JP2583715B2 - Manufacturing method of thick steel plate with excellent low temperature 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 thick steel plate having excellent low-temperature toughness, and more particularly, to a method for producing a thick steel plate having excellent toughness both in the surface portion and in the central portion of the plate thickness. It is about the method.

[0002]

2. Description of the Related Art In general, a controlled rolling method has conventionally been used as a method for producing a steel sheet having good low-temperature toughness.
However, in this conventional controlled rolling method, after rolling the slab extracted from the heating furnace in a high temperature range, the rolling is interrupted and the slab is air-cooled and cooled to the temperature of the unrecrystallized region, Alternatively, as described in Japanese Patent Publication No. 3-069967, a method of performing forcible cooling to a temperature of _three or more Ar and then performing finish rolling.

However, the method of air-cooling the slab by interrupting the rolling needs to wait for the temperature to drop to the target temperature, which wastes a lot of time and increases productivity. In addition, since the temperature distribution in the thickness direction is small, it is not possible to sufficiently reduce the central portion in the thickness direction, and there is a problem that a defect occurs in the central portion in the thickness direction. .

[0004] In the KOKOKU 3-069967 discloses the following Ar ₃ point + 100 ° C. Instead of air, Ar ₃
By performing forced cooling to a temperature equal to or higher than the temperature, the surface layer portion of the rough hot rolled material is cooled and hardened, and a temperature difference in the thickness direction is imparted, and finish rolling is performed in an austenitic single phase state. .

However, in the method disclosed in Japanese Patent Publication No. 3-069967, the surface temperature is cooled only to the temperature of the Ar ₃ point, so that the central portion of the sheet thickness is not sufficiently cooled and finish rolling is performed. In this case, there is a problem that reduction in a temperature range effective for improving toughness is not performed, and high toughness cannot be ensured at the central portion of the thickness of a thick material.

On the other hand, if the surface temperature after forced cooling is too low, the entire vicinity of the surface is transformed, and after rolling, the transformed structure is processed to deteriorate the toughness of the surface. Have the potential to do so.

When it is necessary to perform rolling in a low temperature range in which the finish rolling is performed at an Ar ₃ point or less, if the surface temperature by forced cooling is limited to Ar ₃ points or more, the rolling immediately follows the forced cooling. Cannot be performed, and it is necessary to wait for the temperature to be lowered by air cooling, which causes a problem that time is wasted and productivity is hindered.

[0008]

SUMMARY OF THE INVENTION As described above, the present invention has been made by the inventor of the present invention in view of various problems in the conventional method for producing a steel sheet having good low-temperature toughness.
As a result of repeated investigations, we have developed a method that can produce thick steel plates with excellent low-temperature toughness at high productivity.

[0009]

The feature of the method for producing a thick steel plate having excellent low-temperature toughness according to the present invention is that a billet is heated to a temperature of 950 ° C. or more, hot-rolled, and then rolled. And the surface portion of the slab is maintained at 1 to 10 ° C./sec until the surface temperature of the slab reaches an Ar ₃ point or lower and a temperature of 600 ° C. or higher.
After cooling with water at a cooling rate of c and setting the temperature difference between the billet surface part and the billet center part to 150 ° C. or more and 400 ° C. or less, the cooling recuperation process of self-recuperation is repeated one or more times. Rolling, the surface temperature of the slab is Ar ₃ point or less, Ar ₃ point -8
Rolling is terminated at a temperature of 0 ° C. or more.

The method for producing a thick steel plate having excellent low-temperature toughness according to the present invention will be described in detail below.

First, when hot rolling is performed on a slab, the heating temperature of the slab is set to 950 ° C. or higher. If the temperature is lower than 950 ° C., the structure of the slab does not become an austenite single phase. Also, the crystal grains cannot be refined by rolling. Therefore, the heating temperature of the steel slab when hot rolling is performed is 950 ° C. or higher.

In this way, hot rolling is performed after extracting the heated steel slab from the heating furnace, and the rolling reduction is preferably 30% or more in order to refine the austenite crystal grains. Further, the rolling is interrupted halfway, but the thickness of the steel slab at that time is set to a thickness that can secure a reduction of 30% or more in the finish rolling.

[0013] After the rolling is interrupted, it is cooled to a favorable temperature range for performing finish rolling. This cooling requires a lot of time when performed by air cooling, the whole rolling time becomes long, the number of rolling times per hour is reduced, that is, it is not possible to increase the number of rolling, and therefore, in a short time It is preferable to perform water cooling for cooling, and the water cooling includes cooling methods such as shower cooling, laminar cooling, and mist cooling, and any of these cooling methods may be used. In addition, this water cooling increases the temperature difference between the surface and the center of the billet,
In the finish rolling, it becomes easier to reduce to the center of the billet,
It is also effective in improving the toughness in the center of the sheet thickness and preventing Zaku defects.

[0014] In a temperature exceeding the Ar ₃ point of the surface temperature of the steel strip immediately after the water cooling, the steel strip thickness of, for example, as in the case of 100mm is shown in FIG. 2, the surface portion and the steel of the steel strip immediately after the water cooling Since the temperature difference from the center of the slab is about 300 ° C., the temperature of the center of the slab is about 1050 ° C. if the Ar ₃ point is 750 ° C.

Further, water cooling temperature at a lapse of about 30 seconds after the surface portion heated is restored to about 100 ° C., since the steel strip center is reduced further to about 30 ° C., the surface portion is Ar ₃ point + 100 ° C., the steel strip If the center reaches the Ar ₃ point + 270 ° C and the rolling is restarted after 30 seconds of water cooling, it becomes difficult to perform rolling in the low temperature region of austenite, and particularly high toughness in the center of the slab cannot be obtained.

By performing air cooling for a long time after water cooling, the temperature of the surface portion and the central portion of the billet can be reduced, but this is a major obstacle in terms of productivity,
In particular, when it is necessary to perform low-temperature region rolling such that the finish rolling is performed at _three or less Ar points, long-time air cooling is required, and the entire rolling time becomes extremely long.

When cooling is performed so that the surface temperature immediately after water cooling is equal to or lower than the Ar ₃ point, the vicinity of the surface is transformed, and the surface is hardened by the subsequent rolling, and the toughness is deteriorated. It is conceivable that, as shown in FIG. 1, when the temperature of the surface after water cooling is set to 600 ° C. or higher, the transformation of the surface is not completed, and a part of ferrite precipitates and this ferrite is processed. There is no problem with the quality.

In this case, if the surface temperature of the steel slab is lower than 600 ° C., the transformation is completed, and a structure of pearlite or bainite is formed. Therefore, it is important that the surface temperature after the rolling of the slab is interrupted and water-cooled is controlled to an Ar temperature of ₃ points or less and 600 ° C. or more.

If the temperature after the water cooling is too high, the productivity is reduced,
Deterioration of the toughness of the slab center is also caused, and improvement of productivity and toughness of the slab center by water cooling have been conventionally said.However, water cooling the slab to a low temperature, The quality of the surface portion is adversely affected, and the temperature of 600 ° C. is a limit temperature at which no adverse effect is exerted.

[0020] The following ₃ points Ar the surface temperature of the steel piece after water cooling, by regulating the temperature above 600 ° C., a region where the ferrite transformation has started is only billet surface portion, slab thickness most of the the Ru temperature der of _three or more points Ar.

Therefore, the surface temperature of the steel slab after water cooling is set to Ar
When the temperature of the steel strip center and _three or more points is compared with the time <br/> to be 1000 ° C. or higher, it is possible to perform substantial temperature drop, improve toughness of the steel strip central portion, also cooled with water by the surface temperature of the steel strip with the following _3-point Ar, cooled with water at Ar ₃ point or more, productivity is improved compared with the case of performing the long air cooling.

The cooling rate when performing such water cooling is as follows:
The cooling rate is determined by the thickness of the slab and the water density, but if the cooling rate is too slow, a sufficient effect cannot be obtained to improve the productivity. Therefore, the cooling rate of the slab surface is 1 ° C./sec or more. If the cooling rate is too fast, only the surface is cooled, and the temperature drop at the center of the slab becomes small,
It becomes difficult to secure toughness in the center of the billet.

In order to lower the temperature at the center of the slab, if the cooling rate is high, the temperature of the surface becomes lower than 600 ° C., and a hardened structure is formed, thereby increasing the hardness of the surface and deteriorating the toughness. In order to prevent this, it is necessary to set the cooling rate of the billet surface to 10 ° C./sec or less. Therefore, the cooling rate is 1 to 10 ° C./sec. In addition. The water density is preferably 0.05 to 0.50 m ³ / min · m ² .

When the surface portion is supercooled by water cooling, a temperature difference between the surface portion and the center portion of the steel slab becomes large, and the temperature of the steel slab decreases due to heat conduction. Therefore, the greater the temperature difference between the surface and the center of the slab, the greater the effect of lowering the temperature of the center of the slab. Therefore, in order to obtain this effect, the temperature difference between the surface and the center of the slab must be 150 ° C. or more, and the temperature difference between the surface and the center of the slab is large. If the temperature is too high, it becomes difficult to secure the flatness by rolling after water cooling, and it takes time to reheat. Therefore, the upper limit of the temperature difference between the surface portion and the center of the slab must be 400 ° C or less immediately after water cooling. There is.

When the thickness of the slab is large or the temperature at the center of the slab is to be lowered, it is not necessary to perform water cooling only once. It is possible to improve the productivity by further reducing the time.

The finish rolling is an important rolling for toughness ensuring, in <br/> Ru temperature rolling completion temperature exceeds the surface portion Ar ₃ point of the steel strip, ensure sufficient toughness in the steel strip center And the rolling completion temperature of the billet surface is A
At a temperature lower than r ₃ -80 ° C., the toughness of the surface portion is greatly deteriorated. Therefore, the rolling completion temperature is less than ₃ points Ar, Ar ₃
The temperature is -80 ° C or higher. In the method for manufacturing a thick steel plate having excellent low-temperature toughness according to the present invention, after finish rolling, any treatment such as air cooling, accelerated cooling, and direct quenching may be performed.

Further, the components and ratios of the steel that can be used in the method for producing a thick steel sheet having excellent low-temperature toughness according to the present invention are not particularly limited, but C is 0.2 wt% or less, Si 0.05 to 0.05%. 0.50 wt%, Mn 0.5-2.0
wt%, S 0.005 wt% or less, Al 0.005 ~
It is a normal thick steel sheet containing 0.10 wt%, the balance being Fe and unavoidable impurities.
V, N, Ni, Cr, Mo, B, Ca, REM and the like can be contained in appropriate amounts.

[0028]

EXAMPLES Examples of the method for producing a thick steel plate having excellent low-temperature toughness according to the present invention will be described with reference to Examples and Comparative Examples.

[0029]

[Examples] Steels A, B, C, and D having the components and component ratios shown in Table 1 were melted and cast by a conventional method, and the produced steel slabs were heated and rolled in accordance with the manufacturing conditions shown in Table 2. The steel sheet was manufactured by performing cooling during rolling and cooling after the completion of rolling. Each of the steel slabs A, B, C, and D is a steel having the contained components and component ratios used in the method for producing a thick steel plate having excellent low-temperature toughness according to the present invention.

A test piece was taken from this steel plate and subjected to a tensile test,
A 2 mm V notch Charpy impact test and a Vickers hardness test were performed. The tensile test was performed using a round bar tensile test piece taken from a position at a quarter of the plate thickness direction. 2mm
The V-notch Charpy impact test was performed on test pieces taken from the surface and the center of the steel slab.

Table 3 shows the test results. No. 1 and No. 1 by the manufacturing method of the thick steel plate excellent in low-temperature toughness according to the present invention.
7, No. 9 and No. 12 all have high toughness in which the vTrs at the surface and the center of the slab are lower than −40 ° C.
The hardness of the surface is not particularly hard as compared with the comparative example of the same steel. Further, the air cooling time immediately after the water cooling is 60 seconds or less, which does not significantly impair the productivity.

However, in the case of the comparative example, the material was inferior or a long time was required for air cooling.
No. 2 and No. 4 had the same finish rolling temperature as No. 1, so there was no problem in the material. However, No. 4 did not perform water cooling and required 700 seconds for the air cooling time. .2 is water-cooled, but since the surface temperature immediately after water-cooling is equal to or higher than the Ar ₃ point, it takes 300 seconds for air-cooling after water-cooling, which hinders productivity.

No. 3 and No. 8 show the surface temperature after water cooling as A.
r ₃ points or more, since the air cooling time after water cooling was set to 30 seconds so as not to hinder productivity, the temperature does not sufficiently decrease in finish rolling, and by performing rolling at a high temperature,
Both the surface and the center of the slab have poor toughness.

In No. 5, No. 10, and No. 13, the surface temperature after water cooling was lowered to 500 ° C., so that the vicinity of the surface was transformed and the reheating was not sufficiently performed. , The hardness of the surface part is increased, and the toughness is deteriorated.

No. 11 is a case where the water cooling rate was increased to lower the surface temperature to 500 ° C. and the temperature after reheating was increased, and the finishing temperature of the finish rolling was recovered to 720 ° C. However, since the surface temperature was lowered to 500 ° C., the layer of the transformed surface portion became deep, and the toughness of the surface portion was deteriorated. No. 6 has a water cooling rate of 12 ° C. /
sec, the surface temperature drops to 400 ° C,
The surface becomes hard and the toughness is deteriorated.

[0036]

[Table 1]

[0037]

[Table 2]

[0028]

[Table 3]

[0029]

As described above, the method for manufacturing a thick steel plate having excellent low-temperature toughness according to the present invention has the above-described structure, and therefore can be used for a thick steel plate having a large thickness at a high productivity. However, both the surface and the center of the slab have good toughness,
In addition, it has the effect that a steel sheet having a surface portion having a low hardness can be manufactured.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between surface temperature, vTrs, and surface hardness when water cooling is stopped.

FIG. 2 is a diagram showing a relationship between a time after the start of water cooling and a surface portion, a billet central portion, and an average temperature.

Claims (1)

(57) [Claims] 1. A slab is heated to a temperature of 950 ° C. or higher, hot-rolled, and then the rolling is interrupted to reduce the surface temperature of the slab to Ar.
The surface of the slab is subjected to water cooling at a cooling rate of 1 to 10 ° C./sec until the temperature of the slab is ₃ points or less, and the temperature of the slab is 600 ° C. or more. 400 ° C
After that, the cooling and reheating process of self-reheating is repeated once or more than once, and then rolling is performed to reduce the surface temperature of the slab to Ar.
₃ points or less, the production method of low-temperature toughness excellent steel plate, characterized in that to terminate the rolling at Ar ₃ point -80 ° C. or higher.