CN111270161B - High-elongation hot-rolled tissue regulating steel with tensile strength of more than or equal to 1000MPa and production method thereof - Google Patents
High-elongation hot-rolled tissue regulating steel with tensile strength of more than or equal to 1000MPa 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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- C21D2211/008—Martensite
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Abstract
A high-elongation hot-rolled tissue regulating steel with tensile strength not less than 1000MPa comprises the following components in percentage by weight: c: 0.12 to 0.14%, Mn: 1.60-1.80%, 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 not adding Nb, V and Mo alloy elements, the invention can ensure that the tensile strength is more than 1000MPa, the elongation is more than or equal to 18 percent, the product of strength and elongation is more than or equal to 18GPa percent, the yield ratio is less than 0.65, the forming requirement of the steel plate with the thickness of 2-12 mm can be met, and the alloy cost is 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 1000MPa 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 No. 201610451301.5 discloses 980 MPa-grade hot-rolled dual-phase steel and a manufacturing method thereof, and the steel comprises the following chemical components in percentage by weight: 0.10 to 0.20%, Si: 0.8-2.0%, 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: 0.01-0.05%, and the balance of Fe and inevitable impurities, wherein the elements simultaneously satisfy the following relations: nb and Ti are more than or equal to 0.05 percent and less than or equal to 0.10 percent. The microstructure of the document is ferrite and martensite, the average grain size of the ferrite is 5-10 mu m, the equivalent grain size of the martensite is 15-20 mu m, the yield strength is more than or equal to 500MPa, the tensile strength is more than or equal to 980MPa, and the elongation A80 is more than or equal to 12%. The invention has higher Si content, is not beneficial to the control of the surface quality of steel, increases the alloy cost due to the composite addition of Nb and Ti, has lower elongation, can not meet the requirements of part of deformed complex parts, and can not meet the requirements of steel with the thickness of 7-12 mm because the thickness is less than or equal to 6 mm.
The Chinese patent application number is 201610451302.X, which discloses 980 MPa-grade hot-rolled high-reaming dual-phase steel and a manufacturing method thereof, wherein the steel comprises the following chemical components in percentage by weight: c: 0.15 to 0.20%, Si: 1.0-2.0%, 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: 0.3-1.0%, N is less than or equal to 0.006%, Nb: 0.01-0.06%, Ti: 0.08-0.20%, and the balance of Fe and inevitable impurities, wherein the elements simultaneously satisfy the following relations: n b + Ti is more than or equal to 0.10 percent and less than or equal to 0.25 percent, and Al/C is more than or equal to 2.5 and less than or equal to 5.0 percent. The microstructure of the document is nano-scale carbide distributed in ferrite, martensite and ferrite, the yield strength is more than or equal to 700MPa, the tensile strength is more than or equal to 980MPa, the elongation A80 is more than or equal to 12 percent, and the hole expansion rate is more than or equal to 30 percent. The content of Si is high, the control of the surface quality of steel is not facilitated, the alloy cost is increased due to the compound addition of Nb and Ti, the elongation is low, the requirement of part of deformed complex parts cannot be met, and meanwhile, the thickness is less than or equal to 6mm, and the requirement of 7-12 mm thick steel cannot be met.
Chinese patent application number 201610450203.X discloses 980 MPa-grade hot-rolled ferrite-bainite dual-phase steel and a manufacturing method thereof, wherein the steel comprises the following chemical components in percentage by weight: 0.15-0.30%, Si: 0.8-2.0%, 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: 0.5-1.0%, N is less than or equal to 0.006%, Nb: 0.01-0.06%, Ti: 0.01-0.05%, and the balance of Fe and inevitable impurities, and simultaneously satisfying the following requirements: nb and Ti are more than or equal to 0.05 percent and less than or equal to 0.10 percent, and Al/C is more than or equal to 2.5 and less than or equal to 5.0 percent. The microstructure of the document is ferrite and bainite, the yield strength is more than or equal to 600MPa, the tensile strength is more than or equal to 980MPa, and the elongation is more than or equal to 15%. The high Si content is not beneficial to the control of the surface quality of steel, the composite addition of Nb and Ti increases the alloy cost, the lower limit of the elongation is only 15%, the requirement of part of deformed complex parts cannot be met, and meanwhile, the thickness is less than or equal to 6mm, and the requirement of 7-12 mm thick steel cannot be met.
The Chinese patent application No. 201310121587.7 discloses a low yield ratio high strength hot rolled Q & P steel and a manufacturing method thereof, which comprises the following chemical components: c: 0.20-0.40%, Si: 1.0% -2.0%, Mn: 1.5-3.0%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, Al: 0.02-0.08%, N is less than or equal to 0.006%, Ti: 0.005-0.015 percent, and the balance of Fe and inevitable impurities. The document obtains a structure containing proeutectoid ferrite + martensite + retained austenite; the yield strength is less than 700MPa, the tensile strength is more than 1000MPa, the yield ratio is 0.50-0.60, and the elongation is more than 8%. The C element content is high, the cold formability of steel is influenced, the Si content is high, the control of the surface quality of the steel is not facilitated, the elongation is low, and the requirement of a part of a deformed complex part cannot be met.
The document with the Chinese patent application number of 2017109778735.5 discloses a thin 980 MPa-grade dual-phase steel and a processing method thereof, and belongs to the technical field of steel rolling. The dual-phase steel comprises the following chemical components: c: 0.17% -0.20%, Si: 0.30-0.60%, Mn: 0.60-1.00%, P is less than or equal to 0.015%, S is less than or equal to 0.004%, Cr: 0.30% -0.70%, Als: 0.020% to 0.060% and the balance of Fe and inevitable impurities. The method adopts the continuous casting and rolling process of the thin slab, and the processing method comprises the processes of smelting, refining, continuous casting and rolling of the thin slab, cooling, reeling and leveling; finally obtaining the thin 980 MPa-grade dual-phase steel with the thickness of 1.0-3.0 mm. The content of gasoline and C elements is high, the cold formability of steel is affected, and meanwhile, the thickness is only 1.0-3.0mm, and the requirement of the steel with the thickness of 7-12 mm cannot be met.
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, which can ensure that the tensile strength is more than 1000MPa, the elongation is more than or equal to 18 percent, the product of strength and elongation is more than or equal to 18GPa percent, the yield ratio is less than 0.65, the forming requirement of the steel plate thickness of 2-12 mm can be met, and the alloy cost is reduced by at least 10 percent on the premise of not adding Nb, V and Mo alloy elements.
The measures for realizing the aim are as follows:
a high-elongation hot-rolled tissue regulating steel with tensile strength not less than 1000MPa comprises the following components in percentage by weight: c: 0.12 to 0.14%, Mn: 1.60-1.80%, 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 percent, and the balance of Fe and inevitable impurities; mechanical properties: the yield strength is more than or equal to 550MPa, the tensile strength is more than or equal to 1000MPa, the elongation is more than or equal to 18 percent, the product of strength and elongation is more than or equal to 18GPa 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: 30-40% of ferrite, 10-15% of bainite, 40-50% of martensite and 5-10% of retained austenite.
Preferably: the content of C is 0.126-0.136% by weight.
Preferably: the weight percentage content of Als is 0.021-0.036%.
The method for producing the high-elongation hot-rolled tissue regulating steel with the tensile strength of more than or equal to 1000MPa 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 1200-1240 ℃, and the heating time is 50-70 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 a cooling speed of 100-200 ℃/s;
the second section is cooled to 620-640 ℃ at a cooling speed of 5-15 ℃/s;
the third section is cooled to 460-500 ℃ at a cooling speed of 50-100 ℃/s;
the fourth stage is cooled to 440-480 ℃ at a cooling speed of 5-15 ℃/s;
the fifth 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%.
Preferably: and (3) cooling in a segmented variable speed manner:
the first-stage cooling speed is 120-190 ℃/s;
the cooling speed of the second section is 5-12 ℃/s;
the cooling speed of the third section is 55-93 ℃/s;
the fourth stage is cooled to 440-480 ℃ at a cooling speed of 5-13 ℃/s;
the cooling speed of the fifth section is 110-185 ℃/s.
Mechanism and action of each element and main process in the invention
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 0.12%, the hardness of martensite is reduced, and the tensile strength of the material cannot meet the standard requirement under the condition of certain components; if the carbon content is more than 0.14%, good formability of the material cannot be satisfied. Therefore, the carbon element is limited to 0.12 < C < 0.14%, and preferably the content is 0.126-0.136%.
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 to 0.70-0.80%.
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 1.60 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 silicon element is limited to 1.60 < Mn < 1.80%.
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 0.021 to 0.036%.
In the present invention, noble alloying elements such as Nb, Ti, V, Mo, Ni, Cu, Cr and the like are not added from the viewpoint of improving the formability of the material and the economy, except for the limitation of the ranges of the above chemical components.
The key technology of the invention is to perform multi-path high cooling speed controlled cooling on the rolled steel coil. The invention adopts a five-section cooling and medium-low temperature coiling heat preservation process to realize accurate regulation and control of different tissues, thereby realizing matching of high strength and high elongation. Firstly, adopting an ultrahigh cooling speed of 100-200 ℃/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 30-40% of ferrite; then cooling at a high speed of 50-100 ℃/s to 460-500 ℃ to avoid the formation of structures such as pearlite; then, controlling phase change at a low cooling speed of 5-15 ℃/s to form 10-15% of bainite; 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 water temperature of the cooling water is controlled to be less than or equal to 20 ℃ so as to ensure the cooling speed of each section during cooling.
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 complex phase structure of ferrite, bainite, martensite and retained austenite after being cooled, movable dislocation in the ferrite after phase transformation can be consumed under the condition of 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.
In the present invention, noble alloying elements such as Nb, Ti, V, Mo, Ni, Cu, Cr and the like are not added from the viewpoint of improving the formability of the material and the economy, except for the limitation of the ranges of the above chemical components.
The invention makes the metallographic structure as follows: 30-40% of ferrite, 10-15% of bainite, 40-50% of martensite and 5-10% of residual austenite by volume ratio, because 40-50% of high-hardness martensite can improve the tensile strength of steel, 30-40% of low-hardness ferrite ensures that the steel has good forming performance, 5-10% of the residual austenite is transformed into martensite through a deformation induced phase transformation mechanism in the steel processing deformation process, the forming performance is ensured, the tensile strength of a part is further improved after forming, 10-15% of the bainite is used for increasing a hardness intermediate phase between the low-hardness ferrite and the high-hardness martensite two phases, the coordinated deformation capability of each phase in the steel processing process is improved, and the forming capability of the ultrahigh-strength steel is further improved.
The purpose of controlling the heating temperature of the casting blank to be 1200-1240 ℃ is 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 320-350 ℃, 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.
Compared with the prior art, the invention can ensure that the tensile strength is more than 1000MPa, the elongation is more than or equal to 18 percent, the product of strength and elongation is more than or equal to 18GPa percent, the yield ratio is less than 0.65, the forming requirement of the steel plate with the thickness of 2-12 mm can be met, and the alloy cost is reduced by at least 10 percent on the premise of not adding Nb, V and Mo alloy elements.
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 1200-1240 ℃, and the heating time is 50-70 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 a cooling speed of 100-200 ℃/s;
the second section is cooled to 620-640 ℃ at a cooling speed of 5-15 ℃/s;
the third section is cooled to 460-500 ℃ at a cooling speed of 50-100 ℃/s;
the fourth stage is cooled to 440-480 ℃ at a cooling speed of 5-15 ℃/s;
the fifth 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 2
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 more than 1000MPa, the elongation can reach more than 18%, the product of strength and elongation of the material exceeds 18GPa%, 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 (2)
1. A production method of high-elongation hot-rolled tissue regulating steel with tensile strength of more than or equal to 1000MPa 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 1200-1240 ℃, and the heating time is 50-70 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 a cooling speed of 100-200 ℃/s;
the second section is cooled to 620-640 ℃ at a cooling speed of 5-15 ℃/s;
the third section is cooled to 460-500 ℃ at a cooling speed of 50-100 ℃/s;
the fourth section is cooled to 440-458 ℃ at the cooling speed of 5-15 ℃/s;
the fifth 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, wherein the leveling elongation is controlled to be less than 1.0%;
the high-elongation hot-rolled tissue regulating steel with the tensile strength of more than or equal to 1000MPa comprises the following components in percentage by weight: c: 0.12 to 0.14%, Mn: 1.60-1.80%, 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 550MPa, the tensile strength is more than or equal to 1000MPa, the elongation is more than or equal to 18 percent, the product of strength and elongation is more than or equal to 18GPa 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: 30-40% of ferrite, 10-15% of bainite, 40-50% of martensite and 5-10% of retained austenite.
2. The production method of the high-elongation hot-rolled microstructure-regulated steel with the tensile strength of more than or equal to 1000MPa according to claim 1, characterized by comprising the following steps: and (3) cooling in a segmented variable speed manner:
the first-stage cooling speed is 120-190 ℃/s;
the cooling speed of the second section is 5-12 ℃/s;
the cooling speed of the third section is 55-93 ℃/s;
the fourth stage is cooled to 440-480 ℃ at a cooling speed of 5-13 ℃/s;
the cooling speed of the fifth section is 110-185 ℃/s.
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