CA1281974C - Steel strip cooling method - Google Patents
Steel strip cooling methodInfo
- Publication number
- CA1281974C CA1281974C CA000525985A CA525985A CA1281974C CA 1281974 C CA1281974 C CA 1281974C CA 000525985 A CA000525985 A CA 000525985A CA 525985 A CA525985 A CA 525985A CA 1281974 C CA1281974 C CA 1281974C
- Authority
- CA
- Canada
- Prior art keywords
- cooling
- steel strip
- steel
- rolls
- strip
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
- C21D9/54—Furnaces for treating strips or wire
- C21D9/56—Continuous furnaces for strip or wire
- C21D9/573—Continuous furnaces for strip or wire with cooling
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Heat Treatment Of Strip Materials And Filament Materials (AREA)
Abstract
60-289,320 STEEL STRIP COOLING METHOD
Abstract of the Disclosure A method of rapidly cooling steel strips heated and soaked in a continuous heat-treatment furnace.
The rapidly cooling is carried out by winding a steel strip about a plurality of cooling rolls in succession to bring the steel strips into contact therewith.
The rapid cooling is started at a temperature of a range of 550-720°C and simultaneously a tensile force .alpha. (kgf/mm2) of the steel strip is adjusted within a range determined by an inequality of (1900-Ts)/1670<.alpha.<(1980-Ts)/720 according to the starting temperature Ts(°C) of the rapid cooling. In another aspect, the rapid cooling is carried out by winding the steel strip about a plurality of cooling rolls in suc-cession to bring the steel strip into contact therewith and at the same time by jetting a gas against a back surface of the steel strip. The rapid cooling is started at a temperature of a range of 550-720°C and simultaneously a tensile force .alpha.' (kgf/mm2) of the steel strip is adjusted within a range determined by an in-equality of (1570-Ts)/1670<.alpha.'<(2410-Ts)/630 according to the starting temperature Ts(°C) of the rapid cooling.
Abstract of the Disclosure A method of rapidly cooling steel strips heated and soaked in a continuous heat-treatment furnace.
The rapidly cooling is carried out by winding a steel strip about a plurality of cooling rolls in succession to bring the steel strips into contact therewith.
The rapid cooling is started at a temperature of a range of 550-720°C and simultaneously a tensile force .alpha. (kgf/mm2) of the steel strip is adjusted within a range determined by an inequality of (1900-Ts)/1670<.alpha.<(1980-Ts)/720 according to the starting temperature Ts(°C) of the rapid cooling. In another aspect, the rapid cooling is carried out by winding the steel strip about a plurality of cooling rolls in suc-cession to bring the steel strip into contact therewith and at the same time by jetting a gas against a back surface of the steel strip. The rapid cooling is started at a temperature of a range of 550-720°C and simultaneously a tensile force .alpha.' (kgf/mm2) of the steel strip is adjusted within a range determined by an in-equality of (1570-Ts)/1670<.alpha.'<(2410-Ts)/630 according to the starting temperature Ts(°C) of the rapid cooling.
Description
60-289,320 STEEL STRIP COOLING METHOD
This invention relates to a method of cooling steel s-trips after continuous annealing of -the steel strips, and more particularly to a method o:E cooling steel strips capable of enabling cooling capacity of 05 roll cooling process to increase -to a maximum possible extent without detrimentally af:Eec-ting quality of s-teel products.
Various cooling systems have been suggested to rapidly cool steel strips after continuous annealing.
A roll cooling method among them, in which a steel strip is wound about cooling rolls so as to be in contact therewith is advantageous in that its cooling speed is sufficiently high with a relatively low cost of operation wi~hout any risk of oxida-tion on a surface of the strip.
In a cooling process of a steel strip by roll cooling at 650-400C after heating and keeping -the steel strip in continuous annealing, a plurality of cooling rolls are usually needed for a limitation of the heat transfer coefficient between -the steel strip and the rolls. In this case, if an unevenness in temperature in width directions of the steel strip occurs when the steel strip becomes in contact with the first cooling , roll about which the steel strip is wound, an exces-sively cooled portion of the steel strip will contract in its longi-tudinal direction. Such an excessively cooled condition oE the steel strip is enhanced by o~ further roll cooling, so that -the unevenness in temperature in -the width directions is further increased.
Such a phenomenon detrimentally af~ects figure of a steel strip to cause serpen-tine movements of the steel lo strip, which obstruct stable passing of the steel strip through the heat-treatment passage and make difficult the contact between the cooling rolls and the steel strip wound thereabout so as to lower the cooling capacity o~ the cooling rolls.
Moreover, the unevenness in temperature in the width directions of the steel s-trip causes compressive stresses in the width directions on a surface of the steel strip in contact with the cooling rolls with a risk of buckling of the steel strip. If the buckling ~o exceeds a certain value, the steel strip may undergo violent serpen-tine movements resulting in breaking down o~ the steel strip in a furnace.
Japanese Laid-open Patent Application No. 59-129737 discloses a feature of providing gas ~6 jetting means in opposition to cooling rolls, capable of controlling cooling intensity in width directions of a steel strip subjected to the roll cooling, thereby reducing the unevenness in temperature in the width directions of the steel strip to prevent defects in figure of the steel strip. However, it does not solve 05 the problem of the uneven cooling suficiently because of low contact pressure between the steel strip and the cooling rolls resulting from a slight tensile force in the steel strip of -the order of less than 0.8 kgf/mm2 in order to prevent the buckling of the s-teel strip in high temperature zones in a usual con-tinuous annealing installation.
Moreover, Japanese Laid-open Patent Application No. 59-20428 discloses a feature of cooling a steel strip under tension of 2-5 kgf/mm2 with more than two 1~ rolls. If even a slight fault of contact in width directions of the steel strip occurs, a large tensile force concentration locally occurs to give rise to a plastic deformation of the steel strip and an exceed-ing defect in figure of the steel strip causing large serpentine movements of the s-teel strip to decrease the production capacity. Moreover 7 motor power for bridle rolls located upstream and downstream of the cooling rolls increases and a large number ~of bridle rolls are required because of a usual restriction of coefficient of friction between the steel strip and bridle rolls to increase a space for the bridle rolls resulting in --an increased ini-tial cost.
It is a primary objec-t of the invention to provide an improved method of cooling s-teel strips in roll cooling, which eliminates all the disadvantages of the 05 prior art and is capable of preventing the uneven cooling in wid-th directions of a steel strip to a minimum possible e~tent to effectively restrict defec-ts in figure of the steel strip and inexpensively exhibiting the cooling capacity to a maximum possible 1~ extent.
In order to achieve this object, in a method of cooling steel strips heated and maintaine~ in a con-tinuous heat-treatment furnace, according to the invention a rapid cooling of a steel strip by winding it about a plurality of cooling rolls in succession to bring it into contact therewith is started at a tempera-ture of a range of 550-720C and simultaneously a tensile force a (kgf/mm2) of the steel strip is adjusted within a range determined by an inequality of ~1900-Ts)/1670<a~(1980-Ts)/720 according to the starting temperature Ts(C) of said rapid cooling.
In another aspect of the invention, a rapid cooling of a steel strip by winding it about a plurality of cooling rolls in succession to bring it into contact therewith and at the same time by jetting a gas against a back surface of -the steel strip is started at ~ 7 ~
a -temperature of a range of 550-720C and simultaneously a -tensile force a' (kgf/mm2) of -the steel strip is adjusted within a range determined by an inequality of (1570-Ts)/1670<a'<~2410-TS)/630 according to -the 05 starting temperature Ts(C) of said rapid cooling.
The inven-tion will be more fully understood by referring to -the following detailed specification and claims taken in connection with -the appended drawings.
Fig. 1 is a schematic view illustrating a cooling o zone in a continuous heat-treatment furnace for carrying out the method according -to the invention;
Fig. 2 is a graph illustrating one example of a heat cycle in a continuous annealing line;
Fig. 3 is a graph illustrating an influence of tensile force of steel strips in a cooling zone on unevenness of the s-teel strips;
Fig. 4 is an explanatory view illustrating variation in tensile force of a steel strip due to heat crowing of a cooling roll;
Fig. 5a is a view illustrating change in contact area between a steel strip and cooling rolls in case of average tensile force of the steel strip of 2 kgf/mm2;
Fig. 5b is a view similar to Fig. 5a but in case of average tensile force of 3 kgf/mm2;
Fig. 6 is a graph illustrating relation between steel strip temperature and yield stress;"
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Fig. 7 is a graph illustrating an influence of tensile force of steel strips and starting temperature of roll cooling on the figure of cooled steel strips, Fig. 8 is a graph illustrating relation hetween 05 tensile force of steel strips in a roll cooling zone and cost of bridle rolls;
Fig. 9 is a view illustrating tensile force distribution of steel strips in a heat-treatment furnace; and Fig. 10 is a schematic .view illustrating influence of tensile force of steel strips on rate of inferior quality steel strip in figure and rate of capacity decline due to steel strip serpentine movement.
Fig. 1 schematically illustrates a continuous heat-treatment furnace for steel strips 1 suitable for carrying out the cooling process including the rapid cooling above described. The furnace comprises in a cooling zone 2 deflector rolls 3, 7, 10, 21 and 25, upstream bridle rolls 4, 5 and 6, downstream bridle ~o rolls 22, 23 and 24, gas jetting means 8 and 9, a plurality of cooling rolls 11~ 12, 13, 14 and 15 for the roll cooling process and gas jetting means 16, 17, 18, 19 and 20 for assisting the roll cooling with the cooling rolls 11 -15.
Fig. 2 illustra-tes a typical heat cycle in the roll cooling process. In this heat-treatment, a steel ~ 7 ~
strip is main-tained soaked at 700-800C iII a soaking zone not shown in Fig. 1 and then gradually cooled to a tempera-ture between approxima-tely 6~0C and an Al transformation point (723C) of the strip in a cooling o~ zone upstream of the rapid cooling ~one 2 shown in Fig. 1. The steel strip 1 is then cooled by jet~ing a gas from the gas jetting means 8 and 9 shown in Fig. 1. Thereafter, the steel strip 1 is extended around the plurality of cooling rolls 11-15 so as to be lo in contact therewith and is simultaneously subjected to the jetting cooling gas from the gas jet~ing means 16-20, so that the steel strip i5 rapidly coolsd to about 400C. After the steel strip 1 is then maintained at this temperature for overaging the strip, it is fed out of the heat-treatment system.
The temperature Ts at which the rapid cooling by the roll cooling is started has been slightly explained referring to ~ig. 2 in connection with the gas jetting means 8 and 9 located upstream oE the cooling rolls.
~o In rapid cooling only by the roll cooling without using gas jetting means, if the starting temperature Ts of the rapid cooling is lower than ~0C, defects in figure of strips would often occur depending upon materials and sizes of the strips. By previous applying a first 2~ cooling by means of the gas jet-ting means 8 and 9 in this case, a starting temperature Ts of the rapid cooling lower than 600-550C can be acceptable without a risk of the defects in figure of strips. However, an excessively low starting tempera-~ure such as lower than 550C would cause a requirement of an unduly oo widened heating zone of -the gas jetting means 8 and 9 as the previous cooling resulting in increased electric power cost. Accordingly, -the temperature range of 550-720C is applicable for the starting temperature Ts of rapid cooling with the roll cooling.
The inventors carried out an experiment on the cooling of steel strips having thicknesses of 0.5-1.2 mm and widths of 800-1200 mm under tension 0.3-4 kgf/mm2 between the bridle rolls 4-6 and 22-24 with starting cooling ~empera~ure Ts of 450-750C by means of the roll t~ cooling system shown in Fig. 1. Fig. 3 illustra-tes observed results of strip figures at the termination of cooling with the starting temperature Ts of 650C
wherein results of only the roll cooling ~case A) are shown in solid lines~ and results of addition of gas jetting onto back surfaces of the strips (case B~ in broken lines.
~ s can be seen from the results, $he low tension of the steel strips in the proximity of 0,5 kgf/mm2 causes uneven cooling which detrimentally affects the figure of strips because of lower contact pressure of the rolls with the strips. On the other hand, as the .
tension increases, the uneven cooling is greatly preven-ted with -the tension within 0.8-2 kgf/mm2 of the case A and wi-thin 0.6-3 ~gf/mm2 of the case B~ so that unevenness of -the steel strips is less than 1~ in both 05 cases. On the other hand9 if the tension further increases, it again adversely affects the figure of strips.
It is considered that the reason why such an excessive tension detrimentally affects the figure of 0 strips is as follows. Heat transmission from the strip to the roll in contact therewith causes thermal crown of the roll in the center of the roll as shown by shaded area in Fig. 4 where the steel strip tends to be excessively cooled resulting in longitudinal contraction of the strip. The continuous roll cooling further increases such an excessive cooling. Fig. 5a illus-trates the change in contact area between the steel strip and the cooling rolls in case of uneven cooling and Fig. 5b is in case that the uneven cooling does not occur. The decrease in contact area a-t the respective cooling rolls 11-15 causes tensile stress concentration at the excessively cooled portions of the strip, so that when average tension is more than 2 kgf/mm2 in the case A and 3 kgf/mm2 in the case B, it becomes in excess of yield st~ess of the steels trip (5-6 kgf/mm2) at a temperature of the order of 650C shown in Fig. 6.
.
Accordingly, such a tensile force causes plas-tic deformation of -the strip which will be further enhanced by the subsequent roll cooling.
Fig. 7 illustrates threshold values for causing defects in figure of steel strips (more -than 1% of the unevenness) with the rapid cooling starting temperatures Ts of 400-750C in relation to tensile forces of the strips, wherein solid lines are for the case A and broken lines are for the case B. As the star-ting temperature Ts is higher, -the strips are prone to defects in figure. On the other hand, as the starting temperature Ts is lower, the range of the tensile force not causing defects of figure becomes wider. Such threshold values are experimentally ascertained in this manner.
According to the threshold values in Fig. 7, the tensile force not causing defects in figure with the starting temperature of 550-720C is approximately obtained as follows.
Ts - 1900 Ts - 1980 < a < mcaseofA
Ts - 1570 Ts-2410 < Cl' C in case ~B
In case of preventing cooling buckle particularly using the first cooling by jetting -the gas from the gas jetting means 8 and 9 as shown in Fig. l, the starting temperature for the rapid cooling is within 550~600C.
In this case, the zone causing the defects in figure is substantially the same as that shown in Fig. 7.
Fig. 8 illustrates the relation between the tensile forces of steel strips and cost of bridle rolls to be arranged upstream and downstream of cooling rolls in order to apply the tensile forces to the steels trips.
This graph shows the increase of ~he cost of bridle rolls with increase of the tensile forces in comparison with the cost of bridle rolls (indicated by an index lO0) required to cause the tensile force of l.0 kgf/mm2 acting upon the steel strips. As shown in Fig. 8, the cost of the bridle rolls is rapidly increased in order to increase the tensile force of the steel strips to more than 3 kgf/mm2, because of not only an increase of power for bridle rolls but also considerably bulky bridle rolls requiring a great space for settling such bulky bridle rolls.
According to -the invention, the starting temperature of the rapid cooling of a strip winding around and in contac~ with a plurality of rolls is limited to 550-720C. The lower limitation of the 05 starting temperature is defined in order to avoid the disadvantage of increase power cost due to undue increase of the gas jetting cooling zone for unduly lowering the temperature to 550C. The upper limitation of the starting temperature is defined in order to avoid lo the risk of defects of figure of steel strip at a temperature higher than 720C.
According to the invention9 moreover, the tensile force a (kgf/mm2) of the steel strip to be cooled only by roll cooling without je~ting the gas against -the back 1~ surface of the strip is defined within the following range.
(l900-Ts)/1670 < a <(1980-Ts)/720 According -to the invention, furthermore, the tensile force a' ~kgf/mm2) of the steel strip to be ~o cooled by roll cooling and hy jetting the gas against the back surface o-f the strip is defined within the following range.
(1570-Ts)/1670 < a' < (2410-Ts)/630 The reason why such ranges of the tensile forces are defined is that such ranges are absolutely necessary to restrain the unevenness of steel strip within 1~ 9 .
which is defect in figure of the steel strip as already explained by referring to Fig. 7.
Example l Steel s-trips 1 heated, soaked and gradually cooled 05 were introduced into the rapid cooling zone 2 as shown in Fig. 1. In the rapid cooling zone 2, the cooling rolls 11-15 were operated and the bridle rolls 4-6 and 22-24 upstream and downstream thereof were operated in order to increase the tensile force of the steel strips in contact with the cooling rolls. The gas jetting chambers 16-20 were inoperative. The operating conditions are shown in Table 1.
t5 - lg -Table 1 Thickncss of strip (mm) 0.8 1.0 Width of strip (mm) llO0 1200 Feeding speed of strip (m/min) 200 150 Tensile force of s-teel strip in 1 5 1 0 rap.id cooling zone ~kgf/mm2) .
Strip temperature on entry side 670 700 of rapid cooling zone ( C) Starting temperature of rapid 600 625 cooling by roll cooling (C) Strip tempera-ture on rapid 400 400 cooling zone side ( C) Number of cooling rolls 5 Diameter of cooling rolls (mm) 1500 1500 Average winding angle () 90 90 . . . ................ . .. ., ~
Figure of cooled strip good good In both the cases I and II, figures of the cooled strips were good. By defining the tensile force kgf/mm2 of steel strips in the rol]. cooling zone as the following inequality, a rate of steel strips of inferior quality due to defect of figure was lowered to less than 0.5%.
Ts - lsoo Tc-19oo Moreover, the capacity decline of the opera-tion resulting from serpentine movements of steel strips due to defects of figure was fairly prevented.
Example 2 Steel strips 1 heated, soaked and gradually cooled were charged into the rapid cooling zone 2.
In the rapid cooling zone, there were provided cooling rolls 11-15, gas jetting chambers 16-20 in opposition thereto and bridle rolls 4-6 and 22-24 upstream and downstream of the gas je-tting chamhers in order ~o increase tensile forces of the steel strip in contact with the cooling rolls.
~ s shown in Fig. 9, the strips were fed in the heating and soaking zones under tensile force of the order of 0.7 kgf/mm2. The tensile force was then increased from 0.7 kgf/mm2 to 2.7 kgf/mm2 according to thicknesses and widths of th0 strips by means of the fron~ and rear bridle rolls 4-6 and 22-24. The steel strips were then subjected to the first cooling by the previous gas jetting means 8 and 9 and thereafter the strips were extended around the cooling rolls 11-15 to ~z~
be cooled and simultaneously cooled on their back sides by means of the gas jetting chambers 16-20 arranged in opposition to the cooling rolls 11-15. the tensile force was lowered to value of the order of 0.7 kgf/mm2 05 by the rear bridle rolls 22-24.
In this manner it was possible to cool the steel strips uniformly without detrimentally affecting figures o~ the steel strips. The operating conditions are shown in Table 2.
1~
' ' : :
Table 2 _ Thickness of s-trip (mm) 0.8 Width of strip (mm) 1100 Feeding speed of strip (m/min) 200 Tensile force of steel strip in rapid 2 0 cooling zone (kgf/mm2) _ _ Strip temperature on entry side of 670 rapid cooling zone (C) Starting temperature of rapid cooling 600 by roll cooling (C) _ Strip temperature on rapid cooling zone 400 Number of cooling rolls S
Delivery pressure of gas jetting 240 against back side of strip (mmH2O) Average heat transfer coefficient of roll cooling and gas jetting 200 (kcal/m2h C) Average winding angle () 60 . _, ,, . .._ _ . . =.,, Figure of cooled strip good Fig. 10 illustrates the effects of the present invention in the two cases, one of which uses only the ~v~
roll cooling and -the other of which uses bo-th -the roll cooling and the gas jetting. In -the Example 2, particularly, by defining -the -tensile force a' (kgf/mm2) of the steel strips in the roll cooling zone as the following inequality, a rate of steel strips of inferior quality due to defect of figure was lowered to less than 0.5~ and the capacity decline of the operation resulting from serpentine movements of steel strips due to defects of igure was significantly prevented.
~rS-- 1570 ~rS-2~10 As can be seen from -the above description, the cooling capacity in roll cooling of steel strips can be exhibited to the maximum possible limitation withou-t causing any defect of figure of strips in their cooling after continuous annealing by roll cooling.
While the inven-tion has been par-ticularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without depar-ting from the spirit and scope of the invention.
.
This invention relates to a method of cooling steel s-trips after continuous annealing of -the steel strips, and more particularly to a method o:E cooling steel strips capable of enabling cooling capacity of 05 roll cooling process to increase -to a maximum possible extent without detrimentally af:Eec-ting quality of s-teel products.
Various cooling systems have been suggested to rapidly cool steel strips after continuous annealing.
A roll cooling method among them, in which a steel strip is wound about cooling rolls so as to be in contact therewith is advantageous in that its cooling speed is sufficiently high with a relatively low cost of operation wi~hout any risk of oxida-tion on a surface of the strip.
In a cooling process of a steel strip by roll cooling at 650-400C after heating and keeping -the steel strip in continuous annealing, a plurality of cooling rolls are usually needed for a limitation of the heat transfer coefficient between -the steel strip and the rolls. In this case, if an unevenness in temperature in width directions of the steel strip occurs when the steel strip becomes in contact with the first cooling , roll about which the steel strip is wound, an exces-sively cooled portion of the steel strip will contract in its longi-tudinal direction. Such an excessively cooled condition oE the steel strip is enhanced by o~ further roll cooling, so that -the unevenness in temperature in -the width directions is further increased.
Such a phenomenon detrimentally af~ects figure of a steel strip to cause serpen-tine movements of the steel lo strip, which obstruct stable passing of the steel strip through the heat-treatment passage and make difficult the contact between the cooling rolls and the steel strip wound thereabout so as to lower the cooling capacity o~ the cooling rolls.
Moreover, the unevenness in temperature in the width directions of the steel s-trip causes compressive stresses in the width directions on a surface of the steel strip in contact with the cooling rolls with a risk of buckling of the steel strip. If the buckling ~o exceeds a certain value, the steel strip may undergo violent serpen-tine movements resulting in breaking down o~ the steel strip in a furnace.
Japanese Laid-open Patent Application No. 59-129737 discloses a feature of providing gas ~6 jetting means in opposition to cooling rolls, capable of controlling cooling intensity in width directions of a steel strip subjected to the roll cooling, thereby reducing the unevenness in temperature in the width directions of the steel strip to prevent defects in figure of the steel strip. However, it does not solve 05 the problem of the uneven cooling suficiently because of low contact pressure between the steel strip and the cooling rolls resulting from a slight tensile force in the steel strip of -the order of less than 0.8 kgf/mm2 in order to prevent the buckling of the s-teel strip in high temperature zones in a usual con-tinuous annealing installation.
Moreover, Japanese Laid-open Patent Application No. 59-20428 discloses a feature of cooling a steel strip under tension of 2-5 kgf/mm2 with more than two 1~ rolls. If even a slight fault of contact in width directions of the steel strip occurs, a large tensile force concentration locally occurs to give rise to a plastic deformation of the steel strip and an exceed-ing defect in figure of the steel strip causing large serpentine movements of the s-teel strip to decrease the production capacity. Moreover 7 motor power for bridle rolls located upstream and downstream of the cooling rolls increases and a large number ~of bridle rolls are required because of a usual restriction of coefficient of friction between the steel strip and bridle rolls to increase a space for the bridle rolls resulting in --an increased ini-tial cost.
It is a primary objec-t of the invention to provide an improved method of cooling s-teel strips in roll cooling, which eliminates all the disadvantages of the 05 prior art and is capable of preventing the uneven cooling in wid-th directions of a steel strip to a minimum possible e~tent to effectively restrict defec-ts in figure of the steel strip and inexpensively exhibiting the cooling capacity to a maximum possible 1~ extent.
In order to achieve this object, in a method of cooling steel strips heated and maintaine~ in a con-tinuous heat-treatment furnace, according to the invention a rapid cooling of a steel strip by winding it about a plurality of cooling rolls in succession to bring it into contact therewith is started at a tempera-ture of a range of 550-720C and simultaneously a tensile force a (kgf/mm2) of the steel strip is adjusted within a range determined by an inequality of ~1900-Ts)/1670<a~(1980-Ts)/720 according to the starting temperature Ts(C) of said rapid cooling.
In another aspect of the invention, a rapid cooling of a steel strip by winding it about a plurality of cooling rolls in succession to bring it into contact therewith and at the same time by jetting a gas against a back surface of -the steel strip is started at ~ 7 ~
a -temperature of a range of 550-720C and simultaneously a -tensile force a' (kgf/mm2) of -the steel strip is adjusted within a range determined by an inequality of (1570-Ts)/1670<a'<~2410-TS)/630 according to -the 05 starting temperature Ts(C) of said rapid cooling.
The inven-tion will be more fully understood by referring to -the following detailed specification and claims taken in connection with -the appended drawings.
Fig. 1 is a schematic view illustrating a cooling o zone in a continuous heat-treatment furnace for carrying out the method according -to the invention;
Fig. 2 is a graph illustrating one example of a heat cycle in a continuous annealing line;
Fig. 3 is a graph illustrating an influence of tensile force of steel strips in a cooling zone on unevenness of the s-teel strips;
Fig. 4 is an explanatory view illustrating variation in tensile force of a steel strip due to heat crowing of a cooling roll;
Fig. 5a is a view illustrating change in contact area between a steel strip and cooling rolls in case of average tensile force of the steel strip of 2 kgf/mm2;
Fig. 5b is a view similar to Fig. 5a but in case of average tensile force of 3 kgf/mm2;
Fig. 6 is a graph illustrating relation between steel strip temperature and yield stress;"
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Fig. 7 is a graph illustrating an influence of tensile force of steel strips and starting temperature of roll cooling on the figure of cooled steel strips, Fig. 8 is a graph illustrating relation hetween 05 tensile force of steel strips in a roll cooling zone and cost of bridle rolls;
Fig. 9 is a view illustrating tensile force distribution of steel strips in a heat-treatment furnace; and Fig. 10 is a schematic .view illustrating influence of tensile force of steel strips on rate of inferior quality steel strip in figure and rate of capacity decline due to steel strip serpentine movement.
Fig. 1 schematically illustrates a continuous heat-treatment furnace for steel strips 1 suitable for carrying out the cooling process including the rapid cooling above described. The furnace comprises in a cooling zone 2 deflector rolls 3, 7, 10, 21 and 25, upstream bridle rolls 4, 5 and 6, downstream bridle ~o rolls 22, 23 and 24, gas jetting means 8 and 9, a plurality of cooling rolls 11~ 12, 13, 14 and 15 for the roll cooling process and gas jetting means 16, 17, 18, 19 and 20 for assisting the roll cooling with the cooling rolls 11 -15.
Fig. 2 illustra-tes a typical heat cycle in the roll cooling process. In this heat-treatment, a steel ~ 7 ~
strip is main-tained soaked at 700-800C iII a soaking zone not shown in Fig. 1 and then gradually cooled to a tempera-ture between approxima-tely 6~0C and an Al transformation point (723C) of the strip in a cooling o~ zone upstream of the rapid cooling ~one 2 shown in Fig. 1. The steel strip 1 is then cooled by jet~ing a gas from the gas jetting means 8 and 9 shown in Fig. 1. Thereafter, the steel strip 1 is extended around the plurality of cooling rolls 11-15 so as to be lo in contact therewith and is simultaneously subjected to the jetting cooling gas from the gas jet~ing means 16-20, so that the steel strip i5 rapidly coolsd to about 400C. After the steel strip 1 is then maintained at this temperature for overaging the strip, it is fed out of the heat-treatment system.
The temperature Ts at which the rapid cooling by the roll cooling is started has been slightly explained referring to ~ig. 2 in connection with the gas jetting means 8 and 9 located upstream oE the cooling rolls.
~o In rapid cooling only by the roll cooling without using gas jetting means, if the starting temperature Ts of the rapid cooling is lower than ~0C, defects in figure of strips would often occur depending upon materials and sizes of the strips. By previous applying a first 2~ cooling by means of the gas jet-ting means 8 and 9 in this case, a starting temperature Ts of the rapid cooling lower than 600-550C can be acceptable without a risk of the defects in figure of strips. However, an excessively low starting tempera-~ure such as lower than 550C would cause a requirement of an unduly oo widened heating zone of -the gas jetting means 8 and 9 as the previous cooling resulting in increased electric power cost. Accordingly, -the temperature range of 550-720C is applicable for the starting temperature Ts of rapid cooling with the roll cooling.
The inventors carried out an experiment on the cooling of steel strips having thicknesses of 0.5-1.2 mm and widths of 800-1200 mm under tension 0.3-4 kgf/mm2 between the bridle rolls 4-6 and 22-24 with starting cooling ~empera~ure Ts of 450-750C by means of the roll t~ cooling system shown in Fig. 1. Fig. 3 illustra-tes observed results of strip figures at the termination of cooling with the starting temperature Ts of 650C
wherein results of only the roll cooling ~case A) are shown in solid lines~ and results of addition of gas jetting onto back surfaces of the strips (case B~ in broken lines.
~ s can be seen from the results, $he low tension of the steel strips in the proximity of 0,5 kgf/mm2 causes uneven cooling which detrimentally affects the figure of strips because of lower contact pressure of the rolls with the strips. On the other hand, as the .
tension increases, the uneven cooling is greatly preven-ted with -the tension within 0.8-2 kgf/mm2 of the case A and wi-thin 0.6-3 ~gf/mm2 of the case B~ so that unevenness of -the steel strips is less than 1~ in both 05 cases. On the other hand9 if the tension further increases, it again adversely affects the figure of strips.
It is considered that the reason why such an excessive tension detrimentally affects the figure of 0 strips is as follows. Heat transmission from the strip to the roll in contact therewith causes thermal crown of the roll in the center of the roll as shown by shaded area in Fig. 4 where the steel strip tends to be excessively cooled resulting in longitudinal contraction of the strip. The continuous roll cooling further increases such an excessive cooling. Fig. 5a illus-trates the change in contact area between the steel strip and the cooling rolls in case of uneven cooling and Fig. 5b is in case that the uneven cooling does not occur. The decrease in contact area a-t the respective cooling rolls 11-15 causes tensile stress concentration at the excessively cooled portions of the strip, so that when average tension is more than 2 kgf/mm2 in the case A and 3 kgf/mm2 in the case B, it becomes in excess of yield st~ess of the steels trip (5-6 kgf/mm2) at a temperature of the order of 650C shown in Fig. 6.
.
Accordingly, such a tensile force causes plas-tic deformation of -the strip which will be further enhanced by the subsequent roll cooling.
Fig. 7 illustrates threshold values for causing defects in figure of steel strips (more -than 1% of the unevenness) with the rapid cooling starting temperatures Ts of 400-750C in relation to tensile forces of the strips, wherein solid lines are for the case A and broken lines are for the case B. As the star-ting temperature Ts is higher, -the strips are prone to defects in figure. On the other hand, as the starting temperature Ts is lower, the range of the tensile force not causing defects of figure becomes wider. Such threshold values are experimentally ascertained in this manner.
According to the threshold values in Fig. 7, the tensile force not causing defects in figure with the starting temperature of 550-720C is approximately obtained as follows.
Ts - 1900 Ts - 1980 < a < mcaseofA
Ts - 1570 Ts-2410 < Cl' C in case ~B
In case of preventing cooling buckle particularly using the first cooling by jetting -the gas from the gas jetting means 8 and 9 as shown in Fig. l, the starting temperature for the rapid cooling is within 550~600C.
In this case, the zone causing the defects in figure is substantially the same as that shown in Fig. 7.
Fig. 8 illustrates the relation between the tensile forces of steel strips and cost of bridle rolls to be arranged upstream and downstream of cooling rolls in order to apply the tensile forces to the steels trips.
This graph shows the increase of ~he cost of bridle rolls with increase of the tensile forces in comparison with the cost of bridle rolls (indicated by an index lO0) required to cause the tensile force of l.0 kgf/mm2 acting upon the steel strips. As shown in Fig. 8, the cost of the bridle rolls is rapidly increased in order to increase the tensile force of the steel strips to more than 3 kgf/mm2, because of not only an increase of power for bridle rolls but also considerably bulky bridle rolls requiring a great space for settling such bulky bridle rolls.
According to -the invention, the starting temperature of the rapid cooling of a strip winding around and in contac~ with a plurality of rolls is limited to 550-720C. The lower limitation of the 05 starting temperature is defined in order to avoid the disadvantage of increase power cost due to undue increase of the gas jetting cooling zone for unduly lowering the temperature to 550C. The upper limitation of the starting temperature is defined in order to avoid lo the risk of defects of figure of steel strip at a temperature higher than 720C.
According to the invention9 moreover, the tensile force a (kgf/mm2) of the steel strip to be cooled only by roll cooling without je~ting the gas against -the back 1~ surface of the strip is defined within the following range.
(l900-Ts)/1670 < a <(1980-Ts)/720 According -to the invention, furthermore, the tensile force a' ~kgf/mm2) of the steel strip to be ~o cooled by roll cooling and hy jetting the gas against the back surface o-f the strip is defined within the following range.
(1570-Ts)/1670 < a' < (2410-Ts)/630 The reason why such ranges of the tensile forces are defined is that such ranges are absolutely necessary to restrain the unevenness of steel strip within 1~ 9 .
which is defect in figure of the steel strip as already explained by referring to Fig. 7.
Example l Steel s-trips 1 heated, soaked and gradually cooled 05 were introduced into the rapid cooling zone 2 as shown in Fig. 1. In the rapid cooling zone 2, the cooling rolls 11-15 were operated and the bridle rolls 4-6 and 22-24 upstream and downstream thereof were operated in order to increase the tensile force of the steel strips in contact with the cooling rolls. The gas jetting chambers 16-20 were inoperative. The operating conditions are shown in Table 1.
t5 - lg -Table 1 Thickncss of strip (mm) 0.8 1.0 Width of strip (mm) llO0 1200 Feeding speed of strip (m/min) 200 150 Tensile force of s-teel strip in 1 5 1 0 rap.id cooling zone ~kgf/mm2) .
Strip temperature on entry side 670 700 of rapid cooling zone ( C) Starting temperature of rapid 600 625 cooling by roll cooling (C) Strip tempera-ture on rapid 400 400 cooling zone side ( C) Number of cooling rolls 5 Diameter of cooling rolls (mm) 1500 1500 Average winding angle () 90 90 . . . ................ . .. ., ~
Figure of cooled strip good good In both the cases I and II, figures of the cooled strips were good. By defining the tensile force kgf/mm2 of steel strips in the rol]. cooling zone as the following inequality, a rate of steel strips of inferior quality due to defect of figure was lowered to less than 0.5%.
Ts - lsoo Tc-19oo Moreover, the capacity decline of the opera-tion resulting from serpentine movements of steel strips due to defects of figure was fairly prevented.
Example 2 Steel strips 1 heated, soaked and gradually cooled were charged into the rapid cooling zone 2.
In the rapid cooling zone, there were provided cooling rolls 11-15, gas jetting chambers 16-20 in opposition thereto and bridle rolls 4-6 and 22-24 upstream and downstream of the gas je-tting chamhers in order ~o increase tensile forces of the steel strip in contact with the cooling rolls.
~ s shown in Fig. 9, the strips were fed in the heating and soaking zones under tensile force of the order of 0.7 kgf/mm2. The tensile force was then increased from 0.7 kgf/mm2 to 2.7 kgf/mm2 according to thicknesses and widths of th0 strips by means of the fron~ and rear bridle rolls 4-6 and 22-24. The steel strips were then subjected to the first cooling by the previous gas jetting means 8 and 9 and thereafter the strips were extended around the cooling rolls 11-15 to ~z~
be cooled and simultaneously cooled on their back sides by means of the gas jetting chambers 16-20 arranged in opposition to the cooling rolls 11-15. the tensile force was lowered to value of the order of 0.7 kgf/mm2 05 by the rear bridle rolls 22-24.
In this manner it was possible to cool the steel strips uniformly without detrimentally affecting figures o~ the steel strips. The operating conditions are shown in Table 2.
1~
' ' : :
Table 2 _ Thickness of s-trip (mm) 0.8 Width of strip (mm) 1100 Feeding speed of strip (m/min) 200 Tensile force of steel strip in rapid 2 0 cooling zone (kgf/mm2) _ _ Strip temperature on entry side of 670 rapid cooling zone (C) Starting temperature of rapid cooling 600 by roll cooling (C) _ Strip temperature on rapid cooling zone 400 Number of cooling rolls S
Delivery pressure of gas jetting 240 against back side of strip (mmH2O) Average heat transfer coefficient of roll cooling and gas jetting 200 (kcal/m2h C) Average winding angle () 60 . _, ,, . .._ _ . . =.,, Figure of cooled strip good Fig. 10 illustrates the effects of the present invention in the two cases, one of which uses only the ~v~
roll cooling and -the other of which uses bo-th -the roll cooling and the gas jetting. In -the Example 2, particularly, by defining -the -tensile force a' (kgf/mm2) of the steel strips in the roll cooling zone as the following inequality, a rate of steel strips of inferior quality due to defect of figure was lowered to less than 0.5~ and the capacity decline of the operation resulting from serpentine movements of steel strips due to defects of igure was significantly prevented.
~rS-- 1570 ~rS-2~10 As can be seen from -the above description, the cooling capacity in roll cooling of steel strips can be exhibited to the maximum possible limitation withou-t causing any defect of figure of strips in their cooling after continuous annealing by roll cooling.
While the inven-tion has been par-ticularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without depar-ting from the spirit and scope of the invention.
.
Claims (6)
1. A method of cooling steel strips heated and maintained in a continuous heat-treatment furnace, wherein a rapid cooling of a steel strip by winding it about a plurality of cooling rolls in succession to bring it into contact therewith is started at a temperature of a range of 550-720°C and simultaneously a tensile force .alpha. (kgf/mm2) of the steel strip is adjusted within a range determined by an inequality of (1900-Ts)/1670<.alpha.<(1980-Ts)/720 according to the starting temperature Ts(°C) of said rapid cooling.
2. A method of cooling steel strips as set forth in claim 1, wherein said starting temperature of the rapid cooling is within a range of 720-640°C and the tensile force of the steel strip is within a range of 0.8-2 kgf/mm2.
3. A method of cooling steel strips heated and maintained in a continuous heat-treatment furnace, wherein a rapid cooling of a steel strip by winding it about a plurality of cooling rolls in succession to bring it into contact therewith and at the same time by jetting a gas against a back surface of the steel strip is started at a temperature of a range of 550-720°C and simultaneously a tensile force .alpha.' (kgf/mm2) of the steel strip is adjusted within a range determined by an inequality of (1570-Ts)/1670<.alpha.'<(2410-Ts)/630 according to the starting temperature Ts(°C) of said rapid cooling.
4. A method of cooling steel strips as set forth in claim 3, wherein said starting temperature of the rapid cooling is within a range of 600-550°C and the tensile force of the steel strip is within a range of 0.6-3 kgf/mm2.
5. A method of cooling steel strips as set forth in claim 1 or 3, wherein before the steel strip is cooled by the cooling rolls, the steel strip is cooled by jetting a gas against the steel strip.
6. A method of cooling steel strips as set forth in claim 1 or 3, wherein the tensile force of the steel strips is adjusted by bridle rolls arranged upstream and downstream of the cooling rolls.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP60289320A JPS62149820A (en) | 1985-12-24 | 1985-12-24 | Method for cooling steel strip |
| JP289,320/85 | 1985-12-24 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1281974C true CA1281974C (en) | 1991-03-26 |
Family
ID=17741662
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000525985A Expired - Lifetime CA1281974C (en) | 1985-12-24 | 1986-12-22 | Steel strip cooling method |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0230780A1 (en) |
| JP (1) | JPS62149820A (en) |
| KR (1) | KR930005069B1 (en) |
| CA (1) | CA1281974C (en) |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2592175B2 (en) * | 1990-07-31 | 1997-03-19 | 日本鋼管株式会社 | Strip cooling device |
| JPH0568363U (en) * | 1991-11-07 | 1993-09-17 | 株式会社ダイシン | Folding wagon |
| KR0159121B1 (en) * | 1992-06-23 | 1999-01-15 | 미요시 순기찌 | Metal band cooling apparatus and cooling method thereof |
| JP2011184773A (en) * | 2010-03-10 | 2011-09-22 | Kobe Steel Ltd | Continuous annealing apparatus, and method for suppressing corrugation deformation of metal sheet during quenching in the same |
| KR101977466B1 (en) * | 2017-05-25 | 2019-05-13 | 주식회사 포스코 | Apparatus for cooling strip |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2499591A1 (en) * | 1981-02-12 | 1982-08-13 | Stein Heurtey | DEVICE FOR QUICK COOLING AND CONTROLLED IN AN ANNEAL OR REDUCING ATMOSPHERE OVEN |
| JPS599130A (en) * | 1982-07-08 | 1984-01-18 | Kawasaki Steel Corp | Roll cooling method of steel strip |
| JPS5920428A (en) * | 1982-07-26 | 1984-02-02 | Nippon Kokan Kk <Nkk> | Cooling method of steel strip in continuous annealing furnace |
| JPS59143028A (en) * | 1983-02-03 | 1984-08-16 | Nippon Steel Corp | Cooler for metallic strip in continuous heat treating furnace |
| JPS60128219A (en) * | 1983-12-15 | 1985-07-09 | Mitsubishi Heavy Ind Ltd | Cooler for hoop |
| JPS60128220A (en) * | 1983-12-15 | 1985-07-09 | Mitsubishi Heavy Ind Ltd | Method for controlling temperature of strip in cooling zone of continuous annealing furnace |
| JPS60169524A (en) * | 1984-02-14 | 1985-09-03 | Mitsubishi Heavy Ind Ltd | Cooler for metallic strip |
-
1985
- 1985-12-24 JP JP60289320A patent/JPS62149820A/en active Granted
-
1986
- 1986-12-22 CA CA000525985A patent/CA1281974C/en not_active Expired - Lifetime
- 1986-12-23 KR KR1019860011139A patent/KR930005069B1/en not_active IP Right Cessation
- 1986-12-23 EP EP86310098A patent/EP0230780A1/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| KR930005069B1 (en) | 1993-06-15 |
| JPS62149820A (en) | 1987-07-03 |
| KR870006215A (en) | 1987-07-10 |
| JPH0414173B2 (en) | 1992-03-12 |
| EP0230780A1 (en) | 1987-08-05 |
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