JPH11286718A - Production of steel plate excellent in surface characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel plate excellent in surface characteristic and its production. SOLUTION: At the time of supplying gas to a combustion burner 2 in a reheating furnace at hot rolling, heating is performed while supplying not only heating air but also water vapor together with the combusting air so that >=25% water vapor atmosphere concentration in a heating furnace 1 is reached, by which the thickness of the scale formed in the heating furnace 1 can be made uniform and the occurrence of surface defect can be prevented. By reducing the occurrence of surface defect caused by the occurrence of local deep wedge- shaped intergranular oxidation resultant from the nonuniform formation of the scale formed in a reheating furnace at the time of hot rolling, the steel plate excellent in surface characteristic can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a hot-rolled steel sheet, a cold-rolled steel sheet, an electroplated steel sheet, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet, in which the scale generated in a reheating furnace during hot rolling is not affected. The present invention relates to a method for producing a steel sheet having excellent surface properties by reducing the occurrence of surface defects based on local deep wedge-shaped grain boundary oxidation by uniformly forming.

[0002]

2. Description of the Related Art On the surface of a hot-rolled steel sheet obtained by hot rolling, surface defects such as linear or streak-like defects are often observed. As a problem. The cause of this pattern is considered to be wedge-shaped grain boundary oxidation into the steel material generated in the reheating furnace in the hot rolling step.

As a means for solving the above-mentioned problem, a method for defining a steel sheet component has been disclosed. For example, Japanese Patent Application Laid-Open No. 6-279923 discloses that, based on the knowledge that Ni or Cu causes surface defects in a hot rolling process, the content of these elements in a steel material is suppressed as much as possible, and an appropriate amount of Si is reduced. There is disclosed a method in which the descaling property of the produced scale is increased by containing it, and the scale is removed by injecting high-pressure water before hot rolling. However, this method is based on the premise that Ni and Cu, which cause surface defects, are reduced as much as possible at the steelmaking stage.
It cannot be used effectively for steel materials mixed with Cu or Cu.

Japanese Patent Application Laid-Open No. Hei 6-346145 discloses that Ni and Cu are contained in an amount of 0.01 wt% or more and (C
u / Ni) ratio is 2 or less, and the Si content is 0.2 to 3.0.
A method for reducing surface defects by using a steel material of wt% and lowering the heating temperature and the first descaling temperature to a low level is disclosed. However,
In this method, the heating temperature is specified, but the heating atmosphere is not specified.

Japanese Patent Application Laid-Open No. 9-52110 discloses that
A method is disclosed in which a steel sheet containing 0.01% by weight or more of Ni and Cu suppresses the average of the ratio of the interface length along the interface irregularities to the linear length during heating to 1.5 or less. However, the actually generated surface defect is not an average value of the ratio of the interface length to the linear length, but a scale that has deeply penetrated into some steel material causes the surface-induced defect. Further, in this method, the heating temperature is specified, but the heating atmosphere is not specified.

Further, as a means for reducing surface defects, a method of adjusting a heating atmosphere has been disclosed. For example, Japanese Patent Application Laid-Open No. Sho 53-1113 discloses a technique in which a heating atmosphere gas is caused to flow on the back surface of a slab to eliminate unevenness in the scale generation amount and reduce surface defects. However, in this method, no specific atmosphere is described.

Further, Japanese Patent Application Laid-Open No. 53-140212 discloses a technique for adjusting the oxygen concentration in the atmosphere. However, in the adjustment of the air ratio using a heating burner, only the oxygen concentration can be changed. The water vapor concentration in the heating furnace, which is the object of the present invention, cannot be changed.
Further, it is not possible to reduce the wedge-shaped grain boundary oxidation and to make the amount of scale uniform by simply adjusting the oxygen concentration.

[0008] Japanese Patent Application Laid-Open Nos.
JP-A-331533 and JP-A-7-54036 disclose techniques for adjusting the dew point in a heating furnace.
However, in this method, in order to generate casting defects in scale and to prevent surface defects, the amount of generated scale is controlled. There is no disclosure of grain boundary oxidation. Further, this method does not disclose the oxygen concentration in the heating furnace.

[0009]

An object of the present invention is to produce a hot-rolled steel sheet, a cold-rolled steel sheet, an electroplated steel sheet, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet in a reheating furnace during hot rolling. It is an object of the present invention to establish a technique capable of reducing the occurrence of surface defects caused by local deep wedge-shaped grain boundary oxidation caused by uneven generation of scale.

[0010]

DISCLOSURE OF THE INVENTION The present invention is directed to a method for reducing surface defects caused by local deep wedge-shaped grain boundary oxidation caused by non-uniform formation of scale generated in a reheating furnace during hot rolling. This is a method for producing a hot-rolled steel sheet having excellent surface properties by preventing occurrence thereof, and the gist of the present invention is as follows.

(1) In a reheating furnace at the time of hot rolling, when supplying gas to a combustion burner, the oxygen atmosphere concentration in the heating furnace is 0.1% or more and 10% or less and the steam atmosphere concentration is This is a method for producing a steel sheet having excellent surface properties, wherein heating is performed while supplying steam together with combustion air and combustion gas so as to be 25% or more.

(2) In the reheating furnace at the time of hot rolling, a regenerator is attached to each of a pair of combustion burners, and a gas from the reheating furnace is introduced to recover heat and recover heat. Supply is performed alternately, and while heat is being recovered by one of the heat accumulators, the oxygen atmosphere concentration in the heating furnace is set to 0.1% or more and 10% or less and steam is passed through the other heat accumulator. By heating while supplying steam, combustion air and combustion gas so that the atmosphere concentration becomes 25% or more, the scale thickness generated in the heating furnace is uniformly generated to prevent the generation of surface flaws. This is a method for producing a steel sheet having excellent surface properties.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventors of the present invention have investigated the causes of surface defects of products. As a result, the cause of the surface defects is that due to unevenness of the scale generated during heating, deep wedge-shaped grain boundary oxidation is locally formed inside the steel material, and this grain boundary oxidation is eroded into the steel material during rolling. Was confirmed by experiment.

The present inventors have investigated the relationship between the atmosphere during reheating during hot rolling and the scale generated. As a result, the form of the scale depends on the water vapor concentration in the heating furnace,
It was found that as the water vapor concentration increased, the amount of scale formation became uniform, and local wedge-shaped grain boundary oxidation also decreased. The cause is considered as follows.

FIG. 1 is a schematic diagram for explaining the form of the production scale when the water vapor concentration in the heating furnace is low.
When a steel material is oxidized at a high temperature, the grain boundary is easily oxidized, so that grain boundary oxidation is formed. In addition, pores are generated in the scale, and the amount of scale generated can vary in a portion of the steel material. At that time, the part where the amount of scale generation is small, that is,
In the concave portion, grain boundary oxidation to the inside of the steel material increases. As a result, the incidence of surface flaws increases.

FIG. 2 is a schematic diagram for explaining the form of the production scale when the water vapor concentration in the heating furnace is high.
When the water vapor concentration in the heating furnace increases, the amount of pores generated in the scale increases due to the effect of the water vapor. When the amount of generated pores increases, the atmospheric gas can pass between the pores in the scale and directly undergo an oxidation reaction with iron on the steel material surface. As a result, a uniform oxidation reaction easily occurs on the surface of the steel material, so that the variation in the scale of production is reduced, and local increase in grain boundary oxidation can be prevented. Therefore, the incidence of surface flaws decreases.

The lower limit of the water vapor concentration in the heating atmosphere is determined from the viewpoint of generation of surface flaws. When the water vapor concentration is 25% or less, the effect of suppressing defects due to a decrease in grain boundary oxidation due to a uniform oxidation reaction is weak, and the rate of occurrence of defects is increased. The upper limit of the water vapor in the heating atmosphere is not particularly limited. However, if the water vapor atmosphere is increased, the temperature in the heating furnace is lowered and the energy efficiency is deteriorated.

Regarding the oxygen concentration in the heating furnace, when the oxygen concentration is low, the oxidation reaction is suppressed, and as a result, the grain boundary oxidation reaction hardly occurs, so that the generation rate of flaws is low and the flaws due to water vapor are low. It is not necessary to perform the effect of suppressing the above. Therefore, the lower limit of the oxygen concentration is set to 0.1%. The upper limit of the oxygen concentration is high. However, if the oxygen concentration is increased, the grain boundary oxidation reaction becomes intense and the amount of air blown during combustion increases, which is uneconomical. Therefore, the upper limit of the oxygen concentration is set to 10%.

The heating temperature is not particularly limited, either.
If the heating temperature is lowered, it becomes difficult to maintain the temperature during hot rolling. In addition, if the heating temperature is increased, the grain boundary oxidation increases, and the flaw generation rate increases.
It is desirably 00 ° C or less.

Further, the faster the gas flow in the heating furnace, the easier the atmospheric gas passes between the pores in the scale, and the greater the effect of reducing the incidence of surface flaws. Therefore, in each zone, a pair of burners are provided as a set and a regenerator is attached to each zone, and heat is stored in each regenerator alternately by introducing high-temperature gas in the heating furnace and recovering heat. Through the other regenerator stored during the
If heating is performed by the burner while supplying not only combustion air but also water vapor and combustion gas together, the flow of gas combustion will alternate between one-on-one set of burners, and the gas flow of the atmosphere gas will be faster. In addition, since the steam atmosphere in the heating furnace can be increased, the variation in the scale produced can be reduced, and local increase in grain boundary oxidation can be prevented. Therefore, the incidence of surface flaws decreases. In addition, according to the method using the switchable burner, the temperature drop of the gas atmosphere caused by the addition of steam is minimized by passing the gas through the regenerator, so that efficient steam addition becomes possible.

[0021]

Embodiments of the present invention will be described below. Embodiment 1 FIG. 3 is a schematic view of a heating furnace to which Embodiment 1 is applied.
Shown in The heating furnace 1 is a device for heating a steel material 3, and is provided with a combustion burner 2 that is a set of a pair facing the width direction of the furnace. Combustion gas and air are supplied to the combustion burner 2 from a combustion gas blower 4 and a combustion air blower 6. At this time, steam is supplied from the steam generator 5 to the supply pipe from the combustion air, and the steam concentration in the heating furnace is adjusted. Here, the steel material 3 is placed on the skid 7 in the heating furnace 1.

In this example, a heating experiment was performed using steel materials having the components shown in Table 1 under the test conditions shown in Table 2, and thereafter, a hot-rolled steel sheet was manufactured by rolling. The slab thickness was 250 mm, hot rolling was performed to a product thickness of 3 mm by rough rolling and finish rolling, and the occurrence of surface defects was visually inspected. As the surface defect defect rate, a value obtained by dividing the length of the defect in the length direction on the surface of the hot-rolled steel sheet by the length of the coil of the hot-rolled steel sheet was used. In this experiment, the COG gas and the LNG gas were used as combustion gases, and the oxygen concentration and the water vapor concentration in the heating furnace were adjusted.

As is clear from Table 2, under the condition 10, since the oxygen concentration is low, the surface defect rate is low without adjusting the water vapor concentration. In the conditions 10 to 12, since the oxygen concentration or the water vapor concentration is out of the appropriate range,
In Examples 1 to 8, which are in an appropriate range, whereas the surface defect rate is 0.32% or more, the surface defect rate was 0.17% or less, and a steel sheet having good surface properties could be manufactured.

[0024]

[Table 1]

[0025]

[Table 2]

Embodiment 2 FIG. 4 is a schematic diagram of a heating furnace to which Embodiment 2 is applied. The heating furnace 8 is a device for heating the steel material 18, and is provided with a combustion burner 9 in which a pair facing the width direction of the furnace is a set. The combustion burner 9 is provided with a regenerator 10. Combustion gas and air are supplied to the combustion burner 9 from a combustion gas blower 12 and a combustion air blower 14. At this time, the combustion burners 9 in the furnace alternately burn the combustion burners 9 in pairs in the width direction of the furnace. That is, while one burner is burning, the switching valve 16 and the solenoid valve 17 are adjusted, and the other burner introduces high-temperature gas in the furnace from the combustion air pipe to the regenerator 10 by the in-furnace gas suction device 13. And recover heat. Next, the ventilation and suction to the combustion burner are switched, and the combustion air is heated by passing the combustion air to the other regenerator 10 which has been previously sucked and heat recovered, and is supplied to the burner 9 together with the combustion gas. At that time, the heat storage 10
Is supplied with steam from the steam generator 5 to adjust the steam concentration in the heating furnace. Here, a steel material 18 is placed on the skid 19 in the heating furnace 8.

In this example, a heating experiment was conducted under the test conditions shown in Table 3 using steel materials having the components shown in Table 1 in the same manner as in Example 1, and thereafter, a hot-rolled steel sheet was manufactured by rolling. The slab thickness was 250 mm, hot rolling was performed to a product thickness of 3 mm by rough rolling and finish rolling, and the occurrence of surface defects was visually inspected. As the surface defect defect rate, a value obtained by dividing the length of the defect in the length direction on the surface of the hot-rolled steel sheet by the length of the coil of the hot-rolled steel sheet was used. In this experiment, the COG gas and the LNG gas were used as combustion gases, and the steam concentration in the heating furnace was adjusted. The switching time of the combustion burner was set to 60 seconds.

As is clear from Table 3, under the condition 21, since the oxygen concentration is low, the surface defect rate is low without adjusting the water vapor concentration. In the conditions 21 to 24, since the water vapor concentration is out of the proper range, the surface defect rate is 0.3%.
0% or more, but within an appropriate range according to the first embodiment.
In Nos. 3 to 20, the surface defect rate was 0.12% or less, and a steel sheet having good surface properties could be manufactured. Further, in the present embodiment, the use of the switchable burner results in a faster gas flow in the furnace, resulting in a lower surface defect rate than in the first embodiment.

[0029]

[Table 3]

[0030]

As described above, according to the present invention,
Steel sheet with excellent surface properties by reducing the occurrence of surface defects due to local deep wedge-shaped grain boundary oxidation caused by non-uniform scale generated in the reheating furnace during hot rolling Can be manufactured.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a cross section of a production scale when a water vapor concentration in a heating atmosphere is low.

FIG. 2 is a diagram schematically showing a cross section of a production scale when a water vapor concentration in a heating atmosphere is high.

FIG. 3 is a view schematically showing a longitudinal section of the heating furnace according to the first embodiment of the present invention.

FIG. 4 is a view schematically showing a longitudinal section of a heating furnace according to a second embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Heating furnace 2 ... Combustion burner 3 ... Steel material, 4 ... Combustion gas blower 5 ... Steam generator 6 ... Combustion air blower 7 ... Skid 8 ... Heating furnace 9 ... Combustion burner 10 ... Regenerator 11 ... Steam generator 12 ... Combustion gas blower 13 ... Furnace gas suction machine 14 ... Combustion air blower 15 ... Switching valve 16 ... Switching valve 17 ... Solenoid valve 18 ... Steel material 19 ... Skid.

Claims (2)

[Claims] When a gas is supplied to a combustion burner in a reheating furnace at the time of hot rolling, the oxygen atmosphere concentration in the heating furnace is 0.1% or more and 10% or less and the water vapor atmosphere concentration is 25% or less. % While heating while supplying steam together with the combustion air and the combustion gas so as to be not less than 10% by weight. 2. A reheating furnace at the time of hot rolling, wherein a regenerator is attached to each of a pair of combustion burners, and a heat storage for recovering heat by introducing a gas from the reheating furnace and a combustion gas. Supply is performed alternately, and while heat is being recovered by one of the heat accumulators, the oxygen atmosphere concentration in the heating furnace is set to 0.1% or more and 10% or less and steam is passed through the other heat accumulator. A method for producing a steel sheet having excellent surface properties, comprising heating while supplying water vapor, combustion air and combustion gas so that the atmosphere concentration becomes 25% or more.