A method for preparing a high-strength steel with low surface white line-like defects and the high-strength steel

Abstract

The application relates to the technical field of steel rolling, in particular to a method for preparing high-strength steel with low surface white line type defects and high-strength steel; the method comprises the following steps: heating a casting blank, preheating a roller and continuously rolling to obtain a strip steel; cold rolling the strip steel, and then annealing and flattening to obtain high-strength steel with low surface white line type defects; wherein the continuous rolling is carried out by using an infinite cold hard roller; the roller preheating comprises preheating the infinite cold hard roller by using a roller material, and the terminal temperature of the preheating is less than or equal to 880 DEG C; the graphite spheroidization grade of the infinite cold hard roller is greater than or equal to 2; the chemical components of the high-strength steel comprise C, Si, Mn, P, S, Al, B, Nb and Ti; by controlling the preheating temperature of the infinite cold hard roller in the rolling stage and the graphite spheroidization grade of the infinite cold hard roller, the thickness of the oxidation film of the infinite cold hard roller and the surface oxidation film are difficult to fall off, so that the surface white line type defects caused by the falling off of the oxidation film are avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel rolling, in particular to a method for preparing high-strength steel with low surface white line type defects and high-strength steel. BACKGROUND

[0002] In the quality requirements of modern cars, especially high-end and luxury cars, surface brightness, aesthetics and good paint performance are very important. In particular, high-end and luxury cars require that the surface of the steel plate be clean and free of defects, and that the outer plate be a cold-rolled plate with O5 level and flatness higher than 5I. During the production of the steel plate, the steel strip will come into contact with the pair of rollers, the roll, the trimming shear and the cross-cut shear, etc., but the final steel plate product is not allowed to have any scratches, indentations and stains. However, in practice, the generation of surface defects on the strip occurs throughout the entire process from smelting, continuous casting to hot rolling, cold rolling, annealing, flattening and packaging. Therefore, when high-strength IF steel is used to produce automobile outer plates, short linear scale defects frequently occur. However, through on-site investigation, it was found that such defects exist on the surface of the steel plate only during cold rolling, and there are no obvious corresponding defects during hot rolling.

[0003] Although current reports on such scale indentation defects all indicate that they are closely related to the use of the roller, most research and solutions are directed at high-speed steel rollers, and it is currently not possible to effectively reduce such defects for non-high-speed steel rollers, especially for infinite cold hard rollers. Therefore, how to provide a preparation method for reducing the surface white line type defects of high-strength steel after cold rolling of infinite cold hard rollers is a technical problem that needs to be solved at present. SUMMARY

[0004] The present application provides a method for preparing high-strength steel with low surface white line type defects and high-strength steel to solve the technical problem that the preparation process of high-strength steel in the prior art cannot reduce the surface white line defects after cold rolling of infinite cold hard rollers.

[0005] In a first aspect, the present application provides a method for preparing high-strength steel with low surface white line type defects, which comprises:

[0006] heating the cast blank, preheating the roller and continuously rolling to obtain a steel strip;

[0007] cold rolling the steel strip, followed by annealing and flattening to obtain high-strength steel with low surface white line type defects;

[0008] wherein the continuous rolling is performed using an infinite cold hard roller;

[0009] the roller preheating comprises preheating the infinite cold hard roller with a hot roller material, and the end temperature of the preheating is ≤880℃;

[0010] The graphite spheroidization grade of the endless hard rolling roller is greater than or equal to grade 2.

[0011] Optionally, the number of the hot rolling materials is greater than or equal to 10.

[0012] Optionally, the number of the continuously rolled cast slabs is less than or equal to 15.

[0013] Optionally, the continuous rolling further comprises adding wear-resistant particles for continuous rolling, and the diameter of the wear-resistant particles is less than 5 microns.

[0014] Optionally, the chemical composition of the wear-resistant particles satisfies:

[0015] [V] / [Nb] = 0.5-1.5:1,

[0016] wherein [V] is the V content in the wear-resistant particles, and [Nb] is the Nb content in the wear-resistant particles.

[0017] Optionally, the continuous rolling comprises rough rolling, rough rolling descaling, finishing rolling descaling and finishing rolling, and the descaling water pressure of the rough rolling descaling and the finishing rolling descaling is 18-22 MPa.

[0018] Optionally, the inlet temperature of the finishing rolling is 1000-1040 DEG C, and the finish rolling temperature is 880-900 DEG C.

[0019] Optionally, the cooling water pressure between adjacent two stands of the finishing rolling is 8-10 MPa, and the cooling water input mass ratio between adjacent two stands of the finishing rolling is 0.5-0.8.

[0020] Optionally, the soaking temperature of the heating is 1180-1200 DEG C, the time of the heating is 140-160 min, and the lambda value of the heating is 0.8-0.9.

[0021] In a second aspect, the application further provides a high-strength steel, which is prepared by the method of the first aspect, and the chemical composition of the high-strength steel comprises, in mass fraction: C: 0.0015%-0.004%, Si: 0.05%-0.12%, Mn: 0.3%-0.5%, P: 0.03%-0.05%, S: 0.01%-0.02%, Al: 0.03%-0.05%, B: 0.0004%-0.0008%, Nb: 0.025%-0.035%, Ti: 0.035%-0.045%, and the rest is Fe and inevitable impurities.

[0022] Compared with the prior art, the above technical solution provided by the embodiments of the application has the following advantages:

[0023] The method for preparing the high-strength steel with low surface white line type defects provided by the embodiment of the application controls the preheating temperature of the endless hard rolling roller in the rolling stage and the graphite spheroidization grade of the endless hard rolling roller. Since the endless hard rolling roller will be subjected to accelerated oxidation when the temperature is above 1080 DEG C, the thickness of the oxidation film of the endless hard rolling roller increases, the oxidation film on the endless hard rolling roller is prone to falling off in the subsequent cold rolling stage, and the oxidation film is pressed into the steel plate to form surface white line type defects. Therefore, the temperature requirement of the casting blank in the rolling process is comprehensively considered, the use temperature of the endless hard rolling roller is controlled to be below 880 DEG C, the thickness of the oxidation film of the endless hard rolling roller is ensured, and the graphite spheroidization grade of the endless hard rolling roller is controlled. Since the graphite in the roller mainly plays a lubricating role, not only can the peeling defects on the surface of the roller body be reduced, but also the thermal shock on the surface of the roller can be reduced. Therefore, under the premise of ensuring the use temperature of the endless hard rolling roller, the graphite spheroidization grade of the endless hard rolling roller is controlled, so that the surface oxidation film of the endless hard rolling roller is difficult to fall off, the surface white line type defects caused by the falling off of the oxidation film are avoided, and only a small number of surface white line type defects are ensured on the surface of the steel plate after cold rolling. BRIEF DESCRIPTION OF DRAWINGS

[0024] The drawings incorporated into the specification and constituting a part of the specification show embodiments consistent with the application and, together with the specification, serve to explain the principles of the application.

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings according to these drawings without any creative effort.

[0026] Figure 1 The flowchart of the method provided by the embodiment of the application is shown in the figure.

[0027] Figure 2 The macroscopic morphology schematic diagram of the surface white line type defects provided by the embodiment of the application is shown in the figure.

[0028] Figure 3 The microscopic morphology schematic diagram of the surface white line type defects provided by the embodiment of the application is shown in the figure.

[0029] Figure 4 The enlarged microscopic morphology schematic diagram of the surface white line type defects provided by the embodiment of the application is shown in the figure.

[0030] Figure 5 The energy spectrum analysis result diagram of the scarring scar body in the surface white line type defects provided by the embodiment of the application is shown in the figure.

[0031] Figure 6An enlarged view of the analysis result of the C element of the interface electron probe of the scar body of the scarring in the surface white line type defect provided by the embodiment of the present application;

[0032] Figure 7 A graph of the analysis result of the C element of the interface electron probe of the scar body of the scarring in the surface white line type defect provided by the embodiment of the present application;

[0033] Figure 8 A graph of the analysis result of the O element of the interface electron probe of the scar body of the scarring in the surface white line type defect provided by the embodiment of the present application;

[0034] Figure 9 A graph of the analysis result of the Si element of the interface electron probe of the scar body of the scarring in the surface white line type defect provided by the embodiment of the present application;

[0035] Figure 10 A graph of the analysis result of the Cr element of the interface electron probe of the scar body of the scarring in the surface white line type defect provided by the embodiment of the present application;

[0036] Figure 11 A graph of the analysis result of the Ni element of the interface electron probe of the scar body of the scarring in the surface white line type defect provided by the embodiment of the present application;

[0037] Figure 12 A graph of the energy spectrum analysis result of the blocky white bright metal in the scar body of the scarring in the surface white line type defect provided by the embodiment of the present application;

[0038] Figure 13 A graph of the oxidation weight gain curve of the infinite cold hardening roll provided by the embodiment of the present application;

[0039] Figure 14 A graph of the oxidation result of the graphite in the infinite cold hardening roll under the heating condition of 800 DEG C provided by the embodiment of the present application;

[0040] Figure 15 A graph of the state of the problem graphite in the structure of the infinite cold hardening roll provided by the embodiment of the present application;

[0041] Figure 16 A graph of the state of the problem wear-resistant particle of the infinite cold hardening roll provided by the embodiment of the present application. DETAILED DESCRIPTION

[0042] To make the purposes, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work belong to the scope of protection of the present application.

[0043] The creative thinking of the present application is that: currently domestic manufacturers have analyzed the white line type defects, specifically including:

[0044] (1) Small white strip defects on the surface of pickling plates occur along the length direction of the surface of the strip steel, the defects are in the form of small strips, the surface is rough and has a hand feel, and analysis shows that the small white strip defects have a corresponding relationship with the meteor-like peeling of the M part of the surface of the high-speed steel roller of the front stand;

[0045] (2) When producing 2.0mm export materials, batch of pockmarked scale defects appear on the surface of the hot-rolled strip steel in the finishing rolling area, such defects cannot be removed after pickling, forming a large number of rejudgments. Analysis shows that the high-speed steel roller can be seen that the density of the scale is large at the initial stage, and the scale is distributed in the form of pockmarks. With the continuation of the rolling process, the degree of scale gradually decreases, and finally transitions to a strip shape.

[0046] In an embodiment of the present application, a method for preparing a high-strength steel with low surface white line type defects is provided, the method comprising:

[0047] S1. Heating a cast blank, preheating a roller, and continuously rolling to obtain a strip steel;

[0048] S2. Cold rolling the strip steel, then annealing and skin passing to obtain a high-strength steel with low surface white line type defects;

[0049] Wherein, the continuous rolling is carried out by using an infinite cold hard roller;

[0050] The roller preheating comprises preheating the infinite cold hard roller with a hot roller material, and the end point temperature of the preheating is ≤880℃;

[0051] The graphite spheroidization grade of the infinite cold hard roller is ≥2 levels.

[0052] In the embodiments of the present application, the positive effect of controlling the end point temperature of the preheating to be ≤880℃ is that within the range of the end point temperature, a stable and dense oxide film can be formed on the surface of the infinite cold hard roller, and the thickness of the oxide film is ensured to be within a suitable range, avoiding the situation that the surface of the infinite cold hard roller forms a relatively thick oxide film due to too high temperature, thereby causing surface layer cracking and falling off.

[0053] The positive effect of graphite spheroidization grade of the infinite chill roll being greater than or equal to level 2 is that graphite plays a lubricating role in the roll, which can reduce the peeling defects on the surface of the roll body and slow down the thermal shock on the surface of the roll; when the graphite spheroidization grade is less than level 2, the graphite spheroidization state on the surface of the roll is not good, and the roll surface is in a worm-like and sea urchin-like morphology, which will cause the soft and hard interfacial cracking during use.

[0054] In some optional embodiments, the number of the hot-rolled materials is greater than or equal to 10.

[0055] In the embodiments of the present application, the number of the hot-rolled materials is controlled, which can not only ensure that the temperature of the surface of the infinite chill roll is within the range of 880℃ or below, but also promote the generation of a more complete surface oxide film layer of the infinite chill roll.

[0056] In some optional embodiments, the number of the continuously rolled cast slabs is less than or equal to 15.

[0057] In the embodiments of the present application, the positive effect of controlling the number of the continuously rolled cast slabs to be less than or equal to 15 is that within this range, the thickness of the oxide film on the surface of the steel plate can be reduced during subsequent rolling; when the number of the rolling is greater than or less than the endpoint value of the range, the adverse effects are that when the number of the rolling is too low, the dense oxide film is not formed on the surface of the roll, and the surface oxidation scale is not uniform during the rolling process; when the number of the rolling is too high, the oxide film on the surface of the roll grows too thick, and the oxide film peeling occurs during the rolling process, which easily causes the roll scale indentation defects.

[0058] In some optional embodiments, the continuous rolling further comprises adding wear-resistant particles for continuous rolling, and the diameter of the wear-resistant particles is less than 5μm.

[0059] In the embodiments of the present application, the positive effect of controlling the diameter of the wear-resistant particles to be less than 5μm is that within this range, the material can have high hardness and fine and dispersed distribution of carbides (M-C or M-N) when crystallized at high temperature, so as to ensure that the steel plate surface is free of white line defects; when the diameter is greater than the endpoint value of the range, the adverse effect is that the fatigue cracking occurs due to the difference in hardness between the wear-resistant particles and the body during the use of the roll.

[0060] In some optional embodiments, the chemical composition of the wear-resistant particles satisfies:

[0061] [V] / [Nb]=0.5-1.5:1,

[0062] wherein, [V] is the V content in the wear-resistant particles, and [Nb] is the Nb content in the wear-resistant particles.

[0063] In the embodiments of the present application, the positive effect of [V] / [Nb]=0.5-1.5:1 is that within the range, the wear-resistant particles can be formed in a refined and dispersed distribution; when the ratio is greater than or less than the end value of the range, the wear-resistant particles will be in a large block aggregation distribution or a state of too little precipitation, affecting the rolling of the steel.

[0064] In some optional embodiments, the continuous rolling includes rough rolling, rough rolling descaling, finishing rolling descaling, and finishing rolling, and the descaling water pressure of the rough rolling descaling and the finishing rolling descaling is 18-22 MPa.

[0065] In the embodiments of the present application, the positive effect of controlling the descaling water pressure of the rough rolling descaling and the finishing rolling descaling to be 18-22 MPa is that the surface oxide scale can be effectively removed, and the oxide scale is prevented from being pressed into the steel plate to form surface defects; when the pressure is greater than or less than the end value of the range, the adverse effects are that the oxide scale cannot be removed at too low pressure, and the descaling pressure system is unstable at too high pressure.

[0066] In some optional embodiments, the inlet temperature of the finishing rolling is 1000-1040°C, and the finish rolling temperature is 880-900°C.

[0067] In the embodiments of the present application, the positive effect of the inlet temperature of the finishing rolling being 1000-1040°C is that the structure of the oxide scale is mainly FeO during the finishing rolling process, the content of FeO is more than 95%, and the plastic deformation ability is good, so that the oxide scale is not easily broken and cracked during the rolling process; when the temperature is greater than or less than the end value of the range, the adverse effect is that the proportion of FeO decreases during the finishing rolling process, and the oxide scale is easily broken and cracked during the rolling process.

[0068] The positive effect of controlling the finish rolling temperature to be 880-900°C is to avoid the steel falling into the two-phase region for rolling; when the temperature is greater than or less than the end value of the range, the adverse effects are that the temperature is too low to easily fall into the two-phase region for rolling, which easily causes abnormal surface structure of the steel plate; and the temperature is too high to increase the surface temperature of the strip and the surface temperature of the roller, and the oxide film is too thick to easily cause the oxide film to be broken and cracked.

[0069] In some optional embodiments, the finishing rolling has a strip casting speed of 8-10 m / s, and the inter-stand cooling water pressure of the finishing rolling is 8-10 MPa.

[0070] In the embodiments of the present application, the positive effect of the finishing rolling steel throwing speed of 8 m / s to 10 m / s is to ensure that the oxide film thickness in the finishing rolling process is controlled at a lower thickness; when the value of the steel throwing speed is greater than or less than the end point value of the range, the adverse effect is that the speed is too high, the rolling stability is at risk, and the running deviation and rolling waste are prone to occur; the speed is too low, and the oxide film formed in the finishing rolling process is too thick, and the rolling process is prone to cracking and indentation.

[0071] The positive effect of the finishing rolling inter-stand cooling water pressure of 8 MPa to 10 MPa is to effectively reduce the surface temperature of the steel plate and inhibit the excessive growth of the oxide film; when the value of the cooling water pressure is greater than or less than the end point value of the range, the adverse effect is that the oxide film on the surface of the steel plate cannot be effectively controlled, and the pressure is unstable, which may affect the surface structure of the steel plate and cause mixed crystals and other situations.

[0072] In some optional embodiments, the intermediate billet thickness of the finishing rolling is 32 mm to 38 mm, the reduction rate of the fourth stand of the finishing rolling is ≤30%, and the reduction rate of the fifth stand of the finishing rolling is ≤20%.

[0073] In the embodiments of the present application, the positive effect of the intermediate billet thickness of the finishing rolling of 32 mm to 38 mm is to transfer the rolling load and reduce the rolling load in the finishing rolling process to avoid excessive compression ratio exceeding the FeO deformation limit; when the value of the intermediate billet thickness is greater than or less than the end point value of the range, the adverse effect is that the finishing rolling process cannot effectively complete a large number of dislocations to play a nucleation and fine-grain effect when the thickness is too low; when the thickness is too thick, the finishing rolling process has a large compression ratio, and cracking occurs due to the inability of FeO to deform with the matrix.

[0074] The positive effect of the reduction rate of the fourth stand of the finishing rolling ≤30% is to ensure the stability of the roll oxide film; when the value of the reduction rate is greater than or less than the end point value of the range, the adverse effect is that the overall finishing rolling process cannot guarantee compression deformation when the reduction rate is too low, and the roll oxide film is prone to peeling and breaking when the reduction rate is too high.

[0075] The positive effect of the reduction rate of the fifth stand of the finishing rolling ≤20% is to ensure the stability of the roll oxide film; when the value of the reduction rate is greater than or less than the end point value of the range, the adverse effect is that the overall finishing rolling process cannot guarantee compression deformation when the reduction rate is too low, and the surface layer is prone to mixed crystals; the roll oxide film is prone to peeling and breaking when the reduction rate is too high.

[0076] In some optional embodiments, the cooling water pressure between the adjacent two stands of the finishing rolling is 8 MPa to 10 MPa, and the cooling water input mass ratio between the adjacent two stands of the finishing rolling is 0.5 to 0.8.

[0077] In the embodiments of the present application, the positive effect of the cooling water pressure between the two adjacent stands of the finishing rolling being 8MPa-10MPa is to effectively reduce the surface temperature of the steel plate and inhibit the excessive growth of the iron scale; when the value of the cooling water pressure is greater than or less than the end point value of the range, the adverse effect is that the iron scale on the surface of the steel plate cannot be effectively controlled due to being too low, and the pressure is unstable due to being too high, which may affect the surface structure of the steel plate and cause mixed crystals and other conditions.

[0078] The positive effect of the cooling water input mass ratio between the two adjacent stands of the finishing rolling being 0.5-0.8 is to ensure the cooling effect of the steel plate surface and avoid the generation of a large amount of iron scale; when the value of the ratio is greater than or less than the end point value of the range, the adverse effect is that a large amount of iron scale is generated, causing the iron scale to be pressed in.

[0079] In some optional embodiments, the heating soaking temperature is 1180-1200°C, the heating time is 140-160min, and the heating λ value is 0.8-0.9.

[0080] In the embodiments of the present application, the positive effect of controlling the heating soaking temperature to be 1180-1200°C is that within this temperature range, the oxide film state of the roll can be stable; when the temperature value is greater than or less than the end point value of the range, the adverse effect is that the heating furnace temperature is too high, which causes the surface temperature to be relatively high during the finishing rolling process of the steel plate, the roll surface temperature is too high, and the oxide film is prone to fall off; the heating temperature is too low, which causes the rolling force to be too large due to the surface temperature of the steel plate being too low during the finishing rolling process, and the roll is prone to fatigue cracking and oxide film falling off, and the Nb-Ti precipitates precipitated during continuous casting may not be effectively dissolved.

[0081] The positive effect of controlling the heating time to be 140-160min is that within this temperature range, the oxide film state of the roll can be stable; when the time value is greater than or less than the end point value of the range, the adverse effect is that the heating temperature is too low, which causes the steel to not be transparent and the austenitization to be insufficient, and the performance fluctuation is obvious during the rolling process; the heating temperature is too high, which causes the surface temperature to be too high and the iron scale to be too thick, and the iron scale is prone to residual and pressing in.

[0082] The positive effect of controlling the heating λ value to be 0.8-0.9 is that within this λ value range, the oxide film state of the roll can be stable; when the λ value is greater than or less than the end point value of the range, the adverse effect is that the steel is prone to a large amount of oxidation during the heating process due to being too high, and the gas pressure in the furnace fluctuates greatly and the oxidation atmosphere is difficult to control due to being too low.

[0083] Based on the same inventive concept, the application further provides a high-strength steel prepared by the method, the chemical composition of the high-strength steel includes, in mass fraction, C: 0.0015% to 0.004%, Si: 0.05% to 0.12%, Mn: 0.3% to 0.5%, P: 0.03% to 0.05%, S: 0.01% to 0.02%, Al: 0.03% to 0.05%, B: 0.0004% to 0.0008%, Nb: 0.025% to 0.035%, Ti: 0.035% to 0.045%, and the rest is Fe and inevitable impurities.

[0084] In the application, the positive effect of the mass fraction of C being 0.0015% to 0.004% is to ensure the forming performance of the steel, low yield strength, high uniform elongation and total elongation; when the mass fraction is greater than or less than the end point value of the range, the adverse effect is that the {111} component in the recrystallization texture sharply decreases with the increase of solid solution C, and if the content of C element is too low, the smelting cost of the process increases and the mechanical properties of the steel cannot be achieved through precipitation strengthening.

[0085] The positive effect of the mass fraction of Si being 0.05% to 0.12% is that solid solution strengthening ensures the strength of the steel; when the mass fraction is greater than or less than the end point value of the range, the adverse effect is that the strength is too low or too high to meet the user's demand.

[0086] The positive effect of the mass fraction of Mn being 0.3% to 0.5% is that solid solution strengthening ensures the strength of the steel; when the mass fraction is greater than or less than the end point value of the range, the adverse effect is that the strength is too low or too high to meet the user's demand.

[0087] The positive effect of the mass fraction of P being 0.03% to 0.05% is that solid solution strengthening greatly improves the strength of the steel; when the mass fraction is greater than or less than the end point value of the range, the adverse effect is that it is too low to play a solid solution strengthening effect and too high to easily cause P grain boundary precipitation to cause brittle cracking of the steel plate.

[0088] The positive effect of the mass fraction of S being 0.01% to 0.02% is that sulfur is a harmful element in deep drawing steel and should be reduced as much as possible, which easily causes a large amount of inclusions to be generated and consumes Ti\Nb elements, and cannot play a precipitation strengthening effect; when the mass fraction is greater than or less than the end point value of the range, the adverse effect is that a large amount of inclusions are formed to affect the deep drawing performance of the product.

[0089] The positive effect of the mass fraction of Al being 0.03% to 0.05% is that Al is added as a deoxidizer, and the main role is to remove oxygen dissolved in the molten steel during oxygen blowing smelting; when the mass fraction is greater than or less than the end value of the range, the adverse effect is that too low cannot effectively deoxidize, and too high affects the mechanical properties of the product.

[0090] The positive effect of the mass fraction of B being 0.0004% to 0.0008% is that the cold brittle transformation temperature of the IF steel can be significantly reduced, and since the steel type adds P element, the B element can be segregated at the grain boundary, and the segregation of P element at the grain boundary is inhibited; when the mass fraction is greater than or less than the end value of the range, the adverse effect is that too high causes cost loss, and too low cannot inhibit the segregation effect of P element.

[0091] The positive effect of the mass fraction of Nb being 0.025% to 0.035% is precipitation strengthening effect, which improves the mechanical properties of the steel type; when the mass fraction is greater than or less than the end value of the range, the adverse effect is that the performance cannot meet the user's demand.

[0092] The positive effect of the mass fraction of Ti being 0.035% to 0.045% is precipitation and fine-grain strengthening effect, which improves the mechanical properties of the steel type; when the mass fraction is greater than or less than the end value of the range, the adverse effect is that the performance cannot meet the user's demand.

[0093] Example 1

[0094] By analyzing the defects as shown in Figure 2 , the defects extend along the rolling direction, the defect width is about 0.2mm to 0.5mm, the length is 2mm to 5mm, and the overall defect presents an elongated white line morphology, and the surface microscopic analysis shows that Figures 3 to 12 ,

[0095] As shown in Figure 5 , the chemical composition analysis of different spectrum points in the spectrum analysis of the scar body is shown in Table 1:

[0096] Table 1

[0097] Spectrum O(％) Si (%) Cr(％) Mn (%) Fe (%) Ni (%) Mo (%) Total (%) spectrum Figure 1 3.37 1.26 4.12 0.83 89.61 0.83 - 100.00 spectrum Figure 2 2.35 - 8.15 2.09 84.60 2.80 - 100.00 spectrum Figure 3 8.07 0.75 3.74 2.58 74.64 8.25 1.96 100.00 spectrum Figure 4 4.31 1.71 1.22 1.15 83.53 8.08 - 100.00 spectrum Figure 5 4.64 0.91 3.18 1.57 84.16 5.53 - 100.00 spectrum Figure 6 10.71 - 8.71 1.35 77.11 2.12 - 100.00 spectrum Figure 7 - - - - 100.00 - - 100.00 spectrum Figure 8 - - - - 100.00 - - 100.00 spectrum Figure 9 - - - - 100.00 - - 100.00

[0098] As shown in Figure 12 , the chemical composition analysis of different spectrum points in the spectrum analysis of the block-shaped white metal in the scar body is shown in Table 2:

[0099] Table 2

[0100]

[0101]

[0102] The results show that: 1) there are scarring and broken patterns in the cold hard roll defects, and there are more iron oxide residues around the scars; 2) after surface polishing, it is found that the internal structure is scarring and string-like defect morphology, and the scar body is ferrite structure, and the internal structure is slightly smaller; 3) the surface of most scar bodies captures Ni-Cr-Mo element residues, which correspond to the composition of F4-F6 roll stand; 4) interface analysis of the scar body shows that there are many cracks and oxidation dot patterns in the scar body, and the surface layer region of the scar body adheres to the Ni-Cr-Si alloy element, and there is oxidation phenomenon; 5) a large amount of Nb element residues can be captured on the surface of individual scar bodies, showing bright white metal state, with a content of 8% to 80%, which is related to the falling of the hard state wear-resistant phase in the roll.

[0103] Oxidation characteristics analysis: with a heating rate of 10℃ / min, the oxidation weight gain curve is as follows Figure 13 It can be seen that the roll has good oxidation resistance below 1000℃, and the overall oxidation weight gain is controlled within 0.25%. When the steel grade is heated to about 1080℃, the oxidation occurs at an accelerated rate, and the peak position of the oxidation weight gain rate is near 1130℃, and the DTG can reach 0.2% / min. Subsequently, the oxidation weight gain rate slows down, and as the temperature continues to rise to 1200℃, the oxidation weight gain rate of the steel grade increases. It can be seen that as the temperature rises, the oxidation resistance of the roll is weakened, so the control idea is mainly to reduce the service temperature of the roll.

[0104] As shown in Figure 14 , it is found that the graphite position is obviously thickened during the oxidation process, and there is inward oxidation inside the body when the experiment is heated at 800℃ for 1h. It can be seen that as the oxidation temperature rises and the oxidation time extends, the selective oxidation of different elements in the roll will occur. Since the main element in graphite is C, which is more prone to oxidation under high temperature conditions, it will cause uneven thinning of the roll oxidation film and nodular surface, which is more likely to cause the roll oxidation film to peel off during use, and the peeled material is pressed into the substrate to form surface white line defects.

[0105] Microstructure analysis: Since the graphite in the roll mainly plays a lubricating role, it can reduce the peeling defects on the surface of the roll body, and can also reduce the thermal shock on the surface of the roll. However, if the spheroidization state of the graphite on the surface of the roll is not good, as shown in Figure 15 , it will present a worm-like or sea urchin-like morphology, which will cause cracking between the soft and hard phases during use at the boundary position.

[0106] The improved ICDP roller is established on the basis of the common ICDP roller, and the wear-resistant particles are formed by adding V and Nb to enhance the performance, and the wear-resistant particles are formed by adding special alloy elements (Nb, V, W, etc.) in the nickel-chromium-molybdenum material, so that the material crystallizes high-hardness and fine-dispersed carbide (M-C or M-N) at high temperature. With the increase of Nb content, the size of the wear-resistant particles is too large to form a large block distribution, but in order to ensure the formation of fine and dispersed wear-resistant particles, the mass fraction ratio of V and Nb should be 1:1, otherwise the wear-resistant particles may be less precipitated or in a large block distribution state.

[0107] According to the defect data and the organization oxidation characteristic data, the high-strength steel smelting process and the rolling process are controlled respectively, and the high-strength steel outer plate without surface white line defects is obtained; the specific idea is as follows:

[0108] The chemical composition of the steel grade in the high-strength steel smelting process is controlled as follows in mass fraction: C: 0.002%, Si: 0.1%, Mn: 0.4%, P: 0.04%, S: 0.01%, Al: 0.04%, B: 0.0006%, Nb: 0.03%, Ti: 0.04%, and the rest is Fe and inevitable impurities.

[0109] The high-strength steel needs to be rolled after the infinite cold hardening roller is heated for 10 blocks, and the final rolling temperature of the heated roller before the high-strength steel is rolled is 860℃, and the number of continuous rolling before the high-strength steel is rolled is 10 blocks

[0110] The heating in the high-strength steel rolling process is controlled: the soaking temperature of the heating is controlled to be 1180℃, the heating time of the heating is controlled to be 150min, and the lambda value of the heating is controlled to be 0.9.

[0111] The phosphorus removal process in the high-strength steel rolling process is controlled: three passes of phosphorus removal are used in the rough rolling, double passes of phosphorus removal are used at the entrance of the finishing rolling, and the pressure of the phosphorus removal is controlled to be 22MPa.

[0112] The finishing rolling stage in the high-strength steel rolling process is controlled: the entrance temperature of the finishing rolling is controlled to be 1030℃, the final rolling temperature of the finishing rolling is controlled to be 890℃, the coiling speed of the finishing rolling is controlled to be 9m / s, and the interstand cooling water pressure of the finishing rolling is controlled to be 9MPa.

[0113] The intermediate billet thickness of the finishing rolling stage is controlled to be 36mm, the reduction of the fourth stand of the finishing rolling is controlled to be 25%, and the reduction of the fifth stand of the finishing rolling is controlled to be 15%.

[0114] Controlling the heat fatigue feature in the rolling process of the high-strength steel: the fourth to sixth stands of the controlled finishing rolling adopt new rolls for the on-machine rolling, and the first to third stands adopt the rolling rolls with the surface level reaching grade 1; the cooling water usage ratio between the first and second stands of the controlled finishing rolling is 0.8, the cooling water usage ratio between the second and third stands of the controlled finishing rolling is 0.6, and the rolling process is lubricated.

[0115] Controlling the microstructure of the infinite cold hard rolling roll in the rolling process of the high-strength steel: the graphite spheroidization level in the infinite cold hard rolling roll is greater than or equal to grade 2.

[0116] Controlling the diameter of the wear-resistant particles to be 3 μm.

[0117] Controlling the wear-resistant particles to satisfy: [V] / [Nb]=1:1, wherein [V] is the V content in the wear-resistant particles, and [Nb] is the Nb content in the wear-resistant particles.

[0118] Example 2

[0119] Comparing the example 2 with the example 1, the difference between the example 2 and the example 1 is that:

[0120] Controlling the chemical composition of the steel grade in the smelting process of the high-strength steel to be, in mass fraction: C: 0.0015%, Si: 0.05%, Mn: 0.3%, P: 0.03%, S: 0.01%, Al: 0.03%, B: 0.0004%, Nb: 0.025%, Ti: 0.035%, and the rest is Fe and inevitable impurities.

[0121] Controlling the heating in the rolling process of the high-strength steel: controlling the soaking temperature of the heating to be 1180℃, controlling the time of the heating to be 140 min, and controlling the λ value of the heating stage to be 0.8.

[0122] Controlling the dephosphorization process in the rolling process of the high-strength steel: controlling the dephosphorization of the rough rolling to be three passes, controlling the dephosphorization at the entrance of the finishing rolling to be two passes, and controlling the pressure of the dephosphorization to be 18 MPa.

[0123] Controlling the finishing rolling in the rolling process of the high-strength steel: controlling the entrance temperature of the finishing rolling to be 1000℃, controlling the finish rolling temperature to be 880℃, controlling the speed of the finishing rolling to be 8 m / s, and controlling the inter-stand cooling water pressure of the finishing rolling to be 8 MPa.

[0124] Controlling the intermediate billet thickness of the finishing rolling stage to be 32 mm.

[0125] Controlling the heat fatigue feature in the rolling process of the high-strength steel includes: controlling the cooling water usage ratio between the first and second stands of the controlled finishing rolling to be 0.5, controlling the cooling water usage ratio between the second and third stands of the controlled finishing rolling to be 0.5, and lubricating the rolling process.

[0126] Example 3

[0127] Comparing Example 3 with Example 1, the difference between Example 3 and Example 1 is that:

[0128] The chemical composition of the high-strength steel in the smelting process is controlled as follows in mass fraction: C: 0.004%, Si: 0.12%, Mn: 0.5%, P: 0.05%, S: 0.02%, Al: 0.05%, B: 0.0008%, Nb: 0.035%, Ti: 0.045%, and the rest is Fe and inevitable impurities.

[0129] The heating in the rolling process of the high-strength steel includes: controlling the soaking temperature of heating to be 1200℃, controlling the heating time to be 160min, and controlling the λ value of heating to be 0.9.

[0130] The phosphorus removal process in the rolling process of the high-strength steel includes: controlling three passes of phosphorus removal in rough rolling, controlling double passes of phosphorus removal at the entrance of finish rolling, and controlling the pressure of phosphorus removal to be 22MPa.

[0131] The finish rolling in the rolling process of the high-strength steel includes: controlling the entrance temperature of finish rolling to be 1040℃, controlling the finish rolling temperature to be 900℃, controlling the speed of finish rolling to be 10m / s, and controlling the inter-stand cooling water pressure of finish rolling to be 10MPa.

[0132] The intermediate billet thickness of finish rolling is controlled to be 38mm.

[0133] The thermal fatigue characteristics in the rolling process of the high-strength steel include: controlling the cooling water usage ratio between the first stand and the second stand in the finish rolling stage to be 0.8, and controlling the cooling water usage ratio between the second stand and the third stand in the finish rolling to be 0.8.

[0134] Comparative Example 1

[0135] Comparing Comparative Example 1 with Example 1, the difference between Comparative Example 1 and Example 1 is that:

[0136] The chemical composition of the high-strength steel in the smelting process is controlled as follows in mass fraction: C: 0.001%, Si: 0.04%, Mn: 0.2%, P: 0.02%, S: 0.008%, Al: 0.02%, B: 0.0003%, Nb: 0.020%, Ti: 0.030%, and the rest is Fe and inevitable impurities.

[0137] The heating stage in the rolling process of the high-strength steel includes: controlling the soaking temperature in the heating stage to be 1150℃, controlling the heating time in the heating stage to be 120min, and controlling the λ value in the heating stage to be 0.7.

[0138] The phosphorus removal process in the rolling process of the high-strength steel is controlled as follows: three passes of phosphorus removal are used in rough rolling, two passes of phosphorus removal are used at the entrance of finish rolling, and the pressure for phosphorus removal is 15 MPa.

[0139] The finish rolling stage in the rolling process of the high-strength steel is controlled as follows: the entrance temperature of the finish rolling stage is 800℃, the finish rolling temperature of the finish rolling stage is 800℃, the speed of the finish rolling stage is 5 m / s, and the cooling water pressure between the stands of the finish rolling stage is 5 MPa.

[0140] The finish rolling stage in the rolling process of the high-strength steel is further controlled as follows: the intermediate billet thickness of the finish rolling stage is 25 mm.

[0141] The thermal fatigue characteristics in the rolling process of the high-strength steel are controlled as follows: the cooling water usage ratio between the first stand and the second stand of the finish rolling stage is 0.2, and the cooling water usage ratio between the second stand and the third stand of the finish rolling stage is 0.3.

[0142] Comparative Example 2

[0143] Comparing the comparative example 2 and the example 1, the difference between the comparative example 2 and the example 1 is as follows:

[0144] The chemical composition of the steel in the smelting process of the high-strength steel is controlled as follows: C: 0.005%, Si: 0.15%, Mn: 8%, P: 0.03%~0.05%, S: 0.04%, Al: 0.08%, B: 0.0010%, Nb: 0.040%, Ti: 0.055%, and the rest is Fe and inevitable impurities.

[0145] The heating in the rolling process of the high-strength steel is controlled as follows: the soaking temperature of the heating is 1250℃, the time of the heating is 180 min, and the λ value of the heating is 0.95.

[0146] The phosphorus removal process in the rolling process of the high-strength steel is controlled as follows: three passes of phosphorus removal are used in rough rolling, two passes of phosphorus removal are used at the entrance of finish rolling, and the pressure for phosphorus removal is 25 MPa.

[0147] The finish rolling in the rolling process of the high-strength steel is controlled as follows: the entrance temperature of the finish rolling is 1100℃, the finish rolling temperature of the finish rolling is 950℃, the speed of the finish rolling is 15 m / s, and the cooling water pressure between the stands of the finish rolling is 15 MPa.

[0148] The intermediate billet thickness of the finish rolling is 40 mm.

[0149] Controlling the thermal fatigue characteristics in the rolling process of high-strength steel: the proportion of the amount of cooling water between the first and second stands of controlled finishing rolling is 0.9, and the proportion of the amount of cooling water between the second and third stands of controlled finishing rolling is 0.9.

[0150] Related experiments:

[0151] The surface white line defect rates of the steel materials obtained in Examples 1-3 and Comparative Examples 1-2 are counted, and the results are shown in Table 3.

[0152] Table 3

[0153]

[0154]

[0155] Specific analysis of Table 1:

[0156] From the data of Example 1-3:

[0157] If the method of the present application is used, the chemical composition of the smelting process and the rolling process of high-strength steel can effectively obtain high-strength steel products with low surface white line defect rates.

[0158] From the data of Comparative Example 1-2:

[0159] If the parameters of the control process provided by the present application are not used, the surface white line defects will increase, affecting the quality of the steel product.

[0160] The one or more technical solutions in the embodiments of the present application at least have the following technical effects or advantages:

[0161] (1) The method provided by the embodiments of the present application can determine the chemical composition of the steel in the smelting process and the process parameters in the rolling process by first analyzing the surface white line defects to determine their morphology and microfeature data, and then analyzing the oxidation characteristics and microstructure of the infinite hard roller, thereby comprehensively controlling the surface white line defects to obtain high-strength steel plate without surface white line defects.

[0162] (2) The method provided by the embodiments of the present application can accurately analyze the surface white line defects and find the relationship between the generation mechanism and the fourth to sixth stands of finishing rolling, thereby providing a strong basis for improving the surface white line defects.

[0163] (3) The method provided by the embodiments of the present application can effectively reduce the occurrence rate of surface white line defects of high-strength IF steel and improve the product quality.

[0164] (4) The method provided by the embodiments of the present application is simple in overall process, strong in applicability, and has a significant effect on reducing the surface white line defect rate.

[0165] It has to be noted that, in the present document, relational terms are intended to encompass the various physical or functional relationships between or among components. It has to be understood that, in normal use, where used without modifications, "coupled" or "connected" means that the components co-operate or interact with each other. It has to be further understood that, in normal use, various embodiments of the present application can be implemented without co-operation or interaction between the components, and that the desired relationship, co-operation or interaction exists between the components within one embodiment. It has to be further noted that the relationship between the components can be a direct relationship.

[0166] It is to be understood that the endpoints of the ranges specified in this disclosure are subject to ± 1% variation. Unless otherwise stated, all ranges include all the sub-ranges within the specified ranges. All values and sub-ranges within the specified ranges are specifically included as if explicitly written out.

[0167] The foregoing description of specific embodiments will enable any person skilled in the art to make or use the application. The modi fications of these specific embodiments will occur to persons skilled in the art upon reading the foregoing description. The applications described herein are not intended to be limited by the particular forms described herein, but ought to include all modifications and equivalents of the subject matter taken in connection with the teachings of the present disclosure. The specific embodiments described herein are given as examples to the best of the knowledge of the present inventors of the preferred ways of making and using the application. The application is not limited by the embodiments described herein, but rather by the claims as interpreted in their full and broad aspects, including all equivalents.

Claims

1. A method of producing a high-strength steel with low surface white line-like defects, characterized by, The method comprises: heating, roll preheating and continuous rolling of the cast blank to obtain a strip steel; cold rolling, annealing and skin passing of the strip steel to obtain a high-strength steel with low surface white line type defects; wherein the continuous rolling is performed by using an infinite cold hard roll; the roll preheating comprises preheating the infinite cold hard roll with a roll preheating material, and the end point temperature of the preheating is ≤880℃; the graphite spheroidization grade of the infinite cold hard roll is ≥2.

2. The production method according to claim 1, characterized by, The number of the roll preheating materials is ≥10.

3. The preparation method according to claim 1, characterized in that The number of the cast blanks for the continuous rolling is ≤15.

4. The method of claim 1, wherein, The continuous rolling further comprises adding wear-resistant particles for continuous rolling, and the diameter of the wear-resistant particles is <5μm.

5. The production method according to claim 4, characterized by, The chemical composition of the wear-resistant particles satisfies: [V] / [Nb] = 0.5-1.5:1, wherein [V] is the V content in the wear-resistant particles, and [Nb] is the Nb content in the wear-resistant particles.

6. The method of claim 1, wherein, The continuous rolling comprises rough rolling, rough rolling descaling, finish rolling descaling and finish rolling, and the descaling water pressure of the rough rolling descaling and the finish rolling descaling is 18-22MPa.

7. The preparation method according to claim 6, characterized in that The inlet temperature of the finish rolling is 1000-1040℃, and the finish rolling final rolling temperature is 880-900℃.

8. The preparation method according to claim 6, characterized in that The cooling water pressure between adjacent two stands of the finish rolling is 8-10MPa, and the cooling water input mass ratio between adjacent two stands of the finish rolling is 0.5-0.

8.

9. The method of claim 1, wherein, The soaking temperature of the heating is 1180-1200℃, the heating time is 140-160min, and the λ value of the heating is 0.8-0.

9.

10. A high-strength steel, characterized by, The high-strength steel is prepared by the method according to any one of claims 1-9, and the chemical composition of the high-strength steel comprises, in mass fraction: C: 0.0015%-0.004%, Si: 0.05%-0.12%, Mn: 0.3%-0.5%, P: 0.03%-0.05%, S: 0.01%-0.02%, Al: 0.03%-0.05%, B: 0.0004%-0.0008%, Nb: 0.025%-0.035%, Ti: 0.035%-0.045%, and the rest is Fe and inevitable impurities.

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