JPH08225834A - Production of low yield ratio high tensile strength steel plate - Google Patents

Abstract

PURPOSE: To produce a low yield ratio high tensile strength steel plate having sufficient performance as a steel for welding structures by subjecting a slab having a specified componental compsn. in which the total content of P and N and the contents of Mg, Ca and Y are prescribed to heating under specified conditions and executing hot rolling. CONSTITUTION: A slab having a compsn. contg., by weight, 0.01 to 0.20% C, 0.03 to 1.0% Si, 0.30 to 2.0% Mn, <=0.008%. P, <=0.005% S, 0.005 to 0.1% Al, 0.001 to 0.006% N, 0.005 to 0.020% Ti and 0.001 to 0.010% O, in which the total content of P and N is regulated to <=0.01%, furthermore added with total 0.002 to 0.01% of one or more kinds among Mg, Ca nad Y, and the balance iron with inevitable impurities is prepd. This slab is heated to the Ac3 transformation point or above to 1250 deg.C, and hot rolling is finished at the Ar3 transformation point or above. Thus, the steel plate low in a yield ratio and excellent in plastic deformability can be obtd. at a reduced production cost.

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 high yield steel plate having a low yield ratio and having a sufficient performance as a steel for welded structure, a low yield ratio and an excellent plastic deformability.

[0002]

2. Description of the Related Art A conventional method for producing a low yield ratio steel is a method of applying an intermediate heat treatment (hereinafter referred to as QLT treatment) of heating in a ferrite (α) + austenite (γ) two-phase region between quenching and tempering heat treatments. As represented by, the purpose is basically to mix α as a soft phase and bainite or martensite as a hard phase. And the overall strength level and yield ratio have been controlled by varying the mixing ratio of these phases.

Various manufacturing methods for obtaining the mixed structure of the soft phase and the hard phase have been proposed in the past. For example, Japanese Unexamined Patent Publication No. 53-23817 discloses that the steel sheet is reheat-quenched and then the A _c1 transformation is performed. Between the A _c3 transformation point and γ and α
The method of air-cooling as two phases is shown,
Japanese Unexamined Patent Publication No. 4-314824 similarly discloses a method of reheating to a two-phase region and then quenching. Also, as a method of manufacturing online without performing reheating treatment,
For example, after performing hot rolling toward two-phase region from γ zone in JP 63-286517, 2 from A _r3 transformation point
A method is disclosed in which the α phase is generated by air cooling to a temperature 0 to 100 ° C. lower, and then rapidly cooled.

However, the conventional techniques for obtaining the dual phase steel as described above are generally complicated, and the productivity is likely to decrease. That is, since the number of steps is increased in the above-mentioned QLT processing, manufacturing cost is increased and productivity is decreased. Further, as another typical manufacturing method, in a so-called DQ process in which hot rolling is performed and then direct quenching is performed, after rolling, the steel is not immediately quenched, but allowed to cool until a certain amount of α is formed and then quenched. A method of forming a phase structure (hereinafter referred to as DLT treatment) can be mentioned.

However, in this method, the intermediate heat treatment can be omitted as compared with the QLT treatment, but the waiting time from the rolling to the quenching becomes long, so that the reduction in productivity cannot be avoided. In addition, since the waiting time is long, temperature nonuniformity in the plate is likely to occur, which tends to cause a large variation in the material within the plate. Further, α and the hard second phase produced in such a process are likely to become coarse, and toughness is likely to be deteriorated.

[0006]

As long as a low yield ratio is achieved by changing the conventional ratio of α which is a soft phase and martensite or bainite which is a hard phase, there are the following two major problems. . That is, the first point is that the productivity is remarkably increased because the slow cooling or holding process in the α / γ two-phase region is indispensable for producing α regardless of whether it is an online process or a reheating process. That is the point of hindrance. The second point is that when manufactured by such a process, α tends to be coarse, and the second phase is also likely to be coarse, so that it is not easy to secure toughness.

The present invention has a low yield ratio from a completely new point of view, which does not depend on the conventional technique of changing the yield ratio depending on the structure ratio in the two-phase structure of α phase and hard phase, and has the prior art. By solving the above problems, having a sufficient performance as a welded structural steel, a low yield ratio low plasticity of low yield ratio high tensile strength steel sheet to find a method of manufacturing at low cost. .

[0008]

[Means for Solving the Problems] In the composite structure steel,
When the proportion of the hard phase is small, the yield stress is almost dominated by the yield stress of the soft phase. Also, in single-phase steel, the characteristics of the α phase, which is the matrix, are considered to have a large effect on the yield stress. The present inventors believe that if it is possible to achieve a low yield ratio by lowering the yield stress of the α phase itself, it is possible to achieve a low yield ratio independent of the microstructure without being limited to composite microstructure steel. The inventors have come to discover the present invention.

Factors that control the yield stress of the α phase itself include crystal grain size, solid solution element, dislocation density, and precipitation state. Regarding the crystal grain size, if the grain size is made coarse, the yield stress can be reduced, but in that case, deterioration of the toughness also occurs at the same time, so this is not a generally applicable means. Both dislocation strengthening and precipitation strengthening have a large effect on the yield stress, but since they greatly change depending on the manufacturing history and the chemical composition of the steel, it is considered difficult to provide a stable means of lowering the yield ratio.

On the other hand, solid solution strengthening is a strengthening means having a stable effect regardless of the structure and manufacturing history. Conversely, by reducing the solid solution strengthening, which is unavoidable under normal manufacturing conditions, by some means, the yield stress is reduced without deteriorating other properties or complicating manufacturing conditions. It is thought to be possible. Of course, if the amount of P or Si, which has a large amount of solid solution strengthening, is reduced, it is possible to lower the yield stress, but such means imposes a load on the steelmaking process, resulting in an increase in manufacturing cost. Therefore, it is not a method that can be adopted at all.

The interstitial elements C and N exhibit particularly large solid solution strengthening. For example, in the case of C, the yield stress can be increased by about 100 MPa only by solid solution of several tens of ppm. In the ordinary steel plate manufacturing process, solid solution of C and N is inevitable even in a small amount in any of the processes, and therefore, the normally measured yield stress is the unavoidable solid solution of C and N. N
It is considered that the solid solution strengthening component due to is already included. Therefore, it is considered that if the inevitable inclusion of C and N, and further P as an impurity, can be canceled against the increase of the yield stress, the yield stress can be reduced and the resulting yield ratio can be reduced.

[0012] Based on the above idea, as a result of experimenting and studying means for lowering the yield stress itself of the α matrix of steel to lower the yield ratio, a suitable oxide was found in the α matrix. By dispersing, it becomes possible to precipitate the solid solution element contained inevitably contained in the oxide, which increases the yield stress, as the production nucleus without depending on the conditions of hot rolling, cooling and heat treatment. Since the amount of solid solution elements in the
The present invention has been developed by finding that the yield stress can be reduced regardless of the microstructure of steel. The gist of the invention is as follows.

The first aspect of the present invention is C: 0.01% by weight.
0.20%, Si: 0.03 to 1.0%, Mn: 0.3
0 to 2.0%, P: 0.008% or less, S: 0.005
% Or less, Al: 0.005 to 0.1%, N: 0.001
~ 0.006%, Ti: 0.005-0.020%,
O: 0.001 to 0.010% is contained, the total content of P and N is 0.01% or less, and one or two or more of Mg, Ca, Y is added as the total of the added amount, 0.002-
The steel slab consisting balance iron and unavoidable impurities added 0.01%, A _c3 transformation point or higher, then heated to a temperature of 1250 ° C. or less, and characterized in that to terminate the hot rolling at a temperature not lower than A _r3 transformation point A method for manufacturing a high-strength steel plate having a low yield ratio.

A second aspect of the invention is C: 0.01% by weight.
0.20%, Si: 0.03 to 1.0%, Mn: 0.3
0 to 2.0%, P: 0.008% or less, S: 0.005
% Or less, Al: 0.005 to 0.1%, N: 0.001
~ 0.006%, Ti: 0.005-0.020%,
O: contains 0.001 to 0.010%, Cr: 0.0
1 to 0.50%, Ni: 0.01 to 3.0%, Mo:
0.01 to 0.50%, Cu: 0.01 to 1.5%,
V: 0.005 to 0.20%, Nb: 0.003 to 0.
05%, B: 0.0003 to 0.0020% of 1 type or 2 types or more, and the total content of P and N is 0.01.
% Or less, and a steel slab composed of the balance iron and unavoidable impurities added with 0.002 to 0.01% as a total of the addition amount of one or more of Mg, Ca, and Y is not less than the A _c3 transformation point. A method for producing a high-strength steel sheet having a low yield ratio, which comprises heating to a temperature of 1250 ° C. or lower and ending hot rolling at a temperature of an _{Ar 3} transformation point or higher.

The third invention is, by weight%, C: 0.01 to.
0.20%, Si: 0.03 to 1.0%, Mn: 0.3
0 to 2.0%, P: 0.008% or less, S: 0.005
% Or less, Al: 0.005 to 0.1%, N: 0.001
~ 0.006%, Ti: 0.005-0.020%,
O: 0.001 to 0.010% is contained, the total content of P and N is 0.01% or less, and one or two or more of Mg, Ca, Y is added as the total amount of addition, 0.002
After heating a steel slab consisting of the balance iron added to 0.01% to unavoidable impurities to a temperature of A _c3 transformation point or more and 1250 ° C. or less and finishing hot rolling at a temperature of A _r3 transformation point or more,
Accelerated cooling at a cooling rate of 5 ° C / sec or more, 650-500
A method for producing a high-strength steel plate with a low yield ratio, which comprises accelerating cooling in the range of ° C.

A fourth invention is, in% by weight, C: 0.01 to.
0.20%, Si: 0.03 to 1.0%, Mn: 0.3
0 to 2.0%, P: 0.008% or less, S: 0.005
% Or less, Al: 0.005 to 0.1%, N: 0.001
~ 0.006%, Ti: 0.005-0.020%,
O: contains 0.001 to 0.010%, Cr: 0.0
1 to 0.50%, Ni: 0.01 to 3.0%, Mo:
0.01 to 0.50%, Cu: 0.01 to 1.5%,
V: 0.005 to 0.20%, Nb: 0.003 to 0.
05%, B: 0.0003 to 0.0020% of 1 type or 2 types or more, and the total content of P and N is 0.01.
% Or less, and a steel slab composed of the balance iron and unavoidable impurities added with 0.002 to 0.01% as a total of the addition amount of one or more of Mg, Ca, and Y is not less than the A _c3 transformation point. After heating to a temperature of 1250 ° C. or lower and finishing hot rolling at a temperature of _{Ar 3} transformation point or higher, accelerated cooling is performed at a cooling rate of 5 ° C./sec or higher, and accelerated cooling is completed in the range of 650 to 500 ° C. A method for producing a high-strength steel sheet having a low yield ratio, which comprises:

A fifth aspect of the present invention is C: 0.01% by weight.
0.20%, Si: 0.03 to 1.0%, Mn: 0.3
0 to 2.0%, P: 0.008% or less, S: 0.005
% Or less, Al: 0.005 to 0.1%, N: 0.001
~ 0.006%, Ti: 0.005-0.020%,
O: 0.001 to 0.010% is contained, the total content of P and N is 0.01% or less, and one or two or more of Mg, Ca, Y is added as the total amount of addition, 0.002
After heating a steel slab consisting of the balance iron added to 0.01% to unavoidable impurities to a temperature of A _c3 transformation point or more and 1250 ° C. or less and finishing hot rolling at a temperature of A _r3 transformation point or more,
A method for producing a high-strength steel sheet having a low yield ratio, which comprises accelerating cooling to 650 ° C. or lower at a cooling rate of 5 ° C./sec or more, and then tempering to 500 ° C. or more and A _c1 transformation point or less.

A sixth aspect of the present invention is C: 0.01% by weight.
0.20%, Si: 0.03 to 1.0%, Mn: 0.3
0 to 2.0%, P: 0.008% or less, S: 0.005
% Or less, Al: 0.005 to 0.1%, N: 0.001
~ 0.006%, Ti: 0.005-0.020%,
O: contains 0.001 to 0.010%, Cr: 0.0
1 to 0.50%, Ni: 0.01 to 3.0%, Mo:
0.01 to 0.50%, Cu: 0.01 to 1.5%,
V: 0.005 to 0.20%, Nb: 0.003 to 0.
05%, B: 0.0003 to 0.0020% of 1 type or 2 types or more, and the total content of P and N is 0.01.
% Or less, and a steel slab composed of the balance iron and unavoidable impurities added with 0.002 to 0.01% as a total of the addition amount of one or more of Mg, Ca, and Y is not less than the A _c3 transformation point. After heating to a temperature of 1250 ° C. or lower and terminating hot rolling at a temperature of A _r3 transformation point or higher, accelerated cooling to 650 ° C. or lower at a cooling rate of 5 ° C./sec or higher, then 500 ° C. or higher, A _c1 A method for producing a high-strength steel sheet having a low yield ratio, which comprises tempering to a temperature not higher than a transformation point.

The seventh aspect of the present invention is C: 0.01% by weight.
0.20%, Si: 0.03 to 1.0%, Mn: 0.3
0 to 2.0%, P: 0.008% or less, S: 0.005
% Or less, Al: 0.005 to 0.1%, N: 0.001
~ 0.006%, Ti: 0.005-0.020%,
O: 0.001 to 0.010% is contained, the total content of P and N is 0.01% or less, and one or two or more of Mg, Ca, Y is added as the total amount of addition, 0.002
After making a steel plate by hot rolling a steel slab consisting of the balance iron and unavoidable impurities added by 0.01% or less, it is reheated to a temperature not lower than the _Ac3 transformation point and not higher than 1000 ° C. to carry out a quenching treatment. A method for producing a high-strength steel sheet having a low yield ratio, which comprises tempering at a temperature of 500 ° C or higher and a temperature of A _C1 or lower.

The eighth aspect of the present invention is C: 0.01% by weight.
0.20%, Si: 0.03 to 1.0%, Mn: 0.3
0 to 2.0%, P: 0.008% or less, S: 0.005
% Or less, Al: 0.005 to 0.1%, N: 0.001
~ 0.006%, Ti: 0.005-0.020%,
O: contains 0.001 to 0.010%, Cr: 0.0
1 to 0.50%, Ni: 0.01 to 3.0%, Mo:
0.01 to 0.50%, Cu: 0.01 to 1.5%,
V: 0.005 to 0.20%, Nb: 0.003 to 0.
05%, B: 0.0003 to 0.0020% of 1 type or 2 types or more, and the total content of P and N is 0.01.
% Or less, and one or two or more of Mg, Ca, and Y as the total amount of addition, a steel slab composed of the remaining iron and unavoidable impurities added by 0.002 to 0.01% is formed into a steel plate by hot rolling. After that, it is reheated to a temperature of not less than A _c3 transformation point and not more than 1000 ° C. to perform quenching treatment, and subsequently _{tempered at not} less than 500 ° C. and not more than A _c1 transformation point. .

The ninth aspect of the present invention is, by weight%, C: 0.01-.
0.20%, Si: 0.03 to 1.0%, Mn: 0.3
0 to 2.0%, P: 0.008% or less, S: 0.005
% Or less, Al: 0.005 to 0.1%, N: 0.001
~ 0.006%, Ti: 0.005-0.020%,
O: 0.001 to 0.010% is contained, the total content of P and N is 0.01% or less, and one or two or more of Mg, Ca, Y is added as the total amount of addition, 0.002
A steel slab consisting of the balance iron added to 0.01% and unavoidable impurities is hot-rolled into a steel plate, which is then reheated to a temperature not lower than the _Ac3 transformation point and not higher than 1000 ° C to perform a normalizing treatment. A method for producing a high-strength steel sheet having a low yield ratio.

The tenth aspect of the present invention is C: 0.01 in% by weight.
.About.0.20%, Si: 0.03 to 1.0%, Mn: 0.
30-2.0%, P: 0.008% or less, S: 0.00
5% or less, Al: 0.005 to 0.1%, N: 0.00
1 to 0.006%, Ti: 0.005 to 0.020%,
O: contains 0.001 to 0.010%, Cr: 0.0
1 to 0.50%, Ni: 0.01 to 3.0%, Mo:
0.01 to 0.50%, Cu: 0.01 to 1.5%,
V: 0.005 to 0.20%, Nb: 0.003 to 0.
05%, B: 0.0003 to 0.0020% of 1 type or 2 types or more, and the total content of P and N is 0.01.
% Or less, and one or two or more of Mg, Ca, and Y as the total amount of addition, a steel slab composed of the remaining iron and unavoidable impurities added by 0.002 to 0.01% is formed into a steel plate by hot rolling. After that, a method for producing a low-yield-ratio high-strength steel sheet, which comprises reheating to a temperature not lower than the A _c3 transformation point and not higher than 1000 ° C. and performing normalizing treatment.

[0023]

The most important point of the present invention is to reduce the amount of the solid solution element in the matrix phase. However, the precipitation nucleus of the solid solution element does not depend on the manufacturing conditions and can be utilized and is thermally stable. Oxides are considered to be the most effective. Therefore, by investigating the relationship with tensile properties by changing the type of oxide dispersed in the steel material by changing the deoxidizing element, the strength, especially the yield stress may change depending on the type of oxide even if the structure is the same. It was discovered.

That is, in% by weight, C: 0.12%, S
i: 0.25%, Mn: 1.4%, Al: 0.03%,
When steel containing Ti: 0.015% was melted in a small vacuum melting furnace, various deoxidizing elements were added alone or in combination before tapping, and then solidified in a steel mold. The ingot after solidification is reheated to 1250 ° C., and the steel plate having a plate thickness of 20 mm by hot rolling is further 900 to 1200 ° C.
1 to 3 show the results of examining the tensile properties of the test pieces taken in parallel with the rolling direction at the center of the plate thickness when the normalizing treatment was performed at various heating temperatures. The total amount of the deoxidizing elements added individually or in combination before tapping was 0.002 to 0.01%. Cooling after hot rolling and after normalizing was both allowed to cool.

As is clear from FIGS. 1 to 3, when Mg, Ca, and Y are added individually or in combination in addition to ordinary deoxidation, the same α-grains are obtained as compared with the case where these elements are not added. The yield stress also decreases significantly with diameter. Since the tensile strength depends not only on the α phase but also on the second phase, a clear tendency due to the additional element is not recognized, and the yield stress is reduced.
The yield ratio also decreases. In this case, the manufacturing conditions were not changed except for the addition of the element to the molten steel, and the optical microstructure also changed the α particle size due to the change in the normalizing temperature, and also the amount and distribution of pearlite. There is no change. Nevertheless, only the yield stress is reduced.

Therefore, the decrease of the yield stress is not caused by changing the structure as in the conventional means, but is caused by directly changing the characteristic of α. The oxides containing these elements are dispersed in the steel by the addition of Mg, Ca, Y alone or in combination, and the elements that cause solid solution strengthening in these oxides are easily deposited as precipitates. Occurs.

Since the reduction of the yield ratio according to the present invention is a means which does not involve the change of the structure, the reduction of the yield stress as shown in FIG. 3 is not limited to the case of normalizing, regardless of the structure.
It is expected that various manufacturing methods and microstructures can be expected. Actually, not only the normalizing but also the reduction of the yield stress was actually observed by various manufacturing methods such as normal rolling, reheating quenching and tempering, direct quenching and tempering, controlled rolling and controlled cooling. The above is the outline of the present invention as a whole, but the manufacturing conditions, chemical components, and the like will be individually described below.

First, the manufacturing conditions will be described. The first point of the manufacturing conditions in the present invention is to add one or more of Mg, Ca and Y in addition to the normal deoxidation in the deoxidation at the molten steel stage. This facilitates the precipitation of the element that increases the yield stress due to solid solution due to the inclusion of the element in the oxide. M
Since g, Ca, and Y all have the same effect on the change in the composition of the oxide and the decrease in the yield stress, the total addition amount is 0.002 to 0.
It should be in the range of 010%. If the amount added is less than the lower limit of this range, the oxide containing the element is not sufficient, and a clear decrease in yield stress cannot be stably obtained. 0.010
%, The effect is saturated and it is economically disadvantageous.
Further, since the oxide becomes coarse and the toughness may deteriorate, it is necessary to add 0.010% or less.

The second important point in the manufacturing conditions is that the heat history is sufficient to precipitate the element that increases the yield stress due to solid solution in the oxide formed by the addition of Mg, Ca, and Y in the molten steel. It is to be. There is no restriction on the microstructure obtained, but cooling conditions or tempering conditions need to be specified.

That is, as the manufacturing method, heating at a temperature of not less than A _c3 transformation point and not more than 1250 ° C. and ending hot rolling at a temperature of not less than A _r3 transformation point are not accompanied by ordinary hot rolling or controlled cooling. Controlled rolling or A _c3 transformation point or higher, 1
Controlled cooling that heats to a temperature of 250 ° C. or lower, finishes hot rolling at a temperature of _{Ar 3} transformation point or higher, then accelerates cooling at a cooling rate of 5 ° C./sec or higher, and finishes accelerated cooling within a certain temperature range,
Alternatively A _c3 transformation point or higher, then heated to a temperature of 1250 ° C. or less, after completion of the hot rolling at A _r3 transformation point or higher, 5
After accelerated cooling to 650 ° C or lower at a cooling rate of ℃ / sec or higher, direct quenching and tempering for tempering or hot rolling to form a steel sheet, and then reheating to a temperature of not lower than the _Ac3 transformation point and not higher than 1000 ° C. performs quenching and subsequently reheating quenching and tempering subjected to tempering or after the steel sheet by hot rolling,, a _c3 transformation point or higher, a method for performing a reheating to normalizing treatment at a temperature of 1000 ° C. or less Although it can be used, in order to reduce the yield ratio without deteriorating other properties under the same manufacturing conditions, cooling after hot rolling as in hot rolling and when controlled rolling without controlled cooling is used. Needs to be cooled at a cooling rate equal to or lower than cooling.

Similarly, the cooling after normalizing must be performed at a cooling rate equal to or lower than the standing cooling. Further, in the case of controlled cooling at which the cooling rate during controlled cooling is 5 ° C./sec or more without tempering treatment, the cooling stop temperature is set to 650 because of precipitation treatment.
It must be in the range of 500 ° C. If the stop temperature exceeds 650 ° C, the original structure control of controlled cooling cannot be achieved.
If the heating is stopped below 0 ° C., the precipitation of the solid solution element is not sufficient, so that the yield stress is not sufficiently lowered. Furthermore, there is concern that the toughness and ductility may be reduced. In the case of controlled cooling for direct tempering, direct quenching, and reheat quenching, the yield stress can be reduced by defining the tempering conditions. That is, if the tempering temperature is lower than 500 ° C, the precipitation of the solid solution element is insufficient, and if it exceeds the A _c1 transformation point, the structure is greatly changed and other characteristics are deteriorated.
_c1 Limited to below the transformation point.

Next, other reasons for limiting the manufacturing conditions will be described. First, the heating temperature of the billet in hot rolling is A _c3.
The temperature is not lower than the transformation point and not higher than 1250 ° C., but this is required to be not lower than the transformation point A _c3 for the purpose of embodying the components. , The upper limit is 125 because it adversely affects the final material.
It was set to 0 ° C. However, in the case of normalizing treatment and reheating, quenching and tempering, it is not necessary to limit the heating temperature in this rolling step, because γ is formed by reheating after rolling. Instead, in the case of manufacturing by normalizing treatment and reheating quenching and tempering treatment, the heating temperature at the time of reheating must be A _c3 transformation point or more in order to secure hardenability as a γ single phase, and To prevent coarsening of the γ grain size, 1
It is necessary to limit the temperature to 000 ° C or lower.

Further, in the case of a manufacturing method which does not involve reheating treatment other than normalizing treatment or reheating quenching and tempering, it is necessary to set the end temperature of hot rolling to the _{Ar 3} transformation point or higher. This is because when hot rolling is carried out up to the temperature of the two-phase region below the _{Ar 3} transformation point, α is processed, so that the yield stress of α rises and the yield ratio rises, making it difficult to lower the yield ratio. This is because the toughness is deteriorated when the processing α is included. Further, in the case of performing controlled cooling or direct quenching, the cooling rate after rolling should be 5 ° C./sec or more in order to obtain a desired quenching structure and secure the strength.

The above are the reasons for limiting the present invention relating to the manufacturing method. However, a low yield ratio and high tensile strength steel plate that exhibits sufficient effect as a welded structural steel and has a low low yield ratio and an excellent plastic deformability is manufactured. In order to do so, it is necessary to specify the chemical composition as well. The reasons for limiting each chemical component will be described below.

First, C is added as an effective component for improving the strength of steel. If it is less than 0.01%, it is difficult to secure the strength required for structural steel, and if it exceeds 0.20%. Excessive addition significantly reduces toughness, weld crack resistance, etc., so the range was made 0.01 to 0.20%. Next, Si is an element effective as a deoxidizing element and for ensuring the strength of the base material. Addition of less than 0.03% results in insufficient deoxidation and is disadvantageous in securing strength. On the contrary, excessive addition of more than 1.0% forms a coarse oxide and causes ductility and toughness deterioration. Therefore, the range of Si is 0.03 to 1.0%
And Further, Mn is an element necessary for securing the strength and toughness of the base metal, and it is necessary to add at least 0.30% or more, but the upper limit is 2 within the allowable range of the material such as the toughness and crackability of the welded portion. It was set to 0.0%.

Since P forms a solid solution and hinders lowering of the yield ratio,
Although it is preferable to reduce the amount as much as possible, in the present invention, since P can be precipitated as a P compound by the oxide, the allowable amount is relaxed as compared with the case where the present invention is not used. However, the upper limit was made 0.008% from the viewpoint of ensuring the toughness of the heat affected zone. Since S forms MnS and deteriorates the ductility value, it is an element that needs to be reduced particularly in the steel plate which is required to secure the plastic deformability as the subject of the present invention. However, the content is 0.005% or less, which is the upper limit of ductility that is not significantly deteriorated and is practically acceptable.

Al is an element effective for deoxidizing, refining the γ grain size, etc., and must be contained in an amount of 0.005% or more to exert the effect, but if it exceeds 0.1%, it is excessive. If added to the alloy, a coarse oxide is formed and ductility is extremely deteriorated, so it is necessary to limit the content to 0.005 to 1.0%. N works effectively in γ grain refinement in combination with Al and Ti, but in order to clarify the effect, it is necessary to contain 0.001% or more, but if added in excess, the solid solution N increases. This leads to an increase in yield ratio and deterioration in toughness. From the viewpoint of ensuring the toughness of the heat affected zone, the upper limit is made 0.006%. However, from the viewpoint of lowering the yield ratio, it is necessary to consider it together with the content of P showing the same adverse effect. In the steel of the present invention, suitable oxides are dispersed as precipitation nuclei, but if the total amount of P and N is extremely large, it is difficult to prevent the yield stress from rising even in the present invention. Therefore, based on experimental results, the total amount of P and N is limited to 0.01% or less as a range in which a low yield ratio can be achieved under the condition that an appropriate oxide is dispersed.

Further, in the present invention, it is an important point to disperse the oxide containing Mg, Ca, Y. For that purpose, first, as shown in FIGS. 1 to 3, in the deoxidation at the molten steel stage, one or more of Mg, Ca, and Y are added in addition to the normal deoxidation. . This facilitates the precipitation of the element that increases the yield stress due to solid solution due to the inclusion of the element in the oxide. Since Mg, Ca, and Y all have the same effect on the change in the composition of the oxide and the reduction of the yield stress, the total addition amount is 0.002 regardless of the individual addition amount.
It should be in the range of% -0.010%. If the amount added is less than the lower limit of this range, the oxide containing the element is not sufficient, and a clear decrease in yield stress cannot be stably obtained.
If the addition exceeds 0.010%, the effect is saturated and it is economically disadvantageous, and since the oxide becomes coarse and the toughness may deteriorate, the addition needs to be 0.010% or less.

In addition to the above additions of Mg, Ca and Y, it is necessary to optimize the amount of O in the steel in order to secure the total amount of oxides. According to the detailed experimental results, the amount of O must be 0.001% or more to secure a sufficient amount of oxide for controlling the yield stress. The more the O content is, the more stable the effect is obtained. However, when the O content is excessive, the oxide is coarsened and the toughness and local ductility are deteriorated. Upper limit of 0.010%
And

The above are the basic components of the steel of the present invention, but for the purpose of increasing the strength of the base material in accordance with the desired strength level, Cr, Ni, Mo, Cu, Ti, V, Nb, B are added as necessary. 1 type or 2 types or more of these can be contained.

First, Cr and Mo are both effective elements for improving the strength of the base material, but 0.01% or more is necessary for producing a clear effect, while on the other hand, if over 0.50%. If added, the toughness tends to deteriorate,
The range is 0.01 to 0.50%. Further, Ni is a very effective element because it can improve the strength and toughness of the base material at the same time, but it is necessary to contain Ni in an amount of 0.01% or more in order to exert the effect. When the content is high, the strength and toughness are improved, but the effect is saturated even if added in excess of 3.0%. Therefore, considering the economical efficiency, the upper limit is made 3.0%.

Next, Cu has almost the same effect as Ni, but addition of more than 1.5% causes a problem in hot workability, so the range is limited to 0.01 to 1.5%. Ti is T
Although it is an element effective in improving the toughness by making the γ grains fine by forming iN, 0.005% or more is required to be effective. On the other hand, if it exceeds 0.020%, similarly to Al, a coarse oxide is formed to deteriorate toughness and ductility, so the upper limit is made 0.020%.

Both V and Nb mainly contribute to improving the strength of the base material by precipitation strengthening, but if added excessively, the toughness deteriorates. Therefore, V is 0.005 to 0.20% and N is a range in which the effect can be exhibited without causing deterioration of toughness.
b is 0.003 to 0.05%. B is 0.0003
% Is very effective in increasing the hardenability of steel and increasing the strength, but if added in excess, it forms BN, which adversely reduces the hardenability and greatly deteriorates the toughness. , The upper limit is 0.0020%.

[0044]

EXAMPLES Next, the effects of the present invention will be described more specifically by way of examples. Table 1 shows the chemical composition of the test steel used in the examples. Each of the test steels was made into a slab by slab rolling after continuous ingot casting or continuous casting. In Table 1, steel Nos. 1 to 13 are chemical composition ranges of the present invention and Mg, Ca, Y
The steel Nos. 14 to 19 do not satisfy the chemical composition range of the present invention or the addition conditions of Mg, Ca and Y of the present invention.

[0045]

[Table 1]

The slabs shown in Table 1 were manufactured into steel sheets under the rolling conditions shown in Table 2, and then heat-treated under the conditions shown in Table 3 to examine the tensile properties and Charpy impact properties. All test pieces were sampled in the rolling direction from the center of the plate thickness. Tensile test for parallel part diameter 1
A round bar tensile test piece having a length of 4 mm and a parallel portion length of 60 mm was used, a Charpy impact test was performed using a JIS No. 4 standard test piece, and properties were evaluated at a 50% fracture surface transition temperature (vTrs). Table 4 shows the test results of strength and toughness.

[0047]

[Table 2]

[0048]

[Table 3]

[0049]

[Table 4]

In Tables 2, 3 and 4, the test No. A1-
A26 are all steel sheets manufactured according to the present invention,
All of them have good elongation and toughness, and the yield ratio is reduced as compared with those of the comparative example having the same manufacturing conditions and tensile strength levels.

On the other hand, the test No. B1 to B10 are comparative examples, and since any of the conditions deviates from the limited range of the present invention, when compared at the same tensile strength level, the yield ratio is high, and the ductility and the toughness are not necessarily as a welded structural steel. not enough. That is, the test No. B1, B2, B4
B5 does not have sufficient addition of Mg, Ca and Y, or its amount is not sufficient, so that the yield ratio is not sufficiently low. Test No. On the contrary, for B3, the addition of these elements is excessive, so that the yield ratio is reduced, but the ductility and toughness are greatly deteriorated.

Test No. In addition to B4, the C content deviates from the range of the present invention and is excessively added, so that the ductility and toughness are also poor. In addition, the test No. Since B5 has a large amount of P and N,
Although Mg, Ca, and Y are properly added, it cannot be said that the yield ratio is sufficiently reduced. Test No. Since B6 has an excessive O content, deterioration of ductility and toughness is observed. Test No. The slabs used for B7 and B8 are chemical components, Mg,
Although the addition conditions of Ca and Y are appropriate, sufficient characteristics have not been obtained because the rolling conditions or heat treatment conditions do not satisfy the scope of the present invention.

Test No. B9 is a comparative example manufactured by quenching and tempering treatment, but the test No. A1
Compared with No. 8, the tempering temperature is out of the range of the present invention, so that the yield ratio is not sufficiently reduced and the toughness is also poor. Test No.
B10 is inferior in toughness because the normalizing temperature is too high. From the above, when compared at the same tensile strength level, it is clear that, according to the present invention, a lower yield ratio can be achieved without causing deterioration of ductility and toughness and lowering of productivity, as compared with the prior art. is there.

[0054]

INDUSTRIAL APPLICABILITY The present invention has sufficient performance as a welded structural steel without using expensive alloying elements or reducing productivity due to complicated heat history, and has a low yield ratio and plastic deformability. This is an epoch-making method capable of producing an excellent high-yield steel plate with a low yield ratio, and has an extremely great industrial effect such as reduction of production cost and improvement of safety as a structure.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of additional elements on the relationship between the yield ratio and the ferrite grain size in a normalized material.

FIG. 2 is a diagram showing the effect of additional elements on the relationship between the tensile strength and the ferrite grain size in a normalized material.

FIG. 3 is a diagram showing the effect of additional elements on the relationship between yield stress and ferrite grain size in a normalized material.

Claims (10)

[Claims] 1. By weight%, C: 0.01 to 0.20% Si: 0.03 to 1.0% Mn: 0.30 to 2.0% P: 0.008% or less S: 0.0. 005% or less Al: 0.005-0.1% N: 0.001-0.006% Ti: 0.005-0.020% O: 0.001-0.010% is contained, P and N are contained. The total content of 0.01% or less, and further Mg, Ca, Y
The total amount of one or more of
A steel slab consisting of the balance iron and unavoidable impurities added by 2 to 0.01% is heated to a temperature of A _c3 transformation point or more and 1250 ° C. or less, and hot rolling is finished at a temperature of A _r3 transformation point or more. A method for producing a high-strength steel sheet having a low yield ratio. 2. C .: 0.01 to 0.20% Si: 0.03 to 1.0% Mn: 0.30 to 2.0% P: 0.008% or less S: 0.0. 005% or less Al: 0.005-0.1% N: 0.001-0.006% Ti: 0.005-0.020% O: Contains 0.001-0.010%, Cr: 0 0.01 to 0.50% Ni: 0.01 to 3.0% Mo: 0.01 to 0.50% Cu: 0.01 to 1.5% V: 0.005 to 0.20% Nb: 0 0.003 to 0.05% B: 0.0003 to 0.0020% of 1 type or 2 types or more, the total content of P and N is 0.01% or less, and further Mg, Ca, Y. one or the total amount of addition of two or more, the steel slab consisting balance iron and unavoidable impurities was added 0.002 to 0.01%, a _c3-varying Above points, heated to a temperature of 1250 ° C. or less, low yield ratio method for producing a high tensile steel plate, which comprises terminating the hot rolling at A _r3 transformation point or higher. 3. By weight%, C: 0.01 to 0.20% Si: 0.03 to 1.0% Mn: 0.30 to 2.0% P: 0.008% or less S: 0.0. 005% or less Al: 0.005-0.1% N: 0.001-0.006% Ti: 0.005-0.020% O: 0.001-0.010% is contained, P and N are contained. The total content of 0.01% or less, and further Mg, Ca, Y
The total amount of one or more of
After heating the steel slab consisting of the balance iron and unavoidable impurities added by 2 to 0.01% to a temperature of A _c3 transformation point or more and 1250 ° C. or less and finishing hot rolling at a temperature of A _r3 transformation point or more, Accelerated cooling at a cooling rate of 5 ° C / sec or more,
A method for producing a high-strength steel plate having a low yield ratio, which comprises terminating accelerated cooling within a range of 00 ° C. 4. By weight%, C: 0.01 to 0.20% Si: 0.03 to 1.0% Mn: 0.30 to 2.0% P: 0.008% or less S: 0.0. 005% or less Al: 0.005-0.1% N: 0.001-0.006% Ti: 0.005-0.020% O: Contains 0.001-0.010%, Cr: 0 0.01 to 0.50% Ni: 0.01 to 3.0% Mo: 0.01 to 0.50% Cu: 0.01 to 1.5% V: 0.005 to 0.20% Nb: 0 0.003 to 0.05% B: 0.0003 to 0.0020% of 1 type or 2 types or more, the total content of P and N is 0.01% or less, and further Mg, Ca, Y. one or the total amount of addition of two or more, the steel slab consisting balance iron and unavoidable impurities was added 0.002 to 0.01%, a _c3-varying Above points, heated to a temperature of 1250 ° C. or less, after completion of the hot rolling at a temperature not lower than A _r3 transformation point, and accelerated cooling at 5 ° C. / sec or more cooling rate, accelerated cooling in the range of six hundred and fifty to five hundred ° C. The method for producing a high-strength steel plate with a low yield ratio, which comprises: 5. C .: 0.01 to 0.20% Si: 0.03 to 1.0% Mn: 0.30 to 2.0% P: 0.008% or less S: 0.0. 005% or less Al: 0.005-0.1% N: 0.001-0.006% Ti: 0.005-0.020% O: 0.001-0.010% is contained, P and N are contained. The total content of 0.01% or less, and further Mg, Ca, Y
The total amount of one or more of
After heating the steel slab consisting of the balance iron and unavoidable impurities added by 2 to 0.01% to a temperature of A _c3 transformation point or more and 1250 ° C. or less and finishing hot rolling at a temperature of A _r3 transformation point or more, A method for producing a high-strength steel sheet having a low yield ratio, which comprises accelerating cooling to 650 ° C. or lower at a cooling rate of 5 ° C./sec or more, and then tempering to 500 ° C. or more and A _c1 transformation point or less. 6. By weight%, C: 0.01 to 0.20% Si: 0.03 to 1.0% Mn: 0.30 to 2.0% P: 0.008% or less S: 0.0. 005% or less Al: 0.005-0.1% N: 0.001-0.006% Ti: 0.005-0.020% O: Contains 0.001-0.010%, Cr: 0 0.01 to 0.50% Ni: 0.01 to 3.0% Mo: 0.01 to 0.50% Cu: 0.01 to 1.5% V: 0.005 to 0.20% Nb: 0 0.003 to 0.05% B: 0.0003 to 0.0020% of 1 type or 2 types or more, the total content of P and N is 0.01% or less, and further Mg, Ca, Y. one or the total amount of addition of two or more, the steel slab consisting balance iron and unavoidable impurities was added 0.002 to 0.01%, a _c3-varying Above points, heated to a temperature of 1250 ° C. or less, after completion of the hot rolling at A _r3 transformation point or higher temperatures, 5 ° C. / sec after accelerated cooling to 650 ° C. or less at a cooling rate higher than, 500 ° C. or higher, A
_A method for producing a high-strength steel sheet having a low yield ratio, which is characterized by tempering to a temperature not higher than the _c1 transformation point. 7. By weight%, C: 0.01 to 0.20% Si: 0.03 to 1.0% Mn: 0.30 to 2.0% P: 0.008% or less S: 0.0. 005% or less Al: 0.005-0.1% N: 0.001-0.006% Ti: 0.005-0.020% O: 0.001-0.010% is contained, P and N are contained. The total content of 0.01% or less, and further Mg, Ca, Y
The total amount of one or more of
A steel slab consisting of the balance iron and unavoidable impurities added by 2 to 0.01% is hot-rolled into a steel plate, and then reheated to a temperature of not lower than the _Ac3 transformation point and not higher than 1000 ° C to perform quenching treatment, A method for producing a high-strength steel sheet having a low yield ratio, which comprises tempering at a temperature of 500 ° C or higher and a temperature of A _C1 or lower. 8. C: 0.01 to 0.20% Si: 0.03 to 1.0% Mn: 0.30 to 2.0% P: 0.008% or less S: 0.0. 005% or less Al: 0.005-0.1% N: 0.001-0.006% Ti: 0.005-0.020% O: Contains 0.001-0.010%, Cr: 0 0.01 to 0.50% Ni: 0.01 to 3.0% Mo: 0.01 to 0.50% Cu: 0.01 to 1.5% V: 0.005 to 0.20% Nb: 0 0.003 to 0.05% B: 0.0003 to 0.0020% of 1 type or 2 types or more, the total content of P and N is 0.01% or less, and further Mg, Ca, Y. Hot rolling a steel slab consisting of the balance iron and unavoidable impurities added with 0.002 to 0.01% as a total of one or two or more of More steel sheet was then ,, A _c3 transformation point or higher, and re-heated to quenching temperature of 1000 ° C. or less, subsequently 500 ° C. or higher, wherein the tempered below transformation point A _c1 low yield ratio and high Method of manufacturing tensile steel sheet. 9. In% by weight, C: 0.01 to 0.20% Si: 0.03 to 1.0% Mn: 0.30 to 2.0% P: 0.008% or less S: 0.0. 005% or less Al: 0.005-0.1% N: 0.001-0.006% Ti: 0.005-0.020% O: 0.001-0.010% is contained, P and N are contained. The total content of 0.01% or less, and further Mg, Ca, Y
The total amount of one or more of
A steel slab consisting of the balance iron and unavoidable impurities added by 2 to 0.01% is hot-rolled into a steel plate, and then reheated to a temperature not lower than the _Ac3 transformation point and not higher than 1000 ° C to perform a normalizing treatment. A method for manufacturing a high-strength steel sheet having a low yield ratio, characterized by: 10. By weight%, C: 0.01 to 0.20% Si: 0.03 to 1.0% Mn: 0.30 to 2.0% P: 0.008% or less S: 0.0. 005% or less Al: 0.005-0.1% N: 0.001-0.006% Ti: 0.005-0.020% O: Contains 0.001-0.010%, Cr: 0 0.01 to 0.50% Ni: 0.01 to 3.0% Mo: 0.01 to 0.50% Cu: 0.01 to 1.5% V: 0.005 to 0.20% Nb: 0 0.003 to 0.05% B: 0.0003 to 0.0020% of 1 type or 2 types or more, the total content of P and N is 0.01% or less, and further Mg, Ca, Y. 1 or 2 or more as the total amount of addition, a steel slab consisting of the balance iron and unavoidable impurities added by 0.002 to 0.01% is hot pressed. After the steel sheet by, A _c3 transformation point or higher, the low yield ratio method for manufacturing a high-tensile steel sheet and performing reheating to the normalizing treatment at a temperature of 1000 ° C. or less.