JP2001164323A - Cleaning method for thick steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cooling method for a thick steel plate by which stable cooling informing the material in the plate thickness direction and also preventing the increase of the deviation of the temperature in the plate after the completion of the cooling compared to the deviation of the temperature in the plate at the start of the cooling is executed, and its good shape is secured. SOLUTION: At the time of water-cooling a thick steel plate of high temperature subjected to hot rolling and having a temperature deviation in the plate, the thick steel plate is cooled at a cooling rate of 5 to 15 deg.C/sec till the surface temperature in the low temperature part reaches 630 to 530 deg.C and is successively cooled at a cooling rate of >=25 deg.C/sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an on-line cooling method for hot-rolled hot steel plates.

[0002]

2. Description of the Related Art A process for continuously cooling a hot-rolled high-temperature steel sheet online to produce a high-strength, high-toughness thick steel sheet is widely used. In this manufacturing process,
Since the structure of the steel material is controlled by continuous cooling, it is possible to reduce the number of alloying elements and omit the heat treatment step, thereby greatly reducing the production cost. In addition, the reduction of alloy elements improves weldability, enables reduction of preheating, application of large heat input welding, and the like, and greatly improves welding work efficiency.

[0003] However, when cooling a hot steel plate after hot rolling online, especially when the thickness is large and high strength characteristics are required, a method of injecting a constant amount of cooling water from the start to the end of cooling is used. The cooling rate of the surface layer in the thickness direction is large, the cooling rate of the central part is small, and there is a speed difference between the two. Therefore, there is a problem that a strength distribution, that is, a material difference occurs in the thickness direction.

In order to solve such a problem, as disclosed in JP-A-63-20410, 0.05 to 0.50
Cooling is started at a low water density of m ³ / m ² min, and then cooling is performed while gradually increasing the amount of injected water until the water density reaches 0.30 to 1.50 m ³ / m ² min. A method for producing a thick steel plate having a small difference in strength has been proposed.

However, the water-cooled steel sheet has a heat transfer coefficient from a surface temperature of around 600 ° C. to 200 ° C. as shown in FIG.
Because it has a cooling characteristic that increases with a sudden variation over the vicinity, as the cooling progresses and the cooling in the above temperature range, the deviation of the heat flux in the plate increases, and the temperature deviation in the plate tends to increase. Has characteristics. For this reason, in the method of gradually increasing the water density with the progress of cooling as described in the above-mentioned publication, the cooling time in the temperature range of around 600 ° C. to 200 ° C. becomes longer, so that the cooling rate is increased over the entire surface of the steel sheet. And it is very difficult to stably cool while increasing the heat removal. In addition, since the surface temperature generally varies within the plate at the start of cooling, after the start of cooling,
The deviation of the cooling rate and cooling temperature history in the plate further increases,
As a result, a shape change due to thermal stress occurs and correction work is often required.

[0006]

SUMMARY OF THE INVENTION The present invention provides stable cooling such that the material in the sheet thickness direction is made uniform and the temperature difference in the sheet after the completion of cooling does not increase more than the temperature difference in the sheet at the start of cooling. An object of the present invention is to provide a method for cooling a thick steel plate for ensuring a good shape.

[0007]

Means for Solving the Problems The present invention has been made to solve the above-mentioned problems, and the means comprises a hot-rolled hot steel plate having a surface temperature deviation within the plate. In the method of water cooling, the thick steel plate is cooled at a temperature of 5 ° C./sec or more until the surface temperature of the low-temperature portion reaches 630 to 530 ° C.
After cooling at a cooling rate of not more than 25 ° C / sec,
This is a method of cooling at a cooling rate of c or more.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In producing a high-strength steel sheet,
When a hot steel plate is water-cooled on-line, the surface temperature is generally cooled from around 800 ° C. to a range of 400 ° C. to normal temperature. FIG. 1 is a diagram showing the relationship between the steel sheet surface temperature and the heat transfer coefficient in the cooling temperature range. The temperature can be divided into two temperature ranges at a surface temperature of about 200 ° C. according to the direction of increase and decrease of the heat transfer coefficient.

The region where the surface temperature is around 800 ° C. to 200 ° C. is a region where the heat transfer coefficient increases as the surface temperature becomes lower, and is generally called a transition boiling region. In this transition boiling region, the lower the surface temperature is, the higher the cooling capacity is. Therefore, if the plate has a temperature deviation, the temperature deviation increases as the cooling proceeds. On the other hand, if the surface temperature is 2
The low temperature region from around 00 ° C. to room temperature is a region where the heat transfer coefficient decreases as the surface temperature decreases, and is generally called a nucleate boiling region. In the nucleate boiling region, the temperature deviation converges, contrary to the phenomenon in the transition boiling region.

In consideration of such cooling characteristics, in order to perform stable cooling with less cooling unevenness, the transition boiling range, especially the heat transfer coefficient, among them, particularly rapidly increases, and the dispersion becomes large. It can be seen that it is desirable to pass through the temperature range around ℃ in a short time, that is, to rapidly lower the steel sheet surface temperature at a large cooling rate and perform cooling in the nucleate boiling range of 200 ° C. or less.

However, as described above, when cooling is performed by uniformly increasing the amount of water supplied from the cooling device during the entire period from the start to the end of cooling, only the surface layer in the thickness direction is rapidly cooled, and the temperature is sharply increased in the thickness direction. Distribution is formed, and it passes through the transition boiling region in a short time and cooling unevenness is small and a relatively good shape can be secured, but since the cooling rate from the start of cooling to the completion of transformation in the surface layer is excessive, it is In comparison, the surface layer portion is rapidly quenched, the strength is increased, and the toughness is deteriorated. As a result, the material deviation in the thickness direction increases.

Therefore, in order to reduce the material deviation in the thickness direction, it is necessary to prevent the strength of the surface layer from increasing.
For that purpose, it is necessary to reduce the cooling rate until the transformation of the surface layer is completed.

The inventors of the present invention devised a method of slowly cooling the cooling start stage and then rapidly cooling it, and conducted various experiments and simulations. It has been confirmed that it is possible to achieve both the uniformity of the material in the direction and the flatness of the steel sheet by reducing the cooling unevenness. The reason for limiting the cooling rate and the steel sheet surface temperature at which the cooling rate is changed in the present invention will be described below.

The cooling rate at the start of cooling is 15 ° C./s
The reason for setting it to ec or less is that at a cooling rate higher than 15 ° C./sec, the strength of the surface layer of the steel sheet increases, and even if the cooling rate is increased in the latter half of cooling, the strength of the central part can be increased to the same level as the surface layer. This is because it is difficult and consequently a material deviation occurs in the thickness direction. The reason for setting the cooling rate to 5 ° C./sec or more is that a cooling rate longer than 5 ° C./sec increases the cooling time, lowers productivity, and does not provide the required strength value of 490 N / mm ² as a high-tensile steel plate. That's why.

Next, the reason why cooling is performed at a cooling rate of 25 ° C./sec or more in the latter half of cooling will be described. Generally, the surface temperature of a steel sheet before cooling varies within the sheet. This is due to a temperature deviation generated in the heating stage of the slab, a temperature deviation due to descaling during hot rolling, a temperature drop in the four peripheral portions of the steel sheet, and the like. FIG. 2 shows the heat flux of the high temperature part and the low temperature part in the plate when the cooling rate is increased at a stretch when cooling the steel plate having such a plate temperature deviation, that is, the unit time and the unit area. It shows the transition of heat removal per area. Here, the heat flux is represented by equation (1). q = α × (Ta−Tw) (1) where q: heat flux [W / m ² ], α: heat transfer coefficient [W
/ (M ² K)], Ta: steel sheet surface temperature, Tw: water saturation temperature. Further, as shown in FIG. 1, since the value of the heat transfer coefficient differs according to the steel sheet surface temperature, the heat flux differs between a high temperature portion and a low temperature portion in the plate. As a result, in the transition boiling region in the first half of cooling, the heat flux in the low temperature portion is larger than that in the high temperature portion, and the temperature deviation in the plate is increasing.

The heat flux rapidly increases at the timing when the cooling rate is increased during the cooling, and thereafter, the heat flux reaches a peak, and the heat flux in the high temperature portion and the low temperature portion is reversed. This indicates that the surface temperature has been cooled to about 200 ° C. or less and has entered the nucleate boiling region. The heat flux in the high temperature portion is larger than that in the low temperature portion, and the in-plate temperature deviation is converging.

As shown in FIG. 3, the area surrounded by the heat flux of the high-temperature portion and the heat flux of the low-temperature portion represents a difference in heat removal between the two. Assuming that the respective areas are ΔJ ₁ , ΔJ ₂ , and ΔJ ₃ , ΔJ ₁ and ΔJ ₂ are the heat removal amounts in the direction of increasing the temperature deviation, and ΔJ ₃ is the heat removal amount in the direction of converging the temperature deviation. In order not to make the temperature difference in the plate after the completion worse than the temperature difference in the plate at the start of cooling, the condition of ΔJ ₁ + ΔJ ₂ ≦ ΔJ ₃ (2) is required. As a result of a simulation under various conditions, in order to satisfy the expression (2), the heat flux is rapidly increased during the cooling as in the present invention, and the heat flux is reduced in a short time by rapidly decreasing the surface temperature. It was found that it was necessary to reach the peak, to secure a large area of ΔJ ₃ , and to cool at a cooling rate of at least 25 ° C./sec in the latter half of cooling. The upper limit is preferably 70 ° C./sec.

In the conventional method, the cooling rate is kept constant or gradually increased until the cooling is completed.
A steep temperature distribution cannot be formed in the thickness direction during cooling, and the entire steel sheet is cooled with a gentle temperature distribution, so the center is already required before or near the surface temperature reaching the nucleate boiling region. It is cooled until a sufficient strength can be ensured, and if it is further cooled to lower the surface temperature, not only the strength is exceeded but also the toughness is deteriorated. Therefore, sufficient cooling cannot be performed in the nucleate boiling region, and as shown in FIG. 4, there is no or small heat removal amount difference ΔJ ₃ in the direction of converging the temperature deviation. It is difficult to ensure a good shape while expanding from the starting state.

The reason why the timing for changing the cooling rate is set so that the minimum surface temperature in the plate is in the temperature range of 630 ° C. to 530 ° C. will be described below.

In order to reduce the temperature deviation in the plate after cooling, the difference in heat removal ΔJ ₁ + Δ in the direction of increasing the temperature deviation.
It is most effective to J ₂ changes the cooling rate at a timing so as to minimize. For example, when the change timing is late and the surface temperature becomes low, the area of ΔJ ₁ increases as shown in FIG. This is because when the surface temperature in the initial stage of cooling is high, the change in the heat transfer coefficient with respect to the surface temperature is small, and although the heat flux difference between the high-temperature portion and the low-temperature portion is small, the cooling is performed at a relatively low cooling rate. This is because the time becomes longer. Conversely, when the change timing is early and the surface temperature is high, the area of ΔJ ₂ increases as shown in FIG. This means that if you increase the cooling rate,
This is because the change in the heat transfer coefficient with respect to the surface temperature increases. In any case, it is not appropriate to reduce ΔJ ₁ + ΔJ ₂ .

In consideration of these facts, a simulation was conducted under various conditions. As a result, ΔJ ₁ + ΔJ ₂ changed with respect to the surface temperature as shown in FIG. It was found that the temperature became minimum around 580 ° C. without being largely influenced by conditions such as the cooling rate.

An experiment was conducted based on the simulation results to confirm the effect.
In the temperature range of ° C., no significant difference was observed in the temperature deviation in the plate after cooling, and a good shape was obtained. The result is that the heat transfer coefficient used in the simulation is the average value of the heat transfer coefficient, and the heat transfer coefficient actually changes due to the surface properties such as scale thickness and roughness,
It is estimated that the surface temperature that minimizes ΔJ ₁ + ΔJ ₂ varies between the high temperature side and the low temperature side. In addition, the influence of the difference in the amount of heat removal of about 200 kJ / m ² that changes within the above temperature range is less than 5 ° C. as the temperature deviation in the plate, and is not an effect that the shape changes greatly. .

[0023] However, since the surface temperature begins to increase while the variation abruptly heat transfer coefficient becomes a low temperature side than the lower limit temperature 530 ° C. of the temperature range, the heat removing amount .DELTA.J ₁ + .DELTA.J to enlarge the temperature difference, as shown in FIG. 7 ₂ surges. From this, in order to perform cooling so as not to enlarge the temperature deviation in the plate, the cooling speed change temperature is determined based on the temperature of a low temperature portion (low temperature portion), preferably the lowest temperature portion in the plate. There is a need. As a method of detecting the temperature in the sheet, it is preferable to measure the temperature distribution in the sheet after the completion of rolling by using a thermo tracer installed on the line.
Temperature history during cooling is estimated by heat transfer calculation based on the temperature distribution in the plate measured by thermo tracer before cooling,
The timing of changing the cooling rate is determined based on the calculation result.

When the temperature at which the cooling rate was changed was in the range of 630 ° C. to 530 ° C., a uniform material distribution was obtained in the thickness direction. This is because the transformation of the surface layer of the steel sheet has been almost completed near the timing of changing the cooling rate, but since the heat has not been sufficiently extracted to the center of the steel sheet, the transformation of the central part has not yet begun. This is because, when the speed is increased and strong cooling is performed, transformation at the center of the steel sheet is promoted, and a structure similar to the surface layer is obtained.

[0025]

An embodiment of the present invention will be described below with reference to FIG.

FIG. 8 is an overall conceptual diagram of equipment to which the method for cooling a thick steel plate according to one embodiment of the present invention is applied. As shown in this figure, a slab 2 heated in a heating furnace 1 is hot rolled by a rolling mill 3 to become a thick steel plate 4. The temperature distribution of the thick steel plate is measured by the thermotracer 5 installed on the line, and then the steel plate is cooled continuously by the online cooling device 7 via the hot leveler 6.

Table 1 shows the chemical components of the thick steel plate used in this example. Table 2 shows cooling conditions, mechanical properties, differences in cross-sectional hardness, and slope temperature deviations after cooling. The cooling speed change surface temperature shown in the cooling conditions represents the lowest temperature in the plate, and the cooling speed was changed by changing the amount of water injected from the cooling device. The difference in cross-sectional hardness is the difference between the maximum hardness and the minimum hardness in the thickness direction, and the temperature deviation in the plate is the difference between the maximum temperature and the minimum temperature in the plate.

[0028]

[Table 1]

[0029]

[Table 2]

Examples 1 to 3 are examples in which the cooling conditions limited by the present invention are applied, and the mechanical properties, the difference in cross-sectional hardness, the temperature deviation in the plate, and the shape of the steel plate are all good, which is difficult with the prior art. Both material and shape can be realized.

Comparative Example 1 is an example in which the cooling rate at the start of cooling is out of the lower limit of the range of the present invention.
The required value of 90 N / mm ² could not be satisfied. Comparative Example 2
In the example, the cooling rate at the start of cooling was out of the upper limit of the range of the present invention. The difference in cross-sectional hardness increased, the temperature deviation in the plate was large, and the steel plate was deformed. Comparative Example 3 is an example in which the cooling rate in the latter half of the cooling is below the lower limit of the range of the present invention, and Comparative Example 4 is an example in which the condition of the cooling rate is out of the range of the present invention in both the first and second half of the cooling. Was insufficient and a good shape could not be obtained. Comparative Example 5 was an example in which the surface temperature of the steel sheet when the cooling rate was changed was outside the upper limit of the range of the present invention. The hardness of the surface layer increased, and a uniform material could not be obtained in the thickness direction. In Comparative Example 6, contrary to Comparative Example 5, the steel sheet surface temperature at the time of changing the cooling rate was out of the lower limit of the range of the present invention, the temperature deviation in the steel sheet was large, and the steel sheet was deformed.

Conventional example 1 is an example in which the cooling rate is gradually increased from the start to the end of cooling. Although the difference in cross-sectional hardness is small and good results are obtained in terms of material, the temperature deviation in the plate increases and the steel plate Marked deformation.

[0033]

As described above, by carrying out the present invention, stable cooling with less cooling unevenness over the entire surface of the high-temperature steel sheet is achieved, the material difference in the sheet thickness direction is small, and the shape is good. Thick steel plates can be manufactured.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a steel sheet surface temperature and a heat transfer coefficient.

FIG. 2 is a graph showing a change in heat flux during a cooling process.

FIG. 3 is a graph showing a difference in heat removal between a high-temperature portion and a low-temperature portion.

FIG. 4 is a graph showing a change in heat flux when cooling is performed at a constant cooling rate.

FIG. 5 is a graph showing a change in heat flux when the cooling rate change temperature is low.

FIG. 6 is a graph showing a change in heat flux when the cooling rate change temperature is high.

FIG. 7 is a graph showing a relationship between a surface temperature and a difference in a heat removal amount in a direction in which a temperature deviation increases.

FIG. 8 is an overall conceptual diagram of equipment to which the method for cooling a thick steel plate according to one embodiment of the present invention is applied.

[Explanation of symbols]

1: heating furnace, 2: slab, 3: rolling mill, 4: steel plate,
5: Thermo tracer, 6: Hot leveler, 7: Online cooling device

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K034 AA02 AA10 BA05 CA01 DA06 DB03 FA05 4K043 AA01 BA03 BA04 CB01 EA07 FA03 FA13 GA06 GA10

Claims (1)

[Claims] 1. A method for water-cooling a high-temperature steel plate which is hot-rolled and has a surface temperature deviation in the plate, wherein the steel plate is cooled at 5 ° C. until the surface temperature of a low-temperature portion becomes 630-530 ° C. A cooling method for a thick steel plate, comprising: cooling at a cooling rate of 25 ° C./sec or more after cooling at a cooling rate of 15 ° C./sec or more.