JP4325503B2 - Steel material with excellent fatigue characteristics and method for producing the same - Google Patents

Description

   The present invention relates to a steel material having excellent fatigue characteristics and a method for producing the same. More specifically, the steel material according to the present invention is excellent in fatigue characteristics of a plasma cutting part, a laser cutting part, and a HAZ part at the time of large heat input welding. Therefore, the present invention can be applied to a ship, an offshore structure, a bridge, and an architecture. TECHNICAL FIELD The present invention relates to a steel material that can be suitably used for a structure such as an object or a tank, whether it is cut or welded, and a method for manufacturing the same.

  For example, steel materials used in structures such as ships, offshore structures, bridges, buildings, tanks, etc. are first cut into a desired shape and size by an appropriate cutting method prior to constructing the structure. As such a cutting method, high-speed cutting that is several times higher than conventional gas cutting is possible, so a high-temperature, high-speed plasma formed using the thermal pinch effect by a metal nozzle Recently, plasma cutting and laser cutting using a flow are frequently used. Since these cutting methods have considerably high cutting accuracy, the cut steel material may be used as it is cut.

  However, since the cooling rate in the vicinity of the cut surface by such cutting increases, the structure in the vicinity of the cut surface is hardened. For this reason, if the steel material as it is cut is used in a portion where the stress is repeatedly applied, fatigue cracks are likely to occur from the vicinity of the cut surface where the hardness has increased, and the fatigue life is reduced due to the crack as a starting point.

For this reason, in many cases, it has been necessary to perform machining such as grinding as a finishing process in the vicinity of the cut surface in the cut steel material, and an increase in processing cost and processing time cannot be denied.
The steel material cut and finished in this way is often used as a welded structure. With regard to the welding used at that time, in order to improve the welding efficiency, a large amount of welding is performed by electrogas or electroslag welding. Thermal welding has been performed. In the case of such high heat input welding, the structure in the vicinity of the boundary portion is hardened at a boundary portion between the weld metal and the base metal with a rapid temperature rise and a rapid cooling rate. Therefore, when a repeated stress is applied to this portion, a fatigue crack is likely to be generated from the vicinity of the boundary where the hardness is increased, and the fatigue life is reduced starting from this crack.

  By the way, as invention which improves the fatigue life of steel materials, in patent document 1, the shot peening process is carried out as post-processing on the surface of the steel plate which has a composite structure which has a bainite and a retained austenite whose volume ratio is 10% or more as a main phase. There has been proposed a method for improving the fatigue properties of a steel material, in which the residual austenite phase in the surface layer is subjected to strain-induced transformation. According to the invention concerning this proposal, the fatigue characteristics of the whole steel material including the cut surface of the steel material cut by plasma can be improved.

In addition, as a method for improving the fatigue characteristics of the plasma cut surface, Patent Document 2 describes the optimization of the components of the base metal and the plasma cut steel material by managing the hardenability index and the austenite grain size within an appropriate range. A technique for improving the fatigue strength of a cut surface is disclosed.
JP-A-6-271930 Japanese Patent Laid-Open No. 2001-107175

  However, according to the invention of Patent Document 1, since it is necessary to perform post-processing on the steel material, it is undeniable that the processing cost and the processing time are increased. Therefore, in this steel material, it cannot be used as a constituent member at a site where stress repeatedly acts as it is with plasma or laser cutting.

  Moreover, in patent document 2, although the post-process of steel materials is unnecessary and the fatigue strength of a cut surface improves, the increase in C amount can be considered as an effective measure for ensuring DI value. However, increasing the amount of C has a problem that the hardness of the cut portion is remarkably increased and the bending workability after cutting is lowered.

Here, the subject of this invention is providing the steel material excellent in the fatigue characteristic in parts, such as a plasma cutting | disconnection, a laser cutting part, and a HAZ part at the time of high heat-input welding, and its manufacturing method.
More specifically, since the problem of the present invention is that the cut portion and the welded HAZ portion are excellent in fatigue characteristics, repeated stress acts among components such as ships, marine structures, bridges, buildings, and tanks. It is to provide a steel material that can be suitably used for a structural member used in a site to be used without special post-treatment after cutting or welding, and a method for manufacturing the same.

As a result of intensive studies on factors affecting the fatigue characteristics of the cut portion, not the entire steel sheet, the present inventors have obtained the new knowledge listed below and completed the present invention.
(I) By optimizing the hardenability index Dl and the Mn / C ratio, the structure of the hardened layer portion of the cut surface can be made into a fine-grained structure, thereby generating dislocations due to the action of repeated stress. It is possible to increase the fatigue life of the cut part.

  (Ii) Although the structure of the cut part is austenitized by heating by heat input at the time of cutting, the growth of austenite grains can be suppressed by forming carbonitride by adding an appropriate amount of Nb, Ti, etc. Thereby, the hardened layer part of a cut surface can be made into a fine grain structure.

  (Iii) In order to increase the hardenability index Dl to 12 or more, it is effective to increase both the C content and the Mn content and the strength of the steel sheet itself is increased, but the toughness is deteriorated. Therefore, by appropriately specifying the composition of the steel plate, the characteristics of the base material and the cut portion can both be maintained at a desired level without deteriorating the base material characteristics.

  (Iv) The above knowledge is the knowledge in the plasma cutting part, but the same can be said in the laser cutting, and both the cutting part and the welding part are subjected to rapid temperature rise and rapid cooling. The effect on is almost the same.

  (V) By using a steel having a steel composition conceived based on the above knowledge, a steel material having high fatigue characteristics can be obtained without being subjected to post-processing such as machining on the cut surface. As a result, it has been found that it is possible to provide a steel material that can reduce both the processing cost and the processing time.

(Vi) Such an effect can be obtained even during high heat input welding, and a steel material having high fatigue characteristics can be obtained even with high heat input welding.
The present invention has been completed based on these findings. The gist of the present invention is a steel material having excellent fatigue strength described in the following (1) to (4), a method for producing the same, and a steel structure using the steel material. is there.

(1) By mass%, C: 0.01 to 0.10%; Si: 0.01 to 0.6%; Mn: 0.4 to 2.0%; P: 0.02% or less; S: 0.01% or less; Cr: 0.16 to 0.6%; Nb : One or two of 0.005 to 0.06% and Ti: 0.005 to 0.03%; sol.Al: 0.10% or less; N: 0.01% or less; Other steel material having a steel composition consisting of unavoidable impurities and the balance Fe The hardenability index Dl defined by the following formula is 12 or more, and Mn / C is 15 or more, and is excellent in fatigue characteristics, for plasma cutting, laser cutting, or high heat input welding use steel.

・ When Mn is 1.2% or less
D1 = 0.311 × √C × (1 + 0.7 × Si) × (1 + 3.33 × Mn) × (1 + 2.16 × Cr) × (1 + 3 × Mo) × 25.4
・ When Mn exceeds 1.2%
D1 = 0.311 × √C × (1 + 0.7 × Si) × (5 + 5.1 × (Mn−1.2)) × (1 + 2.16 × Cr) × (1 + 3 × Mo) × 25.4
(2) The steel composition may be one or two of Mo: 0.01 to 0.5%, B: 0.0003 to 0.0030%, and W: 0.05 to 0.50% in mass% instead of part of Fe. A steel material having excellent fatigue properties as described in (1) above, containing the above.
(3) The above steel composition (1) or (2) , wherein the steel composition contains one or two of Cu: 0.05 to 0.6% and Ni: 0.05 to 0.6% in mass% instead of a part of Fe Steel material with excellent fatigue characteristics as described in 1.

(4) The steel material having excellent fatigue characteristics according to any one of the above (1) to (3), wherein the steel composition contains V: 0.005 to 0.08% by mass% instead of part of Fe .

(5) The steel slab having the steel composition described in any one of (1) to (4) above is heated to a temperature range of 1200 ° C. or lower and rolled, and the rolling is performed in a temperature range of Ar ₃ or higher. After completion, plasma cutting with excellent fatigue characteristics is characterized by performing accelerated cooling with an average cooling rate between 650-500 ° C being 5-50 ° C / S and stopping the accelerated cooling at 450 ° C or less Of steel for laser, laser cutting or high heat input welding .

(6) The steel slab having the chemical composition according to any one of the above (1) to (4) is heated to a temperature range of 1200 ° C. or lower and rolled, and the rolling is completed at a temperature range of Ar ₃ points or higher. After that, the plasma is reheated to Ac ₁ point or higher, then cooled to an average cooling rate between 650 ° C. and 500 ° C. of 5 ° C./s or higher, and the cooling is stopped at 500 ° C. or lower. A method of manufacturing a steel sheet for cutting, laser cutting, or high heat input welding .

(7) A method for producing a steel sheet for plasma cutting, laser cutting or high heat input welding, characterized by heating to 450 ° C. or lower and tempering in addition to (5) or (6) above.
(8) A structure using the steel material according to any one of (1) to (4 ) above.

  According to the present invention, since the fatigue characteristics of the cut portion are excellent, for example, plasma, laser cutting on a member on which a repeated stress acts among components of a steel structure such as a ship, an offshore structure, a bridge, a building, and a tank. As a result, it has become possible to provide a steel material that can be suitably used even with a high heat input welding and a method for producing the same.

  Next, an embodiment of a steel material excellent in fatigue characteristics of a cut portion and a manufacturing method thereof according to the present invention will be described in detail. In the following description, the case where the steel material is a steel plate is taken as an example, but this is an example, and the present invention is a hot-rolled steel material other than the steel plate, such as a tube material, a bar material, a shape material, and a wire material. The same applies to.

  First, the reason for limiting the composition of the steel used in the production method according to the present invention will be described. In this specification, “%” indicating the steel composition is “% by mass” unless otherwise specified.

(Steel composition)
C: 0.01-0.10%
C is a component that increases the strength and hardenability index Dl of steel by containing 0.01% or more. However, if the C content exceeds 0.10%, it becomes difficult to ensure the necessary strength and toughness. Therefore, in the present invention, the C content is limited to 0.01% or more and 0.10% or less. In order to secure the hardenability index Dl and Mn / C ratio, which will be described later, and to ensure the toughness of the base material, it is desirable that the lower limit of the C content is 0.03% and the upper limit is 0.08%.

Si: 0.01-0.6%
Si contributes to deoxidation of steel by containing 0.01% or more. However, if the Si content exceeds 0.6%, the toughness of the steel is impaired. Therefore, in the present invention, the Si content is limited to 0.01% or more and 0.6% or less. From the same viewpoint, it is desirable that the lower limit of the Si content is 0.02% and the upper limit is 0.4%.

Mn: 0.4 to 2.0%
By containing 0.4% or more of Mn, the strength of the steel can be improved and the hardenability index Dl can be secured. However, if the Mn content exceeds 2.0%, the toughness and workability of the steel are impaired. Therefore, in the present invention, the Mn content is limited to 0.4% or more and 2.0% or less. From the same viewpoint, the upper limit of the Mn content is desirably 1.50%.

P: 0.02% or less P is an unavoidable impurity and deteriorates the toughness of the steel, such as promoting central segregation. Therefore, in the present invention, 0.02% is made the upper limit. Desirably, it shall be 0.018% or less.

S: 0.01% or less S is an inevitable impurity, and when it is present in a large amount exceeding 0.01%, it causes weld cracking and forms inclusions that can be the starting point of cracks such as MnS. In order to stop the HAZ toughness without affecting the toughness, it is preferably 0.006% or less, more preferably 0.004% or less.

Cr: 0.01-0.6%
Cr is a component that improves hardenability and increases strength by containing 0.01% or more. However, when the content exceeds 0.6%, a significant increase in strength is observed, but the toughness deteriorates. Therefore, in the present invention, the Cr content is limited to 0.01% or more and 0.6% or less. From the same viewpoint, it is desirable that the lower limit of Cr content is 0.03% and the upper limit is 0.5%.

Nb: 0.005-0.06%
When Nb is contained in an amount of 0.005% or more, carbonitride is formed to suppress the grain growth of ferrite and austenite, and the structure is refined to improve the strength and toughness. However, if the Nb content exceeds 0.06%, the strength of the steel is remarkably increased and the toughness is impaired. Therefore, in the present invention, the Nb content is limited to 0.005% or more and 0.06% or less. From the same viewpoint, it is desirable that the lower limit of the Nb content is 0.01% and the upper limit is 0.05%.

Ti: 0.005 to 0.03%
Ti is an element that exhibits the same effect as Nb when contained in an amount of 0.005% or more. However, if the Ti content exceeds 0.03%, weld cracking is likely to occur. Therefore, in the present invention, the Ti content is limited to 0.005% or more and 0.03% or less. From the same viewpoint, it is desirable that the lower limit of the Ti content is 0.01% and the upper limit is 0.02%.

In the present invention, it is sufficient that at least one of the above-described Nb and Ti is contained.
Sol.Al: 0.10% or less
Sol.Al, like Si, contributes effectively to deoxidation. Moreover, since the structure is refined and uniformed by adding Al, it has uniform characteristics with respect to fatigue characteristics. If it exceeds 0.10%, the cleanliness of the steel is not lowered and the structure is not refined. For this reason, the upper limit of sol.Al is 0.10%. Preferably it is 0.005-0.08%.

N: 0.01% or less When N is present in a large amount, both the base material and the HAZ part deteriorate the toughness. Usually, Ti is added to steel and fixed in the form of TiN to make it harmless. However, when N is present in steel exceeding 0.01%, TiN is dissolved in steel during heating in the HAZ part. As a result, the HAZ portion is hardened and the toughness deteriorates. Further, by adding N, it is possible to increase the hardness of the steel material. By increasing the hardness, the fatigue characteristics are improved, but when it exceeds 0.01%, the toughness deteriorates due to a significant increase in hardness. For this reason, N sets 0.01% as an upper limit. Preferably it is 0.0005 to 0.008%.

  In the present invention, in addition to these elements, in order to improve the quality, Cu, Ni, V, Mo, B and W are arbitrarily added elements, and one or two of Cu and Ni, or V alone. , Or one or more of Mo, B and W, or a combination of two or more thereof.

Therefore, these optional additive elements will also be described.
Cu: 0.05-0.6%
When steel is used in a corrosive environment, Cu can improve corrosion resistance by adding 0.05% or more as necessary. However, if the Cu content exceeds 0.6%, these effects are saturated and the strength of the steel is excessively increased, and the toughness is impaired. Therefore, when Cu is added, its content is desirably limited to 0.05% or more and 0.6% or less.
From the same viewpoint, it is desirable that the lower limit of the Cu content is 0.1% and the upper limit is 0.5%.

Ni: 0.05-0.6%
Ni is added at 0.05% or more to show the effect of improving the corrosion resistance in a corrosive environment. However, if the Ni content exceeds 0.6%, these effects are saturated and the strength of the steel is excessively increased, and the toughness is impaired. Therefore, when Ni is added, its content is desirably limited to 0.05% or more and 0.6% or less. From the same viewpoint, it is desirable that the lower limit of Ni content is 0.1% and the upper limit is 0.5%.

V: 0.005-0.08%
When V is added in an amount of 0.005% or more, the structure is refined and contributes to an increase in the fatigue strength of the steel material. However, if the V content exceeds 0.08%, the effect is saturated and the strength is excessively increased, and the toughness is impaired. Therefore, when V is added, its content is 0.005.
It is desirable to limit it to not less than% and not more than 0.08%. From the same viewpoint, it is desirable that the lower limit of the V content is 0.01% and the upper limit is 0.06%.

Mo: 0.01-0.5%
Mo is a component that increases hardenability and strength by containing 0.01% or more. However, when the Mo content exceeds 0.5%, a significant increase in strength is observed, but the toughness deteriorates. Therefore, in the present invention, the Mo content is limited to 0.01% to 0.5%. From the same viewpoint, it is desirable that the lower limit of the Mo content is 0.02% and the upper limit is 0.4%.

B: 0.0003 to 0.0030%
B is an effective element for increasing the hardenability of the γ grain boundary and increasing the strength of the base material even in a small amount. In order to obtain this effect, it is necessary to add 0.0003% or more. However, if it exceeds 0.0030%, the heat affected zone is hardened, so the upper limit was made 0.0030%.

W: 0.05-0.50%
W is an effective element for increasing the strength of the base material and improving the corrosion resistance. In order to obtain this effect, it is necessary to add 0.05% or more. However, if it exceeds 0.50%, the toughness deteriorates.

Hardenability index Dl: 12 or more The hardenability index Dl is a characteristic value defined by the present inventors for indicating the hardenability. That is, the hardenability index Dl is obtained by water quenching various samples (round bars) with different thicknesses and examining the cross section of the samples, so that 50% of the structure in the center of the sample is martensite. The limit thickness (diameter) is shown. The hardenability index Dl is defined by the above equation (1).

  When the hardenability index Dl is 12 or more and Mn / C is 15 or more, the hardened structure formed by heat in cutting is martensitic. For this reason, the occurrence of dislocation associated with repetitive stress is suppressed, the fatigue life is increased, and the fatigue limit is improved. On the other hand, if the hardenability index Dl is less than 12, the fatigue limit becomes the conventional level. Therefore, in the present invention, the hardenability index Dl is limited to 12 or more. In addition, each element symbol in the formula which shows the hardenability index | exponent D1 represents content (mass%) in steel composition.

A preferred range is 12.5 or more.
Mn / C ratio: 15 or more
Fatigue characteristics are dramatically improved by securing a DI value above a certain level, but the increase in the DI value associated with an increase in the amount of C results in poor bending workability after cutting due to a significant increase in the hardness of the cutting part. It was confirmed. Therefore, in the present invention, by increasing the Mn / C ratio to 15 or more, an attempt is made to achieve a high fatigue life without impairing the bending workability after cutting. “Mn” and “C” represent the content (mass%) of each element in the steel composition. A preferred range is 16 or more.

  In Patent Document 2, fatigue characteristics are improved by prescribing a retained austenite grain size at the time of cutting after securing a predetermined hardenability index. However, in the present invention, by defining Mn / C to be 15 or more, the bending characteristics that have not been considered in Patent Document 2 are secured and the fatigue strength is improved.

  Moreover, when Mn / C increases, the proportion of bainite in the steel structure increases. In order to improve the fatigue strength, the bainite fraction is preferably set to 30% or more. For this reason, it is also controlled by Mn / C and set to 15 or more.

The reason why an increase in the bainite fraction is effective in suppressing fatigue crack growth is as follows.
It is known that bainite softens when subjected to repeated deformation as in a fatigue crack growth test. This is because the dislocations introduced by the transformation coalesce and disappear due to repeated deformations, thereby relaxing the strain accumulated at the fatigue crack tip. In other words, it is considered that bainite is effective in suppressing crack growth because the crack growth driving force decreases due to work softening characteristics.

Here, it will be as follows if the manufacturing method of the steel materials concerning this invention is demonstrated.
(Steel heating)
In the present invention, in order to manufacture a steel material having such a steel composition and hardenability index Dl, a steel piece (slab) is formed by a conventional melting means or a processing means, and this is turned into a steel material by rolling, for example. That's fine. When the steel material is a rolled steel material, the steel slab is heated to a temperature range of 1200 ° C. or lower prior to rolling. When the steel slab is heated to a high temperature exceeding 1200 ° C., the austenite grains of the steel become very coarse, which impairs the toughness of the steel. Therefore, the steel heating temperature prior to rolling is limited to 1200 ° C. or less.

(rolling)
The steel thus heated is rolled. The rolling is finished in a temperature range of Ar ₃ points or higher. This is because hot rolling in the non-recrystallized region is preferable for improving the toughness of the steel, and in rolling other than the non-recrystallized region, the steel structure becomes coarse and deteriorates the toughness.

Although there are various calculation methods for the Ar ₃ point, in the present invention, the calculation is performed using the following formula.
Ar ₃ = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo + 0.35 (t-8)
Here, the symbol is the content (% by mass) of each element in the steel composition, and t is the plate thickness (mm).

This formula referred to "Iron and Steel" 67th (1981) No. 1, P143, Ouchi et al., "Effects of rolling conditions and chemical composition on ferrite transformation start temperature after hot rolling".
(cooling)
After rolling is completed in this manner, accelerated cooling is performed with an average cooling rate between 650-500 ° C. being 5-50 ° C./s, and the accelerated cooling at this time stops at 450 ° C. or less.

When the cooling stop temperature is set to 450 ° C. or lower, for example, in order to reduce the temperature unevenness in the steel sheet, it means to include cooling to near normal temperature.
In one aspect of the present invention, accelerated cooling may be performed immediately after hot rolling. This is a so-called TMCP type manufacturing method.

In another aspect of the present invention, accelerated cooling may not be performed immediately after rolling, and the steel sheet may be heated again after quenching and then quenched. The heating temperature at this time is preferably at least _one Ac point. If it is less than ₁ Ac, transformation by quenching does not occur.

  Further, the tempering temperature is limited to 450 ° C. or less. This is because a structure and mechanical properties satisfying the characteristics can be obtained at 450 ° C. or lower, and the properties are lost when tempering is performed at a temperature exceeding the above properties. Therefore, the tempering temperature is preferably 450 ° C. or lower.

  Although the manufacturing method according to the present invention has been described using a steel plate as an example, steel materials such as wire rods, rods, shapes and pipes can be manufactured under similar manufacturing conditions, and the present invention is not particularly limited in this respect.

  Thus, according to the present invention, a steel material excellent in fatigue characteristics and a method for producing the same, and more specifically, excellent in fatigue characteristics of plasma or laser cutting parts or large heat input welds, for example, ships, marine structures Among structural members of so-called steel structures such as objects, bridges, buildings, tanks, etc., it can be suitably used as it is cut or welded to a member that repeatedly undergoes stress, such as a longi material of a ship member. A steel material and a method for manufacturing the same are provided. For this reason, according to this invention, both processing cost and processing time can be reduced.

Next, the effects of the present invention will be described more specifically with reference to examples.
Example 1
Using the test materials a to j having the steel composition, hardenability index Dl and Mn / C shown in Table 1, according to the manufacturing conditions shown in Table 2, the strength conforming to the SM490A standard and the thickness of 25 mm Test materials were manufactured from No. 1 to No. 10.

  Axial force fatigue test pieces of the dimensions (unit: mm) and shape shown in FIG. 1 are cut from each test material by plasma cutting under the cutting conditions shown in Table 3, and then the axial force fatigue tester shown in FIG. 2 is used. An axial force fatigue test was conducted.

That is, an axial force fatigue test piece having the planar shape and dimensions shown in FIG. 1 is produced by plasma cutting, and a 20-ton electrohydraulic fatigue tester shown in FIG. the axial force pulsating tensile load control method of the amplitude 284~421N / mm ^2, were subjected to fatigue tests. 2, reference numeral 1 is an axial force fatigue test piece, reference numeral 2 is a load cell for detecting a load, reference numeral 3 is a hydraulic cylinder for applying a load to the axial fatigue test piece 1, and reference numeral 4 is a waveform generation. Reference numeral 5 denotes a load controller, reference numeral 6 denotes a servo valve, and reference numeral 7 denotes a hydraulic pressure source.

In this axial force fatigue test, the stress amplitude at which the number of repetitions of fracture was 107 was measured as the fatigue limit Δσw. The measurement results are also shown in Table 2.
Further, a bending test (bending radius: 1.0 t; where t is a plate thickness) was performed using a test piece 30 having a plasma cut surface on the side surface portion of the test piece shown in FIG. 3, and the superiority of bending workability was confirmed. . The results are also shown in Table 2. In addition, the side part 32 in a figure is a plasma cut surface.

Nos. 1 to 8 in Table 2 are examples of the present invention that satisfy the scope of the present invention, and the fatigue limit Δσw is a high value of 457 to 418 (N / mm ² ). Good results are obtained with no cracking.

On the other hand, in No. 9, since Mn / C is below the lower limit of the range of the present invention, the fatigue strength Δσw is 321 (N / mm ² ), and cracking occurs in the bending test.
In No. 9, since Mn / C and the hardenability index DI are below the lower limit of the range of the present invention, the fatigue strength Δσw is 297 (N / mm ² ), and cracking occurs in the bending test. Yes.

In No. 11, since Mn / C is below the lower limit of the range of the present invention, the fatigue strength Δσw is 324 (N / mm ² ), and cracks are also generated in the bending test.
In No. 12, since Mn / C is below the lower limit of the range of the present invention, the fatigue strength Δσw is 319 (N / mm ² ), and the crack is generated in the bending test.

(Example 2)
Based on the manufacturing conditions shown in Table 5, using the steel composition, hardenability index DI and Mn / C shown in Table 4, the strength according to the SM490A standard and the plate thickness of 25 mm Sample materials were manufactured from No.13 to No.17. A welded joint was prepared from each specimen under the welding conditions shown in Table 6.

  After processing into a test piece shape having the dimensions (unit: mm) and shape shown in FIG. 4, an axial force fatigue test of the welded joint portion was performed using an axial force fatigue tester shown in FIG. 4A is a side view of the test piece, and FIG. 4B is a plan view. In the figure, the blackened portion indicates the welded portion.

In this axial force fatigue test, the stress amplitude at which the number of repeated fractures Nf was 107 was measured as the fatigue limit Δσw. The measurement results are also shown in Table 5.
Nos. 13 to 15 in Table 5 are all examples of the present invention satisfying the scope of the present invention, and the fatigue limit Δσw of the welded joint portion is 421 to 408 (N / mm ² ), indicating a high value and Even in the bending test, no cracks are generated and good results are shown.

On the other hand, in No. 16 in Table 5, since the Mn / C is below the lower limit of the range of the present invention and the hardenability is insufficient, the fatigue limit Δσw of the welded joint is 296 (N / mm ² ) and deteriorated. In addition, cracks also occur in the bending test.

Similarly, in No. 17, since Mn / C is below the lower limit of the range of the present invention, the fatigue limit Δσw
However, it deteriorated to 264 (N / mm ² ) and cracking occurred in the bending test.

It is a top view which shows the shape of an axial-force fatigue test piece. It is a typical explanatory view of an axial force fatigue testing machine. It is a top view which shows the shape of a bending test piece. FIG. 4 (a) is a side view showing the shape of a welded joint fatigue test piece, and FIG. 4 (b) is a plan view of the same.

Claims (8)

In mass%, C: 0.01 to 0.10%; Si: 0.01 to 0.6%; Mn: 0.4 to 2.0%; P: 0.02% or less; S: 0.01% or less; Cr: 0.16 to 0.6%; Nb: 0.005 to 0.06% and Ti: 0.005 1 kind or two kinds of ~0.03%; sol.Al:0.10% less; N: 0.01% or less; other a steel having a steel composition consisting of incidental impurities and the balance Fe, the following formula A steel material for plasma cutting, laser cutting, or high heat input welding with excellent fatigue characteristics, characterized by having a hardenability index Dl of 12 or more and Mn / C of 15 or more.
When Mn: 1.2% or less
D1 = 0.311 × √C × (1 + 0.7 × Si) × (1 + 3.33 × Mn) × (1 + 2.16 × Cr) × (1 + 3 × Mo) × 25.4
When Mn exceeds 1.2%
D1 = 0.311 × √C × (1 + 0.7 × Si) × (5 + 5.1 × (Mn−1.2)) × (1 + 2.16 × Cr) × (1 + 3 × Mo) × 25.4 The steel composition is in place of a part of Fe , in mass%, Mo: 0.01 to 0.5%, B: 0.0003 to 0.0030% and W: 0.05 to 0.50%. The steel material excellent in fatigue characteristics according to claim 1 , containing one or more kinds. The steel composition, instead of a part of Fe, by mass%, Cu: from 0.05 to 0.6% and Ni: containing from 0.05 to 0.6% of one or claim 2 Steel material with excellent fatigue characteristics as described in 1. The steel material with excellent fatigue characteristics according to claim 2 or 3 , wherein the steel composition contains V: 0.005 to 0.08% in mass% instead of a part of Fe . The steel slab having the steel composition according to any one of claims 1 to 4 is heated to a temperature range of 1200 ° C or lower and rolled, and after the rolling is finished at a temperature range of Ar ₃ or higher, 650 to For plasma cutting and laser cutting excellent in fatigue characteristics, characterized by performing accelerated cooling with an average cooling rate between 500 ° C. and 5 to 50 ° C./S, and stopping the accelerated cooling at 450 ° C. or less , Or the manufacturing method of the steel material for large heat input welding . The steel slab having the chemical composition according to any one of claims 1 to 4 is heated to a temperature range of 1200 ° C or lower and rolled, and after the rolling is finished at a temperature range of Ar ₃ points or higher, the Ac 1 point or higher is reached. After reheating to 650 ° C to 500 ° C, the average cooling rate is cooled to 5 ° C / s or more, and the cooling is stopped at 500 ° C or less, for plasma cutting, for laser cutting, Or the manufacturing method of the steel plate for high heat input welding . The method for producing a steel sheet for plasma cutting, laser cutting, or high heat input welding according to claim 5 or 6, wherein the tempering is performed by further heating to 450 ° C or less following the cooling. A structure using the steel material according to any one of claims 1 to 4 .

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