JPH07278656A - Method of manufacturing low yield ratio high strength steel - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a method for easily producing a steel having a low yield ratio and excellent ductility and toughness. [Structure] The steel slab is hot-rolled as it is, or is cooled once and then reheated to a temperature of Ac ₃ point or higher, and then A
r Finish hot rolling at a temperature of ₃ points or more, cool from a temperature of 700 ° C or more at a cooling rate of 1 ° C / sec or more and 20 ° C / sec or less, and finish cooling at a temperature of 600 ° C or less, and allow to cool. Then, the temperature is further raised to a temperature of 750 ° C. or higher and 850 ° C. or lower, the holding is completed within 60 minutes, and cooling is performed again at a cooling rate of 2 ° C./sec or more and 50 ° C./sec or less to obtain strength, toughness and It is possible to easily manufacture a high-strength steel having a low yield ratio and excellent ductility.

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 low-yield ratio high-strength steel excellent in strength, toughness and ductility which can be easily produced by combining accelerated cooling and tempering.

[0002]

2. Description of the Related Art In recent years, in steel materials (steel plates, steel pipes, shaped steels, etc.) used for construction structures, high tensile strength steels having a low yield ratio have been demanded from the viewpoint of earthquake resistance. In order to meet such demands, for example, the methods described in JP-A-53-23817 or JP-A-1-75817 have been proposed. In either case, a steel having a yield ratio reduced to about 50% is proposed. No manufacturing method has been proposed. Further, in order to significantly reduce the yield ratio, it is necessary to disperse a hard phase such as martensite in a soft phase such as ferrite,
In such a case, there is also a problem that the toughness and ductility are significantly reduced.

As a means for improving toughness, Japanese Patent Laid-Open No.
222450, JP-A-5-230530,
The method described in Japanese Patent Laid-Open No. 5-230531 is disclosed. Similar to the present invention, these methods obtain low YR steel by treatment at a two-phase region temperature of α + γ, and the treatment temperature and the holding time are optimized by rapid heating, holding for a short time, and the composition The purpose is to prevent the coarsening of the metal structure that occurs at times and improve the toughness. However, the present invention is different from the present invention which aims to provide a method of obtaining low YR and high toughness for a steel mainly containing ferrite in that the target metallographic structure is bainite and martensite.

[0004]

SUMMARY OF THE INVENTION The present invention eliminates the problems of the conventional method, and makes it possible to maintain the toughness and ductility (uniform elongation, total elongation) while reducing the yield ratio to about 50%. An object of the present invention is to provide a method for easily manufacturing a high-strength steel having a low yield ratio for steel having a strength of about 60 kgf / mm ² .

[0005]

The gist of the present invention is as follows. (1) C: 0.01 to 0.4% and Mn: 0.
02-3.0%, Si: 0.001-0.75%, A
1: 0.001 to 0.1%, the balance being a steel slab composed of Fe and unavoidable impurities, either as it is, or after cooling once, reheating to a temperature of Ac ₃ points or higher and then hot rolling is started, Ar _After hot rolling at a temperature of ₃ points or more, 700
Cooling is performed at a cooling rate of 1 ° C / sec or more and 20 ° C / sec or less from a temperature of ℃ or more, cooling is finished at a temperature of 600 ° C or less, and the mixture is allowed to cool, and further heated to a temperature of 750 ° C or more and 850 ° C or less. , Holding within 60 minutes, 2 ℃ / sec or more, 50 ℃
A method for producing a high-strength steel having a low yield ratio, which is characterized by having excellent strength, toughness, and ductility by being re-cooled at a cooling rate of not more than / sec.

(2) Nb: 0.001 to 0.
05%, Ti: 0.001 to 0.05%, V: 0.00
1-0.1% of any 1 type, or 2 or more types are contained, and it has the outstanding strength, toughness, and ductility, The manufacturing method of the low yield ratio high tensile steel of (1) characterized by the above-mentioned. (3)
% By weight, Mo: 0.01 to 1.0%, Ni: 0.01
~ 2%, Cr: 0.01-1%, Cu: 0.01-1
%, B: 0.00001 to 0.003%, either 1
(1) or (2) characterized by further containing one or more kinds, and having excellent strength, toughness and ductility.
A method for producing a high tensile strength steel having a low yield ratio. According to the production method of the present invention, high carbon martensite or a mixture thereof with retained austenite (hereinafter referred to as M
It is characterized by significantly reducing the yield ratio while maintaining toughness and ductility by dispersing (*) abbreviated) extremely finely.

[0007]

The basic idea of the present invention is as follows. First, regarding the means for lowering the yield ratio of steel, a simple method is to disperse a hard phase such as martensite in a soft phase such as ferrite.
In the present invention, in order to obtain such a metallographic state, the steel is brought to a temperature of Ac ₁ point temperature or higher after rolling and cooling, a part of the metallographic structure is once made into a high-carbon austenite, and this is cooled to martensite or M * As a result, the soft ferrite yields at low stress, while the hard martensite and M * are dispersed, so that the tensile strength can be increased and a high-strength steel with a low yield ratio can be obtained. However, in such a mixed structure of the soft phase and the hard phase, the toughness and the ductility are generally inferior because of the nonuniformity.

On the other hand, it is known that softening the metal structure and making the crystal grain size fine are effective for improving the toughness and ductility of ordinary steel. According to the study by the present inventors, even in the low yield ratio steel manufactured by the above method, its ductility (uniform elongation, elongation at break)
The toughness is affected by the ferrite grain size, the size of the dispersed martensite or M *, the precipitation rate, the distribution, etc.
It has been found that the toughness and ductility can be improved by the fact that the ferrite grains are fine, the martensite or M * is also fine, and the precipitation rate is appropriate.

Next, a method for obtaining such a metallographic state will be described. First, in order to obtain a fine ferrite structure, it is effective to forcibly cool the austenite structure after rolling and transform it from a supercooled state to increase the nucleation frequency of ferrite transformation. Further, prior to this, it is also more effective to make the austenite fine particles by rolling and to carry out rolling in the unrecrystallized temperature range of austenite.

Next, regarding a method for finely dispersing martensite and M *, for this purpose, first, rolling,
When the steel after cooling is heated to a temperature of Ac ₁ point temperature or higher and a part of the metal structure is transformed into high carbon austenite,
It is necessary to finely disperse this high carbon austenite. This is because this portion is transformed into martensite or M * by the forced cooling that follows.

Since such austenite nucleation sites are high carbon sites in the metal structure before heating, that is, pearlite, high carbon martensite, and retained austenite, these should be finely dispersed in advance. In the method of the present invention, by applying forced cooling after rolling, it is possible to suppress the formation of coarse band-shaped pearlite, which is caused during cooling, to disperse it finely, and to further cool it. This is achieved by intentionally fine-dispersing bainite, martensite, or M *, which are harmful in the accelerated cooling method, by lowering the speed relatively and making the cooling end temperature lower than that of normal accelerated cooling. Was achieved.

Next, regarding the method of controlling the volume fraction of martensite or M * to an appropriate amount, this also controls the volume fraction of austenite when reheated to Ac ₁ point or more. To be achieved. Therefore, it is necessary to control the temperature and the holding time at this time. In the present invention, it was found that this can be obtained at 750 ° C or higher and 850 ° C or lower. For example, when the reheating temperature is less than 750 ° C, a low yield ratio is obtained,
The M * fraction was too low and the ductility and toughness deteriorated.

In such a metal structure of steel, the number of martensites or M * s generated is small, and the generation sites (the sites where austenite is formed during the two-phase region processing) are ubiquitous. The processing temperature of less than 750 ° C. Then Martensite and M
It is considered that this is because the carbon concentration of * is high and the hardness difference from ferrite becomes too large. Further, at a temperature above 850 ° C., the fraction of martensite or M * is considerably large, spreads over a wide range, a fine dispersed state cannot be obtained, and the toughness and ductility also deteriorate. Also, 750 ° C or higher and 850 ° C
If the holding time below is too long, the metal structure and the carbide become coarse, so it is not preferable to carry out the treatment for more than 60 minutes. Further, after completion of the holding, it is necessary to perform forced cooling in order to transform the austenitized portion into martensite or M *.

The above is the basic idea of the present invention, which is summarized as follows. First, a low yield ratio steel can be manufactured by austenitizing a part of the steel at a temperature of Ac ₁ point or higher and further cooling it to disperse martensite and M * in the steel. However, in such a case, toughness and ductility deteriorate. Therefore, in the structure before the austenitizing treatment at a temperature of Ac ₁ point or more, the ferrite is made fine by accelerated cooling, and at the same time, the cooling rate of the accelerated cooling and the cooling end temperature are controlled to make the coarse pearlite. It is effective to prevent band formation, intentionally finely disperse high carbon bainite, martensite, or M * to finely disperse austenite nucleation sites.

Further, it is necessary to appropriately control the volume ratio of austenitization (volume ratio of later martensite and M *), and for this purpose, it is necessary to control the austenitizing temperature. As described above, by controlling the austenite treatment and the microstructure before the treatment precisely, it becomes possible to manufacture steel that does not deteriorate the toughness and ductility while significantly lowering the yield ratio. As a result of detailed investigation of the chemical composition and manufacturing conditions of steel in the method of the present invention based on the above new discoveries, the present inventors have devised a method for manufacturing steel as shown in the present invention.

The reasons for limiting each component will be described below. C is an element effective for strengthening steel and is 0.01
If it is less than%, sufficient strength cannot be obtained. On the other hand, if the content exceeds 0.4%, the weldability is deteriorated. Mn is an element effective for strengthening steel, and if it is less than 0.02%, a sufficient effect cannot be obtained. On the other hand, if its content exceeds 3.0%, the workability of steel deteriorates. Si is a deoxidizing element,
It is also effective as a steel strengthening element, but 0.001%
If the content is less than that, there is no effect. On the other hand, if it exceeds 0.75%, the surface properties of steel are impaired.

Al is added to suppress the growth of austenite at the time of reheating prior to deoxidation or rolling.
If it is less than 1%, there is no effect, and if it exceeds 0.1%, the surface properties of steel deteriorate. Nb, Ti, and V all effectively function in terms of grain refinement and precipitation strengthening when added in a trace amount, and thus may be used in a range that does not deteriorate the toughness of the welded portion. From such a viewpoint, the addition amount is 0.001 to 0.05% for Nb and Ti, and 0.00 for V.
1 to 0.1%.

Mo, Ni, Cr, Cu and B are all elements for improving the hardenability of steel, and in the case of the present invention, the addition of them can enhance the strength of steel. But,
Since excessive addition impairs the toughness and weldability of steel, Mo
Is 0.01% to 1.0%, Ni is 0.01% to 2%, and Cr is 0.01%.
1% or less, Cu is 0.01% or more and 1% or less, and B is 0.00001% or more and 0.003% or less.

Next, the manufacturing conditions in the present invention will be described. Since the present invention is effective for a slab cast under any casting condition, it is not necessary to specify the casting condition. Further, the hot rolling may be started as it is without cooling the slab, or the slab once cooled may be reheated to the Ac ₃ point or higher and then the rolling may be started. In the present invention, the rolling conditions are not particularly specified except that the rolling is finished at a temperature of Ar ₃ or higher, but the effectiveness of the present invention is not lost by any rolling. Is.

However, in the present invention, since it is necessary that the ferrite crystal grains are fine, it is necessary to adjust the rolling conditions to refine the austenite and to control the rolling to control the austenite in the unrecrystallized region. This may be carried out because rolling is effective.

Next, the reasons for limiting the cooling conditions will be described. In the present invention, ferrite is finely generated during cooling,
It is required that M * be finely generated without generation of coarse band-shaped pearlite. Therefore, first, the cooling start temperature is limited to 700 ° C. or higher in order to stably generate fine ferrite. This is because if cooling is performed below this temperature, coarse ferrite will be generated before the start of cooling.

The cooling rate is set to 1 ° C./second or more because the effect of refining ferrite cannot be obtained at a cooling rate of less than 1 ° C./second. In addition, the upper limit of the cooling rate is 2
The reason for setting 0 ° C./sec is that ferrite is not stably generated at a cooling rate higher than this.

The cooling end temperature is set to 600 ° C. or lower because the ferrite transformation is not completed at a temperature higher than 600 ° C. and a large amount of untransformed austenite remains, and this portion has a coarse band shape. Because it becomes perlite. Further, in such a case, untransformed austenite becomes pearlite, and bainite, martensite, or M * having high carbon cannot be finely dispersed.

Next, regarding the heat treatment at the Ac ₁ point temperature or higher, the reason why the treatment temperature is set to 750 ° C. or higher is that sufficient ductility cannot be obtained at a temperature lower than this. If the treatment temperature is too low, the volume fraction of austenite formation is too low, so the cast segregation part of the steel is ubiquitous in the austenite generation site, and it is estimated that the ductility deteriorates due to this nonuniformity difference. To be done. In addition, the processing temperature is 85
The reason why the temperature is 0 ° C. or lower is that at a temperature above this, the volume fraction of austenite is too large and a fine dispersed state cannot be obtained.

The reason why the processing time is within 60 minutes is that
This is because if the content exceeds this, coarsening of the metal structure and coarsening of carbides due to coalescence growth of ferrite grains and austenite grains occur, which is not preferable from the viewpoint of strength and toughness. Further, the cooling rate after the end of the holding is set to 2 ° C./second or more because at a cooling rate of less than 2 ° C./second, the austenitized portion cannot be transformed into martensite or M *. The upper limit of the cooling rate is 5
The reason for setting the temperature to 0 ° C./second is that it is difficult to achieve a cooling rate exceeding 50 ° C./second with a normal water cooling device.

[0026]

EXAMPLES Table 1 shows the components of the steels of the examples.
In the table, the steel underlined is a comparative steel, and the items that do not correspond to the present invention are underlined. Next, Table 2 shows the strength, yield ratio, ductility, and toughness obtained when steels having such components were manufactured under various manufacturing conditions, together with the manufacturing conditions. As the strength, yield strength (YS (MPa)) and tensile strength (TS (MPa)) are shown. Also, the yield ratio is YR
(Yield strength YS / Tensile strength TS), for ductility uniform elongation (uEl (%)) and elongation at break (El (%)),
The toughness was evaluated by the absorbed energy (vE ₀ (J)) at 0 ° C. in the Charpy impact test. According to this, all the methods of the present invention show clearly better characteristics than the comparative methods. By the method of the present invention, it is possible to produce a steel having a low yield ratio and excellent ductility and toughness, and the present invention is effective.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

[Table 4]

[0031]

[Table 5]

[0032]

According to the method of the present invention, a steel having a low yield ratio and excellent ductility and toughness can be easily manufactured.

Claims (3)

[Claims] 1. C .: 0.01 to 0.4%, Mn: 0.02 to 3.0%, Si: 0.001 to 0.75%, Al: 0.001 to 0. A steel slab containing 1% and the balance of Fe and unavoidable impurities is left as it is, or after being cooled once, it is reheated to a temperature of Ac ₃ point or higher and then hot rolling is started, and hot rolling is performed at a temperature of Ar ₃ point or higher. After rolling, cooling is performed from a temperature of 700 ° C. or higher at a cooling rate of 1 ° C./sec or more and 20 ° C./sec or less, cooling is finished at a temperature of 600 ° C. or less, and cooling is further performed, and further 750 ° C. or more 8
The material is heated to a temperature of 50 ° C. or lower, held within 60 minutes, and cooled again at a cooling rate of 2 ° C./sec or more and 50 ° C./sec or less to have excellent strength, toughness and ductility. A method for producing a high-strength steel having a low yield ratio. 2. By weight%, any one or two of Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, V: 0.001 to 0.1%. It contains the above and has excellent strength, toughness and ductility.
A method for producing a high-strength steel having a low yield ratio. 3. By weight%, Mo: 0.01 to 1.0%, Ni: 0.01 to 2%, Cr: 0.01 to 1%, Cu: 0.01 to 1%, B: 0. 3. The low yield ratio high tensile steel according to claim 1 or 2, further containing any one kind of 0.0001 to 0.003%, or two or more kinds, and having excellent strength, toughness and ductility. Production method.