CN114855075B - Hot-rolled strip steel for ultrahigh-strength high-torsion fatigue shaft tube and preparation method thereof - Google Patents

Abstract

The invention particularly relates to a hot-rolled strip steel for an ultrahigh-strength high-torsion fatigue shaft tube and a preparation method thereof, belonging to the technical field of steel preparation, wherein the steel comprises the following chemical components in percentage by mass: c:0.10 to 0.15 percent; si:0% -0.05%; mn:0.50% -1.00%; p: less than or equal to 0.010 percent; s: less than or equal to 0.005 percent; and (3) Alt:0.02% -0.04%; nb:0.050% -0.100%; v:0.050% -0.10%; mo:0.10% -0.50%; cr:0.20% -0.40%; the balance of Fe and inevitable impurities; according to the method, elements such as Nb, V and Mo are added, and the content of the elements is controlled, so that the grain sizes of ferrite and precipitated phases are ensured to be in a smaller size range, the hot-rolled strip steel is ensured to obtain high strength, and meanwhile, higher ductility and toughness are kept, and the problem that the ductility and toughness of the steel are lower at present is solved.

Description

Hot-rolled strip steel for ultrahigh-strength high-torsion fatigue shaft tube and preparation method thereof

Technical Field

The invention belongs to the technical field of steel preparation, and particularly relates to a hot-rolled strip steel for an ultrahigh-strength high-torsion fatigue shaft tube and a preparation method thereof.

Background

With the progress of roll forming and welding technology, the preparation technology of the welded steel pipe is more and more advanced, and the welded pipe manufactured by the roll forming and high-frequency welding technology and the combination of the seam seamless technology has excellent comprehensive properties such as strength, plasticity, secondary processing performance and the like; compared with a seamless steel pipe, the welded steel pipe has better dimensional tolerance, uniform wall thickness and the like, so the welded steel pipe is more and more widely applied to the fields of automobiles, engineering machinery, buildings, household appliances and the like.

With the stricter requirements of national energy-saving and environment-friendly policies, the raw materials of welded steel pipes are gradually upgraded to 600MPa and 700MPa products from Q195, Q235, Q345 and Q460 grade products which are widely applied, such as parts applied to passenger car frameworks, welded steel pipes and the like. The welded steel pipe is developed in most advanced Japan abroad, and the highest strength level of the welded steel pipe raw material also reaches 700MPa. At present, products of 700MPa and above are mainly produced by adopting a heat treatment process, the alloy addition amount is high, the flow is long, the cost is high, and in addition, because the structure is a tempered sorbite structure, the structure is seriously recovered, recrystallized and softened in the subsequent tube making and welding process, the structure and the performance of the cross section of the whole welded tube are uneven, and the improvement of the fatigue performance of the product is influenced.

The problems existing in the prior art mainly comprise: 1) The production of high-strength steel of 700MPa and above mostly adopts a quenching and tempering heat treatment process, the alloy addition is high, and the production cost is high; 2) The steel pipe cracking rate of the existing 700 MPa-level welded steel pipe and the flattening cracking rate after pipe making are higher, the yield strength is improved to 850MPa, the strength is improved, and the plasticity is inevitably reduced.

Disclosure of Invention

The application aims to provide a hot-rolled strip steel for an ultrahigh-strength high-torsion fatigue shaft tube and a preparation method thereof, so as to solve the problem that the ductility and toughness of the steel are low at present.

The embodiment of the invention provides a hot-rolled strip steel for an ultrahigh-strength high-torsion fatigue axle tube, which comprises the following chemical components in parts by mass:

c:0.10% -0.15%; si:0% -0.05%; mn:0.50% -1.00%; p: less than or equal to 0.010 percent; s: less than or equal to 0.005 percent; and (3) Alt:0.02% -0.04%; nb:0.050% -0.100%; v:0.050% -0.10%; mo:0.10% -0.50%; cr:0.20% -0.40%; the balance of Fe and inevitable impurities.

Optionally, the steel comprises the following chemical components in percentage by mass:

C：0.10％-0.14％；Si：0.01％-0.04％；Mn：0.60％-0.90％；P：≤0.010％；S：≤0.005％；

and (3) Alt:0.02% -0.04%; nb:0.060% -0.090%; v:0.060-0.10%; mo:0.15 to 0.40 percent; cr:0.20% -0.35%; the balance of Fe and inevitable impurities.

Optionally, the microstructure of the steel comprises in volume fraction: ferrite less than or equal to 10 percent and nano-scale precipitated phase more than or equal to 90 percent.

Optionally, the ferrite grain size is 1.0 μm to 2.5 μm, the nanoscale precipitated phase is composite carbonitride, and elements of the composite carbonitride include Nb, V and Mo; the grain diameter of the composite carbonitride is 1nm-10nm.

Based on the same inventive concept, the embodiment of the invention also provides a preparation method of the hot-rolled strip steel for the ultrahigh-strength high-torsion fatigue shaft tube, which comprises the following steps:

smelting and continuously casting molten iron to obtain a casting blank;

heating the casting blank to obtain a hot casting blank;

rolling the hot casting blank to obtain a steel plate;

and cooling the steel plate, and then coiling to obtain the strip steel coil.

Optionally, the heating temperature is 1120-1200 ℃, and the heating heat preservation time is 1.5-2.0 h.

Optionally, the rolling comprises rough rolling and finish rolling, wherein the outlet temperature of the rough rolling is 950-1010 ℃, and the thickness of the rough-rolled intermediate billet is 32-38 mm.

Optionally, in the finish rolling, an outlet temperature of the finish rolling is 800 ℃ to 900 ℃.

Optionally, the cooling is layer cooling, the layer cooling adopts front section cooling, and the coiling temperature is 550-600 ℃.

Optionally, the method further includes: and conveying the strip steel coil into a heat preservation pit for slow cooling, wherein the slow cooling time exceeds 48h.

One or more technical solutions in the embodiments of the present invention at least have the following technical effects or advantages:

according to the hot-rolled strip steel for the ultrahigh-strength high-torsion fatigue shaft tube, the Nb, V, mo and other elements are added, and the content of the Nb, V, mo and other elements is controlled, so that the grain sizes of ferrite and a precipitated phase are ensured to be in a smaller size range, the hot-rolled strip steel is ensured to obtain high strength, and simultaneously higher ductility and toughness are maintained, and the problem of lower ductility and toughness of the existing steel is solved.

The above description is only an overview of the technical solutions of the present invention, and the present invention can be implemented in accordance with the content of the description so as to make the technical means of the present invention more clearly understood, and the above and other objects, features, and advantages of the present invention will be more clearly understood.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.

FIG. 1 is a flow chart of a method provided by an embodiment of the present invention;

FIG. 2 is a metallographic structure of steel provided by an example of the present invention;

FIG. 3 is a graph of second phase precipitates in a steel provided in accordance with an example of the present invention.

Detailed Description

The present invention will be described in detail below with reference to specific embodiments and examples, and the advantages and various effects of the present invention will be more clearly apparent therefrom. It will be understood by those skilled in the art that these specific embodiments and examples are for the purpose of illustrating the invention and are not to be construed as limiting the invention.

Throughout the specification, unless otherwise specifically noted, terms used herein should be understood as having meanings as commonly used in the art. Accordingly, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. If there is a conflict, the present specification will control.

Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment and the like used in the present invention are commercially available or can be prepared by existing methods.

In order to solve the technical problems, the general idea of the embodiment of the application is as follows:

according to an exemplary embodiment of the present invention, there is provided a hot-rolled steel strip for an ultra-high strength high torsional fatigue axle tube, the steel having a chemical composition comprising, in mass percent:

c:0.10% -0.15%; si:0 to 0.05 percent; mn:0.50% -1.00%; p: less than or equal to 0.010 percent; s: less than or equal to 0.005 percent; and (3) Alt:0.02% -0.04%; nb:0.050% -0.100%; v:0.050% -0.10%; mo:0.10% -0.50%; cr:0.20 to 0.40 percent; the balance of Fe and inevitable impurities.

The design principle of each chemical element in the application is as follows:

c is one of the most economical strengthening elements in steel, is an essential element when the strength exceeds 850MPa, and provides a remarkable solid solution strengthening effect. But the content of the element C cannot be too high so as to avoid the disadvantages of welding performance and cold forming performance, and the ultrahigh strength and good plasticity and toughness of the strip steel cannot be ensured. Therefore, the content of C in the steel of the present invention is controlled to 0.10% to 0.15%, and further preferably 0.10% to 0.14%, in consideration of the strength, cold formability and weldability of the material.

Si is a solid solution strengthening element and is added into steel to facilitate the improvement of strength. However, when the content of Si is relatively high, an fayalite phase is easily formed, the adhesiveness of the iron sheet is increased, the removal difficulty in the descaling stage is increased, a red iron sheet is formed, and the surface quality of the strip steel is not facilitated. In addition, the higher Si content is detrimental to ERW welding, resulting in larger sparks. Therefore, the Si content in the steel is controlled between 0 and 0.05 percent by comprehensively considering the strength, the weldability and the surface quality of the strip steel.

Mn is a solid solution strengthening element and contributes to an increase in the strength of steel. When the Mn content is too high, a severe band-shaped structure is formed, the transverse elongation is reduced, and the cold formability is affected. Therefore, in the present invention, the Mn content is designed to be 0.50% to 1.00%, and preferably 0.60% to 0.90%, in consideration of the toughness and cold formability of the material.

P and S are impurity elements in steel, and the P element is easy to cause central segregation of the steel, deteriorates the weldability and the plastic toughness of the steel and is preferably reduced as much as possible; the S element is likely to form MnS inclusions in the Mn element, and is preferably as small as possible, because it lowers weldability, formability, fatigue property and low-temperature toughness of the steel. Therefore, the content of P in the steel is controlled to be less than or equal to 0.010 percent and the content of S in the steel is controlled to be less than or equal to 0.005 percent by comprehensively considering the weldability and the ductility and toughness of the material.

Al plays a role of a deoxidizer during steel making, and the incomplete deoxidation can cause the reduction of the cold forming performance of the material, thereby having an important role in ensuring that the pipe is not cracked. However, too high Al content results in too many AlN inclusions in the steel, and decreases elongation of the material and fatigue resistance. Therefore, the Al content is controlled between 0.02 percent and 0.04 percent by comprehensively considering deoxidation and inclusion control.

Nb is bonded to C in steel to precipitate carbide, and has a control effect of suppressing austenite recovery and recrystallization grain growth in a hot rolling step, thereby ultrafining a ferrite phase. In order to obtain such an effect, it is necessary to contain 0.05% or more. On the other hand, when the Nb content exceeds 0.10%, on the one hand, the rolling difficulty in the hot rolling process is significantly increased, and on the other hand, the strength is significantly increased and the ductility of the steel is significantly decreased due to precipitated carbides. Therefore, the content of Nb in the steel of the present invention is controlled to 0.05% to 0.10%, and further, preferably 0.06% to 0.09%, in consideration of the rolling difficulty and the strengthening effect.

V is completely dissolved in a solid solution in the high-temperature austenite region, and forms carbide precipitation by bonding with C only in the ferrite region, thereby supplementing Nb, suppressing austenite recovery and recrystallization grain growth in the hot rolling step, making the ferrite phase have a desired grain size, and improving the strength of the guaranteed steel. In order to meet the requirement of ultrahigh strength of above 850MPa, the content of V is controlled to be above 0.05%, the strengthening effect is obvious, but the content of V is not more than 0.10% as much as possible, otherwise, the formability is obviously reduced. Therefore, in the steel of the present invention, the V content is controlled to 0.05% to 0.10%, and more preferably 0.06% to 0.10%, in consideration of the strengthening effect and cold formability.

Mo improves the solid solubility of microalloy elements Nb and V in austenite, delays the precipitation of microalloy carbonitride, and enables more microalloy elements to be separated out from ferrite at a lower temperature, thereby generating a larger precipitation strengthening effect; in addition, mo can be dissolved in the crystal lattices of microalloy carbonitride precipitated from ferrite, so that the volume fraction of precipitated phases is improved, and the size of the microalloy precipitate is obviously refined, thereby enhancing the precipitation strengthening effect; on the other hand, the Mo-containing microalloy carbonitride has better thermal stability, is not easy to coarsen at high temperature, and is beneficial to improving the stability of the mechanical property of the product. Therefore, in the present invention, the Mo content is controlled to 0.10% to 0.50%, more preferably 0.15% to 0.40%, in consideration of the strength and cold formability.

Cr belongs to solid solution strengthening elements, and is added into steel, so that the strength is improved, and the formation of iron scale is inhibited, and the good surface quality is ensured. However, when the content of Cr element is high, weldability and cold formability are adversely affected. Therefore, in the present invention, the Cr content is controlled to 0.20 to 0.40%, preferably 0.20 to 0.35%, in consideration of the strength, plasticity and surface quality.

In some embodiments, the steel has a chemical composition, in mass fractions:

c:0.10% -0.14%; si:0.01 to 0.04 percent; mn:0.60% -0.90%; p: less than or equal to 0.010 percent; s: less than or equal to 0.005 percent; and (3) Alt:0.02% -0.04%; nb:0.060% -0.090%; v:0.060-0.10%; mo:0.15 to 0.40 percent; cr:0.20% -0.35%; the balance of Fe and inevitable impurities.

In some embodiments, the microstructure of the steel comprises in volume fraction: ferrite less than or equal to 10 percent and nano precipitated phase more than or equal to 90 percent.

The microstructure is an important factor in ensuring excellent cold formability and fatigue property, in order to ensure high strength of strip steel and welded steel pipes made of the strip steel and ensure excellent cold formability, prevent cracking in the pipe and cracks in the pipe post-processing, the structure is controlled to be an ultra-fine ferrite + nano precipitated phase, and ensure that the average grain size of the ferrite structure is 1.0-2.5 mu m, and the proportion of nano precipitates of the Nb, V and Mo composite carbonitride is more than or equal to 90% between 1-10 nm. The smaller and more uniform the average grain size of the ferrite phase, the higher ductility and toughness of the hot-rolled strip steel can be ensured while high strength is obtained, and fine grain strengthening is the only strengthening mode which can improve the strength and simultaneously does not lose the ductility and toughness.

In some embodiments, the ferrite has a grain size of 1.0 μm to 2.5 μm, the nano-sized precipitate phase is a composite carbonitride, elements of the composite carbonitride include Nb, V, and Mo; the grain diameter of the compound carbonitride is 1nm-10nm.

The yield strength of the product cannot be increased to more than 850MPa by singly relying on fine-grain strengthening, so other strengthening modes are needed. For steel solid solution strengthening, dislocation strengthening, structure strengthening and precipitation strengthening, the brittleness vector of the precipitation strengthening is the smallest, namely, the loss of the ductility and toughness is the smallest by improving the strength through the precipitation strengthening, so the precipitation strengthening is also introduced to improve the strength; however, if coarse second-phase precipitates are formed by the microalloying elements, not only the ductility and toughness are impaired, but also the fatigue properties are extremely unfavorable. Therefore, in the present application, the average grain size of the ferrite phase is limited to a range of 2 to 5 μm, and the proportion of the nano-scale precipitates in the range of 1 to 10nm in the average grain size of the (Nb, V, mo) composite carbonitride is 90% or more.

According to another exemplary embodiment of the present invention, there is provided a method of manufacturing a hot rolled steel strip for an ultrahigh strength and high torsional fatigue axle tube as described above, the method including:

s1, smelting and continuously casting molten iron to obtain a casting blank;

s2, heating the casting blank to obtain a hot casting blank;

in some embodiments, the heating temperature is 1120 ℃ to 1200 ℃, and the holding time for heating is 1.5h to 2.0h.

In actual operation, the heating temperature of the continuous casting billet is set according to the solid solution and precipitation conditions of Nb, ti, mo and V in steel and the coarsening behavior of original austenite grains. When the heating temperature is less than 1120 ℃, the amount of dissolved coarse second phase precipitates precipitated during continuous casting is insufficient, which affects the amount of precipitates in the hot rolling process and the cooling process, and the desired strength cannot be secured, and the precipitates dissolved therein grow further and remain in the final product, which affects the fatigue properties of the product. On the other hand, the heating temperature is not easily over 1200 ℃, and although the temperature does not reach the austenite coarsening temperature of the component system, the austenite grains become coarser as the temperature is higher, which is not favorable for ensuring the control of the ultrafine grains of the finished product. In addition, in order to ensure the uniformity of the solid solution state of the microalloying elements Nb, ti, mo, V and the like and sufficient solid solution time, the time for heating the continuous casting slab is limited to 1.5 to 2.0 hours, the homogenization effect of the structure and the microalloying elements cannot be ensured when the time is too short, and the coarsening of austenite grains is influenced when the time is too long.

S3, rolling the hot casting blank to obtain a steel plate;

in some embodiments, the rolling comprises rough rolling having an exit temperature of 950 ℃ to 1010 ℃, the rough rolled intermediate slab having a thickness of 32mm to 38mm, and finish rolling having an exit temperature of 800 ℃ to 900 ℃.

In the embodiment, rough rolling adopts a rolling process in a mode of 1+5, a 6-pass descaling process is carried out, wherein R1 is used for descaling once, R2 is used for descaling in 1, 2, 3, 4 and 5 passes, so that the iron scale on the surface of an intermediate billet is completely removed, and the rough rolling adopts a slow speed mode so as to ensure that the outlet temperature range of rough rolling is 950-1010 ℃; the thickness range of the rough rolling intermediate billet is 32-38 mm, so as to ensure that the austenite structure is fully refined and homogenized by recrystallization in the rough rolling stage, and provide conditions for obtaining uniformly fine ferrite grains subsequently.

In the embodiment, the secondary reduction rate of the finish rolling processes F1 and F2 is more than 40%, but the rolling speed is lower than 3m/s, so that the mixed crystal problem of the F1 and the F2 is prevented; the F7 reduction rate is not more than 15%, and the accelerated rolling is adopted to improve the plate shape control capability of the F7 outlet strip steel and improve the finish rolling stability.

The finish rolling temperature (i.e. the finish rolling exit temperature) of the hot rolling has a significant effect on the subsequent austenite to ferrite transformation and the second phase strain induced precipitation behavior, thereby affecting the mechanical properties of the final product. When the finishing temperature exceeds 900 ℃, due to high temperature, the defects of deformation belts, dislocation and the like generated by deformation can be recovered and disappear, the superfine control of subsequent tissues is not facilitated, and the formability and the fatigue performance of the strip steel are influenced. On the other hand, when the finish rolling temperature is less than 800 ℃, the non-uniformity of the structure is caused, and the number of second-phase precipitates induced by phase transformation is also rapidly increased, thereby reducing the precipitation strengthening effect, and at the same time, the growth rate of the second phase precipitated at a high temperature is high, and the large-sized second phase also causes the reduction of formability and fatigue. Therefore, the finish rolling temperature is preferably in the range of 900 to 800 ℃.

Generally, in actual practice, from the viewpoint of ensuring good surface quality of a strip, the descaling operation is performed before finish rolling by using high-pressure water of 18MPa or more to completely remove scale on the surface of the strip so as not to affect the surface quality by being pressed into the surface of the strip during finish rolling.

And S3, cooling the steel plate, and then coiling to obtain the strip steel coil.

In some embodiments, the cooling is layer cooling, the layer cooling adopts front section cooling, and the coiling temperature is 550-600 ℃.

The laminar cooling adopts a front-section rapid cooling mode, and the rolled strip steel is rapidly cooled to a ferrite region for coiling so as to avoid the reduction of deformation belts, dislocation and the like due to high-temperature retention and reduce the transformation rate of austenite to ferrite.

The coiling temperature is one of the important factors that determine the percentage and size of the structure of the ferrite phase of the hot-rolled steel strip and the state of precipitation of the (Nb, ti, mo) composite carbide. When the coiling temperature is lower than 550 ℃, bainite structures are easy to appear, the formability of the material is reduced, the proportion of second-phase precipitates is reduced, and the formability and the strength are not ensured. But the coiling temperature is not easy to be higher than 600 ℃, which is not beneficial to the refinement of ferrite, reduces the strength of the material and can not reach the requirement of 850MPa yield; further, the high coiling temperature tends to cause the (Nb, ti, mo, V) composite carbide to be easily coarsened, and the desired strength and fatigue performance cannot be secured. Therefore, the coiling temperature is preferably in the range of 600 to 550 ℃.

And S4, conveying the strip steel coil into a heat preservation pit for slow cooling, wherein the slow cooling time exceeds 48h.

And (3) rapidly moving the coiled material to a heat preservation pit, ensuring the temperature to be 550-500 ℃ before entering the pit, preserving the heat of the heat preservation pit for 48 hours, and then taking the coiled material out of the pit and cooling the coiled material to room temperature. Because the coiling temperature is not at the nose tip temperature of the second-phase precipitates precipitated in the ferrite region, a large amount of second-phase precipitated elements are in a solid solution state in the matrix; because the precipitation of the second phase is determined by thermodynamics and kinetics, the precipitation of the second phase needs time, the second phase quickly enters a heat-preservation pit after being coiled, the pit entering temperature is ensured to be more than 500 ℃, the micro-alloying elements in a solid solution state are slowly precipitated, and the precipitated second phase is necessarily nanoscale and not easy to coarsen, so that the contribution of precipitation strengthening to the strength can be maximally improved; however, the pit entry temperature is not easily higher than 550 ℃ to avoid cooling and coarsening of the ferrite grains and the secondary phase precipitated in coiling. The slow cooling time is more than 48 hours, which is mainly to complete the precipitation in a low temperature state in a large amount of time, and if the time is short, the effect of sufficient precipitation cannot be achieved.

In general, the production flow of the whole preparation is as follows: smelting → continuous casting heating → rough descaling → fixed width press → rough rolling → coiled sheet box → flying shear → fine descaling → fine rolling → laminar cooling → coiling to form a coiled steel coil.

The hot rolled steel strip for an ultra-high strength and high torsion fatigue shaft tube and the method for manufacturing the same according to the present application will be described in detail with reference to examples, comparative examples, and experimental data.

Examples 1 to 4

A preparation method of hot rolled strip steel for ultrahigh-strength high-torsion fatigue shaft tubes comprises the following steps:

(1) Smelting and continuous casting: smelting molten steel according to set components and casting into a blank, wherein the chemical elements in the embodiments are shown in the following table according to the mass percentage:

	C	Si	Mn	P	S	Alt	Nb	V	Mo	Cr
											example 1	0.10	0.01	0.9	0.009	0.0010	0.035	0.08	0.10	0.35	0.35
Example 2	0.12	0.03	0.7	0.008	0.0030	0.030	0.065	0.08	0.30	0.30
											Example 3	0.14	0.05	0.6	0.007	0.0020	0.030	0.055	0.06	0.20	0.25
Example 4	0.15	0.04	0.5	0.007	0.0040	0.025	0.050	0.05	0.10	0.20

(2) Heating a plate blank: heating and insulating the continuous casting billet at 1120-1200 ℃, controlling the insulating time to be 1.5-2.0 h,

(3) Hot rolling: rough rolling adopts a rolling process in a mode of 1+5, a 6-pass descaling process is carried out, R1 is used for one-pass descaling, R2 is used for 1, 2, 3, 4 and 5-pass descaling, complete removal of scale on the surface of an intermediate billet is ensured, and a slow mode is adopted for rough rolling to ensure that the temperature range of a rough rolling outlet is 950-1010 ℃; the thickness range of the rough rolling intermediate billet is 32-38 mm; the secondary reduction rate of the finish rolling working procedures F1 and F2 is more than 40 percent, but the rolling speed is lower than 3m/s, the F7 reduction rate is not more than 15 percent, the accelerated rolling is adopted, and the finishing temperature is 800-900 ℃;

(4) Laminar cooling: the laminar cooling adopts a front-section rapid cooling mode, the rolled strip steel is rapidly cooled to a ferrite area for coiling, and the coiling temperature is 600-550 ℃; and (3) rapidly moving the coiled material to a heat preservation pit, ensuring the temperature to be 550-500 ℃ before entering the pit, preserving the heat of the heat preservation pit for 48 hours, and then taking the coiled material out of the pit and cooling the coiled material to room temperature.

The specific process parameters for each example are shown in the following table:

comparative examples 1 to 4

A preparation method of hot rolled strip steel for ultrahigh-strength high-torsion fatigue shaft tubes comprises the following steps:

(1) Smelting and continuous casting: smelting molten steel according to set components and casting into a blank, wherein the chemical elements in the embodiments are shown in the following table according to the mass percentage:

	C	Si	Mn	P	S	Alt	Nb	V	Mo	Cr
											example 1	0.05	0.01	0.3	0.009	0.0010	0.01	0.04	0.03	0.05	0.10
Example 2	0.19	0.08	1.5	0.008	0.0030	0.100	0.150	0.12	0.60	0.50
											Example 3	0.14	0.05	0.6	0.007	0.0020	0.030	0.055	0.06	0.20	0.25
Example 4	0.15	0.04	0.5	0.007	0.0040	0.025	0.050	0.05	0.10	0.20

(2) Heating a plate blank: heating and preserving the continuous casting billet at 1120-1200 ℃, controlling the preserving time to be 1.5-2.0 h,

(3) Hot rolling: the rough rolling adopts a rolling process in a mode of 1+5, a 6-pass descaling process is carried out, R1 is subjected to one-pass descaling, R2 is subjected to 1, 2, 3, 4 and 5-pass descaling to ensure that the scale on the surface of the intermediate billet is completely removed, the rough rolling adopts a slow mode, the secondary reduction rate of the finish rolling procedures F1 and F2 is more than 40 percent, the rolling speed is lower than 3m/s, the F7 reduction rate is not more than 15 percent, and the high-speed rolling is adopted;

(4) Laminar cooling: the laminar cooling adopts a front-section rapid cooling mode, and the rolled strip steel is rapidly cooled to a ferrite region for coiling; and (4) rapidly moving to a heat preservation pit after coiling, preserving heat in the heat preservation pit for 48 hours, taking out the pit, and cooling to room temperature.

The specific process parameters for each example are shown in the following table:

examples of the experiments

The steels obtained in examples 1 to 4 and comparative examples 1 to 4 were subjected to property tests, and the test results are shown in the following table:

according to the table above, the yield strength of the steel prepared by the method provided by the embodiment of the application is greater than 850Mpa and reaches 898Mpa at most; the tensile strength is more than 900Mpa and reaches 935Mpa at most; the proportional elongation is more than or equal to 20.0 percent; meanwhile, the cold bending test of 180 degrees d =1a does not crack; the 2/3 diameter flattening experiment welding seam and the base metal of the high-strength welded steel pipe manufactured by the steel strip are not cracked. As can be seen from comparison between the comparative examples and examples, when a certain parameter is out of the range of the present application, problems of 180 ℃ cold roll cracking and 2/3d crush cracking occur.

Detailed description of the drawings 2 and 3:

as shown in FIG. 2, which is a metallographic structure diagram of steel, the structure of the test steel was a single ferrite structure, the grain size of the ferrite was 1.0 μm to 2.5 μm, and the structure was very uniform and fine.

As shown in FIG. 3, which is a diagram of second phase precipitates of the steel, it can be seen from the diagram that a large number of nano-scale precipitated phases are present on the matrix of the test steel, and the precipitated phases are complex carbonitrides having particle diameters of mainly 1nm to 10nm and are uniformly dispersed.

One or more technical solutions in the embodiments of the present invention at least have the following technical effects or advantages:

(1) The yield strength of the steel provided by the embodiment of the invention is more than or equal to 850MPa, the tensile strength is more than or equal to 900MPa, and the elongation is more than or equal to 20%;

(2) The steel provided by the embodiment of the invention has excellent plate shape and surface quality, excellent cold formability and weldability, and is suitable for the requirements of structures such as welded steel pipes produced by roll forming;

(3) The method provided by the embodiment of the invention adopts the traditional TMCP low-cost process to produce the hot-rolled strip steel for the high-strength welded steel pipe, and solves the technical bottlenecks of pipe-making cracking, flattening cracking after pipe-making, torsion cracking and the like.

Finally, it should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including the preferred embodiment and all changes and modifications that fall within the scope of the invention.

It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.

Claims (6)

1. A hot-rolled strip steel for ultrahigh-strength high-torsion fatigue shaft tubes is characterized by comprising the following chemical components in percentage by mass:

c:0.10% -0.14%; si:0.01% -0.04%; mn:0.60% -0.90%; p: less than or equal to 0.010 percent; s: less than or equal to 0.005 percent; and (3) Alt:0.02% -0.04%; nb:0.060% -0.090%; v:0.060-0.10%; mo:0.15 to 0.40 percent; cr:0.20% -0.35%; the balance of Fe and inevitable impurities; the microstructure of the steel is a single ferrite structure, the grain size of the ferrite is 1.0-2.5 mu m, a nano precipitated phase is dispersed and distributed on a tissue matrix, the nano precipitated phase is composite carbonitride, and elements of the composite carbonitride comprise Nb, V and Mo; the grain size of the composite carbonitride is 1nm-10nm, the yield strength of the steel is more than or equal to 850MPa, the tensile strength is more than or equal to 900MPa, the elongation is more than or equal to 20%, the rolling process of the hot-rolled strip steel comprises rough rolling and finish rolling, the rough rolling adopts a slow speed mode, and the temperature range of a rough rolling outlet is 950-1010 ℃; the thickness range of the rough rolling intermediate billet is 32-38 mm, the two-pass reduction rate of the finish rolling procedures F1 and F2 is more than 40%, the rolling speed is lower than 3m/s, the F7 reduction rate is not more than 15%, and the speed is increased for rolling.

2. A method for preparing a hot rolled steel strip for an ultra-high strength high torsional fatigue axle tube according to claim 1, comprising:

smelting and continuously casting molten iron to obtain a casting blank;

heating the casting blank to obtain a hot casting blank;

rolling the hot casting blank to obtain a steel plate;

and cooling the steel plate, and then coiling to obtain the strip steel coil.

3. The method for preparing the hot-rolled strip steel for the ultrahigh-strength high-torsion fatigue axle tube according to claim 2, wherein the heating temperature is 1120-1200 ℃, and the holding time for heating is 1.5-2.0 h.

4. The method for preparing a hot-rolled steel strip for an ultrahigh-strength high-torsional-fatigue shaft tube according to claim 3, wherein in the finish rolling, the outlet temperature of the finish rolling is 800 ℃ to 900 ℃.

5. The method for preparing the hot rolled strip steel for the ultrahigh-strength high-torsion fatigue shaft tube according to claim 2, wherein the cooling is layer cooling, the layer cooling adopts front-stage cooling, and the coiling temperature is 550-600 ℃.

6. The method for preparing the hot-rolled steel strip for the ultrahigh-strength high-torsional-fatigue shaft tube according to claim 2, further comprising the steps of: and sending the strip steel coil into a heat preservation pit for slow cooling, wherein the slow cooling time exceeds 48h.

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