阅读说明：本技术 具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带及其制造方法 (Ultra-high strength hot rolled steel sheet and steel strip having good fatigue and hole expansion properties, and method for manufacturing same ) 是由 张瀚龙 张玉龙 王利 于 2018-06-27 设计创作，主要内容包括：具有良好的疲劳及扩孔性能的超高强热轧钢板和钢带及其制造方法,该钢板和钢带成分重量百分比为：C：0.07～0.14％,Si：0.1～0.4％,Mn：1.55～2.00％,P≤0.015％,S≤0.004％,Al：0.01～0.05％,N≤0.005％,Cr：0.15～0.50％,V：0.1～0.35％,Nb：0.01％～0.06％,Mo：0.15～0.50％,且Ti≤0.02％,其余为Fe和不可避免的杂质；且同时需满足：1.0≤[(Cr/52)/(C/4)+(Nb/93+Ti/48+V/51+Mo/96)/(C/12)]≤1.6。本发明超高强热轧钢板和钢带的抗拉强度≥780MPa、屈服强度≥660MPa,拉伸疲劳极限(循环1000万次)FL≥570MPa,或疲劳极限比抗拉强度FL/Rm≥0.72,扩孔率满足：若原始孔为冲压孔：扩孔率＞85％；若原始孔为铰孔：扩孔率＞120％。(The ultrahigh-strength hot-rolled steel plate and the steel strip with good fatigue and hole expanding performance and the manufacturing method thereof are disclosed, wherein the steel plate and the steel strip comprise the following components in percentage by weight: c: 0.07 to 0.14%, Si: 0.1-0.4%, Mn: 1.55-2.00%, P is less than or equal to 0.015%, S is less than or equal to 0.004%, Al: 0.01-0.05%, N is less than or equal to 0.005%, Cr: 0.15-0.50%, V: 0.1 to 0.35%, Nb: 0.01% -0.06%, Mo: 0.15-0.50%, Ti is less than or equal to 0.02%, and the balance is Fe and inevitable impurities; and simultaneously the following requirements are met: 1.0 to less than or equal to [ (Cr/52)/(C/4) + (Nb/93+ Ti/48+ V/51+ Mo/96)/(C/12) ] < 1.6. The tensile strength of the ultrahigh-strength hot-rolled steel plate and the steel strip is not less than 780MPa, the yield strength is not less than 660MPa, the tensile fatigue limit (circulating for 1000 ten thousand times) FL is not less than 570MPa, or the fatigue limit specific tensile strength FL/Rm is not less than 0.72, and the hole expansion ratio meets the following requirements: if the original hole is a punched hole: the hole expanding rate is more than 85 percent; if the original hole is a reamed hole: the hole expanding rate is more than 120 percent.)

1. The ultrahigh-strength hot-rolled steel plate and the steel strip with good fatigue and hole expanding performance comprise the following components in percentage by weight: c: 0.07 to 0.14%, Si: 0.1-0.4%, Mn: 1.55-2.00%, P is less than or equal to 0.015%, S is less than or equal to 0.004%, Al: 0.01-0.05%, N is less than or equal to 0.005%, Cr: 0.15-0.50%, V: 0.1 to 0.35%, Nb: 0.01% -0.06%, Mo: 0.15-0.50%, Ti is less than or equal to 0.02%, and the balance is Fe and inevitable impurities; and the above elements simultaneously need to satisfy the following relations: 1.0 to less than or equal to [ (Cr/52)/(C/4) + (Nb/93+ Ti/48+ V/51+ Mo/96)/(C/12) ] < 1.6.

2. The ultra high strength hot rolled steel sheet and the steel strip having excellent fatigue and hole expansibility as recited in claim 1, wherein the chemical composition of the ultra high strength hot rolled steel sheet and the steel strip is C: 0.07-0.09% by weight.

3. The ultra high strength hot rolled steel sheet and the steel strip having excellent fatigue and hole expansibility as claimed in claim 1, wherein the chemical composition of the ultra high strength hot rolled steel sheet and the steel strip is Si: 0.1 to 0.3 percent by weight.

4. The ultra high strength hot rolled steel sheet and the steel strip having good fatigue and hole expansibility as claimed in claim 1, wherein the chemical composition of the ultra high strength hot rolled steel sheet and the steel strip is Mn: 1.70-1.90% by weight.

5. The ultra high strength hot rolled steel sheet and the steel strip having excellent fatigue and hole expansibility as recited in claim 1, wherein the chemical composition of the ultra high strength hot rolled steel sheet and the steel strip is, among Cr: 0.35-0.50% by weight.

6. The ultra high strength hot rolled steel sheet and the steel strip having good fatigue and hole expansibility as claimed in claim 1, wherein the chemical composition of the ultra high strength hot rolled steel sheet and the steel strip is V: 0.12-0.22% by weight.

7. The ultra high strength hot rolled steel sheet and the steel strip having excellent fatigue and hole expansibility as recited in claim 1, wherein the chemical composition of Mo: 0.15 to 0.3 percent by weight.

8. The ultra high strength hot rolled steel sheet and the steel strip having excellent fatigue and hole expansibility as recited in claim 1, wherein Ti is 0.005% or less in chemical composition by weight.

9. The ultra high strength hot rolled steel sheet and the steel strip having excellent fatigue and hole expansibility as claimed in claim 1, wherein the chemical composition of the ultra high strength hot rolled steel sheet and the steel strip is 0.003% or less Ti and 0.003% or less N, in weight%.

10. The ultra high strength hot rolled steel sheet and the steel strip having excellent fatigue and hole expansibility as recited in any one of claims 1 to 9, wherein the microstructure of the ultra high strength hot rolled steel sheet and the steel strip is a bainite microstructure mainly including lower bainite.

11. The ultra high strength hot rolled steel sheet and steel strip having excellent fatigue and hole expansibility as claimed in any one of claims 1 to 10, wherein the tensile strength of the ultra high strength hot rolled steel sheet and steel strip is 780MPa or more, the yield strength is 660MPa or more, and the hole expansibility performance index is: if the original hole is a punched hole: the hole expanding rate is more than 85 percent; if the original hole is a reamed hole: the hole expanding rate is more than 120 percent; the fatigue resistance performance index is as follows: the high-frequency fatigue limit (1000 ten thousand cycles) FL is more than or equal to 570MPa, or the fatigue limit specific tensile strength FL/Rm is more than or equal to 0.72.

12. The ultra high strength hot rolled steel sheet and the steel strip having excellent fatigue and hole expansibility as claimed in claim 1, 8 or 10, wherein the tensile strength of the ultra high strength hot rolled steel sheet and the steel strip is 780MPa or more, the yield strength is 660MPa or more, and the hole expansibility performance index is: if the original hole is a punched hole: the hole expanding rate is more than 85 percent; if the original hole is a reamed hole: the hole expanding rate is more than 120 percent; the fatigue resistance performance index is as follows: the high-frequency fatigue limit (1000 ten thousand cycles) FL is more than or equal to 600MPa, or the fatigue limit specific tensile strength FL/Rm is more than or equal to 0.75.

13. The ultra high strength hot rolled steel sheet and the steel strip having good fatigue and hole expansibility as recited in any one of claims 1, 9 and 10, wherein the ultra high strength hot rolled steel sheet and the steel strip have fatigue resistance performance indexes of: the high-frequency fatigue limit (1000 ten thousand cycles) FL is more than or equal to 640MPa, or the fatigue limit specific tensile strength FL/Rm is more than or equal to 0.8.

14. The method for producing an ultra high strength hot rolled steel sheet and steel strip having good fatigue and hole expansibility according to any one of claims 1 to 13, comprising the steps of:

1) smelting and casting

Smelting and casting of chemical components according to any one of claims 1 to 9;

2) rolling of

The heating temperature is 1100-1250 ℃; the start rolling temperature of finish rolling is 950-1000 ℃, and the finish rolling temperature is 900-950 ℃;

3) cooling and coiling

The cooling speed is more than or equal to 30 ℃/s, and the coiling temperature is 450-580 ℃;

4) and (6) acid washing.

15. The method for manufacturing an ultra high strength hot rolled steel sheet and steel strip having excellent fatigue and hole expansibility as claimed in claim 14, further comprising the steps of holding the temperature and slowly cooling after cooling and coiling after the rolling in step 3): and keeping the temperature above 450 ℃ for 2-4 h.

Technical Field

The invention belongs to the field of metal materials, and particularly relates to an ultrahigh-strength hot-rolled steel plate and a steel strip with good fatigue and hole expanding performance and a manufacturing method thereof, which are mainly applied to manufacturing products such as automobile chassis, suspension parts and the like.

Background

The 'light weight' of the automobile can directly reduce the emission and the oil consumption, and is the target of the development of the automobile manufacturing industry at present. One important measure for the "lightweight" of automobiles is to use high-strength and ultra-high-strength steel plates instead of mild steel. After a large amount of high-strength steel is adopted, the weight reduction effect of 20-25% can be realized. In the past decade, the body-in-white structural member has been widely made of advanced high-strength steel with high strength and high elongation to realize light weight, and excellent energy-saving and emission-reducing effects are achieved. At present, the concept of light weight is further applied to automobile chassis and suspension systems, and increasingly severe environmental requirements and market requirements also require that the automobile chassis material adopts high-strength steel to realize light weight.

However, for structural members of automotive chassis and suspension systems, the forming process requires materials with high hole expansibility. In addition, the service characteristics of the chassis and the structural members of the suspension system also require the material to have higher fatigue performance. Although high-strength steel mainly based on bainite structure becomes a common steel grade of the current automobile chassis and suspension system parts due to high strength and good hole expansion performance, the design and manufacture of steel with high strength, good hole expansion performance and good fatigue performance are extremely difficult due to the complex components and structures of the bainite steel and the mutual restriction among the three properties of high strength, high hole expansion rate and high fatigue limit of the material.

Chinese patent CN102612569A discloses a high-strength hot-rolled steel plate, the tensile strength of which is greater than 780MPa, the bending fatigue limit ratio of 1000 ten thousand times is greater than 0.45, and the hole expanding rate (the original hole is punched hole) is 30-50%. Although the steel sheet has high strength and a certain bending fatigue limit, the hole expansibility is relatively low.

Chinese patent CN103108971A discloses a high-strength hot-rolled steel sheet with excellent fatigue resistance, wherein the tensile strength of the steel sheet is greater than 780MPa, and the 200 ten thousand times tensile fatigue limit is 0.66-0.78. However, the fatigue limit is only that of 200 ten thousand loads, and according to the common knowledge, the fatigue limit is inversely proportional to the number of cycles, so if the number of fatigue test loads of the material is further increased, the fatigue limit is further decreased, and the patent does not consider the hole-expanding performance of the material.

Chinese patent CN101906567A discloses a high-strength hot-rolled steel sheet with excellent hole expansion workability, the tensile strength of the steel sheet is greater than 780MPa, and the hole expansion ratio (original hole is punched hole) is 43-89%. Chinese patent document CN104136643A discloses a high-strength hot-rolled steel plate with tensile strength greater than 780MPa and hole expansion ratio (original hole is reamed) between 37 and 103 percent. However, neither of the above patents considers the fatigue properties of the material.

In the above four patents, Ti is an optional or essential beneficial element to improve the strength of the material or to inhibit the growth of prior austenite grains. However, Ti and N, which is a common impurity in steel, form square (or triangular) massive, brittle TiN particles with sharp corners at high temperature, which may have a harmful effect on the formability of steel such as bending and hole expansion, and may greatly reduce the fatigue limit of steel. These adverse effects caused by Ti element have not been considered in the prior art.

In addition, for the material (hereinafter referred to as the material) having tensile strength of 800MPa level, mainly bainite, and carbide precipitation as a reinforcing phase, the strength, fatigue limit, and hole expansibility are all properties that are mutually restricted. First, the strength of the material is generally inversely proportional to the hole expansion performance, and in order to obtain higher strength, particularly yield strength, precipitation strengthening effect of carbide is needed in this type of steel. However, the large precipitation and coarsening of carbides greatly impair the hole-enlarging property of the material. Further, generally, the higher the yield strength of a material, the higher the fatigue limit of the material; however, in this type of material, the improvement of yield strength greatly depends on the precipitation of a large amount of carbides, but the precipitation of a large amount of carbides greatly lowers the fatigue limit of this type of material, as in the case of coarsening. Therefore, the material has great design and manufacturing difficulties for obtaining the material with high strength, high hole expansibility and high fatigue limit at the same time.

Disclosure of Invention

The invention aims to provide an ultrahigh-strength hot-rolled steel plate and a steel strip with good fatigue and hole expansion performance and a manufacturing method thereof, wherein the tensile strength of the steel plate is more than or equal to 780MPa, the yield strength of the steel plate is more than or equal to 660MPa, the tensile fatigue limit (1000 ten thousand cycles) FL is more than or equal to 600MPa, or the fatigue limit specific tensile strength FL/Rm is more than or equal to 0.75, and the hole expansion ratio satisfies the following conditions: if the original hole is a punched hole: the hole expanding rate is more than 85 percent; if the original hole is a reamed hole: the hole expanding rate is more than 120 percent, and the method is mainly used for preparing automobile chassis and suspension system parts.

In order to achieve the purpose, the technical scheme of the invention is as follows:

an ultrahigh-strength hot-rolled steel plate and a steel strip with good fatigue and hole expanding performance comprise the following components in percentage by weight: c: 0.07 to 0.14%, Si: 0.1-0.4%, Mn: 1.55-2.00%, P is less than or equal to 0.015%, S is less than or equal to 0.004%, Al: 0.01-0.05%, N is less than or equal to 0.005%, Cr: 0.15-0.50%, V: 0.1 to 0.35%, Nb: 0.01% -0.06%, Mo: 0.15-0.50%, Ti is less than or equal to 0.02%, and the balance is Fe and inevitable impurities; and the above elements simultaneously need to satisfy the following relations: 1.0 to less than or equal to [ (Cr/52)/(C/4) + (Nb/93+ Ti/48+ V/51+ Mo/96)/(C/12) ] < 1.6.

Preferably, the chemical composition of the ultra-high-strength hot-rolled steel sheet and the steel strip is C: 0.07-0.09% by weight.

Preferably, the chemical composition of the ultra-high-strength hot-rolled steel sheet and the steel strip is Si: 0.1 to 0.3 percent by weight.

Preferably, the chemical composition of the ultra-high strength hot rolled steel sheet and the steel strip is Mn: 1.70-1.90% by weight.

Preferably, the chemical composition of the ultra-high-strength hot-rolled steel sheet and the steel strip is Cr: 0.35-0.50% by weight.

Preferably, the chemical composition of the ultra-high strength hot rolled steel sheet and the steel strip is V: 0.12-0.22% by weight.

Preferably, the chemical composition of the ultra-high strength hot rolled steel sheet and the steel strip is Mo: 0.15 to 0.3 percent by weight.

Preferably, Ti in the chemical compositions of the ultrahigh-strength hot-rolled steel plate and the steel strip is less than or equal to 0.005 percent in weight percentage.

More preferably, the chemical compositions of the ultrahigh-strength hot-rolled steel plate and the steel strip are less than or equal to 0.003 percent of Ti and less than or equal to 0.003 percent of N in percentage by weight.

Further, the tensile strength of the ultrahigh-strength hot-rolled steel plate and the steel strip is not less than 780MPa, the yield strength is not less than 660MPa, and the hole expansion ratio performance indexes are as follows: if the original hole is a punched hole: the hole expanding rate is more than 85 percent; if the original hole is a reamed hole: the hole expanding rate is more than 120 percent; the fatigue resistance performance index is as follows: the high-frequency fatigue limit (1000 ten thousand cycles) FL is more than or equal to 570MPa, or the fatigue limit specific tensile strength FL/Rm is more than or equal to 0.72.

More preferably, the fatigue resistance indexes of the ultra-high strength hot rolled steel sheet and the steel strip are as follows: the high-frequency fatigue limit (1000 ten thousand cycles) FL is more than or equal to 600MPa, or the fatigue limit specific tensile strength FL/Rm is more than or equal to 0.75.

Preferably, the fatigue resistance indexes of the ultra-high strength hot rolled steel sheet and the steel strip are as follows: the high-frequency fatigue limit (1000 ten thousand cycles) FL is more than or equal to 640MPa, or the fatigue limit specific tensile strength FL/Rm is more than or equal to 0.8.

The microstructure of the ultrahigh-strength hot-rolled steel plate and the steel strip is a bainite microstructure mainly comprising lower bainite.

In the composition design of the steel of the invention:

carbon (C): carbon has a great influence on the strength, formability and weldability of the steel sheet. Carbon and other alloying elements form alloy carbide to improve the strength of the steel plate, and if the carbon content is lower than 0.07 percent, the strength of the steel cannot meet the target requirement; the carbon content is higher than 0.14%, so that a martensite structure and coarse cementite are easily generated, and the elongation and the hole expansion rate are reduced, so that the carbon content is controlled to be 0.07-0.14%.

Silicon (Si): silicon is an essential element for steel-making deoxidation, has a certain solid-solution strengthening effect, and when the content is less than 0.1%, a sufficient deoxidation effect is difficult to obtain; when the silicon content is more than 0.5%, a polygonal ferrite structure is easily generated, which is disadvantageous in improving the hole expansibility and also in deteriorating the platability, which is disadvantageous in producing a hot-dip galvanized steel sheet. Therefore, the present invention limits the silicon content to 0.1 to 0.4%.

Manganese (Mn): manganese is an effective element for improving the strength and has low cost, so the invention takes manganese as a main additive element. However, when the manganese content is higher than 2.00%, a large amount of martensite is generated, which is unfavorable for the hole expansion performance; when the manganese content is less than 1.55%, the strength of the steel sheet is insufficient. Therefore, the manganese content is limited to 1.55-2.00%.

Aluminum (Al): aluminum has a deoxidizing effect during steel making and is an element added to improve the purity of molten steel. Aluminum can also fix nitrogen in steel to form a stable compound, so that grains are effectively refined, but when the aluminum content is less than 0.01%, the effect is small: when the aluminum content exceeds 0.05%, the deoxidation effect is saturated, and when the content is higher, the base material and the welding heat affected zone are adversely affected. Therefore, the aluminum content is limited to 0.01 to 0.05% in the present invention.

Niobium (Nb): niobium can effectively delay the recrystallization of deformed austenite, prevent the growth of austenite grains, improve the recrystallization temperature of austenite, refine grains and simultaneously improve the strength and the elongation. However, since the niobium content is higher than 0.06%, the cost increases and the effect is not significant any more, the niobium content is limited to 0.06% or less in the present invention.

Vanadium (V) has the functions of forming carbide precipitation and solid solution strengthening to improve the strength of steel, but the effect of increasing the content of vanadium is not obvious after the content of vanadium is more than 0.35 percent, and the effect of precipitation strengthening is not obvious when the content of V is less than 0.10 percent. Therefore, the vanadium content is limited to 0.1 to 0.35% in the present invention.

Chromium and molybdenum (Cr, Mo): chromium and molybdenum increase the incubation period of pearlite and ferrite in the CCT curve, inhibit the formation of ferrite of pearlite, make bainite structure easily obtained during cooling, and are beneficial to improving the hole expansion rate. Meanwhile, chromium and molybdenum are beneficial to refinement of austenite grains and generation of fine bainite during rolling, the strength of the steel is improved through solid solution strengthening and carbide precipitation, and when the addition amount exceeds 0.5%, the cost is improved, and the weldability is obviously reduced. When the content of Cr and Mo is less than 0.15%, the influence on the CCT curve is not significant. Therefore, the content of chromium and molybdenum is limited to 0.15-0.5%.

In addition, the metering relation between the alloy elements and the carbon elements also satisfies the following formula: 1.0 ≦ [ (Cr/52)/(C/4) + (Nb/93+ Ti/48+ V/51+ Mo/96)/(C/12) ] ≦ 1.6: the addition of the alloy elements can improve the strength of the material through a solid solution strengthening effect and a carbide precipitation effect. However, the carbide precipitation effect has a greater negative impact on the hole expansion performance and fatigue limit than solid solution strengthening. The more the alloying elements are, the more easily they are bonded to a large amount of carbon in the steel to form a coarse carbide precipitate phase. Therefore, the ratio of the alloy element to the carbon element needs to reach the range set by the formula so as to ensure that the material can obtain the strength and the hole expansion performance which reach the design standard at the same time.

Titanium (Ti): titanium belongs to a harmful element for reducing the fatigue limit, although the addition of the Ti element can improve the strength of the steel, massive and brittle TiN particles with sharp corners can be produced to become a potential fatigue crack source, so that the fatigue performance of the steel is greatly reduced, and the higher the content of the Ti element is, the larger the size of the formed TiN particles is, the more serious the adverse effect on the fatigue performance is. In addition, addition of a large amount of Ti causes a large amount of coarse TiC to be precipitated, and the hole-expanding performance is impaired. Therefore, a strict upper limit of the Ti element content is required. In the case where Ti is not additionally added, Ti is required to be 0.02% or less, preferably 0.005% or less.

The upper limit of the impurity elements in the steel is controlled to be P: less than or equal to 0.015 percent, S: less than or equal to 0.004%, N: less than or equal to 0.005 percent, and the purer the steel quality, the better the effect. Furthermore, in order to obtain the highest fatigue limit, the content of N element is required to be less than or equal to 0.003 percent when the content of Ti element is ensured to be less than 0.003 percent.

The invention relates to a manufacturing method of an ultrahigh-strength hot-rolled steel plate and a steel strip with good fatigue and hole expanding performance, which comprises the following steps:

1) smelting and casting

Smelting and casting into a casting blank according to the chemical components;

2) rolling of

Heating the casting blank at 1100-1250 ℃; the start rolling temperature of finish rolling is 950-1000 ℃, and the finish rolling temperature is 900-950 ℃;

3) cooling and coiling

Cooling the rolled blank with water at a cooling speed of more than or equal to 30 ℃/s and at a coiling temperature of 450-580 ℃;

4) and (6) acid washing.

Further, cooling and coiling in the step 3), then carrying out heat preservation and slow cooling, and then carrying out acid washing, wherein in the heat preservation and slow cooling step, the temperature is controlled to be higher than 450 ℃ for 2-4 h. The heat preservation slow cooling can be used for placing the hot-rolled coil in a non-heating type heat preservation device to preserve heat for 2-4 hours at the temperature of more than 450 ℃.

In the step 2), the heating temperature of the slab influences the austenite grain size. When manufacturing the ultra-high strength complex phase steel, the added alloy elements such as V and Nb form carbides to improve the strength of the steel sheet. When the plate blank is heated, the alloy elements need to be dissolved into austenite to form complete solid solution, and fine carbide or nitride can be formed in the subsequent cooling process to play a role in strengthening, so that the heating temperature of the plate blank is limited to 1100-1250 ℃.

In the step 2), when the finish rolling temperature is not less than 900 ℃, a fine and uniform structure can be obtained, and when the finish rolling temperature is less than 900 ℃, a banded structure formed in hot working is retained, which is unfavorable for improving the hole expanding performance, so that the finish rolling temperature is limited to not less than 900 ℃. The upper limit of the finish rolling temperature is not particularly specified in general, but the finish rolling temperature does not exceed 950 ℃ in view of the slab heating temperature.

In the above step 3), the cooling rate is defined to be not less than 30 ℃/s in order to prevent transformation of supercooled austenite into polygonal ferrite or pearlite and precipitation of carbide at higher temperatures, resulting in a microstructure mainly composed of lower bainite.

In the step 3), the coiling temperature is one of the most critical process parameters for obtaining high strength, high hole expansion rate and high fatigue limit. When the coiling temperature is higher than 580 ℃, the strength of ferrite is reduced due to strong precipitation and coarsening of alloy carbides, which has a negative effect on both the hole expansibility and the fatigue limit of the steel sheet, and on the other hand, when the coiling temperature is lower than 450 ℃, a large amount of martensite structures are formed, which can enhance the strength of the material, but adversely affect the hole expansibility, thus resulting in a problem of poor workability. The coiling temperature is limited to 450-580 ℃.

Further, the tensile strength of the steel grade can be further improved by a hot rolling heat preservation method, and the method comprises the following specific steps: after curling, the hot coil is placed in a heat preservation pit, heat preservation and slow cooling are carried out by utilizing the heat of the hot coil, and the heat preservation is carried out for 2-4 hours at the temperature of more than 450 ℃, so that the fine dispersion precipitation of vanadium carbide can be promoted, the strength of the material is obviously improved, and the obvious reduction of the hole expansion rate and the fatigue limit can not be caused. In the hot-rolling heat-preserving process, the minimum heat-preserving temperature and the heat-preserving time have influence on the performance of the final product. If the heat preservation temperature is lower than 450 ℃, the precipitation power of vanadium (molybdenum) carbide is insufficient, and fine and dispersed vanadium (molybdenum) carbide cannot be precipitated. If the heat preservation time is less than 2h, the precipitation of vanadium (molybdenum) carbide is limited, and the strength of the steel can not be improved; and if the heat preservation time is longer than 4 hours, the vanadium (molybdenum) carbide grows and coarsens after being separated out, and the hole expanding rate and the fatigue limit of the steel can be obviously reduced.

The primary requirements of automobile chassis and suspension system parts on the material are high strength and high hole expansion performance, and in order to achieve strength above 780MPa and hole expansion performance above 60% (original holes are punched holes), steel grades of ferrite or ferrite plus bainite structures (wherein the content of the bainite structures is more than 50%) are generally adopted at present. Because the ferrite matrix is softer, more alloying elements are generally required to be added to form solid solution strengthening and fine alloy carbide strengthening ferrite matrix to obtain higher strength. In the prior art, Ti is used as a necessary or optional beneficial element for improving the strength of the steel grade, but the Ti and N in the steel can form massive and brittle TiN particles with sharp corners at high temperature, so that the hole expansion performance of the steel grade is not facilitated. In addition, with the higher and higher requirements of the chassis parts for the automobile on the fatigue performance of steel, the research of the invention proves that: massive, brittle, sharp-edged TiN particles are more likely to be a potential source of fatigue cracking and greatly reduce the fatigue limit of this type of steel. In addition, researches show that because TiN particles are generated in the steel-making and continuous casting (or die casting) processes, the size and the appearance of the TiN particles can hardly be changed by the subsequent processes, and the TiN particles can not be eliminated, so that the content of Ti element in the steel grade can be reduced as much as possible to obtain higher hole expansion performance and fatigue performance.

Therefore, the invention adopts the design idea of Ti-free components, strictly controls the Ti content in the steel, reduces the generation of TiN particles and obtains high fatigue limit; and the high-strength hot-rolled steel plate with high strength, high hole expansion rate and high fatigue limit is obtained by Mo-V compounding and optimization of the manufacturing process. The steel plate adopts a bainite microstructure with lower bainite as a main structure to ensure the strength and toughness of the steel plate, and the addition of alloy elements Cr and Mo enables a ferrite phase transformation area to move to the right, so that the critical cooling rate can be reduced, and the bainite structure is easy to obtain. In addition, Mo, V and Nb alloy elements are added to refine grains, so that dispersed fine carbides are generated, and the strength of the steel grade is further improved. However, the carbide precipitates too much and then becomes coarse, which is not only disadvantageous for further improvement of strength but also lowers the hole expansion performance and fatigue limit of the steel. Therefore, it is necessary to optimize the hot rolling process to obtain fine and dispersed alloy carbides, thereby achieving the purpose of improving the hole expanding performance.

Through detection, the performance of the ultrahigh-strength hot-rolled steel plate and the steel strip provided by the invention meets the following indexes:

normal temperature mechanical properties:

the tensile strength is more than or equal to 780 MPa; the yield strength is more than or equal to 660 MPa.

Hole expansibility performance:

if the original hole is a punched hole: the hole expansion rate is more than 85 percent;

if the original hole is a reamed hole: the hole expansion ratio is greater than 120%.

Fatigue resistance:

the high-frequency fatigue limit (1000 ten thousand cycles) FL is more than or equal to 570 MPa;

or the fatigue limit specific tensile strength FL/Rm is more than or equal to 0.72.

When Ti in the steel components is less than or equal to 0.005 percent, the fatigue resistance can meet the following indexes:

the high-frequency fatigue limit (1000 ten thousand cycles) FL is more than or equal to 600 MPa;

or the fatigue limit specific tensile strength FL/Rm is more than or equal to 0.75.

When Ti in the steel components is less than or equal to 0.003 percent and N is less than or equal to 0.003 percent, the fatigue resistance can meet the following indexes:

the FL of the high-frequency fatigue limit (1000 ten thousand cycles) is more than or equal to 640 MPa; or

The fatigue limit specific tensile strength FL/Rm is more than or equal to 0.8.

The ultrahigh-strength hot-rolled steel plate and the steel strip manufactured by the method have high strength, high hole expansibility and high fatigue limit at the same time, hot-dip galvanizing is carried out on the ultrahigh-strength hot-rolled steel plate and the steel strip product to obtain a hot-rolled hot-dip galvanized steel plate finished product, and the ultrahigh-strength hot-rolled steel plate product, the steel strip product and the hot-dip galvanized steel plate finished product can be used for preparing automobile chassis and suspension system parts to realize light weight of automobiles.

Drawings

FIG. 1 is a photograph (magnified 1000 times) of the microstructure of the steel of example G-1 of the present invention.

FIG. 2 is a photograph (1000 times magnification) of the morphology of TiN particles in the microstructure of the comparative example P steel.

Detailed Description

The present invention will be further described with reference to the following examples.

The steels with different compositions shown in the table 1 are smelted and then are heated and hot rolled according to the process shown in the table 2 to obtain the steel plate with the thickness of less than 4 mm. Taking a tensile sample along the transverse JIS 5# to measure yield and tensile strength, taking a middle area of the plate to measure the hole expanding rate and the fatigue limit, wherein the transverse sample is adopted for measuring the fatigue limit, and the sample size and the experimental method refer to the GB 3075-; the test data are shown in table 2. The hole expansion rate is measured by a hole expansion test, and a test piece with a hole in the center is pressed into a female die by a male die, so that the center hole of the test piece is expanded until the edge of the hole is necked or cracks are penetrated. Because the preparation mode of the original hole in the center of the test piece has great influence on the hole expansion ratio test result, the original hole in the center of the test piece is prepared by punching and reaming respectively, and the subsequent test and test method are executed according to the hole expansion ratio test method specified in the ISO/DIS 16630 standard. The fatigue limit is determined by adopting an axial high-frequency tensile fatigue test, a GB 3075-2008 metal axial fatigue test method is adopted, the test frequency is 85Hz, and the maximum strength of the sample which is not failed after being loaded for 1000 ten thousand times in a circulating manner is taken as the fatigue limit RL.

In Table 1, examples A to H are steels of the invention, comparative examples J to P have contents of carbon or manganese or other alloying elements outside the ranges of the composition of the invention, and it is noted that: in the table, M (all) indicates the calculated value of (Cr/52)/(C/4) + (Nb/93+ Ti/48+ V/51+ Mo/96)/(C/12) in the composition.

As can be seen from tables 1-3, when the alloy compositions of C, Mn and the like deviate from the scope of the invention, such as C and Mn contents are lower, the yield strength of the steel of the comparative example J and the steel of the comparative example K are less than 660MPa, and the tensile strength is less than 780 MPa; when the contents of C and Mn are higher than the composition range of the present invention, the hot rolled microstructure contains a large amount of martensite, which adversely affects the formability of steel and deteriorates the hole expansibility, which is not in accordance with the object of the present invention, for example, the hole expansibility of comparative examples I and L is smaller than that of the present invention.

When the Ti content deviates from the range of the present invention, the fatigue limit of the steel is adversely affected. As in comparative example M, N, O, P. The Ti content in the comparative examples M and P is higher, so that the fatigue limit of the steel is far lower than 570MPa and the fatigue limit ratio is far lower than the minimum design standard of 0.72 although the steel reaches the strength standard designed by the invention; in comparative examples N and O, the Ti content was low but still exceeded the minimum upper limit of the present invention, so that the fatigue limit and the fatigue limit ratio did not meet the requirements of the present invention. Meanwhile, in the design of the two groups of components, the proportion of the alloy element and the carbon element, namely M (all), does not reach the range designed by the invention, so that the hole expansion performance of the two groups of materials does not reach the standard.

As can be seen from tables 2-3, when the final rolling temperature of the coil is low, as in comparative steels A-2 and F-1 in Table 2, the hole expansion ratio does not meet the design standard of the invention; when the coiling temperature is more than 550 ℃, a pearlite structure and a large amount of carbide precipitation are generated to deteriorate the hole enlarging property as in comparative example F-2. In addition, if the holding temperature is too low, precipitation of carbide is suppressed, resulting in insufficient strength of the steel, and if the holding time is too long, a large amount of coarse carbide is generated, resulting in a proportional influence on the hole expansion ratio, as in comparative examples F-3, G-3 and H-3.

As can be seen from FIG. 1, the steel G-1 has no large square TiN particles in the structure because the Ti content is controlled to be extremely low, and the carbide precipitation is mainly fine dispersed (Mo, V) C. In contrast, as shown in fig. 2, in the comparative example P steel, due to the design concept of Ti element enhancement, large square TiN particles are common in the structure, and the grain boundaries have sharp corners. In addition, the steel of the present invention has a fine dispersion precipitation distribution (as shown in fig. 1) of Mo, V composite carbide precipitates, while the steel of comparative example P has a larger size of TiC precipitates (black-gray cluster, round precipitates in the matrix) in the matrix and an insufficient uniform dispersion (as shown in fig. 2), thereby deteriorating the hole-expanding performance of the material.

In conclusion, on the basis of carbon manganese steel, the invention controls reasonable component range, adds micro alloy elements and limits the content of Ti element, further controls coiling temperature on the basis of conventional automobile steel production line, and can further produce ultrahigh-strength hot rolled steel plate and steel strip with good hole expanding performance and fatigue performance by adopting heat preservation ring cooling technology, wherein the yield strength Rp0.2 is more than or equal to 660MPa, the tensile strength Rm is more than or equal to 780MPa, the hole expanding rate is more than or equal to 85 percent (original holes are punched holes), the hole expanding rate is more than or equal to 120 percent (original holes are reamed holes), the high-frequency fatigue limit strength RL is more than or equal to 570MPa, or the tensile fatigue limit ratio RL/Rm is more than or equal to 0.72, and the invention is suitable for manufacturing products such as automobile chassis, suspension parts and.