CN109321839B - 240 MPa-grade bake-hardening steel and manufacturing method thereof - Google Patents
240 MPa-grade bake-hardening steel and manufacturing method thereof Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 109
- 239000010959 steel Substances 0.000 title claims abstract description 109
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 23
- 238000000137 annealing Methods 0.000 claims abstract description 40
- 238000000034 method Methods 0.000 claims abstract description 30
- 238000005097 cold rolling Methods 0.000 claims abstract description 26
- 229910000760 Hardened steel Inorganic materials 0.000 claims abstract description 24
- 238000005098 hot rolling Methods 0.000 claims abstract description 15
- 239000000126 substance Substances 0.000 claims abstract description 12
- 238000005246 galvanizing Methods 0.000 claims abstract description 11
- 239000012535 impurity Substances 0.000 claims abstract description 9
- 238000003723 Smelting Methods 0.000 claims abstract description 6
- 238000001816 cooling Methods 0.000 claims description 20
- 238000010438 heat treatment Methods 0.000 claims description 19
- 230000009467 reduction Effects 0.000 claims description 10
- 238000005096 rolling process Methods 0.000 claims description 10
- 229910001335 Galvanized steel Inorganic materials 0.000 claims description 8
- 239000008397 galvanized steel Substances 0.000 claims description 8
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 claims description 7
- 229910052725 zinc Inorganic materials 0.000 claims description 7
- 239000011701 zinc Substances 0.000 claims description 7
- 238000010583 slow cooling Methods 0.000 claims description 4
- 239000002253 acid Substances 0.000 claims description 3
- 238000005266 casting Methods 0.000 claims description 3
- 238000005554 pickling Methods 0.000 claims description 3
- 238000003860 storage Methods 0.000 abstract description 30
- 230000035945 sensitivity Effects 0.000 abstract description 9
- 230000003712 anti-aging effect Effects 0.000 abstract description 5
- 239000000047 product Substances 0.000 description 38
- 230000008569 process Effects 0.000 description 25
- 239000010955 niobium Substances 0.000 description 24
- 229910052799 carbon Inorganic materials 0.000 description 19
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 14
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 8
- 239000006104 solid solution Substances 0.000 description 8
- 238000001953 recrystallisation Methods 0.000 description 6
- 230000008901 benefit Effects 0.000 description 5
- 238000012986 modification Methods 0.000 description 5
- 230000004048 modification Effects 0.000 description 5
- 230000032683 aging Effects 0.000 description 4
- 238000004886 process control Methods 0.000 description 4
- 239000012467 final product Substances 0.000 description 3
- 229910052758 niobium Inorganic materials 0.000 description 3
- 230000035882 stress Effects 0.000 description 3
- 229910001566 austenite Inorganic materials 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 239000007789 gas Substances 0.000 description 2
- 230000004075 alteration Effects 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 125000004432 carbon atom Chemical group C* 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 238000000354 decomposition reaction Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 239000007769 metal material Substances 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical group [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 1
- 150000002822 niobium compounds Chemical class 0.000 description 1
- 238000010899 nucleation Methods 0.000 description 1
- 230000006911 nucleation Effects 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 238000007747 plating Methods 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 230000002035 prolonged effect Effects 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 239000000758 substrate Substances 0.000 description 1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- 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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- 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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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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Abstract
The invention discloses 240 MPa-grade bake-hardening steel and a manufacturing method thereof, wherein the steel comprises the following chemical components in percentage by mass: c: 0.0015-0.0030%; si is less than or equal to 0.03 percent; mn: 0.40-0.6%; p: 0.05-0.065%; s: less than or equal to 0.01 percent; and (3) Alt: 0.02-0.06%; nb: 0.006-0.014%; b: 0.0003 to 0.0008 percent; n: less than or equal to 0.004 percent, and the balance of Fe and inevitable impurities; the manufacturing method comprises the following steps: firstly, smelting a steel billet, and then sequentially carrying out hot rolling, cold rolling, annealing and galvanizing on the steel billet. The finished steel manufactured by the method reduces the temperature sensitivity of the bake-hardened steel, improves the anti-aging time to more than 6 months under the condition of a storage environment ranging from-10 ℃ to 40 ℃, and ensures that the yield point elongation Ae value before stamping is less than or equal to 0.8 percent.
Description
Technical Field
The application belongs to the technical field of cold continuous rolling hot galvanizing in metal material processing, and particularly relates to 240 MPa-grade bake-hardening steel and a manufacturing method thereof.
Background
The bake-hardening steel is strain aging steel, has lower yield strength before stamping and forming, has excellent processing performance, can improve the yield strength to a certain extent after stamping and forming baking finish, and is mainly used for producing outer covering parts such as automobile outer plates and the like. At present, the yield strength of the domestic common outer plate number mainly has two strength levels of 180MPa and 220MPa, for example, the invention applies 'an anti-aging cold-rolling bake-hardening steel 220BH and a production method thereof' (application number CN201611177925), discloses an anti-aging cold-rolling bake-hardening steel 220BH, the yield strength of which is 220MPa, and the anti-aging performance of more than 6 months is obtained by adopting ultra-low carbon, trace Nb and Mo elements and matching with a high-temperature annealing rapid cooling process.
At present, the BH steel outer plate with the yield strength of 240MPa is rarely applied in China, and the aging period of the BH steel is sensitive to temperature change, so that the orange peel stamping defect is easy to occur after the BH steel is stored for more than three months under high-temperature conditions (more than or equal to 30 ℃), and the product quality is seriously influenced.
Disclosure of Invention
In order to solve the technical problems, the invention provides 240 MPa-grade bake-hardening steel and a manufacturing method thereof, wherein the 240 MPa-grade bake-hardening steel can solve the problem of part stamping orange peel caused by aging due to temperature sensitivity of the existing bake-hardening steel.
The technical scheme adopted for achieving the purpose of the invention is that the 240 MPa-grade bake-hardening steel comprises the following chemical components in percentage by mass: c: 0.0015-0.0030%; si is less than or equal to 0.03 percent; mn: 0.40-0.6%; p: 0.05-0.065%; s: less than or equal to 0.01 percent; and (3) Alt: 0.02-0.06%; nb: 0.006-0.014%; b: 0.0003 to 0.0008 percent; n: less than or equal to 0.004 percent, and the balance of Fe and inevitable impurities.
Preferably, the ratio of the adding amount of the Nb component to the content ratio of the C component satisfies 9 & lt, Ceff & lt, 15, wherein Ceff is C-Nb/7.75.
In the 240 MPa-grade bake-hardening steel provided by the invention, the effects of the chemical components are as follows:
in the 240 MPa-grade bake-hardening steel provided by the invention, Nb is one of key elements and forms a compound with C, and the content of C and Nb is reasonably set, so that the content of C is controlled to be 0.0015-0.0030%, meanwhile, Nb is controlled to be 0.006-0.014%, and the content Ceff of solid-solution carbon of a final finished product is controlled to be 9-15 PPm by cooperating with each other.
The 240 MPa-grade bake-hardening steel provided by the invention has lower Si content which is less than or equal to 0.03%, and the lower Si content can ensure the platability of the surface of a galvanized product.
The Mn element dissolved in the steel has the capability of improving the strength of the material and preventing hot brittleness, and in order to ensure that the product has high tensile strength and good formability, the content of the Mn element in the 240 MPa-grade bake-hardened steel provided by the invention is designed to be 0.40-0.6%.
The P element exists in the 240 MPa-level bake-hardening steel component system product in a replacement solid solution mode, the effect is that the yield strength of a final product is improved in a solid solution strengthening mode, the yield strength is larger than or equal to 240MPa, the tensile strength is larger than or equal to 340MPa, the upper limit of the content of the P element in the 240 MPa-level bake-hardening steel provided by the invention is limited to 0.065%, and the low-temperature brittleness of the product can be reduced.
The S element is an impurity element and is controlled to be close to the lower limit range as much as possible according to the condition of steel-making equipment.
Al element is used as a deoxidizer in the 240 MPa-grade bake-hardened steel, and aims to mainly remove oxygen dissolved in molten steel during smelting and improve the purity of the molten steel. The 240 MPa-grade bake-hardening steel provided by the invention has the total aluminum content of 0.02-0.06%, and a small amount of Al content can refine grains during hot rolling, so that the impact toughness is improved.
The B element is used for improving the secondary processing brittleness in the 240 MPa-grade bake-hardening steel, and because the P element is easy to segregate in a grain boundary and reduces the strength of the grain boundary, and the B element is precipitated at the grain boundary and can play a role in increasing the cohesion of the grain boundary, the content of the B element in the 240 MPa-grade bake-hardening steel is 0.0003-0.0008%, and the secondary cold deformation embrittlement tendency caused by the P element can be avoided.
The N element is the residual element in the invention, and does not need to be increased manually. However, the residual element can indirectly influence the solid solution carbon content of the final product, so that the N element content is strictly controlled to be less than or equal to 0.004% in the 240 MPa-level bake-hardened steel provided by the invention.
Based on the same inventive concept, the invention also provides a manufacturing method of the 240 MPa-grade bake-hardening steel, which comprises the following steps:
smelting and continuously casting to obtain a steel billet containing the chemical components in percentage by mass;
hot rolling the steel billet to obtain a hot rolled coil;
cold rolling the hot rolled coil to obtain a cold-hard coil;
and annealing the cold-rolled hard coil to finally obtain the 240 MPa-grade bake-hardening steel.
Further, the hot rolling the steel slab includes: and heating the billet from room temperature, performing rough rolling, finish rolling and laminar cooling, and coiling at low temperature to obtain the hot-rolled coil.
Further, heating the steel billet at 1250 +/-30 ℃; the finishing temperature is 930 +/-20 ℃; the coiling temperature is 600 +/-20 ℃.
When the heating temperature reaches 1250 ℃, the niobium compound Nb (C, N) is decomposed to obtain a certain amount of solid solution carbon, if the heating temperature is low, austenite grains are not uniform in size, so that mixed crystals are easily generated in the processed steel, and if the heating temperature is high, grains are excessively large, so that the grains of the steel are difficult to refine after processing. Therefore, the heating temperature in the technical scheme is 1250 ℃.
Since the precipitation of Nb (C, N) is considered to be almost no recrystallization after the austenite grains are deformed from about 930 ℃, the finish rolling temperature in this embodiment is determined to be 930 ℃.
The degree of aggravating hot rolling iron scale is increased when the curling temperature is too high, the surface quality of a product is influenced, the curling temperature is too low, and the performance of the product is influenced, so that the curling temperature is set to be 600 ℃ in the technical scheme.
Further, the cold rolling the hot rolled coil after the hot rolling includes: and naturally cooling the hot-rolled coil, and then carrying out acid pickling and cold rolling to obtain a cold-hard coil, wherein the total cold rolling reduction rate is 58-84%.
The certain cold rolling reduction rate is a driving force for subsequent continuous annealing recrystallization and determines the number of recrystallization nucleation points, so that the cold rolling reduction rate is controlled to be 58-84% according to different strip steel thickness specifications in the technical scheme.
Further, the annealing the cold-rolled hard coil comprises: and continuously annealing the cold hard coil to obtain strip steel, and carrying out hot galvanizing on the annealed strip steel to finally obtain the 240 MPa-grade baking hardened steel.
Further, the continuous annealing comprises: the cold and hard coil enters the continuous annealing furnace from a preheating section of the continuous annealing furnace and is continuously heated to 200 ℃ in the reducing atmosphere in the continuous annealing furnace for 20-30 seconds; the aim of the operation is to reduce the internal stress of the product before recrystallization to the greatest extent, so that buckling of the product in the furnace due to excessive internal stress does not occur in the subsequent treatment process.
Further heating the chilled coils to 760 +/-5 ℃ in 130-200 seconds in a heating section of the continuous annealing furnace, and preserving heat for 50-70 seconds; the decomposition temperature of the component system NbC is in the range of 800-820 ℃ (as shown in figure 2), so that annealing below 800 ℃ is required to be ensured to prevent NbC precipitates from being dissolved into carbon and niobium atoms to increase the solid solution content of the whole carbon atoms of the product, in addition, the strength is lower when the temperature is high, the recrystallization is insufficient when the temperature is too low, and the n value and the r value are lower, therefore, the annealing temperature is set to 760 ℃, the temperature is kept for 50-70 s, and the process ensures that the ultra-low carbon bake-hardened steel structure is fully recrystallized.
Cooling the chilled coils to 680 +/-5 ℃ in a slow cooling section of the continuous annealing furnace for 14-20 seconds; the process reduces the internal stress of the product during cooling, prevents the buckling deformation of the product caused by the sudden temperature reduction of the strip steel in the cooling process from the high-temperature recrystallization temperature, and ensures that the product can keep good plate shape.
And cooling the cold hard coil to 470 +/-5 ℃ in 13-20 seconds in a fast cooling section of the continuous annealing furnace to obtain the annealed strip steel. The purpose of this operation is to rapidly cool to the target temperature, control the solid solution carbon content of the final product, and ultimately control the properties of the finished product.
Further, the medium in the reducing atmosphere in the continuous annealing furnace is H2And N2The mixed gas of (1). At H2And N2The purpose of treatment in the medium is to ensure that the surface of the product strip steel is not oxidized, and finally the steel substrate with a bright surface can be obtained.
Further, the hot galvanizing the annealed strip steel comprises the following steps: putting the strip steel into a zinc pot for galvanizing, wherein the temperature of the strip steel when the strip steel is put into the zinc pot is 460 +/-10 ℃, and obtaining hot galvanized strip steel;
and leveling the hot-dip galvanized steel strip, and coiling the hot-dip galvanized steel strip into a finished product, wherein the leveling elongation is 1.4 +/-0.2%, so as to obtain the 240 MPa-grade baking hardened steel. By proper flattening, the initial yield platform of an annealed product can be eliminated, the problem of the Luders belt defect of a finished product in stamping is solved, meanwhile, the product obtains certain roughness, and the surface quality is improved, so that the surface quality requirement of the automobile outer plate is met.
According to the technical scheme, the 240 MPa-grade bake-hardening steel and the manufacturing method thereof provided by the invention have the advantages that through the optimized component design, when the range of the components fluctuates, the final solid solution carbon of the finished product is ensured to be within the range of 9-15 ppm by controlling the content ratio of C, Nb in the smelting process and matching with low-temperature curling and low-temperature annealing in the subsequent treatment process, further reducing the temperature sensitivity of the bake hardening steel, successfully producing the ultra-low carbon hot galvanizing 240MPa grade bake hardening steel with low temperature sensitivity by adjusting and optimizing the parameters of hot rolling and cold rolling annealing processes and flattening process parameters, the standard yield strength range of the hot-dip galvanized steel sheet is 240-300 MPa, the temperature sensitivity of bake-hardening steel is reduced, the anti-aging time of the storage environment is prolonged to more than 6 months under the condition of the temperature ranging from-10 ℃ to 40 ℃, and the yield point elongation Ae value before stamping is less than or equal to 0.8%.
Drawings
FIG. 1 is a flow chart of a method for manufacturing a 240MPa class bake-hardened steel in an embodiment of the present invention;
FIG. 2 is a NbC solubility curve of a 240MPa class bake-hardened steel in an example of the invention.
Detailed Description
In order to make the present application more clearly understood by those skilled in the art to which the present application pertains, the following detailed description of the present application is made with reference to the accompanying drawings by way of specific embodiments.
Example 1:
the 240 MPa-grade bake-hardening steel comprises the following chemical components in percentage by mass: c: 0.0021%; si: 0.010%; mn: 0.44%; p: 0.053 percent; s: 0.004%; and (3) Alt: 0.036%; nb: 0.009%; b: 0.0004 percent; n: 0.0017%, the balance being Fe and unavoidable impurities. Wherein the addition amount of the Nb component and the proportion of the content of the Nb component to the C component meet Ceff-Nb/7.75-9 ppm.
The embodiment of the invention also correspondingly provides a manufacturing method of the 240 MPa-grade bake-hardening steel, which comprises the following steps as shown in figure 1:
step 1: smelting and continuously casting to obtain the steel billet containing the chemical components in percentage by mass.
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 1270 ℃; obtaining a hot rolled plate by rough rolling and finish rolling, wherein the finish rolling temperature is 940 ℃; after the hot rolled sheet was cooled by laminar flow cooling, it was coiled at a low temperature of 615 ℃.
And step 3: cold rolling the hot rolled coil to obtain a cold hard coil;
the method specifically comprises the following 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 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 detailed steps are as follows:
step 41: the chilled coil enters the continuous annealing furnace from the preheating section of the continuous annealing furnace and is continuously heated to 200 ℃ in the reducing atmosphere in the continuous annealing furnace for 27 seconds, and in the embodiment, the medium in the reducing atmosphere in the continuous annealing furnace is H2And N2The mixed gas of (1).
Step 42: the chilled coil was further heated to 763 ℃ for 159 seconds in the heating zone of the continuous annealing furnace and held for 56 seconds.
Step 43: the chilled coils were cooled to 682 ℃ over 17 seconds in the slow cooling section of the continuous annealing furnace.
Step 44: and cooling the cold hard coil to 469 ℃ in 16 seconds in a fast cooling section of the continuous annealing furnace to obtain the annealed strip steel.
Step 45: and (3) putting the strip steel into a zinc pot for galvanizing, wherein the temperature of the strip steel when the strip steel is put into the zinc pot is 462 ℃, and obtaining the hot galvanized strip steel.
Step 46: and cooling the hot-dip galvanized steel strip to room temperature through a cooling section after plating, flattening the hot-dip galvanized steel strip, and coiling the flattened hot-dip galvanized steel strip into a finished product, wherein the flattening elongation is 1.35%, so that the 240 MPa-grade baking hardened steel is obtained.
The steel grade performance of the 240MPa grade bake-hardened steel obtained after the treatment according to the process is tested, the performance results are shown in Table 1, and the yield elongation Ae values after the steel is stored for 6 months in different storage temperature environments are shown in Table 2.
TABLE 1 ultra-low carbon BH steels Performance
TABLE 2 elongation at yield Ae values after 6 months storage in different storage temperature environments
| Storage temperature | -10℃ | 20℃ | 40℃ |
| Ae value (after 6 months) | 0.2 | 0.4 | 0.6 |
As can be seen from the table above, after the 240 MPa-grade bake-hardened steel produced by the embodiment is stored for 6 months under different storage temperature conditions, the yield elongation Ae value is less than or equal to 0.8 percent, and the product quality in the stamping process of a user is effectively ensured.
Example 2:
the 240 MPa-grade bake-hardening steel comprises the following chemical components in percentage by mass: c: 0.0023%; si: 0.010%; mn: 0.48 percent; p: 0.057%; s: 0.006%; and (3) Alt: 0.039%; nb: 0.010%; b: 0.0005%; n: 0.0014%, the balance being Fe and unavoidable impurities. Wherein the addition amount of the Nb component and the proportion of the content of the Nb component to the C component meet the Ceff-Nb/7.75-10 ppm.
The heating temperature of the hot rolling process of the steel grade is controlled to be 1260 ℃; the final rolling temperature is 935 ℃; the coiling temperature is 610 ℃; the cold rolling process controls the cold rolling reduction rate to be 80%. In the preheating section in the continuous annealing process, strip steel enters a furnace from room temperature into a medium in the furnace from H2And N2Heating to 200 ℃ continuously for 27 seconds in a reducing atmosphere; the strip steel is further heated to 760 ℃ in 159 seconds by the heating section, and the temperature is kept for 56 seconds; the slow cooling section cools the strip steel to 680 ℃ within 17 seconds; cooling the strip steel in a rapid cooling section for 16 secondsTo 470 ℃; finally, the temperature of the strip steel in a zinc pot is 460 ℃; the flat elongation was 1.4%.
The steel grade performance of the 240MPa grade bake-hardened steel obtained after the treatment according to the process is tested, the performance results are shown in Table 3, and the yield elongation Ae values after the steel is stored for 6 months in different storage temperature environments are shown in Table 4.
TABLE 3 ultra-low carbon BH steels Performance
TABLE 4 elongation at yield Ae value after 6 months storage in different storage temperature environments
| Storage temperature | -10℃ | 20℃ | 40℃ |
| Ae value (after 6 months) | 0.1 | 0.3 | 0.5 |
As can be seen from the table above, after the 240 MPa-grade bake-hardened steel produced by the embodiment is stored for 6 months under different storage temperature conditions, the yield elongation Ae value is less than or equal to 0.8 percent, and the product quality in the stamping process of a user is effectively ensured.
Example 3:
the 240 MPa-grade bake-hardening steel comprises the following chemical components in percentage by mass: c: 0.0025 percent; si: 0.0045 percent; mn: 0.48 percent; p: 0.052 percent; s: 0.004%; and (3) Alt: 0.041 percent; nb: 0.010%; b: 0.0005%; n: 0.0019%, the balance being Fe and unavoidable impurities. Wherein the addition amount of the Nb component and the proportion of the content of the Nb component to the C component meet Ceff-Nb/7.75-12 ppm.
The heating temperature of the hot rolling process of the steel grade is controlled to be 1250 ℃; the finishing temperature is 927 ℃; the coiling temperature is 600 ℃; the cold rolling process controls the cold rolling reduction rate to be 81%. The flattening elongation is 1.4%, and the rest of the process and the corresponding parameters are the same as those of example 1.
The steel grade performance of the 240MPa grade bake-hardened steel obtained after the treatment according to the process is tested, the performance results are shown in Table 5, and the yield elongation Ae values after the steel is stored for 6 months in different storage temperature environments are shown in Table 6.
TABLE 5 ultra-low carbon BH steels Performance
TABLE 6 elongation at yield Ae values after 6 months storage in different storage temperature environments
| Storage temperature | -10℃ | 20℃ | 40℃ |
| Ae value (after 6 months) | 0.2 | 0.4 | 0.7 |
As can be seen from the table above, after the 240 MPa-grade bake-hardened steel produced by the embodiment is stored for 6 months under different storage temperature conditions, the yield elongation Ae value is less than or equal to 0.8 percent, and the product quality in the stamping process of a user is effectively ensured.
Example 4:
the 240 MPa-grade bake-hardening steel comprises the following chemical components in percentage by mass: c: 0.0027 percent; si: 0.010%; mn: 0.49 percent; p: 0.055%; s: 0.005 percent; and (3) Alt: 0.034%; nb: 0.010%; b: 0.0004 percent; n: 0.0022%, the balance being Fe and unavoidable impurities. Wherein the addition amount of the Nb component and the proportion of the content of the Nb component to the C component meet Ceff-C-Nb/7.75-14 ppm.
The heating temperature of the hot rolling process of the steel grade is controlled to be 1240 ℃; the finishing temperature is 920 ℃; the coiling temperature is 590 ℃; the cold rolling process controls the cold rolling reduction rate to be 82%. The flattening elongation is 1.45%, and the rest of the process and the corresponding parameters are the same as those of example 1.
The steel grade properties of the 240MPa grade bake-hardened steel obtained after the treatment according to the process are tested, the property results are shown in Table 7, and the yield elongation Ae values after 6 months of storage in different storage temperature environments are shown in Table 8.
TABLE 7 ultra-low carbon BH steels Performance
TABLE 8 elongation at yield Ae after 6 months storage in different storage temperature environments
| Storage temperature | -10℃ | 20℃ | 40℃ |
| Ae value (after 6 months) | 0 | 0.3 | 0.6 |
As can be seen from the table above, after the 240 MPa-grade bake-hardened steel produced by the embodiment is stored for 6 months under different storage temperature conditions, the yield elongation Ae value is less than or equal to 0.8 percent, and the product quality in the stamping process of a user is effectively ensured.
Example 5:
the 240 MPa-grade bake-hardening steel comprises the following chemical components in percentage by mass: c: 0.0029%; si: 0.010%; mn: 0.49 percent; p: 0.055%; s: 0.005 percent; and (3) Alt: 0.047 percent; nb: 0.011 percent; b: 0.0006 percent; n: 0.0016%, the balance being Fe and unavoidable impurities. Wherein the addition amount of the Nb component and the proportion of the content of the Nb component to the C component meet the Ceff-Nb/7.75-15 ppm.
The heating temperature of the hot rolling process of the steel grade is controlled to be 1230 ℃; the finishing temperature is 910 ℃; the coiling temperature is 580 ℃; the cold rolling process controls the cold rolling reduction rate to be 83 percent. The flattening elongation is 1.5%, and the rest of the process and the corresponding parameters are the same as those of example 1.
The steel grade properties of the 240MPa grade bake-hardened steel obtained after the treatment according to the process are tested, the property results are shown in Table 9, and the yield elongation Ae values after 6 months of storage in different storage temperature environments are shown in Table 10.
TABLE 9 ultra-low carbon BH steels Performance
TABLE 10 elongation at yield Ae values after 6 months storage in different storage temperature environments
| Storage temperature | -10℃ | 20℃ | 40℃ |
| Ae value (after 6 months) | 0 | 0.4 | 0.6 |
As can be seen from the table above, after the 240 MPa-grade bake-hardened steel produced by the embodiment is stored for 6 months under different storage temperature conditions, the yield elongation Ae value is less than or equal to 0.8 percent, and the product quality in the stamping process of a user is effectively ensured.
The 240 MPa-grade bake-hardening steel and the manufacturing method thereof are implemented in a certain steel mill from 2016 to 11 months, and the production and supply of 5951 tons of products are completed by 2017, so that the product performance is stable, and the design requirements are met. Economic benefits are as follows: in 2017, the average profit of the product is 1253 yuan/ton, the finished sales volume is 5951 ton, and the profit of the product is 1253 yuan/ton, 5951 ton or 746 ten thousand yuan. Social benefits are as follows: the product and the production method solve the problem that orange peel is easy to appear in the 240MPa strength grade product after the product is stored for more than three months under the high temperature condition (more than or equal to 30 ℃), and integrally realize the best combination of formability and high strength in the manufacturing process of the automobile outer plate.
Through the embodiment, the invention has the following beneficial effects or advantages:
according to the 240 MPa-grade bake-hardening steel and the manufacturing method thereof, the ultralow-carbon hot-dip galvanized bake-hardening steel with low temperature sensitivity is successfully produced by optimizing component design, adjusting and optimizing hot rolling and cold rolling annealing process parameters and leveling process parameters, and the yield strength is more than or equal to 240 MPa; the sensitivity of BH steel to temperature is reduced to a certain extent, the shelf life of the product is more than or equal to 6 months under the condition that the storage environment is in the range of-10 ℃ to 40 ℃, the yield point elongation Ae value before stamping is less than or equal to 0.8 percent, and the optimal combination of the product strength, deep drawing performance and low temperature sensitivity is realized. The stamping die has the advantages that the stamping stability is guaranteed while the product strength and the dent resistance are improved, the temperature sensitivity of the product is reduced, the ageing resistance is improved, and the stamping die has great practical application significance.
While the preferred embodiments of the present application have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all alterations and modifications as fall within the scope of the application.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present application without departing from the spirit and scope of the application. Thus, if such modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is intended to include such modifications and variations as well.
Claims (6)
1. The manufacturing method of the 240 MPa-level bake-hardening steel is characterized in that the 240 MPa-level bake-hardening steel comprises the following chemical components in percentage by mass: c: 0.0015-0.0030%; si is less than or equal to 0.03 percent; mn: 0.40-0.6%; p: 0.052-0.065%; s: less than or equal to 0.01 percent; and (3) Alt: 0.02-0.06%; nb: 0.006-0.014%; b: 0.0003 to 0.0008 percent; n: less than or equal to 0.004 percent, and the balance of Fe and inevitable impurities;
the manufacturing method comprises the following steps:
smelting and continuously casting to obtain a steel billet containing the chemical components in percentage by mass;
hot rolling the steel billet to obtain a hot rolled coil; specifically, the billet is heated from room temperature, and is subjected to rough rolling, finish rolling and laminar cooling, and then is coiled into the hot-rolled coil at low temperature; wherein the heating temperature is 1250 +/-30 ℃; the finishing temperature is 930 +/-20 ℃; the coiling temperature is 600 +/-20 ℃;
cold rolling the hot rolled coil to obtain a cold-hard coil;
continuously annealing the cold-rolled hard coil to finally obtain the 240 MPa-level baking hardened steel; the continuous annealing includes:
the cold and hard coil enters the continuous annealing furnace from a preheating section of the continuous annealing furnace and is continuously heated to 200 ℃ in the reducing atmosphere in the continuous annealing furnace for 20-30 seconds;
further heating the chilled coils to 760 +/-5 ℃ in 130-200 seconds in a heating section of the continuous annealing furnace, and preserving heat for 50-70 seconds;
cooling the chilled coils to 680 +/-5 ℃ in a slow cooling section of the continuous annealing furnace for 14-20 seconds;
and cooling the cold hard coil to 470 +/-5 ℃ in 13-20 seconds in a fast cooling section of the continuous annealing furnace to obtain the annealed strip steel.
2. The manufacturing method according to claim 1, wherein: the cold rolling the hot rolled coil after the hot rolling includes: and naturally cooling the hot-rolled coil, and then carrying out acid pickling and cold rolling to obtain a cold-hard coil, wherein the total cold rolling reduction rate is 58-84%.
3. The manufacturing method according to claim 1, wherein: and after the cold-rolled hard coil is subjected to continuous annealing, hot galvanizing is performed on the annealed strip steel, and finally the 240 MPa-grade baking hardened steel is obtained.
4. The manufacturing method according to claim 3, wherein: the annealing band steel is subjected to hot galvanizing, and the method comprises the following steps: putting the strip steel into a zinc pot for galvanizing, wherein the temperature of the strip steel when the strip steel is put into the zinc pot is 460 +/-10 ℃, and obtaining hot galvanized strip steel;
and leveling the hot-dip galvanized steel strip, and coiling the hot-dip galvanized steel strip into a finished product, wherein the leveling elongation is 1.4 +/-0.2%, so as to obtain the 240 MPa-grade baking hardened steel.
5. The manufacturing method according to claim 1, wherein: the medium in the reducing atmosphere in the continuous annealing furnace is H2And N2The mixed gas of (1).
6. The manufacturing method according to claim 1, wherein: the proportion of the addition amount of the Nb component and the content ratio of the C component meets the condition that Ceff is not less than 9ppm and not more than 15ppm, wherein Ceff is C-Nb/7.75.
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| CN111850262B (en) * | 2020-06-22 | 2022-07-26 | 鞍钢蒂森克虏伯汽车钢有限公司 | Production method of ultra-low carbon baking hardening continuous hot-dip galvanized steel sheet |
| CN111690874A (en) * | 2020-06-22 | 2020-09-22 | 武汉钢铁有限公司 | Band steel with excellent low-temperature secondary processing performance and tensile strength of 370MPa and production method thereof |
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