A hole expansion steel and preparation method thereof

Abstract

The present application relates to the technical field of hot-rolled pickled strip steel preparation process, and relates to a hole-expanding steel and a preparation method thereof. The hole-expanding steel has a tensile strength of 800MPa or above, and the hole-expanding steel includes the following chemical components: C, Mn, P, S, Cr, Nb, Mo, N, Ti, Al, Si, and the rest is Fe and inevitable impurities; the content of Si is ≤0.2% by weight; the content of Al is 0.3% by weight to 1% by weight. The microstructure of the hole-expanding steel in the embodiment of the present application is ferrite + bainite, wherein the volume fraction of bainite is 25-45%, the tensile strength of the hole-expanding steel is ≥800MPa, the tensile fatigue limit (R=-1) FL is ≥380MPa, the hole-expanding rate λ is ≥60%, and the elongation A80 is ≥14%. The content of the present application solves the problem of poor hole-expanding performance and fatigue performance of hole-expanding steel above 800MPa in the prior art.

Description

Technical Field

[0001] The present application relates to the technical field of hot-rolled pickled strip steel preparation process, and relates to a hole-expanding steel and a preparation method thereof. Background Art

[0002] As important components of a car, the chassis structure and suspension system account for more than 30% of the total weight of the car. Chassis materials are increasingly made of high-strength steel to achieve "lightweight" and the shapes of chassis structural parts are complex. During the forming process of parts, key positions have large holes and flanging. As the strength grade of the material increases, maintaining the material forming ability has become the biggest challenge in the lightweight chassis.

[0003] Chinese patent CN 110643894A discloses an ultra-high strength hot-rolled steel plate and steel strip with good fatigue and hole expansion properties. Although it takes into account both strength hole expansion and fatigue performance, it needs to achieve carbide precipitation strengthening through heat preservation and slow cooling, and has high requirements for the range of alloy element content.

[0004] Chinese patent CN114107792A discloses a 780MPa grade high surface ultra-high hole expansion steel, which obtains ferrite and nano-precipitation structure by reducing Si and high temperature coiling. However, the patent adds relatively expensive alloy elements such as Cu and Ni, which increases the material alloy cost and does not consider the fatigue performance of the material.

[0005] In summary, as the strength level increases, it becomes increasingly difficult to improve the hole expansion performance and fatigue performance of the material. When the material reaches 800MPa level and above, carbide precipitation strengthening and grain refinement are urgently needed while ensuring that the hardness difference between the two phases improves the strength. At the same time, carbide precipitation is also a double-edged sword. The segregation and coarsening of carbides will further harm the hole expansion performance and fatigue performance. Summary of the invention

[0006] The present application provides a hole-expanding steel and a preparation method thereof, so as to solve the problem of poor hole-expanding performance and fatigue performance of hole-expanding steels above 800 MPa in the prior art.

[0007] In a first aspect, the present application provides a hole-expanding steel, characterized in that the hole-expanding steel has a tensile strength of 800 MPa or above, and the hole-expanding steel comprises the following chemical components:

[0008] C, Mn, P, S, Cr, Nb, Mo, N, Ti, Al, Si, the rest is Fe and inevitable impurities;

[0009] The content of Si is ≤0.2 wt %; the content of Al is 0.3 wt % to 1.0 wt %.

[0010] Optionally, in the chemical composition of the hole expansion steel, the C content is 0.06 weight % to 0.1 weight %; the Mn content is 0.8 weight % to 1.5 weight %; the P content is ≤0.015 weight %; the S content is ≤0.003 weight %; the Cr content is 0.1 weight % to 0.5 weight %; the Nb content is 0.01 weight % to 0.05 %; the Mo content is 0.1 weight % to 0.5 weight %; the N content is ≤0.005 weight %; and the Ti content is 0.02 weight % to 0.05 weight %.

[0011] Optionally, it is characterized in that the chemical composition of the hole expansion steel satisfies the following two conditions:

[0012] [Cr] + [Mo] ≤ 0.5 wt%, [Nb] + [Ti] ≤ 0.07 wt%;

[0013] Here, [Cr] represents the Cr content, [Mo] represents the Mo content, [Nb] represents the Nb content, and [Ti] represents the Ti content.

[0014] Optionally, the microstructure of the hole expansion steel is bainite and ferrite, wherein the volume fraction of bainite is 25-45%.

[0015] In a second aspect, the present application provides a method for preparing hole-expanding steel, which is used to prepare the hole-expanding steel described in the first aspect, and the method comprises:

[0016] Heating the slab having the chemical composition, and then performing rough rolling on the heated slab under a condition of setting a first finishing rolling temperature;

[0017] Under the conditions of setting the rolling speed, setting the second finishing rolling temperature and setting the rolling target thickness, the slab after the rough rolling is subjected to finish rolling to obtain a hot-rolled plate;

[0018] The hot rolled plate is cooled in stages so that the hot rolled plate has a first temperature to obtain hole-expanded steel.

[0019] Optionally, the first finishing rolling temperature is set to 1020°C to 1100°C.

[0020] Optionally, the set rolling speed is 5m / s to 11m / s; the set second final rolling temperature is 860 to 930°C; and the set rolling target thickness is 1.8mm to 5.5mm.

[0021] Optionally, cooling the hot-rolled plate in stages so that the hot-rolled plate has a first temperature to obtain hole-expanded steel comprises:

[0022] Air cooling the hot rolled plate to make the hot rolled plate reach a second temperature;

[0023] Water cooling the hot-rolled plate at the second temperature to make the hot-rolled plate reach the first temperature to obtain hole-expanded steel;

[0024] The first temperature is 400°C to 450°C, and the second temperature is 700°C to 760°C.

[0025] Optionally, after cooling the hot-rolled plate in stages so that the hot-rolled plate has a first temperature, the method further comprises:

[0026] The hot-rolled plate having the first temperature is coiled and then flattened to obtain hole-expanded steel.

[0027] Optionally, the hot-rolled plate having the first temperature is coiled and then flattened to obtain the hole-expanded steel, comprising:

[0028] The hot-rolled plate having the first temperature is coiled, and then flattened under the conditions of set flattening elongation and rolling force to obtain hole-expanded steel;

[0029] The set flattening elongation is 1% to 3%, and the set flattening rolling force is 400t to 500t.

[0030] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0031] The present application designs the hole expansion steel composition with reasonable process, and uses low Si and high Al composition to facilitate hole expansion and improve hole expansion rate, while Cr and Mo elements improve hardenability and expand bainite transformation zone;

[0032] Combined with a reasonable process flow, bainite equivalent grain refinement is achieved, and the strength decrease caused by reducing C and Si elements is compensated on the basis of ensuring the strength level; the bainite-Maho Island interface is used to increase the crack propagation resistance, thereby improving the fatigue strength of the steel, while avoiding the poor surface quality of the steel caused by the iron olivine (2FeO-SiO2) oxide scale generated by the Si element in the preparation, and the hardness difference caused by the increase of bainite hardness caused by the solid solution of Si element affecting the hole expansion performance, thereby solving the problem of poor hole expansion performance and fatigue performance of hole expansion steel above 800MPa in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0034] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0035] Figure 1 A schematic flow chart of a method for preparing hole-expanding steel provided in an embodiment of the present application;

[0036] Figure 2 This is a microstructure diagram of a hole-expanding steel provided in Example 1 of the present application. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0038] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0039] In this application, unless otherwise specified, directional words such as "upper" and "lower" used refer specifically to the directions of the drawings in the accompanying drawings. In addition, in the description of the specification of this application, the terms "include", "comprise", etc. mean "including but not limited to".

[0040] In this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. In this article, "and / or" describes the association relationship of associated objects, indicating that there can be three relationships, for example, A and / or B, which can represent: A exists alone, A and B exist at the same time, and B exists alone. Wherein A, B can be singular or plural. In this article, "at least one" refers to one or more, and "multiple" refers to two or more. "At least one", "the following at least one (individual)" or similar expressions refer to any combination of these items, including any combination of single (individual) or plural (individual). For example, "at least one (individual) of a, b, or c", or "at least one (individual) of a, b, and c", can all represent: a, b, c, ab (i.e. a and b), ac, bc, or abc, wherein a, b, c can be single or multiple respectively.

[0041] Unless otherwise defined, all professional terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention. For example, the room temperature described in the present invention refers to a temperature of 10 to 35°C. In the following implementation manner, the tensile data are taken as the average value of the 1 / 4 1 / 2 3 / 4 positions along the transverse direction, and the hole expansion rate and fatigue limit are measured in three groups along the rolling direction in the middle of the plate. The hole expansion rate is measured in accordance with GB / T 15825.4-2008 standard; the fatigue limit is determined using a transverse specimen, and the fatigue test specifically refers to GB 3075-2021 Metal Axial Fatigue Test Method.

[0042] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0043] In a first aspect, the present application provides a hole-expanding steel, characterized in that the hole-expanding steel has a tensile strength of 800 MPa or above, and the hole-expanding steel comprises the following chemical components:

[0044] C, Mn, P, S, Cr, Nb, Mo, N, Ti, Al, Si, the rest is Fe and inevitable impurities;

[0045] The content of Si is ≤0.2 wt %; the content of Al is 0.3 wt % to 1.0 wt %.

[0046] In the embodiment of the present application, the positive effect of controlling the Si content to ≤ 0.2 wt% is that Si is a necessary element for steelmaking deoxidation and also plays a role in solid solution strengthening in hole expansion steel. However, too high Si content will deteriorate the surface quality of the product to a certain extent and is not conducive to hole expansion forming. Specifically, in the present embodiment, the Si content can be 0.2 wt%, 0.1 wt%, 0.15 wt%, etc.

[0047] The positive effects of controlling the Al content to 0.3 wt% to 1.0 wt% are as follows: Al has a deoxidizing effect in the steelmaking process, Al is insoluble in cementite, and strongly expands the ferrite zone, promotes the formation of ferrite, can increase the bainite transformation temperature, and reduce the bainite incubation time. Al reduces the residual austenite phase transformation to form a hard phase martensite hard phase in the punching process to reduce the hole expansion performance, which has a significant effect on improving the hole expansion rate and surface quality. Too high an aluminum content will lead to an increase in defects in the continuous casting slab to a certain extent, and is not conducive to welding performance; too high an aluminum content will coarsen the bainite structure to a certain extent, which is not conducive to organizational control. Specifically, in this embodiment, the Al content can be 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, etc.

[0048] In some embodiments, in the chemical composition of the hole expansion steel, the content of C is 0.06 weight % to 0.1 weight %; the content of Mn is 0.8 weight % to 1.5 weight %; the content of P is ≤0.015 weight %; the content of S is ≤0.003 weight %; the content of Cr is 0.1 weight % to 0.5 weight %; the content of Nb is 0.01 weight % to 0.05 %; the content of Mo is 0.1 weight % to 0.5 weight %; the content of N is ≤0.005 weight %; and the content of Ti is 0.02 weight % to 0.05 weight %.

[0049] In the embodiment of the present application, the positive effect of controlling the C content to 0.06 wt% to 0.1 wt% is as follows: the C content largely determines the tensile strength level of the steel plate and is a very important indicator affecting the welding carbon equivalent. Too high a C content will cause martensite to appear in the structure, which is not conducive to forming and welding; too low a C content will cause insufficient material strength to a certain extent. Specifically, in the present embodiment, the C content can be 0.06 wt%, 0.07 wt%, 0.08 wt%, 0.09 wt%, 0.1 wt%, etc.

[0050] The positive effects of controlling the Mn content to 0.8 wt% to 1.5 wt% are as follows: Manganese improves the hardenability of steel, delays the pearlite transformation and reduces the bainite transformation temperature, and refines the steel's organizational substructure; it has good formability while ensuring the tensile strength of the product. However, when the manganese content is too high, it will not only cause segregation of the organization to a certain extent, but also the cooling organization will easily form martensite, which is unfavorable to the plasticity and hole expansion performance and deteriorates the welding performance; if the manganese content is too low, it will cause the bainite volume fraction to decrease to a certain extent, and the material strength will be insufficient. Specifically, in this embodiment, the Mn content can be 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, etc.

[0051] Positive effects of controlling the P content to ≤ 0.015 wt %: P in steel is generally dissolved in ferrite and has a strong solid solution strengthening effect, which is used to improve the strength of steel and reduce the toughness of steel. However, too high a P content is not good for welding performance and is a harmful element in this application, so the phosphorus content should be reduced as much as possible. Specifically, in this embodiment, the P content can be 0.015 wt %, 0.014 wt %, 0.013 wt %, 0.012 wt %, 0.010 wt %, etc.

[0052] Positive effects of controlling the S content to ≤ 0.003 wt %: S content and the form of sulfide are the main factors affecting formability. S often combines with Mn to form MnS, which is more detrimental to hole expansion performance and is a harmful element in the present invention. Specifically, in this embodiment, the S content can be 0.003 wt %, 0.002 wt %, 0.001 wt %, etc.

[0053] The positive effect of controlling the N content to ≤ 0.005 wt % is that a low content of N can form dispersed TiN with Ti and refine austenite grains, but an excessively high content of N will form large-sized sharp-angle inclusions with Al, seriously reducing the strength and toughness of the material and the hole expansion performance. Specifically, in this embodiment, the N content can be 0.005 wt %, 0.004 wt %, 0.003 wt %, 0.002 wt %, 0.001 wt %, etc.

[0054] The positive effect of controlling the Cr content to 0.1 wt% to 0.5 wt% is that Cr and Mo delay the ferrite and pearlite transformations and expand the bainite transformation process window. Specifically, in this embodiment, the Cr content can be 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, etc.

[0055] The positive effects of controlling the Mo content to 0.1 wt% to 0.5 wt% are as follows: Cr and Mo delay the ferrite and pearlite transformation, expand the bainite transformation process window, and Mo improves the weld strength to a certain extent. Specifically, in this embodiment, the Mo content can be 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, etc.

[0056] The positive effect of controlling the Nb content to 0.01 wt% to 0.05% is that the pinning effect of the microalloying element Nb is used to promote grain refinement during hot rolling, and at the same time, it helps to reduce the strength difference between the bainite / ferrite matrix and improve the hole expansion performance. However, considering the cost issue, the Nb content is controlled below 0.05%. Specifically, in this embodiment, the Nb content can be 0.01 wt%, 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, etc.

[0057] The positive effect of the Ti content of 0.02 wt% to 0.05 wt% is that Ti can combine with N to form TiN, fix N and refine austenite grains at the same time, but Ti and N nitrogen have too strong a binding ability, and excessive Ti can easily lead to coarsening of TiN, resulting in performance deterioration; at the same time, welding experiments show that adding a trace amount of Ti can effectively improve the strength of the weld structure. Specifically, in this embodiment, the Ti content can be 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, etc.

[0058] In some embodiments, it is characterized in that the chemical composition of the hole expansion steel satisfies the following two conditions:

[0059] [Cr] + [Mo] ≤ 0.5 wt%, [Nb] + [Ti] ≤ 0.07 wt%;

[0060] Here, [Cr] represents the Cr content, [Mo] represents the Mo content, [Nb] represents the Nb content, and [Ti] represents the Ti content.

[0061] The carbide precipitation strengthening and solid solution strengthening of Cr and Mo are crucial to the strength improvement of this material. Taking the alloy cost into consideration, the positive effects of controlling Cr+Mo≤0.5% and controlling Nb+Ti≤0.07% by weight are: Under the premise of controllable cost, it can ensure sufficient grain refinement and provide precipitation dispersed in ferrite in the cooling section to ensure strength.

[0062] In some embodiments, the microstructure of the hole expansion steel is bainite and ferrite, wherein the volume fraction of bainite is 25-45%, see Figure 2 .

[0063] The positive effect of bainite: improving the uniformity of the organization and the strength of the material; the positive effect of ferrite: coordinating the strain during the deformation process and ensuring the plasticity; the positive effect of controlling the volume fraction of bainite to 25% to 45%: ensuring the strength of the material while making the bainite evenly distributed among the ferrites; if the volume fraction of bainite is too high, it will reduce the elongation to a certain extent and be unfavorable for forming; if the volume fraction of bainite is too low, it will lead to insufficient material strength to a certain extent. Specifically, in this embodiment, the volume fraction of bainite can be 25%, 27%, 29%, 31%, 33%, 35%, 37%, 39%, etc.

[0064] In the second aspect, the present application provides a method for preparing hole-expanding steel, see Figure 1 , used to prepare the hole-expanding steel described in the first aspect, the method comprising:

[0065] S1, heating the slab having the chemical composition, and then performing rough rolling on the heated slab under the condition of setting a first finishing rolling temperature;

[0066] S2, under the conditions of setting the rolling speed, setting the second finishing rolling temperature and setting the rolling target thickness, performing finish rolling on the slab after the rough rolling to obtain a hot-rolled plate;

[0067] S3. Cooling the hot-rolled plate in stages so that the hot-rolled plate has a first temperature to obtain hole-expanded steel.

[0068] In some embodiments, the first finishing rolling temperature is set to 1020°C to 1100°C.

[0069] The positive effect of controlling the final rolling temperature of rough rolling to 1020℃~1100℃ is to ensure that the steel plate has good strength, hole expansion rate and elongation. If the temperature is too high, it is easy to form acicular ferrite to a certain extent, and the plasticity is poor; if the temperature is too low, it is easy to form elongated ferrite grains or enter the γ+α two-phase region to a certain extent. Specifically, the final rolling temperature of rough rolling can be 1020℃, 1030℃, 1040℃, 1050℃, 1060℃, 1070℃, 1080℃, 1090℃, 1100℃, etc.

[0070] In some embodiments, the set rolling speed is 5 m / s to 11 m / s; the set second finishing temperature is 860 to 930° C.; and the set rolling target thickness is 1.8 mm to 5.5 mm.

[0071] In the embodiment of the present application, the finishing rolling adopts constant speed rolling, and the positive effect of controlling the rolling speed of the finishing rolling to be 5m / s to 11m / s is: to ensure that the strip steel passes through the rolling mill smoothly at the target temperature point. If the rolling speed of the finishing rolling is too high, the rolling load will increase to a certain extent, affecting the stability of the organizational performance; if the rolling speed of the finishing rolling is too low, it will cause the material temperature drop to be too high to meet the target temperature to a certain extent. Specifically, the rolling speed of the finishing rolling can be 5m / s, 6m / s, 7m / s, 8m / s, 9m / s, 10m / s, 11m / s, etc. The rolling target thickness is related to the rolling speed. The positive effect of controlling the rolling target thickness to be 1.8mm to 5.5mm is: if the rolling target thickness is too high, it will cause uneven organization of the edge and core of the material to a certain extent; if the rolling target thickness is too low, it will affect the preparation stability and plate shape control to a certain extent. Specifically, the rolling target thickness can be 1.8mm, 2.1mm, 2.4mm, 2.7mm, 3.0mm, 3.3mm, 3.6mm, etc. The positive effect of controlling the final rolling temperature of fine rolling to 860℃~930℃: ensuring that the steel plate has good strength, hole expansion rate and elongation. If the temperature is too high, it is easy to form acicular ferrite with poor plasticity to a certain extent; if the temperature is too low, it is easy to form elongated ferrite grains or enter the γ+α two-phase region to a certain extent. Specifically, the final rolling temperature of fine rolling can be 860℃, 880℃, 900℃, 920℃, etc.

[0072] In some embodiments, the step of cooling the hot rolled plate in stages so that the hot rolled plate has a first temperature to obtain a hole-expanded steel comprises:

[0073] Air cooling the hot rolled plate to make the hot rolled plate reach a second temperature;

[0074] Water cooling the hot-rolled plate at the second temperature to make the hot-rolled plate reach the first temperature to obtain hole-expanded steel;

[0075] The first temperature is 400°C to 450°C, and the second temperature is 700°C to 760°C.

[0076] In the embodiment of the present application, the hot-rolled plate is cooled in stages, including: first air-cooling the hot-rolled plate to 700-760°C, and then water-cooling to 400-450°C. 700-760°C is the rapid nucleation zone of the ferrite (α) phase, and its positive effects are: air cooling ensures that the ferrite phase has sufficient nucleation growth time, and at the same time realizes the enrichment of carbon elements in austenite. The enrichment of carbon elements in austenite by air cooling increases the subsequent ferrite lath phase transformation dynamics in bainite, refines the bainite laths and introduces dispersed Maho islands, effectively divides the bainite laths, and reduces the residual austenite in the grain boundary triangle area. The appearance of bainite lath-lath interface and lath-Maho island interface makes the cracks in the fatigue test continuously pass through different phase interfaces during the expansion process, increasing the crack expansion resistance and thus improving the fatigue strength of the steel. If the second temperature is too high, the ferrite grain size will be too large to a certain extent. If the second temperature is too low, the carbon element will not be enriched enough in austenite to a certain extent, resulting in insufficient driving force for bainite transformation and the formation of pearlite structure to be avoided. Specifically, the second temperature can be 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, etc. The positive effect of water cooling to 400-450℃: In order to increase the bainite transformation power and form bainite structure, the first temperature is too high, and pearlite is easy to form to a certain extent; the first temperature is too low, and martensite and other structures are easy to form to a certain extent. Specifically, the first temperature can be 400℃, 410℃, 420℃, 430℃, 440℃, 450℃, etc.

[0077] In some embodiments, after cooling the hot rolled plate in stages so that the hot rolled plate has a first temperature, the method further comprises:

[0078] The hot-rolled plate having the first temperature is coiled and then flattened to obtain hole-expanded steel.

[0079] In some embodiments, the hot-rolled plate having the first temperature is coiled and then flattened to obtain the hole-expanded steel, comprising:

[0080] The hot-rolled plate having the first temperature is coiled, and then flattened under the conditions of set flattening elongation and rolling force to obtain hole-expanded steel;

[0081] The set flattening elongation is 1% to 3%, and the set flattening rolling force is 400t to 500t.

[0082] In the embodiment of the present application, the surface quality of the steel is ensured by leveling, and the positive effect of controlling the leveling elongation to 1% to 3% is: it can effectively ensure the material shape and eliminate the yield platform. If the leveling elongation is too high, it will cause work hardening of the material to a certain extent, reduce the elongation and hole expansion performance. If the leveling elongation is too low, it will be impossible to ensure the material shape to a certain extent. Specifically, the leveling elongation can be 1%, 2%, 3%, etc. The positive effect of controlling the leveling rolling force to 400t to 500t is related to the leveling elongation setting to improve the leveling effect. Specifically, the leveling rolling force can be 400t, 420t, 440t, 460t, 480t, 500t, etc.

[0083] The hole-expanding steel is realized based on the preparation method of the hole-expanding steel mentioned above. The specific steps of the preparation method of the hole-expanding steel can refer to the above-mentioned embodiment. Since the preparation method of the hole-expanding steel adopts part or all of the technical solutions of the above-mentioned embodiment, it at least has all the beneficial effects brought by the technical solutions of the above-mentioned embodiment, which will not be described one by one here.

[0084] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for which specific conditions are not specified in the following examples are usually measured according to national standards. If there is no corresponding national standard, then the conditions recommended by the manufacturer are followed.

[0085] Example 1

[0086] The chemical composition by weight percentage is: C 0.06%, Si 0.1%, Mn 1.4%, P 0.011%, S 0.003%, Nb 0.02%, Cr 0.25%, Mo 0.2%, Ti 0.03%, Al 0.5%, and the rest is Fe and unavoidable impurities.

[0087] The preparation method comprises: pre-treating molten iron, smelting in a converter, refining, and continuously casting to obtain a slab with the above chemical composition; heating the slab, and rough rolling the heated slab under the condition that the final rolling temperature of the rough rolling is 1050°C; finishing rolling the slab after rough rolling at a rolling speed of 10m / s, a final rolling temperature of 860°C, and a target thickness of 3.6mm to obtain a hot-rolled plate; air cooling the hot-rolled plate to 740°C first, and then water cooling to 420°C; then coiling to obtain a hot-rolled coil; flattening the hot-rolled coil cooled to room temperature under the conditions of a flattening elongation of 1.5% and a flattening rolling force of 450t; then pickling using shallow trough turbulent pickling, and the strip running speed is 100m / min. After pickling, the surface color of the strip is grayish white, and then it is dried, electrostatically sprayed with anti-rust oil on both sides, coiled, and packaged to become a finished product.

[0088] Example 2

[0089] The chemical composition by weight percentage is: C 0.08%, Si 0.1%, Mn 1.4%, P 0.012%, S0.002%, Nb 0.03%, Cr: 0.1%, Mo 0.4%, Ti 0.02%, Al 0.8%, and the rest is Fe and unavoidable impurities.

[0090] The preparation method comprises: pre-treating molten iron, smelting in a converter, refining, and continuously casting to obtain a slab with the above chemical composition; heating the slab, and rough rolling the heated slab under the condition that the final rolling temperature of the rough rolling is 1060°C; finishing rolling the slab after rough rolling at a rolling speed of 8m / s, a final rolling temperature of the fine rolling of 880°C, and a target thickness of 1.5mm to obtain a hot-rolled plate; air cooling the hot-rolled plate to 720°C first, and then water cooling it to 450°C; then coiling it to obtain a hot-rolled coil; flattening the hot-rolled coil cooled to room temperature under the conditions that the flattening elongation is 1.0% and the flattening rolling force is 400t; then pickling is performed by shallow trough turbulent pickling, and the strip running speed is 80m / min. After pickling, the surface color of the strip is grayish white, and then it is dried, electrostatically sprayed with anti-rust oil on both sides, and then coiled and packaged to become a finished product.

[0091] Example 3

[0092] The chemical composition by weight percentage is: C 0.07%, Si 0.15%, Mn 1.3%, P 0.010%, S0.001%, Nb 0.02%, Cr: 0.15%, Mo 0.3%, Ti 0.03%, Al 0.7%, and the rest is Fe and unavoidable impurities.

[0093] The preparation method comprises: pre-treating molten iron, smelting in a converter, refining, and continuously casting to obtain a slab with the above chemical composition; heating the slab, and rough rolling the heated slab under the condition that the final rolling temperature of the rough rolling is 1080°C; finishing rolling the slab after rough rolling at a rolling speed of 9m / s, a final rolling temperature of the fine rolling of 880°C, and a target thickness of 1.8mm to obtain a hot-rolled plate; air cooling the hot-rolled plate to 750°C first, and then water cooling to 450°C; then coiling to obtain a hot-rolled coil; flattening the hot-rolled coil cooled to room temperature under the conditions of a flattening elongation of 2.0% and a flattening rolling force of 450t; then pickling using shallow trough turbulent pickling, and the strip running speed is 90m / min. After pickling, the surface color of the strip is grayish white, and then it is dried, electrostatically sprayed with anti-rust oil on both sides, coiled, and packaged to become a finished product.

[0094] Example 4

[0095] The chemical composition is calculated by weight percentage: C 0.1%, Si 0.2%, Mn 1.5%, P 0.015%, S0.003%, Nb 0.05%, Cr: 0.40%, Mo 0.10%, Ti 0.02%, Al 1%, and the rest is Fe and unavoidable impurities.

[0096] The preparation method comprises: pre-treating molten iron, smelting in a converter, refining, and continuously casting to obtain a slab with the above chemical composition; heating the slab, and rough rolling the heated slab under the condition that the final rolling temperature of the rough rolling is 1100°C; finishing rolling the slab after rough rolling at a rolling speed of 11m / s, a final rolling temperature of the fine rolling of 930°C, and a target thickness of 5.5mm to obtain a hot-rolled plate; air cooling the hot-rolled plate to 760°C first, and then water cooling to 450°C; then coiling to obtain a hot-rolled coil; flattening the hot-rolled coil cooled to room temperature under the conditions of a flattening elongation of 3.0% and a flattening rolling force of 500t; then pickling using shallow trough turbulent pickling, and the strip running speed is 100m / min. After pickling, the surface color of the strip is grayish white, and then it is dried, electrostatically sprayed with anti-rust oil on both sides, coiled, and packaged to become a finished product.

[0097] Example 5

[0098] The chemical composition by weight percentage is: C 0.07%, Si 0.1%, Mn 0.8%, P 0.015%, S0.003%, Nb 0.01%, Cr: 0.20%, Mo 0.20%, Ti 0.05%, Al 0.3%, and the rest is Fe and unavoidable impurities.

[0099] The preparation method comprises: pre-treating molten iron, smelting in a converter, refining, and continuously casting to obtain a slab with the above chemical composition; heating the slab, and rough rolling the heated slab under the condition that the final rolling temperature of the rough rolling is 1020°C; finishing rolling the slab after rough rolling at a rolling speed of 5m / s, a final rolling temperature of the fine rolling of 860°C, and a target thickness of 1.8mm to obtain a hot-rolled plate; air cooling the hot-rolled plate to 700°C first, and then water cooling to 400°C; then coiling to obtain a hot-rolled coil; flattening the hot-rolled coil cooled to room temperature under the conditions of a flattening elongation of 1.0% and a flattening rolling force of 400t; then pickling using shallow trough turbulent pickling, and the strip running speed is 100m / min. After pickling, the surface color of the strip is grayish white, and then it is dried, electrostatically sprayed with anti-rust oil on both sides, coiled, and packaged to become a finished product.

[0100] Comparative Example 1

[0101] The chemical composition by weight percentage is: C 0.06%, Si 0.3%, Mn 1.4%, P 0.011%, S 0.003%, Nb 0.02%, Cr 0.25%, Mo 0.2%, Ti 0.03%, Al 0.2%, and the rest is Fe and unavoidable impurities.

[0102] The preparation method comprises: pre-treating molten iron, smelting in a converter, refining, and continuously casting to obtain a slab with the above chemical composition; heating the slab, and rough rolling the heated slab under the condition that the final rolling temperature of the rough rolling is 1050°C; finishing rolling the slab after rough rolling at a rolling speed of 10m / s, a final rolling temperature of 860°C, and a target thickness of 3.6mm to obtain a hot-rolled plate; air cooling the hot-rolled plate to 740°C first, and then water cooling to 420°C; then coiling to obtain a hot-rolled coil; flattening the hot-rolled coil cooled to room temperature under the conditions of a flattening elongation of 1.5% and a flattening rolling force of 450t; then pickling using shallow trough turbulent pickling, and the strip running speed is 100m / min. After pickling, the surface color of the strip is grayish white, and then it is dried, electrostatically sprayed with anti-rust oil on both sides, coiled, and packaged to become a finished product.

[0103] Comparative Example 2

[0104] The chemical composition by weight percentage is: C 0.05%, Si 0.3%, Mn 1.6%, P 0.011%, S 0.003%, Nb 0.03%, Cr 0.4%, Mo 0.3%, Ti 0.05%, Al 0.2%, and the rest is Fe and unavoidable impurities.

[0105] The preparation method comprises: pre-treating molten iron, smelting in a converter, refining, and continuously casting to obtain a slab with the above chemical composition; heating the slab, and rough rolling the heated slab under the condition that the final rolling temperature of the rough rolling is 1000°C; finishing rolling the slab after rough rolling at a rolling speed of 4m / s, a final rolling temperature of the fine rolling of 840°C, and a target thickness of 3.0mm to obtain a hot-rolled plate; air cooling the hot-rolled plate to 790°C first, and then water cooling to 350°C; then coiling to obtain a hot-rolled coil; flattening the hot-rolled coil cooled to room temperature under the conditions of a flattening elongation of 0.5% and a flattening rolling force of 390t; then pickling using shallow trough turbulent pickling, and the strip running speed is 100m / min. After pickling, the surface color of the strip is grayish white, and then it is dried, electrostatically sprayed with anti-rust oil on both sides, coiled, and packaged to become a finished product.

[0106] Table 1 Performance results of the hole-expanding steels of Examples 1-5 and Comparative Examples 1-2.

[0107]

[0108] From Examples 1-5 in Table 1, the microstructure of the hole-expanding steel prepared in the examples of the present application is ferrite + bainite, wherein the volume ratio of the bainite structure is 25-45%, the tensile strength of the hole-expanding steel is ≥800MPa, the tensile and compressive fatigue limit (R=-1) FL≥380MPa, the hole-expanding rate λ≥60%, and the elongation A80≥14%. The hole-expanding performance and fatigue performance of the hole-expanding steel in Examples 1-5 of the present application are better than those of the hole-expanding steel in Comparative Examples 1-2. In summary, the content of the present application solves the problem of poor hole-expanding performance and fatigue performance of hole-expanding steels above 800MPa in the prior art.

[0109] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A method for preparing hole-expanding steel, characterized in that: The method comprises: The slab is heated, and then the heated slab is rough rolled under the condition of setting a first finishing rolling temperature, wherein the first finishing rolling temperature is set to be 1020° C. to 1100° C.; Under the conditions of setting the rolling speed, setting the second finishing rolling temperature and setting the rolling target thickness, the slab after the rough rolling is finished rolled to obtain a hot rolled plate, the set rolling speed is 5m / s to 11m / s; the set second finishing rolling temperature is 860 to 930°C; the set rolling target thickness is 1.8mm to 5.5mm; Air cooling the hot rolled plate to make the hot rolled plate reach a second temperature; The hot-rolled plate having the second temperature is water-cooled to make the hot-rolled plate reach the first temperature to obtain hole-expanded steel; the first temperature is 400° C. to 450° C., and the second temperature is 700° C. to 760° C.; The hot-rolled plate having the first temperature is coiled, and then flattened under the conditions of set flattening elongation and rolling force to obtain hole-expanding steel; the set flattening elongation is 1% to 3%, and the set flattening rolling force is 400t to 500t; The chemical composition of the hole-expanding steel includes: a C content of 0.06 wt% to 0.1 wt%; a Mn content of 0.8 wt% to 1.5 wt%; a P content of ≤0.015 wt%; a S content of ≤0.003 wt%; a Cr content of 0.1 wt% to 0.5 wt%; a Nb content of 0.01 wt% to 0.05%; a Mo content of 0.1 wt% to 0.5 wt%; a N content of ≤0.005 wt%; and a Ti content of 0.02 wt% to 0.05 wt%. The content of Si is ≤0.2 wt %; the content of Al is 0.3 wt % to 1.0 wt %, and the remainder is Fe and inevitable impurities.

2. The hole-expanding steel prepared by the method according to claim 1 is characterized in that: The chemical composition of the hole expansion steel meets the following two requirements: [Cr] + [Mo] ≤ 0.5 wt%, [Nb] + [Ti] ≤ 0.07 wt%; Here, [Cr] represents the Cr content, [Mo] represents the Mo content, [Nb] represents the Nb content, and [Ti] represents the Ti content.

3. The hole-expanding steel according to claim 2, characterized in that: The microstructure of the hole-expanding steel is bainite and ferrite, wherein the volume fraction of bainite is 25-45%.

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