A method for reducing surface black spot defects of galvanized IF steel

Abstract

The application provides a method for reducing surface black spot defects of galvanized IF steel, and belongs to the field of steel rolling. The method comprises the following steps: pouring IF steel liquid containing Cr to obtain a casting blank; at least part of the surface of the casting blank contains an FeO-Cr2O3 layer; the casting blank is placed in a heating furnace for heating, and the reaction activity of S in the heating atmosphere of the heating furnace is adjusted to reduce the formation amount of high-temperature sulfides on the surface of the casting blank. By adding a certain amount of Cr element in the casting blank, a dense FeO-Cr2O3 layer is formed at the interface of the casting blank, and the diffusion speed of S element into the matrix and the critical S element activity at the interface are further reduced; by adjusting the reaction activity of S atmosphere in the heating atmosphere, the formation amount of high-temperature sulfides on the surface of the casting blank is reduced. Therefore, the small wrinkle defects on the surface of the IF steel are reduced, and the technical problem that the surface black spot defects of the galvanized IF steel are difficult to reduce in the prior art is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel rolling, in particular to a method for reducing surface black spot defects of galvanized IF steel. BACKGROUND

[0002] Hot-dip galvanized automobile sheet has good corrosion resistance, coating performance and welding performance, and is widely used in automobile body manufacturing. More than 40% of the automobile sheet used in China is galvanized automobile sheet, and the surface quality requirement of automobile outer plate is very strict, which needs to reach O5 level, that is, there is no surface defect affecting use on single side. However, the production of high-grade O5 plate is difficult because the generation of plate surface defects runs through the whole production process of smelting, continuous casting, hot rolling, pickling, annealing, galvanizing and skinning, and there are many influencing factors, and it is difficult to qualitatively analyze. There are more than thirty kinds of common surface defects, such as non-metallic inclusions, subcutaneous bubbles, iron oxide scale, surface cracks, zinc ash, zinc slag and furnace roller nodules. It not only affects the surface appearance of the subsequent forming parts, but also closely related to the stamping effect, shape and processing cost of the subsequent processing. The hot-dip galvanized surface point defect accounts for a high proportion in the surface quality dispute and product closure rate of galvanized sheet, which can reach 20% to 30%. At the same time, the point defect has many forms, which can be divided into black spot, white spot, bright spot and yellow rust spot according to the difference of reflection degree, and the black spot defect accounts for 80% of the occurrence rate of point defect.

[0003] In the prior art, the research and breakthrough of galvanized black spot defects focus on the breakthrough of zinc ash and zinc slag defects, but under the background of process optimization in the production line, the control measures of zinc ash and zinc slag defects have been gradually optimized and solidified, and the defect occurrence rate has been significantly reduced. However, new type of galvanized black spot defects have appeared in an explosive trend, and there is no suitable method for analyzing and controlling the surface black spot defects in the production process of galvanized O5 outer plate. SUMMARY

[0004] The present application provides a method for reducing the surface black spot defects of galvanized IF steel, which solves the technical problem that the surface black spot defects of galvanized IF steel are difficult to reduce in the prior art.

[0005] The present application provides a method for reducing the surface black spot defects of galvanized IF steel, which comprises:

[0006] Pouring the IF steel liquid containing Cr to obtain a casting blank; at least part of the surface of the casting blank contains an FeO-Cr2O3 layer;

[0007] Placing the casting blank in a heating furnace for heating, and adjusting the reaction activity of S in the heating atmosphere of the heating furnace to reduce the formation amount of high-temperature sulfide on the surface of the casting blank.

[0008] Optionally, the heating of the casting blank in the heating atmosphere containing sulfur and adjusting the reactive activity of sulfur in the heating atmosphere comprises:

[0009] heating the casting blank in a heating furnace, and controlling the content of sulfur in the heating atmosphere of the heating furnace to be a set content of sulfur;

[0010] According to the set content of sulfur in the heating atmosphere, the air excess coefficient, the temperature and the soaking time of the heating are set respectively.

[0011] Optionally, the set content of sulfur comprises: SO2≤100mg / m 3 , H2S≤150mg / m 3 .

[0012] Optionally, the set content of sulfur comprises: SO2: 30mg / m 3 ~100mg / m 3 , H2S: 80mg / m 3 ~150mg / m 3 .

[0013] Optionally, if the content of SO2 in the heating atmosphere is ≤60mg / m 3 , and / or the content of H2S is ≤100mg / m 3 , the air excess coefficient of the heating is set to be 0.9~1.1;

[0014] If the content of SO2 in the heating atmosphere is >60mg / m 3 and ≤100mg / m 3 , and / or the content of H2S is >100mg / m 3 and ≤150mg / m 3 , the air excess coefficient of the heating is set to be 1.1~1.3.

[0015] Optionally, if the content of SO2 in the heating atmosphere is ≤60mg / m 3 , and / or the content of H2S is ≤100mg / m 3 , the temperature of the heating is set to be 1160℃~1180℃;

[0016] If the content of SO2 in the heating atmosphere is >60mg / m 3 and ≤100mg / m 3 , and / or the content of H2S is >100mg / m 3 and ≤150mg / m 3 , the temperature of the heating is set to be 1190℃~1230℃.

[0017] Optionally, if the content of SO2 in the heating atmosphere is ≤60mg / m3 and / or H2S content is ≤ 100 mg / m 3 If the SO2 content in the heating atmosphere is > 60 mg / m

[0018] and ≤ 100 mg / m 3 and ≤ 100 mg / m 3 and / or H2S content is > 100 mg / m 3 and ≤ 150 mg / m 3 and ≤ 150 mg / m

[0019] Optionally, the chemical composition of the casting blank comprises Cr, and the content of Cr is 0.1% to 0.2% by mass fraction.

[0020] Optionally, the chemical composition further comprises C, Al, Mn, Si, Ti and Fe, and the content of C is 0.015% to 0.02% by mass fraction, the content of Al is 0.02% to 0.05% by mass fraction, the content of Mn is 0.1% to 0.2% by mass fraction, the content of Si is 0.02% to 0.05% by mass fraction, and the content of Ti is 0.06% to 0.08% by mass fraction.

[0021] Compared with the prior art, the above technical solution provided by the embodiments of the present application has the following advantages:

[0022] The present application provides a method for reducing the black spot defects on the surface of galvanized IF steel. By adding a certain amount of Cr in the casting blank, a dense FeO-Cr2O3 layer is formed at the interface of the casting blank under high-temperature oxidation conditions, which further reduces the diffusion speed of S element into the matrix and the critical S element activity at the interface, thereby avoiding the formation of high-temperature sulfides such as FeS and TiS, which can embrittle the surface of the steel plate. By heating the casting blank in a heating atmosphere and adjusting the reaction activity of the S atmosphere in the heating atmosphere, the amount of high-temperature sulfides formed on the surface of the casting blank is reduced. Thus, the small curling defects on the surface of the IF steel are reduced, thereby solving the technical problem that the black spot defects on the surface of the galvanized IF steel are difficult to reduce in the prior art. BRIEF DESCRIPTION OF DRAWINGS

[0023] The accompanying drawings, which are incorporated herein and constitute part of the specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the embodiments or the prior art description will be briefly introduced as follows. Obviously, those skilled in the art can obtain other drawings from these drawings without any creative effort.

[0025] Figure 1 A morphology diagram of a black spot defect on the surface of a galvanized IF steel (a) and a morphology diagram of the steel substrate after removing the zinc layer at the black spot defect (b) provided for an embodiment of the present application;

[0026] Figure 2 A morphology diagram of surface buckling of a hot-rolled pickled plate (a) and a diagram showing the influence of the surface buckling on the surface morphology of a cold-rolled plate with a thickness of 2mm (b), 1mm (c), and 0.6mm (d) after cold rolling provided for an embodiment of the present application;

[0027] Figure 3 A morphology diagram of a cross section of surface buckling of a hot-rolled plate provided for an embodiment of the present application;

[0028] Figure 4 A backscattering image of surface buckling of a hot-rolled plate (a) and distribution diagrams of element S (b) and element Ti (c) near the surface buckling provided for an embodiment of the present application;

[0029] Figure 5 A backscattering image of surface cracks of a casting blank (a), a distribution diagram of element S near the surface cracks (b), and an energy spectrum analysis diagram near the surface cracks (c) provided for an embodiment of the present application;

[0030] Figure 6 A flowchart of an analysis and control method of a black spot defect on the surface of a galvanized IF steel provided for an embodiment of the present application. DETAILED DESCRIPTION

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the following will be combined with the accompanying drawings for the embodiments of the present application to make a clear and complete description of the technical solutions in the embodiments of the present application. Obviously, the described embodiments are a part of the embodiments of the present application, rather than all 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 fall within the scope of protection of the present application.

[0032] Various embodiments of the present application can exist in the form of a range; it should be understood that the description in the form of a range is merely for the convenience and brevity, and should not be understood as a hard limitation on the scope of the present application; therefore, it should be considered that the described range has specifically disclosed all possible sub-ranges and single values within the range. For example, it should be considered that the range description from 1 to 6 has 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., and single numbers within the described range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. In addition, whenever a numerical range is indicated in the present text, it refers to any cited number (fraction or integer) within the indicated range.

[0033] In addition, in the description of the present application, the terms "comprise", "contain" and the like mean "comprise but not limited to". In this paper, such as "first" and "second" and other relational terms are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between the entities or operations. In this paper, "and / or", the association between the associated objects, means that there can be three kinds of relationships, for example, A and / or B, can represent: the existence of A alone, the existence of A and B at the same time, and the existence of B alone. Where A, B can be singular or plural. In this paper, "at least one" means one or more, and "multiple" means two or more. "At least one", "at least one of the following" or similar expressions means any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can represent a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, where a, b, and c can be single or multiple.

[0034] Unless otherwise specified, the various raw materials, reagents, instruments and equipment used in the present application can be purchased or prepared by existing methods.

[0035] The creative idea of the present application is:

[0036] 1. Analysis method of black spot defects on the surface of galvanized IF steel:

[0037] (1) Analysis of the causes of small black spot defects on the surface of IF steel after galvanizing:

[0038] Figure 1 The morphology of the black spot defects on the surface of the galvanized IF steel (a) and the morphology of the steel substrate after removing the zinc layer at the black spot defect site (b) provided by the embodiments of the present application.

[0039] As shown in Figure 1 The morphology characteristics of the small black spot defects on the surface after galvanizing are: the size is in the range of 100-200 μm, the black spot defects are circular or elliptical, the surface roughness after flattening is low and different from the roughening morphology of the steel plate surface at other positions, and there is a slight rolling direction extension morphology.

[0040] The defect density is: dispersedly distributed on the surface of the steel plate, and exists on the upper and lower surfaces, and the defect density can reach 50-100 / 100 cm 2 .

[0041] Substrate surface features: After removing the surface zinc layer with hydrochloric acid containing corrosion inhibitor, it was found that the substrate surface had damaged and cracked suspected skin morphology features, and the skin defects were elongated in the rolling direction.

[0042] Causes of black point defects after galvanizing: The main reason is the presence of small skin on the substrate surface, which has a certain blocking effect on the air flow of the galvanizing air knife and the flow of zinc liquid, resulting in poor flow of zinc liquid at this position, easy thickening of the zinc layer, and after coming out of the zinc pot, contact and friction with the top roller or smoothing roller, causing the surface flatness of the thickened area to be slightly higher, and there is a difference in the morphology of the plated layer, which is observed as a black point defect under diffuse reflection.

[0043] (2) Analysis of the evolution law of the skin of the cold-rolled sheet

[0044] Figure 2 The surface skin morphology of the hot-rolled pickled sheet provided for the embodiments of the present application and the influence of the surface skin on the surface morphology of the cold-rolled sheet with a thickness of 2 mm (b), 1 mm (c), and 0.6 mm (d) after cold rolling are shown in the following figures.

[0045] Surface condition of pickled sheet: For a hot-rolled pickled sheet with a thickness of 4 mm, defect observation was performed, and it was found that there were small skin morphologies on the surface after pickling, as shown in Figure 2 , which presented micro-area damage features, and the size of the small skin was about 10-30 μm.

[0046] Evolution law of skin defects after cold rolling: For the small skin steel sheet after pickling, sampling was performed, and cold rolling was simulated to analyze whether cold rolling had a certain eliminating effect on the defects. As shown in Figure 2 , after cold rolling with a simulated compression ratio of 50% and a thickness of 2 mm: there were many linear skin and pit morphologies along the rolling direction, with a width of about 20 μm and an extension length of 20-50 μm. After cold rolling with a simulated compression ratio of 75% and a thickness of 1 mm, and cold rolling with a simulated compression ratio of 85% and a thickness of 0.6 mm: the same skin and pit morphologies were also presented, which were elongated along the rolling direction. With the increase of the compression ratio, the skin along the length direction was broken to form multiple point skin and pit morphologies, with an extension length of 50-100 μm. There was a good morphology correspondence between the small skin on the surface of the black point substrate and the small skin.

[0047] It can be seen that the surface black point defects of the galvanized sheet are inherited from the small skin on the surface of the hot-rolled sheet.

[0048] (3) Analysis of the skin morphology features and element enrichment state of the hot-rolled sheet

[0049] Figure 3 The skin section morphology of the hot-rolled sheet provided for the embodiments of the present application is shown in the following figure.

[0050] When measuring the skin morphology of the hot-rolled sheet, it was found that Figure 3As shown, the iron sheet interface is uneven, and the flatness and continuity are abnormal. The hot-rolled surface interface presents two types of topographic features. One is the surface uneven topography, and the depth difference can reach 8-13 μm. The other is that the surface continuity and integrity of the iron sheet are lost, and the micro-cracks or micro-curl interrupt the iron sheet interface. The crack severity mainly exists within 10 μm, and the crack extends inwardly with the iron sheet wrapped inside. Energy spectrum analysis shows that the crack mainly contains Fe-O elements.

[0051] Figure 4 The surface curl backscattering image (a) of the hot-rolled plate provided by the embodiment of the present application and the distribution diagrams of element S (b) and element Ti (c) near the surface curl.

[0052] As shown in Figure 4 Further analysis of the curl defect element distribution by electron probe shows that the enrichment of S element near the curl can be obviously captured. The surface layer enrichment of S element and the corresponding relationship with other elements: the enrichment position point has a good corresponding relationship with the Ti element enrichment point, and the corresponding relationship with Mn-Cu is not obvious. There are more particles near the surface layer, and there are more TiS particle precipitates on the surface of the IF steel hot-rolled plate.

[0053] It can be seen that the small curl defect on the surface of the IF steel has a clear relationship with the enrichment of S element on the surface and the formation of multi-state sulfide brittle surface layer grains.

[0054] (4) Analysis of the surface topographic features and element enrichment state of the casting blank

[0055] Figure 5 The backscattering image (a) of the surface crack of the casting blank provided by the embodiment of the present application, the distribution diagram of element S near the surface crack (b), and the energy spectrum analysis diagram near the surface crack (c).

[0056] As shown in Figure 5 The surface state of the casting blank: there are uneven topography on the surface, anchor-shaped embedded matrix topography in local position, and suspected grain boundary oxidation and cracking topographic features.

[0057] S affects the surface existing state: field emission scanning electron microscope observation and electron probe analysis of the casting blank surface crack area show that there is obvious S element enrichment penetration into the matrix, and the energy spectrum analysis shows that the existing state of S is mainly FeS and TiS.

[0058] 2. Control method of black spot defects on the surface of galvanized IF steel:

[0059] (1) Key points of atmosphere control of the hot-rolling heating furnace

[0060] Based on the production line S element source analysis mainly from the heating process, the use of gas is generally high coke conversion mixed gas, which is mainly blast furnace gas and coke oven gas. The SO2 of blast furnace gas, the H2S content in the gas is very high. High temperature surface sulfidation high temperature corrosion.

[0061] The sulfidation and oxidation reaction of steel at high temperature can be found through the Fe-O-S phase diagram. When the S partial pressure is low enough, the oxidation reaction mainly occurs, and when the oxygen partial pressure is low enough, the metal sulfide is mainly generated: Fe + SO2 = FeS + 2FeO, and the melting point of FeS is 980℃, which is lower than the slab soaking temperature. During the long-term heat preservation process of the heating furnace, it is easy to liquefy and invade the austenite grain boundary, causing surface cracks. At the same time, due to the existence of a large amount of Ti element in the IF steel, Ti + S = TiS is easy to form in large quantities, causing the increase of hard phase particles in the surface layer, the significant increase of surface microhardness within 100μm of the surface layer, and the increase of the difficulty of surface layer deformation during rolling. In addition, the existence of cracks is easy to cause surface layer cracking during rolling.

[0062] Based on the above analysis, it can be seen that the control points of the diffusion of S element on the surface to the matrix to form sulfide are: 1) control the S element content in the gas; 2) adjust the oxygen content by using the Fe-O-S phase diagram to reduce the S activity critical value of the reaction front interface to avoid the formation of sulfide; 3) reduce the heating temperature to avoid the liquefaction and embrittlement of the grain boundary of FeS.

[0063] (2) Alloy element adjustment direction

[0064] By adding a certain amount of Cr element in the IF steel, a dense FeO-Cr2O3 interface is formed under high temperature oxidation conditions, which further reduces the diffusion speed of S element into the matrix and the critical S element activity of the interface, avoids the formation of FeS and TiS and other high temperature sulfides, and embrittle the surface of the steel plate.

[0065] Figure 6 A flowchart of an analysis and control method of a black spot defect on the surface of a galvanized IF steel provided by the embodiment of the present application.

[0066] See Figure 6 The present application provides a method for reducing the black spot defect on the surface of a galvanized IF steel, which comprises:

[0067] S1, pouring the IF steel liquid containing Cr to obtain a casting blank; at least part of the surface of the casting blank contains a FeO-Cr2O3 layer;

[0068] In some embodiments, the chemical composition of the casting blank comprises: Cr, the content of Cr is 0.1% to 0.2% by mass fraction.

[0069] The positive effect of controlling the content of Cr to be 0.1% to 0.2%: adding a certain amount of Cr element in the IF steel, forming a dense FeO-Cr2O3 at the interface under high-temperature oxidation conditions, further reducing the diffusion speed of S element into the matrix and reducing the critical S element activity at the interface, avoiding the formation of high-temperature sulfides such as FeS and TiS, and the embrittlement of the surface of the steel plate. For example, the content of Cr can be 0.1%, 0.12%, 0.14%, 0.16%, 0.18%, 0.2%, etc.

[0070] In some embodiments, the chemical composition further includes: C, Al, Mn, Si, Ti and Fe, the content of C is 0.015% to 0.02%, the content of Al is 0.02% to 0.05%, the content of Mn is 0.1% to 0.2%, the content of Si is 0.02% to 0.05%, and the content of Ti is 0.06% to 0.08%.

[0071] The positive effect of controlling the content of C to be 0.015% to 0.02%: for the requirement of ultra-low carbon content in deep drawing steel to ensure high plasticity. For example, the content of C can be 0.015%, 0.016%, 0.017%, 0.018%, 0.019%, 0.02%, etc.

[0072] The positive effect of controlling the content of Al to be 0.02% to 0.05%: as a deoxidizer, it mainly removes oxygen in the steel, adds a certain amount to inhibit the solid solution of nitrogen in the iron crystal, eliminates strain aging, and improves low-temperature plasticity. For example, the content of Al is 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0073] The positive effect of controlling the content of Mn to be 0.1% to 0.2%: to remove the influence of residual S elements in the steel, form MnS to avoid the formation of low-melting-point FeS, and prevent the generation of surface cracks during hot rolling. For example, the content of Mn can be 0.1%, 0.12%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.2%, etc.

[0074] The positive effect of controlling the content of Si to be 0.02% to 0.05%: Si element is too low, the smelting cost increases, and too high, there is a risk of iron skin pressing into the surface during hot rolling. For example, the content of Si can be 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0075] The positive effect of controlling the content of Ti to be 0.06% to 0.08%: to ensure deep drawing effect and no aging, Ti is used to solid-solute C and N atoms. For example, the content of Ti can be 0.06%, 0.07%, 0.08%, etc.

[0076] S2, heating the casting blank in a heating furnace, and adjusting the reaction activity of S in the heating atmosphere of the heating furnace to reduce the amount of formation of high-temperature sulfides on the surface of the casting blank.

[0077] In some embodiments, the heating the casting blank in the S-containing heating atmosphere, and adjusting the reaction activity of S in the heating atmosphere, comprises:

[0078] heating the casting blank in a heating furnace, and controlling the S content in the heating atmosphere of the heating furnace to be a set S content;

[0079] According to the set S content of the heating atmosphere, the air excess coefficient, the temperature, and the soaking time of the heating are respectively set.

[0080] In some embodiments, the set S content comprises: SO2≤100 mg / m 3 , and H2S≤150 mg / m 3 .

[0081] The atmosphere in the heating furnace is mainly supplied by blast furnace gas and coke oven gas, and the atmosphere can contain impurities such as CO, CO2, SO2, and H2S. The present application mainly aims at the surface defects of IF steel, and it is found that the surface defects are closely related to the penetration of S atmosphere in the heating furnace. The content of the S element oxidation related chemicals in the heating furnace is analyzed. For example, the SO2 content can be controlled to be 10 mg / m 3 , 20 mg / m 3 , 30 mg / m 3 , 40 mg / m 3 , 50 mg / m 3 , 60 mg / m 3 , 70 mg / m 3 , 90 mg / m 3 , 100 mg / m 3 , and the H2S content can be controlled to be 30 mg / m 3 , 40 mg / m 3 , 50 mg / m 3 , 60 mg / m 3 , 70 mg / m 3 , 80 mg / m 3 , 90 mg / m 3 , 100 mg / m 3 , 120 mg / m 3 , 140 mg / m 3 , 150 mg / m 3 .

[0082] In some embodiments, the set S content comprises: SO2: 30 mg / m 3~100mg / m 3 , H2S: 80mg / m 3 ~150mg / m 3 .

[0083] The atmosphere in the heating furnace is mainly supplied by the blast furnace gas and the coke oven gas. The S content in the heating atmosphere can be controlled by controlling the S content in the blast furnace gas and the coke oven gas. However, if the S content is controlled too strictly, the production cost will be greatly increased. Therefore, the SO2 content can be controlled to be 30mg / m 3 ~100mg / m 3 , and the H2S content is 80mg / m 3 ~150mg / m 3 .

[0084] In some embodiments, if the SO2 content in the heating atmosphere is ≤60mg / m 3 , and / or the H2S content is ≤100mg / m 3 , the air excess coefficient of the heating is set to be 0.9-1.1.

[0085] If the SO2 content in the heating atmosphere is >60mg / m 3 and ≤100mg / m 3 , and / or the H2S content is >100mg / m 3 and ≤150mg / m 3 , the air excess coefficient of the heating is set to be 1.1-1.3.

[0086] The air excess coefficient is the ratio of the actual value of air to the theoretical value in the air and gas combustion reaction. The greater the air excess coefficient, the more the combustion-supporting atmosphere in the furnace, which is beneficial to the complete combustion of the gas. However, too much air entering the furnace will cause the exhaust gas temperature to be too high, the thermal efficiency to be reduced, and the oxidation loss rate of the billet to be increased. On the contrary, if the air excess coefficient is too small, the content of the combustion-supporting atmosphere in the furnace will be reduced, which will cause incomplete combustion of the gas and waste a part of the fuel. In the present application, under the condition of low temperature and low S atmosphere, a low air excess coefficient is used, and high-temperature fast burning is used to avoid the penetration of S elements into the matrix under high-temperature conditions, especially when the temperature is greater than 980℃. Under high-temperature conditions, the diffusion ability of S elements is increased, and the penetration ability is enhanced. A high air excess coefficient is mainly used to increase the oxidation loss method to consume the invasion of S elements into the matrix.

[0087] In the embodiments of the present application, the SO2 content in the heating atmosphere during the hot rolling process of the IF steel is ≤60mg / m 3 , and the H2S content is ≤100mg / m 3In this case, a low-temperature, rapid-burning, weakly reducing atmosphere is used to avoid the inward diffusion of sulfur and the liquefaction and intrusion of FeS into the grain boundaries, which could cause surface cracking. For example, the excess air coefficient during the hot rolling process of IF steel can be controlled to be 0.9, 0.95, 1.0, 1.05, 1.08, 1.1, etc.

[0088] The SO2 content in the heating atmosphere during the hot rolling process of IF steel is >60 mg / m³. 3 And ≤100mg / m 3 and H2S content >100mg / m³ 3 And ≤150mg / m 3 In this case, an oxidizing atmosphere is used, utilizing the oxygen partial pressure to increase the formation of more FeO and the spinel-dense Fe3O4, which consumes the diffusion depth of S element into the matrix, reducing the S element content at the interface. Simultaneously, the faster FeO formation rate under high temperature conditions can consume already formed FeS, resulting in FeS mainly existing within the iron scale. For example, the excess air coefficient during the hot rolling process of IF steel is controlled at 1.1, 1.15, 1.20, 1.23, 1.25, and 1.30.

[0089] In some embodiments, if the SO2 content in the heating atmosphere is ≤60 mg / m³ 3 and / or H2S content ≤100mg / m³ 3 The heating temperature is then set to 1160℃~1180℃;

[0090] If the SO2 content in the heating atmosphere is >60 mg / m³ 3 And ≤100mg / m 3 and / or H2S content > 100 mg / m³ 3 And ≤150mg / m 3 The heating temperature is then set to 1190℃~1230℃.

[0091] In the embodiments of the present application, the current process in the current production line is mainly low-temperature process. From the perspective of reducing production cost and environmental protection, if the content of S element oxides SO2 and H2S is at a low level after the heating furnace gas burns for a period of time, this production process can be used to avoid the inward diffusion of S element. High-temperature heating process is a supplementary condition, and high-temperature heating process is used under the condition that the heating furnace atmosphere exists in a high-S atmosphere. In addition, when the production rhythm or the heating furnace atmosphere fluctuates and high-temperature conditions are required for tapping, the oxidation loss method is used to consume the influence of S element in the heating furnace. The temperature of the heating in the hot rolling process of the IF steel is controlled to ensure that the steel plate is burned through and austenitized. For example, the temperature of low-temperature heating can be controlled at 1160°C, 1165°C, 1170°C, 1175°C, 1180°C, etc., and the temperature of high-temperature heating can be controlled at 1190°C, 1195°C, 1200°C, 1205°C, 1210°C, 1220°C, 1230°C, etc.

[0092] In some embodiments, if the content of SO2 in the heating atmosphere is ≤60 mg / m 3 , and / or the content of H2S is ≤100 mg / m 3 , the soaking time of the heating is set to 30 min-40 min.

[0093] If the content of SO2 in the heating atmosphere is >60 mg / m 3 and ≤100 mg / m 3 , and / or the content of H2S is >100 mg / m 3 and ≤150 mg / m 3 , the soaking time of the heating is set to 40 min-50 min.

[0094] In the embodiments of the present application, the soaking time ensures that the steel plate is burned through, and the oxidation is intensified with a long soaking time, and at the same time, the inward penetration of S may also be intensified. There is a certain interaction relationship between the oxidation time and the S penetration amount, and overall, the inward penetration of S can be inhibited by oxidation loss. For example, the soaking time of low-temperature heating can be 30 min, 32 min, 34 min, 36 min, 38 min, 40 min, etc., and the soaking time of high-temperature heating can be 40 min, 42 min, 44 min, 45 min, 46 min, 48 min, 50 min, etc.

[0095] The present application will be further described in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present application and are not used to limit the scope of the present application. The experimental methods in the following examples are not specified, and generally, the industry standards are measured. If there is no corresponding industry standard, the general international standards, conventional conditions, or the conditions suggested by the manufacturer are followed.

[0096] The embodiment of the present application provides a method for reducing surface black spot defects of galvanized IF steel, and the method comprises the following steps:

[0097] S11, pouring IF steel liquid containing Cr to obtain a casting blank; at least part of the surface of the casting blank contains an FeO-Cr2O3 layer;

[0098] S12, placing the casting blank in a heating furnace for heating, and adjusting the reaction activity of S in the heating atmosphere of the heating furnace to reduce the formation amount of high-temperature sulfides on the surface of the casting blank;

[0099] S13, rolling the heated casting blank to obtain a hot-rolled plate, and sequentially performing pickling and cold rolling on the hot-rolled plate to obtain a cold-rolled plate, and then sequentially performing continuous hot-dip galvanizing annealing on the cold-rolled plate to obtain the galvanized IF steel; the main process parameters of the method for reducing surface black spot defects of the galvanized IF steel are shown in Table 2.

[0100] Table 1 Chemical composition (wt%) of the casting blank, the rest is Fe and inevitable impurities

[0101]

[0102]

[0103] Table 2 Main process parameters of the method for reducing surface black spot defects of the galvanized IF steel

[0104]

[0105] The galvanized IF steel obtained by the examples and the comparative examples is tested for black spot defects, and the results are shown in Table 3.

[0106] Table 3 Test results of black spot defects of the galvanized IF steel

[0107] Group Defect density of black dot defects, pieces / 100 cm 2 ]]> Example 1 10 Example 2 7 Example 3 5 Example 4 8 Comparative Example 1 50 Comparative Example 2 70 Comparative Example 3 90

[0108] The above only is the specific embodiment of the present application, so that the person 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 these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method of reducing surface black spot defects of a galvanized IF steel, characterized in that, The method comprises: casting a molten steel containing Cr to obtain a casting blank; at least part of the surface of the casting blank contains an FeO-Cr2O3 layer; placing the casting blank in a heating furnace for heating, and adjusting the reactive activity of S in the heating atmosphere of the heating furnace to reduce the formation amount of high-temperature sulfides on the surface of the casting blank; the placing of the casting blank in the heating furnace for heating, and the adjusting of the reactive activity of S in the heating atmosphere of the heating furnace, comprise: placing the casting blank in a heating furnace for heating, and controlling the content of S in the heating atmosphere of the heating furnace to be a set content of S; according to the set content of S in the heating atmosphere, setting the air excess coefficient, temperature and soaking time of the heating respectively; The setting S content includes: SO2≤100 mg / m 3 , H2S≤150 mg / m 3 ; If the SO2 content in the heating atmosphere is ≤ 60 mg / m 3 , and / or the H2S content is ≤ 100 mg / m 3 , the excess coefficient of the heated air is set to 0.9-1.1, the temperature of the heating is 1160-1180℃, and the soaking time of the heating is 30-40 min. if the SO2 content in the heating atmosphere is > 60 mg / m 3 and ≤ 100 mg / m 3 and / or the H2S content is > 100 mg / m 3 and ≤ 150 mg / m 3 then the excess factor of the heated air is set to 1.1 to 1.3, the temperature of the heating is 1190°C to 1230°C and the soaking time of the heating is 40 min to 50 min.

2. The method of claim 1, wherein, The set S content includes: SO2: 30 mg / m 3 ~100 mg / m 3 H2S: 80 mg / m 3 ~150 mg / m 3 .

3. The method of claim 1, wherein, the chemical composition of the casting blank comprises: Cr, the content of Cr is 0.1% to 0.2% by mass fraction.

4. The method of claim 3, wherein, the chemical composition further comprises: C, Al, Mn, Si, Ti and Fe, the content of C is 0.015% to 0.02% by mass fraction, the content of Al is 0.02% to 0.05% by mass fraction, the content of Mn is 0.1% to 0.2% by mass fraction, the content of Si is 0.02% to 0.05% by mass fraction, and the content of Ti is 0.06% to 0.08% by mass fraction.

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