JP3291188B2 - Manufacturing method of steel sheet with excellent toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel sheet having a good shape and excellent toughness.

[0002]

2. Description of the Related Art In order to obtain a steel sheet having excellent toughness at low temperatures,
Controlled rolling methods are becoming widely used.
This controlled rolling method accurately controls the rolling temperature during rolling.
And the final rolling finish temperature is Ar _ThreeThings just above the temperature
This makes the microstructure finer and increases toughness.
Wide improvement can be achieved.

[0003] The conventional controlled rolling method is shown in FIG.
As shown in the figure, a predetermined thickness with respect to the product thickness t
To temporarily stop the primary rolling (rough rolling) work and then air-cool
From the temperature T1 at that time, the target temperature T
After secondary cooling (finish rolling), the product
The finishing temperature at the time of final rolling when the plate thickness becomes t is Ar _ThreeWarm
(See Japanese Patent Application Laid-Open No. 50-984).
No. 21).

However, the control rolling disclosed in Japanese Patent Application Laid-Open No. 50-98421 is different from the normal rolling, and the rolling operation is temporarily stopped during the rolling, and the temperature of the steel material is reduced to T as shown in FIG.
Since cooling was performed by air cooling from 1 to T2, the cooling time was long, so that the rolling efficiency was reduced, the productivity was lowered, and the toughness was low.

[0005] Therefore, as proposed in Japanese Patent Application Laid-Open No. 60-56017, the cooling during the rolling of the steel material is forcibly cooled with cooling water, thereby shortening the cooling time and improving the productivity. There is a method for improving toughness.

However, in the forced cooling using the cooling water, it is difficult to uniformly cool the steel material in the longitudinal direction and the width direction depending on the cooling stop temperature, and the temperature of the steel material becomes uneven. Warpage is likely to occur at times, and the rolling operation is temporarily interrupted for this warpage to correct the warpage and then roll again, or the rolling operation must be stopped, and the rolling efficiency is poor. There was a problem that productivity was hindered, and the rolling work was troublesome, resulting in poor workability.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to roll a steel sheet having excellent toughness without the above-mentioned problems, with good productivity and while maintaining a good shape without warpage. Things.

[0008]

Means for Solving the Problems In order to achieve the above object, the present invention uses the following items. (1) 40 mm thick after primary rolling at 850 ° C or higher
The above steel material was forcibly cooled, and then subjected to secondary rolling to obtain 7
In the method for producing a steel sheet having excellent toughness by terminating the rolling at 20 to 850 ° C., when the secondary rolling is performed, the bite incidence angle and rolling shape ratio of the steel for each pass of the rolling are predicted. Then, it is determined from the predicted bite incidence angle and the rolling shape ratio whether any of the following applies to the following, and the above-mentioned two are determined so as to have a relationship between the determined peripheral speeds of the upper work roll and the lower work roll. Method for producing steel sheet with excellent toughness characterized by adjusting the peripheral speeds of the upper work roll and the lower work roll for each pass of the secondary rolling mill for performing the next rolling. > 1.5 upper roll peripheral speed <lower roll peripheral speed biting incident angle> 0 °, rolling shape ratio> 1.5 upper roll peripheral speed> lower roll peripheral speed biting incident angle <0 °, rolling shape ratio <case on a roll peripheral speed of 1.5 <lower roll peripheral speed bite incident angle> 0 °, rolling shape ratio If the roll peripheral speed of 1.5> lower roll peripheral speed, however, rolling shape ratio = (contact projection arc length / average thickness) (2) thickness 40mm completing the primary rolling at 850 ° C. or higher
The above steel material was forcibly cooled, and then subjected to secondary rolling to obtain 7
In the method for producing a steel sheet having excellent toughness by terminating the rolling at 20 to 850 ° C., when the secondary rolling is performed, the bite incidence angle and rolling shape ratio of the steel for each pass of the rolling are predicted. Then, from the predicted bite incidence angle and the rolling shape ratio, it is determined which one of the following applies, and the steel material of the steel material is so determined as to have a relationship between the average temperature of the upper surface and the lower surface of the determined steel material. Method for producing a steel sheet having excellent toughness characterized by adjusting the amount of forced cooling of the upper and lower surfaces. In the case of bite incidence angle <0 °, rolling shape ratio> 1.5 Average temperature of upper surface> Average temperature of lower surface When the incident angle> 0 ° and the rolling shape ratio> 1.5, the average temperature of the upper surface <the average temperature of the lower surface When the incident angle <0 ° and the rolling shape ratio <1.5, the average temperature of the upper surface <the average of the lower surface Temperature In the case of bite incidence angle> 0 ° and rolling shape ratio <1.5 Average temperature of upper surface> Average of lower surface Temperature However, rolling shape ratio = (contact projection arc length / average plate thickness) (3) Plate thickness 40 mm after primary rolling at 850 ° C. or more
The above steel material was forcibly cooled, and then subjected to secondary rolling to obtain 7
In the method for producing a steel sheet having excellent toughness by terminating the rolling at 20 to 850 ° C., when the secondary rolling is performed, the bite incidence angle and rolling shape ratio of the steel for each pass of the rolling are predicted. From the predicted bite incidence angle and the rolling shape ratio, it is determined which of the following applies: the average temperature of the upper surface and the lower surface of the determined steel material, the peripheral speed of the upper work roll and the lower work roll. So that the peripheral speed of the upper work roll and the peripheral speed of the lower work roll for each pass of the secondary rolling mill before the bite of the steel material and the forced cooling of the lower surface of the steel material are adjusted. Manufacturing method of steel sheet with excellent toughness In case of bite incident angle <0 °, rolling shape ratio> 1.5 Average temperature of upper surface> Average temperature of lower surface, and upper roll peripheral speed <lower roll peripheral speed In the case of biting incidence angle> 0 ° and rolling shape ratio> 1.5 Average of upper surface Degrees <on the lower surface of the average temperature, and the upper roll peripheral speed> lower roll peripheral speed bite angle of incidence <0 °, the average temperature of the average temperature <lower surface of the upper surface when the rolling shape ratio <1.5, and, above Roll circumference
Speed <lower roll peripheral speed bite incidence angle> 0 °, rolling shape ratio <1.5 Average temperature of upper surface> average temperature of lower surface and upper roll circumference
Speed> lower roll peripheral speed, where rolling shape ratio = (contact projection arc length / average plate thickness) When the steel material is forcibly cooled with cooling water during the primary rolling and the secondary rolling, the heat history due to the difference in the manufacturing process shown in FIG. As shown in the figure showing the difference, the average cooling rate in the thickness direction of the broken line portion is increased, and the high-temperature residence time is significantly reduced. Therefore, as shown in FIG. 3, the toughness is improved by about −30 ° C. in vTrs as compared with the case where air cooling is performed during rolling (the same manufacturing conditions except for cooling during rolling).

However, in the method of manufacturing a steel sheet by performing forced cooling after primary rolling and then performing secondary rolling again as described above, it is extremely difficult to uniformly cool the steel material in the longitudinal and width directions. Since this temperature difference triggers warpage during secondary rolling after forced cooling, it is difficult to ensure the shape of the steel material in a good state (a state without warpage) by secondary rolling after forced cooling. It is important to prevent warpage correction work before the next rolling and to complete the rolling work without causing loss time such as warpage correction.

Therefore, in order to examine the conditions of the secondary rolling after the forced cooling, rolling conditions (mesh incident angle, rolling shape ratio = contact arc length / average plate thickness, FIGS. 4 and 5 show the results of examining the state of warpage of the rolled material during the secondary rolling while changing the temperature difference between the upper and lower rolls and the upper and lower surfaces.

The contact projection length is

[0012]

(Equation 1) Is the value obtained in

The bite incidence angle is the angle α shown in FIG. 6, and is specifically obtained by observing the image directly from the side of the rolling mill with a camera and analyzing the image.

From FIG. 4 (1), when the upper roll peripheral speed> the lower roll peripheral speed and the rolling shape ratio> 1.5, the warpage occurs, and when the rolling shape ratio <1.5, the warpage occurs. From (2), it was found that when the upper roll peripheral speed <the lower roll peripheral speed and the rolling shape ratio> 1.5, the warpage was downward, and conversely, when the rolling shape ratio <1.5, the warpage was downward.

From FIG. 4 (3), when the angle of incidence is <0 and the rolling shape ratio is> 1.5, warpage occurs, and the rolling shape ratio <
In the case of 1.5, warpage occurs, and from FIG. 4 (4), in the case of a bite incidence angle> 0 ° and a rolling shape ratio> 1.5, warpage occurs.
It was found that when the rolling shape ratio was less than 1.5, warpage occurred.

From the above, if the angle of incidence is <0 ° and the rolling shape ratio is> 1.5, warping will occur as it is. Therefore, if the upper roll peripheral speed is smaller than the lower roll peripheral speed, Warpage can be prevented, bite incidence angle> 0 °, rolling shape ratio> 1.
In the case of No. 5, since downward warpage occurs as it is, it has been found that the downward warpage can be prevented by setting the upper roll peripheral speed> the lower roll peripheral speed.

On the other hand, when the bite incidence angle <0 °, the rolling shape ratio
In the case of <1.5, the upper roll peripheral speed <the lower roll peripheral speed
Can prevent roll-down, and can be rolled at a bite incidence angle> 0 °
When the shape ratio is <1.5, the warp direction is reversed and the upper roll
Speed> lower roll peripheral speed to prevent warpage.
Issued.

Further, FIG. 5 shows the relationship between the temperature difference between the upper and lower surfaces of the steel sheet during re-rolling and the warpage. It has been found that when the temperature difference of the lower surface (upper surface temperature−lower surface temperature) △ T> 0, warpage occurs, and when △ T <0, downward warpage occurs.

From this, as in the case of the means 2, when the bite incidence angle <0 ° and the rolling shape ratio> 1.5, the average temperature of the upper surface of the steel material during cooling during the secondary rolling> the average of the lower surface If the temperature is set, the warpage can be prevented, the bite incidence angle> 0 °,
Conversely, when the rolling shape ratio> 1.5, the average temperature of the upper surface < lower
It has been found that warpage can be prevented by setting the average temperature of the surface .

On the other hand, the bite incidence angle <0 ° and the rolling shape ratio <
In the case of 1.5, the average temperature of the upper surface <the average temperature of the lower surface and
By doing so, warpage can be prevented, biting incidence angle> 0 °, rolled type
When the shape ratio is less than 1.5, the warp direction is reversed,
It is possible to prevent downward warpage if the temperature> the average temperature of the lower surface
Was found.

The temperatures of the upper surface and the lower surface of the steel material are average values obtained by measuring the entire steel material before being bitten by a rolling mill during secondary rolling by a thermotracer.

Various studies have revealed that a better shape can be obtained by simultaneously controlling the peripheral speeds of the upper work roll and the lower work roll and controlling the temperature of the upper and lower surfaces of the steel material as in the means 3. .

As discussed above under various conditions, by optimally controlling the rolling conditions during the secondary rolling and properly controlling the average temperature difference between the upper and lower surfaces before the secondary rolling, the warpage during the rolling is reliably prevented. It has been found that it is possible to obtain an excellent effect of being able to do so.

That is, even if the bite incidence angle changes due to the shape of the minute end of the front portion / tail portion of the steel material and the operation change such as the length of the steel material and the bounce during transportation as shown in FIG. By adjusting the peripheral speed of the upper and lower rolls and the upper and lower surface temperatures of the steel material immediately before the rolling mill is engaged in the secondary rolling, a manufacturing technique for flattening the shape of the steel material during the secondary rolling was established. Also, if the thickness of the steel material to be forcibly cooled before the secondary rolling is 40 mm or less, the heat removal of the steel material by air cooling becomes faster, and the temperature waiting time during the control rolling is inevitably shortened. And air cooling have little difference in temperature waiting time. For this reason, the thickness of the target steel material was set to 40 mm or more.

The primary rolling temperature is set to 850 ° C. or higher because it is assumed that rolling is performed in a temperature range in which austenite is recrystallized. In addition, the secondary rolling end temperature is in the range of 720 to 850 ° C. because rolling in an unrecrystallized region is essential to improve toughness.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION

(Examples) Examples of the present invention are shown below together with conventional examples and comparative examples. The components of the test steels are those used as typical structural steels. Table 1 shows the chemical components of the steels used in this example.

Heating temperature and thickness of slab, rolling conditions for rough rolling as primary rolling, cooling at transfer thickness (plate thickness waiting for temperature to predetermined temperature after rough rolling), temperature waiting condition for secondary rolling, secondary rolling Table 2-1 and Table 2-2 show production conditions such as rolling conditions for finish rolling as described above and the production results. Table 3-1 and Table 3-2 and Table 3-3 show the finish rolling conditions.

Tables 2-1 and 2-2 and Tables 3-1 and 3-
As shown in 2, 3-3, steel numbers 1, 13, 2
Reference numeral 1 denotes an example corresponding to the means 1 (Claim 1-Adjustment of peripheral speed of upper and lower work rolls of a finishing mill).
Is an example corresponding to the means 2 (Claim 2-Adjustment of temperature difference between front and back of steel material), and steel numbers 5, 7, 9, 11, 15, 17, 19,
Is an example corresponding to the means 3 (Claim 3-Combination of the peripheral speed adjustment of the upper and lower work rolls of the finishing mill and the temperature difference adjustment of the front and back surfaces of the steel material). It was greatly shortened, no warpage occurred during finish rolling, no warpage correction was required, and good toughness was obtained.

On the other hand, steel Nos. 2, 6,
Nos. 8, 10, 12, 16 and 20 were air-cooled before rough rolling and finishing rolling.
In Nos. 2 and 16, the warpage of the steel material increased during the finish rolling, and the rolling was stopped halfway. In addition, although steel numbers 2, 6, 10, and 20 did not have to stop rolling halfway, the temperature waiting time at the transfer thickness became longer, and furthermore, the productivity of steel materials,
Both toughness deteriorated significantly. In addition, steel number 3, which is a comparative example,
Sample No. 18 was subjected to forced cooling with cooling water after rough rolling, but the rolling conditions during finish rolling were not appropriate. That is, although the bite incidence angle was negative and the rolling shape ratio was 1.5 or less, the circumference of the upper roll of the finishing mill was set to be higher than that of the lower roll. Rolling was stopped due to warpage of the steel material. Steel No. 18 required warpage correction after the first to third passes.

Further, Steel No. 14 had a positive bite incidence angle and the upper work roll of the finishing mill was higher in speed than the lower work roll even though the shape ratio was 1.5 or less. The rolling operation was interrupted after the second and fourth passes of finish rolling, and the warpage was corrected.

[0031]

[Table 1]

[0032]

[Table 2-1]

[0033]

[Table 2-2]

[0034]

[Table 3-1]

[0035]

[Table 3-2]

[0036]

[Table 3-3]

[0037]

As described above, according to the present invention,
A steel sheet having a good shape without warpage and having excellent toughness can be manufactured with high productivity and good rolling workability without waiting for useless temperatures, and the effect in this field is extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram showing a temperature transition of a steel sheet during rolling.

FIG. 2 is a diagram showing a difference in heat history due to a difference in a manufacturing process.

FIG. 3 is a view showing a toughness improving effect by cooling in the middle.

FIG. 4 is a diagram showing the relationship between rolling conditions and warpage during re-rolling.

FIG. 5 is a diagram showing a relationship between a difference in vertical temperature of a steel sheet and warpage during re-rolling.

FIG. 6 is a schematic diagram showing a bite incident angle.

FIG. 7 is a schematic diagram showing that the bite incidence angle changes as the length of the slab changes.

[Explanation of symbols]

 1 Steel 2 Upper work 3 Lower work

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 B21B 3/00 B21B 37/00-37/76

Claims (3)

(57) [Claims] 1. Thickness 4 after primary rolling at 850 ° C. or higher
In a method for producing a steel sheet having excellent toughness by forcibly cooling a steel material of 0 mm or more and then performing secondary rolling and terminating the rolling at 720 to 850 ° C., when the secondary rolling is performed, Predict the bite incidence angle and rolling shape ratio of the steel material for each pass of, and determine which of the following from the predicted bite incidence angle and the rolled shape ratio, and determine the upper workpiece The toughness characterized by adjusting the peripheral speed of the upper work roll and the lower work roll for each pass of the secondary rolling mill that performs the secondary rolling so that the relationship between the peripheral speed of the roll and the lower work roll is obtained. excellent production method bite angle of incidence of the steel plate <0 °, rolling shape ratio> if the roll peripheral speed of 1.5 <lower roll peripheral speed bite incident angle> 0 °, the case of the rolling shape ratio> 1.5 roll peripheral speed> lower roll peripheral speed bite angle of incidence <0 °, the field of the rolling shape ratio <1.5 Upper roll peripheral speed <lower roll peripheral speed bite incident angle> 0 °, rolling shape ratio <case on a roll peripheral speed of 1.5> lower roll peripheral speed, however, rolling shape ratio = (contact projection arc length / average thickness ) 2. A sheet thickness of 4 after primary rolling at 850 ° C. or higher.
In a method for producing a steel sheet having excellent toughness by forcibly cooling a steel material of 0 mm or more and then performing secondary rolling and terminating the rolling at 720 to 850 ° C., when the secondary rolling is performed, Predict the bite incidence angle and rolling shape ratio of the steel material for each pass, and determine which of the following from the predicted bite incidence angle and the rolling shape ratio to apply, the steel material of the determined ~ A method for producing a steel sheet having excellent toughness, characterized in that the forced cooling amount of the upper and lower surfaces of the steel material is adjusted so as to have a relationship between the average temperature of the upper surface and the lower surface. In the case of ratio> 1.5, the average temperature of the upper surface> the average temperature of the lower surface When the angle of incidence> 0 ° and the rolling shape ratio> 1.5, the average temperature of the upper surface <the average temperature of the lower surface The angle of incidence <0 ° Rolling shape ratio <1.5 Average temperature on the upper surface <Average temperature on the lower surface Biting incident angle> 0 °, Rolling shape ratio <1.5 Average temperature of upper surface> Average temperature of lower surface However, rolling shape ratio = (contact projection arc length / average plate thickness) 3. A sheet thickness of 4 after primary rolling at 850 ° C. or higher.
In a method for producing a steel sheet having excellent toughness by forcibly cooling a steel material of 0 mm or more and then performing secondary rolling and terminating the rolling at 720 to 850 ° C., when the secondary rolling is performed, Predict the bite incidence angle and rolling shape ratio of the steel material for each pass, and determine which of the following from the predicted bite incidence angle and the rolling shape ratio to apply, the steel material of the determined ~ The average temperature of the upper surface and the lower surface, the forced cooling amount of the upper and lower surfaces of the steel material, and each pass of the secondary rolling mill before biting the steel material so that the peripheral speed of the upper work roll and the lower work roll is related. A method of manufacturing a steel sheet having excellent toughness, characterized in that the peripheral speeds of the upper work roll and the lower work roll are adjusted respectively. In the case where the bite incidence angle <0 ° and the rolling shape ratio> 1.5, the average temperature of the upper surface> The average temperature of the lower surface and the upper roll peripheral speed <the lower roll peripheral Speed When the bite incidence angle> 0 ° and the rolling shape ratio> 1.5: Upper surface average temperature <Lower surface average temperature, and upper roll peripheral speed> lower roll peripheral speed Bite incidence angle <0 °, rolled shape When the ratio is less than 1.5, the average temperature of the upper surface is smaller than the average temperature of the lower surface, and the upper roll has a circumference.
Speed <lower roll peripheral speed bite incidence angle> 0 °, rolling shape ratio <1.5 Average temperature of upper surface> average temperature of lower surface and upper roll circumference
Speed> lower roll peripheral speed where rolling shape ratio = (contact projection arc length / average plate thickness)