A 450mpa grade low cost high surface quality zinc-iron alloy dual phase steel and a method of producing the same

Abstract

The application discloses a 450MPa-grade low-cost high-surface-quality zinc-iron alloy high-strength steel, and each chemical component has the following weight percentage: C: 0.06-0.08%, Si: 0.20-0.30%, Mn: 1.00-1.20%, P: less than or equal to 0.005%, S: less than or equal to 0.003%, Als: 0.020-0.070%, and the mass fraction of the elements should meet 5C <= Si / 2 + Mn / 5 <= 6C, and the rest is Fe and inevitable impurities. The main process of the production method comprises the following steps: steelmaking, hot rolling, pickling + cold continuous rolling, continuous annealing + hot galvanizing. The zinc-iron alloy high-strength steel product produced by the application has the following physical properties: yield strength is 280-330MPa, tensile strength is 460-510MPa, elongation is 32-38%, n value is 0.18-0.20, roughness is 0.7-1.5um, powdering level is 1-3, surface quality is excellent, powdering resistance is excellent, and the zinc-iron alloy high-strength steel product can meet the production and manufacturing of automobile inner plates and structural parts.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of manufacturing zinc-iron alloy high-strength steel, and particularly relates to a 450MPa-grade low-cost high-surface-quality zinc-iron alloy high-strength steel and a production method thereof. BACKGROUND

[0002] In recent years, in the production of dual-phase steel, in order to ensure the strength and hardenability of high-strength steel, higher Cr, Mo, Nb and other alloying elements are usually added to obtain ferrite + martensite dual-phase structure. The addition of these alloying elements leads to a significant increase in the cost of steel.

[0003] CN 102839329 A discloses a tensile strength 450MPa-grade cold-rolled dual-phase steel and a manufacturing method thereof, which produces 450MPa dual-phase steel by adopting a C-Si-Mn-Cr composition system, adds 0.1-0.3% of Cr element, and has a high alloy cost. The main process flow is steelmaking, hot rolling, pickling, and continuous annealing, mainly producing continuous annealing plates, and the surface quality is not described.

[0004] CN 105369135 A discloses a 450MPa-grade galvanized dual-phase steel for cars and a production method thereof, which adopts a C-Mn-Cr-Mo composition design and adds Nb element, the content of Cr element is 0.01-1.00%, the content of Mo element is 0.01-0.30%, and the content of Nb element is 0.001-0.03%, and the alloying cost is high. To avoid the influence of Si element enrichment on the surface quality, the Si content is limited to ≤0.01%. The main process flow is steelmaking, hot rolling, pickling, and galvanizing, and the subsequent zinc-iron alloy reaction is not involved, and the surface quality can meet the surface quality requirements of middle and high-grade car structural parts and outer cover parts.

[0005] CN 108754343 A discloses a 450MPa-grade zinc-iron alloy coated dual-phase steel plate for car outer plates and a manufacturing method thereof, which also adopts a C-Mn-Cr-Mo composition design, the content of Cr element is 0.20-0.40%, and the content of Mo element is 0.05-0.20%, and the alloying is more. To ensure the surface quality, a low-Si design is adopted, and the process flow is steelmaking, hot rolling, pickling, galvanizing, and alloying. Through process control, a product with excellent anti-powdering performance and meeting the surface quality of car cover parts can be obtained.

[0006] In the production technology of existing 450MPa-grade cold-rolled hot-dip galvanized high-strength steel, higher Cr, Mo, Nb and other alloying elements are usually added to the product to improve the hardenability, thereby increasing the product manufacturing cost. In addition, when a product containing high Si is produced, due to unreasonable composition design and process control, surface defects such as Figure 2In order to ensure surface quality, low-Si design is adopted for the galvanized dual-phase steel (as shown in the left part of the figure). However, with the development of the steel industry, low-cost manufacturing has become the lifeline of each steel plant. SUMMARY

[0007] The present application provides a 450MPa grade low-cost high-surface-quality zinc-iron alloy high-strength steel and a production method thereof, aiming at the limitations of high cost and poor surface quality caused by the addition of Si element in the prior art production of 450MPa grade zinc-iron alloy dual-phase steel.

[0008] The technical scheme adopted by the present application to solve the above-mentioned problems is as follows:

[0009] A 450MPa grade low-cost high-surface-quality zinc-iron alloy high-strength steel, the weight percentage of each chemical component is as follows: C: 0.06-0.08%, Si: 0.20-0.30%, Mn: 1.00-1.20%, P≤0.005%, S≤0.003%, Als: 0.020-0.070%, and the mass fraction of the elements should satisfy 5C≤Si / 2+Mn / 5≤6C (in the formula, C, Si and Mn represent the weight percentage of each), and the rest is Fe and inevitable impurities.

[0010] According to the above scheme, C is preferably 0.065-0.075%, Si is preferably 0.22%-0.28%, and Mn is preferably 1.05-1.15%.

[0011] The present application also provides a production method of a 450MPa grade low-cost high-surface-quality zinc-iron alloy high-strength steel, the main process of which includes steelmaking, hot rolling, pickling + cold continuous rolling, continuous annealing + hot galvanizing, and the features are as follows:

[0012] 1) Smelting and continuous casting into billets according to the chemical composition of the steel;

[0013] 2) Heating the continuous casting slab, the heating temperature of the casting billet is 1220-1250℃, and the heating time is 160-220min;

[0014] 3) Hot rolling: the rough rolling temperature is controlled to be 1060-1100℃, the finish rolling temperature is 900-930℃, and the coiling temperature is 560-600℃, so as to ensure obtaining uniform grain structure;

[0015] 4) Pickling and cold continuous rolling: the pickling speed is 120-200m / min, the acid concentration is 160-220g / L, the acid temperature is 80-90℃, and the cold rolling reduction is 62-75%;

[0016] 5) Continuous annealing: the soaking temperature of the continuous annealing is 760-780℃, and the fast cooling temperature is 480-500℃;

[0017] 6) hot dip galvanizing: the strip steel temperature entering the zinc pot is 460-480℃, the dew point of the furnace nose is -35--45℃; the temperature of the zinc liquid is 458-462℃, the Al content in the zinc liquid composition is 0.100-0.115% by mass fraction; the alloying temperature after hot dip galvanizing is 480-500℃, the holding time is 10-25s, the iron content in the coating is controlled to be 9-11wt%; after the alloying is completed, the super-fast air mist cooling is adopted, the cooling speed is greater than 50℃ / s; the finishing elongation is 0.4-0.6%, and the zinc-iron alloy high-strength steel with 450MPa grade, low cost and high surface quality is obtained.

[0018] According to the above scheme, in the hot rolling step, U-shaped cooling is adopted after finish rolling, the head and tail of the steel coil are set to have a coiling temperature of 580-600℃, and the middle part of the steel coil has a coiling temperature of 560-580℃.

[0019] According to the above scheme, the dew point of the heating section of the continuous annealing is -15--5℃, and pre-oxidation is realized.

[0020] The zinc-iron alloy high-strength steel with 450MPa grade, low cost and high surface quality produced by the above method has a yield strength of 280-330MPa, a tensile strength of 460-510MPa, an elongation A 80 of 32-38%, an n value of 0.18-0.20, a roughness of 0.7-1.5μm, and a powdering level of 1-3.

[0021] The main mechanism and role of each element in the application are as follows:

[0022] C: the most effective strengthening element, the formation element of martensite, the content of C element determines the morphology of martensite and the strength of the dual-phase steel. At the same time, since it is used for automobile inner plate and structural parts, the forming property, flanging property and welding property of the material are required to be higher, so a lower carbon content is required. At the same time, during hot dip galvanizing, higher carbon content can also accelerate the zinc-iron reaction, and the thickening of the zinc-iron alloy layer can deteriorate the anti-powdering property of the material. Therefore, the C content is controlled to be 0.06-0.08% by comprehensively considering.

[0023] Si: can be dissolved in ferrite to improve the strength of the product; at the same time, the Si element can improve the activity of the C element, accelerate the formation of proeutectoid ferrite, further enrich the untransformed austenite, thereby improving the hardenability, so that the dual-phase steel obtains the required dual-phase structure. Moreover, the Si element can expand the austenite region, which is beneficial to the subsequent heat treatment process to control the volume fraction of martensite and its carbon content, and is helpful to maintain the stability of the strength and plasticity and other properties of the dual-phase steel. Therefore, the Si content is controlled to be 0.20-0.30%.

[0024] Mn: can be dissolved in ferrite and austenite, can expand the austenite region, can increase the critical temperature, can reduce the martensite transformation temperature of the steel, can improve the hardenability of the steel, and can increase the content of residual austenite. Therefore, under the condition of ensuring the strength, the content of Mn is controlled in the lower range of 1.00-1.20%.

[0025] At the same time, since Si and Mn elements can affect the hardenability of the steel, in order to ensure the strengthening effect, the mass fraction of the elements should satisfy 5C≤Si / 2+Mn / 5≤6C. When Si / 2+Mn / 5<5C, the hardenability of the steel is not enough, which causes insufficient tensile strength of the steel; when Si / 2+Mn / 5>6C, too much Si and Mn elements are easy to enrich in ferrite, which causes the yield strength of the dual-phase steel to be too high and the plasticity to be poor. At the same time, the composition ratio can ensure that the bainite transformation temperature of the steel is above 500℃, so as to avoid the bainite phase transformation before rapid cooling after alloying, thereby reducing the strength and plasticity of the steel.

[0026] P: P is a harmful element in the steel, which is easy to segregate at the grain boundary, increase the brittleness of the steel sheet, cause the stamping performance of the steel sheet to be poor, and cause the weldability to be poor. At the same time, when galvanizing, too high P content will form a large amount of Γ phase in the plated layer, and the anti-powdering ability of the plated layer will be poor. Therefore, the P content should be as low as possible. However, considering that P is also an effective solid solution strengthening element, preferably, P≤0.005%.

[0027] S: S is a harmful element in the steel, when the S content is too high, MnS inclusions are easy to form, which damages the plasticity of the steel sheet and causes anisotropy of the performance. And with the increase of the S content, the corrosion resistance of the steel sheet will also be poor. Therefore, it is necessary to reasonably control the S content, and the S content should be less than 0.003%.

[0028] Al: the most effective deoxidizing element. However, with the increase of the Als content, the inclusions in the steel will also increase, and the size of the inclusions will become larger. Therefore, the Als content should be reasonably controlled, and preferably, Als: 0.020-0.070%.

[0029] The reasons for the main processes in the application are as follows:

[0030] The reason why a higher heating temperature (1220-1250℃) and a higher rough rolling temperature (1060-1080℃) are adopted is that the internal stress between the surface oxide scale and the matrix can be increased by cooperating with subsequent descaling cooling, so that the oxide scale is more easily removed.

[0031] The reason why the finish rolling temperature is selected to be 900-930℃ is that when the finish rolling temperature is too low, slight fluctuation of the hot rolling temperature may enter the two-phase region for rolling, which is easy to produce mixed crystal structure. The non-uniformity of the structure will not only affect the uniformity of the mechanical properties, but also affect the uniformity of the galvanized surface.

[0032] The reason for choosing the coiling temperature of 560-600℃ is that the coiling temperature is too high, which will lead to coarse grain, thereby affecting the uniformity of the structure. At the same time, the U-shaped cooling is adopted, which can effectively ensure the uniformity of the performance of the head and tail of the dual-phase steel, and avoid the problem of high strength caused by too fast cooling of the tail.

[0033] The reason for the pickling speed of 120-200m / min, the pickling temperature requirement of 80-90℃, and the acid concentration of 160-220g / l is that this matching process has the best pickling quality, which can effectively remove the iron oxide scale. Poor pickling effect will lead to the residual of the iron oxide scale, which will then be pressed into the matrix during the pickling process, thereby causing the surface defects of the plating leakage.

[0034] The reason for setting the cold rolling reduction rate to 62-75% is that it is beneficial to increase the deformation energy storage in the steel, reduce the recrystallization temperature, and improve the forming performance of the steel plate. However, if the cold rolling reduction rate is too high, the load of the rolling mill will increase, and the equipment loss will also increase.

[0035] The reason for the continuous annealing soaking temperature of 760-780℃ is that at this annealing temperature, the product of the present application can obtain the best performance matching. At the same time, at a lower annealing temperature, the enrichment of Mn element on the surface of the steel plate will also be reduced, thereby reducing the formation of secondary oxides.

[0036] The reason for the fast cooling temperature of 480-500℃ is that ensuring a higher fast cooling temperature can avoid the dual-phase steel entering the bainite transformation zone, thereby reducing the strength. At the same time, since Si element is added in the composition, the dew point of the heating section is set to -15 to -5℃, which is to realize pre-oxidation, so that the Si element is oxidized on the surface in the heating section in advance, and then reduced to sponge iron by the hydrogen reducing atmosphere in the soaking section, thereby improving the platability and avoiding the formation of SiO2 film on the surface of the steel plate during the soaking section, which will cause the plating leakage defects.

[0037] The reason for the strip steel entering the zinc pot temperature of 460-480℃ is to prevent the temperature from being too high, which will cause the zinc liquid temperature to rise and the bottom slag to increase, thereby causing the zinc slag and submerged roll print defects on the surface of the steel plate. The dew point of the furnace nose is -35 to -45℃, which is to avoid the oxidation of the zinc liquid surface to form suspended slag and improve the surface quality. At the same time, the effective Al content in the zinc liquid is controlled in the range of 0.100-0.115%, because when the effective aluminum content in the zinc pot is lower than 0.100%, the Fe2Al5 inhibiting layer formed directly between the substrate and the plated layer is not uniform enough, and the plated layer is prone to over-alloying; when the effective aluminum content in the zinc pot is higher than 0.105%, the Fe2Al5 inhibiting layer is too thick, and the alloying process is difficult, at this time, it is necessary to increase the alloying temperature or reduce the strip speed, which will easily lead to serious powdering of the alloyed plate.

[0038] The alloying temperature is 480-500℃, the holding time is 10-25s, and the iron content of the plated layer is required to be controlled at about 10%. This is because when the alloying temperature is lower than 480℃, the plated layer is difficult to be alloyed, the iron content of the plated layer is low, and a large amount of ζ phase is easily left on the surface of the plated layer, which results in poor corrosion resistance, welding performance and coating performance of the product. When the alloying temperature is higher than 500℃, a large amount of brittle phase Γ is easily formed, which results in poor anti-powdering performance of the product. In addition, when the alloying temperature is too high, the steel enters the bainite phase change region, which results in the decrease of the strength of the steel. Meanwhile, the research shows that when the Fe content of the plated layer is 9-11%, the anti-powdering performance of the plated layer is the best.

[0039] The reason why the super-fast air mist cooling is used after the hot-dip galvanizing alloying is completed and the cooling speed is greater than 50℃ / s is that the high cooling speed is beneficial to the formation of the martensite. In the present application, Cr, Mo, Nb and other alloying elements are not added, and the hardenability of C, Si and Mn elements must be used to the maximum extent to form the martensite, so as to ensure the strength of the dual-phase steel. Therefore, the super-fast air mist cooling is required after the alloying, and the cooling speed is required to be greater than 50℃ / s.

[0040] The reason why the light finishing elongation is selected to be 0.4-0.6% is that the plate shape and the surface roughness of the strip steel can be improved, so that the plated layer surface roughness is controlled to be 0.75-1.5(μm), which is beneficial to the coating of the automobile inner plate.

[0041] Compared with the prior art, the present application has the following beneficial effects:

[0042] (1) The present application effectively solves the hardenability problem caused by the insufficient alloying by effectively matching the C-Si-Mn composition and optimizing the subsequent process, and the yield strength of the obtained zinc-iron alloy high-strength steel product is 280-330MPa, the tensile strength is 460-510MPa, the elongation A 80 is 32-38%, and the n value is 0.18-0.20. The product has excellent mechanical properties, and the low-cost manufacturing is well achieved.

[0043] (2) The 450MPa grade high-surface-quality zinc-iron alloy high-strength steel and the production method thereof provided by the present application effectively solve the plating leakage defect caused by the oxidation film formed by the external oxidation of the Si element through the subsequent process optimization, and the surface quality of the product is good. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 is the plated layer surface morphology of the zinc-iron alloy high-strength steel of the present application;

[0045] Figure 2 In the figure, the left is the surface plating leakage defect of the existing high-Si hot-dip galvanizing high-strength steel product; and the right is the surface of the zinc-iron alloy high-strength steel of the present application. DETAILED DESCRIPTION

[0046] For better understanding of the present application, the following further illustrates the content of the present application in combination with examples, but the present application is not limited to the following examples only.

[0047] Example

[0048] The 450MPa low-cost high-surface-quality zinc-iron alloy high-strength steel according to the embodiments of the present application is produced according to the following production method, and the main process includes steelmaking→hot rolling→pickling + cold continuous rolling→continuous annealing + hot galvanizing, and the process flow is as follows:

[0049] 1) smelting and continuous casting according to the chemical composition of the steel;

[0050] 2) heating the continuous casting slab, the heating temperature of the casting blank is 1220-1250℃, and the heating time is 160-220min;

[0051] 3) hot rolling: the coarse rolling temperature is controlled to be 1060-1100℃ in the hot rolling process, the finish rolling temperature is 900-930℃, and the coiling temperature is 560-600℃, so that uniform grain structure is ensured;

[0052] 4) pickling and cold continuous rolling: the pickling speed is 120-200m / min, the acid concentration is 160-220g / L, the acid temperature is 80-90℃, and the cold rolling reduction is 62-75%;

[0053] 5) continuous annealing: the soaking temperature of the continuous annealing is 760-780℃, the fast cooling temperature is 480-500℃, and the dew point of the heating section is -15--5℃;

[0054] 6) hot galvanizing: the strip steel enters the zinc pot at a temperature of 460-480℃, the dew point of the furnace nose is -35--45℃, the zinc liquid temperature is 458-462℃, the Al content in the zinc liquid composition is 0.100-0.115% by mass fraction, the alloying temperature after hot galvanizing is 480-500℃, the holding time is 10-25s, the iron content in the coating is controlled to be 9-11wt%, after the alloying is completed, super-fast air mist cooling is adopted, the cooling speed is greater than 50℃ / s, the finishing elongation is 0.4-0.6%, and the 450MPa low-cost high-surface-quality zinc-iron alloy high-strength steel is obtained.

[0055] Comparative Example 1 is Example 3 of CN 105369135 A, in which N is 0.0035%, because CN 105369135 A requires controlling N≤0.005%, and the present application does not need to strictly control N; Comparative Example 2 is Example 4 of CN 108754343 A.

[0056] Table 1 is a list of values of each embodiment and the comparative example of the present application, the rest is Fe and inevitable impurities; Table 2 is a list of main process parameters of each embodiment and the comparative example of the present application; Table 3 is a list of performance detection of each embodiment and the comparative example of the present application. Note: X in Table 1 represents Si / 2+Mn / 5, each embodiment of the present application satisfies 5C≤Si / 2+Mn / 5≤6C.

[0057] Table 1 is a list of values of each embodiment and the comparative example of the present application, the rest is Fe and inevitable impurities; Table 2 is a list of main process parameters of each embodiment and the comparative example of the present application; Table 3 is a list of performance detection of each embodiment and the comparative example of the present application. Note: X in Table 1 represents Si / 2+Mn / 5, each embodiment of the present application satisfies 5C≤Si / 2+Mn / 5≤6C.

[0058]

[0059] Table 2 is a list of main process parameters of each embodiment and the comparative example of the present application; Table 3 is a list of performance detection of each embodiment and the comparative example of the present application. Note: X in Table 1 represents Si / 2+Mn / 5, each embodiment of the present application satisfies 5C≤Si / 2+Mn / 5≤6C.

[0060]

[0061] Table 2 is a list of main process parameters of each embodiment and the comparative example of the present application; Table 3 is a list of performance detection of each embodiment and the comparative example of the present application. Note: X in Table 1 represents Si / 2+Mn / 5, each embodiment of the present application satisfies 5C≤Si / 2+Mn / 5≤6C.

[0062]

[0063] Table 3 is a list of performance detection of each embodiment and the comparative example of the present application; Table 3 is a list of performance detection of each embodiment and the comparative example of the present application. Note: X in Table 1 represents Si / 2+Mn / 5, each embodiment of the present application satisfies 5C≤Si / 2+Mn / 5≤6C.

[0064]

[0065] From Table 3, it can be seen that the mechanical properties and the anti-powdering performance of the 450MPa low-cost high-surface-quality zinc-iron alloy high-strength steel produced by the present application are excellent, compared with the comparative example 1 and the comparative example 2, only adding Si element, under the premise of not adding Cr, Mo, Nb alloying elements, the performance is further improved, and through process optimization, the surface performance is good, the plating property of high-Si product is solved, which has obvious progress significance.

[0066] From Figure 1 it can be seen that the plating layer surface of the zinc-iron alloy high-strength steel described in the present application has no obvious micro-crack defects, the plating layer has suitable strength and plasticity, and the powdering performance is good; from Figure 2 it can be seen that the surface is good and has no plating defect.

[0067] Through the implementation of the present application, the 450MPa low-cost high-surface-quality zinc-iron alloy high-strength steel product has the following performance: yield strength is 280-330MPa, tensile strength is 460-510MPa, elongation is 32-38%, n value is 0.18-0.20, roughness is 0.7-1.5μm, powdering level is 1-3, surface quality is excellent, anti-powdering performance is excellent, and can meet the production and manufacturing of automobile inner plate and structural parts.

[0068] The above merely describes the preferred embodiments of the present application, and it should be noted that, for those skilled in the art, several improvements and changes can be made without departing from the inventive concept, and these all belong to the protection scope of the present application.

Claims

1. A method for producing 450MPa grade, low-cost, high-surface-quality zinc-iron alloy high-strength steel, the main process including steelmaking → hot rolling → pickling + cold continuous rolling → continuous annealing + hot-dip galvanizing, characterized in that, 1) The steel is smelted and continuously cast into billets according to its chemical composition; 2) Heat the continuously cast slab at a temperature of 1220–1250℃ for 160–220 min. 3) Hot rolling: The roughing temperature is controlled at 1060-1100℃ and the final rolling temperature is controlled at 900-930℃. After final rolling, U-shaped cooling is adopted. The coiling temperature at the head and tail of the steel coil is set at 580-600℃, and the coiling temperature in the middle of the steel coil is set at 560-580℃. 4) Pickling + Cold Rolling: Pickling speed 120-200 m / min, acid concentration 160-220 g / L, acid temperature 80-90℃; cold rolling reduction 62-75%; 5) Continuous annealing: The continuous annealing soaking temperature is 760~780℃, and the rapid cooling temperature is 480~500℃; the dew point of the heating section is -15~-5℃; 6) Hot-dip galvanizing: The temperature of the strip entering the zinc pot is 460-480℃, and the dew point of the furnace nose is -35℃ to -45℃; the Al content in the zinc bath is 0.100-0.115% by mass, and the zinc bath temperature is 458-462℃; the alloying temperature after hot-dip galvanizing is 480-500℃, the holding time is 10-25s, and the iron content of the coating is controlled at 9-11wt%; after alloying, ultra-fast air mist cooling is adopted, with a cooling rate greater than 50℃ / s; after finishing, the elongation is 0.4-0.6%, resulting in a 450MPa grade, low-cost, high-surface-quality zinc-iron alloy high-strength steel with an n value of 0.18-0.20, a roughness of 0.7-1.5μm, and a powdering level of 1-3. The 450MPa grade low-cost, high-surface-quality zinc-iron alloy high-strength steel has the following chemical composition and weight percentage: C 0.06~0.08%, Si 0.20~0.30%, Mn 1.00~1.15%, P≤0.005%, S≤0.003%, Als 0.020~0.070%. The weight percentages of C, Si, and Mn must satisfy 5C≤Si / 2+Mn / 5≤6C, with the remainder being Fe and unavoidable impurities.

2. The method for producing a low-cost, high-surface-quality zinc-iron alloy high-strength steel of 450MPa grade according to claim 1, characterized in that... The weight percentage of C is 0.065% to 0.075%.

3. The method for producing a low-cost, high-surface-quality zinc-iron alloy high-strength steel of 450MPa grade according to claim 1, characterized in that... The weight percentage of Si is 0.22% to 0.28%.

4. The production method of 450MPa grade low-cost high surface quality zinc-iron alloy high-strength steel according to claim 1, characterized in that... The yield strength of zinc-iron alloy high-strength steel products is 280–330 MPa, the tensile strength is 460–510 MPa, and the elongation is A. 80 It ranges from 32% to 38%.

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