CN113083915B - A method for reducing water-marked defects on the surface of steel plate - Google Patents

Abstract

The invention discloses a method for reducing water-marked defects on the surface of a steel plate, which is characterized in that: the steel plate includes iron and the following components by mass percentage: C≤0.20%, Si: 0.25-0.60%, Mn≤2.0% , Ni ≤ 0.80%, Cr ≤ 0.80%, Mo ≤ 0.6%, Cu: ≤ 0.80%, and CEV ≤ 0.65%; the method includes heating, rolling, cooling processes, by using conventional heating parameters, appropriately increase the temperature At the same time, the descaling pressure of the finishing mill is reduced from 20-22Mpa to 11-13Mpa, the surface oxide layer is removed, the dense oxide layer is retained, and the purpose of reducing the "water mark"-like defects on the surface of the steel plate is achieved. At the same time, the scheme does not need to modify the equipment, which is simple and easy to implement; it also enhances the corrosion resistance of the surface of the steel plate.

Description

A method for reducing water-marked defects on the surface of steel plates

technical field

The invention belongs to the technical field of steel rolling, and in particular relates to a method for reducing water-marked defects on the surface of a steel plate.

Background technique

With the development of steel rolling technology, the corresponding auxiliary rolling technology has also been developed. For example, the removal process of iron oxide scale generated when the surface of the steel plate is heated is the so-called descaling process. The scale pressure water is replaced, and the descaling pressure has developed from the initial 10Mpa to the current 25Mpa, and the development is very rapid.

At present, the descaling pressure of mainstream steel mills is basically between 20-23Mpa, but the surface of the produced steel plate cannot avoid the "water pattern" defect, that is, after descaling, the iron oxide scale on the surface of the steel plate cannot be removed, resulting in descaling. After scaling, strip marks appear on the surface of the steel plate in the same rolling direction. The depth of the defect is 0.02-0.15mm. Usually, it is not obvious to the naked eye. After the side irradiation of strong light, water-like defects can be found, which are usually strip-shaped. Especially in ordinary steel and low-alloy high-strength steel. At present, in order to solve this problem, major steel mills mainly adopt the following three methods: 1. Increase the rolling temperature of finishing rolling to reduce the production of iron oxide scale. This method needs to increase the rolling temperature to above 860 °C, but this will reduce the temperature of the steel plate. 2. Increase the descaling pressure, this method needs to increase the dephosphorization pressure to more than 25Mpa, and the equipment needs to be upgraded, which increases the cost; 3. , Change the chemical composition of the steel plate, this method uses low-silicon smelting, but doing so will increase the cost.

To sum up, there is no simple and economical solution for this defect in the prior art.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for reducing water streak defects on the surface of a steel plate, so as to solve the problems in the prior art.

The present invention is achieved through the following technical solutions: a method for reducing water-marked defects on the surface of a steel plate, characterized in that: the steel plate includes iron and the following components by mass percentage: C≤0.20%, Si: 0.25-0.60 %, Mn≤2.0%, Ni≤0.80%, Cr≤0.80%, Mo≤0.6%, Cu:≤0.80%, and CEV≤0.65%;

The method includes heating, rolling and cooling processes:

During the heating process, a walking beam heating furnace is used to heat the slab to a soaking temperature of 1180-1220°C. The temperature is 1150-1220℃, the temperature of the soaking section of the fourth heating section is 1180-1240℃, the slab time in the furnace is ≥0.95min/mm, and the air-fuel ratio is 1.1~1.3;

The rolling process adopts the section rolling method in the recrystallization zone and the non-recrystallization zone. The starting temperature of the rolling stage in the recrystallization zone is greater than or equal to 1060°C, the dephosphorization water pressure is 20-22Mpa, and the dephosphorization passes are the first pass and the last pass. The end temperature range is 1000-1050℃; the starting temperature of the rolling stage in the non-recrystallization zone is ≥840℃, the phosphorus removal pass is the first pass and the second pass, and the phosphorus removal water pressure is 11-13Mpa;

The cooling process adopts the ACC high-density fast cooling header method for cooling, the inlet water pressure is 0.2-0.3MPa, the water flow density range is 1000-2000L/(m2.min), and the final cooling temperature is 500-700℃.

The advantages of the present invention are: the scheme of the present invention has the following advantages: 1. The scheme does not need to significantly change the heating parameters; 2. The scheme does not need to increase the descaling pressure, solves the above problems with ordinary descaling pressure, and reduces the investment 3. This scheme basically does not change the chemical composition of the steel plate, and basically has no influence on the formula design; 4. This scheme does not need to modify the equipment and is simple and easy to implement; 5. This scheme also enhances the corrosion resistance of the steel plate surface.

Detailed ways

The invention discloses a method for reducing water-marked defects on the surface of a steel plate, wherein the steel plate includes the remainder of iron and inevitable impurities, and also includes the following components by mass percentage: C≤0.20%, Si: 0.25 -0.60%, Mn≤2.0%, Ni≤0.80%, Cr≤0.80%, Mo≤0.6%, Cu:≤0.80%, and CEV≤0.65%.

Preferably, C≤0.20%, Si: 0.40-0.60%, Mn≤2.0%, Ni≤0.70%, Cr≤0.60%, Mo≤0.5%, Cu:≤0.60%.

A method for reducing water streak defects on the surface of a steel plate, which includes the processes of heating, rolling and cooling:

During the heating process, a walking beam heating furnace is used to heat the slab to a soaking temperature of 1180-1220°C. The temperature is 1150-1220℃, the temperature of the soaking section of the fourth heating section is 1180-1240℃, the slab time in the furnace is ≥0.95min/mm, and the air-fuel ratio is 1.1~1.3;

The rolling process adopts the rolling method in the recrystallization zone and the non-recrystallization zone. The starting temperature of the rolling (that is, rough rolling) stage in the recrystallization zone is ≥ 1060 °C, the phosphorus removal water pressure is 20-22Mpa, and the phosphorus removal pass is the first. Pass and last pass, end temperature range is 1000-1050℃; starting temperature of non-recrystallization zone rolling (finishing rolling) stage ≥840℃, phosphorus removal pass is the first pass and second pass, phosphorus removal The water pressure is 11-13Mpa;

The cooling process adopts the ACC high-density fast cooling header method for cooling, the inlet water pressure is 0.2-0.3MPa, the water flow density range is 1000-2000L/(m ² .min), and the final cooling temperature is 500-700°C.

Example 1

A method for reducing water streak defects on the surface of a steel plate, the steel plate includes iron, and further includes the following components by mass percentage: C: 0.18%, Si: 0.50%, Mn: 1.8%, Ni: 0.70%, Cr: 0.60%, Mo: 0.5%, Cu: ≤0.60%. The method as described above was adopted, wherein the water pressure for dephosphorization in rough rolling was 21 Mpa, and that in finishing rolling was 12 Mpa. In the obtained steel sheet, less than 10% of watermark defects appeared, and the defect range was small and not obvious.

The principle of the present invention is as follows:

After research, it is found that after the billet is heated, an oxide layer will be formed on the surface of the billet. There are two main types: the first is the furnace pig iron oxide scale produced after the billet is heated from the heating furnace. The billet is heated in the heating furnace for a long time, and the temperature of the billet reaches 1180 ℃ ~ 1280 ℃. The iron oxide scale formed on the surface of the billet is divided into three layers of Fe2O3, Fe3O4 and FeO from the surface to the inside, and the oxide layer is thicker. The second is the regenerated iron oxide scale produced by the secondary oxidation of the steel surface in the steel rolling production. Mainly, after the rolled material is rolled by the rough rolling mill, it needs to be warmed (in order to avoid rolling in the partial recrystallization area). The structure is mainly divided into two layers of Fe2O3 and Fe3O4, and the oxide layer is thinner. But very dense.

In the existing process, there are two main reasons for the occurrence of "water marks" defects during descaling: the first structure of iron oxide scale, in terms of strength, the FeO layer has the lowest strength. But the viscosity is very high. The temperature of the billet coming out of the heating furnace is usually 1180 ℃ ~ 1280 ℃. When the billet encounters high-pressure water, the surface iron oxide scale bursts due to the instantaneous quenching and shrinking, and the flaky fluid formed by the high-pressure water flows from the FeO crack with the lowest strength. At the beginning, along with the Fe2O3 and Fe3O4 on it, it was washed away. In actual production, this kind of furnace pig iron oxide scale has been basically washed away when it passes through the phosphorus removal box and the rough rolling mill for descaling after being transported out of the heating furnace. If there is still residue, it will be pressed into the surface of the steel plate by the roll. After tracking, it is found that due to its thick thickness, after being pressed into the steel plate, the steel plate tends to be pressed in point-like, with a deep depth and local dense point-like defects. Often this defect is referred to as a "pocket". The iron oxide scale of the second structure is mainly divided into two layers of Fe2O3 and Fe3O4. It is a regenerated iron oxide scale, which is very dense. Because the regenerated iron oxide scale is very dense, the descaling effect is poor. In the actual production process, the area under the descaling nozzle (the area with the highest descaling pressure) has a higher probability of cleaning, while the edge area of the nozzle striking surface is often not cleaned. When descaling is complete, stripes of "water streak" defects develop on the dirty areas. The depth of the defect area is 0.02-0.15mm. Usually, it is not obvious to the naked eye, but it can be found in a strip-like distribution after being irradiated by strong light on the side. Relevant tests have been done abroad, and the regenerated iron oxide scale cannot be completely removed when the descaling pressure is 40-50Mpa. This shows the density of the regenerated iron oxide scale.

Through in-depth research on the formation of water marks, combined with its own equipment capabilities and process technology, this scheme mainly adopts conventional heating parameters, and appropriately increases the Si content to ensure the viscosity of the iron oxide scale. And appropriately increase the waiting time (waiting time ≥ 4 minutes) to ensure the thickness and uniformity of the iron oxide scale. The descaling strike pressure of the finishing mill is reduced from 20-22Mpa to 11-13Mpa (the purpose can be achieved by adjusting the size of the nozzle, without changing the descaling water tank), and the dense iron oxide scale formed when the steel plate is warm can be retained and uniform. Covered on the steel plate, thereby reducing the "water mark"-like defects on the surface of the steel plate. The descaling water pressure remains unchanged for the roughing mill and descaling box. This method changes the existing understanding of reducing "water marks" by increasing the descaling water pressure. By reducing the water pressure, the inner oxide layer is attached to the plate and a dense protective layer is formed, thereby improving the Surface quality and corrosion resistance. It has been verified that after adopting this scheme, the "water mark"-like defects on the surface of the steel plate are greatly improved, and the occurrence probability of water marks is reduced from 60% to less than 5%, which greatly improves the surface quality, reduces the rate of unqualified products, and prevents corrosion. Performance is also greatly improved at the same time.

Claims (1)

1. A method for reducing water-marked defects on the surface of a steel plate, characterized in that: the steel plate comprises iron and each component in the following mass percentages: C≤0.20%, Si: 0.25-0.60%, Mn≤2.0%, Ni ≤0.80%, Cr≤0.80%, Mo≤0.6%, Cu:≤0.80%, and CEV≤0.65%; The method includes heating, rolling and cooling processes: During the heating process, a walking beam heating furnace is used to heat the slab to a soaking temperature of 1180-1220°C. The temperature is 1150-1220℃, the temperature of the soaking section of the fourth heating section is 1180-1240℃, the slab time in the furnace is ≥0.95min/mm, and the air-fuel ratio is 1.1~1.3; The rolling process adopts the rolling method in the recrystallization zone and the non-recrystallization zone. The starting temperature of the rolling stage in the recrystallization zone is greater than or equal to 1060 ° C, the dephosphorization water pressure is 20-22MPa, and the dephosphorization passes are the first pass and the last pass. One pass, the ending temperature range is 1000-1050℃; the starting temperature of the rolling stage in the non-recrystallization zone is ≥840℃, the phosphorus removal pass is the first pass and the second pass, and the phosphorus removal water pressure is 11-13MPa; The cooling process adopts ACC high-density fast cooling header mode for cooling, the inlet water pressure is 0.2-0.3MPa, the water flow density range is 1000-2000L/(m ² ·min), and the final cooling temperature is 500-700°C.