CN111334701B - High-elongation hot-rolled tissue regulating steel with tensile strength of more than or equal to 800MPa and production method thereof - Google Patents
High-elongation hot-rolled tissue regulating steel with tensile strength of more than or equal to 800MPa and production method thereof Download PDFInfo
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- 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
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Abstract
A high-elongation hot-rolled tissue regulating steel with tensile strength not less than 800MPa comprises the following components in percentage by weight: c: 0.07 to 0.09%, Mn: 1.95-2.10%, Si: 0.70-0.80%, P is less than or equal to 0.008%, S is less than or equal to 0.002%, Als: 0.02-0.04%; the production method comprises the following steps: smelting in a converter and then carrying out RH vacuum furnace treatment; heating a casting blank; rolling in sections; and (3) cooling in a segmented variable speed manner: coiling; preserving heat; and (6) leveling. On the premise of ensuring that the tensile strength is more than 800MPa, the invention ensures that the yield strength is more than or equal to 500MPa, the elongation is more than or equal to 25 percent, the product of strength and elongation is more than or equal to 20GPa percent, the yield ratio is less than 0.65, the forming requirement of thick high-strength steel can be met, the alloy elements are simple, noble alloys such as Nb, V, Mo and the like are not added, and the alloy cost can be reduced by at least 10 percent.
Description
Technical Field
The invention relates to hot rolled steel for automobiles and a production method thereof, in particular to high-elongation hot rolled tissue regulating steel with tensile strength of more than 800MPa and a production method thereof; and is suitable for the thickness of the steel plate of 2-12 mm.
Background
In recent years, the urgent degree of light weight in the automobile industry, particularly in the market segments of commercial vehicles, semitrailers, passenger cars and the like, is increasing day by day, and relevant manufacturing enterprises put forward clear demands on weight reduction of vehicles so as to meet the requirements of regulations such as mandatory national standard GB1589-2016 and the like. On one hand, structural parts such as wheels, axles and saddles of commercial vehicles and other vehicle types need to be subjected to forming processing such as drawing and reaming in the manufacturing process, and have clear requirements on the drawing and flanging forming performances of the ultrahigh-strength steel with tensile strength increased to be more than 800MPa, on the other hand, the thickness of the steel used for the commercial vehicles and other vehicle types is thicker and is generally about 3-16 mm, and the thickness of the ultrahigh-strength steel after being lightened also needs to be 2-12 mm, so that the current market puts forward urgent requirements on the high-formability ultrahigh-strength steel with thick specification.
After preliminary retrieval:
the Chinese patent application numbers are: the document of CN201210411202.6 discloses a hot-rolled dual-phase steel plate with 780 MPa-grade tensile strength and a manufacturing method thereof, and the components are as follows: 0.07-0.12 percent of C, 0.2-0.7 percent of Si, 1.0-1.8 percent of Mn, 0.02-0.08 percent of Als, 0.5-1.2 percent of Cr, 0.02-0.05 percent of Nb, 0.01-0.03 percent of Ti, 0.02 percent of P, 0.005 percent of S and the balance of Fe. The thickness of the finished product is 2.5-6 mm; the elongation is higher than 15%. The elongation of the invention can not meet the forming requirement of deformed complex parts, meanwhile, the thickness of the finished product is only 2.5-6 mm, the finished product can not be used for structural members such as axles and saddles of commercial vehicles and other vehicle types, the requirements can not be met for steel products with the thickness of 7-12 mm, and the invention has more alloy elements and higher relative cost.
The Chinese patent application No. CN201110402359.8 discloses a hot-rolled dual-phase steel and a production process thereof, which can be used for manufacturing structural members such as high-strength automobile wheels, and the like, wherein the steel comprises the following chemical components in percentage by weight: c: 0.06-0.10%, Si: 0.10-0.50%, Mn: 1.00-1.60%, P is less than or equal to 0.02%, S is less than or equal to 0.02%, Nb: 0.02-0.05%, and the balance of Fe and inevitable impurities; the yield strength range of the hot-rolled dual-phase steel obtained by the invention is 440-500 MPa, the tensile strength is 650-750 MPa, the elongation is 18-25%, and the yield ratio is less than 0.7. The tensile strength of the invention is lower than 800MPa, and the light weight requirement of the commercial vehicle can not be effectively met.
The document with the Chinese patent application number of CN201610450190.6 discloses 780 MPa-grade hot-rolled high-hole-expansion dual-phase steel and a manufacturing method thereof, and the steel comprises the following chemical components in percentage by weight: c: 0.10 to 0.15%, Si: 0.8-1.8%, Mn: 1.0-2.0%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, O is less than or equal to 0.003%, Al: 0.02-0.06%, N is less than or equal to 0.006%, Nb: 0.01-0.06%, Ti: 0.05-0.15 percent of the total weight of the steel, and the balance of Fe and inevitable impurities, wherein the yield strength of the dual-phase steel is more than or equal to 600MPa, the tensile strength is more than or equal to 780MPa, and the elongation percentage A is80Not less than 15 percent and not less than 40 percent of hole expansion rate. The content of C element in the invention is relatively high, which affects the cold formability of steel, the content of Si is relatively high, which is not beneficial to the control of the surface quality of steel, meanwhile, the lower limit of elongation is only 15%, which can not meet the requirements of parts with complex deformation, and the invention has more alloy elements and relatively high cost.
The Chinese patent application No. CN201410651490 discloses a hot-rolled dual-phase steel with tensile strength more than or equal to 780MPa, which comprises the following components in percentage by weight: c: 0.05-0.08%, S is not more than 0.5%, Mn: 0.6-1.2%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, N is less than or equal to 0.006%, Als is 0.01-0.1%, Ti: 0.05-0.25 percent of Cr with the content of less than or equal to 0.6 percent or B with the content of less than or equal to 0.003 percent or the compound addition of the Cr and the B; the tensile strength of the invention is more than or equal to 780MPa, the elongation is more than or equal to 15 percent, and the yield ratio is less than or equal to 0.70. The production of the hot-rolled dual-phase steel with the thickness of 1.2-4.0 mm is realized. The lower limit of the elongation of the document is only 15%, the forming requirement of deformed complex parts cannot be met, meanwhile, the thickness of a finished product is only 1.2-4.0 mm, the requirement of thicker specification steel cannot be met, and the alloy elements are more and the relative cost is higher.
Disclosure of Invention
The invention aims to solve the defects in the prior art, and provides the high-elongation hot-rolled tissue regulating steel and the production method thereof, wherein the elongation is more than or equal to 25% on the premise of ensuring the tensile strength to be more than 800MPa, the forming requirement of the steel plate with the thickness of 7-12 mm can be met, the alloy elements are simple, noble alloys such as Nb, V and Mo are not added, and the alloy cost can be reduced by at least 10%.
The measures for realizing the aim are as follows:
a high-elongation hot-rolled tissue regulating steel with tensile strength not less than 800MPa comprises the following components in percentage by weight: c: 0.07 to 0.09%, Mn: 1.95-2.10%, Si: 0.70-0.80%, P is less than or equal to 0.008%, S is less than or equal to 0.002%, Als: 0.02-0.04%, and the balance of Fe and inevitable impurities; mechanical properties: the yield strength is more than or equal to 500MPa, the tensile strength is more than or equal to 800MPa, the elongation is more than or equal to 25 percent, the product of strength and elongation is more than or equal to 20GPa percent, and the yield ratio is less than 0.65; the thickness of the steel plate is 2.0-12.0 mm; the metallographic structure is as follows: 44-55% of ferrite, 40-50% of martensite and 5-10% of retained austenite.
Preferably: the content of C is 0.073-0.086% by weight.
Preferably: the weight percentage content of Si is 0.72-0.78%.
Preferably: the weight percentage content of Als is 0.02-0.033%.
The method for producing the high-elongation hot-rolled tissue regulating steel with the tensile strength of more than or equal to 800MPa comprises the following steps:
1) carrying out RH vacuum furnace treatment after smelting in a converter, wherein the RH treatment time is not less than 15min, then continuously casting molten steel into a blank, and carrying out electromagnetic stirring in the continuous casting process;
2) heating the casting blank, wherein the heating temperature is controlled to be 1260-1300 ℃, and the heating time is 60-80 min;
3) performing sectional rolling, wherein the finishing temperature of rough rolling is controlled to be 1060-1100 ℃, and the finishing temperature of finish rolling is controlled to be 800-840 ℃;
4) and (3) carrying out segmented variable speed cooling:
the first section is cooled to 660-680 ℃ at the cooling speed of 120-200 ℃/s;
the second section is cooled to 620-640 ℃ at a cooling speed of 5-15 ℃/s;
the third section is cooled to the coiling temperature at the cooling speed of 100-200 ℃/s;
5) coiling at 320-350 ℃;
6) taking off the coil after coiling, and preserving heat at 310-350 ℃ for 30-60 min;
7) carrying out leveling treatment, and controlling the leveling elongation rate to be less than 1.0%.
The method for producing the high-elongation hot-rolled tissue regulating steel with the tensile strength of more than or equal to 800MPa is characterized by comprising the following steps of: and (3) cooling in a segmented variable speed manner:
the first-stage cooling speed is 135-195 ℃/s;
the cooling speed of the second section is 5-12 ℃/s;
the cooling speed of the third section is 115-190 ℃/s.
The mechanism and action of each element and main process in the invention are as follows:
carbon: carbon is the most important strengthening element in the present invention, and can increase the hardness of the martensite hard phase and influence the transformation ratio of martensite and retained austenite. According to the application range of the steel grade forming processing, the material is required to have good cold forming performance while meeting the strength requirement. If the carbon content is less than 0.07 percent, the hardness of martensite is reduced, the tensile strength of the material can not meet the standard requirement under the condition of certain components, and the stability of the retained austenite in the heat preservation process can be influenced; if the carbon content is more than 0.09%, good formability of the material cannot be satisfied. Therefore, the carbon element is limited to 0.07 < C < 0.09%, preferably.
Silicon: silicon accelerates the segregation of carbon to austenite in the structure-regulated steel, so that ferrite is further purified, the gap solid solution strengthening is avoided, the generation of coarse carbides during cooling can be avoided, meanwhile, the silicon which is solid-dissolved in the ferrite can influence the interaction of dislocation, and the uniform elongation performance under a given strength level is increased, therefore, the lower limit of the silicon element is 0.70%; however, too high silicon content will form Fe during high temperature rolling2SiO4The adhesion between the scale and the steel substrate surface is increased, the effect of the descaling process is reduced, and the surface quality of the steel sheet is deteriorated, so that the upper limit of the silicon element needs to be strictly controlled under the condition of necessary addition. Therefore, the silicon element is limited within the range of 0.70 < C < 0.80%, and preferably the content is 0.72-0.78%.
Manganese: manganese is the most effective element for improving the strength and the toughness, and can effectively delay pearlite transformation in the structure-regulated steel and provide conditions for forming ferrite in the process of low cold-speed phase transformation. If the content of the manganese element is less than 1.95 percent, the strength requirement of the material cannot be met; however, the addition of excessive manganese increases the risk of segregation of the manganese element and increases the alloy cost of the steel. Therefore, the limited range of Mn is 1.95 < Mn < 2.10%.
Phosphorus: phosphorus is a harmful element in the present invention, and the upper limit of the content thereof is set to 0.008% in order to avoid deterioration of the weldability, press formability, toughness, and secondary workability of the material. Therefore, the phosphorus content is controlled to be less than 0.008%.
Sulfur: elemental sulfur is a very harmful element in the present invention. Sulfur is often present in the form of MnS, and this sulfide inclusion is very disadvantageous to formability of steel and causes anisotropy of properties, so that the lower the sulfur content in steel, the better. Therefore, the sulfur content in steel is controlled to 0.002% or less.
Aluminum: aluminum is added for deoxidation, and when the content of Als is less than 0.02%, the effect thereof cannot be exerted; on the other hand, since addition of a large amount of aluminum easily forms alumina agglomerates, the upper limit of Als is defined to be 0.04%. Therefore, the content of Als is limited to 0.02 to 0.04%, preferably 0.02 to 0.033%.
The invention makes the metallographic structure as follows: the steel comprises 45-55% of ferrite, 40-50% of martensite and 5-10% of residual austenite in volume ratio, the high-hardness martensite accounts for 40-50% of the high-hardness martensite can improve the tensile strength of the steel, the low-hardness ferrite accounts for 45-55% of the low-hardness martensite ensures that the steel has good forming performance, and the residual austenite accounts for 5-10% of the low-hardness ferrite is transformed into the martensite through a deformation induced phase transformation mechanism in the steel processing deformation process, so that the forming performance is ensured, and the tensile strength of parts is further improved after forming.
The invention controls the heating temperature of the casting blank to 1260-1300 ℃ in order to fully diffuse C element and Mn element in the casting blank, reduce the segregation of C, Mn element and improve the formability of rolled steel.
The invention is characterized in that the heat preservation is carried out when the temperature under the wire is 310-350 ℃, the heat preservation time is 30-60 min, in order to diffuse the C element in the previously transformed martensite into the non-transformed austenite in the temperature range, so that the content of the C element in the non-transformed austenite is increased, the stability of the non-transformed austenite is improved, the non-transformed austenite is kept to the room temperature, and finally 5-10% of residual austenite is formed.
The invention is therefore subject to stepped variable speed cooling, i.e.
The first-stage cooling speed is 120-200 ℃/s; the cooling speed of the second section is 5-15 ℃/s; the third-stage cooling speed is 100-200 ℃/s, and the precise regulation and control of different tissues are realized by adopting a three-stage cooling and medium-low temperature coiling heat preservation process, so that the matching of high strength and high elongation is realized: firstly, adopting an ultrahigh cooling speed of 120-200 ℃/s to quickly cool a steel plate to 660-680 ℃ from 800-840 ℃ after rolling, fully retaining a deformation substructure in a rolled steel plate crystal grain, increasing the number of nucleation, and simultaneously avoiding coarsening of the rolled crystal grain; then, controlling phase change at a low cooling speed of 5-15 ℃/s to form 45-55% of ferrite; and finally, carrying out ultra-high speed cooling at the speed of 100-200 ℃/s again, cooling to 320-350 ℃, and carrying out heat preservation for 30-60 min, so as to form 40-50% of martensite and 5-10% of retained austenite, thereby realizing matching of high strength and high elongation.
The control process of the flattening procedure after cooling is also one of the important technologies of the invention. In order to ensure the shape quality of the steel plate after high-speed cooling, the steel coil needs to be leveled. Because the steel plate is a multi-phase structure of ferrite, martensite and retained austenite after being cooled, movable dislocation in the ferrite after phase transformation is consumed under excessive flat pressing, and the retained austenite generates a deformation induction phenomenon. Data obtained through a large number of experiments show that the yield strength of the structure-regulated steel can be increased by about 50MPa by 1% of the flat elongation, the yield ratio of the material is remarkably improved, and the elongation of the finished steel product is greatly reduced along with the improvement of the flat elongation, so that the forming capability of the subsequent steel in the forming process is influenced. Therefore, the flattening elongation is limited to be less than 1 percent in the flattening process.
Compared with the prior art, on the premise of ensuring that the tensile strength is more than 800MPa, the yield strength is more than or equal to 500MPa, the elongation is more than or equal to 25 percent, the product of strength and elongation is more than or equal to 20GPa percent, the yield ratio is less than 0.65, the forming requirement of thick high-strength steel can be met, the alloy elements are simple, noble alloys such as Nb, V, Mo and the like are not added, and the alloy cost can be reduced by at least 10 percent.
Drawings
FIG. 1 is a metallographic structure diagram of the present invention.
Detailed Description
The present invention is described in detail below:
table 1 is a list of values of chemical components of each example and comparative example of the present invention;
table 2 is a table of the main process parameters of each example of the present invention and comparative example;
table 3 is a table of the performance test of each example and comparative example of the present invention.
The embodiments of the invention are produced according to the following steps:
1) carrying out RH vacuum furnace treatment after smelting in a converter, wherein the RH treatment time is not less than 15min, then continuously casting molten steel into a blank, and carrying out electromagnetic stirring in the continuous casting process;
2) heating the casting blank, wherein the heating temperature is controlled to be 1260-1300 ℃, and the heating time is 60-80 min;
3) performing sectional rolling, wherein the finishing temperature of rough rolling is controlled to be 1060-1100 ℃, and the finishing temperature of finish rolling is controlled to be 800-840 ℃;
4) and (3) carrying out segmented variable speed cooling:
the first section is cooled to 660-680 ℃ at the cooling speed of 120-200 ℃/s;
the second section is cooled to 620-640 ℃ at a cooling speed of 5-15 ℃/s;
the third section is cooled to the coiling temperature at the cooling speed of 100-200 ℃/s;
5) coiling at 320-350 ℃;
6) taking off the coil after coiling, and preserving heat at 310-350 ℃ for 30-60 min;
7) carrying out leveling treatment, and controlling the leveling elongation rate to be less than 1.0%.
TABLE 1 list of chemical compositions (wt%) of inventive examples and comparative examples
TABLE 2 Main Process parameter List of the inventive examples and comparative examples
TABLE 3 test result list of mechanical properties of each example and comparative example of the present invention
As can be seen from Table 3, the tensile strength of the steel produced by the method is above 800MPa, the elongation can reach above 25%, the product of strength and elongation of the material exceeds 20GPa%, the steel has high strength and good forming performance, the thickness of the steel covers 2-12 mm, and the application thickness range of various parts such as commercial vehicle axles and saddles can be met.
The above examples are merely preferred examples and are not intended to limit the embodiments of the present invention.
Claims (6)
1. A high-elongation hot-rolled tissue regulating steel with tensile strength not less than 800MPa comprises the following components in percentage by weight: c: 0.07 to 0.09%, Mn: 1.95-2.10%, Si: 0.70-0.80%, P is less than or equal to 0.008%, S is less than or equal to 0.002%, Als: 0.02-0.04%, and the balance of Fe and inevitable impurities; mechanical properties: the yield strength is more than or equal to 500MPa, the tensile strength is more than or equal to 800MPa, the elongation is more than or equal to 25 percent, the product of strength and elongation is more than or equal to 20GPa percent, and the yield ratio is less than 0.65; the thickness of the steel plate is 2.0-12.0 mm; the metallographic structure is as follows: ferrite accounting for 44-55% by volume, martensite accounting for 40-50% by volume, and residual austenite accounting for 5-10% by volume;
the production method comprises the following steps:
1) carrying out RH vacuum furnace treatment after smelting in a converter, wherein the RH treatment time is not less than 15min, then continuously casting molten steel into a blank, and carrying out electromagnetic stirring in the continuous casting process;
2) heating the casting blank, wherein the heating temperature is controlled to be 1260-1300 ℃, and the heating time is 60-80 min;
3) performing sectional rolling, wherein the finishing temperature of rough rolling is controlled to be 1060-1100 ℃, and the finishing temperature of finish rolling is controlled to be 800-840 ℃;
4) and (3) carrying out segmented variable speed cooling:
the first section is cooled to 660-680 ℃ at the cooling speed of 120-200 ℃/s;
the second section is cooled to 620-640 ℃ at a cooling speed of 5-15 ℃/s;
the third section is cooled to the coiling temperature at the cooling speed of 100-200 ℃/s;
5) coiling at 320-350 ℃;
6) taking off the coil after coiling, and preserving heat at 310-350 ℃ for 30-60 min;
7) carrying out leveling treatment, and controlling the leveling elongation rate to be less than 1.0%.
2. The high-elongation hot-rolled tissue-conditioning steel with tensile strength of not less than 800MPa according to claim 1, which is characterized in that: the content of C is 0.073-0.086% by weight.
3. The high-elongation hot-rolled tissue-conditioning steel with tensile strength of not less than 800MPa according to claim 1, which is characterized in that: the weight percentage content of Si is 0.72-0.78%.
4. The high-elongation hot-rolled tissue-conditioning steel with tensile strength of not less than 800MPa according to claim 1, which is characterized in that: the weight percentage content of Als is 0.02-0.033%.
5. The method for producing the high-elongation hot-rolled microstructure-regulated steel having a tensile strength of not less than 800MPa according to claim 1, comprising the steps of:
1) carrying out RH vacuum furnace treatment after smelting in a converter, wherein the RH treatment time is not less than 15min, then continuously casting molten steel into a blank, and carrying out electromagnetic stirring in the continuous casting process;
2) heating the casting blank, wherein the heating temperature is controlled to be 1260-1300 ℃, and the heating time is 60-80 min;
3) performing sectional rolling, wherein the finishing temperature of rough rolling is controlled to be 1060-1100 ℃, and the finishing temperature of finish rolling is controlled to be 800-840 ℃;
4) and (3) carrying out segmented variable speed cooling:
the first section is cooled to 660-680 ℃ at the cooling speed of 120-200 ℃/s;
the second section is cooled to 620-640 ℃ at a cooling speed of 5-15 ℃/s;
the third section is cooled to the coiling temperature at the cooling speed of 100-200 ℃/s;
5) coiling at 320-350 ℃;
6) taking off the coil after coiling, and preserving heat at 310-350 ℃ for 30-60 min;
7) carrying out leveling treatment, and controlling the leveling elongation rate to be less than 1.0%.
6. The method for producing a high-elongation hot-rolled microstructure-regulated steel having a tensile strength of not less than 800MPa according to claim 5, wherein: and (3) cooling in a segmented variable speed manner:
the first-stage cooling speed is 135-195 ℃/s;
the cooling speed of the second section is 5-12 ℃/s;
the cooling speed of the third section is 115-190 ℃/s.
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| US3765874A (en) * | 1972-05-19 | 1973-10-16 | Armco Steel Corp | Vacuum degassed, interstitial-free, low carbon steel and method for producing same |
| CN102304664A (en) * | 2011-09-13 | 2012-01-04 | 北京科技大学 | High strength and high plasticity aluminum-containing medium manganese transformation-induced plasticity (TRIP) cold-rolled steel plate and preparation method thereof |
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