CN113088635B - Baking-hardened steel and method for prolonging aging time of baking-hardened steel - Google Patents
Baking-hardened steel and method for prolonging aging time of baking-hardened steel Download PDFInfo
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- 229910000760 Hardened steel Inorganic materials 0.000 title claims abstract description 107
- 238000000034 method Methods 0.000 title claims abstract description 56
- 230000032683 aging Effects 0.000 title abstract description 41
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 144
- 239000010959 steel Substances 0.000 claims abstract description 144
- 238000000137 annealing Methods 0.000 claims abstract description 60
- 238000005266 casting Methods 0.000 claims abstract description 22
- 238000005457 optimization Methods 0.000 claims abstract description 10
- 229910052799 carbon Inorganic materials 0.000 claims description 30
- 239000006104 solid solution Substances 0.000 claims description 26
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 25
- 239000000126 substance Substances 0.000 abstract description 32
- 239000000203 mixture Substances 0.000 abstract description 14
- 238000004519 manufacturing process Methods 0.000 abstract description 12
- 230000002035 prolonged effect Effects 0.000 abstract description 4
- 238000005096 rolling process Methods 0.000 description 31
- 238000005097 cold rolling Methods 0.000 description 24
- 230000008569 process Effects 0.000 description 17
- 238000001816 cooling Methods 0.000 description 15
- 239000000463 material Substances 0.000 description 12
- 238000010438 heat treatment Methods 0.000 description 11
- 239000012535 impurity Substances 0.000 description 10
- 230000009467 reduction Effects 0.000 description 10
- 125000004432 carbon atom Chemical group C* 0.000 description 9
- 238000005098 hot rolling Methods 0.000 description 9
- 238000003860 storage Methods 0.000 description 9
- 238000003723 Smelting Methods 0.000 description 8
- 239000002253 acid Substances 0.000 description 8
- 238000005246 galvanizing Methods 0.000 description 8
- 238000005554 pickling Methods 0.000 description 8
- 238000002360 preparation method Methods 0.000 description 8
- 125000004429 atom Chemical group 0.000 description 5
- 238000003483 aging Methods 0.000 description 4
- 230000008859 change Effects 0.000 description 4
- 230000000694 effects Effects 0.000 description 3
- 150000001247 metal acetylides Chemical class 0.000 description 3
- 125000004433 nitrogen atom Chemical group N* 0.000 description 3
- 238000004080 punching Methods 0.000 description 3
- 238000002791 soaking Methods 0.000 description 3
- 239000000243 solution Substances 0.000 description 3
- 238000007619 statistical method Methods 0.000 description 2
- 229910001209 Low-carbon steel Inorganic materials 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 239000010960 cold rolled steel Substances 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 230000001627 detrimental effect Effects 0.000 description 1
- 238000004090 dissolution Methods 0.000 description 1
- 230000007774 longterm Effects 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
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- 238000001953 recrystallisation Methods 0.000 description 1
- 238000011160 research Methods 0.000 description 1
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- 238000010972 statistical evaluation Methods 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 239000002699 waste material Substances 0.000 description 1
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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Abstract
The invention relates to a bake-hardened steel and a method for prolonging the aging time of the bake-hardened steel, which comprises the following steps: counting the BH value of the bake-hardened steel, and determining the BH value optimization range of which the occurrence probability of the surface orange peel phenomenon does not exceed a specific value when the bake-hardened steel is used for 3 months; counting Nb/C atomic ratios corresponding to different BH values in the BH value optimization range; fixing the Nb/C atomic ratio, and optimizing the chemical composition and casting process of the bake-hardened steel; and casting the bake-hardened steel according to the optimized bake-hardened steel chemical composition and casting process. According to the invention, by researching the relationship between the chemical composition of the bake-hardening steel, the annealing temperature in the bake-hardening steel casting process, the leveling elongation and the bake-hardening steel BH value, the aging time of the bake-hardening steel is prolonged from 3 months to the limit of 6 months, so that the product rejection rate caused by the plant yield plan fluctuation is reduced, and the production cost is further reduced.
Description
Technical Field
The invention relates to the field of production of bake-hardened steel, in particular to bake-hardened steel and a method for prolonging the aging time of the bake-hardened steel.
Background
The bake-hardening steel is a cold-rolled steel plate added with trace Nb or Ti, and a small amount of carbon atoms or nitrogen atoms existing in the material are uniformly dispersed in Fe lattice gaps before stamping to form solid solution with Fe; fe lattice dislocation in the stamping process; after stamping, solid-dissolved carbon atoms or nitrogen atoms are heated and diffused in the baking process and interact with dislocation to form a pinning structure, so that the yield strength of the material is increased and the dent resistance is improved. The bake-hardened steel is soft before stamping and easy to form, and the strength is increased in the baking finish process after forming, so that the dent resistance is improved. The bake hardening steel is added with solid solution strengthening elements such as Mn, P and the like in an auxiliary way, so that a material with a certain grade can be obtained. Based on the above properties, bake-hardened steels have been widely used in the manufacture of parts such as outer door panels, outer hair cover panels, and trunk lids of passenger cars.
However, the aging time of the existing bake hardening steel is only 3 months, and when the existing bake hardening steel is used beyond the aging time, the surface of a stamping part is easy to generate tensile strain marks (commonly called as orange peel marks on the stamping surface), and the defects can not be repaired, so that the stamping part can only be discarded.
In the prior art, the yield strength of the currently common domestic outer plate brand mainly has two strength levels of 180MPa and 220MPa, for example, chinese invention application with the publication number of CN106702266A, namely 'an aging-resistant cold-rolled bake-hardening steel 220BH and a production method thereof', discloses an aging-resistant cold-rolled bake-hardening steel 220BH, the yield strength of which is 220MPa, and the aging resistance of more than 6 months is obtained by adopting ultra-low carbon and trace Nb and Mo elements and matching with a high-temperature annealing rapid cooling process.
Chinese patent application with publication number CN109321839A discloses a 240 MPa-grade bake-hardened steel and a manufacturing method thereof, the method controls the content of C element to be 0.0015-0.0030% and the content of Nb element to be 0.006-0.014% by reasonably setting the content of C and Nb elements, and controls the content Ceff of solid-solution carbon of a final finished product to be 9-15 ppm by cooperating with each other.
The above methods are optimized for the bake-hardened steel with a specific strength grade, and are not suitable for the bake-hardened steel with different strength grades, so it is urgently needed to develop a universal method for prolonging the aging time of the bake-hardened steel.
Disclosure of Invention
The embodiment of the invention provides a bake-hardening steel and a method for prolonging the aging time of the bake-hardening steel, and aims to solve the problem of short aging time of the bake-hardening steel in the related art.
The technical scheme provided by the invention is as follows:
in a first aspect, there is provided a method of extending the ageing time of a bake-hardened steel, comprising the steps of:
counting the BH value of the bake-hardened steel, and determining the BH value optimization range of which the occurrence probability of the surface orange peel phenomenon does not exceed a specific value when the bake-hardened steel is used for 3 months;
counting Nb/C atomic ratios corresponding to different BH values in the BH value optimization range;
fixing the Nb/C atomic ratio, and optimizing the chemical composition and casting process of the bake-hardened steel;
and casting the bake-hardened steel according to the optimized bake-hardened steel chemical composition and casting process.
In some embodiments, in the step of determining that the occurrence probability of the surface orange peel phenomenon does not exceed the BH value range of a specific value, the occurrence probability of the surface orange peel phenomenon is less than or equal to 1%.
In some embodiments, in the step of determining that the occurrence probability of the surface orange peel phenomenon does not exceed the BH value range of the specific value, the occurrence probability of the surface orange peel phenomenon is less than or equal to 0.3%.
In some embodiments, the BH value is optimized in the range of 30 to 53MPa.
In some embodiments, the Nb/C atomic ratio is 0.5 to 0.82, preferably the Nb/C atomic ratio is 0.7.
In some embodiments, the Nb/C atomic ratio and the chemical composition of the bake-hardened steel are fixed, optimizing the annealing temperature.
In some embodiments, the annealing temperature is 860 to 900 ℃ when the Nb/C atomic ratio is 0.7.
In some embodiments, the optimized bake-hardened steel chemistry has a solid solution carbon content of 2.5 to 6ppm.
In a second aspect, there is provided a bake-hardened steel cast using a bake-hardened steel chemical composition and casting process optimized in the above method of extending the age of a bake-hardened steel.
The technical scheme provided by the invention has the beneficial effects that: according to the invention, the aging time of the bake-hardening steel is prolonged to 6 months from the current 3 months, the aging time of the bake-hardening steel is reduced by 5%, and the loss of about 260 tons of bake-hardening steel caused by the aging time is reduced by calculating according to the average 52kg of the hood outer plate, the front and rear door outer plates and the trunk outer plate and the annual output of 10 ten thousand.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
FIG. 1 shows the relationship between annealing temperature and bake-hardened steel BH value;
FIG. 2 shows the relationship between annealing temperature and bake-hardened steel r-value;
FIG. 3 is a graph showing the relationship between the BH value and the solid-solution carbon content in a certain annealing temperature range.
Detailed Description
In order to solve the problems of short ageing time and high rejection rate after long-term storage of the bake-hardened steel, the invention provides a method for prolonging the ageing time of the bake-hardened steel, which comprises the following steps: counting the BH value of the bake-hardened steel, and determining the BH value optimization range of the surface orange peel phenomenon occurrence probability not exceeding a specific value when the bake-hardened steel is used for 3 months; counting Nb/C atomic ratios corresponding to different BH values in the BH value optimization range; fixing the Nb/C atomic ratio, and optimizing the chemical composition and casting process of the bake-hardened steel; and casting the bake-hardened steel according to the optimized bake-hardened steel chemical composition and casting process.
In some embodiments, in the step of determining that the occurrence probability of the surface orange peel phenomenon does not exceed the BH value range of the specific value, the occurrence probability of the surface orange peel phenomenon is less than or equal to 1%.
In some embodiments, in the step of determining that the occurrence probability of the surface orange peel phenomenon does not exceed the BH value range of the specific value, the occurrence probability of the surface orange peel phenomenon is less than or equal to 0.3%.
In some embodiments, the BH value is optimized in the range of 30 to 53MPa.
In some embodiments, the Nb/C atomic ratio is from 0.5 to 0.82.
In some embodiments, the Nb/C atomic ratio and the chemical composition of the bake-hardened steel are fixed, optimizing the annealing temperature.
In some embodiments, the annealing temperature is 750 to 850 ℃ when the Nb/C atomic ratio is 0.3.
In some embodiments, the annealing temperature is 860 to 900 ℃ when the Nb/C atomic ratio is 0.7.
In some embodiments, the optimized bake-hardened steel chemistry has a solute carbon content of 2.5 to 6ppm.
To make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
In the terminology of the present invention, solid-solution carbon refers to carbon atoms uniformly dispersed in the gaps of the Fe lattice; the BH value refers to the steel bake-hardening value in GB/T24174-2009, namely the yield strength increase of the steel caused by the heat treatment effect; the Nb/C atomic ratio, i.e., the ratio of the number of atoms of Nb and C; the aging time is the longest time that no punching orange peel (the scientific name is tensile strain mark) is generated on the surface of the steel plate from the completion of steel coil production to the punching deformation.
The method is characterized in that when the bake-hardening steel is used for the aging time exceeding 3 months, a large number of BH values are accumulated, the relationship between the BH value and the probability of orange peel on the surface of actual stamping is analyzed, the Nb/C atomic ratio corresponding to the BH values is counted, and after the atomic ratio is fixed, the relationship between the annealing temperature, chemical components, leveling elongation and the bake-hardening steel performance (main BH value) of the bake-hardening steel is clearly researched, so that the optimal range of each process parameter is controlled, and finally the aging time of the bake-hardening steel is prolonged.
In order to ensure the punching performance of the steel plate and the baking hardenability of the post process, the invention has the following requirements on the chemical components of the baking hardening steel: (1) adopting ultra-low carbon steel: the ultra-low carbon content can greatly improve the stamping formability, the plasticity and the toughness of the steel plate and reduce the cost; (2) adding microalloy elements Nb and Ti: nb and Ti can be combined with C and N to form a compound, so that C atoms and N atoms of solid solution in Fe lattice gaps are reduced; (3) and (3) impurity reduction: the content of impurity elements is controlled at a very low level to ensure that the material has a very high n value and plasticity.
The invention controls the components of the bake-hardened steel plate by the following two methods: (1) in the steel-making stage, the number of C atoms exceeds that of Ti and Nb; (2) in the hot rolling stage, ti atoms or Nb atoms and C atoms form carbides, and the C atoms are completely removed from Fe lattice gaps, so that the yield strength of the bake-hardened steel plate is reduced; (3) in the continuous annealing stage, carbides of Ti atoms or Nb atoms are decomposed by high-temperature annealing, and C atoms are diffused into Fe lattice gaps again to form a solid solution with Fe, so that the yield strength of the bake-hardened steel sheet is improved.
Although the chemical composition of the bake-hardened steel is complicated, the determination of the BH value is the amount of change in the amount of solid-solution carbon in the material before and after baking, and therefore, how to control the amount of change in the solid-solution carbon becomes critical in production. Before annealing, the carbon in the bake hardened steel is present in the form of carbides: c + Nb = NbC, and during annealing recrystallization, heating and soaking, part of C in NbC is dissolved and can be made to exist as a solid solution by rapid cooling, and the atomic ratio Nb/C of Nb and C in the steel sheet is calculated by the following formula:
Nb/C≈([Nb]/93)/([C]/14)
wherein, the [ Nb ] and the [ C ] are the weight contents of Nb and C in the material before annealing respectively, when the atomic ratio Nb/C is less than 1, solid solution carbon exists in the material before annealing, the BH value is more than 0, and the BH performance can be obtained; when the atomic ratio Nb/C is more than or equal to 1, no solid solution carbon exists in the material before annealing, and high-temperature annealing is carried out to dissolve C in NbC in the material into solid solution carbon so as to obtain BH performance.
Example 1:
1. determining the BH value optimization range of the bake-hardening steel and the Nb/C atomic ratio corresponding to the BH value optimization range:
the existing research shows that if the BH value of the bake hardening steel is too high, the material performance of the bake hardening steel is easily influenced at normal temperature, which mainly shows that the stamping performance of the material is reduced, and the surface appearance quality of the stamped material is influenced (namely, the surface orange peel phenomenon occurs); if the BH value of the bake-hardened steel is too low, the strength change of the bake-hardened steel before and after baking is not significant, and the dent resistance (i.e., finger pressure rigidity) of the stamped part is affected. For the reasons mentioned above, GB/T24174-2009 states that the lower limit of the BH value is not less than 30MPa, but does not limit the upper limit.
(1) For the bake-hardened steels used in the production process, the service time, nb/C atomic ratio and BH value are recorded, whether the surface orange peel phenomenon occurs is recorded, and the BH value range and Nb/C atomic ratio range respectively corresponding to the bake-hardened steels with actual aging time of more than 3 months and aging time of less than or equal to 3 months are counted by a statistical analysis method, and the results are shown in Table 1:
TABLE 1 results of statistical evaluation of aging time, nb/C atomic ratio, BH value of bake-hardened steel
| Aging time | Atomic ratio of Nb to C | BH value |
| 3 months old | 0.15～0.48 | ≥57MPa |
| More than 3 months | 0.5～0.82 | 31MPa≤BH≤53MPa |
As shown in Table 1, the Nb/C atomic ratio of the bake-hardened steel with the aging time of less than or equal to 3 months is 0.15-0.48, and the BH value is more than or equal to 57MPa; the Nb/C atomic ratio of the bake-hardening steel with the aging time of more than 3 months is 0.5-0.82, and BH is more than or equal to 31MPa and less than or equal to 53MPa. Therefore, in order to prolong the aging time of the bake-hardened steel, the BH value is required to satisfy 31 MPa-53 MPa.
(2) Practical performance statistics of the bake-hardening steel used by the company in the last 12 years show that whether the bake-hardening steel is used within the aging time of 3 months or used after the aging time of 3 months, the probability of orange peel stamping on the surface of the bake-hardening steel has a certain relation with the BH value, and the results are listed in Table 2.
TABLE 2 relationship between the probability of orange peel stamping on the surface of bake-hardened steel and the BH value
As shown in Table 2, when the steel is used within 3 months of aging time, the probability of the orange peel phenomenon on the surface of the bake-hardening steel which meets the requirement that BH is more than or equal to 55MPa is 0, and when the steel is used within 3 months of aging time, the probability of the orange peel phenomenon on the surface of the bake-hardening steel does not exceed 0.3 percent, so the steel is the bake-hardening steel with excellent performance; the baking hardening steel meeting the requirement that BH is more than 55MPa and less than or equal to 75MPa has the probability of orange peel stamping on the surface of 1 percent when used within 3 months of aging time, and the probability of orange peel stamping on the surface of the baking hardening steel is increased to about 6 percent when used within 3 months of aging time; the baking hardening steel meeting BH more than 75MPa has the probability of orange peel stamping on the surface of 5 percent when used within 3 months of aging time, and the probability of orange peel stamping on the surface of the baking hardening steel is increased to about 50 percent when the baking hardening steel exceeds the probability of orange peel stamping on the surface of 3 months of aging time, so that the rejection rate is extremely high.
The statistical analysis combining table 1 and table 2 can find that: when the steel is cast, the Nb/C atomic ratio of the bake-hardened steel is controlled to be more than or equal to 0.5, preferably 0.5-0.82, and the steel can be used for more than 3 months of aging time, so that the probability of orange peel stamping on the surface of the bake-hardened steel is not more than 0.3 percent, the rejection rate is greatly reduced, the waste is avoided, and the trial-error rate and the production cost are reduced.
2. Optimizing production process parameters in the casting stage of bake-hardened steel
Under the condition that the Nb/C atomic ratio is 0.5-0.82, a bake-hardened steel raw material satisfying the required performance is prepared. For example, a steel sheet having a strength level of 180MPa or 220MPa is prepared.
Under the condition of chemical composition determination, annealing is a key factor influencing the bake hardenability of the bake hardening steel, and among various annealing process parameters, the heating time and the annealing atmosphere have little influence on the bake hardening steel performance, and the soaking temperature, the soaking time and the cooling speed have great influence on the BH value of the bake hardening steel. As mentioned above, the BH value of the bake-hardening steel should be controlled to satisfy 30MPa or more and BH 53MPa or less, and the BH value is too low, which results in no bake-hardening effect and poor dent resistance of the bake-hardening steel; if the BH value is too high, the accompanying age hardening value (AI) is too high, which is detrimental to the service performance of the product.
(1) In order to verify the importance of the Nb/C atomic ratio to the bake-hardened steel, control the Nb/C atomic ratio to be 0.3 or 0.7, optimize the annealing temperature in the production process of the bake-hardened steel, record BH values and r values (anisotropic plastic strain ratios) of the bake-hardened steel prepared at different annealing temperatures, and FIG. 3 shows the relationship between the annealing temperature and BH values and r values of the bake-hardened steel. Combining the statistical results of fig. 1, 2 and table 2, the optimum ranges of annealing temperatures for different Nb/C atomic ratios are found and are listed in table 3:
TABLE 3 optimum annealing temperature ranges for different Nb/C atomic ratios
| Atomic ratio of Nb to C | Optimum annealing temperature Range (. Degree. C.) |
| 0.3 | 750～850 |
| 0.7 | 860～900 |
When the Nb/C atomic ratio is 0.3, the BH values are all more than or equal to 60MPa and the r value is less than or equal to 1.8 in the whole annealing temperature range of 750-1000 ℃, so that the performance requirement of the bake-hardened steel cannot be met. When the Nb/C atomic ratio is 0.7, the annealing temperature is controlled within the range of 860-900 ℃, the BH value can be ensured to be 30-55 MPa, and the r value within the optimal annealing temperature range can be ensured to be more than or equal to 1.8.
(2) Optimizing the chemical composition of the bake-hardened steel:
the bake hardening effect of the bake hardening steel is realized by the change of the number of solid solution carbon atoms in the steel caused by the annealing temperature, so the solid solution carbon content of the original steel plate also has great influence on the BH value of the bake hardening steel.
FIG. 3 shows the relationship between the BH value of bake-hardened steel and the solid-solution carbon content over a range of annealing temperatures. As shown in fig. 3, the BH value increases with the amount of solid-solution carbon in steel, and for steel with a lower solid-solution carbon content, sufficient solid-solution carbon content can be obtained by dissolving carbon in NbC into Fe lattice only by raising the annealing temperature (higher than the NbC dissolution temperature), resulting in a higher BH value. Therefore, the content of solid-solution carbon in the original steel sheet is controlled to 2.5 to 6ppm.
(3) Optimizing the leveling elongation in the production process of the bake-hardened steel:
the guarantee of the timeliness and the appearance after processing is mainly in the leveling process, the appropriate rolling reduction can improve the flatness and the surface smoothness of the steel plate, the roughness of the steel plate can be controlled, and the occurrence of Luders belt on the surface of the steel plate during stamping is avoided, so that the yield is improved, and the leveling amount is generally controlled to be 1-2%.
In temper rolling, the temper rolling amount is mainly determined by rolling force and front and rear tension, edge waves caused by overlarge rolling force are easy to occur when a steel plant actually produces and bakes hardened steel plates, and the rolling force is ensured within a certain range while the elongation is controlled by adjusting the tension, the roll bending force and the like. The statistics and analysis of the leveling process parameters and the appearance grade of the steel plate show that the unit tension before and after the leveling equipment cannot exceed a certain range, and the larger the rolling force of the SKP is, the less obvious the concave-convex degree after stretching is.
Example 2:
the embodiment provides 240 MPa-grade bake-hardening steel which comprises the following chemical components in percentage by weight: c:0.0021%; si:0.010%; mn:0.44%; p:0.053%; s:0.004%; and (3) Alt:0.036%; nb:0.0114%; b:0.0004 percent; n:0.0017%, the balance being Fe and unavoidable impurities. Wherein the addition amount of the Nb component and the addition amount of the C component satisfy the Nb/C atomic ratio of 0.7.
The preparation method of the 240 MPa-grade bake-hardening steel provided by the embodiment of the invention comprises the following steps:
step 1: smelting and continuously casting to obtain the billet containing the chemical components in percentage by weight.
Step 2: carrying out hot rolling on the steel billet to obtain a hot rolled coil;
the method specifically comprises the following steps: heating the steel billet from room temperature to 1280 ℃; obtaining a hot rolled plate by rough rolling and finish rolling, wherein the finish rolling temperature is 930 ℃; after laminar cooling, the hot rolled sheet was coiled at a low temperature of 625 ℃ into a hot rolled coil.
And step 3: cold rolling the hot rolled coil to obtain a cold hard coil;
the method specifically comprises the following steps: the cold-rolled coil is naturally cooled and then subjected to acid pickling and cold rolling to obtain a cold-hard coil, and in the embodiment, the cold rolling reduction rate is controlled to be 80% according to the thickness specification of the strip steel.
And 4, step 4: annealing the cold-rolled hard coil, specifically, slowly cooling the cold-rolled hard coil within the range of 860-900 ℃ to carry out continuous annealing on the cold-rolled hard coil to obtain strip steel, and carrying out hot galvanizing on the annealed strip steel to finally obtain the baking-hardened steel of 240MPa grade.
The steel grade performance of the 240 MPa-grade bake-hardened steel obtained by the process is tested, the average value of BH values is 48MPa, and the occurrence probability of surface orange peel phenomenon =0.25% after the steel is stored for 6 months in different storage temperature environments.
Example 3:
unlike example 2, the Nb/C atomic ratio of this example was 0.8:
the 240 MPa-grade bake-hardening steel provided by the embodiment comprises the following chemical components in percentage by weight: c:0.0021%; si:0.010%; mn:0.44%; p:0.053 percent; s:0.004 percent; and (3) Alt:0.036%; nb:0.0130 percent; b:0.0004 percent; n:0.0017%, the balance being Fe and unavoidable impurities. Wherein the amount of the Nb component added and the amount of the C component added satisfy an Nb/C atomic ratio of 0.8.
The preparation method of the baking hardened steel corresponding to the 240MPa level provided by the embodiment of the invention comprises the following steps:
step 1: smelting and continuously casting to obtain the steel billet containing the chemical components in percentage by weight.
Step 2: carrying out hot rolling on the steel billet to obtain a hot rolled coil;
the method specifically comprises the following steps: heating the billet steel from room temperature to 1280 ℃; obtaining a hot rolled plate by rough rolling and finish rolling, wherein the finish rolling temperature is 930 ℃; after laminar cooling, the hot rolled sheet was coiled at a low temperature of 625 ℃ into a hot rolled coil.
And 3, step 3: cold rolling the hot rolled coil to obtain a cold hard coil;
the method specifically comprises the following steps: the cold-rolled coil is naturally cooled and then subjected to acid pickling and cold rolling to obtain a cold-hard coil, and in the embodiment, the cold rolling reduction rate is controlled to be 80% according to the thickness specification of the strip steel.
And 4, step 4: annealing the cold-rolled hard coil, specifically, slowly cooling the cold-rolled hard coil within the range of 860-900 ℃ to carry out continuous annealing on the cold-rolled hard coil to obtain strip steel, and carrying out hot galvanizing on the annealed strip steel to finally obtain the baking-hardened steel of 240MPa grade.
The steel grade performance of the 240 MPa-grade bake-hardened steel obtained by the process is tested, the mean BH value is 48MPa, and the occurrence probability of the surface orange peel phenomenon is =0.3% after the steel is stored for 6 months in different storage temperature environments.
Example 4
Unlike example 2, the Nb/C atomic ratio of this example was 0.9.
The 240 MPa-grade bake-hardening steel provided by the embodiment comprises the following chemical components in percentage by weight: c:0.0021%; si:0.010%; mn:0.44%; p:0.053%; s:0.004%; and (3) Alt:0.036%; nb:0.0146%; b:0.0004 percent; n:0.0017%, the balance being Fe and unavoidable impurities. Wherein the addition amount of the Nb component and the addition amount of the C component satisfy the Nb/C atomic ratio of 0.9.
The preparation method of the 240 MPa-grade bake-hardening steel provided by the embodiment of the invention comprises the following steps:
step 1: smelting and continuously casting to obtain the steel billet containing the chemical components in percentage by weight.
Step 2: carrying out hot rolling on the steel billet to obtain a hot rolled coil;
the method specifically comprises the following steps: heating the billet steel from room temperature to 1280 ℃; obtaining a hot rolled plate by rough rolling and finish rolling, wherein the finish rolling temperature is 930 ℃; after laminar cooling, the hot rolled sheet was coiled at a low temperature of 625 ℃ into a hot rolled coil.
And step 3: cold rolling the hot rolled coil to obtain a cold hard coil;
the method specifically comprises the following steps: the cold-rolled coil is naturally cooled and then subjected to acid pickling and cold rolling to obtain a cold-hard coil, and in the embodiment, the cold rolling reduction rate is controlled to be 80% according to the thickness specification of the strip steel.
And 4, step 4: annealing the cold-rolled hard coil, specifically, slowly cooling the cold-rolled hard coil within the range of 860-900 ℃ to carry out continuous annealing on the cold-rolled hard coil to obtain strip steel, and carrying out hot galvanizing on the annealed strip steel to finally obtain the baking-hardened steel of 240MPa grade.
The steel grade performance of the 240 MPa-grade bake-hardened steel obtained by the process is tested, the mean BH value is 48MPa, and the occurrence probability of the surface orange peel phenomenon is =7% after the steel is stored for 6 months in different storage temperature environments.
Example 5:
unlike example 2, the Nb/C atomic ratio in this example was 0.4.
The 240 MPa-grade bake-hardening steel provided by the embodiment comprises the following chemical components in percentage by weight: c:0.0021%; si:0.010%; mn:0.44%; p:0.053%; s:0.004 percent; and (3) Alt:0.036%; nb:0.0065%; b:0.0004 percent; n:0.0017%, the balance being Fe and unavoidable impurities. Wherein the amount of the Nb component added and the amount of the C component added satisfy an Nb/C atomic ratio of 0.4.
The preparation method of the baking hardened steel corresponding to the 240MPa level provided by the embodiment of the invention comprises the following steps:
step 1: smelting and continuously casting to obtain the steel billet containing the chemical components in percentage by weight.
Step 2: carrying out hot rolling on the steel billet to obtain a hot rolled coil;
the method specifically comprises the following steps: heating the steel billet from room temperature to 1280 ℃; obtaining a hot rolled plate by rough rolling and finish rolling, wherein the finish rolling temperature is 930 ℃; after laminar cooling, the hot rolled sheet was coiled at a low temperature of 625 ℃ into a hot rolled coil.
And step 3: cold rolling the hot rolled coil to obtain a cold hard coil;
the method comprises the following specific steps: the hot-rolled coil is naturally cooled and then subjected to acid pickling and cold rolling to obtain a cold-hard coil, and in the embodiment, the cold rolling reduction is controlled to be 80% according to the thickness specification of the strip steel.
And 4, step 4: annealing the cold-rolled hard coil, specifically, slowly cooling the cold-rolled hard coil within the range of 860-900 ℃ to carry out continuous annealing on the cold-rolled hard coil to obtain strip steel, and carrying out hot galvanizing on the annealed strip steel to finally obtain the baking-hardened steel of 240MPa grade.
The steel grade performance of the 240 MPa-grade bake-hardened steel obtained by the process is tested, the average value of BH values is 48MPa, and the surface orange peel phenomenon occurrence probability =23% after the steel is stored for 6 months in different storage temperature environments.
Example 6:
unlike example 2, the annealing temperature in this example was 850 ℃.
The 240 MPa-grade bake-hardening steel provided by the embodiment comprises the following chemical components in percentage by weight: c:0.0021%; si:0.010%; mn:0.44%; p:0.053 percent; s:0.004 percent; and (3) Alt:0.036%; nb:0.0114%; b:0.0004 percent; n:0.0017%, the balance being Fe and unavoidable impurities. Wherein the amount of the Nb component added and the amount of the C component added satisfy an Nb/C atomic ratio of 0.7.
The preparation method of the baking hardened steel corresponding to the 240MPa level provided by the embodiment of the invention comprises the following steps:
step 1: smelting and continuously casting to obtain the steel billet containing the chemical components in percentage by weight.
Step 2: carrying out hot rolling on the steel billet to obtain a hot rolled coil;
the method specifically comprises the following steps: heating the steel billet from room temperature to 1280 ℃; obtaining a hot rolled plate by rough rolling and finish rolling, wherein the finish rolling temperature is 930 ℃; after laminar cooling, the hot rolled sheet was coiled at a low temperature of 625 ℃ into a hot rolled coil.
And step 3: cold rolling the hot rolled coil to obtain a cold hard coil;
the method comprises the following specific steps: the cold-rolled coil is naturally cooled and then subjected to acid pickling and cold rolling to obtain a cold-hard coil, and in the embodiment, the cold rolling reduction rate is controlled to be 80% according to the thickness specification of the strip steel.
And 4, step 4: and annealing the cold-rolled hard coil, specifically, carrying out isothermal annealing at 850 ℃ on the cold-rolled hard coil to obtain strip steel, and carrying out hot galvanizing on the annealed strip steel to finally obtain the 240 MPa-grade bake-hardened steel.
The steel grade performance of the 240 MPa-grade bake-hardened steel obtained by the process is tested, and the surface orange peel phenomenon occurrence probability =6% after the steel is stored for 6 months in different storage temperature environments.
Example 7:
unlike example 2, the annealing temperature in this example was 920 ℃.
The 240 MPa-grade bake-hardening steel provided by the embodiment comprises the following chemical components in percentage by weight: c:0.0021%; si:0.010%; mn:0.44%; p:0.053 percent; s:0.004 percent; and (3) Alt:0.036%; nb:0.0114%; b:0.0004 percent; n:0.0017%, the balance being Fe and unavoidable impurities. Wherein the amount of the Nb component added and the amount of the C component added satisfy an Nb/C atomic ratio of 0.7.
The preparation method of the baking hardened steel corresponding to the 240MPa level provided by the embodiment of the invention comprises the following steps:
step 1: smelting and continuously casting to obtain the billet containing the chemical components in percentage by weight.
Step 2: carrying out hot rolling on the steel billet to obtain a hot rolled coil;
the method specifically comprises the following steps: heating the billet steel from room temperature to 1280 ℃; obtaining a hot rolled plate by rough rolling and finish rolling, wherein the finish rolling temperature is 930 ℃; after laminar cooling, the hot rolled sheet was coiled at a low temperature of 625 ℃ into a hot rolled coil.
And step 3: cold rolling the hot rolled coil to obtain a cold hard coil;
the method specifically comprises the following steps: the cold-rolled coil is naturally cooled and then subjected to acid pickling and cold rolling to obtain a cold-hard coil, and in the embodiment, the cold rolling reduction rate is controlled to be 80% according to the thickness specification of the strip steel.
And 4, step 4: and annealing the cold-rolled hard coil, specifically, carrying out continuous annealing at 920 ℃ on the cold-rolled hard coil to obtain strip steel, and carrying out hot galvanizing on the annealed strip steel to finally obtain the baking-hardened steel at 240 MPa.
The performance of the steel grade of the 240 MPa-grade bake-hardened steel obtained by the process is tested, and the orange peel occurrence probability =1.8% after the steel grade is stored for 6 months in different storage temperature environments.
Example 8:
unlike example 2, the annealing temperature in this example was 880 ℃.
The 240 MPa-grade bake-hardening steel provided by the embodiment comprises the following chemical components in percentage by weight: c:0.0021%; si:0.010%; mn:0.44%; p:0.053 percent; s:0.004 percent; and (3) Alt:0.036%; nb:0.0114%; b:0.0004 percent; n:0.0017%, the balance being Fe and unavoidable impurities. Wherein the addition amount of the Nb component and the addition amount of the C component satisfy the Nb/C atomic ratio of 0.7.
The preparation method of the 240 MPa-grade bake-hardening steel provided by the embodiment of the invention comprises the following steps:
step 1: smelting and continuously casting to obtain the billet containing the chemical components in percentage by weight.
Step 2: carrying out hot rolling on the steel billet to obtain a hot rolled coil;
the method specifically comprises the following steps: heating the steel billet from room temperature to 1280 ℃; obtaining a hot rolled plate by rough rolling and finish rolling, wherein the finish rolling temperature is 930 ℃; after the hot rolled plate was cooled by laminar flow cooling, it was coiled at a low temperature of 625 ℃.
And step 3: cold rolling the hot rolled coil to obtain a cold hard coil;
the method comprises the following specific steps: the hot-rolled coil is naturally cooled and then subjected to acid pickling and cold rolling to obtain a cold-hard coil, and in the embodiment, the cold rolling reduction is controlled to be 80% according to the thickness specification of the strip steel.
And 4, step 4: and annealing the cold-rolled hard coil, specifically, continuously annealing the cold-rolled hard coil at 880 ℃ to obtain strip steel, and hot galvanizing the annealed strip steel to finally obtain the 240 MPa-grade bake-hardened steel.
The steel grade performance of the 240 MPa-grade bake-hardened steel obtained by the process is tested, and the surface orange peel phenomenon occurrence probability =0.1% after the steel is stored for 6 months in different storage temperature environments.
Example 9:
unlike example 2, the annealing temperature in this example was 860 ℃.
The 240 MPa-grade bake-hardening steel provided by the embodiment comprises the following chemical components in percentage by weight: c:0.0021%; si:0.010%; mn:0.44%; p:0.053 percent; s:0.004%; and (3) Alt:0.036%; nb:0.0114%; b:0.0004 percent; n:0.0017%, the balance being Fe and unavoidable impurities. Wherein the amount of the Nb component added and the amount of the C component added satisfy an Nb/C atomic ratio of 0.7.
The preparation method of the 240 MPa-grade bake-hardening steel provided by the embodiment of the invention comprises the following steps:
step 1: smelting and continuously casting to obtain the steel billet containing the chemical components in percentage by weight.
Step 2: carrying out hot rolling on the steel billet to obtain a hot rolled coil;
the method specifically comprises the following steps: heating the steel billet from room temperature to 1280 ℃; obtaining a hot rolled plate by rough rolling and finish rolling, wherein the finish rolling temperature is 930 ℃; after laminar cooling, the hot rolled sheet was coiled at a low temperature of 625 ℃ into a hot rolled coil.
And step 3: cold rolling the hot rolled coil to obtain a cold hard coil;
the method comprises the following specific steps: the cold-rolled coil is naturally cooled and then subjected to acid pickling and cold rolling to obtain a cold-hard coil, and in the embodiment, the cold rolling reduction rate is controlled to be 80% according to the thickness specification of the strip steel.
And 4, step 4: and annealing the cold-rolled hard coil, specifically, continuously annealing the cold-rolled hard coil at 860 ℃ to obtain strip steel, and hot galvanizing the annealed strip steel to finally obtain the 240 MPa-grade bake-hardened steel.
The steel grade performance of the 240 MPa-grade bake-hardened steel obtained by the process is tested, and the surface orange peel phenomenon occurrence probability =0.2% after the steel is stored for 6 months in different storage temperature environments.
In conclusion, according to the invention, by researching the relationship between the chemical composition of the bake-hardened steel, the annealing temperature in the casting process of the bake-hardened steel, the leveling elongation and the BH value of the bake-hardened steel, the aging time of the bake-hardened steel is prolonged from 3 months to 6 months, and the aging time of 5% of the bake-hardened steel is reduced, so that the rejection rate of products caused by the planned fluctuation of factory yield is reduced, and the loss of 110 ten thousand caused by the aging time of about 260 tons of bake-hardened steel is reduced every year by calculating the average 52kg of hood outer plates, front and rear door outer plates and trunk outer plates and the 10 ten thousand of annual yield.
The above description is merely illustrative of particular embodiments of the invention that enable those skilled in the art to understand or practice the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (9)
1. A method of extending the age time of bake hardened steel comprising the steps of:
counting the BH value of the bake-hardened steel, and determining the BH value optimization range with the occurrence probability of surface orange peel phenomenon less than or equal to 1% when the bake-hardened steel is used for 3 months;
counting Nb/C atomic ratios corresponding to different BH values in the BH value optimization range;
fixing the Nb/C atomic ratio, and optimizing the solid solution carbon content and the annealing temperature of the bake-hardened steel;
and casting the bake-hardened steel according to the optimized bake-hardened steel solid solution carbon content and the annealing temperature.
2. The method of extending the age of bake hardened steel of claim 1 wherein: the occurrence probability of the surface orange peel phenomenon is less than or equal to 0.3 percent.
3. The method of extending the age of bake hardened steel of claim 1 wherein: the optimized range of the BH value is 30-53 MPa.
4. The method of extending the age of bake hardened steel of claim 1 wherein: the Nb/C atomic ratio is 0.5 to 0.82.
5. The method of extending the age of bake hardened steel of claim 4 wherein: the Nb/C atomic ratio was 0.7.
6. The method of extending the age time of bake-hardened steel of claim 1, wherein: the Nb/C atomic ratio and the solid solution carbon content of the bake hardening steel are fixed, and the annealing temperature is optimized.
7. The method of extending the age of bake hardened steel of claim 6, wherein: when the Nb/C atomic ratio is 0.7, the annealing temperature is 860 to 900 ℃.
8. The method of extending the age of bake hardened steel of claim 1 wherein: the content of solid-solution carbon in the optimized bake-hardening steel is 2.5-6 ppm.
9. A bake-hardened steel characterized by: cast with a bake-hardened steel solid solution carbon content and annealing temperature optimized in the method for extending the age of bake-hardened steel of any one of claims 1 to 8.
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