JPH02153023A - Roll cooling method for steel strip - Google Patents

Abstract

PURPOSE:To subject steel strips of a wide range of thicknesses to roll cooling with good flatness by dividing cooling rolls to a front cooling roll group and a rear cooling roll group and changing the coeffts. of heat conduction between the steel strip and the cooling rolls in the two roll groups. CONSTITUTION:The cooling rolls are segmented to the front cooling roll group and the rear cooling roll group at the time of continuously passing the high- temp. steel strip in the plural cooling rolls and cooling the steel strip. The front cooling roll group is then set in such a region where the roll inlet side temp. of the steel strip exceeds 600 deg.C and the rear cooling roll is classified and set in such a region where the roll inlet side temp. of the steel strip attains <=600 deg.C. The coefft. alpha1 of heat conduction between the steel strip and the cooling rolls in the front cooling roll group is Specified to <=1200Kal/m<2>h deg.C and the coefft. alpha2 of heat conduction between the steel strip and the cooling rolls in the rear cooling roll group is specified to >=1800Kal/m<2>h deg.C. A wide range of the strip thicknesses from thin to thick sheets are dealt with in this way and the cooling in the prescribed temp. range is executed without impairing the flatness, material quality, etc., of the steel sheet.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for cooling a high-temperature steel strip during continuous annealing of the steel strip, etc. This relates to a roll cooling force method 1 in which the rolls are divided into a front cooling roll group and a rear cooling roll group, and the steel strip is cooled by changing the heat transfer coefficient between the steel strip and the cooling roll in each cooling roll group.

(Prior art) The roll cooling method, which is adopted as one of the methods for cooling high-temperature steel strips in continuous annealing furnaces, etc., is a method in which the steel strip is cooled by passing the steel strip through a plurality of cooling rolls arranged in a staggered manner. It has the characteristics that the equipment cost and energy consumption are small, and the surface condition of the steel strip is maintained in good condition.

To describe this roll cooling method in more detail, as shown in Fig. 1, a steel strip (2) is passed between a plurality of cooling rolls (1) that are vertically supported at appropriate intervals ([). The steel strip (2) is cooled by passing the steel strip in contact with a roll, and the contact angle (θ) of the steel strip (2) on the roll I\ is changed by moving the cooling roll (1) up and down. Contact circumference (
The configuration is such that the sum of ρ1) can be adjusted.

By the way, in general, in such a roll cooling method, the procedure for determining the specifications of the cooling roll is as follows: (1) the required temperature drop of the steel strip and the allowable per 10-ru, which is determined from the quality constraints of the obtained steel strip; The required number of rolls is determined based on the amount of temperature drop, and (2) the roll diameter and heat transfer between the steel strip and the rolls are determined so that a predetermined amount of heat can be removed from the thickest steel strip to be threaded. Determine the coefficient α.

(Problem to be Solved by the Invention) As a result of determining the specifications of the cooling roll based on the thick steel strip as described above, when cooling the fallen steel strip, it is difficult to rapidly cool it in the high temperature range at the initial stage of cooling. This resulted in the inconvenience that the resulting product often had poor flatness.

Therefore, in order to cool the thin steel strip, it is necessary to lower the starting temperature of roll cooling. That is, in this case, a method is adopted in which the steel strip heated to a high temperature in a continuous annealing furnace or the like is lowered to a predetermined temperature by jetting the steel strip with cooling gas until the time when the rolls in the cooling section start cooling. However, this gas injection method not only increases operating costs, but also causes problems with the material quality, such as raising the yield point of the steel strip product due to its low cooling capacity and deteriorating aging properties. This is not desirable because it causes

The present invention was developed in view of the problem of sloping in the conventional steel strip roll cooling method, and its purpose is to improve the thickness of steel strips in a wide range of thicknesses from thin to original. The object is to provide a method that can perform cooling within a predetermined temperature range without impairing flatness, material quality, etc.

(Means for Solving the Problems) When carrying out this invention, the inventors of the present invention have conducted repeated research and examinations, and have divided the cooling roll group into a front stage and a rear stage, and created a structure between the steel strip and the rolls in the front stage cooling roll group. By keeping the heat transfer coefficient as low as possible and instead increasing the heat transfer coefficient in the cooling roll group in the latter stage compared to the former stage, it is possible to perform roll cooling that corresponds to both steel strip thicknesses. This invention was completed based on this knowledge.

That is, the present invention relates to a roll cooling method for steel strip in which a high-temperature steel strip is continuously passed through a plurality of cooling rolls to be cooled. A steel strip roll cooling method characterized in that the heat transfer coefficient α1 between the steel strip and the cooling roll in the cooling roll group is made smaller than the heat transfer coefficient α between the steel strip and the cooling roll in the downstream cooling roll group. Furthermore, the first stage cooling roll group is placed in an area where the temperature on the roll entrance side of the steel strip exceeds 600℃, and the second stage cooling roll group is placed in an area where the temperature on the roll entrance side of the steel strip is 000℃ or less. In addition to setting the classification, the heat between the steel strip and the cooling roll in the front stage cooling roll group1
The delivery coefficient α is set to 1200Kca17nfh℃ or less,
Also, the heat transfer coefficient α2 between the steel strip and the cooling roll in the latter stage cooling roll group is 1800 KCa/'rr? This is a roll cooling method for steel strip, characterized in that the temperature is at least h°C. .

(Function) Next, a series of experiments conducted by the present inventors and the results of their considerations in carrying out the present invention will be described. For example, the heat pattern of a typical continuous annealing furnace used to produce cold-rolled steel sheets for drawing is as shown in Figure 2. A steel strip heated to around 700°C and kept soaked at the same temperature has a heat pattern of 400°C.
The rolls are cooled to the overaging treatment temperature of around 600°C, but there is a close relationship between the steel strip temperature at the start of cooling and the permissible temperature drop per cooling roll. The higher the zone temperature, the lower the yield stress, so
Even a small amount of temperature drop tends to cause flatness defects.

From the above point of view, if the high temperature range at the start of normal cooling, that is, the steel strip temperature is around 600 to 700°C, it is appropriate that the allowable temperature drop per cooling roll is around 30 to 40°C. has been done. However, the value of the above-mentioned allowable temperature drop varies not only depending on the steel strip temperature but also on the thickness of the steel strip and the heat transfer coefficient between the steel strip and the cooling roll.

That is, in general, in the roll cooling method, the cooling rate Ec, roll contact time t, contact circumference 11, contact angle θ, etc. of each roll of the steel strip to be passed are expressed by the following equations (1) to (4). They are said to be in a relationship.

That is, ・・・・・・・・・(1) c ・・・・・・・・・(2) θ”ut in 360/' Dπ ・・・・・・・・・
・(4) Here, α ; Heat 1 expansion coefficient (Kcal/rn' Hh ・℃)
T1: Cooling start temperature ('C) T2: Cooling end temperature ('C) T3: Roll surface temperature ('C> h: Plate thickness (bleached) C2: Specific heat (KCal /'kg ・'C) V Niline speed (m/'m1n) N: Number of rolls D Two roll diameter (m> As can be seen from the above equation, the cooling rate Ec of the steel strip in contact with the rolls is proportional to the heat transfer coefficient α, It is inversely proportional to the thickness.Therefore, the heat transfer coefficient α must be set to achieve an appropriate cooling rate when passing a thick steel strip, especially in high-temperature ranges where the amount of temperature drop must be strictly controlled. If this is determined, the cooling rate Ec will become too high when passing a thin plate with a small thickness, causing flatness defects.

Furthermore, when the heat transfer coefficient α is large compared to the plate thickness, in order to maintain the cooling rate Ec within an appropriate range, the contact angle θ between the steel strip and the cooling roll must be reduced, and the contact angle θ between the steel strip and the cooling roll must be reduced. The contact becomes unstable and a so-called flapping phenomenon occurs, causing temperature non-uniformity in the longitudinal direction of the steel strip, which also causes flatness defects.

From this point of view, the present inventors used a thin steel strip (0.8 mmt x 914 W) that tends to cause problems in the high temperature range at the start of cooling, and calculated the heat transfer coefficient α between the steel strip and the cooling roll and the temperature after cooling. An experiment was conducted to find the relationship with the flatness of the steel strip.

In the experiment, a roll with a diameter of 1400 mm was used, and the line speed was set at 200 m7'ml. The heat transfer coefficient α was controlled by adjusting the roll surface roughness and the temperature and flow rate of the coolant passed through the cooling roll.

The results are shown in Table 1.

From the results in Table 1, in order to obtain a steel strip with good flatness after cooling in the high temperature range of 600 to 700°C, it is necessary to set the heat transfer coefficient α between the steel strip and the cooling roll to 1200 Kca.
l/rr? It has been found that it is desirable to use relatively slow cooling conditions below h°C.

In addition, when roll cooling is performed from the annealing holding temperature of around 700°C to the overaging temperature of around 400°C in the continuous annealing of the drawn cold-rolled steel described above, the allowable temperature drop per 10 mm (30 to 50°C) is used. It is said that the appropriate number of rolls for kneading is 7, but if all the cooling rolls from the high temperature range to the low temperature range are used under the above-mentioned slow cooling conditions with a small heat transfer coefficient α, use the same roll. With a large number of devices, it became impossible to remove heat from thick steel strips, and 400
It becomes difficult to cool down to a predetermined temperature of °C. Therefore,
The present inventors succeeded in reducing the thickness of the thin sheet by applying rapid cooling conditions by increasing the heat transfer coefficient in the low-temperature region in the latter half of cooling, where the cooling conditions of the steel strip do not have a large effect on the flatness of the steel strip. The following experiment was conducted on the assumption that cooling to a predetermined temperature range could be achieved without deteriorating the flatness of steel strips with a fairly wide range of thickness up to the plate.

Seven cooling rolls were used in the experiment, and 700 heated steel strips were heated.
Cooling roll 3 for cooling from around ℃ to around 600℃
4 cooling rolls (allowable temperature drop i
:40~50℃) divided into two groups, the latter cooling roll group,
Applying the results in Table 1, the heat transfer coefficient α1 between the steel strip and the cooling roll in the front roll group was set to 1000 and 1200.
kcal/ryfh'C, and the heat transfer coefficient α2 of the latter roll group was changed appropriately in the range of 1200 to 1800 Kcal/rrfh°C. 1.6mmt
A cooling experiment was carried out for x914 t ).

The roll diameter was 1400 mm, the roll interval was 1500 mm, and the line speed was set at 200 m/min.

The experimental results are shown in Table 2.

In Table 2, the symbol x indicating the experimental results indicates that the steel strip could not achieve the predetermined temperature drop, and the number in parentheses indicates the temperature drop reached. The symbol O indicates that the steel strip can be cooled to the specified temperature without any problems.

From the results in Table 2, when the heat transfer coefficient α1 in the first stage cooling roll group is set to a slow cooling condition of 1000 to 1200 KCal/rrf'h°C, the heat 1 in the second stage cooling roll group is
It can be seen that if the reach coefficient α2 is kept within the rapid cooling condition of 1800 KCal/'rn'h°C or more, a predetermined temperature drop can be achieved even when cooling a considerably thick plate.

Combining the results of the two experiments described above, when cooling a high-temperature steel strip to a predetermined temperature by the roll cooling method using multiple cooling rolls, the cooling rolls are divided into two roll groups, the front stage and the rear stage, and each By changing the heat transfer coefficient between the steel strip and the rolls in the roll group, and setting the first stage to a slow cooling condition and the second stage to a rapid cooling condition, it is possible to handle a wide range of thickness changes from thin to thick. Correspondingly, the steel strip can be cooled to a predetermined temperature with good flatness, which is extremely efficient.

Although the above experiment was conducted on roll cooling during continuous annealing of cold-rolled steel sheets for drawing, the present invention is not limited to this, and can be applied to roll cooling of steel strips of steel types having similar heat patterns. Needless to say, it is possible.

(Example) Using an experimental apparatus consisting of seven 300 mm diameter cooling rolls, two types of steel strips with thicknesses of 1.6 mrn and 0.8 mm were cooled under cooling conditions according to the present invention and cooling conditions according to the conventional method. The results are shown in Table 3.

The results in Table 3 show that in the conventional method, that is, when the heat transfer coefficient between the steel strip and the roll is determined based on the thick plate standard, flatness defects occur when attempting to cool the roll of a thin steel strip from around 700°C. However, it is unavoidable to take measures to lower the starting temperature of roll cooling by means of gas injection cooling, etc. Performing such additional operations poses problems in terms of cost and material management. However, it can be seen that according to the present invention, it is possible to cool plates ranging from thick plates (1.6 mm) to thin plates (0.8 mm) to a predetermined temperature around 400° C. without impairing the flatness.

(Effects of the Invention) As explained above, according to the method of the present invention, steel strips having a wide range of thickness can be fixed with good flatness without the need for special additional equipment such as gas injection cooling. This makes it possible to perform roll cooling of 100%, which is an outstanding effect in the primary cooling treatment of continuous annealing lines for steel strips.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the roll arrangement of the roll cooling method, Figure 2
The figure is a thermal curve diagram showing the heat pattern of drawn cold-rolled steel in a continuous annealing furnace. 1...Cooling roll, 2...Steel strip

Claims (1)

[Claims] 1. In a steel strip roll cooling method in which a high-temperature steel strip is cooled by continuously passing it through a plurality of cooling rolls, the cooling rolls are divided into a front stage cooling roll group and a rear stage cooling roll group. The heat transfer coefficient α between the steel strip and the cooling roll in the front cooling roll group is
A roll cooling method for a steel strip, characterized in that _1 is smaller than a heat transfer coefficient α_2 between the steel strip and the cooling roll in the latter stage cooling roll group. 2. The temperature of the roll entrance side of the steel strip in the first stage cooling roll group is 600℃.
The heat transfer between the steel strip and the cooling rolls in the first stage cooling roll group is divided into regions where the temperature exceeds 600°C, and the second stage cooling roll group is divided into a region where the temperature at the roll entrance of the steel strip is 600°C or less. The coefficient α_1 is 1200Kcal/
m^2h℃ or less, and the heat transfer coefficient α_2 between the steel strip and the cooling roll in the latter stage cooling roll group is 1800Kca.
The roll cooling method for steel strip according to claim 1, wherein the temperature is 1/m^2h°C or higher.