CN109554529B - Hot-rolled strip steel iron scale reduction method based on reduction pretreatment process - Google Patents

Abstract

The invention relates to a hot-rolled strip steel iron scale reduction method based on a reduction pretreatment process, which comprises the following steps: rapidly heating the hot-rolled strip steel to 400-550 ℃ at a speed of 80-150 ℃/s, carrying out isothermal reduction for 30-600 s, obtaining a reduction product which is multi-gap sponge iron, carrying out intermediate cooling, heating to 700-1100 ℃ at the speed of 80-150 ℃/s, reducing the temperature for 30-600 s at an isothermal temperature, bonding iron particles in the multi-gap sponge iron, growing crystal grains, through material transmission, gaps, air holes and crystal boundaries in a reduction product are gradually reduced, the total volume is shrunk, a compact polycrystalline pure iron layer with a microstructure is formed, and reduction of the iron scale is completed.

Description

A method for reducing scale of hot-rolled strip steel based on reduction pretreatment process

Technical field:

The invention belongs to the technical field of metallurgy, and in particular relates to a method for reducing scale of hot-rolled strip steel based on a reduction pretreatment process.

Background technique:

At present, the method of removing iron oxide scale in major iron and steel enterprises at home and abroad is generally to use hydrochloric acid or sulfuric acid for pickling. The pickling technology can provide an ideal metal surface. However, long-term and intensive use of acid solution is likely to cause air pollution and groundwater pollution. cause great harm to the ecological environment. In view of environmental protection requirements, domestic and foreign enterprises and scientific research institutes have carried out extensive research on pickling _- free descaling technology. No-pickling reduction and descaling technology of iron oxide scale of rolled steel. The gas reduction descaling technology of hot-rolled steel originated from the field of pyrometallurgical iron ore, and the process of extracting metal from oxides. The main process is that at a specific reduction temperature, combustible gas ( _H2 or CO) is used as a reducing agent to reduce oxides to obtain crude metals or alloys. The gas reduction process of the iron oxide scale of hot-rolled steel is similar to that of gas-reduced iron ore. In the continuous annealing furnace, a reducing atmosphere is introduced to reduce the iron oxide scale on the surface of the hot-rolled steel strip to pure Fe instead of pickling. The process has the characteristics of cleanliness, high efficiency and high quality, and has received extensive attention in the steel industry as soon as it was proposed.

The reduction process of the hot-rolled strip iron oxide scale by _H2 or CO is thermodynamically step-by-step. First, lower valence oxides are formed, and finally elemental Fe is formed. Fe, as a transition metal element, will appear in three oxides during high temperature oxidation, namely FeO, Fe ₃ O ₄ and Fe ₂ O ₃ . Under the action of O, the order of oxide formation follows the principle of step-by-step transformation. At the same time, since 570 °C is the critical temperature of FeO disproportionation reaction, there will be different transformation sequences above or below the critical temperature, so the reduction of iron oxide scale also has a step-by-step nature. Under different temperature conditions, the reaction process of _H2 reduction of iron oxide scale is shown in the following equation, and the reduction process of CO2 is the same as that of _H2 .

When the reaction temperature is above 570°C:

3Fe ₂ O ₃ +H ₂ =2Fe ₃ O ₄ +H ₂ O(g)

Fe ₃ O ₄ +H ₂ =3FeO+H ₂ O(g)

FeO+H ₂ =Fe+H ₂ O(g)

When the reaction temperature is below 570°C:

3Fe ₂ O ₃ +H ₂ =2Fe ₃ O ₄ +H ₂ O(g)

However, in production practice and laboratory theoretical research, the "no-pickling" technology for removing iron oxide scale by gas reduction still faces the following problems:

(1) Selection of reducing agent, CO is a toxic gas, and safety costs need to be increased during use; the reaction product CO ₂ will cause environmental pollution, and a tail gas filtration and recovery device needs to be built; C in the reducing agent will enter into the reaction. Cementite is formed on the surface of the matrix, which affects the final mechanical properties of the material. Compared with CO, _H2 has stronger diffusion and transport ability and faster reaction efficiency. The reduction product is water vapor, which has no pollution to the environment and can reduce the investment of exhaust gas treatment equipment. The advantages of green energy H ₂ in terms of reduction efficiency make it the preferred reducing agent in the gas reduction and descaling process, but because H ₂ is an explosive gas , requires security cost input during use.

(2) Concentration of reducing agent. In order to improve the safety factor, the _H2 concentration in the mixed gas used in the annealing furnace at present is low (≤8%), which makes the reduction reaction efficiency slow, and the time required for all the reduction of the iron oxide scale is too long, which is not suitable for Practical production line application.

(3) The reduction temperature, the temperature system of the reduction annealing furnace is unreasonably set. At present, the annealing furnace adopts single-stage heating, and the temperature in the furnace cavity is generally lower than 750 °C, the diffusion rate of the reducing agent is slow, and the reduction reaction efficiency is low.

(4) "gas-solid" phase reaction and reduction product, the reducing agent reacts with the iron oxide scale on the surface of the steel, the reduction product on the surface is metallic iron, which will nucleate and grow at high temperature to form a dense pure iron layer. The premature formation of the pure iron layer will delay the diffusion rate of H into the iron oxide scale and reduce the reduction reaction rate.

Invention content:

The purpose of the present invention is to aim at in the prior art, in the hot-dip galvanizing technology without pickling reduction annealing, the reduction efficiency of the iron oxide scale on the surface of the hot-rolled strip steel is low in the continuous annealing furnace, and the galvanized sheet needs to be formed after the influence of the excessive amount of the residual iron oxide scale. To solve the problem of performance, a method for reducing iron oxide scale of hot-rolled strip steel based on reduction pretreatment process is provided. Using the reduction characteristics of polygrain boundary eutectoid structure in iron oxide scale in H ₂ The reduction pretreatment section of the hot-rolled steel strip can be used to prepare a pickling-free hot-dip galvanized sheet with high surface quality and good formability, which provides new ideas for industrialization and application, so that the iron oxide scale on the surface of the hot-rolled strip can be reduced at a faster rate. A dense pure iron layer is formed, which improves the adhesion and formability of the hot-dip galvanized sheet while reducing the investment cost of the continuous annealing furnace.

To achieve the above object, the present invention adopts the following technical solutions:

A method for reducing scale of hot-rolled strip steel based on a reduction pretreatment process, comprising the following steps:

Step 1, warm up:

The hot-rolled strip is heated to 200～300℃ to complete the preheating;

Step 2, restore preprocessing:

(1) Rapid heating: heat the preheated strip to 400-550°C at a heating rate of 80-150°C/s;

(2) Reduction pretreatment: the heated strip is subjected to isothermal reduction to generate pre-reduced strip; wherein, the reduction temperature is 400-550°C, the reduction time is 30-600s, and the reduction atmosphere is H ₂ and N ₂ mixed atmosphere, wherein the volume concentration of _H2 is 5~50%;

Step 3, Intermediate cooling:

Cool the pre-reduced strip to 300-500°C at a cooling rate of 10-20°C/s to obtain the cooled strip;

Step 4, high temperature reduction:

(1) Rapid heating: After cooling, the strip steel is heated to 700-1100°C at a heating rate of 80-150°C/s;

(2) High temperature reduction: after heating up, the strip steel is subjected to isothermal reduction to complete the reduction of iron oxide scale, and is cooled to room temperature; wherein, the reduction temperature is 700-1100 ° C, the reduction time is _30-600 s, and the reducing atmosphere is H and A mixed atmosphere of _N2 , in which the volume concentration of _H2 is 5-100%.

In the step 1, the composition of the hot-rolled strip steel contains C: 0.03-0.10%, Si: 0.04-0.60%, Mn: 0.15-2.5%, S: ≤ 0.015%, P: ≤ 0.019% by weight percentage, Cr: 0.2 to 1.0%, Als: 0.015 to 0.045%, Ti: ≤ 0.08%, Nb: ≤ 0.08%, and the balance is Fe and unavoidable impurities during smelting.

In the step 1, the thickness of the hot-rolled strip is 1.3-3.0 mm, the rolling temperature of the hot-rolled strip is 950-1050°C, the final rolling temperature is 800-900°C, the coiling temperature is 350-500°C, and the oxidation temperature is 350-500°C. The thickness of the iron scale is 4-8 μm, the cooling rate is 0.01-0.05°C/s, and the ratio of Fe ₃ O ₄ /α-Fe of the lamellar eutectoid structure in the iron oxide scale is 60-100%.

In the step 1, the preheating method adopts radiant tube heating.

In the step 1, the heating rate is 30-60°C/s.

In the step 2(1), by controlling the heating rate, the original eutectoid structure (Fe ₃ O ₄ /α-Fe) in the iron oxide scale is prevented from reverse phase transformation, so that the proportion of the lamellar eutectoid structure is kept at 60 to 100%.

In the step 2(2), the microstructure of the pre-reduced strip steel obtained after reduction pretreatment is multi-gap sponge iron.

In the step 4(1), the reduction product on the surface of the strip steel is still multi-gap sponge iron after cooling.

In the step 4 (2), after isothermal reduction, the iron particles in the multi-gap sponge iron of the reduction product on the surface of the strip steel will bond with each other, and the grains will grow up, through the gaps and pores in the material transport reduction product. and grain boundaries gradually decrease, the total volume shrinks, and finally a dense polycrystalline pure iron layer with microstructure is formed.

In the described step 4 (2), after the iron oxide scale is reduced, the strip steel is used to prepare the galvanized sheet, and the specific process is:

(1) Cool the strip steel after the reduction of the iron oxide scale to 450-480°C at a rate of 10-30°C/s, and then immerse it in a zinc solution for hot-dip galvanizing; wherein, the temperature of the zinc solution is 450-480°C ℃, the components and weight percentages in the zinc solution are Al0.4～6.0%, Sb≤0.07%, and the balance is pure Zn;

(2) The strip steel after galvanizing adopts the conventional combination of forced cooling and natural cooling to cool the galvanized strip steel to obtain a galvanized sheet.

In the present invention, in order to improve the reduction efficiency, intermediate cooling is performed between the reduction pretreatment and the high temperature reduction operation. In the first stage, the strip temperature drops by 50-100°C. Due to the different shrinkage rates of the substrate and the reduction product upon cooling, a large number of micro-cracks will be formed on the surface of the reduction product, which can be used for the inward diffusion of the reduction gas and the outward diffusion of the reduction product water vapor. Provides a diffusion channel.

In the steps 2 and 4, in the process of reduction pretreatment and high temperature reduction, the dew point value is controlled to be below -70°C, and the specific operation is: use a gas dew point tester to measure the water vapor content in the furnace cavity, High temperature graphite is used to control the water vapor content in the furnace (C+H ₂ O=CO+H ₂ ), so that the dew point value is controlled below -70°C, and the CO content in the furnace atmosphere is kept at 5-30%.

In the present invention, a method for efficiently reducing scale of hot-rolled strip steel based on a reduction pretreatment process, the thickness specification of the hot-rolled strip steel used is 1.3-3.0 mm. At present, the steel products with this thickness specification are ultimately in the form of coils. Production. In order to prevent the surface scale of the hot-rolled strip steel from being too thick, the process of "high-temperature fast rolling" is adopted in the production, and the thickness of the scale of scale is controlled at 4-8 μm. The temperature range of the strip steel product during coiling is 400 to 700 °C. Since the strip steel is cooled in coils, the cooling rate is slow. According to the Fe-O phase diagram, FeO in the iron oxide scale will occur when the temperature is below 570 °C. Phase transformation, into pro-eutectoid Fe ₃ O ₄ or eutectoid structure (Fe ₃ O ₄ /α-Fe). Therefore, the cooling process setting of the hot-rolled steel will affect the final scale structure. Due to the large difference in properties between different microstructures, the control of the scale structure will have a great impact on the reduction efficiency of the continuous annealing furnace. . Therefore, this patent controls the ratio of Fe ₃ O ₄ /α-Fe of the lamellar eutectoid structure in the iron oxide scale to 60-100%.

The method of the present invention is different from the above-mentioned method of reducing iron oxide scale by stage. In order to suppress the inverse eutectoid phase transition in the iron oxide scale and prevent the occurrence of Fe ₃ O ₄ +Fe→FeO, the present invention designs a reduction pretreatment section with rapid heating. The temperature of this stage is 350-550° C. For the rapid heating method, the most suitable heating rate is 80-150°C/s. The purpose of adopting this reduction temperature is to control the temperature of reduction pretreatment below the phase transition critical temperature (570°C) to ensure the stability of the eutectoid structure (Fe ₃ O ₄ /α-Fe). The eutectoid structure is multi-grain boundary structure, which can provide more diffusion channels for its diffusion, and also increase the contact area between H and oxide, so that the iron oxide scale can be quickly reduced to porous iron. At the same time, the purpose of adopting this heating rate is to reduce the diffusion rate of ferrite ions inside the iron oxide scale during the heating process, to prevent the solid-state phase transition in the iron oxide scale during the heating process, especially to ensure that the iron oxide scale of the strip steel is reduced in the pretreatment section. The content of the eutectoid structure in the eutectoid does not decrease.

According to the binary alloy phase diagram of Fe-Zn, when hot-dip galvanizing in zinc solution without Al, Fe-Zn phases such as η, ζ, σ and Γ will be formed inside the coating. It is hard and brittle, which deteriorates the performance of the coating and is detrimental to the forming performance of the coating. Therefore, in order to improve the adhesion and processability of the coating, Al with a mass fraction of 0.15 to 0.20% is added to the zinc solution, so that when the substrate enters the zinc pot, the Fe ₂ Al ₅ layer will be formed on the surface first, which is very important for Fe-Zn The reaction plays an inhibitory role and inhibits the formation of Fe-Zn brittle phase. The high-efficiency reduction method for the scale of hot-rolled strip steel based on the reduction pretreatment process of the present invention is a method for removing scale by using H ₂ reduction instead of pickling. After the scale is reduced in the continuous annealing furnace, the surface of the scale will form dense pure iron At this time, the surface roughness becomes larger, which will affect the distribution of Al in the coating and the formation of the Fe ₂ Al ₅ inhibitory layer, thereby destroying the uniformity and adhesion of the coating structure. Therefore, in the present invention, the mass percentage of Al in the zinc liquid is designed to be 0.4-6.0%.

Beneficial effects of the present invention:

(1) the present invention greatly improves the reduction reaction efficiency of the iron oxide scale by means of the characteristic of the iron oxide scale itself solid-state phase transition, so that the iron oxide scale can be reduced to pure iron in a relatively short period of time;

(2) The present invention adopts high-temperature graphite to absorb the water vapor of the reduction product, and the obtained reaction product CO will continue to play the role of a reducing agent in the furnace, that is, the dew point in the furnace is effectively controlled, and the amount of hydrogen used is also reduced.

Description of drawings:

Fig. 1 is the surface SEM topography diagram of the pre-reduced strip steel obtained after reduction pretreatment in Example 1 of the present invention;

Fig. 2 is the surface SEM topography of the strip steel obtained after intermediate cooling in the embodiment of the present invention 1;

3 is a process diagram of a method for reducing scale of hot-rolled strip steel based on a reduction pretreatment process according to Embodiment 1 of the present invention, wherein ①-preheating, ②-rapid heating, ③-reduction pretreatment, ④-intermediate cooling, ⑤ Secondary rapid heating, ⑥high temperature reduction, ⑦ _- cooling, T1 is ₂₀₀ ～300℃, T2 is ₄₀₀ ～550℃, T3 is ₃₀₀ ～500℃, T4 is 700～1100℃;

Fig. 4 is the SEM topography of the cross section of the iron oxide scale of the hot-rolled strip steel in Example 1 of the present invention;

Fig. 5 is the surface SEM topography of the reduction product obtained after reduction in Example 1 of the present invention;

Fig. 6 is the cross-sectional SEM topography of the reduction product obtained after reduction in Example 1 of the present invention;

7 is a photo of the curved surface of the formability test of the hot-dip galvanized sheet prepared by strip steel after iron oxide scale reduction is completed in Example 1 of the present invention;

Fig. 8 is the SEM topography of the cross-section of the iron oxide scale of the hot-rolled strip steel in Example 2 of the present invention;

Fig. 9 is the surface SEM topography of the reduction product obtained after reduction in Example 2 of the present invention;

Fig. 10 is the cross-sectional SEM topography of the reduction product obtained after reduction in Example 2 of the present invention;

11 is a photo of the curved surface of the formability test of the hot-dip galvanized sheet prepared by the strip steel after the reduction of scale is completed in Example 2 of the present invention;

Fig. 12 is the surface SEM topography of the reduction product obtained after reduction in Example 3 of the present invention;

Fig. 13 is the cross-sectional SEM topography of the reduction product obtained after reduction in Example 3 of the present invention;

14 is a photo of the curved surface of the formability test of the hot-dip galvanized sheet prepared by the strip steel after iron oxide scale reduction is completed in Example 3 of the present invention;

Fig. 15 is the bending surface photograph of the formability test of the hot-dip galvanized sheet prepared by the strip steel after the reduction of the iron oxide scale in Example 4 of the present invention; wherein: A-steel matrix, B-eutectoid structure, C-iron oxide scale, D- reduction product.

Detailed ways:

The present invention will be further described in detail below in conjunction with the examples.

In the embodiment of the present invention, the composition of the hot-rolled strip steel contains C: 0.03%-0.10%, Si: 0.04%-0.60%, Mn: 0.15%-2.5%, S: ≤ 0.015%, P: ≤ 0.019% by weight , Cr: 0.2% ~ 1.0%, Als: 0.015% ~ 0.045%, Ti: ≤ 0.08%, Nb: ≤ 0.08%, and the rest are Fe and inevitable impurities during smelting.

In the embodiment of the present invention, the thickness of the hot-rolled strip is 1.3-3.0 mm, the rolling temperature of the hot-rolled strip is 950-1050° C., the final rolling temperature is 800-900° C., the coiling temperature is 350-500° C. The thickness is 4～8μm, the cooling rate is 0.01～0.05℃/s, and the ratio of Fe ₃ O ₄ /α-Fe of lamellar eutectoid structure in the iron oxide scale is 60～100%.

In the embodiment of the present invention, the hot-rolled steel strip passes through five stages in sequence in the continuous annealing furnace, which are a preheating section, a rapid heating reduction pretreatment section, an intermediate cooling section, and a rapid heating high temperature reduction section and cooling section.

In the embodiment of the present invention, the rapid heating and heating rate of the hot-rolled steel strip in the reduction pretreatment section and the high temperature reduction section is 80-150° C./s.

In the embodiment of the present invention, in the reduction pretreatment section and the high-temperature reduction section of the continuous annealing furnace, a gas dew point tester is used to measure the water vapor content in the furnace cavity, and high-temperature graphite is used to control the water vapor content in the continuous annealing furnace (C+H ₂ O=CO+H ₂ ), the dew point value is controlled below -70°C, and the CO content in the furnace atmosphere is kept at 5-30%.

In the embodiment of the present invention, the weight percentage of Al in the zinc liquid is controlled at 0.4-6.0%, the weight percentage of Sb is less than or equal to 0.07%, and the rest is pure Zn.

Example 1

A method for reducing scale of hot-rolled strip steel based on a reduction pretreatment process, the process diagram of which is shown in Figure 3, comprising the following steps:

(1) Take the hot-rolled strip, heat it up to 200°C at 30°C/s, and preheat, wherein the hot-rolled strip contains C: 0.03%, Si: 0.04%, Mn: 0.15% by weight, S: 0.015%, P: 0.019%, Cr: 0.2%, Als: 0.015%, and the rest are Fe and inevitable impurities during smelting. The thickness of the hot-rolled strip is 1.3 mm, and the microscopic morphology of the cross-section of the iron oxide scale is shown in Figure 4. The thickness of the iron oxide scale is 6 μm, and the ratio of Fe ₃ O ₄ /α-Fe in the lamellar eutectoid structure in the iron oxide scale is 95%;

(2) Rapidly heated reduction pretreatment section, rapidly heated to 550 °C at 150 °C/s, the volume concentration of _H2 was 50%, isothermal reduction for 30 s, the surface SEM morphology of the pre-reduced strip obtained after the reduction pretreatment The appearance diagram is shown in Figure 1;

(3) After strip reduction pretreatment, the temperature was lowered by 100°C at a cooling rate of 20°C/s, and the SEM topography of the strip after cooling was obtained as shown in Figure 2;

(4) Rapidly heated high-temperature reduction section, rapidly heated to 800°C at 150°C/s, the volume concentration of H ₂ was 20%, and isothermal reduction was performed for 180s to complete the reduction of iron oxide scale. As shown in Figure 5 and Figure 6 respectively, the reduction product is a dense polycrystalline pure iron layer with a large number of micro-cracks on the surface; in the continuous annealing furnace, the strip steel after the reduction of the iron oxide scale is cooled at 30 °C/s to 450 ° C, then immersed in a zinc pot for hot-dip galvanizing, the weight percentage of Al in the zinc liquid composition is controlled at 0.4%, and the rest is pure Zn to obtain a hot-dip galvanized sheet, and the produced hot-dip galvanized sheet is tested by a universal chemical testing machine. The plate was subjected to a 180° cold bending test, and the curved surface is shown in Figure 7, which shows that the surface of the coating is bright and smooth without defects, and the hot-dip galvanized plate has good formability.

Example 2

A method for reducing scale of hot-rolled strip steel based on a reduction pretreatment process, comprising the following steps:

(1) Take the hot-rolled strip, heat it up to 300°C at 60°C/s, and preheat, wherein: the hot-rolled strip contains C: 0.10%, Si: 0.60%, Mn: 1.0% by weight, S: 0.015%, P: 0.019%, Cr: 0.5%, Als: 0.045%, Ti: 0.08%, and the rest are Fe and inevitable impurities during smelting. The thickness of the hot-rolled strip is 1.5mm, the microscopic morphology of the iron oxide scale is shown in Figure 8, the thickness of the iron oxide scale is 6 μm, and the ratio of Fe ₃ O ₄ /α-Fe in the lamellar eutectoid structure in the iron oxide scale is 60%;

(2) Rapidly heated reduction pretreatment section, rapidly heated to 500°C at 100°C/s, the volume concentration of H ₂ was 30%, and isothermal reduction was performed for 60s;

(3) After strip reduction pretreatment, reduce the temperature by 80°C at a cooling rate of 15°C/s;

(4) Rapidly heated high-temperature reduction section, rapidly heated to 900°C at 120°C/s, the volume concentration of H ₂ was 5%, and isothermal reduction was performed for 600s to complete the reduction of iron oxide scale. As shown in Figure 9 and Figure 10 respectively, the reduction product is a dense polycrystalline pure iron layer with a large number of microcracks on the surface; in the continuous annealing furnace, the strip steel after the reduction of iron oxide scale is cooled at 10°C/s to 480 ° C, then immersed in a zinc pot for hot-dip galvanizing, the weight percentage of Al in the zinc liquid composition is controlled at 1.5%, and the rest is pure Zn to obtain a hot-dip galvanized sheet, and the produced hot-dip galvanized sheet is tested by a universal chemical testing machine. The plate was subjected to a 180° cold bending test, and the curved surface is shown in Figure 11, which shows that the surface of the coating is bright and smooth without defects, and the hot-dip galvanized plate has good formability.

Example 3

A method for reducing scale of hot-rolled strip steel based on a reduction pretreatment process, comprising the following steps:

(1) Take the hot-rolled strip, heat it up to 300°C at 45°C/s, and preheat, wherein: the hot-rolled strip contains C: 0.05%, Si: 0.30%, Mn: 0.5% by weight, S: 0.009%, P: 0.010%, Cr: 1.0%, Als: 0.045%, Ti: 0.045%, Nb: 0.08%, and the rest are Fe and inevitable impurities during smelting. The thickness of the hot-rolled strip is 2.0 mm, the thickness of the iron oxide scale is 6.5 μm, and the ratio of Fe ₃ O ₄ /α-Fe of the lamellar eutectoid structure in the iron oxide scale is 70%;

(2) Rapidly heated reduction pretreatment section, rapidly heated to 450°C at 80°C/s, the volume concentration of H ₂ was 5%, and isothermal reduction was performed for 600s;

(3) After strip reduction pretreatment, reduce the temperature by 50°C at a cooling rate of 10°C/s;

(4) Rapidly heated high-temperature reduction section, rapidly heated to 1100°C at 100°C/s, the volume concentration of H ₂ was 100%, and isothermal reduction was performed for 30s to complete the reduction of iron oxide scale. The micro-morphologies are shown in Figure 12 and Figure 13, respectively. The reduction product is a dense polycrystalline pure iron layer with a large number of micro-cracks on the surface. Cooled to 460°C, then immersed in a zinc pot for hot-dip galvanizing. The weight percentage of Al in the zinc liquid composition was controlled at 3.0%, and the rest was pure Zn to prepare a hot-dip galvanized sheet. The 180° cold bending test was carried out on the galvanized sheet, and the curved surface is shown in Figure 14, which shows that the surface of the coating is bright and flat without defects, and the hot-dip galvanized sheet has good formability.

Example 4

A method for reducing scale of hot-rolled strip steel based on a reduction pretreatment process, comprising the following steps:

(1) Take the hot-rolled strip, heat it up to 300°C at 60°C/s, and preheat, wherein the hot-rolled strip contains C: 0.045%, Si: 0.18%, Mn: 0.2% by weight, S: 0.009%, P: 0.010%, Cr: 0.4%, Als: 0.045%, Ti: 0.025%, Nb: 0.045%, and the rest are Fe and inevitable impurities during smelting. The thickness of the hot-rolled strip is 3.0mm, the thickness of the iron oxide scale is 8μm, and the ratio of Fe ₃ O ₄ /α-Fe of the lamellar eutectoid structure in the iron oxide scale is 60%;

(2) Rapidly heated reduction pretreatment section, rapidly heated to 400°C at 120°C/s, the volume concentration of _H2 was 20%, and isothermal reduction was performed for 120s;

(3) After strip reduction pretreatment, reduce the temperature by 60°C at a cooling rate of 10°C/s;

(4) Rapidly heated high-temperature reduction section, rapidly heated to 700°C at 80°C/s, the volume concentration of H2 is 75%, isothermally reduced for 60s, and the reduction of iron oxide scale is completed. In the continuous annealing furnace, the reduction of iron oxide scale is completed. The strip steel is cooled to 470°C at 20°C/s, and then immersed in a zinc pot for hot-dip galvanizing. The weight percentage of Al in the zinc liquid composition is controlled at 6%, the weight percentage of Sb is controlled at 0.07%, and the rest is pure Zn. The hot-dip galvanized sheet was obtained, and the produced hot-dip galvanized sheet was subjected to a 180° cold bending test using a universal chemical testing machine. Formability.

Example 5

A method for reducing scale of hot-rolled strip steel based on a reduction pretreatment process, comprising the following steps:

(1) Take the hot-rolled strip, heat it up to 200°C at 60°C/s, and preheat, wherein: the hot-rolled strip contains C: 0.03%, Si: 0.04%, Mn: 0.15% by weight, S: 0.007%, P: 0.008%, Cr: 0.2%, Als: 0.045%, and the rest are Fe and inevitable impurities during smelting. The thickness of the hot-rolled strip is 1.3 mm, the thickness of the iron oxide scale is 4 μm, and the ratio of Fe ₃ O ₄ /α-Fe of the lamellar eutectoid structure in the iron oxide scale is 100%;

(2) Rapidly heated reduction pretreatment section, rapidly heated to 500°C at 150°C/s, the volume concentration of _H2 is 20%, and isothermal reduction is performed for 100s;

(3) After strip reduction pretreatment, reduce the temperature by 50°C at a cooling rate of 10°C/s;

(4) Rapidly heated high-temperature reduction section, rapidly heated to 900°C at 150°C/s, the volume concentration of H ₂ is 50%, isothermal reduction for 120s, to complete the reduction of iron oxide scale, in the continuous annealing furnace, after completion of the reduction of iron oxide scale The strip steel is cooled to 450°C at 30°C/s, and then immersed in a zinc pot for hot-dip galvanizing. The weight percentage of Al in the zinc liquid composition is controlled at 5%, the weight percentage of Sb is controlled at 0.02%, and the rest is pure Zn. The hot-dip galvanized sheet was prepared, and the 180° cold bending test was carried out on the produced hot-dip galvanized sheet by a universal chemical testing machine, which showed that the surface of the coating was bright, flat and defect-free, and the hot-dip galvanized sheet had good formability.

Claims (5)

1. a hot-rolled strip steel scale reduction method based on reduction pretreatment process, is characterized in that, comprises the following steps: Step 1, warm up: The hot-rolled strip is heated to 200-300° C. to complete the preheating. The hot-rolled strip contains C: 0.03-0.10%, Si: 0.04-0.60%, Mn: 0.15-2.5% by weight. , S: ≤0.015%, P: ≤0.019%, Cr: 0.2~1.0%, Als: 0.015~0.045%, Ti: ≤0.08%, Nb: ≤0.08%, the balance is Fe and inevitable impurities during smelting ; The thickness of the hot-rolled strip is 1.3~3.0mm, the rolling temperature of the hot-rolled strip is 950~1050℃, the final rolling temperature is 800~900℃, the coiling temperature is 350~500℃, and the thickness of the iron oxide scale is 4~ 8μm, the cooling rate is 0.01~0.05℃/s, and the ratio of Fe ₃ O ₄ /α-Fe in the lamellar eutectoid structure in the iron oxide scale is 60~100%; Step 2, restore preprocessing: (1) Rapid heating: heat the preheated strip to 400-550°C at a heating rate of 80-150°C/s; (2) Reduction pretreatment: the heated strip steel is subjected to isothermal reduction to generate pre-reduced strip steel; wherein, the reduction temperature is 400~550°C, the reduction time is 30~600s, and the reduction atmosphere is H ₂ and N ₂ mixed atmosphere, wherein the volume concentration of _H2 is 5~50%; Step 3, Intermediate cooling: Cool the pre-reduced strip to 300-500°C at a cooling rate of 10-20°C/s to obtain the cooled strip; Step 4, high temperature reduction: (1) Rapid heating: After cooling, the strip steel is heated to 700-1100°C at a heating rate of 80-150°C/s; (2) High temperature reduction: after heating up, the strip steel is subjected to isothermal reduction to complete the reduction of iron oxide scale and to be cooled; wherein, the reduction temperature is 700~1100°C, the reduction time is 30~600s, and the reduction atmosphere is H ₂ and N A mixed atmosphere of ₂ , in which the volume concentration of _H2 is 5~100%. 2 . The method for reducing scale of hot-rolled steel strip based on reduction pretreatment process according to claim 1 , wherein, in the step 1, the heating rate is 30-60° C./s. 3 . 3. the hot-rolled strip steel scale reduction method based on reduction pretreatment process according to claim 1, is characterized in that, in described step 2, the strip steel structure after the pre-reduction obtained after reduction pretreatment is many Gap sponge iron. 4. The method for reducing scale of hot-rolled strip steel based on reduction pretreatment process according to claim 1, wherein in the step 2, in the reduction pretreatment and high temperature reduction process, the control dew point value is controlled at Below -70℃, the specific operation is as follows: use a gas dew point tester to measure the water vapor content in the furnace cavity, use high-temperature graphite to control the water vapor content in the furnace, so that the dew point value is controlled below -70℃, and the furnace atmosphere The CO content was kept at 5~30%. 5. The method for reducing scale of hot-rolled strip steel based on reduction pretreatment process according to claim 1, is characterized in that, in described step 4, after the reduction of scale, the strip is used to prepare galvanized sheet, The specific process is: (1) Cool the strip steel after the reduction of the iron oxide scale to 450-480° C. at a rate of 10-30° C./s, and then immerse it in a zinc solution for hot-dip galvanizing; wherein, the temperature of the zinc solution is 450-480° C. ℃, the components and weight percentages in the zinc solution are Al0.4~6.0%, Sb≤0.07%, and the balance is pure Zn; (2) The strip steel after galvanizing adopts the conventional combination of forced cooling and natural cooling to cool the galvanized strip steel to obtain a galvanized sheet.

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