CN118086791A - High-temperature oxidation resistant hot forming steel and preparation method and application thereof - Google Patents
High-temperature oxidation resistant hot forming steel and preparation method and application thereof Download PDFInfo
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- 238000011282 treatment Methods 0.000 claims description 83
- 238000000034 method Methods 0.000 claims description 36
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- 238000010438 heat treatment Methods 0.000 claims description 20
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- 239000011159 matrix material Substances 0.000 claims description 16
- 238000002791 soaking Methods 0.000 claims description 16
- 238000005097 cold rolling Methods 0.000 claims description 14
- 238000005554 pickling Methods 0.000 claims description 12
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Abstract
The invention relates to the field of steel, and discloses a hot forming steel and a preparation method and application thereof, wherein Cr element in the hot forming steel exists in a form of Cr-containing precipitate and accounts for less than 35% of the total Cr content in the steel, B content in the hot forming steel is less than 0.00001wt% and Ti content is less than 0.00001 wt%.
Description
Technical Field
The invention relates to the field of steel, in particular to high-temperature oxidation resistant hot forming steel, and a preparation method and application thereof.
Background
Currently, the global market of ultra-high strength hot forming steel is about 600 ten thousand tons/year, and the ultra-high strength hot forming steel is widely applied in the field of automobiles. In the ultra-high-strength steel for the automobile, the 22MnB5 series (1500 MPa level) is the largest in use amount, but loose oxide skin is generated on the surface of a steel plate material in the part thermoforming process, and the oxide skin is easy to partially fall off and adhere to a stamping die, so that the friction coefficient between the steel plate material and the stamping die is increased, the service life of the die is shortened, and the manufacturing cost of the thermoformed part is increased; in addition, the surface of the thermoformed part with loose oxide skin generally cannot meet the surface coating requirement, and subsequent surface shot blasting treatment is required, so that the production cost is increased, the production efficiency is reduced, the dimensional accuracy of the part is affected, and the shot blasting treatment can not be basically performed on the part with the thickness of less than 1.2 mm. To solve the oxidation problem of hot-formed steel, currently, an Al-Si plating technology developed by European Alteromil (Arcelormittal) company is generally adopted, and the technology has been controlled for more than 20 years in the world hot-formed steel field; and, the market price of a hot formed steel product produced based on the al—si plating technique is generally 2 times that of a non-plated product (i.e., bare board), but its cost is only about 1000 yuan/ton higher than that of the non-plated product. Therefore, there is a need to develop a hot-formed steel product having good resistance to high-temperature oxidation and low cost.
Disclosure of Invention
The invention aims to solve the problems that the service life of a hot forming die is low, parts cannot meet the surface shot-blasting-free direct coating requirement and the cost is high caused by the existing ultra-high strength steel, and provides high-temperature oxidation resistant hot forming steel, and a preparation method and application thereof. The surface of the steel is not required to be coated with a coating, the high-temperature oxidation resistance is good, and the hot formed part of the steel can be directly coated without a surface shot blasting process.
In order to achieve the above object, a first aspect of the present invention provides a hot-formed steel, wherein the ratio of Cr-containing precipitates in terms of Cr element in the hot-formed steel is 35% or less compared to the total content of Cr, and the content of B is 0.00001wt% or less and the content of Ti is 0.00001wt% or less based on the total amount of the hot-formed steel.
The second aspect of the present invention provides a method for producing a hot-formed steel, wherein the method comprises: sequentially carrying out primary annealing treatment, cold rolling treatment and secondary annealing treatment on the steel plate obtained by hot rolling; wherein Cr-containing precipitates are precipitated in the steel sheet after the first annealing treatment, and the second annealing treatment is performed so that the ratio of Cr-containing precipitates calculated by Cr elements in the obtained steel sheet is less than 35% compared with the total content of Cr elements;
wherein the content of B in the hot forming steel is below 0.00001wt% and the content of Ti is below 0.00001 wt%.
In a third aspect, the present invention provides a hot-formed steel prepared according to the second aspect of the present invention.
In a fourth aspect the invention provides the use of a hot formed steel in the automotive field.
In a fifth aspect, the present invention provides a thermoformed automotive component.
A sixth aspect of the present invention provides a method of producing an automotive part, the method comprising subjecting the hot-formed steel of the first or third aspect of the present invention to a hot-forming treatment.
Compared with the traditional 22MnB5 steel, the hot forming steel provided by the invention contains almost no boron element and titanium element, the added Cr element mainly exists in a solid solution state, only a very small amount of Cr element exists in a precipitate form, a compact and thin oxide layer on the surface is formed in the hot forming process of a part made of steel plate material, the oxide layer is tightly combined with a steel plate material matrix, no falling phenomenon exists, the steel can be ensured to have good high-temperature oxidation resistance under the hot forming process without coating a coating on the surface of the steel, and the hot forming part produced by adopting the hot forming steel provided by the invention can avoid a surface shot blasting process and be directly coated; in addition, the redissolved Cr element in the second annealing treatment process, namely the Cr element existing in the solid solution state, is also beneficial to improving the hardenability of the steel plate, promoting the matrix structure of the steel to be converted into a martensitic structure as much as possible after the hot forming of the part, and ensuring the strength of the hot formed part.
Drawings
FIG. 1 is an optical microscope characterization of the oxide layer on the surface of the rolled steel sheet in example 1;
FIG. 2 is an SEM image of the oxidized layer on the surface of a steel sheet after coiling in example 1;
FIG. 3 is an SEM-EDS diagram of the oxidized layer on the surface of the steel sheet after coiling in example 1;
FIG. 4 is an SEM image of the iron matrix of the steel sheet after coiling in example 1;
FIG. 5 is a tensile stress-strain curve of the steel sheet after the first annealing treatment in example 1;
FIG. 6 is an SEM image of a steel sheet after the first annealing treatment in example 1;
FIG. 7 is an SEM image of a steel sheet after the second annealing treatment in example 1;
FIG. 8 is a tensile stress-strain curve of the steel sheet after the second annealing treatment in example 1 and comparative examples 1 to 3;
FIG. 9 is an SEM image of a steel sheet after the second annealing treatment in example 1 and comparative examples 1-3;
FIG. 10 is a surface state diagram of the steel sheet subjected to the second annealing treatment in example 1 and comparative examples 1 to 3 after press-hardening by a flat plate;
FIG. 11 is a graph showing the results of a drawing of the coating properties of a steel sheet subjected to press hardening, which was obtained in example 1;
FIG. 12 is a view of a thermoformed part made using example 1;
FIG. 13 is a graph showing the results of the electrodeposition coating property test of the thermoformed part obtained in example 1.
Detailed Description
The endpoints of the ranges and any values disclosed in the present invention are not limited to the precise range or value, and the range or value should be understood to include values close to the range or value. For numerical ranges, one or more new numerical ranges may be found between the endpoints of each range, between the endpoint of each range and the individual point value, and between the individual point value, in combination with each other, and are to be considered as specifically disclosed herein.
The first aspect of the present invention provides a hot-formed steel, wherein the proportion of Cr-containing precipitates in terms of Cr element in the hot-formed steel is 35% or less compared to the total content of Cr, and the content of B is 0.00001wt% or less and the content of Ti is 0.00001wt% or less based on the total amount of the hot-formed steel. The precipitates of Cr element mainly exist in two types, namely M 7C3 type and M (C, N) type, wherein M is Cr element.
The steel obtained by the invention basically does not contain B element and Ti element, and the added Cr element exists in a form of Cr-containing precipitate with very little content and mainly exists in a solid solution state, so that the Cr element can be combined with other elements in the steel in the high-temperature heating, transferring and stamping forming processes when the steel plate material is subjected to part thermoforming, a compact and thin oxide layer is formed on the surface of the steel, the components of the compact oxide layer are generally in a composite state (Cr, si, fe) O, and the compact oxide layer can play a role of isolating oxygen in the environment from contacting with a steel matrix, so that the oxidation behavior is prevented from penetrating into the steel matrix, and the steel is ensured to have good high-temperature oxidation resistance. In addition, the bonding force between the Cr-containing compact oxide layer and the steel matrix is large, the Cr-containing compact oxide layer is not easy to fall off from the surface of steel in the process of hot forming the steel plate, and the produced hot formed part can be directly coated without a surface shot blasting process.
The ratio of Cr-containing precipitates in terms of Cr element in the hot-formed steel is preferably 35% or less, more preferably 20% or less, for example, 35% or less, 30% or less, 20% or less, 10% or less, and generally not less than 5% of the total Cr element content.
In some embodiments of the present invention, preferably, the hot-formed steel contains: 0.19-0.25wt% of C, 1.5-2wt% of Si, 1-2wt% of Cr, 0.5-1.5wt% of Mn, 0.01-0.1wt% of Al, less than or equal to 0.02wt% of P, less than or equal to 0.008wt% of S, less than or equal to 0.005wt% of N, and the balance of Fe and other unavoidable impurities.
The C content in the hot-formed steel may be a value between the range formed by 0.19wt%, 0.20wt%, 0.21wt%, 0.22wt%, 0.23wt%, 0.24wt%, 0.25wt% and any two thereof; the Si content may be a value between the range formed by 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt%, 2.0wt% and any two of them; cr content may be 1wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt%, 2.0wt% and any two thereof; the Mn content may be a value between the range formed by 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.5wt% and any two values thereof; the Al content may be a value between the range formed by 0.01wt%, 0.05wt%, 0.1wt% and any two values thereof; the P content may be less than 0.02wt% or 0.01wt%, or 0; the S content may be less than 0.008wt% or 0.004wt%, or 0; the N content may be less than 0.005wt% or 0.002wt%, or 0.
In some embodiments of the invention, the hot-formed steel preferably has a yield strength of 800-1100MPa, preferably 950-1100MPa, a tensile strength of 1400-1650MPa, preferably 1500-1650MPa, and a post-fracture elongation of not less than 5%, preferably 5-10%.
In some embodiments of the present invention, it is preferable that the hot-formed steel has an oxide layer formed on the surface thereof during hot forming of the part, and the thickness thereof is 10 μm or less, more preferably 5 μm or less, still more preferably 3 μm or less, and generally not less than 1 μm.
The Cr element existing in the solid solution state can not only improve the hardenability of the steel, but also be combined with other elements in the steel when the steel is processed at high temperature, a compact and thin oxide layer is formed on the surface of the steel, the oxide layer is tightly combined with a steel substrate, the steel can be ensured to have good high-temperature oxidation resistance without coating an additional coating on the surface of the steel, and the hot formed part produced by adopting the hot formed steel provided by the invention can be directly coated without a surface shot blasting process.
In some embodiments of the invention, the hot formed steel preferably has a size of 1, preferably 0, when tested for size of the paint film adhesion using the paint film adhesion test of the paint film technology requirements for the electrocoating of passenger vehicle parts, Q/JLY J7110978A-2006.
The second aspect of the present invention provides a method for producing a hot-formed steel, wherein the method comprises: sequentially carrying out primary annealing treatment, cold rolling treatment and secondary annealing treatment on the steel plate obtained by hot rolling; wherein Cr-containing precipitates are precipitated in the steel sheet after the first annealing treatment, and the second annealing treatment is performed so that the ratio of Cr-containing precipitates calculated by Cr elements in the obtained steel sheet is less than 35% compared with the total content of Cr elements;
wherein the content of B in the hot forming steel is below 0.00001wt% and the content of Ti is below 0.00001 wt%.
The first annealing treatment can adopt cover annealing, and the first annealing treatment can convert bainite tissues and martensite tissues generated in the hot rolled steel plate into tempered sorbite and ferrite, so that the yield strength and tensile strength of the hot rolled steel plate are reduced, and the cold rolling requirement is met; meanwhile, cr-containing precipitates are precipitated in the steel plate after the first annealing treatment, compared with the total content of Cr elements in the steel plate, the proportion of the Cr-containing precipitates in the steel plate calculated by the Cr elements is more than 50 percent and basically 70-95 percent, the Cr elements in the form of the precipitates cannot improve the hardenability of the steel and improve the high-temperature oxidation resistance, and for this reason, the Cr-containing precipitates precipitated in the previous steel plate are required to be subjected to further subsequent treatment, and are subjected to the second annealing treatment to be re-dissolved, so that a large amount of Cr elements in the steel in the form of the Cr-containing precipitates are converted into Cr elements in the form of solid solution, and the Cr elements in the form of the solid solution with enough content are necessary conditions for ensuring the excellent high-temperature oxidation resistance of the steel, and meanwhile, the steel can be ensured to have good hardenability and strength after quenching; after the second annealing treatment, the content of Cr-containing precipitates in the steel sheet, calculated as Cr element, is 35% or less, preferably 20% or less, and generally not less than 5% of the total content of Cr element in the steel sheet.
The composition and properties of the hot-formed steel produced by the production method according to the second aspect of the present invention are the same as those of the hot-formed steel according to the first aspect of the present invention, and will not be described in detail herein.
In some embodiments of the present invention, it is preferable that the hot rolled steel sheet is coiled at a temperature of 550-650 ℃ before the first annealing treatment. The winding temperature may be 550 ℃, 560 ℃, 580 ℃, 600 ℃, 620 ℃, 650 ℃ or any two values thereof.
In some embodiments of the invention, the first annealing treatment temperature is preferably 700-750 ℃ and the incubation time is 20-40 hours. A large amount of martensite is generated in the steel during the hot rolling process, the strength of the steel is too high, the subsequent cold rolling is difficult, and the strength of the steel can be reduced through the first annealing treatment. The first annealing treatment temperature may be 700 ℃, 720 ℃, 740 ℃, 750 ℃ and any two values thereof; the incubation time may be a number between the range formed by 20h, 25h, 30h, 35h, 40h and any two of these values.
In some embodiments of the present invention, preferably, the preparation method further comprises: and (3) pickling the steel plate after the first annealing treatment and before the cold rolling treatment. The manner of pickling is not limited as long as it removes all oxide layers on the surface of the steel after the first annealing treatment, and in some embodiments of the present invention, pickling may be turbulent pickling, immersion pickling, or the like.
In some embodiments of the invention, more preferably, the acid solution concentration of the acid wash is 60-120g/L. The acid solution for pickling may be any one of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, etc., and the concentration of the acid solution may be a value ranging between 60g/L, 80g/L, 100g/L, 120g/L, and any two values thereof.
In some embodiments of the present invention, more preferably, the conditions for the acid washing include: the temperature is 50-70 ℃, the pulling speed is 50-80m/min, and the time is 20-50s. The pickling temperature may be a value between the range formed by any two values of 50 ℃, 60 ℃, 70 ℃; the pull-out speed may be a value between a range formed by 50m/min, 60m/min, 70m/min, 80m/min and any two values thereof; the time may be a number between the range formed by 20s, 30s, 40s, 50s and any two of these. The drawing speed refers to a speed at which the steel material is drawn out of the acid solution.
The cold rolling can be carried out at room temperature (20-35 ℃) after the acid washing, the cold rolling times can be selected according to the situation, for example, the cold rolling can be carried out for 2-5 times, and the thickness of the cold rolled steel plate can be 1.2-1.4mm.
In the first annealing process, a large amount of Cr-containing precipitates are precipitated in the steel matrix, and Cr element needs to exist in a solid solution state to exert the effect of improving the high-temperature oxidation resistance and the hardenability of the steel. As described above, this second annealing treatment will result in the hot-formed steel having a Cr-containing precipitate content in terms of Cr element of 35% or less, preferably 20% or less, for example, 35% or less, 30% or less, 20% or less, 10% or less, and generally not less than 5% of the total Cr content.
In a preferred embodiment of the invention, the second annealing treatment is preferably performed by means of a continuous annealing, comprising a heating stage, a soaking stage and a cooling stage. In particular, the process is carried out under an inert atmosphere, for example a nitrogen atmosphere.
Preferably, the second annealing treatment comprises a heating stage, a soaking stage and a cooling stage which are sequentially carried out, wherein the temperature of the soaking stage is 800-Ac 1, and the holding time is 1-10min, and the Ac1 temperature refers to the temperature at which the steel matrix structure starts to be transformed from pearlite to austenite during heating.
In some embodiments of the invention, preferably, the Ac1 temperature is 830-850 ℃. Depending on the composition of the steel, the Ac1 temperature of the steel may be 830℃and 840℃and 850℃as well as any two values thereof.
In some embodiments of the invention, preferably, the conditions of the soaking stage include: the temperature is 830-850 ℃, and the holding time is 1-3min.
When the soaking section temperature is greater than 850 ℃, the steel matrix structure will generate phase change, so that the soaking section temperature can be 800 ℃, 810 ℃, 820 ℃, 830 ℃, 840 ℃, 850 ℃ and any two values thereof form a range between values; the holding time may be a value between a range formed by 1min, 1.5min, 2min, 4min, 5min, 6min, 7min, 8min, 9min, 10min, and any two values thereof.
In some embodiments of the invention, the heating stage preferably has a heating rate of 2-5 ℃/s.
According to the invention, the cooling stage preferably comprises a slow cooling stage, a quick cooling stage, an equalization stage and a final cooling stage which are sequentially carried out, wherein the cooling rate of the slow cooling stage is 3-5 ℃/s, the cooling rate of the quick cooling stage is 25-35 ℃/s, the cooling rate of the equalization stage is 0.12-2 ℃/s, and the cooling rate of the final cooling stage is 2-3 ℃/s.
In a third aspect, the present invention provides a hot-formed steel prepared according to the second aspect of the present invention.
In a fourth aspect the invention provides the use of a hot formed steel in the automotive field.
A fifth aspect of the present invention provides an automotive part formed from the hot-formed steel of the first or third aspect of the present invention.
The hot forming steel surface provided by the invention does not need to be coated with a coating, has excellent high-temperature oxidation resistance, and has the advantages that after a part is prepared by hot forming, the thickness of an oxide layer on the part surface is thin and compact, the surface shot blasting treatment is not needed, the electrophoresis coating can be directly carried out, the adhesion of the surface electrophoresis paint can reach 0 level, and the coated part has good acid resistance, alkali resistance and water resistance.
A sixth aspect of the present invention provides a method for producing an automotive part, the method comprising subjecting the hot-formed steel of the first or third aspect of the present invention to a hot-forming treatment. Wherein the heating temperature of the steel plate material is 910-950 ℃ and the heat preservation time is 3-10min during the hot forming treatment. The heating temperature of the steel plate material during the hot forming treatment can be a numerical value between a range formed by any two values of 910 ℃, 920 ℃, 930 ℃, 940 ℃, 950 ℃ and the like, and the heat preservation time can be a numerical value between a range formed by any two values of 3min, 5min, 8min, 10min and the like.
The present invention will be described in detail by examples. The experimental methods employed in the examples and experimental examples are as follows:
The determination method of the Cr element content in the form of solid solution is GB/T1467-2008 general rule of the standard of the chemical analysis method of metallurgical products, J0802-018-2001 in the general rule of the determination method of Cr in steel chemical analysis method steel, the ratio of the Cr element content in the form of precipitate to the total Cr element content in steel=100% -the Cr element content in the form of solid solution/total Cr content×100%;
The adhesion of the electrophoretic paint film is tested by adopting the technical requirement of the electrophoretic coating of the parts of the passenger car of Q/JLY J7110978A-2006.
Before preparing a hot-formed steel product, hot-rolled steel is prepared, and the preparation process flow of the hot-rolled steel is as follows: iron ore is sequentially subjected to blast furnace ironmaking, steelmaking and continuous casting, after continuous casting cooling solidification, the cross section size of the produced continuous casting billet is (150-250) mm x (1200-1800) mm, the continuous casting billet is hot charged by adopting hot feeding, is heated to 1250+/-20 ℃ by a heating furnace and is kept warm for a certain time, and then enters a hot rolling process, wherein the heating and the heat-preserving time of the continuous casting billet are 3.5-5h, in the hot rolling process, 6-8 times of rough rolling and 6-8 times of finish rolling are carried out, the thickness size of a final hot rolled plate is 3.5-5.0mm, and the final rolling temperature is 860+/-10 ℃.
The content of the hot rolled steel components used in examples 1 to 6 is shown in Table 2.
Example 1
And (3) carrying out laminar cooling on the hot rolled steel, coiling at 560 ℃, carrying out cover annealing on the coiled steel plate at 720 ℃, wherein the cover annealing holding time is 24 hours, and precipitating Cr-containing precipitates in the steel plate after the cover annealing treatment.
The steel plate is subjected to turbulent acid washing after the cover annealing treatment, the used acid solution is HCl solution with the concentration of 60g/L and the temperature of 65 ℃, and the pulling-out speed is 65m/min for 24s.
And (3) carrying out 4-pass cold rolling on the steel plate after pickling to obtain a cold-rolled plate with the thickness of 1.2-1.4mm, and carrying out continuous annealing treatment on the cold-rolled plate under the protection of nitrogen.
The continuous annealing treatment comprises the following specific processes: the heating stage was carried out at a rate of 2.3 ℃/s from room temperature to 810 ℃, at which temperature it was maintained for 95s, then it was put into a slow cooling stage, cooled to 650 ℃ at a rate of 3.3 ℃/s, then it was put into a rapid cooling stage, cooled to 430 ℃ at a rate of 31.5 ℃/s, then it was put into an equalization stage, cooled to 360 ℃ at a rate of 0.17 ℃/s, finally it was put into a final cooling stage, cooled to 180 ℃ at a rate of 2.5 ℃/s, and the content of Cr-containing precipitates in the steel after the two annealing treatments was shown in table 3.
The changes in the steel materials at each stage were analyzed as follows:
The hot rolled plate after coiling was subjected to characterization by an optical microscope and a scanning electron microscope (SEM for short), and the results are shown in fig. 1 and 2. As can be seen from fig. 1, the oxide layer on the surface of the coiled steel plate is thin and compact, the thickness is less than 9 μm, no inter-crystal oxide layer exists, and the steel plate has good bonding force with a matrix; the basic morphology of the oxide layer is shown in fig. 2, the SEM-EDS diagram of the oxide layer is shown in fig. 3, and further analysis of the oxide layer is shown to be a dense Fe 2(Sr/Cr)O4 layer.
The basic mechanical properties of the hot rolled sheet after coiling are shown in Table 1.
TABLE 1 basic mechanical Properties of Steel sheet after coiling
| Steel plate thickness (mm) | Yield strength (MPa) | Tensile strength (MPa) | Elongation (%) |
| 5.0 | 976 | 1409 | 14 |
| 3.5 | 945 | 1415 | 14.4 |
Further SEM characterization of the microstructure of the hot rolled sheet after coiling, as shown in fig. 4, it was found that the matrix structure of the steel sheet was still mainly ferrite structure, but a large amount of bainite structure (gray embossed and feathered/needle-like regions in the figure) and martensite structure (gray embossed regions) were generated.
The steel plate after the cover annealing treatment is taken, and the tensile stress-strain curves of different parts are tested, and the result is shown in figure 5, wherein the yield strength of the steel plate after the cover annealing treatment is 402-427MPa, and the tensile strength is less than 700MPa.
Further, the microstructure of the steel sheet after the cap annealing treatment was subjected to SEM characterization, and the results are shown in FIG. 6, wherein 6 (a) is an SEM image of the matrix structure of the steel sheet, FIG. 6 (b) is an SEM image of Cr-containing precipitates, and FIG. 6 (c) is an SEM-EDS image of Cr-containing precipitates. As can be seen from fig. 6 (a) and 6 (b), the matrix structure of the steel sheet at this time is mainly tempered sorbite (mixed region of flaky ferrite and flaky cementite in the drawing), ferrite, and Cr-containing precipitates are precipitated, and it is seen that the cap annealing treatment promotes effective transformation of a large amount of martensite and bainite remaining in the hot rolled sheet after coiling into sorbite and ferrite, thereby reducing the strength of the hot rolled sheet.
The microstructure SEM characterization of the continuously annealed hot-formed steel is carried out, and the result is shown in FIG. 7, wherein the bright white salient points in FIG. 7 are Cr-containing precipitates. Comparing fig. 7 and 6 (b) shows that the Cr-containing precipitates in the hot-formed steel after the continuous annealing treatment are greatly reduced, indicating that most of the Cr-containing precipitates precipitated during the cap annealing treatment have dissolved back after the continuous annealing treatment, thereby promoting the Cr element to exist mainly in the solid solution state in the hot-formed steel.
As can be seen from the embodiment, the quenching degree of the steel is good due to the addition of Cr element, martensite is generated in the hot rolling process, so that the strength of the steel is too high, and subsequent cold rolling is difficult to carry out, so that the strength of the steel is reduced by adopting hood annealing, but the hood annealing promotes the steel to be separated out a large amount of Cr-containing precipitates, namely, the Cr element in the steel exists in a large amount in the form of Cr-containing precipitates, so that the Cr content in the steel in solid solution is insufficient, and the steel has poor high-temperature oxidation resistance when the part is hot formed; therefore, the invention further adopts continuous annealing treatment to promote the re-dissolution of the Cr-containing precipitate, so that Cr element exists in the steel matrix again in a solid solution state, thereby ensuring that the steel has good high-temperature oxidation resistance when the part is hot formed. Therefore, the cover annealing and the continuous annealing are indispensable links for ensuring that the hot forming steel prepared by the method has good high-temperature oxidation resistance and good mechanical properties.
Example 2
And (3) carrying out laminar cooling on the hot rolled steel, coiling at 640 ℃, carrying out cover annealing on the coiled steel plate at 700 ℃ for 36 hours, and separating Cr-containing precipitate from the steel plate after cover annealing treatment.
And (3) carrying out turbulent acid washing on the steel plate after the cover annealing treatment, wherein the used acid solution is HCl solution, the concentration is 120g/L, the temperature is 55 ℃, the pulling-out speed is 80m/min, and the time is 45s.
After pickling, carrying out 4-pass cold rolling on the steel plate to obtain a cold-rolled plate with the thickness of 1.2-1.4mm, and carrying out continuous annealing treatment on the cold-rolled plate under the protection of nitrogen, wherein the specific process comprises the following steps:
The heating stage was carried out at a rate of 3.5 ℃/s from room temperature to 830 ℃, maintained at this temperature for 8 minutes, then entered into a slow cooling stage, cooled to 660 ℃ at a rate of 3.8 ℃/s, entered into a rapid cooling stage, cooled to 420 ℃ at a rate of 34 ℃/s, entered into an equalization stage, cooled to 350 ℃ at a rate of 0.12 ℃/s, finally entered into a final cooling stage, cooled to 190 ℃ at a rate of 3 ℃/s, and the content of Cr-containing precipitates in the steel after the two annealing treatments was shown in table 3.
Example 3
And (3) carrying out laminar cooling on the hot rolled steel, coiling at 600 ℃, carrying out cover annealing on the coiled steel plate at 750 ℃ for 30 hours, and separating Cr-containing precipitate from the steel plate after cover annealing treatment.
And (3) carrying out turbulent acid washing on the steel plate after the cover annealing treatment, wherein the used acid solution is HCl solution, the concentration is 100g/L, the temperature is 50 ℃, the pulling-out speed is 55m/min, and the time is 30s.
After pickling, carrying out 4-pass cold rolling on the steel plate to obtain a cold-rolled plate with the thickness of 1.2-1.4mm, and carrying out continuous annealing treatment on the cold-rolled plate under the protection of nitrogen, wherein the specific process comprises the following steps:
The heating stage was carried out at a rate of 5 c/s from room temperature to 850 c for 4min at this temperature, then it was carried out in a slow cooling stage, cooled to 640 c at a rate of 4.5 c/s, then it was carried out in a rapid cooling stage, cooled to 440 c at a rate of 27 c/s, then it was carried out in an equalization stage, cooled to 370 c at a rate of 1.5 c/s, finally it was carried out in a final cooling stage, cooled to 170 c at a rate of 2.2 c/s, and the Cr-containing precipitates in the steel after the two annealing treatments were as shown in table 3.
Example 4
The hot rolled steel of example 1 was selected and carried out in the manner of example 1, except that:
The soaking stage in the continuous annealing treatment was maintained at 840℃for 95 seconds, and the Cr-containing precipitate content in the steel material obtained was shown in Table 3.
Example 5
The hot rolled steel of example 1 was selected and carried out in the manner of example 1, except that:
The soaking stage in the continuous annealing treatment was maintained at 810℃for 3 minutes, and the Cr-containing precipitate content in the obtained steel was shown in Table 3.
Example 6
The hot rolled steel of example 1 was selected and subjected to the same procedure as in example 1, except for the continuous annealing treatment as follows:
The steel material obtained was first heated from room temperature to 810 c at a rate of 4.5 c/s for 10min, then cooled to 650 c at a rate of 4.5 c/s, then cooled to 430 c at a rate of 35 c/s, then cooled to 360 c at a rate of 1.5 c/s, and finally cooled to 180 c at a rate of 3 c/s, and the Cr-containing precipitate content was shown in table 3.
Comparative example 1
The hot rolled steel of example 1 was selected and carried out in the same manner as in example 1 except that: the soaking stage temperature in the second annealing treatment was 720 ℃, and the Cr-containing precipitate content in the obtained steel was shown in table 3.
Comparative example 2
The hot rolled steel of example 1 was selected and carried out in the same manner as in example 1 except that: the soaking stage in the second annealing treatment was carried out at 750℃and the Cr-containing precipitate content in the steel product obtained was shown in Table 3.
Comparative example 3
The hot rolled steel of example 1 was selected and carried out in the same manner as in example 1 except that: the soaking stage in the second annealing treatment was carried out at 780℃and the steel plate composition obtained is shown in Table 2.
TABLE 2 summary of steel compositions
TABLE 3 comparison of Cr-containing precipitates from two anneals
Note that: the content of Cr-containing precipitates in the steel calculated as Cr element is a percentage of the total Cr content in the steel compared to the total Cr element content.
Example 4 has a higher continuous annealing temperature, a lower Cr-containing precipitate fraction after continuous annealing than example 1, example 5 has a longer continuous annealing hold time, a lower Cr-containing precipitate fraction after continuous annealing, and example 6 has a faster temperature rise and fall rate at each stage of continuous annealing than example 1, and a longer hold time, which is sufficient to facilitate an increase in the dissolution back amount of Cr-containing precipitates. As can be seen from the change of the ratio of the Cr-containing precipitates after the continuous annealing in examples 2-4, the ratio of the Cr-containing precipitates after the continuous annealing treatment is relatively low when the continuous annealing temperature is 830-850 ℃, and the soaking stage is kept for 1-3min by comprehensively considering the cost factors caused by time and the changes of the Cr-containing precipitates.
Test example 1
Taking the continuous annealed hot-formed steels produced in example 1 and comparative examples 1 to 3, the steels produced in each example were subjected to two parallel tests, and the tensile stress-strain curves thereof were tested, and as a result, as shown in fig. 8, it was found that the tensile stress-strain curves of the steel sheets of comparative examples 1 to 3 all exhibited a distinct lower yield plateau (a phenomenon in which the tensile stress suddenly decreased), whereas the steel sheet of example 1 did not exhibit a lower yield plateau phenomenon, because the Cr-containing precipitates precipitated in the hood-type annealing treatment were dissolved back by the continuous annealing treatment, thereby changing the mechanical properties of the steel sheet.
Test example 2
SEM scanning was performed on the continuously annealed hot-formed steels produced in example 1 and comparative examples 1 to 3, and the results are shown in fig. 9. In fig. 9, (a), (b) and (c) are SEM images of the steel sheets of comparative examples 1, 2 and 3, respectively, and in fig. 9 (d) is an SEM image of the steel sheet of example 1, in which bright white bumps are Cr-containing precipitates, it was found that the number of Cr-containing precipitates remaining in the steel sheet gradually decreases with an increase in soaking temperature of the continuous annealing treatment.
The reduction of the number of the Cr-containing precipitates can effectively reduce the pinning blocking effect of the Cr-containing precipitates on dislocation slip in the stretching deformation process, so that the phenomenon that the stretching stress suddenly drops suddenly in the elastic-plastic deformation conversion process can be avoided as much as possible, and the lower yield platform in the stretching process can be avoided.
Test example 3
Carrying out flat plate die quenching on the continuous annealing state hot forming steel prepared in the embodiment 1 and the comparative examples 1-3, wherein the heating temperature of a quenching sample is 920+/-5 ℃, and the temperature is kept for 4-5min, and the sample is protected by nitrogen in the heating and heat-preserving process; and then taking out the sample and carrying out flat quenching, wherein the surface of the sample generates a compact oxide layer in the flat quenching cooling process, and observing the surface state of the sample, as shown in a graph 10, in fig. 10, (a) - (c) are steel plates after flat quenching in turn in comparative examples 1-3, and in fig. 10, (d) is the steel plate after flat quenching in example 1, the surface of each of comparative examples 1-3 is found to have oxide layer falling areas with different degrees, the subsequent surface shot-free direct coating requirements cannot be met, the surface oxide layer of the steel plate in example 1 is quite compact after flat quenching, the falling phenomenon does not appear, and the subsequent shot-free direct coating requirements are met.
Further, the coating performance of the steel plate subjected to plate quenching prepared in example 1 was tested, and the coating experiment was performed according to the standard of "Q/JLY J7110978A-2016 passenger car component electrophoretic coating technical requirement", and the result is shown in FIG. 11, wherein 11 (a) is the steel plate before the cross-hatch experiment, 11 (b) is the steel plate after the cross-hatch experiment, the paint film on the steel plate after the cross-hatch experiment was uniformly adhered, and the paint film adhesion reached the level of 0 grade.
Test example 4
In order to quantitatively analyze the oxidation resistance of the steel plate, an oxidation weight increase experiment is performed by using a thermogravimetric Analyzer (THERMAL GRAVIMETRIC Analyzer, abbreviated as TGA), and the method is as follows:
taking the continuous annealing state hot forming steel prepared in the examples 1-6 as a sample, comparing the sample with the traditional 22MnB5 steel, cleaning the surfaces of the samples and the comparison sample, drying the samples, and then respectively placing the samples and the comparison sample in a TGA (thermal growth analysis) device for carrying out an oxidation weight increase experiment; in the experiment, the temperature of the sample is raised to 950 ℃ from room temperature at 20 ℃/min, the temperature is kept for 30min, and then the temperature is reduced to the room temperature at-20 ℃/min, and the experimental result is shown in Table 4.
The analysis found that the conventional 22MnB5 steel began to oxidize at 493 c, while the steels prepared in examples 1-6 had a starting oxidation temperature not lower than 747 c, and it was found that the starting oxidation temperature of the steels developed in the present invention was increased by at least 250 c as compared with the conventional steels, with a higher starting oxidation temperature. After the experiment is finished, the oxidation weight gain of the traditional 22MnB5 steel is 17.22mg/cm 2, and the oxidation weight gain of the steel prepared in the examples 1-6 is not more than 2.25mg/cm 2, and is reduced by more than 86 percent, mainly because the surface of the steel developed by the invention can generate a compact and thin Cr-containing oxide layer, and the penetration of oxidation into the steel matrix is prevented, so that the steel provided by the invention has excellent high-temperature oxidation resistance.
TABLE 4 results of oxidative weight gain experiments
Test example 5
The continuous annealed hot-formed steel samples prepared in examples 1 to 6 and comparative examples 1 to 3 were subjected to the plate press quenching treatment described in test example 3, respectively, and the basic mechanical properties of the relevant samples were measured, and the results are shown in Table 5.
TABLE 5 mechanical property test
| Examples | Yield strength, MPa | Tensile strength, MPa | Elongation after break% |
| Example 1 | 841 | 1467 | 8.4 |
| Example 2 | 1069 | 1622 | 5.7 |
| Example 3 | 1023 | 1578 | 6.3 |
| Example 4 | 968 | 1513 | 6.9 |
| Example 5 | 927 | 1516 | 7.8 |
| Example 6 | 1015 | 1531 | 7.2 |
| Comparative example 1 | 692 | 1379 | 9.4 |
| Comparative example 2 | 592 | 1392 | 6.4 |
| Comparative example 3 | 761 | 1354 | 6.1 |
As can be seen from Table 3, the mechanical properties of the steel sheets prepared in comparative examples 1 to 3 were inferior to those of the steel sheets of examples 1 to 6 of the present invention.
Test example 6
The continuous annealed hot formed steel prepared in examples 1 to 6 above and conventional 22MnB5 steel were used for hot formed part industry trial production. The thermoforming process conditions are: before the steel plate is subjected to part stamping forming, the steel plate is heated and austenitized, the heating and heat preservation temperature is 910-950 ℃, the heat preservation time is 5min, and the whole process is protected by nitrogen; after the heating and heat preservation are finished, the steel plate is taken out and transferred to a stamping die to be stamped and cooled and formed, the stamping and cooling process is carried out in an air environment, a compact and thin oxide layer is formed on the surface of the steel plate in the transferring and stamping and cooling process, the compact oxide layer can prevent oxidation from penetrating into the steel plate, the obtained thermoformed part can be prevented from being subjected to surface shot blasting treatment, and the obtained thermoformed part is directly coated.
The test results of the hot-formed parts using the continuous annealed hot-formed steel of example 1 are shown in fig. 12, in which 12 (a) is a steel sheet before the test, 12 (b) is a part obtained after the test, and 12 (c) is a coated part, and it can be seen that the surface of the hot-formed part has metallic luster, and the oxide film is dense and does not fall off.
SEM scanning characterization was performed on the thermoformed parts of each example, and the average thickness of the surface oxide layer was observed, and found that the average thickness of the surface oxide layer of the thermoformed parts made of the conventional 22MnB5 steel reached 102. Mu.m, whereas the average thickness of the surface oxide layer of the thermoformed parts made of the steels of examples 1 to 6 of the present invention was less than 3. Mu.m, and the specific results are shown in Table 6.
TABLE 6 average thickness of oxide layer on surface of thermoformed part
| Examples | Average thickness of oxide layer, μm |
| Example 1 | 2.3 |
| Example 2 | 1.6 |
| Example 3 | 1.2 |
| Example 4 | 1.9 |
| Example 5 | 2.1 |
| Example 6 | 1.7 |
| 22MnB5 steel | 102 |
Further, the surface shot-free direct electrophoretic coating is carried out on the hot formed part of the continuous annealing hot formed steel prepared in the embodiment 1, and the acid resistance, alkali resistance and water resistance of the coated part are tested, and the result is shown in a graph shown in fig. 13, wherein 13 (a) is an SEM image of the part after the electrophoretic coating, and it can be seen that the adhesion of the electrophoretic paint on the surface of the tested part reaches a level of 0, and the thickness of the paint film is about 20-30 mu m;13 And (b) the test results of the acid, alkali and water resistance test and cross-cut test of the part paint film show that the paint film on the surface of the part has good water resistance, acid resistance and alkali resistance.
The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the scope of the technical idea of the invention, a number of simple variants of the technical solution of the invention are possible, including combinations of the individual technical features in any other suitable way, which simple variants and combinations should likewise be regarded as being disclosed by the invention, all falling within the scope of protection of the invention.
Claims (10)
1. A hot-formed steel, characterized in that the proportion of Cr-containing precipitates in terms of Cr element in the hot-formed steel is 35% or less compared to the total content of Cr element, and the content of B is 0.00001wt% or less and the content of Ti is 0.00001wt% or less based on the total amount of the hot-formed steel.
2. The hot-formed steel according to claim 1, wherein the hot-formed steel contains: 0.19 to 0.25wt% of C, 1.5 to 2wt% of Si, 1 to 2wt% of Cr, 0.5 to 1.5wt% of Mn, 0.01 to 0.1wt% of Al, less than or equal to 0.02wt% of P, less than or equal to 0.008wt% of S, less than or equal to 0.005wt% of N, and the balance of Fe and other unavoidable impurities;
preferably, the yield strength of the hot forming steel is 800-1100MPa, the tensile strength is 1400-1650MPa, and the elongation after fracture is not less than 5%;
Preferably, the hot-formed steel has an oxide layer formed on the surface thereof during hot forming of the part, and has a thickness of 10 μm or less, preferably 5 μm or less, more preferably 3 μm or less.
3. A method of preparing a hot-formed steel, the method comprising: sequentially carrying out primary annealing treatment, cold rolling treatment and secondary annealing treatment on the steel plate obtained by hot rolling; wherein Cr-containing precipitates are precipitated in the steel sheet after the first annealing treatment, and the second annealing treatment is performed so that the ratio of Cr-containing precipitates calculated by Cr elements in the obtained steel sheet is less than 35% compared with the total content of Cr elements;
Wherein, in the hot forming steel, the content of B is below 0.00001wt% and the content of Ti is below 0.00001 wt%;
Preferably, the hot-formed steel comprises: 0.19 to 0.25wt% of C, 1.5 to 2wt% of Si, 1 to 2wt% of Cr, 0.5 to 1.5wt% of Mn, 0.01 to 0.1wt% of Al, less than or equal to 0.02wt% of P, less than or equal to 0.008wt% of S, less than or equal to 0.005wt% of N, and the balance of Fe and other unavoidable impurities;
preferably, the yield strength of the hot forming steel is 800-1100MPa, the tensile strength is 1400-1650MPa, and the elongation after fracture is not less than 5%;
Preferably, the hot-formed steel has an oxide layer formed on the surface thereof during hot forming of the part, and has a thickness of 10 μm or less, preferably 5 μm or less, more preferably 3 μm or less.
4. A production method according to claim 3, wherein the second annealing treatment includes a heating stage, a soaking stage, and a cooling stage which are sequentially performed, wherein the conditions of the soaking stage include: the temperature is 800-Ac 1, and the holding time is 1-10min; wherein the Ac1 temperature refers to the temperature at which the steel matrix structure starts to transform from pearlite to austenite when heated;
Preferably, the Ac1 temperature is 830-850 ℃;
Preferably, the conditions of the soaking stage include: the temperature is 830-850 ℃, and the holding time is 1-3min.
5. The preparation method according to claim 4, wherein the heating stage has a heating rate of 2-5 ℃/s;
And/or the cooling stage comprises a slow cooling stage, a quick cooling stage, an equalization stage and a final cooling stage which are sequentially carried out, wherein the cooling rate of the slow cooling stage is 3-5 ℃/s, the cooling rate of the quick cooling stage is 25-35 ℃/s, the cooling rate of the equalization stage is 0.12-2 ℃/s, and the cooling rate of the final cooling stage is 2-3 ℃/s.
6. The production method according to any one of claims 3 to 5, wherein the conditions of the first annealing treatment include: the temperature is 700-750 ℃ and the time is 20-40h;
preferably, the steel sheet obtained by hot rolling is coiled at 550-650 ℃ before the first annealing treatment;
preferably, the preparation method further comprises: pickling the steel plate obtained after the first annealing treatment and before the cold rolling treatment;
More preferably, the acid solution concentration of the acid washing is 60-120g/L;
More preferably, the conditions for the pickling include: the temperature is 50-70 ℃, the pulling speed is 50-80m/min, and the time is 20-50s.
7. A hot formed steel produced by the method of any one of claims 3 to 6.
8. Use of a hot-formed steel as claimed in any one of claims 1 to 3 and 7 in the automotive field.
9. Automotive parts formed from the hot-formed steel as claimed in any one of claims 1 to 3 and 7.
10. The preparation method of the automobile part comprises the following steps: hot forming the hot formed steel according to any one of claims 1 to 3 and 7.
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