WO1987000772A1

WO1987000772A1 - A method for cooling rolling mill stock

Info

Publication number: WO1987000772A1
Application number: PCT/SE1986/000349
Authority: WO
Inventors: Ulf Tjernquist
Original assignee: Mefos - Stiftelsen För Metallurgisk Forskning
Priority date: 1985-08-08
Filing date: 1986-08-01
Publication date: 1987-02-12
Also published as: AU6220986A; SE8503740L; EP0231337A1; SE464280B; SE8503740D0

Abstract

A method for cooling rolled stock downstream of a given roll pair of a rolling mill having connected thereto a stock cooling section comprising at least one cooling tube (2), a liquid-coolant jacket (3) surrounding the tube, an external guide tube (8) leading from a given roll pair to the at least one cooling tube (2), and an internal guide tube (4) located in the cooling tube. The method comprises the steps of forcing the liquid coolant from the jacket into the external guide tube (8) in countercurrent with the direction of stock movement and into the internal guide tube (4) concurrently with said stock movement. In accordance with the novel features of the invention, immediately the stock leaves the nip of the given roll pair it is passed into the external guide tube (8) of a first cooling tube in line, this guide tube being placedin direct connection with said roll nip. The arrangement is such that liquid coolant can be forced from the jacket (3) into the external guide tube (8) in a controlled manner and from there into the roll nip while, at the same time, forcing liquid coolant from the jacket (3) in a controlled manner into the internal guide tube (4).

Description

A METHOD FOR COOLING ROLLING MILL STOCK

FIELD OF INVENTION

The present invention relates to a method for cooling stock in rolling mills of the kind which have connected thereto a cooling section comprising at least one cooling tube, a liquid-coolant jacket surrounding the tube, a guide tube leading from a given roll pair of the mill to the cooling tube, and a second guide tube located in the cooling tube, in which method the liquid-coolant is forced from the jacket into the first guide tube leading to the roll pair and into the second guide tube located within the cooling tube. It will be understood that by "stock" is meant here and in the following any hot rolled work thus cooled.

BACKGROUND PRIOR ART

Thermomechanical Treatment (TMT) is a novel technic recently introduced into rolling mill technology. TMT is actually a composite term for several different processes, such as controlled rolling, air-blast quenching and direct hardening. TMT was initially introduced into sheet- metal rolling mills since such mills are readily adaptable to the process of TMT in view of the fact that the mill layout permits the sheet to be air-cooled between roll passes, such as to attain a low final rolling temperature, in addition to which the sheet speed downstream of the rolling mill is also low, which means that the cooling sections used are short and ineffective. In recent times the TMT-process has also been used in rod and wire mills. Since these mills are often fully continuous, there is no opportunity for the stock to be air-cooled between passes, and hence it is necessary to cool the stock with liquid coolant, e.g. water between the individual passes through the mill. Since the travel distance between the roll pairs is often short and the stock travels at high speed, highly effective cooling must be effected in the cooling section between passes. This also applies to the cooling section downstream of the rolling mill. The distance or space between the final roll pair and the cooling bed is often very short and the stock speed very high (up to 150m/s), which places great demands on the cooling section even in order to achieve some degree of cooling. Since the cooling sections are often ineffective it has not been possible to direct harden the steel, i.e. to cool the stock, bar etc. down to T_mean = 100 - 211°C before the stock reaches the cooling bed or section. Because of this other material technical processes of the type Temcore have been applied, as described in Belgian Patent

Specification No 790867, where the outer layer is cooled to form martensite, this outer layer being annealed by the heat conducted outwardly from the centre of the stock. In order to cool the stock, bar etc. effectively it is necessary to use a coolant capable of carrying away large quantities of heat, water being one of the most common coolants used in this regard.

Other publications which disclose methods for cooling hot rollers work include US Patent No 2,856,803, EP-A10141511, French Published Specification No 2231440, and US Patent No 4136544.

One problem with water cooling is the film of vapour which forms closely adjacent the stock surface. This vapour film, which has poor thermal conductivity, prevents fresh water from reaching the surface of the stock, therewith resulting in poor cooling. An extremely good cooling effect is achieved, however, when it is possible to break down the vapour film so that a new vapour film can form and then quickly broken down again. If no vapour film is formed, i.e. the stock bar etc. passes through the water so quickly that heat transfer takes place solely between the stock and the water, the cooling effect is poor, since water is a poor conductor of heat.

In order to cool the stock effectively it is necessary to effect the cooling process in a manner such that while a vapour film is formed it is quickly washed away so that a fresh film can form. One method of cooling the stock is to spray the stock with water through nozzles or sprays (vide for example Pro Conf "Mechanical working and steel processing", Cleveland, Oct. 1979, pp 107-126.). Another method is one in which the stock is passed through a tube in which water is forced into the space between the stock and the inner wall of the tube in accordance with German Auslegschrift 1602356. The cooling tube is normally designated annular gap quenching tube, and the space between stock and tube the material gap. At suitable relative speeds between the stock and the water there is generated a strong radial and axial turbulence which breaks down the vapour film, to provide effective cooling of the stock, bar etc. One effect that has previously been observed is that a relatively cold surface e.g. 600° C can be cooled more easily than a hotter surface (e.g. 1000° C) due to the fact that the vapour film forms more slowly on the surface whose temperature is 600ºC than on the surface having a temperature of 1000° C. This means that the same rate of cooling (°C/s) can be achieved with cooler surface at a lower relative speed between stock and water. Vide for example Pro Conf "resources efficiency in iron and steel" G. Elsen: "Improved roll cooling practices for roll materials". SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for improved cooling of stock, bar, etc. in a rolling mill.

Accordingly, there is proposed in accordance with the invention a method for cooling rolled stock downstream of a given roll pair of a rolling mill having connected thereto a stock cooling section comprising at least one cooling tube, a liquid-coolant jacket surrounding the tube, an external guide tube leading from a roll pair to the at least one cooling tube, and an internal guide located in the cooling tube, the method comprising the steps of forcing the liquid coolant from the jacket into the external guide tube in countercurrent with the direction of stock movement and into the internal guide tube concurrently with said stock movement, characterized by the further step of passing the stock, immediately it leaves the nip of said roll pair, into the external guide tube of a first cooling tube in line, said guide tube being placed in direct connection with said roll nip, such that liquid coolant can be forced from the jacket into the external guide tube in a controlled manner and from there into the roll nip while, at the same time, forcing liquid coolant in a controlled manner from said jacket into the internal guide tube.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the invention will be more readily understood and further features thereof become apparent an exemplifying embodiment will now be described with reference to the following drawings, in which

Figure 1 is a schematic illustration of a cooling sect section in a rolling mill which applies the method accrding to the invention;

Figure 2 is a longitudinal sectional view of one of the cooling tubes incorporated in the cooling section according to Figure 1; and

Figure 3 is a diagram which illustrates the relationship between water pressure, water velocity, and the cross-sectional area of the stock when carrying out the method according to the invention.

DESCRIPTION OF A PREFERED EMBODIMENT

The cooling section, which can be used both for cooling stock, bar or other hot work between roll pairs and after the final or finishing roll pair 1, comprises a number of cooling tubes 2. The number of tubes used depends on the extent of cooling desired and on the space available. The cooling tubes comprise a water jacket 3 having a guide tube arranged internally therein, and a housing 5 for pressurized air having an air inlet 6. Although water is referred to as the coolant used in this embodiment, it will be appreciated that other liquid coolants may be used. The jacket 3 and housing 5 are firmly welded in a cooling tube chest 7. A guide tube 8 extending from the roll pair 1 is attached to the water jacket 3. The length of the guide tube 8 should be as short as possible, so that the hot stock is introduced into the inner guide tube 4 in the shortest time possible. Water under high pressure (20-500 bars) is delivered to the jacket 3 through a conduit 9. The water is then forced into the aforesaid material gap, i.e. the space between the hot stock (not shown) abd the inner wall of the guide tube 4 , where the actual cooling of the stock takes place, the surface temperature of the stock being lowered to about 100 C. Part of the water is also forced in the reverse direction, from the jacket 3 and into the guide tube 8 in to contact with the rolls 1. This water assists in damping the surface temperature of the stock as it leaves the roll pair 1. The water pressed from the inner guide tube 4 is then passed into the housing 5 for pressurized air. If a small cooling eff- ect is required, the supply for air under pressure (

7 bars) is switched on, whereupon pressurized air is blown counter directionally to the direction of stock travel, so as to prevent the water from leaving the cooling tube 2 together with the stock. If substantial cooling is required, the pressurized supply is switched off. In this case the water is instead passed through the housing 5 for air under pressure, and from there through an intermediate guide tube 8 and into the water jacket 3 of the next cooling tube in line, here refer- enced 10.

When viewed in longitudinal section, the cooling tube 10 and the subsequent cooling tube 11 are of similar appearance to the cooling tube 2. Thus ,. water from the cooling tube 2 is forced into the water -jacket and into the internal guide tube, i.e. the cooling section is in effect a sigle long stock guide tube with a few short interruptions therealong. These interruptions in the continuity of the tube are necessary in order for steam (vapour) to escape from- the water.

In the case of stock, or equivalent hot work, of small dimensions, it is unnecessary to add additional water to the subsequent cooling tubes. As the water passes through consecutive cooling tubes, it becomes hot and evaporates off, i.e. the cooling effect is lowered. In this event, it is beneficial to switch on the supply of air under pressure in one of the cooling tubes, so as to blow the water away from the cooling tube 2, whereafter the process is repeated downstream of the cooling tube 2. In this case, however, the pressure employed (the velocity of the water) need not be as high as that applied in the cooling tube 2, as evident from Figure 3 since the temperature of the stock is now about 100° C as opposed to an earlier temperature of 600° C.

In the case of stock of coarser dimensions, the water supplied to the cooling tubes should be under low pressure (~10 bars). The water from the cooling tube 2 is therefore replenished in the subsequent cooling tubes to compensate for the substantial loss of water, through evaporation. Since fresh water is constantly added, it is not necessary to blow the water away until the last cooling tube is reached.

The speed at which stock moves through a continuous rolling mill constantly increases and when leaving the last pair of rolls in the mill the stock speed may be as high as 150m/s. This means that the intermediate time periods between passes are very short, and the rolls in the finishing pair will exert a cooling effect on the surface of the hot stock. For example, if there is maintained a roll surface temperature of about 100° C and the stock has a temperature of 1000° C upon entry into the roll nip of said roll pair, the temperature of the stock at the point of contact between rolls and stock, and for a short time thereafter, will be about 650° C. This enables the surface temperature of the stock to be more readily lowered to about 100° C. Because of this, when practising the present invention, the cooling tube is placed as close to the roll pair as possible, possibly immediately downstream of the rolled nip, and the cooling section is constructed so that part of the water delivered to the cooling tube is for- ced rearwardly from the tube and into cooling contact with the roll surfaces. Since the surface temperature of the stock is now 100 C as it leaves the cooling tube 2, further surface cooling of the stock is of no or but little interest and the stock is suitably held at this temperature so as to allow heat to be conducted from the centre of the stock and dissipated. Since the stock now has a low surface temperature, only a low relative speed between water and stock is required to maintain the stock at the aforesaid surface temperature. When cooling tubes are located between the roll pairs in the finishing or final rolling mill, e.g. in order to lower the final rolling temperature of the stock, the effect is the same, although in this case a lower relative speed can be used to lower the surface tempera- ture of the stock in the cooling tube 2.

It was found when carrying out tests that the speed of the water should be from five to fifteen times greater than the speed of the stock, in order to be able to cool the outer surface of the stock in the cooling tube to a temperature of about 100 C. Relative speeds between water and stock lower than five generate insufficient turbulence in the aforesaid material gap and the vapour layer is not broken down, while relative speeds greater than fifteen generate excessively powerful turbulence in said material gap and no vapour layer is formed, so that thermal conduction takes place solely between the stock and the water. Since the stock travels at a high speed, each point on the stock is only located in the internal guide tube 4 in the cooling tube 2 for a very short space of time. Thus, only the outer surface of the stock is cooled to the aforesaid temperature of about 100° C, i.e. only a small amount of the water delivered to the cooling tube 2 is heated and evaporated off, the remaining water is "unnecessary". By permitting the water from the cooling tube 2 to press into the next cooling tube, the water can be "re-used", thereby lowering the water consumption to a commensurate extent.

Claims

1. A method for cooling rolled stock downstream of a given roll pair of a rolling mill having connected thereto a stock cooling section comprising at least one cooling tube (2),a liquid-coolant jacket (3) surrounding the tube, an external guide tube (8) leading from a given roll pair to the at least one cooling tube (2) , and an internal guide tube (4) located in the cooling tube, the method comprising the steps of forcing the liquid coolant from the jacket into the external guide tube (8) in countercurrent with the direction of stock movement and into the internal guide tube (4) concurrently with said stock movement, characterized by passing the stock, immediately it leaves the nip of the given roll pair, into the external guide tube (8) of a first cooling tube in line, said guide tube being placed in direct connection with said roll nip, such that liquid coolant can be forced from the jacket (3) into the external guide tube (8) in a controlled manner and from there into the roll nip while, at the same time, forcing liquid coolant from said jacket (3) in a controlled manner into the internal guide tube (4) .

2. A method according to Claim 1, characterized by imparting to the liquid coolant a speed which is from five to fifteen times greater than the speed of the stock.

3. A method for cooling stock in which the liquid coolant is used repeatedly, characterized by placing a plurality of cooling tubes one after the other such that liquid coolant accompanies the stock through sequential cooling tubes.