JP2002266022A - Method for manufacturing high tensile steel with high toughness and high ductility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing high tensile steel combining high toughness with high ductility which has sufficient strength required of steel for welded structure, excellent ductility characteristics, such as uniform elongation, and excellent toughness at low temperature and further has high safety. SOLUTION: In the method for manufacturing the high tensile steel having high toughness and high ductility, ultrafine-grained steel having prescribed components and also having 1-3 μm average ferrite grain size and <=50% fraction of second phase is subjected to heat treatment in two-phase region under the following conditions: heating temperature, (Ac1 transformation point + 10 deg.C) to (Ac1 transformation point + 100 deg.C); holding time, <=5 hr; average cooling rate through the temperature region from the heating temperature to 200 deg.C, (0.1 to 100) deg.C/s. By this method, two-phase structure can be formed while suppressing the coarsening of ultrafine-grained structure, and toughness can be combined with uniform elongation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high-toughness, high-ductility, high-tensile steel having sufficient strength as a steel for welded structures, excellent ductility such as uniform elongation, and excellent low-temperature toughness. And a method for producing the same. Steel produced by this method can be used in general for welded steel structures such as marine structures, pressure vessels, shipbuilding, bridges, buildings, line pipes, etc., since high ductility and high toughness can be compatible,
It is especially useful as a steel material for structures such as buildings and bridges that require earthquake resistance. Although the form of the steel material is not particularly limited, the present invention is useful for a steel plate used as a structural member and requiring low-temperature toughness, particularly a thick plate, a steel pipe material or a shaped steel.

[0002]

2. Description of the Related Art It is known that it is effective to disperse an appropriate amount of a hard phase such as a martensite phase in a soft phase ferrite (α) to improve ductility characteristics, particularly uniform elongation. Although various methods for producing a duplex stainless steel comprising a soft α and a hard phase have been conventionally proposed, an intermediate heat treatment for heating a ferrite (α) + austenite (γ) dual phase region between quenching and tempering heat treatment is performed. Method (hereinafter referred to as QLT treatment), basically, α as a soft phase
And bainite or martensite as a hard phase,
Alternatively, the purpose is to mix a mixed structure of both phases.

[0003] The overall strength level, yield ratio, and ductility properties have been controlled by changing the mixture ratio of these phases. Various production methods for obtaining a mixed structure of the soft phase and the hard phase have been conventionally proposed. For example, JP-A-53-23817 discloses that after a steel sheet is reheated and quenched, the Ac1 transformation point and the Ac3 transformation point are obtained. A method of reheating between points to form two phases of γ and α and then air cooling is disclosed, and Japanese Patent Application Laid-Open No. 4-314824 discloses a method of quenching after reheating similarly to a two-phase region. ing. In addition, as a method of manufacturing online without reheating treatment,
For example, JP-A-63-286517 discloses that after hot rolling is performed from a γ region to a two-phase region, air cooling is performed to a temperature 20 to 100 ° C. lower than the Ar3 transformation point to generate an α phase.
Thereafter, a method of quenching is disclosed.

After reheating and quenching, QLT including a two-phase heat treatment of reheating between the Ac1 transformation point and the Ac3 transformation point to form two phases of γ and α, followed by air cooling or water cooling.
Although the structure is relatively easy to control, the toughness is extremely deteriorated if the heat treatment is performed in the two-phase region. Therefore, it is necessary to perform a tempering treatment at a temperature lower than the Ac1 transformation point. For this reason,
The QLT process has a problem that the process is complicated and the productivity is greatly reduced. When tempering is performed at a temperature lower than the Ac1 transformation point, the high uniform elongation obtained by the two-phase region heat treatment is rather deteriorated due to a decrease in the strength of the hard phase and the strengthening of precipitation in the α matrix.

[0005]

In steels which enhance the ductility characteristics typified by uniform elongation by performing two-phase heat treatment, it is difficult to achieve both high uniform elongation and toughness by a conventional technique. It was difficult. Therefore, an object of the present invention is to provide a method for producing a high-strength steel having uniform elongation equal to or higher than that of a conventional two-phase region heat-treated material and further having good toughness.

[0006]

The reason that it is difficult to secure toughness in steel subjected to dual-phase heat treatment is generally because the toughness of the hard phase is inferior. Existence is inevitable. Therefore, the present inventors have studied a method of securing the toughness of the entire two-phase structure by increasing the toughness of ferrite, which is a soft phase. Specifically, as a material before the two-phase region heat treatment, the ferrite grain size is 3
Using ultra-fine-grained steel of about μm or less, by optimizing the chemical composition and heat treatment conditions in the two-phase region, minimize the deterioration of toughness, and then set the appropriate chemical composition and heat treatment conditions to increase uniform elongation. investigated.

[0007] The finer the grain size, the more thermally unstable, and the more the ultrafine grain structure is likely to be coarsened by heat treatment or the like. It is not easy to form a hard phase in a fine grain structure. The present inventors, in the ultrafine-grained steel manufactured by utilizing the recrystallization of ferrite by applying a large rolling reduction of the cumulative reduction in the ferrite region to the two-phase region, high toughness and high uniform elongation. It is natural to consider a two-phase region heat treatment method that can be compatible, and to devise two-phase region heat treatment conditions in order to suppress coarsening of the ultrafine grain structure. The present inventors have found that it is necessary to satisfy certain requirements for the fine-grained structure, and have reached the present invention.

The gist of the present invention is as follows. (1) In mass%, C: 0.01 to 0.2%, Si: 0.01 to 1%, Mn: 0.1 to 2%, Al: 0.001 to 0.1%, N: 0 0.001 to 0.01%, Ti: 0.003 to 0.1%, V: 0.005 to 0.5%, Nb: 0.003 to 0.1% Species or more, and as impurities, P
: 0.02% or less, S: 0.01% or less, the balance consisting of iron and unavoidable impurities, the average ferrite grain size is 1 to 3 µm, and the proportion of the second phase other than ferrite in the structure is 50%. % Of steel having an ultrafine grained ferrite structure of not more than (Ac1 transformation point + 10 ° C)
(Ac1 transformation point + 100 ° C), high toughness characterized by applying a two-phase heat treatment at a holding time of 5 hours or less and an average cooling rate from a heating temperature to 200 ° C of 0.1 to 100 ° C / s. -A method for producing high-ductility, high-tensile steel.

(2) When performing the two-phase region heat treatment,
The heating rate from 00 ° C to the heating temperature is 1 to 100
The method for producing a high-toughness, high-ductility, high-strength steel according to the above (1), wherein the temperature is ° C / s. (3) After the two-phase heat treatment, the heating temperature is further increased to 250.
(1) characterized in that tempering at up to 600 ° C. is performed.
Or the method for producing a high toughness / high ductility / high tensile strength steel according to (2). (4) As a steel component, Ni: 0.1 to 5%, Cu: 0.1 to 1.5%, Cr: 0.01 to 2%, Mo: 0.01 to 2%, by mass%. W: 0.01 to 2%, Zr: 0.003 to 0.1%, Ta: 0.005 to 0.2%, B: 0.0002 to 0.005% The method for producing a high-toughness / high-ductility high-tensile steel according to any one of the above (1) to (3). (5) As a steel component, Mg: 0.0
005-0.01%, Ca: 0.0005-0.01
%, Y: one or two or more of 0.005 to 0.1% are contained, and the high toughness / high ductility / high ductility according to any one of the above (1) to (4) is provided. Manufacturing method for tensile steel.

[0010]

Embodiments of the present invention will be described below in detail. The present invention provides an appropriate heat treatment to an ultrafine-grained steel having an optimized structure and chemical composition to suppress the coarsening of the ultrafine-grained structure and appropriately disperse the hard second phase to achieve high toughness. And high uniform elongation. Therefore, first, the chemical composition and the reason for limiting the ultrafine grain structure before heat treatment and its action are described, and then the limiting reason for the method for appropriately dispersing the hard second phase while suppressing the coarsening of the ultrafine grain structure is described. .

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 necessary for structural steel, and if C is added in excess of 0.2%. Is in the range of 0.01% to 0.2%, since both the toughness and the uniform elongation are deteriorated due to increase of the hard phase and embrittlement.

Si is an element effective as a deoxidizing element and for ensuring the strength of the base material. Addition of less than 0.01% results in insufficient deoxidation and is disadvantageous for securing strength. 1% conversely
Excessive addition exceeding that causes formation of a coarse oxide and causes deterioration of ductility and toughness. Therefore, the range of Si is set to 0.01 to 1%.

Mn is an element necessary for securing the strength and toughness of the base material, and must be added at least 0.1% or more. However, excessive addition exceeding 2% causes toughness deterioration due to the hard phase as well as excessive C content, and also deteriorates the toughness and cracking property of the welded portion. Therefore, the upper limit is set to 2%.

Al is an element that is effective for deoxidation and finer structure through refinement of austenite grain size, and it is necessary to contain at least 0.001% in order to exhibit the effect.
When added in excess of 0.1%, coarse oxides are formed and ductility is extremely deteriorated.
It must be limited to the range of 0.1%.

N is effective in refining austenite grains in combination with Al and Ti, but it is necessary to contain 0.001% or more in order to clarify the effect. on the other hand,
Excessive addition increases the solute N, leading to an increase in the yield ratio and deterioration in the toughness of the base metal and the weld heat affected zone. The upper limit is set to 0.01% as an allowable range from the viewpoint of securing toughness.

Further, in the present invention, Ti, V, Nb
It is an essential requirement that one or more of the above are added properly. In other words, when performing heat treatment in a two-phase region to a thermally unstable ultrafine-grained structure, in order to stably suppress coarsening of the ultrafine-grained structure, only the structure before the heat treatment and the conditions for the two-phase region heat treatment are limited. However, it is not sufficient, and it is necessary to disperse carbonitrides finely to exert a pinning effect on grain growth. Therefore, Ti, V, effective for forming fine carbonitrides
One or more kinds of Nb are appropriately added. In the present invention, the amount of each element added is limited for the following reasons.

[0017] Ti is an element effective for refining austenite grains by the formation of carbonitrides, but in the present invention, Ti is effective for suppressing grain growth during heat treatment in a two-phase region of an ultrafine grain structure. In order to form carbonitride and exhibit the effect, it is necessary to add 0.003% or more. 0.1%
If it exceeds, coarse oxides or carbonitrides are formed to deteriorate toughness and ductility, so the upper limit is made 0.1%.

V has the same effect as Ti in the present invention. In order to exhibit the effect by finely dispersing the carbonitride, 0.005% or more must be added. 0.5
%, A coarse carbonitride is formed to deteriorate toughness and ductility.
Limited to the range of 0.5%.

Nb also has the same effect as Ti or V in the present invention. In order to exhibit the effect by finely dispersing the carbonitride, it is necessary to add 0.003% or more. On the other hand, if it exceeds 0.1%, precipitation embrittlement becomes remarkable, and coarse carbonitrides are formed to further deteriorate toughness and ductility. Therefore, in the present invention, the Nb content is set to 0.003 to 0.3.
Limit to 1% range.

It should be noted that P and S are impurity elements and are preferably reduced as much as possible. P has a remarkable tendency to deteriorate toughness, and the upper limit is set to 0.
02%.

Since S forms MnS and particularly deteriorates the ductility value, S is an element that needs to be particularly reduced in a steel sheet which is required to ensure ductility as is the object of the present invention. However, the upper limit of the content is set to 0.01% as a practically allowable upper limit of ductility deterioration.

The basic components of the steel of the present invention have been described above. Ni, Cu, Cr, Mo, W, Zr, Ta, and B may be used, if necessary, for the purpose of increasing the strength of the base material according to the desired strength level. Or one or more of these. The reasons for limiting these components are described below.

Ni is a very effective element that can simultaneously improve the strength and toughness of the base material, but must be contained in an amount of 0.1% or more in order to exert its effect. When the content is increased, the strength and toughness are improved, but the effect is saturated even if it exceeds 5%, so the upper limit is set to 5% in consideration of economy.

Cu also has almost the same effect as Ni, and in order to exhibit that effect, 0.1% or more is added.
Addition of more than 5% causes a problem in hot workability.
Limited to the range of 1 to 1.5%.

[0025] Cr is an element effective for improving the strength of the base material, but it must be 0.01% or more in order to produce a clear effect, while if added over 2%, the toughness tends to deteriorate. Therefore, the range is 0.01 to 2%.

Mo is also an element effective for improving the strength of the base material, but it must be 0.01% or more in order to produce a clear effect, while if added over 2%, the toughness tends to deteriorate. Therefore, the range is 0.01 to 2%.

W is an element effective for improving the strength of the base material, similarly to Mo. However, in order to produce a clear effect, 0.01%
On the other hand, if added in excess of 2%, the toughness tends to deteriorate, so the content is set in the range of 0.01 to 2%.

Zr is an element that exerts an effect on precipitation strengthening and grain refinement, but in order to exhibit the effect, it is necessary to add 0.003 or more. On the other hand, excessive addition of more than 0.1% causes deterioration of toughness due to coarsening of precipitates.
It is limited to the range of 003% to 0.1%.

[0029] Ta is also effective for precipitation strengthening and grain refinement, but it must be 0.005% or more in order to exert its effect, and if it exceeds 0.2%, on the contrary, toughness deteriorates. The range is 0.005% to 0.2%.

B is very effective in increasing the strength by increasing the hardenability of steel by adding a very small amount of 0.0002% or more.
When added in excess, BN is formed, and conversely, the hardenability is lowered and the toughness is greatly deteriorated.
%.

Further, in the present invention, one or more of Mg, Ca and Y can be contained for the purpose of stably improving ductility and toughness of a welded portion (HAZ toughness). In each case, the ductility is improved by fine dispersion of oxides and sulfides, and the weld heat affected zone (HAZ)
To improve the HAZ toughness. In order to exhibit the effect, Mg and Ca are each 0.00%.
It is necessary to contain not less than 05% and 0.005% or more of Y. On the other hand, if added in excess, the oxides and sulfides coarsen and themselves become the starting point of brittle fracture, which degrades the HAZ toughness. Limited to 1%.

The above are the reasons for limiting the chemical composition of the present invention. Next, the reason for limiting the ultrafine grain structure that the steel should have before heat treatment for improving the ductility properties is described. In order to improve ductility characteristics such as uniform elongation, it is necessary to have a two-phase structure of a soft phase of ferrite and a hard phase. It was difficult to achieve both.

Therefore, the present inventors have studied a method for minimizing the microstructure of ferrite, which is a soft phase, to offset the toughness deterioration due to the hard phase. As a result, in ultrafine-grained steel with a ferrite grain size of about 3 μm or less, it is possible to improve both toughness and uniform elongation by optimizing heat treatment in the two-phase region for hard phase formation. It was found by various experiments.

That is, in the present invention, the microstructure requirement of the steel before the heat treatment in the two-phase region is determined as follows.
And the proportion of the second phase other than ferrite in the structure is 50%
% Or less. That is, the reason why the lower limit of the average ferrite grain size is set to 1 μm is that ultrafine grains having a ferrite grain size of less than 1 μm become extremely unstable thermally and become abnormal when subjected to the two-phase region heat treatment described later. May cause growth,
As a result, the toughness may be significantly deteriorated.
On the other hand, if the average ferrite grain size is more than 3 μm, such grain growth does not occur in the steel having the chemical composition of the present invention, and the toughness is stably ensured.

Further, if the average ferrite grain size is 3 μm or less, the hard second phase formed during the heat treatment in the two-phase region becomes fine, and the toughness hardly deteriorates. However, if the average ferrite particle size exceeds 3 μm, such an effect cannot be expected, and the hard second phase, which is unfavorable in toughness, may be coarsely dispersed, making it difficult to reliably prevent toughness deterioration.

In the present invention, as a structure requirement, the proportion of the second phase other than ferrite in the structure is further set at 50%.
Limited to the following. This is because if the proportion of the second phase other than ferrite is more than 50%, the second phase formed by the heat treatment in the two-phase region becomes coarse, and the effect of improving the toughness by the ultrafine grain structure becomes ineffective. . If the ratio of the second phase other than ferrite is 50% or less, the toughness of the steel is mainly affected by ultrafine ferrite, so that good toughness is ensured regardless of the size dispersion state of the hard second phase. Becomes possible.

In the present invention, there is no limitation on the means for forming the ultrafine grain structure, but various specific means have been proposed, including those by the present inventors. For example, JP-A-7-126797 and JP-A-8-295982
According to the method disclosed in Japanese Patent Laid-Open Publication No. H10-115, it is possible to produce steel satisfying the structural requirements of the present invention.

The above are the reasons for limiting the chemical composition and the structural requirements of the steel before the heat treatment in the two-phase region in the present invention. Next, requirements for manufacturing conditions for forming an appropriate two-phase mixed structure of a soft phase and a hard phase, which are necessary to achieve high uniform elongation characteristics, will be described.

In the present invention, a steel having an appropriate chemical composition, an average ferrite grain size of 1 to 3 μm, and an ultrafine-grained ferrite structure in which the proportion of the second phase other than ferrite in the structure is 50% or less is used. The heating temperature is (Ac1
Transformation point + 10 ° C) to (Ac1 transformation point + 100 ° C), two-phase zone heat treatment with a holding time of 5 hours or less and an average cooling rate from the heating temperature to 200 ° C of 0.1 to 100 ° C / s. Thereby, an appropriate two-phase mixed structure of a soft phase and a hard phase is formed, and both toughness and ductility are achieved.

The lower limit of the heating temperature is set to (Ac1 transformation point + 10
° C) because it is necessary to form a sufficient amount of the hard phase in order to secure the strength, but it is necessary to secure austenite sufficiently in the heating step for that purpose. In order to secure a sufficient amount of austenite, the heating temperature is set to be higher by 10 ° C. than the Ac1 transformation point. The higher the heating temperature, the more the hard phase increases, but if the ratio of the soft phase decreases, the ductility characteristics deteriorate, and the ultrafine grain structure that had before heating may decrease or disappear due to grain growth. Based on a detailed experiment, the heating temperature at which the uniform elongation and strength are balanced and the ultrafine grain structure is maintained and the toughness is not deteriorated is limited to (Ac1 transformation point + 100 ° C) as an upper limit.

Based on the above reason, the heating temperature of the heat treatment applied to the ultrafine grain structure is (Ac1 transformation point + 10 ° C.) to (A
(c1 transformation point + 100 ° C.), but the heating and holding time is limited to 5 hours or less in the present invention in order to minimize the coarsening of the ultrafine grain structure during the heat treatment. If the holding time is longer than 5 hours, the ultrafine grain structure statically recrystallizes and grows, and as a result, the ferrite grains may be significantly coarsened and the toughness may be deteriorated. If the holding time is 5 hours or less, although there is a possibility that some grain growth may occur, the deterioration amount of toughness can be suppressed to an acceptable level. In addition, maintaining the ultrafine grain structure of the steel before heat treatment,
In order to completely suppress the deterioration of toughness, it is preferable to further limit the upper limit of the heating temperature to (Ac1 transformation point + 50 ° C) and the holding time to 1 hour or less.

In the present invention, cooling after heating and holding is performed at an average cooling rate from the heating temperature to 200.degree.
~ 100 ° C / s. The reason for limiting the cooling rate is to surely transform austenite formed in the heating step into bainite or martensite or a mixed structure of both phases. If the average cooling rate from the heating temperature to 200 ° C. is 0.1 ° C./s or more, a desired hard phase can be formed within the chemical composition range of the present invention. If the cooling rate is less than 0.1 ° C./s, depending on the chemical composition, a bainite phase may not be formed and the cementite may be transformed into a coarse pearlite phase or a pseudo pearlite phase, and improvement in ductility cannot be expected. , The toughness also tends to deteriorate.

From the viewpoint of hard phase formation, the higher the cooling rate, the better, but practically 100 ° C./s or less is sufficient. Even if the cooling rate is further increased, the effect of improving the material is saturated. On the other hand, there is a concern that the shape of the steel deteriorates and the residual stress increases. Therefore, in the present invention, the upper limit of the cooling rate in the heat treatment is set to 100 ° C./s.

There is no problem if the cooling rate after the heat treatment is controlled up to 200.degree. That is, the transformation to the hard phase bainite or martensite is almost one by 200 ° C.
This is because the final material is hardly affected by the cooling conditions after 200 ° C. However, if the cooling rate is extremely low, carbides may precipitate in the grains and deteriorate ductility. Therefore, it should be noted that the cooling rate at 200 ° C. or less is 0.01 ° C./s or more.

Further, in the present invention, if necessary,
In the two-phase region heat treatment, the rate of temperature rise from 300 ° C. to the heating temperature is limited to 1 to 100 ° C./s. In normal heat treatment, it is common to start the heating after inserting the steel into the heat treatment furnace, or a furnace heating method in which the steel is inserted into a heat treatment furnace set near the heating temperature. When the size of the steel is large as described above, the heating rate is at most about 10 to 20 ° C./s, but after inserting into a furnace set to a higher temperature than the heating temperature, such as high-frequency heating, current heating, or heating, the furnace By performing a rapid heating at a heating rate of 1 to 100 ° C./s by a method such as adjusting the temperature according to the temperature of the steel, the generation sites of the austenite phase during the heating increase,
The fine dispersion of the hard phase, which cannot be achieved by the ordinary furnace heating method, is achieved, and the change in the material structure can be suppressed as much as possible.

That is, in the case where it is preferable not to change the ultrafine grain structure of the material as much as possible as in the present invention, especially rapid heating in the heat treatment in the two-phase region limits the inevitable deterioration of toughness in the heat treatment in the two-phase region. It is very effective to suppress If the heating rate is 1 ° C./s or more, the fine dispersion of the hard phase and the resultant effect of suppressing the deterioration of toughness can be stably enjoyed.

The higher the rate of temperature rise, the more preferable.
Rapid heating exceeding ℃ / s is not realistic from the current capacity of heat treatment equipment, and even if it can be achieved, the microstructure refining effect tends to saturate, and the temperature distribution of steel In the present invention, the upper limit of the heating rate is set to 100 ° C./s because it is difficult to ensure uniformity. The reason why the control of the heating rate was set to be from 300 ° C. or higher is that the degree of movement of C or dislocation that affects the formation of the structure below the transformation point is 300 degrees or less regardless of the heating rate in the range below 300 ° C. This is because it is negligibly small compared to a high temperature range of not less than ° C.

Further, in the present invention, for the purpose of removing the residual stress of the steel and adjusting the strength, the heating temperature is from 250 to 600.
° C tempering. Heating temperature for tempering is 2
If the temperature is lower than 50 ° C., the tempering effect is not sufficient.
If the temperature is higher than 0 ° C., the strength of the hard phase decreases, and the balance between strength and uniform elongation deteriorates. Note that the effect on the material of the tempering holding time and cooling conditions is very small compared to the heating temperature, and it is not necessary to particularly specify the conditions in a practical condition range. The time was 48 hours or less, and the cooling rate was 0.01 ° C to prevent ductility deterioration due to intragranular precipitation of cementite.
/ s or more is more preferable.

[0049]

Embodiments of the present invention will be described below. Using steel pieces having the chemical compositions shown in Table 1, steel sheets were manufactured by the methods shown in Table 2. Tables 1 and 2 also show comparative examples in which the chemical composition or the steel structure does not satisfy the present invention. After further heat treatment of the steel sheets shown in Table 2, the mechanical properties were investigated. Table 3 shows the heat treatment conditions and mechanical properties at that time. Table 3 also shows heat treatment conditions and mechanical properties of Comparative Examples not satisfying the present invention.

As mechanical properties, tensile properties and 2 mm
The V-notch Charpy impact characteristics were investigated. The mechanical properties were obtained by taking test pieces from the center of the sheet thickness at right angles to the rolling direction. The tensile test piece was a round bar test piece having a parallel portion of 6 mmφ × 24 mm, and the Charpy test piece was a standard test piece having a test piece thickness of 10 mm. Tensile test is performed at room temperature, Charpy test is performed at various temperatures, and fracture surface transition temperature (vTrs)
I asked.

In Table 3, test numbers A1-1 to A10-
The steel sheet of No. 2 satisfies all the requirements of the present invention and, despite being subjected to the heat treatment in the two-phase region, almost retains the ultrafine-grained structure.
Very good with uniform elongation and tensile strength of about 500
It has a very high value of about 17.9% or more in the MPa class, and 13.6% or more in the about 800 MPa class. According to the present invention, a very high level of high toughness (vTrs) and high ductility (tensile properties). It is clear that the production of steel compatible with the above is possible. Also, from Table 3, it was confirmed that, in the present invention, the toughness was further improved when the heating rate was increased during the heat treatment in the two-phase region.

On the other hand, in Table 3, test numbers B1-1 to B1-1
No. 6-4 does not satisfy any of the requirements of the present invention. For the following reasons, the machine number is clearly higher than the test numbers A1-1 to A10-2 steel plates manufactured by the present invention. Inferior properties. That is, test number B
1-1, since the steel sheet is manufactured by normal hot rolling, the ferrite grain size before the two-phase region heat treatment does not satisfy the present invention, and the toughness is greatly deteriorated by the two-phase region heat treatment. .

Test No. B2-1 also shows that the ferrite grain size and the second phase fraction before the heat treatment in the two-phase region do not satisfy the present invention because the hot rolling conditions are not appropriate, and Poor toughness. Test numbers B3-1, B4-1, and B5-1 are examples in which the chemical composition does not satisfy the present invention.
Since the amount is too large, both the uniform elongation and the toughness are remarkably inferior to those of the present invention, and B4-1 has an excessively large Mn amount, so that the toughness is not sufficient. Test No. B5-1 does not contain any of Ti, V, and Nb, which are essential for suppressing the growth of ultrafine grain structure during the two-phase heat treatment.
During the heat treatment in the two-phase region, the ultrafine grain structure is coarsened, and as a result, the toughness is inferior to that of the present invention.

Test numbers B6-1 and B6-2 are chemical compositions,
Although the structural requirements before heat treatment satisfy the present invention, the heat treatment conditions for properly distributing the soft phase and the hard phase do not satisfy the present invention. is there. That is, in the test number B6-1, since the heat treatment temperature in the two-phase region was too high, the ultrafine grain structure was almost eliminated and the ferrite grain size was coarsened, and the strength and toughness were inferior to the steel of the present invention having the same composition. ing. Test number B6-
No. 2 shows that in the heat treatment corresponding to the two-phase region heat treatment, a hard phase is not formed because the heating temperature has not reached the Ac1 transformation point, the strength is relatively low, and the uniform elongation is higher than that of the two-phase region heat treated material. Significantly inferior.

In test number B6-3, the retention time of the heat treatment in the two-phase region was excessively long, and even with the chemical composition of the present invention, the grain growth of ultrafine ferrite occurred, so that the toughness was significantly deteriorated. . On the other hand, in the test number B6-4, the cooling in the two-phase region heat treatment deviated from the present invention and was extremely slow cooling, so that the hard second phase essential for uniform elongation improvement was not formed, and the coarse pearlite phase was not formed. Therefore, the uniform elongation is significantly inferior to that of the present invention. In addition, the tensile strength also deteriorates, which is not preferable. From the above examples, it is clear that according to the present invention, it is possible to produce a high toughness and high ductility steel excellent in both 2 mm V notch Charpy impact characteristics and uniform elongation.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

[0059]

Industrial Applicability According to the present invention, high safety, high toughness and high ductility, which have sufficient strength as welded structural steel, have excellent ductility characteristics such as uniform elongation, and also have excellent low temperature toughness. Without relying on large amounts of expensive alloying elements,
It can be manufactured at low cost, and the industrial effect is extremely remarkable.

Claims (5)

[Claims] 1. Mass%, C: 0.01 to 0.2%, Si: 0.01 to 1%, Mn: 0.1 to 2%, Al: 0.001 to 0.1%, N : 0.001 to 0.01%, Ti: 0.003 to 0.1%, V: 0.005 to 0.5%, Nb: 0.003 to 0.1% Further, it contains two or more kinds, further contains as impurities P: 0.02% or less, S: 0.01% or less, the balance consisting of iron and unavoidable impurities, and an average ferrite particle size of 1 to 3 μm. The heating temperature of the steel having an ultrafine-grained ferrite structure in which the proportion of the second phase other than ferrite in the structure is 50% or less is from (Ac1 transformation point + 10 ° C.) to (Ac1
(Transformation point + 100 ° C), holding time is 5 hours or less, and the average cooling rate from the heating temperature to 200 ° C is 0.1 to 10
High toughness characterized by applying a two-phase heat treatment at 0 ° C / s
Manufacturing method of high ductility and high tensile steel. 2. A temperature of 300 ° C. for performing a two-phase region heat treatment.
The method for producing a high-toughness, high-ductility, high-tensile steel according to claim 1, wherein the rate of temperature rise from the temperature to the heating temperature is 1 to 100 ° C / s. 3. The method for producing a high-toughness, high-ductility, high-strength steel according to claim 1, wherein tempering at a heating temperature of 250 to 600 ° C. is further performed after the two-phase region heat treatment. 4. As a steel component, Ni: 0.1 to 5%, Cu: 0.1 to 1.5%, Cr: 0.01 to 2%, Mo: 0.01 to 2 by mass%. %, W: 0.01 to 2%, Zr: 0.003 to 0.1%, Ta: 0.005 to 0.2%, B: 0.0002 to 0.005% The method for producing a high-toughness, high-ductility, high-strength steel according to any one of claims 1 to 3, characterized by comprising: 5. As the steel component, one of the following mass%: Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, Y: 0.005 to 0.1% The method for producing a high-toughness, high-ductility, high-strength steel according to any one of claims 1 to 4, wherein the method includes at least two types.