N≤0.0035%, and the rest are Fe and unavoidable impurities; the elements also meet the following restrictions: calculated according to the corresponding weight percentage of each element, 10C+3N+2S≤Ti+2Nb≤12C+3N+2S, 60B≤P≤80B.

[0010] The method for manufacturing the above-mentioned 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel comprises the following steps:

[0011] 1) After continuous casting, the billet is mechanically peeled to remove the surface iron oxide scale, and then enters the heating furnace. The heating temperature is controlled at 1200-1250℃ and the heating time is ≥150min;

[0012] 2) Rough rolling and finishing rolling; control the rough rolling temperature at 1060-1110°C for full descaling; control the finishing rolling temperature at 920-950°C;

[0013] 3) Coiling: adopt U-shaped cooling, control the coiling temperature at 700-740℃ at the head and tail, and 680-720℃ in the middle;

[0014] 4) Pickling: the pickling temperature is 80-90°C and the acid concentration is greater than 120g / l;

[0015] 5) Cold rolling: control the total cold rolling reduction rate to 65-83%;

[0016] 6) Continuous annealing: control the soaking temperature at 810-830°C, the dew point of the annealing atmosphere in the furnace at below -40°C, and the oxygen content ≤5ppm;

[0017] 7) Hot-dip galvanizing: Control the temperature of the steel plate entering the zinc pot at 480-500°C, and the dew point of the furnace nose at below -30°C; control the Al content in the zinc solution between 0.10-0.12wt%;

[0018] 8) Alloying; entering the alloying furnace using alloying power control mode, the power is controlled at 1500-1700KW, the iron content of the coating is controlled at 9 to 11wt%;

[0019] 9) Finishing: Finishing rate is 1.2-1.4%, steel plate surface roughness is 0.9-2.0 μm, oiling amount is 1.5±0.5 g / m 2 .

[0020] According to the above scheme, IF steel or ordinary stamping steel is used for transition when welding the strip in step 5.

[0021] According to the above scheme, in step 6, alkali washing is performed to remove residual substances on the surface before entering the annealing furnace. After cleaning, the iron powder on the plate surface is ≤1000ppm and the oil powder is ≤100ppm.

[0022] According to the above scheme, in step 7, before hot-dip galvanizing, ensure the effect of the squeeze roller and it is strictly forbidden to bring water into the zinc pot.

[0023] According to the above plan, in step 8, the power is increased or decreased according to the degree of flouring of the plate, and the edge burners are fully opened.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention adds P element to IF steel and obtains a steel with a yield strength of 180-320 MPa, a tensile strength of 340-440 MPa, and an elongation of 35-46% through component and process optimization control. This P-containing hot-dip galvanized iron alloy high-strength IF steel features a value (average plastic strain ratio) ≥ 1.8, high surface quality, high formability, and good cold brittleness resistance. Its excellent overall performance allows it to replace traditional IF and BH steels to improve automotive strength and safety, while also achieving high-strength and thinning performance, enabling low-carbon, green production. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 : The matrix structure morphology of the product obtained in Example 1 of the present invention.

[0027] Figure 2 : Surface coating morphology of the GA product obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0028] The following examples further illustrate the technical solutions of the present invention but are not intended to limit the scope of protection of the present invention.

[0029] The specific embodiment provides a method for manufacturing 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel:

[0030] 1) The chemical composition by weight is as follows: C: ≤0.003%, Si: ≤0.03%, Mn: 0.35-0.65%, P: 0.03-0.075%, S≤0.010%, Als: 0.020-0.045%, Nb: 0.008-0.015%, Ti: 0.02-0.03%, B: 0.0004-0.0010%, N≤0.0035%, and the remainder is Fe and unavoidable impurities; wherein the elements also meet the following restrictions: calculated by the corresponding weight percentage of each element, 10C+3N+2S≤Ti+2Nb≤12C+3N+2S, 60B≤P≤80B; after smelting and refining according to the set composition, the elements such as P, S, B, and N are strictly controlled.

[0031] 2) After continuous casting, in order to ensure the subsequent surface quality, the billet is mechanically peeled to remove the surface oxide scale; then it enters the heating furnace, the heating temperature is controlled at 1200-1250℃, and the heating time is ≥150min.

[0032] 3) Carry out rough rolling and control the rough rolling temperature at 1060-1110°C. To ensure the descaling effect, the descaling water is required to be fully opened to ensure complete descaling; control the finishing rolling temperature at 920-950°C.

[0033] 4) After finishing rolling, coiling is carried out. In order to ensure the uniformity of performance at the head and tail, U-shaped cooling is adopted. The coiling temperature at the head and tail is controlled at 700-740℃, and the coiling temperature in the middle is 680-720℃.

[0034] 5) Perform pickling. The pickling temperature is required to be controlled at 80-90°C and the acid concentration is required to be greater than 120g / l.

[0035] 6) Cold rolling is carried out. According to different thickness specifications, the total cold rolling reduction rate is controlled at 65-83%. Since it is P-containing steel, IF steel or ordinary stamping steel is used for transition during welding and threading. High-strength steel transition is strictly prohibited.

[0036] 7) Perform continuous annealing: Ensure the alkali washing effect before entering the annealing furnace, clean the residual substances on the pickling surface, and ensure that the iron powder on the plate surface is ≤1000ppm and the oil powder is ≤100ppm after cleaning; control the soaking temperature at 810-830℃, the dew point of the annealing atmosphere in the furnace is controlled below -40℃, and the oxygen content is ≤5ppm.

[0037] 8) Hot-dip galvanizing: Ensure the effect of the squeeze roller before hot-dip galvanizing, and strictly prohibit the plate surface from entering the zinc pot with water; control the temperature of the steel plate entering the zinc pot at 480-500℃, and the dew point of the furnace nose below -30℃; control the Al content in the zinc liquid composition between 0.10 and 0.12wt%.

[0038] 9) After hot-dip galvanizing, the steel enters the alloying furnace for alloying. The alloying power control mode is adopted, and the alloying power is controlled at 1500-1700KW. The power is increased or decreased according to the degree of plate flouring. The edge burners are required to be fully open, and the iron content of the coating is controlled at 9-11wt%.

[0039] 10) Perform finishing: the finishing rate is 1.2-1.4%. The surface roughness of the steel plate after alloying is controlled at 0.9-2.0 μm, and the oiling amount is 1.5±0.5 g / m 2 .

[0040] The mechanism and function of each element and main process in the present invention:

[0041] C: The most basic solid solution strengthening element. To ensure interstitial-free steel, an ultra-low carbon design is adopted to control the C content below 0.003%.

[0042] Si: A ferrite-strengthening element, the addition of Si can significantly improve product strength. However, during hot rolling, Si promotes the formation of iron oxide scale, deteriorating surface quality and affecting the effectiveness of subsequent pickling processes. Furthermore, as the Si content increases, Si enrichment becomes apparent during hot-dip galvanizing annealing, making it prone to plating defects on the steel sheet surface, seriously impacting surface quality. Furthermore, the combined effects of Si and P make alloying high-strength IF steel difficult. Therefore, the present invention controls the Si content to below 0.03%.

[0043] Mn: Manganese dissolves in ferrite, causing solid solution strengthening and significantly increasing the material's hardness and strength. However, excessive Mn content can deteriorate the steel's weldability. Therefore, while maintaining strength, it's best to keep the Mn content within a low range of 0.35-0.65%.

[0044] Al: The most effective deoxidizing element. However, as the AlS (solid-solution Al) content increases, the number of inclusions in the steel increases, and the size of these inclusions also increases. Therefore, the AlS content should be properly controlled, preferably within a range of 0.020-0.045%.

[0045] Nb and Ti: They exhibit precipitation strengthening effects, effectively fixing interstitial atoms such as C and N, and refining grains, thereby increasing the strength and ductility of the steel. Furthermore, Nb can increase the reaction window of IF steel zinc-iron alloys, facilitating production control. However, Nb is expensive, and Nb-Ti composite strengthening is generally employed. However, adding too much Nb or Ti weakens the strengthening effect, increasing alloy cost and affecting secondary processing brittleness. Therefore, the present invention preferably uses 0.008-0.015% Nb and 0.02-0.03% Ti.

[0046] Nitrogen: Excessive nitrogen content can degrade the elongation and weldability of high-strength steel. Furthermore, nitrogen forms compounds with niobium (Nb) and titanium (Ti), reducing the combined strengthening effects of these elements. Therefore, the nitrogen content in this invention is required to be below 0.0035%.

[0047] S: A harmful element in steel. Excessive S content can easily form MnS inclusions, impairing the steel's plasticity and causing anisotropy in properties, leading to a decrease in r-value. S, along with C and N, can also form compounds with Nb and Ti, severely impacting the steel's secondary processing brittleness. Therefore, the S content must be properly controlled. In this invention, the preferred S content is less than 0.010%.

[0048] At the same time, in order to ensure the effect of Nb and Ti in fixing interstitial atoms and prevent excess Nb, Ti from forming compounds with C, N, and S, affecting the secondary brittleness of steel and increasing the brittle transition temperature, according to actual production experience, it is necessary to ensure that 10C+3N+2S≤Ti+2Nb≤12C+3N+2S.

[0049] P: P is also an effective solid solution element, which can effectively improve the strength of steel. But at the same time, phosphorus is the most important element affecting the secondary processing brittleness of IF steel. In IF steel, due to the lack of competition from solid solution carbon at the grain boundary position, phosphorus segregation at the grain boundary occurs faster and more easily than in non-IF steel, causing grain boundary embrittlement, which in turn leads to secondary processing brittleness. In addition, during galvanizing, if the P content is too high, a large amount of explosive structure ξ phase will be formed in the initial coating, making the zinc-iron alloy reaction difficult and causing the coating's anti-powdering ability to deteriorate. Preferably, the P content of the present invention is controlled between 0.03 and 0.075%.

[0050] B is also an effective solid-solution element. When a certain amount of B is added, the solid-solution B not only preferentially segregates at grain boundaries, reducing the adverse effects of P segregation at grain boundaries, but also increases the number of low-angle grain boundaries, effectively hindering crack propagation and thus improving secondary processing brittleness. However, excess B can result in high yield strength and poor stamping performance. Therefore, the preferred B content is 0.0004-0.0010%, and the requirement is 60B≤P≤80B. This ensures both improved secondary processing brittleness and good stamping performance of the steel sheet.

[0051] The reason for choosing mechanical peeling of continuous casting billets is that it can effectively remove surface iron oxide and other defects to ensure subsequent surface quality.

[0052] The reason why the heating temperature is controlled at 1200-1250℃ and the heating time is ≥150min is that this not only ensures that the steel plate is burned through and is conducive to the uniform distribution of alloy elements, but also the lower heating temperature can save a lot of energy costs.

[0053] The reason why the descaling water is required to be fully opened during hot rolling to ensure complete descaling is to ensure the surface quality of the galvanized original sheet. If the hot rolling scale remains, it will lead to uneven alloying after galvanizing, seriously affecting the surface quality of the coating.

[0054] The reason why the final rolling temperature is selected as 920-950℃ is that the high final rolling temperature is conducive to the precipitation of Nb and Ti compounds, and more effectively refines the grains, thereby improving the material's formability and secondary processing brittleness.

[0055] The reason for choosing a higher coiling temperature is that it is conducive to the precipitation of carbides and refinement of grains; U-shaped cooling is used to ensure uniformity of head and tail performance, and to avoid deterioration of plate shape due to inconsistent head and tail strength during the pickling process, thereby affecting the subsequent hot-dip galvanizing and zinc-iron alloy reaction process.

[0056] The pickling temperature is required to be controlled between 80 and 90°C, and the acid concentration must be greater than 120g / l. This is because pickling effectively removes iron oxide scale. Poor pickling results in residual iron oxide scale, which can then be pressed into the substrate during pickling, causing surface defects such as missed plating and seriously affecting the surface quality of the steel plate.

[0057] The reason why IF steel or ordinary stamping steel is used for transition during pickling and welding is strictly prohibited because the P element is easily concentrated at the welding point, resulting in weakened strength at the welding position. If it is transitioned with high-strength steel, the strength difference before and after is too large, which can easily lead to steel coil breakage during pickling.

[0058] The reason we ensure iron powder ≤ 1000ppm and oil powder ≤ 100ppm on the plate surface after cleaning is that the segregation of the P element hinders the diffusion of Fe into the zinc layer. Therefore, P-containing steels are more difficult to react with zinc and iron than other steels. When the residual iron and oil content on the surface is too high, it further hinders the diffusion of zinc and iron reaction, which can easily lead to uneven alloying on the plate surface.

[0059] The continuous annealing soaking temperature is 810-830°C because at this annealing temperature, the present invention achieves excellent elongation and r-value while avoiding energy waste. Furthermore, at higher annealing temperatures, carbides decompose, increasing dissolved carbon and further reducing phosphorus segregation, thereby ensuring that the steel plate of the present invention has a low secondary processing brittle transition temperature. Furthermore, strict control of the annealing furnace atmosphere, with a dew point below -40°C and an oxygen content ≤5 ppm, ensures sufficient reduction of the steel plate surface and improves its wettability.

[0060] The reason why the squeezing roller effect is guaranteed before hot-dip galvanizing and the plate surface is strictly prohibited from bringing water into the zinc pot is that water on the plate surface will cause uneven reaction of P-containing steel zinc-iron alloy and deteriorate the powdering performance, seriously affecting the surface quality.

[0061] The reason the strip enters the zinc pot at a temperature of 480-500°C is to ensure that the steel plate remains at a relatively high temperature after exiting the zinc pot, which is conducive to the zinc-iron alloying reaction and avoids uneven alloying. The dew point at the furnace nose is controlled below -30°C to prevent the oxidation and accumulation of Mn and P on the steel plate surface at the furnace nose, which could hinder the zinc-iron alloying reaction and even lead to plating errors. Furthermore, an Al content of 0.11-0.12 wt% effectively reduces the formation of an inhibition layer, ensuring a more favorable zinc-iron reaction.

[0062] The alloying power control mode is employed, with the power controlled between 1500 and 1700 kW and adjusted according to the degree of plate powdering. This is because high power control allows Fe to more easily penetrate the barrier of P and diffuse into the zinc layer, resulting in more uniform alloying. When the alloying power exceeds 1800 kW, a large amount of brittle Γ phase is easily formed, deteriorating the product's powdering resistance. The edge burners are required to operate fully to ensure uniform zinc-iron reaction and diffusion along the edges, avoiding color variations caused by uneven alloying. Research has also shown that high-strength steel zinc-iron alloy coatings with an Fe content of 9 to 11% exhibit optimal powdering resistance.

[0063] The reason for choosing a skin-clearing elongation of 1.2-1.4% is to eliminate the yield platform of the material. At the same time, the surface roughness of the alloyed steel plate is controlled at 0.9-2.0μm to meet the coating requirements of the user. Due to the loose structure of the zinc-iron alloy plate coating, the oiling amount needs to be increased to 1.5±0.5g / m 2 , thus ensuring rust prevention and stamping effect.

[0064] Table 1 is a list of chemical composition values ​​of various embodiments and comparative examples of the present invention.

[0065] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention.

[0066] Table 3 is a table of performance test results of various embodiments of the present invention and comparative examples.

[0067] Table 1 Chemical composition values ​​of various embodiments and comparative examples of the present invention (wt%)

[0068]

[0069]

[0070] Table 2 Process parameters of various embodiments of the present invention and comparative examples

[0071]

[0072] Table 3 is a list of performance tests of various embodiments and comparative examples of the present invention.

[0073]

[0074] Figure 1 This is the matrix structure morphology of the product obtained in Example 1. It can be seen that the substrate has fine and uniform grains, a structure of ferrite + a small amount of carbide, and a grain size of level 8. Figure 2 The surface coating morphology of the GA product obtained in Example 1 shows that the surface coating is mainly composed of δ phase, the degree of alloying is good, and there are visible polishing marks. The coating has appropriate strength and plasticity and excellent anti-powdering performance.

[0075] As can be seen from the comparative examples, comparative example 1 adds a large amount of Nb and Ti alloying elements, resulting in a high production cost and being only applicable to continuous slab production; comparative example 2 adds Si as a strengthening element, making surface quality control difficult, and does not restrict elements such as S and N, nor does it mention the problem of secondary processing brittleness. In addition, the yield strength range is much smaller than that of the present invention; comparative example 3 adds Si as a strengthening element, making surface quality control difficult, and does not add B elements, resulting in high alloying power, poor pulverization performance, and high secondary brittle transition temperature. In the full-process manufacturing process, the present invention, under the premise of meeting mechanical properties, achieves uneven alloying surface problems and secondary processing brittleness problems caused by P element segregation through precise component proportioning and process optimization as much as possible, ultimately obtaining a P-containing hot-dip galvanized iron alloy high-strength IF steel with high surface quality, high formability, and good cold brittleness, and a method for manufacturing the same, achieving significant results. The above embodiments are only the best examples, and are not intended to limit the embodiments of the present invention.

Claims

1. A 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel, characterized in that The chemical composition by weight percentage is: C: ≤0.003%, Si: ≤0.03%, Mn: 0.35~0.65%, P: 0.03~0.075%, S≤0.010%, Als: 0.020~0.045%, Nb: 0.008~0.015%, Ti: 0.02~0.03%, B: 0.0004~0.0010%, N≤0.0035%, the rest are Fe and unavoidable impurities; the elements also meet the following restrictions: calculated by weight percentage of each element, 10C+3N+2S≤Ti+2Nb≤12C+3N+2S, 60B≤P≤80B; The manufacturing method of the 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel comprises the following steps: 1) After continuous casting, the billet is mechanically peeled to remove the surface iron oxide scale, and then enters the heating furnace. The heating temperature is controlled at 1200~1250℃ and the heating time is ≥150min; 2) Rough rolling and finishing rolling; control the rough rolling temperature at 1060~1110℃, and completely remove the scale; control the finishing rolling temperature at 920~950℃; 3) Coiling: adopt U-shaped cooling, control the coiling temperature at the head and tail at 700-740℃, and the coiling temperature in the middle at 680-720℃; 4) Pickling: the pickling temperature is 80-90°C and the acid concentration is greater than 120g / l; 5) Cold rolling: After pickling and before cold rolling, the welding strip is made of IF steel or ordinary stamping steel; the total cold rolling reduction is controlled at 65~83%; 6) Continuous annealing: control the soaking temperature at 810~830℃, the dew point of the annealing atmosphere in the furnace below -40℃, and the oxygen content ≤5ppm; 7) Hot-dip galvanizing: Control the temperature of the steel plate entering the zinc pot at 480~500℃, and the dew point of the furnace nose below -30℃; control the Al content in the zinc solution between 0.10~0.12 wt%; 8) Alloying: When entering the alloying furnace, the alloying power control mode is adopted, the power is controlled at 1500-1700KW, and the iron content of the coating is controlled at 9~11wt%; 9) Finishing: Finishing rate is 1.2~1.4%, steel plate surface roughness is 0.9~2.0µm, oiling amount is 1.5±0.5g / m 2 .

2. The 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel according to claim 1, characterized in that Step 6) Before entering the annealing furnace, alkaline washing is performed to remove residual substances on the surface. After cleaning, the iron powder on the plate surface is ≤1000ppm and the oil powder is ≤100ppm.

3. The 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel according to claim 1, characterized in that Step 7) Ensure the squeeze roller effect before hot-dip galvanizing, and it is strictly forbidden to bring water into the zinc pot.

4. The 340MPa grade P-containing hot-dip galvanized iron alloy high-strength IF steel according to claim 1, characterized in that Step 8) Increase or decrease the power according to the degree of flouring of the plate, and fully open the edge burners.

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