CN118460912A

CN118460912A - Hot-rolled steel sheet, galvanized steel sheet, and method for producing same

Info

Publication number: CN118460912A
Application number: CN202310096912.2A
Authority: CN
Inventors: 毕文珍; 陈孟; 金鑫焱
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2023-02-07
Filing date: 2023-02-07
Publication date: 2024-08-09
Also published as: WO2024164984A1

Abstract

The invention provides a hot-rolled steel sheet, a galvanized steel sheet, and methods for producing the same. The hot rolled steel plate sequentially comprises a reduced iron layer, an outer oxide layer, an inner oxide layer and a matrix from the outer surface of the steel plate to the inner matrix; the reduced iron layer consists essentially of reduced iron and has an average thickness of 3.0 to 10.0 μm; the outer oxide layer comprises ferrosilicon oxide, and has an average thickness of 0.1-2.0 μm; and the inner oxide layer comprises an oxide of silicon and/or manganese and has an average thickness of 5.0-40.0 μm. The surface of the hot rolled steel sheet has a reduced iron layer of a proper thickness, and can be brought into a pickling-free state. The galvanized steel sheet of the present invention comprises the hot rolled steel sheet and a zinc-based plating layer plated on the surface thereof.

Description

Hot-rolled steel sheet, galvanized steel sheet, and method for producing same

Technical Field

The present invention relates to a steel sheet and a method for producing the same, and more particularly to a hot-rolled steel sheet, a galvanized steel sheet and a method for producing the same.

Background

Automobile weight reduction is one of key technologies adapting to the trend of modern automobile safety, energy conservation and environmental protection. The demand for high-strength hot-dip galvanized hot-rolled steel sheets having high-strength hot-rolled steel sheets as base steel sheets has been increasing for suspension arms and the like, particularly chassis-like components requiring corrosion resistance. The hot-rolled galvanized steel sheet is obtained by plating a hot-rolled steel sheet, and a cold rolling process and an annealing process are not required, so that it is more economical than plating a cold-rolled steel sheet.

In general, hot-rolled hot-dip plated steel sheets are first pickled to remove scale formed during hot rolling and then plated, but waste acid pollution generated during pickling is serious and increases manufacturing costs. Further, the hot rolled steel sheet containing Si and Mn tends to have defects such as local oxide residue on the surface after pickling, local contamination due to excessive pickling, and local plating leakage and uneven alloying.

In order to solve the above problems, patent document 1 proposes that in order to avoid the defect of poor alloying of the alloyed hot-dip galvanized steel sheet containing Si, mn, P, etc., shot blasting treatment and additional brushing during pickling of the hot-rolled steel sheet, and then continuous hot dip plating and alloying treatment may be performed.

Patent document 2 proposes a production method of environment-friendly high-surface-quality pickling-free automotive girder steel, which mainly controls the surface iron oxide scale structure of the automotive girder steel by optimizing chemical components, a heating system and a rolling process of a matrix. On the one hand, through the design of low Si component, the amount of Fe ₂SiO₄ formed between the iron scale and the matrix in the heating process of the steel billet is reduced, and the descaling pressure is reduced. On the other hand, the rough descaling temperature of the billet is ensured to be higher than the melting point 1170 ℃ of Fe ₂SiO₄ by high-temperature heating, and a plurality of passes of high-pressure water are matched in the rough rolling stage to remove the surface Fe ₂SiO₄; the finish rolling stage adopts a high Wen Zhongga to prevent FeO from being crushed and converted into Fe ₃O₄ and finally into Fe ₂O₃; and finally, coiling at low temperature, increasing the cooling speed of the rolled steel plate, and controlling the eutectoid reaction of FeO to generate black iron oxide scale Fe ₃O₄ which has good plasticity and is tightly combined with a matrix, thereby meeting the use requirement of no acid washing.

Patent document 3 proposes a pickling-free hot rolled strip steel with high surface quality and a manufacturing method thereof, wherein a pure iron layer with good formability is formed on the surface of the strip steel in the hot rolling process, so that the surface adhesiveness and the formability are improved, and the oxide layer in the stamping process is effectively prevented from falling off, thereby achieving the surface quality similar to that of a pickling plate and a cold rolled plate.

Patent document 4 proposes a hot dip galvanized steel sheet with a porous iron layer, which is mainly based on the principle that the surface of the steel sheet is directly subjected to reduction annealing to obtain porous metallic iron under the control of a protective atmosphere of a reducing gas and a reducing temperature. Meanwhile, parameters such as the Al content in a zinc pot, the temperature of the zinc pot and the like in the zinc plating process ensure that a continuous and complete Fe ₂Al₅ barrier layer is formed between the porous iron layer and the plating layer after the steel plate with the porous iron layer is galvanized, so that the hot dip galvanized steel plate has better surface quality.

However, although the method proposed in patent document 1 improves the surface quality of the plating layer, it is necessary to perform shot blasting and brushing on the base material before plating, and there is a problem that the yield is low and the cost is high. Patent document 2 discloses that the adhesiveness of the scale is improved, but only the non-plated steel sheet is required to meet the pickling-free use requirement, the scale on the surface of the steel sheet is still at risk of falling off by punching, and the scale cannot be effectively coated, so that the corrosion resistance is not realized. Patent document 3 discloses that pickling is not performed by the pure iron layer formed on the surface, but the coverage rate of the pure iron layer on the iron oxide layer on the surface of the steel plate is only 70%, the iron oxide layer still exists on the surface of the pure iron layer and the surface of the substrate, and compared with pure iron, the forming performance of the iron oxide layer is poor, and finally the iron oxide layer may fall off in the forming process. On the one hand, patent document 4 discloses that oxide skin on the surface of hot rolled strip steel is reduced under the control of reducing atmosphere and reducing temperature to obtain porous metallic iron; on the other hand, the Al content in the zinc liquid is controlled to be increased to be more than 0.5wt.% so as to ensure the surface quality. The iron scale formed in the actual hot rolling process is thicker (more than 10 mu m), the iron scale is difficult to be completely reduced in the short-time reduction annealing process, and the excessive content of Al in a zinc pot can lead to the increase of the viscosity of zinc liquid, so that zinc slag is not easy to precipitate and decline, and adheres to the surface of a steel plate, thereby causing the deteriorated surface quality.

Prior art literature

Patent document 1: JP06158254

Patent document 2: CN101906584B

Patent document 3: CN106119687B

Patent document 4: CN105506529B

Disclosure of Invention

The present invention aims to solve the problems of the prior art described above. A first aspect of the present invention provides a hot rolled steel sheet having a reduced iron layer consisting essentially of reduced iron and having an average thickness of 3.0 to 10.0 μm, an outer oxide layer comprising an oxide of ferrosilicon and having an average thickness of 0.1 to 2.0 μm, an inner oxide layer comprising an oxide of silicon and/or manganese and having an average thickness of 5.0 to 40.0 μm, and a matrix in this order from an outer surface to an inner matrix of the steel sheet.

The hot rolled steel plate does not contain an iron oxide layer, and the iron oxide on the surface of the hot rolled steel plate can be basically or almost completely converted into reduced iron by controlling the coiling and heat preservation process after the hot rolled steel plate is finally rolled, so that the pickling-free state is achieved. Specifically, the mass percentage of Fe in the reduced iron layer is 90% or more, preferably 95% or more, more preferably 98% or more, and most preferably 100%. The galvanized steel sheet is subjected to hot dip plating and/or alloying, and thus a galvanized steel sheet having excellent surface quality and good plating adhesion can be obtained.

If the reduced iron layer is too thin, it means that iron oxide generated during hot rolling is not effectively reduced, affecting the stamping property of the subsequent finished material, and if it is discontinuous, the reduced iron layer too thin may cause various defects due to poor surface platability, so that the lower limit of the reduced iron layer thickness is defined to be 3 μm, preferably 5 μm. If the reduced iron layer is too thick, the temperature of hot rolling coiling and heat preservation of the heat preservation cover is too high, the mechanical property of the material can be affected if the heat preservation time is too long, the further improvement effect on the surface quality is limited, and the upper limit of the thickness is defined as 10 mu m.

With the reduction of the iron oxide by the Si/Mn element in the matrix, the Si/Mn forms an outer oxide layer containing Si/Mn/Fe at the interface of the reduced iron layer and the matrix on the one hand, and forms an inner oxide layer containing Si/Mn oxide at the outermost layer of the matrix on the other hand. If the inner oxide layer is too thin, e.g. below 5 μm, this indicates insufficient reduction of iron oxide, and if the inner oxide layer is too thick, e.g. above 40 μm, the formability of the finished material is detrimental.

Preferably, the inner oxide layer has an average thickness of 10 to 40 μm, more preferably 20 to 30 μm.

Preferably, the reduced iron layer is continuous and uniform in thickness.

Preferably, the composition of the above matrix is as follows: c:0.15-0.3wt.%, si:1.0-3.0wt.%, mn:1.0 to 3.0wt percent of P less than or equal to 0.02wt percent, S less than or equal to 0.01wt percent, and the balance being Fe and unavoidable impurities.

The design concept of each chemical element in the matrix is as follows:

C：0.15-0.3wt.％。

C is the most basic strengthening element in steel and is also an austenite stabilizing element. When the content of C is too low, the content of the residual austenite is insufficient, and the strength of the material is low; and when the content of C is too high, the welding performance of the material is remarkably deteriorated. The upper limit of the content of C is defined as 0.3wt.% to ensure that the steel plate has certain plasticity and toughness, and the lower limit is defined as 0.15wt.% to ensure that the steel plate has certain strength.

Si：1.0-3.0wt.％。

Si can promote the enrichment of C in austenite, so that the stability of the austenite is increased, the strength of the steel plate is improved, and the toughness of the steel plate is improved to a certain extent. In order to ensure that the material has a strength of 700MPa or more and to achieve formability with an elongation of 10% or more, a relatively large amount of Si needs to be added in the present invention. Meanwhile, in order to reduce the ferric oxide on the surface of the hot rolled strip steel by utilizing the strong oxidizing property of Si in the coiling and heat preservation processes, the Si content cannot be too low. Therefore, the lower limit of the Si content is defined as 1.0wt.%, but when the Si content is large, the probability of surface defects of red scale generated at the time of hot rolling increases, and the rolling force increases, resulting in deterioration of ductility of the hot rolled steel sheet, and therefore the upper limit of the Si content is defined as 3.0wt.%.

Mn：1.0-3.0wt.％。

Mn is an element for improving the hardenability of the steel plate, expanding an austenite phase region and effectively ensuring the strength of the steel plate after quenching, and meanwhile Mn is an element for expanding the austenite phase region, so that the temperatures of Ac3 (which refers to the actual transformation temperature when carbon steel is heated) and Ac1 (the temperature at which austenite starts to form when the steel is heated) can be reduced, and pearlite transformation can be delayed. In addition, the addition of Mn increases the stability of austenite. Thus, the lower limit of the Mn content in the present invention is defined as 1.0wt.%. However, too high Mn causes cracking of the billet in the continuous casting process and affects the weldability of the material, and the upper limit of Mn is controlled to 3.0wt.% in the present invention.

P≤0.02wt.％。

P is an impurity element in steel, and decreases the plasticity and toughness of steel, and particularly causes "cold embrittlement" at low temperatures, and therefore is strictly controlled to be not more than 0.02wt.%, and the lower limit is not defined, but the lower the content is, the better.

S≤0.01wt.％。

S is also an impurity element and needs to be kept at a low level. Since S present in steel causes heat embrittlement problems due to formation of FeS, the S content is limited to 0.01wt.% or less, and the lower limit is not defined, the lower the content is, the better.

Preferably, the matrix further has more than one of 0 < Al.ltoreq.1.0 wt.%,0 < Nb.ltoreq.0.05 wt.%,0 < Ti.ltoreq.0.05 wt.%, and 0 < B.ltoreq.0.001 wt.%.

A1 when present in a solid solution state, can increase the stacking fault energy, inhibit cementite precipitation and gamma-to-martensite transformation, and improve the austenite stability. And Al can form tiny and dispersed insoluble particles with C, N so as to refine grains. The addition of proper A1 can reduce the addition amount of Si element, but the strengthening effect of Al is weaker than Si, the capability of stabilizing austenite is weaker than Si, the addition amount is not too high, or a large amount of oxide inclusions are easy to form, and the steel-making continuous casting is not facilitated. The invention controls the mass percentage of Al within 1.0 wt.%.

Nb, ti and B can further improve the properties of the hot rolled steel sheet. Nb and Ti elements can form fine carbides with C to promote tissue refinement, but the formation of such fine carbides is detrimental to C enrichment into and stabilization of retained austenite. The main function of B is to improve the hardenability of steel, B is easy to gather at austenite grain boundaries, delays the transformation from austenite to ferrite, and has obvious effect by adding a small amount of B into steel, but the too high content of B can cause the increase of the strength of steel, which is unfavorable for obtaining good shaping property. Since the addition of the above alloy elements increases the cost of the material, the content of Nb and Ti is controlled to be 0.05wt.% or less and the content of B is controlled to be 0.001wt.% or less in the present invention. In the present invention, at least one of the above elements may be preferably added in view of both performance and cost control.

A second aspect of the present invention provides a galvanized steel sheet comprising a substrate formed of the above hot rolled steel sheet, and a pure zinc plating layer plated on a surface of the substrate.

A third aspect of the present invention provides another galvanized steel sheet comprising a base sheet formed of the above hot rolled steel sheet, and a zinc-iron alloy plating layer plated on a surface of the base sheet, wherein Fe in the zinc-iron alloy plating layer is not less than 7.0wt.%, preferably 7.0 to 10.0wt.%, and/or the pulverization level of the zinc-iron alloy plating layer is not more than 4 grades.

The zinc-based coating formed on the hot rolled steel plate can be adjusted according to different use requirements, and common zinc-based coatings comprise pure zinc coatings and zinc-iron alloy coatings. It is generally considered that the corrosion resistance is improved by performing the alloying treatment after the hot dip plating. An excessively low Fe content in the alloy plating layer indicates insufficient alloying and the corrosion resistance of the plating layer is reduced, so that the Fe content in the alloy plating layer needs to be 7wt.% or more, preferably 7.0 to 10.0wt.% in order to ensure sufficient alloying.

Preferably, the yield strength of the galvanized steel sheet is 450-750MPa, the tensile strength is 700-1100MPa, and the elongation is not less than 10%, for example, 10% -25%, more preferably 15% -25%. More preferably, when having a zinc-iron alloy plating layer, the galvanized steel sheet has a yield strength of 600 to 750MPa and a tensile strength of 900 to 1100MPa; when the zinc-plated steel sheet is provided with a pure zinc plating layer, the yield strength of the zinc-plated steel sheet is 500-700MPa, and the tensile strength is 900-1100MPa.

A fourth aspect of the present invention provides a method of manufacturing the above hot rolled steel sheet, comprising the steps of, in order:

1) Smelting and casting molten steel to obtain a plate blank;

2) Carrying out hot rolling on the slab;

3) Crimping; and

4) And (5) preserving heat to obtain the hot rolled steel plate.

Wherein the curling and heat preservation are carried out at 600-800 ℃, and the heat preservation time is 2-10 hours, preferably 3-6 hours.

Further, if crimping is performed at 600-700 ℃, the holding time is 3-10 hours, preferably 3-6 hours; if crimping is carried out at 700-800℃, the incubation time is 2-4 hours, preferably 2-3 hours.

In the invention, coiling is carried out at 600-800 ℃ and heat preservation is carried out, and the coiling temperature is preferably 600-700 ℃. If the coiling temperature is too low, the hot coil strength is too high, and cold rolling is difficult. In the present invention, the iron oxide is converted into reduced iron by the internal oxidation of Si and/or Mn, which are strong oxidizing elements, and the reduction of the surface iron oxide, and the effect cannot be exerted at too low a temperature. If the coiling temperature is too high, the hot coil strength is low, the performance of the finished product is affected, and the too high coiling temperature can cause too thick iron oxide layer, which is not beneficial to the generation and control of reduced iron.

The hot rolled steel coil after hot rolling and coiling is conveyed to a heat preservation device for heat preservation, the hot rolled steel coil is curled in the coiling temperature range, and enters a cover retreating device, such as a heat preservation cover or other heat preservation devices, in the temperature range of not lower than 50 ℃ of the coiling temperature. The heat preservation is carried out within the temperature range, so that on one hand, the base material can be softened, and on the other hand, the coiled strip steel is ensured to be converted in the surface ferric oxide at the heat preservation stage. In the heat preservation process, the hot rolled strip steel can be contacted with air except for the head part and the tail part, and is cooled quickly, the temperature of the position in the coil is not greatly changed with the temperature set in the coiling process in the heat preservation process, and the strip steel in the coil is closely contacted with the strip steel, so that the oxygen partial pressure is lower, and therefore, the iron oxide generated in the hot rolling and hot rolling coiling process is higher than the Gibbs free energy of oxides formed by combining Si, mn and O in the matrix, and the Si and Mn on the surface layer of the strip steel matrix can abstract O in the surface oxide layer to form an inner oxide layer on the surface of the matrix. Meanwhile, due to the O content gradient formed between the surface oxide layer and the surface of the strip steel matrix, O continuously diffuses to the matrix, so that iron oxide is continuously reduced by Si and Mn in the matrix, and finally, under the condition of enough heat preservation time, a reduced iron layer, an outer oxide layer containing ferrosilicomanganese oxide, an inner oxide layer of Si and/or Mn oxide and a matrix are sequentially formed from the outer surface of the steel plate to the inner matrix. The invention aims to make use of the strong oxidation characteristics of Si and Mn elements to fully convert ferric oxide into reduced iron in the coiling and heat-preserving processes of hot rolled strip steel, thereby achieving the effect of acid cleaning prevention and improving the platability and alloying capability of the hot rolled steel plate. Therefore, the heat-insulating cover of the present invention cannot keep the heat for a too short period, and the present inventors have found through intensive studies that a long heat-insulating time is required at the time of low-temperature coiling, and the heat-insulating time can be shortened at the time of high-temperature coiling, so that iron oxide generated during hot rolling can be basically converted into reduced iron.

Preferably, in the above method for manufacturing a hot rolled steel sheet,

In step 2), the slab is heated to 1100-1300 ℃, preferably 1180-1270 ℃, more preferably 1210-1270 ℃, and rolled after heat preservation for 1-4 hours, wherein the final rolling temperature is above 850 ℃, preferably 850-950 ℃;

Step 4) is carried out in a heat preservation cover by adopting a reducing atmosphere, wherein the reducing atmosphere is pure H ₂. The volume fraction of H ₂ in pure H ₂ is above 99.9%.

In the step 2), if the heating temperature is too high, the heat preservation time is too long, which can cause the overfiring of the slab, the coarse grain structure in the slab, the reduction of the hot processing performance and the serious decarburization of the surface of the slab; if the heating temperature is too low and the heat preservation time is too short, the internal temperature of the slab is not uniform, and after descaling and blooming by high-pressure water, the finish rolling temperature is too low, so that the deformation resistance of the blank is too high and the shape is poor.

In addition, the final rolling temperature is more than or equal to 850 ℃, the influence on the rolling force is considered, and when the final rolling temperature is lower than 850 ℃, the deformation resistance is obviously increased, and the rolling load is greatly increased. The upper limit of the finishing temperature can be set to 950 ℃, preferably 900 ℃, which is beneficial to energy saving.

In a fifth aspect of the present invention, there is provided a method of manufacturing a galvanized steel sheet having a pure zinc plating layer on a surface thereof, comprising: heating the hot-rolled steel plate to a soaking temperature of 800-930 ℃ at a heating rate of 1-20 ℃/s, preserving heat for 30-200s, rapidly cooling the hot-rolled steel plate to 220-320 ℃ at a cooling rate of more than 40 ℃/s, preserving heat for 15-120s, heating to 380-480 ℃ and entering a plating solution for hot dip plating, and cooling to obtain the galvanized steel plate with a pure zinc plating layer on the surface.

In a sixth aspect of the present invention, there is provided a method for producing a galvanized steel sheet having a zinc-iron alloy plating layer on the surface, comprising hot dip plating and alloying the above hot rolled steel sheet.

Preferably, the hot dip plating is performed as follows: heating the hot rolled steel plate to a soaking temperature of 800-930 ℃ at a heating rate of 1-20 ℃/s, preserving heat for 30-200s, rapidly cooling the hot rolled steel plate to 220-320 ℃ at a cooling rate of more than 40 ℃/s, preserving heat for 15-120s, and then heating to 380-480 ℃ and entering a plating solution for hot dip plating.

Preferably, the alloying temperature is 460-520 ℃, and the alloying heat preservation time is 10-30s. Specifically, alloying is performed as follows: and (3) alloying the hot-rolled steel plate subjected to hot dip plating at 460-520 ℃ for 10-30s to obtain the galvanized steel plate with the zinc-iron alloy coating on the surface.

Preferably, the zinc-iron alloy plating layer on the surface of the galvanized steel sheet has a pulverization level of 4 or less, more preferably 3 or less. The lower the chalking level, the better the surface quality.

Preferably, the hot dip coating and/or the alloying is performed in a reducing atmosphere of N ₂-H₂ mixed gas, wherein the volume fraction of H ₂ is 0.5 to 20%, and the dew point of the reducing atmosphere is-40 to 0 ℃.

The components of the plating solutions adopted in the hot dip plating process of the galvanized steel sheets with different plating layers are different, and the composition of the plating solutions can be adjusted according to actual requirements by referring to the prior art.

Preferably, the steel sheet is heated to a temperature range of + -10deg.C of the zinc pot and hot dip plated in the zinc pot.

Preferably, in the present invention, the hot rolled steel coil needs to be cut before hot dip plating. Specifically, the length ranges of the head and the tail of the strip steel are respectively 5-10m, and the trimming amount of the strip steel is minimum 30mm. The reason is that the edge of the strip steel, the strip head and the strip tail are cooled quickly and partially contacted with air in the coiling and the subsequent heat preservation process of the heat preservation cover, so that the oxygen partial pressure is higher, the formation of the surface reduced iron is not promoted, and the excision process can be increased according to the requirement.

In the hot dip plating process, the steel coil is heated to the temperature range of 800-930 ℃, and the influence of the structure and the performance of the steel strip matrix at the temperature is mainly considered. When the heating temperature is too low, austenite transformation cannot be fully generated, the final strength of the steel plate is low, and when the heating temperature is too high, the matrix structure of the steel plate is completely austenite transformed, the stability of austenite is reduced, the strength of the steel plate is high, and the elongation is insufficient.

In the process of preparing the galvanized steel sheet, the atmosphere of the hot rolled steel coil in the heating and soaking (annealing) process is N ₂-H₂ mixed gas, wherein the content of H ₂ is 0.5-20%, and the dew point of the annealing atmosphere is-40-0 ℃. Because the surface of the hot rolled steel plate is provided with a layer of reduced iron, the layer of reduced iron can prevent Si and Mn in a matrix from diffusing to the surface of the hot rolled steel plate in the heating and soaking processes, so that a lower H ₂ content and a lower dew point can be selected, but the excessive H ₂ content can cause dangers, the excessive dew point can cause surface decarburization, and the upper limit of the H ₂ content is set to 20 percent and the upper limit of the dew point is set to 0 ℃.

The steel plate enters a zinc pot at 380-480 ℃ for hot dip plating, if the temperature of the steel plate is too low, an inhibition layer cannot be effectively formed on the surface, a burst structure can be generated at the initial stage of alloying, and defects such as alloying stripes can be generated on the surface of the alloyed steel plate; if the temperature of the steel plate is too high, waves are formed on the surface of the steel plate after hot dip galvanizing due to a large amount of latent heat, and the shape of the steel plate is affected.

In the process of obtaining the galvanized steel sheet with the zinc-iron alloy coating, the alloying temperature is set to 460-520 ℃, and the alloying heat preservation time is set to 10-30s for the following reasons: the invention has the biggest characteristics that after hot rolling, coiling and heat preservation, the reduced iron layer formed on the surface of the hot rolled plate is equivalent to dip plating and alloying on the pure iron layer, so that the alloying temperature can be greatly reduced compared with the traditional ultra-high strength steel, the lowest alloying temperature is set to 460 ℃, the alloying temperature is too high, zn-Fe diffusion is promoted to be too sufficient, overalloying is caused, and the highest temperature is set to 520 ℃. The minimum value of 10s is set for the heat preservation time, so that the Fe content after alloying is more than or equal to 7%, and the overalloying of the coating is serious due to the excessively high alloying, so that the maximum value is set to 30s.

The hot rolled steel plate provided by the invention can be directly dip-plated and/or alloyed without acid washing to obtain a galvanized steel plate with excellent surface quality, and the galvanized steel plate has good formability and low manufacturing cost.

The manufacturing method provided by the invention has low cost, can manufacture high-quality hot rolled steel plates and galvanized steel plates, and has good application prospect.

Drawings

Fig. 1 shows a schematic structural view of a galvanized steel sheet according to the invention;

FIG. 2 is an electron micrograph of a cross section of a hot rolled steel sheet according to example 2 of the present invention;

FIG. 3 is an electron micrograph of a cross section of a hot rolled steel sheet according to comparative example 2 of the present invention;

FIG. 4 is an electron micrograph of a cross section of a galvanized steel sheet according to an embodiment 2 of the invention;

FIG. 5 is an electron micrograph of a cross section of a galvanized steel sheet according to a comparative example 2 of the invention;

FIG. 6 is a cross-sectional electron micrograph of the zinc coating layer adhesion NG (in failure state) of a galvanized steel sheet according to a comparative example 2 of the invention.

Reference numerals:

1-zinc coating, 2-reduced iron layer, 3-outer oxide layer, 4-inner oxide layer and 5-substrate.

Detailed Description

The high-strength hot-rolled acid-free hot-dip galvanized steel sheet having excellent surface quality and the method of manufacturing the hot-rolled galvanized steel sheet according to the present invention are further explained and illustrated by specific examples below, but the explanation and illustration do not unduly limit the technical scheme of the present invention.

For the purpose of making the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

Examples and comparative examples

The hot rolled steel sheets of examples 1to 7 and the hot rolled steel sheets of comparative examples 1to 3 of the present invention have chemical compositions shown in the following Table 1, and the hot rolled steel sheets of examples 1to 7 and comparative examples 1to 3 and galvanized steel sheets were produced as follows:

(1) The slab was produced by smelting and casting according to the compositions shown in Table 1.

(2) And (3) hot rolling: and heating the slab at a heating temperature, carrying out hot rolling after heat preservation, and finishing rolling at a finishing rolling temperature.

(3) Crimping: the steel sheet is finish rolled, cooled, and then curled.

(4) And (3) heat preservation: transferring the coiled steel coil into a heat preservation cover for heat preservation, and then cutting off the head, tail and edge of the hot rolled strip steel for standby to obtain a hot rolled steel plate;

wherein, the curling and heat preservation are carried out at 600-800 ℃ for 2-10 hours.

(5) Hot dip plating: and heating the hot rolled steel plate to a soaking temperature and preserving heat, wherein the atmosphere of the heating section and the preserving heat section adopts N ₂-H₂ mixed gas, the content of H ₂ is 0.5-20%, and the dew point of the atmosphere is-40-0 ℃. Then the steel plate is rapidly cooled to 220-320 ℃ and is kept for 15-120s; and heating to 380-480 ℃ and then entering a zinc pot for hot dip plating.

(6) Alloying: and (3) carrying out heat preservation for 10-30s at the temperature of 460-520 ℃ to carry out alloying, thus obtaining the required galvanized steel sheet with excellent surface quality.

The specific process conditions for the examples and comparative examples are shown in table 2.

If the galvanized steel sheet with the pure zinc coating is required to be obtained, an alloying step is not required; if a galvanized steel sheet with a zinc-iron alloy coating is to be obtained, an alloying step is further performed after hot dip plating. The plating solutions selected for forming different plating layers on the hot rolled strip are different, and the specific plating solution composition can be adjusted according to the requirements and with reference to the prior art.

The galvanized steel sheets of examples 2, 6 and comparative example 2 of the invention have pure zinc plating layers, and the galvanized steel sheets of examples 1, 3-5, 7 and comparative examples 1, 3 have zinc-iron alloy plating layers, wherein the galvanized steel sheets having zinc-iron alloy plating layers are subjected to an alloying step after hot dip plating.

Table 1 lists the mass percentages (wt.%) of chemical elements of the hot rolled steel sheets of examples and comparative examples, the balance being Fe and other impurities except P, S.

Table 2 lists specific process parameters employed for preparing the hot rolled steel sheet and the galvanized steel sheet of the examples and the comparative examples.

Table 3 shows the thickness of each layer, the mass percentage of Fe (reduced iron) in the reduced iron layer, and the surface quality and mechanical properties of the galvanized steel sheets in the examples and comparative examples.

The average thickness of the reduced iron layer, the outer oxide layer and the inner oxide layer is an average value of thicknesses obtained at 10 different positions in an electron micrograph of a cross section of a hot rolled steel sheet.

The surface quality and adhesion test of the plating layer is carried out by referring to GB/T39130-2020 zinc layer adhesion test method of galvanized product.

Specifically, when the galvanized steel sheet has an alloyed plating layer, its surface quality (pulverization level) is evaluated in accordance with a plating adhesion test standard-V-bend test: the coating of the bent sample was peeled off by tape, the reflectance value of the chalked tape was measured on a whiteboard, or the peeling was visually checked by peeling off-chalking (powdering) of the coating during bending of the sample. The meaning of "alloying NG" in table 3 is that no effective alloying of the steel sheet surface is achieved.

When the galvanized steel sheet has a pure zinc coating, the adhesion of the surface quality is measured according to a bending test method standard, a sample is bent on a 60-degree die on a V-bending machine, and flattened on a flat head, and the adhesion of the coating is determined by visual inspection. In table 3, "NG" indicates poor plating adhesion, and "OK" indicates good plating adhesion.

The mechanical properties of the galvanized steel sheet were evaluated as follows: GB/T228.1-2010 section 1 of Metal Material tensile test: room temperature test method, tensile test was performed.

FIG. 1 shows a schematic structural view of a high strength hot rolled acid pickling free galvanized steel sheet according to the invention, wherein 1 is a zinc system plated, which may be a pure zinc plating or a zinc-iron alloy plating, mainly consisting of delta phase and zeta phase; 2 and 3 are the interface layers of the plating layer and the matrix, 2 is the reduced iron layer, and 3 is the outer oxide layer containing ferrosilicon manganese; 4 is an inner oxide layer of the surface layer of the matrix, which comprises an oxide of ferrosilicon, preferably consisting of an oxide of Si and/or Mn; and 5 is a substrate of a hot rolled steel plate. The key point of the invention is that the reduced iron layer on the surface of the hot rolled steel plate is regulated by controlling the coiling temperature of hot rolling, the heat preservation temperature after coiling and the heat preservation time, so that the hot rolled plate can be finally subjected to hot dip plating and alloying without pickling, and the galvanized steel plate with excellent surface quality is obtained.

The depth of each layer in the hot rolled steel plates of examples 1-7 is within the scope of the invention, so that the excellent surface quality of the coating after hot dip plating or hot dip plating alloying is ensured, the pulverization grade of the alloying coating of all examples is less than 4, and the adhesiveness of the pure zinc coating is OK.

The compositions of comparative examples 1 and 2 are within the range defined by the present invention, but the coiling temperature of comparative example 1 is 500 ℃, the coiling temperature is too low, the holding time of comparative example 2 is only 0.5hr, and after coiling, si and Mn in the matrix cannot effectively reduce iron oxide on the surface of the steel sheet for the two comparative examples, so the surface of the hot rolled steel sheet is mainly covered with iron oxide. It can be seen from Table 3 that the hot rolled steel sheets of comparative examples 1 and 2 have a reduced iron layer of < 3 μm in thickness and are discontinuous, and thus the hot rolled steel sheets formed have poor platability. The iron oxide on the surface of the hot rolled steel sheet hinders the diffusion of zinc and iron during the alloying process, which eventually results in poor surface of the alloyed plating template of comparative example 1 and poor adhesion of the pure zinc plating template of comparative example 2 (as shown in fig. 6, the plating is peeled off on the cross section with the substrate). The coiling temperature and holding time of comparative example 3 were within the ranges defined in the present invention, but the Si content in the alloy composition of the steel sheet matrix was too low, and there was insufficient Si in the matrix to reduce iron oxide on the surface of the hot rolled steel sheet, so that the surface of the hot rolled steel sheet was still iron oxide, and finally an effective alloyed plated steel sheet was not formed.

It should be noted that the above list is merely a specific example of the present invention, and while the present invention has been illustrated and described by referring to certain preferred embodiments thereof, it should be understood by those skilled in the art that the above description is of the present invention in further detail with reference to the specific embodiments, and the specific implementation of the present invention should not be construed as limited to these descriptions. Various changes in form and detail may be made therein by those skilled in the art, including a few simple inferences or alternatives, without departing from the spirit and scope of the present invention.

Claims

1. A hot rolled steel sheet having a reduced iron layer, an outer oxide layer, an inner oxide layer and a base in this order from an outer surface of the hot rolled steel sheet to an inner base;

The reduced iron layer contains 90wt.% or more of reduced iron and has an average thickness of 3.0 to 10.0 [ mu ] m;

The outer oxide layer comprises ferrosilicon oxide, and the average thickness is 0.1-2.0 mu m; and

The inner oxide layer comprises oxides of silicon and/or manganese and has an average thickness of 5.0-40.0 μm;

The composition of the matrix is as follows: c:0.15-0.3wt.%, si:1.0-3.0wt.%, mn:1.0 to 3.0wt percent of P less than or equal to 0.02wt percent, S less than or equal to 0.01wt percent, and the balance being Fe and unavoidable impurities.

2. The hot rolled steel sheet as claimed in claim 1 wherein the matrix further has one or more of 0 < Al.ltoreq.1.0 wt.%,0 < Nb.ltoreq.0.05 wt.%,0 < Ti.ltoreq.0.05 wt.%,0 < B.ltoreq.0.001 wt.%.

3. A galvanized steel sheet comprising a substrate formed from the hot rolled steel sheet as claimed in claim 1 or 2, and a pure zinc plating layer plated on the surface of the substrate.

4. A galvanized steel sheet as set forth in claim 3, wherein said galvanized steel sheet has a yield strength of 450-750MPa, a tensile strength of 700-1100MPa, and an elongation of 10% or more.

5. A galvanized steel sheet comprising a base sheet formed from the hot rolled steel sheet as claimed in claim 1 or 2, and a zinc-iron alloy plating layer plated on the surface of the base sheet, wherein Fe in the zinc-iron alloy plating layer is not less than 7.0wt.%, and/or the pulverization rating of the zinc-iron alloy plating layer is not more than 4 grades.

6. The galvanized steel sheet as set forth in claim 5, wherein the galvanized steel sheet has a yield strength of 450-750MPa, a tensile strength of 700-1100MPa, and an elongation of 10% or more.

7. The method for manufacturing a hot rolled steel sheet according to claim 1 or 2, comprising the steps of, in order:

1) Smelting and casting molten steel to obtain a plate blank;

2) Carrying out hot rolling on the slab;

3) Crimping; and

4) Heat preservation is carried out, and the hot rolled steel plate is obtained;

8. The manufacturing method according to claim 7, wherein the curling is performed at 600 to 700 ℃ and the holding time is 3 to 10 hours; or curling at 700-800 deg.C for 2-4 hr.

9. The manufacturing method according to claim 7 or 8, wherein,

In the step 2), the plate blank is heated to 1100-1300 ℃, and is rolled after heat preservation for 1-4 hours, and the final rolling temperature is above 850 ℃; and/or the number of the groups of groups,

Step 4) is carried out in a heat preservation cover by adopting a reducing atmosphere, wherein the reducing atmosphere is pure H ₂.

10. The method for producing a galvanized steel sheet as set forth in claim 3, comprising: heating the hot rolled steel plate according to claim 1 or 2 to a soaking temperature of 800-930 ℃ at a heating rate of 1-20 ℃/s, preserving heat for 30-200s, cooling the hot rolled steel plate to 220-320 ℃, preserving heat for 15-120s, and then heating to 380-480 ℃ and entering a plating solution for hot dip plating.

11. A method for producing a galvanized steel sheet as defined in claim 5, comprising hot dip plating and alloying the hot rolled steel sheet as defined in claim 1 or 2.

12. The manufacturing method according to claim 11, wherein the alloying temperature is 460 to 520 ℃ and the alloying holding time is 10 to 30s.

13. The manufacturing method according to claim 11 or 12, wherein the hot dip plating and/or the alloying is performed under a reducing atmosphere of N ₂-H₂ mixed gas, wherein the volume fraction of H ₂ is 0.5 to 20%, and the dew point of the reducing atmosphere is-40 to 0 ℃.