JP2001226737A - Low yield ratio and high tensile strength non-heattrated steel and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low yield ratio and high tensile strength non-heattreated steel whose yield ratio is reduced to <=80% while maintaining its high YS of >=480 N/mm2. SOLUTION: In this steel satisfying, by mass, 0.06 to 0.14% C, <=0.5% Si, 1 to 2% Mn, <=0.03% (inclusive of 0%) Cr, 0.05 to 0.3% Mo, 0.01 to 0.1% V, 0.01 to 0.1% Nb, 0.001 to 0.02% Ti, 0.001 to 0.008% N and <=0.05% Al, an insular MA structure is allowed to occupy in 0.5 to 3.0% by volume fractional ratio, and also, the average grain size in the insular MA structure is controlled to 1 to 4μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat-treated steel used for line pipes and architectural structures, which satisfies both low yield ratio and high tensile properties and excellent ductility and toughness. More particularly, the present invention relates to a low-yield-ratio high-strength non-heat treated steel having a reduced yield ratio while maintaining a high yield stress.

[0002]

2. Description of the Related Art In recent years, steel materials used for building structures are required to have strict properties such as strength, toughness and weldability from the viewpoint of earthquake resistance and the like.

[0003] As a conventional technique for reducing the yield ratio (hereinafter sometimes abbreviated as YR) of steel in response to such demands, a steel material is hot-rolled, cooled, and then reheated to a two-phase region to obtain a ferrite. A method of dispersing bainite or martensite in a structure has been proposed (JP-A-8-283).
No. 838, Japanese Patent Publication No. 60-57490). However, in this method, although the YR can be reduced, the yield strength (hereinafter sometimes abbreviated as YS) becomes extremely low.
In addition, there is a problem that mass production is difficult and the cost is high.

[0004] When the steel of the present invention is used as a steel for line pipes, the tensile properties and toughness are improved, and the HIC (hydrogen-induced cracking) resistance and SSC (sulfide stress corrosion cracking) resistance are improved. It is necessary to improve the performance. In the prior art, in order to obtain excellent HIC resistance and SSC resistance, a method has been proposed in which C and Mn are reduced and Cr and Mo are added in combination to increase the strength (see, for example, Japanese Patent Application Laid-Open No. H05-205).
However, in this method, it is difficult to reduce YR.

Further, as a steel sheet manufacturing technique for improving the HIC resistance and the SSC resistance, it has been proposed to perform cooling after rolling by accelerated cooling (for example, Japanese Patent Publication No. 63-136).
No. 9). However, when accelerated cooling is performed, the material of the steel tends to vary, which may cause problems such as uneven bending of the steel plate when manufacturing the line pipe.

[0006] As a method for producing a steel sheet having high strength and excellent low-temperature toughness, an average heating temperature of a slab before rolling is set to 10%.
Reduce the initial austenite grain size to below 00 ° C,
A method for promoting the formation of a two-phase structure of fine ferrite-martensite has been proposed (JP-A-5-255744). However, when the average slab heating temperature is set to 1000 ° C. or lower, Nb and V added for strengthening precipitation become undissolved carbonitrides, so that a strength sufficient for the amount of expensive Nb and V added is obtained. It is uneconomical.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and an object of the present invention is to maintain a high yield stress of 480 N / mm ² or more while maintaining a yield ratio of 80%. % High-strength steel with a low yield ratio of not more than 10%.

[0008]

Means for Solving the Problems The steel material of the present invention which can solve the above-mentioned problems is defined as:
0.14%, Si: 0.5% or less, Mn: 1-2%, C
r: 0.03% or less (including 0%), Mo: 0.05 to
0.3%, V: 0.01 to 0.1%, Nb: 0.01 to
0.1%, Ti: 0.001 to 0.02%, N: 0.0
01-0.008%, steel satisfying Al: 0.05% or less, wherein the island-like MA structure has a volume fraction of 0.5-3.0%.
And the average particle size of the island-shaped MA structure is 1 to 4 μm.

As another element, at least one element selected from the group consisting of Cu: 0.5% or less, Ni: 0.5% or less, and Ca: 0.005% or less may be contained.

The production method of the present invention specifies a method for producing the above-mentioned non-heat-treated steel having a low yield ratio and a high tensile strength, and the average cooling rate from the Ar ₃ point to 500 ° C. in the cooling step after finish rolling. Is set to 0.3 to 3 ° C./sec. Furthermore, in the production method of the present invention, the slab is heated at an average heating temperature of 1
It is preferable to hold at 000 ° C. or more for 30 minutes or more before subjecting to hot rolling.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Under the above-mentioned circumstances, the present inventors have conducted intensive research with the aim of developing a non-heat treated steel which has realized a low YR while maintaining a high YS. It has been found that it is effective to appropriately control the size and the occupancy of the island-like MA structure in the metal structure. By utilizing these findings, it has been found that a low-yield-ratio high-strength steel in which YS is reduced to 80% or less while YS is maintained at 480 N / mm ² or more can be obtained without heat treatment. Furthermore,
The present invention has been conceived as a result of repeated pursuit of a chemical component and a production method for obtaining an island-shaped MA structure having an optimum size and an optimum amount contributing to exhibiting the above-mentioned characteristics without heat treatment.

Hereinafter, the reason why the metal structure, chemical composition, and the like of the steel material are determined in the present invention will be described.

The island-like MA structure refers to a composite structure of martensite and austenite. The island-like MA structure in the steel of the present invention shows a metal structure close to a martensite single phase. The reason why the island-like MA structure is effective for increasing the strength and lowering the YR of steel is that mobile dislocations are introduced into ferrite around the island-like MA structure, thereby suppressing an extreme improvement in YS.
It is presumed that TS is improved because the island-shaped MA structure itself is very hard.

The island-like MA structure effectively acts to obtain such excellent tensile properties. However, if the volume fraction and the size are out of the range of the present invention, it becomes difficult to obtain these excellent properties. . That is, if the volume fraction of the island-shaped MA structure is too small, the mobile dislocation density around the MA structure decreases, so that the YR cannot be reduced. Therefore, the volume fraction of the island-shaped MA structure is 0.5% or more, preferably 1.0% or more. However, even if the volume fraction of the island-like MA tissue is too large,
Since the ductility and toughness decrease, the content is controlled to 3.0% or less, preferably 2.5% or less.

On the other hand, if the average grain size of the island-shaped MA structure is too small, it is difficult for mobile dislocations to be introduced, so that the YR does not decrease.
Therefore, the average particle size of the island-shaped MA structure is controlled to 1 μm or more, preferably 2 μm or more. However, even if the average particle size of the island-like MA structure is too large, ductility, particularly toughness is reduced,
The average particle size is suppressed to 4 μm or less, preferably 3.5 μm or less.

Next, the importance of controlling chemical components for the formation of the above-mentioned island-like MA structure will be described with reference to the metallographic photographs of FIGS.

FIG. 1 shows that C: 0.1%, Mn: 1.5%,
Repeller corrosion (JOURNAL OF METAL) on steel containing V: 0.05% and Nb: 0.025% (hereinafter referred to as base steel)
S, March 38, 1980, p. 38, 39) and a microstructure photograph.

The island-shaped MA structure appears white as shown in FIG. 2 described later by performing repeller corrosion, but no white portion is seen in FIG. That is, the island-shaped MA structure is not generated, and the effect of the island-shaped MA structure cannot be obtained.

FIG. 2 shows the metal structure of steel containing 0.25% of Mo in addition to the above chemical components of the base steel. However, many white portions, that is, island-like MA structures of a suitable size are generated. This indicates a desirable structure in which the effect of the island-shaped MA structure is exhibited.

FIG. 3 shows the metal structure of steel containing 0.2% of Mo and 0.15% of Cr in addition to the above chemical components of the base steel. Nevertheless, almost no island-like MA structure was generated. The reason for this is that Mo is an element that improves hardenability without promoting the formation of cementite, whereas Cr is an element that forms cementite while improving hardenability. Are easily formed, and as a result, a pearlite structure is easily formed.
It is presumed that generation of tissue A was inhibited.

That is, to obtain an optimum size and an optimum amount of the island-like MA structure necessary for achieving a low YR while maintaining a high YS, the present inventors need to add an appropriate amount of Mo into steel. It has been found that it is necessary to reduce the content of Cr as much as possible.

As described above, Mo is an element indispensable for the formation of the island-like MA structure, so that it is 0.05% or more, preferably 0.1% or more.
15% or more is contained. However, if added in excess, the weldability is impaired, so 0.3% or less, preferably 0.27%
Keep below.

Further, Cr is an element effective for improving the strength. However, as described above, the content of Cr is desirably as small as possible.
Keep it below 03%.

In the present invention, V and Nb are added to secure the strength. Although C and Mn are preferable for securing strength, they deteriorate the weldability.
And Nb were added to secure both high strength and weldability.

In particular, V is an element effective for improving the strength without extremely increasing YR by the complex addition with Mo. That is, if VC is precipitated alone, the carbide size becomes very small, so that the YR extremely increases.
However, when Mo coexists, Mo forms a solid solution in VC and V
Since C has an appropriate size, an extreme increase in YR is alleviated.

When Nb is added, the recovery and recrystallization of the austenite structure during rolling are suppressed, and the number of nucleation sites of ferrite during the transformation from austenite to ferrite is increased.
The toughness is improved.

In order to exert the above effects, V is set to 0.0
It is preferable to contain 1% or more and 0.01% or more of Nb. However, if any of the elements V and Nb are added excessively, ductility, toughness and weldability are impaired, so the upper limits are each set to 0.1%.

The reasons for defining the other chemical components are described below.

C: 0.06 to 0.14% C is an element necessary for securing the tensile strength and for forming the island-like MA structure, and hence is 0.06% or more, preferably 0.06% or more.
More than 9% is contained. However, if the C content is excessive, the weldability and ductility deteriorate, so that the content is suppressed to 0.14% or less, preferably 0.12% or less.

Si: 0.5% or less (excluding 0%) Si is an element necessary for deoxidation, but if added excessively, it deteriorates weldability and HAZ toughness. Is suppressed to 0.3% or less.

Mn: 1-2% Mn is an element necessary for improving the hardenability of the steel and ensuring the strength, so that Mn is contained in an amount of 1% or more, preferably 1.2% or more.

FIG. 4 shows a steel having the same chemical composition as that of the base steel, except that the Mn content was increased from 1.5% to 2.3%, as in FIGS. It is a metallographic photograph which carried out the repeller corrosion and was photographed by the microscope. From FIG. 4, it can be seen that, when Mn is excessively added, a large number of island-like MA structures are formed, but the crystal grain size is small, and it is not possible to reduce the YR. Further, since excessive addition of Mn also causes deterioration of toughness and weldability, the Mn content is suppressed to 2% or less, preferably 1.8% or less.

Ti: 0.001% to 0.02% N: 0.001% to 0.008% Since Ti is an element necessary for ensuring weldability, 0.001%
The content is preferably at least 0.002%. Also,
In order to form TiN at a high temperature with Ti and to suppress coarsening of austenite grains, N is preferably contained at 0.001% or more. However, since excessive ductility of TiN deteriorates ductility and toughness, Ti is 0.02% or less, preferably 0.015% or less, and N is 0.008% or less, preferably 0.006% or less. To keep.

Al: 0.05% or less (excluding 0%) Al is an element necessary for deoxidation, but if added excessively, it causes a decrease in toughness. Therefore, Al is 0.05% or less, preferably 0.1% or less.
Keep it below 04%.

The essential elements in the present invention are as described above. If necessary, at least one element selected from the group consisting of Cu, Ni and Ca should be contained in an appropriate amount in order to obtain the following improvement effects. Is also effective.

Cu: 0.5% or less, Ni: 0.5% or less Cu and Ni are elements which improve the strength without increasing Ceq unlike C and Mn.
Is added as an alternative element. However, excessive addition causes cracking during continuous casting, so the upper limit is 0.5
%, Preferably 0.3% or less of Cu and 0.4% or less of Ni.

Ca: 0.005% or less Ca controls the form of sulfide and improves the resistance to hydrogen-induced cracking. Therefore, it is desirable to contain 0.0005% or more. However, excessive addition may lead to coarse CaS or CaO
In order to deteriorate the ductility and toughness by forming inclusions such as
005% or less, preferably 0.004% or less.

The elements contained in the steel of the present invention include, in addition to those described above, unavoidable impurities brought in depending on the conditions of raw materials, materials, production facilities and the like. In addition, it is also possible to use steel containing other elements positively within a range that does not adversely affect the achievement of the object of the present invention. Examples of other elements that can be actively added include H-resistant
Mg, Ce, which has the effect of improving IC properties and SSC resistance
La, Zr and the like.

Next, in order to obtain an appropriate amount of island-like MA structure of a desired size, the present inventors set the average slab heating temperature before hot rolling during production and the cooling rate after finish rolling as follows. It was found that it would be better to control it. Note that the present invention is effective for steel slabs cast under any casting conditions, and thus no particular casting conditions are specified.

Cooling rate after finish rolling: Ar ₃ to 5
Controlling the cooling rate to 00 ° C. is extremely important for forming an island-like MA structure in an appropriate form. If the cooling rate is too low, a pseudo pearlite structure or a bainite structure is formed without forming an island-like MA structure. Therefore, the average cooling rate from the Ar ₃ point to 500 ° C. is 0.
3 ° C./sec or more, preferably 0.5 ° C./sec or more.

On the other hand, if the cooling rate is too high, the material varies as described above, and the ferrite transformation start temperature A
Since the r ₃ point is lowered, the concentration of C from ferrite to untransformed austenite is suppressed, and the MA structure cannot be obtained. Therefore, the upper limit of the average cooling rate from the Ar ₃ point to 500 ° C. is 3.0 ° C./sec, preferably 2.5 ° C./sec.
And In the case of a steel sheet, the average cooling rate refers to the cooling rate at the center of the sheet thickness.

Average slab heating temperature before hot rolling: If the heating temperature is low as described above, it becomes difficult for Nb and V to effectively act as precipitation strengthening elements. Therefore, in order to ensure high YS and TS by sufficiently dissolving Nb and V in austenite during heating, it is necessary to set the average heating temperature to 1000 ° C. or higher, more preferably 1050 ° C. or higher. However, even if the heating temperature of the slab is too high, the austenite grains become coarse, so it is desirable to suppress the temperature to 1200 ° C or less.

[0043]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and the present invention is not limited thereto. Modifications can be made and implemented, all of which are included in the technical scope of the present invention.

That is, in the following examples, the steel of the present invention is formed into a steel sheet as a final shape. However, the same chemical composition as specified in the present invention, the slab heating temperature before rolling, and the cooling rate after rolling are used. The application of the conditions to a bar, a wire, a steel pipe, or the like having a different final shape is also included in the scope of the present invention.

Example Steel having the chemical composition shown in Table 1 below was melted in an air melting furnace to obtain an ingot of 120 mm × 170 mm × 320 mm. Then, at a heating temperature shown in Table 2, 30 to 110
Rolling was started for another minute. The rolling was performed from 120 t to the final 16 t, and the cooling after the rolling was basically allowed to cool. However, in Experiment No. In No. 27, annealing was performed in a heat treatment furnace after rolling. 28 performed water cooling. Table 2 also shows the cooling rates measured at the center of the sheet thickness.

In the metallographic observation, the surface of a cross section perpendicular to the rolling direction of the steel sheet was polished, and then the above-described repeller corrosion was performed to reveal an island-like MA structure.
Analysis was performed using e-Pro Plus (Media Cybernetics).

The average island volume MA volume fraction was determined as follows. That is, after the island-like MA structure was revealed by repeller corrosion, a photograph of the center of the thickness of 16 t to 1/4 t was photographed at a magnification of 400 times using an optical microscope, and 70 × 90 m
Image analysis was performed on the entire m photo. In the measurement, the area ratio of only the white portion representing the island-shaped MA was obtained. The average value for the three visual fields measured in this manner is calculated as the average island-shaped MA.
The volume fraction was used.

The average island-like MA size was determined as follows. That is, a photograph was taken in the same manner as described above, and the white island-like MA tissue was converted into a circle by image analysis, and the average value of the diameter was calculated. The average value of the three visual fields measured in such a manner was defined as the average particle size of the island-shaped MA structure.

As a mechanical property, a 0.5% yield stress Y
S, tensile strength TS, yield ratio YR, elongation EL, at -20 ° C
Of Charpy absorbed energy of 480N
/ Mm ^TwoAs described above, YR is 80% or less, elongation is 35% or more,-
Charpy absorbed energy at 20 ° C is more than 190
Passed when satisfying all. Table 2 shows the results.

The tensile test was carried out according to the API (American Petroleum Institute) standard line pipe steel test method. The gauge length is 50mm, the width of the parallel part is 38m while the test piece remains the full thickness
m. The full-size test piece used for the Charpy test was collected from the center of the plate, and a 2 mm V notch was inserted so that the fracture surface was perpendicular to the plate surface and parallel to the rolling direction.

[0051]

[Table 1]

[0052]

[Table 2]

Experiment Nos. In Tables 1 and 2 1-12
Satisfies all of the requirements of claim 1, so that all of them satisfy the requirement of 480 N / mm ² or more while maintaining high YS.
% And low elongation EL of 35%
It can be seen that the above excellent ductility and the excellent low-temperature toughness having a Charpy absorbed energy at −20 ° C. of 190 J or more are also satisfied.

On the other hand, in Experiment No. 13-28
It lacks any of the requirements defined in claim 1 and fails to achieve the object of the present invention due to poor tensile strength, YR, elongation, or toughness as described below.

That is, in Experiment No. No. 13 had no Mo added thereto, and the experiment no. In No. 21, since the Mo content was below the specified amount and was insufficient, in any case, the island-like MA structure was not sufficiently formed, and the YR was high.

Experiment No. In No. 14, since Nb was not added, the ferrite crystal grains did not become fine, resulting in poor toughness. Experiment No. In No. 15, YS was low because V was not added.

Experiment No. In No. 16, the excessive addition of Cr prevented the formation of the island-like MA structure and increased the YR. Experiment No. No. 17 shows that an optimum size and an appropriate amount of an average island-like MA structure were not generated due to excessive addition of Cr, the YR value was high, the elongation was small, the Charpy absorbed energy was small, and the toughness was poor. It became.

Experiment No. In No. 18, since the C content was excessive, the elongation was small and the Charpy absorbed energy was small, resulting in poor ductility and poor weldability. Experiment N
o. In No. 19, since the C content was short of the specified range and was insufficient, the island-like MA structure did not grow sufficiently and was small in size, and the YR was high.

Experiment No. In the case of No. 20, since Mo was added in excess and the insular MA structure was generated in excess of the specified amount,
The elongation was small and the Charpy absorbed energy was small, resulting in poor toughness.

Experiment No. 22 has V and Nb contents,
Experiment No. In No. 23, the V content was insufficient to reach the specified content and was insufficient, resulting in a low YS. Experiment No. 24 is V
Is excessive, and the experiment No. In No. 25, since both V and Nb were excessively added, a low yield ratio was not obtained.

Experiment No. In No. 26, since both Ti and N were excessively added, a large amount of TiN was generated, and the Charpy absorbed energy was small, resulting in poor weldability.

Experiment No. In Test No. 27, the cooling rate after finish rolling was too low. In No. 28, since the cooling rate after the finish rolling was too high, the island-like MA structure was not sufficiently formed, and the YR was high.

[0063]

The present invention is constituted as described above. In the steel whose chemical composition is specified as described above, the average particle size is 1%.
４4 μm insular MA tissue was obtained at a volume fraction of 0.5 to 3.0.
%, The high YS of 480 N / mm ² or more
The yield ratio is reduced to 80% or less while maintaining the elongation, the elongation EL is 35% or more, and the Charpy absorbed energy is 190%.
It has become possible to provide an epoch-making steel having both excellent ductility and toughness of J or more.

The realization of such a low-yield-ratio high-strength steel has made it possible to provide a steel sheet for line pipes, a steel sheet for construction, and the like that can withstand severe use conditions.

[Brief description of the drawings]

FIG. 1 C: 0.1%, Mn: 1.5%, V: 0.05
5 is a photograph showing a metal structure of a steel (base steel) containing% and Nb: 0.025%.

FIG. 2 shows the addition of Mo to 0.2 in addition to the above chemical components of the base steel.
It is a photograph which shows the metallographic structure of steel which contained 5%.

FIG. 3 In addition to the above chemical components of the base steel,
5 is a photograph showing a metal structure of steel containing 0.15% and 0.15% of Cr.

FIG. 4 is a photograph showing the metallographic structure of steel having the same chemical composition as the base steel, except that the Mn content was increased from 1.5% to 2.3%.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Haruya Kawano 1 Kanazawacho, Kakogawa-shi, Hyogo Prefecture Kobe Steel Works Kakogawa Works F-term (reference) 4K032 AA01 AA04 AA05 AA08 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA27 AA29 AA31 AA35 AA36 BA01 CA02 CD01 CD02

Claims (4)

[Claims] 1. C: 0.0% by mass (hereinafter the same)
6-0.14%, Si: 0.5% or less, Mn: 1-2
%, Cr: 0.03% or less (including 0%), Mo: 0.
05 to 0.3%, V: 0.01 to 0.1%, Nb:
0.01-0.1%, Ti: 0.001-0.02%,
N: 0.001% to 0.008% and Al: 0.05% or less, wherein the island-like MA structure has a volume fraction of 0.1%.
5 to 3.0%, and the average particle size of the island-like MA structure is 1
Low yield ratio, high tensile strength non-heat treated steel characterized by having a thickness of up to 4 μm. 2. The method according to claim 1, further comprising at least one element selected from the group consisting of Cu: 0.5% or less, Ni: 0.5% or less, and Ca: 0.005% or less. Item 4. A low-yield-ratio high-strength non-heat treated steel according to item 1. 3. The method for producing a low-yield-ratio high-strength non-heat-treated steel according to claim 1 or 2, wherein an average cooling rate from the Ar ₃ point to 500 ° C. in a cooling process after finish rolling is performed. A method for producing a low-yield-ratio high-strength non-heat treated steel, which is at a rate of 0.3 to 3 ° C / sec. 4. A slab is heated at an average heating temperature of at least 1000 ° C.
The method for producing a low-yield-ratio high-strength non-heat treated steel according to claim 3, wherein the steel is subjected to hot rolling after being held for 0 minutes or more.