CN113106336B - Ultrahigh-strength dual-phase steel capable of reducing softening degree of laser welding head and production method thereof - Google Patents
Ultrahigh-strength dual-phase steel capable of reducing softening degree of laser welding head and production method thereof Download PDFInfo
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- 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
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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Abstract
The invention discloses ultrahigh-strength dual-phase steel for reducing the softening degree of a laser welding head and a production method thereof, wherein the ultrahigh-strength dual-phase steel comprises the following chemical components in percentage by mass: c: 0.07-0.20%, Mn is less than or equal to 3.0%, Si is less than or equal to 2.0%, S is less than or equal to 0.020%, P is less than or equal to 0.020%, Als: 0.050-1.0%, Mo less than or equal to 0.50%, Cr: 0.50-1.0%, Nb + Ti + V less than or equal to 0.50%, N less than or equal to 0.010%, and the balance of Fe and inevitable impurities; the production process comprises the processes of hot rolling, acid rolling and continuous annealing. The ultrahigh-strength dual-phase steel with the yield strength of 420-900MPa and the tensile strength of 780-1200MPa is obtained by reasonably designing components and matching corresponding parameters of hot rolling, acid rolling and continuous annealing processes, and the ultrahigh-strength dual-phase steel produced by the method has the hardness reduction value of a softening zone of a joint below 20Hv during laser welding and the softening degree obviously lower than other products with the same strength grade.
Description
Technical Field
The invention belongs to the technical field of metallurgy, and particularly relates to ultrahigh-strength dual-phase steel capable of reducing softening degree of a laser welding head and a production method thereof.
Background
With the increasing requirements on automobile safety and the reduction of fuel consumption, more and more high-strength steels are widely applied to automobile bodies, particularly in the process of automobile body collision, in order to reduce the invasion amount to passenger cabins to achieve the purpose of protecting passengers, more and more ultrahigh-strength steels, particularly steels with tensile strength of 780MPa or above are more and more widely applied, particularly series dual-phase steels, and the steel matrix is composed of ferrite and martensite and has good strength and plastic combination.
However, in the case of welding of the ultra-high strength dual phase steel, martensite decomposition in the matrix generates a softened heat affected zone due to the welding heat cycle, softening of the joint causes a reduction in the tensile properties, formability, fatigue properties and other service properties of the welded member, and the degree of softening after welding increases as the strength level increases, and in general, in the case of the ferrite-martensite dual phase steel having a tensile strength of 780MPa or more, the hardness of the softened zone decreases to 30Hv or more, that is, the softening ratio (softening zone hardness/base material hardness, base material hardness of 280Hv) is about 10%, and the softening ratio increases as the material strength level increases, for example, the softening ratio in the DP980 laser welding is in the range of 15 to 20%. Likewise, the tensile strength and formability of the sheet or member containing the weld may be reduced accordingly, and failure occurs in the softened region. Taking fatigue performance as an example, when the fatigue strength of the DP980 base material is 365MPa, the fatigue strength of the weld with laser welding is 268MPa, which is reduced by 27%, and the greater the softening degree, the greater the reduction degree.
Therefore, controlling the degree of softening is an urgent problem to be solved by this steel grade. In laser welding, the parent material undergoes a rapid heating and cooling thermal cycle, and the martensite in the ferrite-martensite dual-phase steel is unstable even at room temperature from the thermodynamic viewpoint, so that weld softening is inevitable. According to the invention, through component design and parameter matching of related procedures, the driving force formed by softening is reduced and the softening caused by martensite tempering is counteracted through precipitation strengthening, so that the overall performance of the joint is improved.
Disclosure of Invention
The invention aims to provide the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head and the production method thereof. The ultrahigh-strength dual-phase steel produced by the method has high strength and high toughness, and simultaneously, under the condition of laser welding, after the parent metal undergoes welding heat circulation, the hardness reduction value of a softening region is below 20Hv, and the softening rate is controlled below 5.8%.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows: the ultrahigh-strength dual-phase steel capable of reducing the softening degree of a laser welding head comprises the following chemical components in percentage by mass: c: 0.07-0.20%, Mn is less than or equal to 3.0%, Si is less than or equal to 2.0%, S is less than or equal to 0.020%, P is less than or equal to 0.020%, Als: 0.050-1.0%, Mo less than or equal to 0.50%, Cr: 0.50-1.0 percent, Nb + Ti + V less than or equal to 0.50 percent, N less than or equal to 0.010 percent, and the balance of Fe and inevitable impurities.
The invention has the following chemical component design ideas:
c: carbon is a solid solution strengthening element, is a guarantee for obtaining the martensite content of the material, ensures the strength of the steel, and needs to be added by at least 0.07 percent, but the hardenability of the welding seam is increased along with the increase of the carbon content, so that cracks are easy to appear in the welding seam, and therefore, the content is controlled below 0.20 percent.
Mn: manganese is an element which strongly improves the hardenability of austenite, and austenite containing a proper amount of Mn can obtain a desired structure through different rapid cooling termination temperatures, so that products with different properties can be obtained.
Si: the solid solution strengthening element can improve the strength of the material on one hand, and can accelerate the segregation of carbon to austenite and purify ferrite on the other hand, thereby improving the performance of the finished product.
Cr: on one hand, the steel plays a strengthening role, and on the other hand, the steel is mainly used for delaying the transformation of bainite in a medium-temperature region, thereby being beneficial to obtaining a martensite structure.
And Als: as an effective deoxidizer, it is usually desirable to add 0.05% or more of Al as an deoxidizer, and Al can form precipitates or change the transformation temperature to obtain a fine uniform structure.
S, P and N are impurity elements, and the lower the content, the better.
Mo: the hardenability is improved, meanwhile, in the hot rolling process, the recrystallization temperature is improved, the structure is refined, the banded structure is lightened, meanwhile, the Mo in the matrix can delay the carbon precipitation degree in martensite in a heat affected zone under the welding heat cycle in the welding process, and meanwhile, fine precipitates can be formed with carbon/nitrogen, and the control is below 0.50 percent in consideration of the cost, the effectiveness and the production and processing difficulty.
Nb, V and Ti: the carbide forming element can strengthen the matrix through precipitation on one hand, and can play a role in refining grain size and improving strength on the other hand.
The structural structure ferrite and martensite of the ultrahigh-strength dual-phase steel are more than or equal to 90 percent.
The specification of the ultra-high strength dual-phase steel is 1.0-2.0 mm.
The performance of the ultrahigh-strength dual-phase steel is as follows: yield strength: 420-900MPa, tensile strength: 780-1200MPa, elongation after break A80: 12.5 to 22 percent.
The performance of the ultrahigh-strength dual-phase steel is as follows: hardness of base material: 264-348Hv, hardness of softening zone: 250-328Hv, softening value Δ Hv: 9-20Hv and a softening rate of 3.3-5.8%.
The invention also provides a production method of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head, wherein the production method comprises the working procedures of hot rolling, cold rolling and continuous annealing; in the hot rolling procedure, the slab heating temperature is 1250-.
In the cold rolling process, the cold rolling reduction is 30-60%, and the thickness of the cold rolled plate is 1.0-2.0 mm.
The continuous annealing process has soaking temperature of 750-850 deg.c and soaking time of 60-90 s.
The continuous annealing process has the rapid cooling starting temperature of 650-700 ℃, the overaging temperature of 260-280 ℃ and the leveling elongation of 0.25-0.40%.
In the chemical composition design of the dual-phase steel, the added alloy elements such as Mo, Cr, Nb/V/Ti and the like exist in the following two forms in the hot rolling and annealing processes: (1) precipitating fine alloy carbon/nitride in the matrix; (2) exists in the matrix in a solid solution form, and the precipitated fine alloy carbon/nitride and the alloy elements in the solid solution form are inherited to the final finished structure.
In the laser welding process, when the welding heat cycle temperature is higher than a critical temperature Ac1 (the temperature of austenite formation is started in the heating process of a material to be welded), austenite is formed, the austenite is converted into finer martensite in the post-welding cooling process, the hardness of the region does not decrease and reversely increases, and the welding heat cycle temperature is lower than the Ac1 temperature, under the action of the welding heat cycle, the martensite in the parent metal does not undergo austenite transformation, only tempering of the martensite is carried out, namely, solid-solution carbon in the martensite is precipitated in the form of carbide, the solid-solution strengthening effect of the carbon in the martensite is reduced, namely, the joint softening is shown (generally characterized by hardness, namely, the region with the hardness lower than that of the parent metal in a heat affected zone), while the invention designs and matches with proper hot rolling, cold rolling and annealing process parameters through specific alloy components, so that part of the alloy elements can be dissolved in the martensite of a finished product, under the action of welding thermal cycle, on one hand, the alloy elements can drag carbon atoms to diffuse, namely, the precipitation power of carbon nitride controlled by carbon diffusion is reduced, so that the original solid solution strengthening state of the carbon atoms is maintained to the maximum degree, and the softening is reduced; on the other hand, the precipitated carbon precipitates fine carbon/nitride in a coherent manner with alloying elements such as Mo, Cr, Nb/V/Ti, and the like, and also has a more significant strengthening effect on the martensite matrix. In summary, on the one hand by reducing the temper softening rate of the martensite itself; on the other hand, fine alloy carbonitride is precipitated in a coherent manner to play a strengthening role, so that the softening degree of the welded joint is remarkably reduced.
In the hot rolling process, on one hand, the size target is realized, namely, a casting blank is rolled into a hot rolled plate strip with the size specification required by cold rolling; on the other hand, in the hot rolling process, alloy elements such as Mo, Cr, Nb/V/Ti and the like drag austenite grain boundaries through solid solution, refine austenite, hot rolled strip grains and homogenize a matrix structure, so that an ideal structure is provided for finally obtaining a high-strength cold rolled finished product. And the acid rolling process is to roll the hot rolled plate strip into the required finished product specification and simultaneously store a large amount of deformation energy as the driving force for annealing the cold hard coil. A continuous annealing step of obtaining a target structure and properties by heat treatment in the continuous annealing step; the plate shape is improved, and the internal stress is reduced; by rapid cooling after heating, an excessive amount of carbon and alloying elements are dissolved in the martensite.
The welding thermal cycle condition of the invention is referred to as follows: in the welding process, relevant technical parameters such as laser power and/or welding speed and the like are adjusted by adopting Nd, CO2 or optical fibers and the like on the premise of realizing complete penetration of the welded material and qualified weld joint forming, and the technical effects of the invention are not influenced.
Adopt the produced beneficial effect of above-mentioned technical scheme to lie in: the ultrahigh-strength dual-phase steel produced by the method has high strength and high toughness, the yield strength is 420-900MPa, the tensile strength is 780-1200MPa, and the elongation after fracture is as follows: 12.5-22%, and under the condition of laser welding, the hardness reduction value of a softening area of the base material after the base material is subjected to welding heat circulation is below 20Hv, and the softening rate is lower than 5.8%.
Detailed Description
The present invention will be described in further detail with reference to specific examples.
Example 1
The specification of the ultra-high strength dual-phase steel for reducing the softening degree of the laser welding head is 1.2mm, and the chemical component composition and the mass percentage content thereof are shown in table 1.
The production method of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head comprises the cold rolling and continuous annealing processes, and the specific process steps are as follows:
(1) a hot rolling procedure: the heating temperature of the plate blank is 1250 ℃, the furnace time of the plate blank is 194min, the start rolling temperature of finish rolling is 1067 ℃, the finish rolling temperature is 866 ℃, and the coiling temperature is 632 ℃.
(2) A cold rolling procedure: the cold rolling reduction rate is 45 percent;
(3) a continuous annealing process: the soaking temperature is 825 deg.C, soaking time is 64s, rapid cooling start temperature is 685 deg.C, overaging temperature is 265 deg.C, and leveling elongation is 0.28%.
The structural structure, performance and welding performance indexes of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head are shown in table 2.
Example 2
The specification of the ultra-high strength dual phase steel for reducing the softening degree of the laser welding head is 2.0mm, and the chemical component composition and the mass percentage content thereof are shown in table 1.
The production method of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head comprises the cold rolling and continuous annealing processes, and the specific process steps are as follows:
(1) a hot rolling procedure: the heating temperature of the plate blank is 1266 ℃, the furnace time of the plate blank is 190min, the starting temperature of finish rolling is 1090 ℃, the finishing temperature is 857 ℃, and the coiling temperature is 580 ℃.
(2) A cold rolling procedure: the cold rolling reduction rate is 30 percent;
(3) a continuous annealing process: the soaking temperature is 750 ℃, the soaking time is 90s, the rapid cooling starting temperature is 668 ℃, the overaging temperature is 274 ℃ and the flattening elongation is 0.35%.
The structural structure, performance and welding performance indexes of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head are shown in the table 2.
Example 3
The specification of the ultra-high strength dual phase steel for reducing the softening degree of the laser welding head is 1.5mm, and the chemical component composition and the mass percentage content thereof are shown in table 1.
The production method of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head comprises the cold rolling and continuous annealing processes, and the specific process steps are as follows:
(1) a hot rolling procedure: the heating temperature of the plate blank is 1255 ℃, the furnace time of the plate blank is 208min, the start rolling temperature of finish rolling is 1050 ℃, the finish rolling temperature is 864 ℃, and the coiling temperature is 650 ℃.
(2) A cold rolling procedure: the cold rolling reduction rate is 55 percent;
(3) a continuous annealing process: the soaking temperature is 810 ℃, the soaking time is 75s, the rapid cooling starting temperature is 700 ℃, the overaging temperature is 260 ℃, and the flattening elongation is 0.33%.
The structural structure, performance and welding performance indexes of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head are shown in the table 2.
Example 4
The specification of the ultra-high strength dual phase steel for reducing the softening degree of the laser welding head is 1.2mm, and the chemical composition and the mass percentage content thereof are shown in table 1.
The production method of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head comprises the cold rolling and continuous annealing processes, and the specific process steps are as follows:
(1) a hot rolling procedure: the heating temperature of the plate blank is 1275 ℃, the furnace time of the plate blank is 230min, the start rolling temperature of finish rolling is 1084 ℃, the finish rolling temperature is 840 ℃, and the coiling temperature is 597 ℃.
(2) A cold rolling procedure: the cold rolling reduction rate is 60 percent;
(3) a continuous annealing process: the soaking temperature is 765 ℃, the soaking time is 64s, the rapid cooling starting temperature is 660 ℃, the overaging temperature is 266 ℃, and the leveling elongation is 0.25%.
The structural structure, performance and welding performance indexes of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head are shown in table 2.
Example 5
The specification of the ultra-high strength dual phase steel for reducing the softening degree of the laser welding head is 1.45mm, and the chemical component composition and the mass percentage content thereof are shown in table 1.
The production method of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head comprises the cold rolling and continuous annealing processes, and the specific process steps are as follows:
(1) a hot rolling procedure: the heating temperature of the plate blank is 1296 ℃, the furnace time of the plate blank is 225min, the start rolling temperature of finish rolling is 1072 ℃, the finish rolling temperature is 872 ℃, and the coiling temperature is 642 ℃.
(2) A cold rolling procedure: the cold rolling reduction rate is 35 percent;
(3) a continuous annealing process: the soaking temperature is 850 ℃, the soaking time is 71s, the rapid cooling starting temperature is 680 ℃, the overaging temperature is 272 ℃, and the flattening elongation is 0.25%.
The structural structure, performance and welding performance indexes of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head are shown in table 2.
Example 6
The specification of the ultra-high strength dual phase steel for reducing the softening degree of the laser welding head is 1.6mm, and the chemical component composition and the mass percentage content thereof are shown in table 1.
The production method of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head comprises the cold rolling and continuous annealing processes, and the specific process steps are as follows:
(1) a hot rolling procedure: the heating temperature of the plate blank is 1268 ℃, the furnace time of the plate blank is 216min, the start rolling temperature of finish rolling is 1086 ℃, the finish rolling temperature is 865 ℃, and the coiling temperature is 633 ℃.
(2) A cold rolling procedure: the cold rolling reduction rate is 40 percent;
(3) a continuous annealing process: the soaking temperature is 775 ℃, the soaking time is 78s, the rapid cooling starting temperature is 650 ℃, the overaging temperature is 280 ℃, and the flattening elongation is 0.40%.
The structural structure, performance and welding performance indexes of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head are shown in table 2.
Example 7
The specification of the ultra-high strength dual-phase steel for reducing the softening degree of the laser welding head is 1.8mm, and the chemical component composition and the mass percentage content thereof are shown in table 1.
The production method of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head comprises the cold rolling and continuous annealing processes, and the specific process steps are as follows:
(1) a hot rolling procedure: the heating temperature of the plate blank is 1274 ℃, the furnace time of the plate blank is 207min, the rolling starting temperature of finish rolling is 1055 ℃, the finishing temperature is 847 ℃, and the coiling temperature is 627 ℃.
(2) A cold rolling procedure: the cold rolling reduction rate is 55 percent;
(3) a continuous annealing process: the soaking temperature is 800 ℃, the soaking time is 82s, the rapid cooling starting temperature is 675 ℃, the overaging temperature is 268 ℃, and the flattening elongation is 0.38%.
The structural structure, performance and welding performance indexes of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head are shown in table 2.
Example 8
The specification of the ultra-high strength dual phase steel for reducing the softening degree of the laser welding head is 1.0mm, and the chemical component composition and the mass percentage content thereof are shown in table 1.
The production method of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head comprises the steps of cold rolling and continuous annealing, and the specific process steps are as follows:
(1) a hot rolling procedure: the heating temperature of the plate blank is 1300 ℃, the furnace time of the plate blank is 196min, the start rolling temperature of finish rolling is 1067 ℃, the finish rolling temperature is 880 ℃, and the coiling temperature is 615 ℃.
(2) A cold rolling procedure: the cold rolling reduction rate is 45 percent;
(3) a continuous annealing process: the soaking temperature is 805 ℃, the soaking time is 60s, the rapid cooling starting temperature is 665 ℃, the overaging temperature is 275 ℃, and the flattening elongation is 0.32%.
The structural structure, performance and welding performance indexes of the ultrahigh-strength dual-phase steel for reducing the softening degree of the laser welding head are shown in table 2.
Table 1 examples 1-8 chemical composition and mass% of ultra high strength dual phase steel (%)
The balance of the ingredients in table 1 is Fe and unavoidable impurities.
TABLE 2 organization structure, Properties and weldability of the ultra-high-strength dual-phase steels of examples 1 to 8
Note: softening rate: (softening value/base material hardness) × 100%; softening value = parent material hardness-softening zone hardness minimum.
Although the present invention has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that: modifications and equivalents may be made thereto without departing from the spirit and scope of the invention and it is intended to cover in the claims the invention any modifications and equivalents.
Claims (7)
1. The ultrahigh-strength dual-phase steel capable of reducing the softening degree of a laser welding head is characterized by comprising the following chemical components in percentage by mass: c: 0.07-0.20%, Mn is less than or equal to 3.0%, Si is less than or equal to 2.0%, S is less than or equal to 0.020%, P is less than or equal to 0.020%, Als: 0.050-1.0%, Mo less than or equal to 0.50%, Cr: 0.50-1.0%, Nb + Ti + V less than or equal to 0.50%, N less than or equal to 0.010%, and the balance of Fe and inevitable impurities;
the specification of the ultra-high strength dual-phase steel is 1.0-2.0 mm;
the performance of the ultrahigh-strength dual-phase steel is as follows: hardness of base material: 264-: 250-328HV, softening value Δ HV: 9 to 20HV and a softening rate of 3.3 to 5.8 percent.
2. The ultra-high strength dual phase steel for reducing the softening degree of a laser welding head as claimed in claim 1, wherein the structure of ferrite and martensite of the ultra-high strength dual phase steel is more than or equal to 90%.
3. The ultra-high strength dual phase steel with reduced laser welding head softening degree according to claim 1 or 2, wherein the ultra-high strength dual phase steel has properties of: yield strength: 420-900MPa, tensile strength: 780-1200MPa, elongation after break A80: 12.5 to 22 percent.
4. A method for producing an ultra high strength dual phase steel with reduced laser welding head softening degree according to any one of claims 1 to 3, wherein the method comprises the steps of hot rolling, cold rolling and continuous annealing; in the hot rolling procedure, the slab heating temperature is 1250-.
5. The method for producing the ultrahigh-strength dual-phase steel with the reduced softening degree of the laser welding head as claimed in claim 4, wherein the cold rolling step is performed with a cold rolling reduction of 30 to 60% and a cold rolled sheet thickness of 1.0 to 2.0 mm.
6. The method for producing the ultrahigh-strength dual-phase steel with the reduced softening degree of the laser welding head as claimed in claim 4 or 5, wherein the continuous annealing process has a soaking temperature of 750-.
7. The method for producing the dual phase steel with ultra high strength and reduced softening degree of the laser welding head as claimed in claim 4 or 5, wherein the continuous annealing process comprises the rapid cooling starting temperature of 650-700 ℃, the overaging temperature of 260-280 ℃ and the temper elongation of 0.25-0.40%.
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