Production method of strong and tough thick steel plate

Abstract

A steel containing predetermined components is rolled in a recrystallization temperature region or a non-recrystallization temperature region of an austenite and is subsequently subjected to repeated hot bending. Alternatively, a surface layer portion is cooled during rolling of the steel described above to an .alpha. single phase or a .gamma./.alpha. dual phase temperature region, rolling is then effected and is finished at the point of time when the surface temperature of the plate rises above an Ac.sub.3 point due to recuperative heat, and repeated bending is carried out. Still alternatively, the steel described above is rolled to a cumulative reduction ratio of at least 20% in the non-recrystallization temperature region and is then subjected to repeated bending. Further alternatively, the surface layer portion is cooled during hot rolling of the steel described above to an .alpha. single or .gamma./.alpha. dual phase temperature region, rolling is then continued at a cumulative reduction ratio of at least 20% and is finished at the point when the surface temperature of the steel plate rises less than (Ac.sub.3 point--200.degree. C.) due to recuperative heat, and subsequently, repeated bending is carried out.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention provides a thick steel plate having excellent strength and toughness and furthermore a thick steel plate devoid of material anisotropy and having excellent brittle crack propagation stop characteristics.

2. Description of the Related Art

Properties of a thick steel plate used as structural members or for other purposes are determined by its chemical components and heat treatment. Recently, production of a thick steel plate having excellent strength and toughness has become possible by a controlled rolling method predominantly comprising rolling at a low temperature and by an accelerated cooling method which conducts cooling in succession to rolling. Such production techniques are described in Japanese Examined Patent Publication (Kokoku) Nos. 49-7291, 57-21007, and 59-14535.

In controlled rolling in general, austenite grains are made fine in a high temperature region by recrystallization and further drawn sufficiently under the non-crystallized state in a low temperature region to obtain fine ferrite by transformation in a subsequent accelerated cooling process.

However, when such rolling in the recrystallization temperature region and rolling in the non-recrystallization temperature region are combined, the problem remains that a long waiting period is necessary for the drop of the rolling temperature and thus the productivity is remarkably impeded. Another problem resides in that the effect of rolling is lost during the period from the end of rolling in the non-recrystallization temperature region to the start of accelerated cooling (mainly because of the decrease of the dislocation density introduced by rolling), and the effect of rolling in the non-recrystallization temperature region cannot be exploited fully.

Still another problem resides in that when rolling is finished in the non-recrystallization temperature region, the rolled aggregate texture is transferred as such to the texture after rolling and material anisotropy increases. When rolling is carried out in the recrystallization temperature region in order to prevent this material anisotropy, there occurs the problem that since the rolling temperature is high, the grain growth after recrystallization is so fast that the crystal grains become coarse. When rolling is finished in a temperature region as low as possible within the range in which recrystallization can take place, however, partial recrystallization is likely to occur and duplex grains develop and cause deterioration of the material. Accordingly, there is a limit to the lowering of the rolling temperature.

The structural members must have excellent brittle crack propagation stop characteristics as one of the required characteristics.

As one of the metallurgical factors that affect brittle crack propagation characteristics when brittle breakdown occurs, it is well known that fine granulation of the crystal grains improves the brittle crack propagation stop characteristics. For this reason, a large number of attempts have been made in the past to make the crystal grains finer, and a thick steel plate having fine crystal grains has become available by a controlled rolling method in a low temperature region or by an accelerated cooling method which conducts cooling in succession to rolling, for example. Such a technique is described in Japanese Examined Patent Publication (Kokoku) Nos. 49-7291, 57-21007, and 59-14535.

Fine granulation of the crystal grains of a plate surface portion is extremely effective for improving the brittle crack propagation stop characteristics. Therefore, Japanese Unexamined Patent Publication (Kokai) No. 61-235534, Japanese Patent Application No. 4-67514, and Japanese Patent Application No. 4-67515 disclose a fine granulation method which combines water cooling during rolling with rolling. All of these related art references disclose the fine granulation method which cools the surface layer portion of the plate with water during rolling so as to bring the texture into an austenite-ferrite dual phase state or a ferrite single phase, conducts rolling during the process in which the temperature of the surface portion of the plate recuperates and rises by heat transfer inside the plate, so as to make the ferrite crystal grains fine and to introduce a rolling strain into the austenite, and eventually makes the crystal grains of the surface portion of the plate fine after transformation.

However, the method described in Japanese Patent Application Nos. 4-67514 and 4-67515 and Japanese Unexamined Patent Publication (Kokai) No. 59-182916 essentially stipulates the requirement that the highest arrival temperature of the plate surface portion by recuperation after water cooling be less than an Ac.sub.3 point to make the crystal grains of the plate surface portion finer. Accordingly, the machined texture of the ferrite remains and the toughness drops.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 61-235534 prevents residual machined texture from occurring by stipulating the essential requirement that the temperature of the plate surface portion after water cooling be recuperated to a point above the Ac.sub.3 point by heat transfer inside the plate. However, since the recuperative temperature exists on a higher temperature side, the resulting crystal grains become greater than those obtained by the method of Japanese Patent Application Nos. 4-67514 and 4-67515, and the brittle crack propagation stop characteristics, too, tend to be inferior.

There are various hot machining methods, and bending is one of them. A strain can be imparted without changing the plate thickness by repeating bending. However, there remains the problem that the strain imparted by bending is generally great in a plate surface portion and is not sufficiently imparted in the center portion in the direction of the plate thickness. For this reason, bending is employed primarily for improving the flatness of the plate but is not used for improving the material properties, in many cases. Japanese Examined Patent Publication (Kokoku) No. 1-16210 discloses a technology which improves a drilling ratio by hot molding a fine grain ferrite, but this reference does not describe the crystal condition between the strain during hot molding and the crystal grains, and so forth.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a thick steel plate which solves the problems with the related art steel plates described above and has excellent strength and toughness.

It is another object of the present invention to provide a thick steel plate which has excellent strength and toughness as well as excellent brittle crack propagation stop characteristics.

It is still another object of the present invention to provide a thick steel plate which has excellent strength and toughness but is free from material anisotropy.

To accomplish the objects described above, the present invention conducts rolling of an ingot or a slab at a high reduction ratio in a temperature region above an Ar.sub.3 point or an Ac.sub.3 point, conducts repeated bending in an austenite non-recrystallization temperature region so as to remarkably increase the dislocation density inside the austenite grains and to make the crystal grains after ferrite transformation extremely fine (below about 5 .mu.m), and achieves a high toughness of the thick steel plate by such a texture.

In this case, it is also possible to finely recrystallize the austenite by conducting repeated bending in the austenite recrystallization temperature region after rolling is carried out in the austenite non-recrystallization temperature region, and in such a case, a thick steel plate free from material anisotropy can be produced.

It is further possible to employ a method which compulsively cools the ingot or the slab before, or during, rolling at a high reduction ratio, so as to convert the surface into the austenite-ferrite dual phase texture or the ferrite single phase texture, then applies repeated bending to the rolled steel plate after transformation to the austenite single phase or the rolled steel plate having the ferrite single phase texture so as to secure a large number of nucleid formation sites for ferrite transformation, or recrystallizes the ferrite to make the metallic texture after transformation or after recrystallization extremely fine (to below about 1 .mu.m). In this way, a strong and tough thick steel plate having excellent brittle crack propagation stop characteristics can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the relationship between a reduction ratio or a rolling strain (or strain due to the rolling strain plus repeated bending) and a temperature when rolling or repeated bending is applied to a slab and schematically shows an austenite recrystallization temperature region and a transformation temperature in a temperature descension process;

FIG. 2 schematically shows the relationship between a reduction ratio or a rolling strain (or strain due to the rolling strain plus repeated bending) and a temperature when rolling or repeated bending is applied to the slab and schematically shows a ferrite recrystallization temperature region and a transformation temperature in a temperature ascension process;

FIG. 3 shows the relationship between the sum (E (%)) of the strain which a steel plate surface portion receives due to repeated bending and a steel plate surface temperature (T (.degree.C.));

FIG. 4 shows an example of the arrangement of rolls of a leveler; and

FIG. 5 shows relational factors for calculating a cumulative strain quantity when bending is applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be explained in further detail.

(1) When bending is repeatedly effected in the austenite non-recrystalline temperature region

Generally, the crystal grain size of the steel plate finally obtained after transformation is determined by the austenite crystal grain size before transformation and the dislocation density introduced into the austenite by rolling. In other words, the finer the austenite crystal grain size before transformation and the greater the dislocation density in the austenite before transformation, the finer the crystal grain size after transformation and the more excellent the material properties.

However, the quantity of the former is determined by the rolling condition in the recrystallization temperature region and the quantity of the latter is determined by the rolling condition in the non-recrystallization temperature region. Therefore, each of these quantities has an inherent limit when the slab thickness before rolling and the plate thickness after rolling are determined.

The inventors of the present invention have found a method which brings the austenite crystal grain size before transformation and the dislocation density in the austenite into a more desirable state by the combination of rolling with repeated bending after rolling. Since bending can impart strain without changing the plate thickness, it is not limited by the slab thickness and the plate thickness after rolling.

FIG. 1 shows the relation between the reduction ratio or the rolling strain (leveler machining strain) and the temperature (the recrystallization temperature and the transformation temperature in the temperature lowering process) in the case where an ingot or a slab (hereinafter referred to as the "slab") consisting of the components according to the present invention is casted and is then directly rolled or repeatedly bent (hereinafter referred to as the "leveler machining") by utilizing the casting temperature in the temperature lowering process, or in the case where the slab described above is once cooled to a temperature below the Ar.sub.1 point and then heated to a temperature above the Ac.sub.3 point.

In the figure, 1 is a line representing the recrystallization limit of the austenite due to rolling, 2 is a line representing the recrystallization limit of the austenite when leveler machining is further carried out after rolling, 3 is a line representing the start of the austenite-ferrite transformation, and 4 is a line representing completion of the ferrite transformation. Symbol A represents the region of the austenite phase, A.sub.1 is the recrystallization temperature region, and A.sub.2 is the non-crystallization temperature region. Symbol B represents the region which is under transformation from the austenite to the ferrite, and symbol C is mainly the region of the ferrite phase.

In the case (1) described above, rolling having a cumulative reduction ratio of at least 20% is completed in the austenite recrystallization temperature region A.sub.1 or in the austenite non-recrystallization temperature region A.sub.2 and subsequently leveler machining is carried out in the austenite non-recrystallization temperature region A.sub.2 so as to impart a desired quantity of strain. In this way, the ferrite crystal grain size can be made small below 5 microns after ferrite transformation due to cooling after leveler machining.

When rolling is finished in the austenite recrystallization temperature region A.sub.1, the full reduction quantity of rolling can be allotted to recrystallization and to the reduction of the grain size. Accordingly, the austenite crystal grain size can be made extremely fine. When leveler machining is thereafter applied in the non-recrystallization temperature region A.sub.2, the dislocation density inside the extremely small austenite grain can be increased. In this way, the crystal grain size after transformation becomes extremely small, and the thick steel plate becomes strong and tough.

On the other hand, when rolling is finished in the non-recrystallization temperature region A.sub.2, the dislocation density built up inside the austenite grains reaches saturation due to the balance of work hardening and dynamic recovery according to the existing rolling technique, even though the reduction ratio in the non-recrystallization temperature region is increased to a certain extent. Hence, the effect of rolling on the reduction of the crystal grain size after transformation is limited.

Further, the effect of reduction falls in the period from the end of rolling until the start of accelerated cooling (mainly because of the decrease of the dislocation density introduced by rolling), and the effect of rolling further drops. However, when leveler machining, which is a different machining mode, is applied to the dislocation density inside the austenite which is in the saturated state due to rolling in the non-recrystallization temperature region, the arrangement of dislocation inside the austenite grains changes, and the dislocation density increases, too.

Accordingly, the nucleid formation sites increase during subsequent transformation, and the crystal grain size after transformation can be reduced to about several microns in the case of the ferrite texture as described above.

In this way, a thick steel plate can be made strong and tough. The leveler machining temperature in this case is predominantly the non-recrystallization temperature region A.sub.1 of the austenite described above, but may be below the Ar.sub.3 point but above the Ar.sub.1 point in which partial transformation occurs. Further, transformation can be caused to occur before the dislocation density introduced by leveler machining decreases, by shortening the leveler machining time and the accelerated cooling time.

By the way, when the plate temperature is high at the time of leveler machining, the effect of machining strain is likely to drop. Therefore, the strain quantity to be imparted by leveler machining must be increased at a higher temperature, and this strain quantity (%) is determined in accordance with the following formula:

E.gtoreq.1.71.times.10.sup.-3 T-0.4 (1) where E: sum of strain which the plate surface portion receives at the time of repeated bending, T: surface temperature (.degree.C.) of the thick steel plate when repeated bending is carried out.

In the case (1) described above, leveler machining is carried out in the austenite non-recrystallization temperature region. Therefore, the upper limit of the sum (E) of the strain is stipulated to be less than a strain quantity obtained by the formula of the strain quantity (formula (3) of the case (3)), when rolling in the non-recrystallization region plus leveler machining are carried out in the austenite recrystallization temperature region:

E.gtoreq.-1.14.times.10.sup.-3 T+2.4 (3)

In other words, the sum (E) must satisfy the following relational formula:

-1.14.times.10.sup.-3 T+2.4>E.gtoreq.1.71.times.10.sup.-3 T-0.4

The relationship described above is shown in FIG. 3. In other words, FIG. 3 shows the relationship between the sum of the strain (E (%)) which the steel plate surface portion receives during leveler machining and the steel plate surface temperature (T (.degree.C.)). The case (1) described above exists inside the region encompassed by the formulas (1) and (3) in FIG. 3.

After leveler machining is completed, the work must be quickly passed through the ferrite transformation end line 4, that is, the Ar.sub.1 transformation point, in order to obtain the ferrite grains having a very small size. Accordingly, though the effect of reducing the grain size after transformation can be obtained to a certain extent by leaving the workpiece standing for cooling, the effect becomes remarkable when cooling is carried out at a mean cooling rate of 0.5 to 80.degree. C./cm in the direction of the plate thickness.

To produce the ferrite-pearlite steel and the ferrite-bainite steel, it is preferred to quickly start cooling after completion of leveler machining as soon as possible and to cool the steel down to about 500.degree. C.

To produce steels consisting principally of bainite and the martensite, quenching is started as soon as possible after completion of leveler machining and then tempering is carried out in an ordinary tempering temperature region.

Leveler machining can be carried out by a hot leveler or repeated bending using roll bending.

(2) Case where the plate surface is cooled and repeated bending is carried out in the austenite non-recrystallization temperature region (case (2))

This case imparts brittle crack propagation stop characteristics with high toughness to the thick steel plate. Therefore, when the slab is directly rolled or is rolled after re-heating, cooling water is sprayed to the plate surface preferably at a rate of 0.05 to 2.0 m.sup.3 /min.multidot.m.sup.2 before the start of rolling or during rolling for at least one second, so as to cool the plate surface portion to a temperature below the Ar.sub.3 point or the Ar.sub.1 point. In this way, the thickness portion of at least 5% in the direction of the plate thickness is converted to the austenite-ferrite dual phases or to the ferrite single phase.

Next, while the plate surface portion is heated by the recuperative heat from inside the plate, rolling is effected for the steel plate having the texture described above at a reduction ratio of at least 20% and after rolling is completed in the texture temperature region, the temperature is raised to a temperature above the Ac.sub.3 point or rolling is completed at a temperature above the Ac.sub.3 point.

When rolling is carried out in the austenite-ferrite dual phase temperature region or in the ferrite single phase temperature region, the driving force of the ferrite-austenite transformation can be sufficiently increased, and then transformation is allowed to proceed to the austenite single phase. In this way, fine austenite grains having a grain size of about 10 .mu.m at a reduction ratio of 20%, for example, can be obtained.

After the rolled material described above is obtained, repeated bending (hereinafter referred to as "leveler machining") is carried out under the same condition as in the case (1) (with the exception that the formula of the lower limit of the strain quantity is different). In other words, the strain quantity E (%) determined by the following formula (2) is imparted by leveler machining in the austenite non-recrystallization temperature region (inclusive of the austenite-ferrite non-recrystallization temperature region of the Ar.sub.3 to Ar.sub.1 points) above the Ar.sub.3 point:

E.gtoreq.1.65.times.10.sup.-3 T-0.5 (2)

The upper limit of the strain quantity is less than the strain quantity obtained by the formula (3) of the case (3) in the same way as in the case (1).

That is, the strain quantity is within the following range (see FIG. 3):

-1.14.times.10.sup.-3 T+2.4>E.gtoreq.1.65.times.10.sup.-3 T-0.5

After the dislocation density inside the fine austenite grains is thus increased remarkably, the workpiece which is leveler-machined is cooled so as to cause the ferrite transformation. In this way, it is possible to obtain a transformation texture containing the ferrite crystal grains of below 5 .mu.m inside the steel plate and the extremely fine ferrite crystal grains of below 1 .mu.m in the surface portion of the steel plate.

The brittle crack propagation stop characteristics of the thick steel plate having the extremely fine ferrite crystal grain texture at the surface portion thereof can be remarkably improved, so that brittle cracks can be prevented and the product becomes extremely effective as building materials.

Cooling of the steel plate before, or during, rolling can be carried out by ordinary industrial methods such as water cooling using a spray or a laminer, water immersion cooling, cooling using a salt dissolved in other than water, and so forth, and is not particularly limited. The cooling condition cannot be determined primarily because it is affected by the plate temperature at the start of cooling, the cooling capacity (cooling rate), and so forth, but the present invention uses the cooling condition where at least 5% of the plate thickness from the surface of the steel plate to be cooled attains the metallic texture described above. For example, cooling water at a rate of 0.05 to 2.0 m.sup.3 /min.multidot.m.sup.2 is sprayed once or several times to the plate surface for at least one second in accordance with the plate thickness.

(3) Case where repeated bending is carried out for recrystallization in the austenite non-recrystallization temperature region (case (3))

This case imparts strong toughness and characteristics free from material anisotropy to the thick steel plate. To attain this object, rolling is carried out in the austenite non-recrystallization temperature region by applying reduction at a cumulative reduction ratio of at least 20% so as to sufficiently secure dislocation inside the austenite grains and to increase the driving force of potential recrystallization. Next, the strain quantity E (%) represented by the formula (3) is imparted subsequently in the austenite non-recrystallization temperature region (inclusive of the temperature region below the Ar.sub.3 point but above the Ar.sub.1 point) by effecting repeated bending (hereinafter referred to as "leveler machining"). As a result, since leveler machining is carried out in the austenite recrystallization temperature region, the fine austenite recrystallization grains can be generated in the low temperature region (see FIG. 1, case (3)).

E.gtoreq.-1.14.times.10.sup.-3 T+2.4 (3) where T: temperature above the Ar.sub.1 point.

In other words, when the reduction quantity in the non-recrystallization temperature region is increased as in the prior art so as to increase the dislocation density as already described, material anisotropy increases, and the steel plate becomes unsuitable as a structural material.

Even when an attempt is made to make the austenite crystal grain before transformation fine by rolling in order to obtain the same effect as the increase of the dislocation density, the reduction ratio is limited from the relationship between the slab thickness before rolling and the plate thickness after rolling because the austenite grain size is determined by rolling recrystallization. Thus, there is a limit to the reduction of the grain size.

The inventors of the present invention have solved such problems by the combination of rolling and leveler machining after rolling as described above. This solution technique is based on the novel finding that the structure of dislocation inside the austenite, which is under the saturated state due to rolling in the austenite non-recrystallization temperature region, is changed and is caused to recrystallize by leveler machining which has a different machining mode from rolling.

As described above, recrystallization occurs by conducting leveler machining for imparting a specific strain quantity even in the temperature region in which austenite remains non-recrystallized by rolling, and the austenite grains having smaller grain sizes than those obtained by conventional rolling can be obtained. In consequence, material anisotropy can be eliminated, the finer ferrite grain texture can be obtain by the ferrite transformation due to cooling after leveler machining, and strong toughness can be accomplished.

(4) Case where plate surface is cooled and repeated bending is carried out in the ferrite recrystallization region (case (4))

This case imparts strong toughness and brittle crack propagation stop characteristics to the steel plate in the same way as in the case (2). To accomplish this object, the plate surface portion is cooled before, or during, rolling of the slab so as to attain the austenite-ferrite dual phase texture or the ferrite single phase texture in the same way as in the case (2), then rolling at a reduction ratio of at least 20% is carried out within the temperature region in which the ferrite is not recrystallized in the recuperative process, that is, within the temperature range of (Ac.sub.3 point minus 200.degree. C.) to the Ac.sub.3 point, in order to increase the driving force of recrystallization.

Next, repeated bending (hereinafter referred to as "leveler machining") is carried out within the temperature region described above so as to impart the strain quantity E (%) (see FIG. 3) expressed by the following formula (4):

E.gtoreq.-1.2.times.10.sup.-3 T+2.7 (4) where T: below Ac.sub.3

Due to this leveler machining, recrystallization occurs even in the temperature region, where the ferrite remains non-recrystallized by rolling alone, as shown in FIG. 2, and extremely fine ferrite grains can be obtained.

According to Japanese Unexamined Patent Publication (Kokai) No. 59-182916 among ther related art references described already, the temperature of the plate surface portion is high below the Ac.sub.3 point. Accordingly, even when recrystallization starts occurring, abnormal grain growth is likely to occur or the texture is likely to become a mixed grain texture, and there is a limit to recrystallization of the ferrite by rolling alone. The present invention solves these problems by the combination of rolling with leveler machining so as to cause recrystallization in the low temperature region.

By the way, when the rolling finish temperature after cooling is less than (Ac.sub.3 point minus 200.degree. C.), recrystallization by subsequent repeated bending is difficult to occur and when it is above the Ac.sub.3 point, on the other hand, ferrite-austenite transformation finishes during rolling, so that the ferrite is not made sufficiently fine. Therefore, the rolling finish temperature is determined to be from (Ac.sub.3 point minus 200.degree. C.) to less than the Ac.sub.3 point. When the cumulative reduction ratio in the ferrite signal phase or in the ferrite/austenite dual phase region is small, the driving force of subsequent recrystallization of ferrite is not sufficient. For this reason, rolling in the ferrite single phase or the ferrite/austenite two-phase region is stipulated to be at least 20% in terms of the cumulative reduction ratio.

Next, the limitation of the components of the steel of the present invention, which is common to all the cases described above, will be explained. In the following description, the term "%" means "wt %".

Carbon (C) is an indispensable element for strengthening the steel material. If its amount is less than 0.02%, a required high strength cannot be obtained, and when the amount exceeds 0.03%, on the other hand, toughness at the weld portion is lost. Therefore, the amount is limited to from 0.02 to 0.30%.

Silicon (Si) is effective for promoting deoxidation and raising the strength. Therefore, at least 0.01% of Si is added, but when the amount is too great, weldability will drop. Therefore, the upper limit is up to 2.0%.

Manganese (Mn) is effective as an element for improving low temperature toughness, and at least 0.3% of Mn must be added. However, when its amount exceeds 3.5%, weld cracks will be promoted. Therefore, the upper limit is 3.5%.

Aluminum (Al) is effective as a deoxidizing agent and more than 0.003% of Al may be added. However, if its amount is too great, Al will form detrimental inclusions. Therefore, the upper limit is 0.1%.

Niobium (Nb) is the element which restricts rolling recrystallization of austenite even in a small amount and is effective for strengthening non-recrystallization rolling. Therefore, at least 0.001% of Nb is added, but if its amount is too great, toughness of weld joint will drop. Therefore, the upper limit is 0.1%.

When added in even a small amount, titanium (Ti) is effective for making the crystal grains fine, and at least 0.001% of Ti is therefore added, and Ti may be added in such an amount as not to deteriorate toughness of the weld portion. Therefore, the upper limit is set to 0.10%.

All of Cu, Ni, Cr, Mo, Co, and W are known elements which improve hardenability, and when added to the steel of the present invention, they can improve the strength of the steel. Therefore, at least 0.05% of these elements are added. However, when their amounts are too great, weldability will drop. Therefore, the upper limits are set to be up to 3.0% for Cu, up to 10% for Ni, up to 10% for Cr, up to 3.5% for Mo, up to 10% for Co, and up to 2% for W.

Vanadium (V) is effective for improving the strength by the precipitation effect, and at least 0.002% is added. However, the upper limit is set to 0.10% because excessive addition will deteriorate toughness.

Boron (B) is a known element which improves hardenability. When added to the steel of the present invention, B can improve the strength of the steel and at least 0.0003% is added. However, the upper limit is set to 0.0025% because excessive addition will increase the precipitation of B and will deteriorate the toughness.

Rem and Ca are effective for making S harmless. Though at least 0.002% of Rem and at least 0.0003% of Ca are added, excessive addition will deteriorate the toughness. Therefore, their limits are set to 0.10% and 0.0040%, respectively.

Since repeated bending receives alternately the tensile strain and the compressive strain, the sum of the strains which the plate surface portion receives in each of the cases described above is defined as the cumulative strain quantity which is the sum of the tensile strain and the compressive strain in the plate surface portion. In the case of bending using the leveler, the cumulative strain quantity is calculated in accordance with FIG. 4.

FIG. 4 shows the arrangement of the rolls of the leveler. Symbol L represents 1/2 of the roll gap and RG is a roll gap. Generally, L is fixed by the setup while RG is variable.

Table 1 tabulates the calculation result of the reduction quantity (push-in quantity) X.sub.i on the basis of the roll gap RG.sub.i of the i-th roll. The variable X.sub.i is determined by RG.sub.i and the plate thickness t. Table 1 represents the conditions of the maximum machining degree when the workpiece is bent along the fourth roll, but the condition of the maximum machining degree can similarly be calculated for other rolls when the workpiece is bent along other rolls by the same method. In other words, when the number of the rolls providing the maximum machining degree is imax, the reduction quantity in such a case is X.sub.imax, the total number of rolls is N, the reduction quantity of the i-th roll is X.sub.i (the true reduction quantity (inter-mesh) mm for imparting the degree of machining .alpha..sub.i to the plate) and other symbols are defined as below, the condition providing the maximum machining degree can be determined by calculating continuously the following formulas:

.sigma..sub.y : yield stress of the material (kg/mm.sup.2) L: 1/2 of the roll pitch (mm) .alpha..sub.i : degree of working of the i-th roll RG.sub.i : roll gap of the i-th roll (mm) t: thickness of the plate (mm) E: Young's modulus of the material (kg/mm.sup.2) G: shake of the leveler (0.3 mm) A: mill spring (mm/ton) P: correction reaction (tan) K: coefficient (2 to 3; 3 is used) x.sub.imax =t-RG.sub.imax -G-AP (determined by setting RG.sub.imax) X.sub.N-1 =.sigma..sub.y L.sup.2 /3tE (calculate the reduction quantity of the last-but-one roll) when i<imax: X.sub.i =X.sub.imax +(X.sub.imax -X.sub.N-1)/(N-1-imax).times.(imax-i) when i>imax: X.sub.i =X.sub.imax -(X.sub.imax -X.sub.N-1)/(N-1-imax).times.(i-imax) .alpha..sub.i =3tE/.sigma..sub.y L.sup.2 .times.X.sub.i with the proviso that the machining degree is 0 between the first roll and the Nth roll and 1 at the (N-1)th roll. That is, .alpha..sub.1 =0 .alpha..sub.N =0 .alpha..sub.N-1 =1

The relation between the machining degree .alpha..sub.i and the strain E.sub.i :

E.sub.i =.sigma..sub.y /E.times..alpha..sub.i

The total strain quantity (corresponding to E in the formula described in the claims):

.SIGMA.E.sub.i =.sigma..sub.y /E.times..SIGMA..alpha..sub.i TABLE 1__________________________________________________________________________Rollno. Reduction q'ty X.sub.i Roll gap RG.sub.i__________________________________________________________________________ ##STR1## RG.sub.i = t - X.sub.t - G - Ap2 ##STR2## RG.sub.2 = t - X.sub.2 - G - Ap3 ##STR3## RG.sub.3 = t - X.sub.3 - G - Ap4 ##STR4## RG.sub.4 = t - X.sub.4 - G - Ap . . .i ##STR5## RG.sub.i = t - X.sub.i - G - Ap . . .n-1 ##STR6## RG.sub.n-1 = t - X.sub.n-1 - G - Apn ##STR7## RG.sub.n = t - X.sub.n - G - Ap__________________________________________________________________________

The cumulative strain quantity when bending is carried out by other methods is calculated in accordance with FIG. 5. Since this machining is bending, positive and negative, opposite strains are imparted to the front and back of the plate, but because they are repeatedly imparted, the sum of the absolute values of the strains is defined as the cumulative strain quantity.

EXAMPLES

Example 1

Examples of the present invention in the case (1) described above will be explained. First of all, the method of the present invention and the comparative method shown in Tables 3(1) to 3(4) were applied to the steel of the present invention having the components shown in Table 2, and the strength and the toughness shown in Tables 3(1) to 3(4) were obtained. When comparison was made for the steels having the same components, the steels obtained by the method of the present invention exhibited an improvement in the tensile strength by at least 2 kgf/mm.sup.2 and the Sharpy impact test ductile-brittle transition temperature by at least 10.degree. C. It could be understood from these results that the steels of the present invention obviously exhibited better material characteristics and the present invention was effective. Repeated bending was carried out using the hot leveler.

By the way, the heat-treatment pattern (after rolling or after repeated bending) was as follows.

a: accelerated cooling to 500.degree. C. at 7.degree. C./S b: accelerated cooling to 460.degree. C. at 14.degree. C./S c: left for cooling d: accelerated cooling to 505.degree. C. at 27.degree. C./S e: direct hardening to room temperature and then tempering at 660.degree. C. f: accelerated cooling to room temperature at 15.degree. C./S and then tempering at 460.degree. C.

In each of the tensile test and the impact test, a JIS No. 4 testpiece (collected from the L direction (rolling direction) at a 1/4 portion of the direction of the plate thickness) was used.

TABLE 2__________________________________________________________________________(wt) % Ac.sub.3 Ar.sub.3 pointSteel No. C Si Mn Cu Ni Cr Mo Co W Nb Ti V B Al Rem Ca (.degree.C.) (.degree.C.)__________________________________________________________________________A 0.08 0.24 1.33 0.12 0.40 -- -- -- -- 0.005 0.007 0.004 -- 0.025 -- 0.0020 858 754B 0.15 0.18 1.10 -- -- -- -- -- -- -- 0.007 0.004 -- 0.030 0.01 -- 840 776C 0.05 0.26 1.55 -- -- -- -- -- -- 0.045 0.022 -- 0.0011 0.030 -- 0.0030 876 771D 0.04 0.29 0.72 0.41 0.14 0.55 -- -- -- 0.020 -- -- -- 0.025 -- -- 880 816E 0.22 0.06 0.33 -- -- -- -- -- -- -- -- -- -- 0.007 -- -- 817 815F 0.10 0.20 1.00 0.61 0.89 0.30 0.30 -- -- -- -- 0.046 0.0009 0.069 -- 0.0020 855 709G 0.11 0.16 0.30 -- 9.9 5.8 0.90 8.8 0.1 -- -- -- -- 0.005 -- -- 728 600 or belowH 0.05 1.05 0.30 1.33 1.72 0.40 0.50 -- -- 0.025 0.009 -- 0.0008 0.077 -- -- 901 703I 0.07 0.26 1.72 -- -- -- -- -- -- -- 0.018 -- -- 0.025 -- -- 868 751J 0.08 0.25 1.71 -- -- -- -- -- -- 0.014 -- -- -- 0.027 -- -- 864 748K 0.12 0.26 1.31 -- -- -- -- -- -- -- -- 0.042 -- 0.018 -- -- 856 768L 0.11 0.28 1.21 -- -- -- -- -- -- -- -- -- 0.0011 0.027 0.05 -- 855 779M 0.12 0.33 1.06 -- -- 0.45 -- -- -- -- 0.011 -- -- 0.035 -- -- 854 781__________________________________________________________________________ TABLE 3(1)__________________________________________________________________________ Reduction Sum of ratio strains in non- imparted Mechanical properties recrystal- Start to plate Ductile- Rolling lization temp. of surface Plate brittle finish temp. repeated portion by thick- Heat Tensile Yield transition Steel Classifi- Slab temp. region bending repeated ness treat- strength strength temp.No. No. cation history (.degree.C.) (%) (.degree.C.) bending (mm) ment (kgf/mm.sup.2) (kgf/mm.sup.2) (.degree.C.)__________________________________________________________________________1 A Invention Re-heating 910 0 775 1.2 60 a 54 40 -81 rolling2 A Invention Re-heating 786 50 771 2.6 60 a 53 41 -106 rolling3 A Comp. Re-heating 791 50 ##STR8## ##STR9## 60 a 51 38 -66 example rolling4 A Comp. Re-heating 915 0 ##STR10## ##STR11## 60 a 51 38 -62 example rolling5 B Invention Re-heating 785 65 770 2.4 30 b 57 40 -105 rolling6 C Invention Re-heating 905 22 894 1.5 28 c 63 46 -100 rolling7 C Comp. Re-heating 911 25 890 ##STR12## 28 c 61 41 -82 example rolling8 D Invention Re-heating 853 70 832 2.6 18 d 62 50 -76 rolling9 D Comp. Re-heating 850 70 ##STR13## ##STR14## 18 d 60 46 -61 example rolling__________________________________________________________________________ NOTE: Underline represents a value other than the value of this invention. TABLE 3(2)__________________________________________________________________________ Reduction Sum of ratio strains in non- imparted Mechanical properties recrystal- Start to plate Ductile- Rolling lization temp. of surface Plate brittle finish temp. repeated portion by thick- Heat Tensile Yield transition Steel Classifi- Slab temp. region bending repeated ness treat- strength strength temp.No. No. cation history (.degree.C.) (%) (.degree.C.) bending (mm) ment (kgf/mm.sup.2) (kgf/mm.sup.2) (.degree.C.)__________________________________________________________________________10 B Comp. Re-heating 781 65 ##STR15## ##STR16## 30 b 55 37 -90 example rolling11 E Invention Re-heating 865 0 765 4.2 30 c 48 34 -15 rolling12 E Comp. Re-heating 854 0 ##STR17## ##STR18## 30 c 46 32 +20 example rolling13 F Invention Re-heating 790 50 779 1.2 25 e 90 82 -119 rolling14 F Invention Re-heating 786 50 771 3.3 25 e 92 86 -126 rolling15 F Comp. Re-heating 788 50 ##STR19## ##STR20## 25 e 87 79 -88 example rolling16 H Invention Re-heating 950 0 772 1.5 20 e 81 77 -135 rolling17 H Comp. Re-heating 950 0 ##STR21## ##STR22## 20 e 76 69 -113 example rolling18 H Invention Direct 936 0 785 3.3 20 e 83 80 -140 rolling__________________________________________________________________________ TABLE 3(3)__________________________________________________________________________ Reduction Sum of ratio strains in non- imparted Mechanical properties recrystal- Start to plate Ductile- Rolling lization temp. of surface Plate brittle finish temp. repeated portion by thick- Heat Tensile Yield transition Steel Classifi- Slab temp. region bending repeated ness treat- strength strength temp.No. No. cation history (.degree.C.) (%) (.degree.C.) bending (mm) ment (kgf/mm.sup.2) (kgf/mm.sup.2) (.degree.C.)__________________________________________________________________________19 G Invention Re-heating 785 20 771 1.6 20 f 160 140 -71 rolling20 G Comp. Re-heating 772 40 ##STR23## ##STR24## 20 f 158 137 -55 Example rolling21 H Invention Direct 775 50 765 2.3 20 e 88 84 -150 rolling22 H Comp. Direct 769 50 ##STR25## ##STR26## 20 e 86 81 -118 example rolling23 I Invention Re-heating 790 50 778 2.1 40 b 60 46 -96 rolling24 I Comp. Re-heating 795 50 ##STR27## ##STR28## 40 b 58 42 -76 example rolling25 J Invention Re-heating 806 66 794 1.8 18 d 68 51 -89 rolling26 J Comp. Re-heating 818 66 ##STR29## ##STR30## 18 d 65 48 -69 example rolling27 K Invention Re-heating 791 50 783 2.5 30 b 61 47 -73 rolling__________________________________________________________________________ TABLE 3(4)__________________________________________________________________________ Reduction Sum of ratio strains in non- imparted Mechanical properties recrystal- Start to plate Ductile- Rolling lization temp. of surface Plate brittle finish temp. repeated portion by thick- Heat Tensile Yield transition Steel Classifi- Slab temp. region bending repeated ness treat- strength strength temp.No. No. cation history (.degree.C.) (%) (.degree.C.) bending (mm) ment (kgf/mm.sup.2) (kgf/mm.sup.2) (.degree.C.)__________________________________________________________________________28 K Comp. Re-heating 795 50 ##STR31## ##STR32## 30 b 59 45 -58 example rolling29 L Invention Re-heating 806 40 791 1.9 25 e 65 59 -115 rolling30 L Comp. Re-heating 800 40 ##STR33## ##STR34## 25 e 61 55 -73 example rolling31 M Invention Re-heating 795 60 788 2.1 12 e 68 62 -115 rolling32 M Comp. Re-heating 795 60 ##STR35## ##STR36## 12 e 65 57 -89 example rolling33 A Comp. Re-heating 790 50 690 ##STR37## 60 a 52 39 -70 example rolling34 C Comp. Re-heating 910 0 901 ##STR38## 28 c 61 42 -80 example rolling35 C Invention Re-heating 906 0 900 1.2 28 c 63 46 -101 rolling__________________________________________________________________________

Example 2

Examples of the present invention in the case (2) will be explained.

The method of the present invention and the comparative method shown in Table 4 were applied to the steels of the present invention having the components shown in Table 2, and the strength, the toughness, and the Kca value shown in Tables 4(1) to 4(4) were obtained. Here, the Kca value was measured by a temperature gradient type ESSO test (refer, for example, to H. Kihara "Brittle Breakdown 2", Baifukan, p.41). When the results in Tables 4(1) to 4(4) were put in order by the thick steel plates having the same components and the same plate thickness at the same test temperature, it could be understood that the Kca value of the steels of the present invention was improved by at least 100 kgf/mm.sup.1.5, the strength of the base metal remained substantially equivalent or more, and the ductile-brittle transition temperature was improved by at least 10.degree. C. It could be understood from Table 4 that the steels of the present invention obviously exhibited better material characteristics and the present invention was effective. Repeated bending was carried out using a hot leveler.

By the way, the heat-treatment pattern (after rolling or after repeated bending) was the same as that of Example 1.

TABLE 4(1)__________________________________________________________________________ Cumulative Proportion reduction cooled to .alpha. single Plate ratio in .alpha. single phase or .gamma./.alpha. thickness at phase or .gamma./.alpha. dual phase Finish temp. start of dual phase temp. region Rolling of repeated water cooling temp. region (one side) finish bendingPlate No. Steel No. Slab history (mm) (%) (%) (.degree.C.) (.degree.C.)__________________________________________________________________________1 A Inv. Re-heating rolling 190 86.8 10 860 8492 A Inv. Re-heating rolling 40 37.5 8 863 8513 A Comp. Ex. Re-heating rolling ##STR39## ##STR40## 0 ##STR41## 7924 A Comp. Ex. Re-heating rolling 190 86.8 10 860 ##STR42##5 B Inv. Re-heating rolling 40 25.0 9 845 7866 B Comp. Ex. Re-heating rolling ##STR43## ##STR44## 0 845 7907 C Inv. Re-heating rolling 150 89.3 11 880 8118 C Comp. Ex. Re-heating rolling 150 89.3 11 ##STR45## 8049 D Inv. Re-heating rolling 40 55.0 7 885 86910 D Comp. Ex. Re-heating rolling 40 55.0 7 885 86811 E Inv. Re-heating rolling 50 40 8 820 81612 E Comp. Ex. Re-heating rolling 35 ##STR46## 7 825 ##STR47##13 F Inv. Re-heating rolling 120 66.7 18 860 76514 F Comp. Ex. Re-heating rolling 120 66.7 17 858 ##STR48##15 F Inv. Direct rolling 80 37.5 10 865 85016 F Comp. Ex. Direct rolling ##STR49## ##STR50## 0 ##STR51## 77117 G Inv. Re-heating rolling 120 73.3 21 730 71618 G Comp. Ex. Re-heating rolling ##STR52## ##STR53## 0 755 739__________________________________________________________________________ Note: Inv.: Invention, Comp. Ex.; Comparative example (1) JIS No. 4 testpiece was used for both tensile test and impact test (1/4t L direction). (2) Temperature gradient type ESSO test was used for arrest test. TABLE 4(2)__________________________________________________________________________ Sum of strains Mechanical properties imparted to Ductile- plate sur- brittle Temp. face portion Plate Heat- Tensile transition exhibiting by repeated thickness treatment strength temp. Kca value Kca valuePlate No. Steel No. bending (mm) pattern (kgf/mm.sup.2) (.degree.C.) (kgf/mm.sup.1.5) (.degree.C.)__________________________________________________________________________1 A 1.1 25 d 56 -105 670 -802 A 1.0 25 d 55 -100 660 -803 A Comp. 1.3 25 d 53 -85 280 -804 A Comp. ##STR54## 25 d 52 -90 360 -805 B 6.5 30 b 55 -100 510 -406 B Comp. 6.5 30 b 53 -85 300 -407 C 3.1 16 c 59 -95 650 -608 C Comp. 3.1 16 c 58 -60 350 -609 D 1.3 18 d 61 -90 640 -8010 D Comp. ##STR55## 18 d 59 -65 460 -8011 E 2.5 30 c 47 -20 250 -2012 E Comp. 2.5 30 c 45 +15 120 -2013 F 3.5 50 e 93 -105 550 -4014 F Comp. ##STR56## 50 e 92 -91 450 -4015 F 2.5 50 c 94 -85 510 -4016 F Comp. 2.4 50 e 92 -50 280 -4017 G 3.8 30 f 151 -72 680 -19618 G Comp. 3.5 30 f 146 -36 490 -196__________________________________________________________________________ Notes: (1) JIS No. 4 testpiece was used for both tensile test and impact test (1/4t L direction). (2) Temperature gradient type ESSO test was used for arrest test. TABLE 4(3)__________________________________________________________________________ Cumulative Proportion reduction cooled to .alpha. single Plate ratio in .alpha. single phase or .gamma..sigma./.alpha. thickness at phase or .gamma./.alpha. dual phase Finish temp. start of dual phase temp. region Rolling of repeated water cooling temp. region (one side) finish bendingPlate No. Steel No. Slab history (mm) (%) (%) (.degree.C.) (.degree.C.)__________________________________________________________________________19 H Re-heating rolling 40 50.0 8 910 89820 H Comp. Re-heating rolling 40 50.0 9 905 ##STR57##21 H Re-heating rolling 120 83.3 11 910 89622 H Comp. Re-heating rolling 120 83.3 13 ##STR58## 85823 I Re-heating rolling 120 66.7 10 870 86024 I Comp. Re-heating rolling 120 66.7 10 871 86225 J Re-heating rolling 80 77.5 7 870 86026 J Comp. Re-heating rolling 80 77.5 7 872 ##STR59##27 K Re-heating rolling 120 75.0 9 862 791 K Comp. Re-heating rolling ##STR60## ##STR61## 0 ##STR62## ##STR63##2829 L Re-heating rolling 40 37.5 9 860 84930 L Comp. Re-heating rolling 30 ##STR64## 6 860 ##STR65##31 M Re-heating rolling 40 70.0 9 860 81032 M Comp. Re-heating rolling ##STR66## ##STR67## 0 861 80633 M Comp. Re-heating rolling 40 40.0 4 858 83534 M Comp. Re-heating rolling 40 70.0 10 857 760__________________________________________________________________________ Notes (1) JIS No. 4 testpiece was used for both tensile test and impact test (1/4t L direction). (2) Temperature gradient type ESSO test was used for arrest test. TABLE 4(4)__________________________________________________________________________ Sum of strains Mechanical properties imparted to Ductile- plate sur- brittle Temp. face portion Plate Heat- Tensile transition exhibiting by repeated thickness treatment strength temp. Kca value Kca valuePlate No. Steel No. bending (mm) pattern (kgf/mm.sup.2) (.degree.C.) (kgf/mm.sup.1.5) (.degree.C.)__________________________________________________________________________19 H 3.3 20 e 90 -155 680 -10020 H Comp. ##STR68## 20 e 88 -130 480 -10021 H 1.4 20 e 91 -160 700 -10022 H Comp. 1.1 20 e 92 -110 420 -10023 I 1.8 40 b 59 -93 470 -4024 I Comp. ##STR69## 40 b 56 -64 290 -4025 J 2.1 18 d 66 -85 660 -8026 J Comp. ##STR70## 18 d 64 -60 310 -8027 K 2.2 30 b 61 -70 390 -4028 K Comp. ##STR71## 30 b 60 -60 240 -4029 L 2.1 25 e 61 -95 510 -8030 L Comp. ##STR72## 25 e 58 -70 240 -8031 M 2.4 12 e 65 -100 650 -8032 M Comp. 2.4 12 e 62 -80 320 -8033 M Comp. 2.4 12 e 63 -81 400 -8034 M Comp. 3.3 12 e 63 ##STR73## 500 -80__________________________________________________________________________ Notes (1) JIS No. 4 testpiece was used for both tensile test and impact test (1/4t L direction). (2) Temperature gradient type ESSO test was used for arrest test.

Example 3

Examples of the present invention in the case (3) will be explained.

When the method of the present invention and the comparative method shown in Tables 5(1) to 5(4) were applied to the steels of the present invention having the components shown in Table 2, the strength and the toughness shown in Tables 5(1) to 5(4) were obtained. When comparison was made by the steels having the same components, it was found that the difference of the tensile strength in the L direction/T direction of the steels of the present invention was within 1 kgf/mm.sup.2 and the ductile-brittle transition temperature of the Sharpy impact test was within 3.degree. C. There could thus be obtained the thick steel plates having extremely small material anisotropy. No. 2 of this example represents the case where non-recrystallization temperature region rolling was not carried out. For this reason, material anisotropy was small, but the ductile brittle transition temperature of the Sharpy impact test was deteriorated by about 50.degree. C. in comparison with the steel No. 3 of the present invention. It could be understood from these results that the steels of the present invention obviously exhibited excellent material characteristics devoid of material anisotropy, and the present invention was effective. Repeated bending was carried out using a hot leveler.

By the way, the heat-treatment pattern (after rolling or after repeated bending) in the table was the same as that of Example 1.

TABLE 5(1)__________________________________________________________________________ Sum of Reduc- strains Mechanical Mechanical tion ratio imparted properties properties in non- to plate (L direction) (T direction) crystal- Start surface Ductile- Ductile- lization Rolling temp. of portion Plate Tensile brittle Tensile brittle temp. finish repeated by thick- Heat- strength transition strength transitionClassi- Steel Slab region temp. bending repeated ness treat- (kgf/ temp. (kgf/ temp.fication No. No. history (%) (.degree.C.) (.degree.C.) bending (mm) ment mm.sup.2) (.degree.C.) mm.sup.2) (.degree.C.)__________________________________________________________________________Comp. 1 A Re-heating 40 785 ##STR74## ##STR75## 60 a 54 -108 51 -71steel rollingComp. 2 A Re-heating -0 915 775 1.9 60 a 54 -66 53 -62steel rollingSteel 3 A Re-heating 40 801 780 4.2 60 a 55 -115 55 -115of Inv. rollingSteel 4 A Direct 50 770 765 3.5 60 a 57 -92 56 -91of Inv. rollingSteel 5 B Re-heating 67 780 766 3.5 30 b 58 -106 58 -106of Inv. rollingComp 6 B Re-heating 67 782 ##STR76## ##STR77## 30 b 57 -110 53 -81steel rollingSteel 7 C Re-heating 25 801 790 3.5 28 c 64 -101 64 -100of Inv. rollingComp. 8 C Re-heating 25 800 790 1.0 28 c 65 -106 62 -71steel rollingSteel 9 D Re-heating 70 845 821 7.5 18 d 60 -77 61 -75of Inv. rolling__________________________________________________________________________ TABLE 5(2)__________________________________________________________________________ Sum of Reduc- strains Mechanical Mechanical tion ratio imparted properties properties in non- to plate (L direction) (T direction) crystal- Start surface Ductile- Ductile- lization Rolling temp. of portion Plate Tensile brittle Tensile brittle temp. finish repeated by thick- Heat- strength transition strength transitionClassi- Steel Slab region temp. bending repeated ness treat- (kgf/ temp. (kgf/ temp.fication No. No. history (%) (.degree.C.) (.degree.C.) bending (mm) ment mm.sup.2) (.degree.C.) mm.sup.2) (.degree.C.)__________________________________________________________________________Comp. 10 D Re-heating 70 851 ##STR78## ##STR79## 18 d 62 -76 58 -48steel rollingSteel 11 E Re-heating 40 796 780 4.1 30 c 48 -30 47 -30of Inv. rollingComp. 12 E Re-heating 40 789 775 ##STR80## 30 c 47 -22 44 +10steel rollingSteel 13 F Re-heating 50 770 758 2.8 25 e 90 -125 90 -123of Inv. rollingComp. 14 F Re-heating ##STR81## 775 765 2.8 25 e 88 -108 85 -84steel rollingComp. 15 F Re-heating 50 772 ##STR82## ##STR83## 25 e 90 -120 88 -88steel rollingSteel 16 G Re-heating 20 785 771 4.6 20 f 160 -72 160 -72of Inv. rollingComp. 17 G Re-heating -0 950 720 3.3 20 f 148 -51 145 -50steel rollingSteel 18 H Re-heating 50 825 812 2.1 20 e 91 -165 90 -163of Inv. rolling__________________________________________________________________________ TABLE 5(3)__________________________________________________________________________ Sum of Reduc- strains Mechanical Mechanical tion ratio imparted properties properties in non- to plate (L direction) (T direction) crystal- Start surface Ductile- Ductile- lization Rolling temp. of portion Plate Tensile brittle Tensile brittle temp. finish repeated by thick- Heat- strength transition strength transitionClassi- Steel Slab region temp. bending repeated ness treat- (kgf/ temp. (kgf/ temp.fication No. No. history (%) (.degree.C.) (.degree.C.) bending (mm) ment mm.sup.2) (.degree.C.) mm.sup.2) (.degree.C.)__________________________________________________________________________Steel 19 H Re-heating 50 820 810 5.1 20 c 91 -161 91 -163of Inv. rollingComp. 20 H Re-heating 50 816 ##STR84## ##STR85## 20 e 90 -155 88 - 112steel rollingSteel 21 I Re-heating 40 785 776 3.5 30 b 61 -97 62 -95of Inv. rollingComp. 22 I Re-heating 40 779 ##STR86## ##STR87## 30 b 60 -94 58 -72steel rollingSteel 23 J Re-heating 60 815 808 2.1 20 d 65 -75 65 -73of Inv. rollingComp 24 J Re-heating 60 812 ##STR88## ##STR89## 20 d 64 -66 62 -38steel rollingSteel 25 K Re-heating 50 790 776 1.7 30 b 60 -72 60 -72of Inv. rollingComp. 26 K Re-heating 50 792 ##STR90## ##STR91## 30 b 57 -60 55 -40steel rollingSteel 27 L Re-heating 40 802 791 3.4 25 e 64 -105 63 -103of Inv. rolling__________________________________________________________________________ TABLE 5(4)__________________________________________________________________________ Sum of Reduc- strains Mechanical Mechanical tion ratio imparted properties properties in non- to plate (L direction) (T direction) crystal- Start surface Ductile- Ductile- lization Rolling temp. of portion Plate Tensile brittle Tensile brittle temp. finish repeated by thick- Heat- strength transition strength transitionClassi- Steel Slab region temp. bending repeated ness treat- (kgf/ temp. (kgf/ temp.fication No. No. history (%) (.degree.C.) (.degree.C.) bending (mm) ment mm.sup.2) (.degree.C.) mm.sup.2) (.degree.C.)__________________________________________________________________________Comp. 28 L Re-heating 40 798 789 ##STR92## 25 e 62 -98 60 -71steel rollingSteel 29 M Re-heating 60 795 780 3.3 12 e 69 -105 68 -102of Inv. rollingComp. 30 M Re-heating 60 795 ##STR93## ##STR94## 12 e 66 -90 63 -68steel rolling__________________________________________________________________________

Example 4

Examples of the present invention in the case (4) will be explained.

When the method of the present invention and the Comparative method shown in Tables 6(1) and 6(2) were applied to the steels of the present invention shown in Table 2, the strength, the toughness, and the Kca values shown in Tables 6(1) and 6(2) were obtained. Here, the Kca value was measured by the temperature gradient type ESSO test in the same way as in Example 2.

When Tables 6(1) and 6(2) were put in order by the thick steel plates having the same components and the same plate thickness at the same test temperature, it was found out that the Kca value was improved by at least 100 mm.sup.1.5, the strength of the base metal remained substantially equivalent, and the-ductile-brittle transition temperature, too, was improved by at least 10.degree. C. It could be understood from Tables 6(1) to 6(2) that the steels of the present invention obviously exhibited better material characteristics and the present invention was effective. Repeated bending was carried out using the hot leveler.

By the way, the heat-treatment pattern (after rolling or after repeating bending) in the table was the same as that of Example 1.

TABLE 6(1)__________________________________________________________________________ Cumulative Proportion reduction cooled to .alpha. single Plate ratio in .alpha. single phase or .gamma./.alpha. thickness at phase or .gamma./.alpha. dual phase Start temp. start of dual phase temp. region Rolling of repeated Classifi- water cooling temp. region (one side) finish bendingPlate No. Steel No. cation Slab history (mm) (%) (%) (.degree.C.) (.degree.C.)__________________________________________________________________________ 1 A Re-heating rolling 200 87.5 18 815 806 2 A Re-heating rolling 40 37.5 30 715 712 3 A Comp. Re-heating rolling ##STR95## -0 796 785 4 A Comp. Re-heating rolling 200 87.5 20 810 ##STR96## 5 A Comp. Re-heating rolling ##STR97## -0 785 ##STR98## 6 B Re-heating rolling 40 25.0 20 775 765 7 B Comp. Re-heating rolling ##STR99## -0 790 ##STR100## 8 C Re-heating rolling 150 89.3 7 875 865 9 C Comp. Re-heating rolling ##STR101## -0 855 84210 D Re-heating rolling 40 55 30 717 71211 D Comp. Re-heating rolling 40 55 30 715 71112 E Re-heating rolling 50 40 20 780 77113 E Comp. Re-heating rolling 35 ##STR102## 22 768 76014 E Direct rolling 120 75.0 34 688 68515 E Comp. Direct rolling ##STR103## -0 796 ##STR104##16 F Re-heating rolling 120 66.7 25 755 75017 F Comp. Re-heating rolling 120 66.7 27 736 ##STR105##18 F Re-heating rolling 80 37.5 33 696 69519 G Re-heating rolling 120 73.3 24 650 64820 G Comp. Re-heating rolling ##STR106## ##STR107## 661 ##STR108##21 H Re-heating rolling 40 50 20 850 83222 H Comp. Re-heating rolling 40 50 20 849 83523 H Comp. Re-heating rolling ##STR109## ##STR110## 851 ##STR111##24 I Re-heating rolling 120 66.7 16 840 83125 I Comp. Re-heating rolling 120 66.7 6 ##STR112## 86426 J Re-heating rolling 80 77.5 24 805 79227 J Comp. Re-heating rolling 80 77.5 25 800 ##STR113##28 K Re-heating rolling 120 75.0 18 815 80229 K Comp. Re-heating rolling ##STR114## ##STR115## 775 ##STR116##30 L Re-heating rolling 40 37.5 10 815 80331 L Comp. Re-heating rolling 30 16.7 -4 767 76032 M Re-heating rolling 40 70.0 28 850 84133 M Comp. Re-heating rolling 40 25.0 -4 850 839__________________________________________________________________________ Notes (1) JIS No. 4 testpiece was used for both tensile test and impact test (1/4t L direction). (2) Temperature gradient type ESSO test was used for arrest test. TABLE 6(2)__________________________________________________________________________ Mechanical properties Sum of strains Ductile- imparted brittle Temp. to plate Plate Heat- Tensile transition exhibiting surface portion thickness treatment strength temp. Kca value Kca valuePlate No. Steel No. by repeated bending (mm) pattern (kgf/mm.sup.2) (.degree.C.) (kgf/mm.sup.1.5) (.degree.C.)__________________________________________________________________________ 1 A 1.9 25 d 57 -110 690 -80 2 A 3.2 25 d 56 -100 670 -80 3 A 2.8 25 d 53 -88 360 -80 4 A ##STR117## 25 d 52 -90 480 -80 5 A ##STR118## 25 d 52 -80 290 -80 6 B 6.8 30 b 56 -105 500 -40 7 B ##STR119## 30 b 55 -90 310 -40 8 C 3.3 16 c 62 - 96 680 -60 9 C 3.3 16 c 61 -80 400 -6010 D 1.9 18 d 62 -90 710 -8011 D ##STR120## 18 d 61 -72 500 -8012 E 2.2 30 c 48 -25 280 -2013 E 2.2 30 c 46 +10 150 -2014 E 6.1 30 c 49 -15 220 -2015 E ##STR121## 30 c 49 +35 120 016 F 2.8 50 e 97 -105 550 -4017 F ##STR122## 50 e 95 -90 400 -4018 F 3.6 50 e 95 -110 650 -4019 G 6.5 32 f 153 -77 700 -19620 G ##STR123## 32 f 148 -35 550 -19621 H 2.8 20 e 90 -160 660 -10022 H ##STR124## 20 e 89 -135 480 -10023 H ##STR125## 20 e 85 -150 310 -10024 I 1.8 40 b 59 -92 480 -4025 I ##STR126## 40 b 56 -65 280 -4026 J 2.5 18 d 67 -80 660 -8027 J ##STR127## 18 d 65 -60 300 -8028 K 2.2 30 b 61 -72 380 -4029 K ##STR128## 30 b 60 -65 250 -4030 L 2.1 25 e 60 -96 500 -8031 L 2.6 25 e 58 -70 250 -8032 M 2.3 12 e 66 -110 690 -8033 M 2.2 12 e 64 -80 310 -80__________________________________________________________________________ Notes (1) JIS No. 4 testpiece was used for both tensile test and impact test (1/4t L direction). (2) Temperature gradient type ESSO test was used for arrest test.

Claims

1. A method of producing a strong and tough thick steel plate, comprising:

casting a steel consisting of 0.02 to 0.30 wt % of C, 0.01 to 2.0 wt % of Si, 0.30 to 3.5 wt % of Mn, 0.003 to 0.10 wt % of Al, and the balance of Fe and unavoidable impurities, into an ingot or a slab;

hot rolling said ingot or said slab at a cumulative reduction ratio of at least 20% in a temperature region higher than an Ar.sub.3 transformation point in succession to said casting or after heating, to obtain a hot rolled steel plate having an austenite texture;

applying repeated bending to said hot rolled steel plate in an austenite non-recrystallization temperature region or in a temperature region higher than said Ar.sub.1 transformation point but lower than an Ar.sub.3 transformation point so as to impart a cumulative strain quantity E (%) expressed by the following formula; and

cooling the resulting bent work so as to transform austenite crystal grains in said bent work to fine ferrite crystal grains;

-1.14.times.10.sup.-3 T+2.4>E.gtoreq.1.71.times.10.sup.-3 T-0.4

where

E: sum (%) of the strains which a plate surface receives by repeated bending,

T: temperature (.degree.C.) of the plate surface when said repeated bending is carried out, within the region of Ar.sub.1 to 1,000.degree. C.

2. A method according to claim 1, wherein, when rolling is carried out in succession to casting of said ingot or said slab or after re-heating it, said ingot or said slab is cooled from a temperature region higher than the Ac.sub.3 point before, or during, rolling, so as to form an austenite-ferrite two-phase texture or a ferrite single phase texture at a portion having a thickness of at least 5% from the surface of said ingot or said slab, rolling is then carried out at a cumulative reduction ratio of at least 20% in the temperature region of said texture in the process of a temperature rise due to recuperation of said ingot or said slab so as to convert said texture to an austenite single phase texture during, or after, rolling, and thereafter repeated bending is carried out in an austenite non-recrystallization temperature region higher than the Ar.sub.1 point so as to impart a cumulative strain quantity E expressed by the following formula:

-1.14.times.10.sup.-3 T+24>E.gtoreq.1.65.times.10.sup.-3 T-0.5 (%)

where T: Ar.sub.1 to 1,000.degree. C.

3. A method according to claim 2, wherein cooling water is sprayed at a rate of 0.05 to 1.0 m.sup.3 /min.multidot.m.sup.2 to said ingot or said slab before, or during, rolling from a temperature region higher than Ac.sub.3.

4. A method according to claim 1, wherein at least one kind of the member selected from each of the following groups (a) to (e) is further added:

(a) 0.001 to 0.10 wt % of material selected from group consisting of Nb and Ti,

(b) at least one member selected from the group consisting of Cu: 0.05 to 3.0 wt %, Ni: 0.05 to 10.0 wt %, Cr: 0.05 to 10.0 wt %, Mo: 0.05 to 3.5 wt %, Co: 0.05 to 10.0 wt % and W: 0.05 to 2.0 wt %,

(c) V: 0.002 to 0.10 wt %,

(d) B: 0.0003 to 0.0025 wt %, and

(e) material selected from the group consisting of Rem: 0.002 to 0.10 wt % and Ca: 0.0003 to 0.0040 wt %.

5. A method according to claim 1, wherein said steel material subjected to said repeated bending is left standing for cooling before said repeated bending.

6. A method according to claim 1, wherein said steel material subjected to said repeated bending is left standing for cooling at a mean rate of 0.5 to 80.degree. C./S of thickness of the plate.

7. A method of producing a strong and tough thick steel plate, comprising:

casting a steel consisting of 0.02 to 0.30 wt % of C, 0.01 to 2.0 wt % of Si, 0.30 to 3.5 wt % of Mn, 0.003 to 0.10 wt % of Al and the balance consisting of Fe and unavoidable impurities, into an ingot or a slab;

hot rolling said ingot or said slab in succession to said casing or after heating, at a cumulative reduction ratio of at least 20% in an austenite non-recrystallization temperature region;

applying repeated bending to said hot rolled steel plate in the austenite non-recrystallization temperature region or a temperature region lower than an Ar.sub.3 transformation point but higher than an Ar.sub.1 transformation point so as to impart a cumulative strain quantity E (%) expressed by the following formula and to obtain fine austenite recrystallized grains; and

cooling the resulting bent work to transform said austenite recrystallized grains to fine ferrite crystal grains:

E.gtoreq.-1.14.times.10.sup.-3 T.sub.2 +2.4

where

E: sum (%) of strains which the plate surface receives by said repeated bending,

T.sub.2 : temperature (.degree.C.) of the plate surface when said repeated bending is carried out, within higher than Ar.sub.1.

8. A method of claim 7, wherein at least one member selected from the following groups (a) to (e) is further added:

(a) 0.001 to 0.10 wt % of material selected from group consisting of Nb and Ti;

(b) at least one member selected from the group consisting of Cu: 0.05 to 3.0 wt %, Ni: 0.05 to 10.0 wt %, Cr: 0.05 to 10.0 wt %, Mo: 0.05 to 3.5 wt%, Co: 0.05 to 10.0 wt % and W: 0.05 to 2.0 wt %,

(c) V: 0.002 to 0.10 wt %,

(d) B: 0.0003 to 0.0025 wt %, and

(e) material selected from the group consisting of Rem: 0.002 to 0.10 wt %, and Ca: 0.0003 to 0.0040 wt %.

9. A method according to claim 7, wherein said steel material subjected to said repeated bending is left standing for cooling.

10. A method according to claim 7, wherein said steel material subjected to said repeated bending is cooled at a mean cooling rate of 0.5 to 80.degree. C./S in the direction of thickness of the plate.

11. A method of producing a strong and tough thick steel plate, comprising:

casting a steel consisting of 0.02 to 0.30 wt % of C, 0.01 to 2.0 wt % of Si, 0.30 to 3.5 wt % of Mn, 0.003 to 0.10 wt % of Al and the balance consisting of Fe and unavoidable impurities to form a crude steel billet;

cooling said crude steel billet in succession to said casting or after heating, from a temperature region higher than an Ac.sub.3 point to convert a texture to an austenite-ferrite dual phase state or a ferrite single phase state at a portion of at least 5% from both surfaces of said crude steel billet in the direction of thickness;

rolling said crude billet at a cumulative reduction ratio of at least 20% in the temperature region of said texture state during a temperature rise process by recuperative heat of said crude steel billet, and raising the surface temperature of the resulting hot-rolled plate to a temperature in the region of from (Ac.sub.3 point minus 200.degree. C.) to a point less than the Ac.sub.3 point after completion of rolling;

applying subsequently repeated bending in said temperature region, where the austenite-ferrite dual phase region exists, so as to impart a cumulative strain quantity E (%) expressed by the following formula and to obtain fine ferrite recrystallized grains; and

cooling the resulting bent work so as to inhibit the grain growth of the recrystallized ferrite grains:

E.gtoreq.-1.2.times.10.sup.-3 T.sub.3 +2.7(%)

where

E: sum (%) of the strains which a plate surface receives by repeated bending,

T.sub.3 : temperature (.degree.C.) of the plate surface when said repeated bending is carried out, within the region of not higher than Ac.sub.3.

12. A method according to claim 11, wherein cooling water is sprayed at a rate of 0.05 to 1.0 m.sup.3 /min.multidot.m.sup.2 from a temperature region higher than Ac.sub.3 point before, or during, rolling of said ingot or said slab.

13. A method according to claim 12, wherein at least one member selected from the following groups (a) to (e) is further added:

(a) 0.001 to 0.10 wt % of material selected from group consisting of Nb and Ti;

(b) at least one member selected from the group consisting of Cu: 0.05 to 3.0 wt %, Ni: 0.05 to 10.0 wt %, Cr: 0.05 to 10.0 wt %, Mo: 0.05 to 3.5 wt %, Co: 0.05 to 10.0 wt %, and W: 0.05 to 2.0 wt %,

(c) V: 0.002 to 0.10 wt %,

(d) B: 0.003 to 0.0025 wt %, and

(e) material selected from the group consisting of Rem: 0.002 to 0.10 wt % and Ca: 0.0003 to 0.0040 wt %.

14. A method according to claim 11, wherein said steel material subjected to said repeated bending is left standing for cooling.

15. A method according to claim 11, wherein cooling is carried out at a mean cooling rate of 0.5 to 80.degree. C./S in the direction of thickness.