KR100774805B1 - Steel product excellent in fatigue characteristics and method for production thereof - Google Patents

Abstract

The steel material which is excellent in the fatigue property of a cut part or a welded HAZ part which can be used without a special post-treatment after cutting or welding is C: 0.01 to 0.10% by mass%; Si: 0.01 to 0.6%; Mn: 0.4-2.0%; Cr: 0.01 to 0.6%; One or two of Nb: 0.005% to 0.06% and Ti: 0.005% to 0.03%; It contains sol.Al: 0.10% or less, and the hardenability index (DI) prescribed | regulated by the following formula is 12 or more, and Mn / C ratio which is ratio of Mn amount with respect to C amount to mass% is 20 or more. Mn: 1.2% or less DI = 0.311 × √C × (1 + 0.7 × Si) × (1 + 3.33 × Mn) × (1 + 2.16 × Cr) × (1 + 3 × Mo) × 25.4 Mn: Over 1.2% DI = 0.311 × √C × (1 + 0.7 × Si) × (5 + 5.1 × (Mn-1.2)) × (1 + 2.16 × Cr) × (1 + 3 × Mo) × 25.4

Description

Steel material with excellent fatigue properties and its manufacturing method {STEEL PRODUCT EXCELLENT IN FATIGUE CHARACTERISTICS AND METHOD FOR PRODUCTION THEREOF}

TECHNICAL FIELD This invention relates to the steel material excellent in fatigue characteristics, and its manufacturing method. The steel according to the present invention has excellent fatigue characteristics of the plasma cutting portion, the laser cutting portion, and the HAZ portion at the time of the high heat input welding, so that the steel materials can be suitably used in structures such as ships, offshore structures, bridges, buildings, tanks, etc. Can be.

For example, steel materials used in structures such as ships, offshore structures, bridges, buildings, tanks, etc. are first cut into a desired shape and dimension by a suitable cutting method before constructing the structure. As such a cutting method, plasma cutting using a high-temperature and high-speed plasma flow formed using a thermal pinch effect by a metal nozzle is possible, since the cutting speed can be several times higher than that of conventional gas cutting. Cutting has been used a lot lately. Since these cutting methods also have extremely high cutting accuracy, the cut steel may be used in a cut state.

However, in such a cutting method, since the cooling rate near the cut surface increases, the structure near the cut surface hardens. For this reason, when a steel material in a cut state is used for a site where cyclic stress acts, fatigue cracking tends to occur in the vicinity of the cut surface where the hardness is increased, and the crack starts as a starting point and the fatigue life decreases.

Therefore, the cut steel material in many cases needs to perform machining, such as grinding, as a finishing process in the vicinity of a cut surface. This finishing process necessarily involves an increase in processing cost and processing time.

In addition, the steel material which has been cut and finished is often provided for use as a welded structure. In the welding of a large structure mentioned above, in order to improve welding efficiency, the high heat input welding by the electrogas welding method, the electro slag welding method, etc. has been generally performed. In the case of such high heat input welding, the vicinity of the boundary between the weld metal and the base metal undergoes rapid temperature rise and cooling, and the structure of the portion (that is, the weld heat affected zone abbreviated as HAZ portion) is hardened. Therefore, when the cyclic stress acts on the welded portion, fatigue cracking is likely to occur when the HAZ portion of which the hardness is increased overlaps with the weld dent position, and the cracking starts and the fatigue life decreases.

In Japanese Patent Laid-Open No. 6-271930, shot peening is performed as a post-treatment on the surface of a steel sheet having a composite structure containing bainite and residual austenite having a volume fraction of 10% or more as its main phase, and the steel surface layer portion. A method of improving the fatigue life of steel materials by modifying organic transformation of the retained austenite phase of is proposed. This method can improve the fatigue property of the steel as a whole, including the cut surface of the steel cut by plasma. However, since this method performs short peening to steel materials as a post-treatment, it comes with the increase of processing cost and processing time.

Japanese Unexamined Patent Application Publication No. 2001-107175 discloses a steel material in which the fatigue strength of a plasma cut section is improved by optimizing the chemical composition of the base material and managing the hardenability index (DI) and the austenite particle diameter in an appropriate range. have. This steel material improves the fatigue strength of a cut surface, without performing post-processing. However, an effective countermeasure for securing the required DI value is an increase in the amount of C, but there is a problem that the hardness of the cutout portion is remarkably increased due to the increase in the amount of C, thereby deteriorating the bending workability after cutting.

This invention provides the steel material excellent in the fatigue characteristic in the site | part hardened | cured the structure called the HAZ part at the time of a plasma cut part, a laser cut part, and high heat input welding, and its manufacturing method. More specifically, the present invention can be used as a structure in which a cyclic stress is applied in a cut state or a weld state without performing a special post-treatment during cutting or after welding, and does not cause a decrease in bending workability after cutting. Provided is a steel material and a method of manufacturing the same.

Steel materials excellent in fatigue properties according to the present invention are, by mass%, C: 0.01 to 0.10%; Si: 0.01 to 0.6%; Mn: 0.4-2.0%; P: 0.02% or less; S: 0.01% or less; Cr: 0.01 to 0.6%; One or two of Nb: 0.005 to 0.06% and Ti: 0.005 to 0.03%; sol.Al: 0.10% or less; N: 0.01% or less; Quenchability index specified by the following formula (wherein each element symbol means content in steel at the mass% of the element) as a steel having a steel composition containing the remaining unavoidable impurities and Fe. (DI) is 12 or more and Mn / C ratio which is ratio of Mn amount with respect to C amount to mass% is 20 or more.

Mn: When 1.2% or less

DI = 0.311 × √C × (1 + 0.7 × Si) × (1 + 3.33 × Mn) × (1 + 2.16 × Cr) × (1 + 3 × Mo) × 25.4

Mn: When greater than 1.2%

DI = 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 may further contain, by mass%, at least one element selected from the following groups (1) to (3).

(1) one or two of Cu: 0.05 to 0.6% and Ni: 0.05 to 0.6%;

(2) V: 0.005% to 0.08%;

(3) One or two or more of Mo: 0.01% to 0.5%, B: 0.0003% to 0.0030%, and W: 0.05% to 0.50%.

Moreover, it is preferable that the said steel material has a structure whose austenite particle diameter in the hardened layer part of a cut surface or the hardened layer part of a welding heat affected zone is 20 micrometers or less.

The present inventors examined not only the whole steel plate but the factor which affects the improvement of the fatigue characteristic of a cut part or a weld part. This invention is based on the following novel knowledge acquired by the examination.

(i) By optimizing the hardenability index (DI) and the Mn / C ratio, the structure of the hardened layer portion of the cut surface can be made into a fine grain structure, thereby suppressing the generation of dislocation due to the action of cyclic stress, Long life of the cut can be achieved.

(Ii) The structure of the cut part is austenite by heating by heat input during cutting, but by adding an appropriate amount of Nb, Ti or the like to form carbonitride, growth of austenite particles can be suppressed, Thereby, the hardened layer part of a cut surface can be made into a fine grain structure. In particular, when the austenite grain size in the hardened layer portion of the cut surface is 20 micrometers or less, the longer fatigue life can be attained.

(Iii) In order to increase the hardenability index (DI) to 12 or more, it is effective to increase both the amount of C and the amount of Mn, and the strength of the steel sheet itself increases, but the toughness deteriorates. Therefore, by properly specifying the composition of the steel sheet, it is possible to maintain both the properties of the base material and the cut portion to a desired level without degrading the base material properties.

(iv) The above-mentioned knowledge is the knowledge of the plasma cutting part, but the same can be said for the laser cutting, and since the cutting and the welding also cause the rapid temperature rise and the rapid cooling, the effect of the cutting part and the welding part also on the base metal is Almost the same.

By using a steel having a steel composition conceived based on the above findings, a steel material having a high fatigue characteristic in a cut state can be obtained even without performing post-treatment such as machining on the cut surface, and thus machining cost and processing time. It can provide steel that can shorten all. The same effect can be obtained even in high heat input welding, and it is possible to provide a steel material having high fatigue characteristics even in high heat input welding.

The steel material excellent in the fatigue characteristic which concerns on this invention heats the steel piece which has the said steel composition to the temperature range of 1200 degrees C or less, and rolls, and after this rolling is complete | finished in the temperature range of ₃ or more Ar points, at least 650-500 degreeC It can manufacture by the method including performing accelerated cooling which makes the average cooling rate between 5-50 degreeC / s, and stopping this acceleration cooling at 450 degrees C or less.

In another method, the steel piece having the above chemical composition is heated to a temperature range of 1200 ° C. or lower, and rolled, and after the rolling is finished in a temperature range of Ar ₃ or higher, after reheating to Ac ₁ or more, at least 650 ° C. to 500 The cooling which makes the average cooling rate between 5 degrees C / s or more is performed, and this cooling is stopped at 500 degrees C or less.

Any of the above methods further includes annealing by heating to 450 ° C. or lower after cooling.

The present invention also relates to a structure constructed of the steel. Since the steel according to the present invention exhibits excellent fatigue characteristics in the conventionally inferior fatigue characteristics, such as a cut portion by plasma or laser, or a HAZ portion during high heat input welding, for example, a ship, an offshore structure, a bridge It can be suitably used in the state which is a plasma or laser cutting, or a high heat input welding state to the member which repeatedly stresses among the structural members of steel structures, such as a building and a tank. As the member to which the cyclic stress acts, for example, the loin material of the ship member is exemplified, but is not limited thereto. Since the processing after cutting or after welding becomes unnecessary, the processing cost and processing time of steel materials can be reduced.

1 is a plan view showing the shape of the axial force fatigue test piece,

2 is a schematic explanatory diagram of an axial force fatigue tester,

3 is a view showing the effect of the austenite grain size (μm) of the cut layer or the hardened layer portion of the weld heat affected zone on the fatigue limit Δσw (N / mm 2) of the steel;

4 is a plan view showing the shape of the bending test piece,

5: (a) is a side view which shows the shape of a weld joint fatigue test piece, and FIG. 5 (b) is a top view of the same shape.

Below, this invention is demonstrated taking the case where steel materials are steel plates as an example. However, it goes without saying that the present invention is similarly applied to hot rolled steels other than steel sheets, for example, pipes, rods, molds, wire rods and the like. .

In the description regarding the steel composition of the steel of the present invention,% is% by mass unless otherwise specified.

(Lecture composition)

C: 0.01% to 0.10%

By containing 0.01% or more, C increases the strength and hardenability index (DI) of the steel. However, when C content exceeds 0.10%, it becomes difficult to ensure required strength and toughness. In order to secure the hardenability index (DI) and the Mn / C ratio described below and to secure the toughness of the base material, the lower limit of the C content is preferably 0.03% or more and the upper limit is 0.08% or less.

Si: 0.01 to 0.6%

Si contributes to deoxidation of steel by containing 0.01% or more. However, when Si content exceeds 0.6%, the toughness of steel will be impaired. Preferable Si content is 0.02% in the lower limit, and 0.4% in the upper limit.

Mn: 0.4-2.0%

By containing 0.4% or more of Mn, the strength of the steel can be improved and the hardenability index (DI) can be secured. However, when Mn content exceeds 2.0%, the toughness and workability of steel will fall. The upper limit of preferable Mn content is 1.50%.

P: 0.02% or less

P is an unavoidable impurity and deteriorates the toughness of the steel, such as to promote central segregation, so that the upper limit is 0.02%. P content becomes like this. Preferably it is 0.018% or less.

S: 0.01% or less

S is an unavoidable impurity, and when present in a large amount in excess of 0.01%, S becomes a cause of welding cracking, and forms an inclusion which may be a starting point of cracking such as MnS. In order to stop it to such an extent that there is no influence in securing toughness of a HAZ part, S content becomes like this. Preferably it is 0.006% or less, More preferably, it is 0.004% or less.

Cr: 0.01% to 0.6%

By containing 0.1% or more of Cr, the hardenability is improved and the strength is increased. On the other hand, when Cr is contained in an amount exceeding 0.6%, a marked increase in strength is seen, but at the same time the toughness deteriorates. Preferable Cr content is 0.03% in the lower limit, and 0.5% in the upper limit.

Nb: 0.005 to 0.06%

By containing 0.005% or more, Nb forms carbonitrides to suppress grain growth of ferrite and austenite, refine the structure, and improve the strength and toughness. However, when the Nb content exceeds 0.06%, the strength increase of the steel is remarkable, and the toughness is inferior. As for a preferable Nb content, a minimum is 0.01% and an upper limit is 0.05%.

Ti: 0.005% to 0.03%

By containing 0.005% or more, Ti brings about the same effect as Nb. However, when the Ti content exceeds 0.03%, weld cracking tends to occur. As for a preferable Ti content, a minimum is 0.01% and an upper limit is 0.02%.

In this invention, at least one should just contain Nb and Ti mentioned above.

Sol.Al: 0.10% or less

Sol.Al, like Si, effectively contributes to deoxidation. In addition, since Al is refined and uniformized by the addition of Al, the fatigue properties are also uniform. However, when Al content exceeds 0.10%, the cleanliness of steel will fall and a refinement | miniaturization of a structure | tissue cannot be obtained. Al content becomes like this. Preferably it is 0.005 to 0.08%.

N: 0.01% or less

When N is present in a large amount, both the base material and the HAZ portion deteriorate the toughness. Normally, Ti is added to steel to fix it in the form of TiN, and detoxification. However, when N is present in steel in excess of 0.01%, TiN is dissolved in steel at the time of heating in the HAZ portion. This results in hardening of the HAZ portion and deteriorates toughness. In addition, the addition of N can increase the hardness of the steel material, and the fatigue properties are thereby improved, but when it exceeds 0.01%, the toughness deteriorates and deteriorates due to a significant increase in hardness. N content becomes like this. Preferably it is 0.0005 to 0.008%.

In addition to the above elements, the steel according to the present invention may be selected from (1) Cu or Ni as one or two, (2) V, or (3) Mo, B, and W as an optional addition element in order to improve quality. You may contain 1 type, or 2 or more types, or 2 or more types of elements selected from these groups in the quantity of the following range.

Cu: 0.05 to 0.6%

When steel is used in a corrosive environment, Cu can improve corrosion resistance by adding 0.05% or more as needed. However, when Cu content exceeds 0.6%, these effects will be saturated, and the intensity | strength of steel will rise excessively, and toughness will fall. More preferable Cu content at the time of adding Cu is 0.1% of a minimum, and 0.5% of an upper limit.

Ni: 0.05 to 0.6%

Ni, when added at 0.05% or more, shows the effect of improving the corrosion resistance under corrosive environments. However, when the Ni content exceeds 0.6%, these effects saturate and the strength of the steel excessively rises excessively, resulting in poor toughness. When Ni is added as needed, the minimum with more preferable Ni content is 0.1% and an upper limit is 0.5%.

V: 0.005 to 0.08%

By adding 0.005% or more, V becomes fine in structure and contributes to the raise of the fatigue strength of steel materials. However, when the V content exceeds 0.08%, the effect saturates and the strength excessively rises excessively, resulting in poor toughness. When V is added as needed, a minimum with more preferable V content is 0.01% and an upper limit is 0.06%.

Mo: 0.01% to 0.5%

By containing 0.01% or more of Mo, hardenability improves and strength increases. However, when the Mo content exceeds 0.5%, a marked increase in strength is observed, but at the same time the toughness deteriorates. When Mo is added as needed, the minimum with more preferable Mo content is 0.02% and an upper limit is 0.4%.

B: 0.0003 to 0.0030%

B increases the hardenability of the? Grain boundaries even in a small amount, and is effective for increasing the base metal strength. It is necessary to add 0.0003% or more in order to obtain this effect. However, adding B in excess of 0.0030% causes hardening of the heat affected zone.

W: 0.05 to 0.50%

W is effective for increasing the base metal strength to improve corrosion resistance. In order to acquire this effect, it is necessary to add 0.05% or more. However, exceeding 0.50% causes deterioration of toughness.

Quenching index (DI): 12 or more

Quenchability index (DI), also called a critical diameter, means the maximum diameter at which 50% of the central tissue becomes martensite when the sample having a round bar shape is quenched. This diameter can be calculated | required by quenching various kinds of round bar samples of which thickness differs on the same conditions, and inspecting the cross section. Since the value of DI is determined by the steel composition (content of specific elements affecting hardenability and its hardenability multiple), in the present invention, DI is defined by the above formula (1).

If the hardenability index (DI) is 12 or more and Mn / C is 20 or more, the hardened structure formed by the heat in cutting will be martensite. For this reason, generation | occurrence | production of the electric potential accompanying cyclic stress is suppressed, the fatigue life of steel materials becomes high, and a fatigue limit improves. On the other hand, when hardenability index DI is less than 12, a fatigue limit will be a conventional level. DI is preferably at least 12.5.

Mn / C ratio: 20 or more

By securing the DI value more than a certain amount, fatigue properties are remarkably improved, but it is confirmed that bending workability after cutting deteriorates due to a marked increase in the hardness of the cut portion when the DI value increases with the increase in the amount of C. Therefore, in the present invention, by setting the Mn / C ratio to 20 or more, it is intended to achieve high fatigue life without impairing bending workability after cutting. Mn / C ratio means the ratio of Mn amount (all mass%) with respect to C amount in steel. Mn / C ratio becomes like this. Preferably it is 22 or more, More preferably, it is 25 or more.

In Japanese Patent Laid-Open No. 2001-107715, after securing a predetermined hardenability index, the fatigue characteristics are improved by defining the residual austenite grain size at the time of cutting. On the other hand, in the present invention, by defining the Mn / C ratio to 20 or more, the bending characteristics not considered in this publication are ensured, and the fatigue strength is improved.

Moreover, when Mn / C ratio becomes large, the ratio (bainite fraction) which bainite occupies in the steel structure becomes high. In order to improve the fatigue strength, the bainite fraction is preferably 30% or more. For this purpose, the Mn / C ratio is controlled to 20 or more.

When the bainite fraction increases, the reason which is effective in suppressing fatigue crack growth is as follows.

It is known that bainite softens work under repeated deformation such as fatigue crack propagation test. This is because the dislocations introduced by transformation are coalesced and extinguished by cyclic deformation, thereby alleviating the deformation accumulated at the fatigue crack tip. In other words, it can be considered that bainite is also effective for suppressing crack propagation because the crack propagation driving force decreases due to work softening characteristics.

Austenitic particle diameter in the cut layer or the hardened layer portion of the weld heat affected zone: 20 μm or less

The steel material according to the present invention preferably has an austenite particle diameter of 20 μm or less in the hardened layer portion of the cut surface, that is, the hardened layer portion of the cut surface formed by plasma cutting or laser cutting, or the hardened layer portion of the weld heat affected zone formed by welding. .

The width of the cured layer portion of the cut surface formed by plasma cutting or laser cutting varies depending on the cutting conditions, but according to the findings of the present inventors, when the value of the hardenability index (DI) is 12 or more, the inner side is cut off from the cut surface. Toward the entire thickness of the sheet within the range of about 0.5mm. The reason why it is preferable that the austenite particle diameter in this hardened layer part is 20 micrometers or less is as follows.

An axial force fatigue test piece having a planar shape as shown in Fig. 1 was produced by plasma cutting, and at room temperature with a repetition frequency of 5 Hz, a stress ratio of 0.1, and a stress amplitude of 284 to 421 N / mm 2 by an axial force deflection tensile load control method. The fatigue test was done using the 20-ton electrohydraulic fatigue test machine shown in FIG. 2 is a axial force fatigue test piece, 2 is a load cell which detects a load, 3 is a hydraulic cylinder which gives a load to the axial force fatigue test piece 1, and 4 is a waveform generator. , 5 is a load controller, 6 is a servo valve, and 7 is a hydraulic source.

In this axial force fatigue test, the stress amplitude at which the number of breaking repetitions was 10 ⁷ was measured as the fatigue limit Δσw (N / mm 2). Moreover, the structure of the hardened layer part of the plasma cutting surface of the test piece after completion | finish of a test was observed by corroding with the mixed corrosion liquid of picric acid, Lipon F, and ferric chloride, and the average particle diameter measured from this observation was austenite It was made into the particle diameter. In addition, the measuring means of austenite particle diameter is not limited to this means, You may measure austenite particle diameter by another means.

3 shows the effect of the austenite grain size (µm) of the hardened layer portion on the cut surface on the fatigue limit Δσw (N / mm 2) of the steel. From the graph shown in FIG. 3, when this austenite particle diameter is 20 micrometers or less, the fatigue limit ((DELTA) w) (N / mm <2>) improves remarkably, and when this austenite particle diameter exceeds 20 micrometers, a fatigue limit ((DELTA) w) (N / mm <2>) turns out to be the same level as before. Therefore, in this invention, 20 micrometers or less are preferable for the austenite particle diameter in the hardened layer part of the cut surface formed by plasma cutting. As the austenite grain size is suppressed to 20 µm or less, generation of dislocations accompanying cyclic stress is suppressed, and high fatigue life can be obtained.

Although the austenite particle diameter of the hardened layer part of the cut surface produced | generated by plasma cutting was demonstrated above, the same can be said also about the hardened layer part of the cut surface produced | generated by laser cutting. In addition, since the abrupt temperature rise and the rapid cooling are also applied to the cutting degree welding, the same applies to the hardened layer portion in the weld heat affected zone.

Next, the manufacturing method of the steel material concerning this invention is demonstrated.

(Strong heating)

By using conventional solvent means and processing means, steel slabs (slabs) of a predetermined composition are prepared, and this is made of steel by rolling, for example. Prior to rolling the steel strip, the steel strip is heated to a temperature range of 1200 ° C or lower. When the steel pieces are heated at a high temperature exceeding 1200 ° C, coarsening of the austenite particles of the steel becomes remarkable, which impairs the toughness of the steel.

(Rolled)

The heated steel piece is rolled as mentioned above. The rolling is finished at a temperature range of at least Ar ₃ . In order to improve the toughness of the steel, hot rolling in the unrecrystallized zone is preferable, and in rolling other than the unrecrystallized zone, the structure of the steel is coarsened and the toughness is degraded.

Further, Ar ₃ point, but the method of calculating various types of, in the present invention, calculated according to the following formula

Ar ₃ = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo + 0.35 (t-8)

In the formula, each element symbol is the content (mass%) in the steel of the element, and t is the plate thickness (mm). This formula refers to "Influence of rolling conditions and chemical composition on the ferrite transformation start temperature after hot rolling", "Iron and Steel" No. 67 (1981) No. 1, P143, Ouchira et al.

(Cooling)

 In this way, after completion | finish of rolling, accelerated cooling which makes the average cooling rate between at least 650-500 degreeC to 5-50 degreeC / s is performed, and this accelerated cooling is stopped at 450 degrees C or less. Lowering the cooling stop temperature to 450 ° C means including cooling to near room temperature, for example, in order to reduce temperature spots inside the steel sheet.

In one embodiment of the present invention, accelerated cooling may be performed immediately after hot rolling. It is a manufacturing method of what is called TMCP type.

In another embodiment of the present invention, the steel sheet may be reheated and quenched after the cooling, once without cooling, immediately after rolling. The heating temperature at this time is preferably Ac ₁ or more. Below Ac ₁ point, transformation by quenching does not occur.

Although annealing is preferable after cooling, the annealing temperature in that case is made into 450 degrees C or less. By annealing at 450 degrees C or less, satisfactory structure and mechanical properties can be obtained. When annealing is carried out at a higher temperature, the characteristics are lowered.

In the case where the steel is not a steel sheet but also a wire, a bar, a mold, a pipe, and the like, it can be produced under the same manufacturing conditions.

Next, the present invention is illustrated by way of examples.

(Example 1)

According to the SM 490A standard from the manufacturing conditions shown in Table 2, using the test materials A1 to S1 having the steel composition, hardenability index (DI), Mn / C ratio, and Ar ₃ point (° C) shown in Table 1 It was specimen No.1-19 of intensity | strength and plate thickness of 25 mm. In Table 2, the steel material whose reheating temperature is described means what was manufactured by the manufacturing method of accelerated cooling after reheating after rolling.

From test materials 1-19, the axial force fatigue test piece which has the planar shape and dimension (unit: mm) shown in FIG. 1 was produced by plasma cutting on the conditions shown in Table 3. In addition, axial force fatigue test pieces having the planar shape and dimensions (unit: mm) shown in FIG. 1 were similarly prepared from specimens 1 to 19 by laser cutting under the conditions shown in Table 4.

Each test piece was subjected to a fatigue test by using the 20-ton electrohydraulic fatigue tester shown in FIG. 2 as an axial tension piece tension load control system having a repetition frequency of 5 Hz, a stress ratio of 0.1, and a stress amplitude of 284 to 421 N / mm 2 at room temperature. It was.

In this axial force fatigue test, the stress amplitude at which the number of breaking repetitions was 10 ⁷ was measured as the fatigue limit Δσw. The measurement results are shown in Table 5.

Moreover, the bending test (bending radius 1.0t; where t is plate | board thickness) was performed using the test piece 30 on the plasma cut surface or the laser cut surface to the test piece side surface part shown in FIG. 4, and bending workability was determined. The results are shown in Table 5 together. In the figure, the side portion 32 is a plasma cutting plane or a laser cutting plane.

In addition, the austenite particle diameter of the hardened layer part was measured about the cutting surface which carried out the plasma cutting or the laser cutting of the test materials 1-19. The results are shown in Table 5 in the same manner.

TABLE 1

TABLE 2

TABLE 3

TABLE 4

TABLE 5

Nos. 1 to 15 in Table 5 are all steels according to the present invention, and the fatigue limit (Δσw) is 392 to 457 (N / mm ² ) in plasma cutting and 389 to 430 (N / mm ² ) in laser cutting. It shows high values and also shows good results without cracking in the bending test. At this time, all of the austenite particle diameters on the plasma cut surface or the laser cut surface were 20 µm or less.

On the other hand, Nos. 16 to 19 have a Mn / C ratio of steel less than 20, and the fatigue limit (Δσw) is 297 to 324 (N / mm 2) in plasma cutting and 299 to 311 (N / mm 2) in laser cutting. It deteriorated and the crack generate | occur | produced also in the bending test. In addition, the austenite particle diameters of the plasma cut surface and the laser cut surface were both more than 20 µm.

(Example 2)

Using the test materials A2 to Q2 having the steel composition, hardenability index (DI), Mn / C ratio and Ar ₃ point (° C) shown in Table 6, the production conditions shown in Table 7 (all accelerated cooling after rolling were started) ), Test materials Nos. 20 to 36 having a strength and a sheet thickness of 25 mm according to the SM 490A standard were produced. The weld joint was created from the welding materials by the welding conditions shown in Table 8.

After this weld seam was processed into the test piece shape of the dimension (unit: mm) and shape shown in FIG. 5, the axial fatigue test of the weld seam was performed using the axial force fatigue test machine shown in FIG. Fig. 5A is a side view of the test piece and Fig. 5B is a plan view. In the figure, blackened parts represent welds.

In this axial force fatigue test, the stress amplitude at which the breaking repetition number Nf is 10 ⁷ times was measured as the fatigue limit Δσw. Table 9 shows the measurement results.

TABLE 6

TABLE 7

TABLE 8

TABLE 9

Nos. 20 to 34 in Table 9 are steels according to the present invention, and exhibit a high fatigue limit (Δσw) of the welded joint at 389 to 421 (N / mm 2), and are excellent in cracking even in the bending test. The results were shown. At this time, all of the austenite particle diameters of the hardened layer part in the weld heat affected zone were 20 µm or less.

On the other hand, in No. 35 and 36 in Table 9, since the Mn / C ratio is less than 20 and the hardenability is insufficient, the fatigue limit (Δσw) of the welded joint deteriorates to 264 to 296 (N / mm 2), Cracking also occurred in the bending test. In addition, the austenite particle diameters of the hardened layer part in the welding heat affected zone were all more than 20 micrometers.

Claims (8)

In mass%, C: 0.01% to 0.10%; Si: 0.01 to 0.6%; Mn: 0.4-2.0%; P: 0.02% or less; S: 0.01% or less; Cr: 0.01 to 0.6%; One or two of Nb: 0.005% to 0.06% and Ti: 0.005% to 0.03%; sol.Al: 0.10% or less; N: 0.01% or less; Other unavoidable impurities and residual Fe, A steel material having a steel composition containing, wherein the hardenability index (DI) defined by the following formula is 12 or more, Mn / C ratio which is the ratio of Mn amount with respect to C amount by mass% is 20 or more, Austenitic particle diameter in the hardened layer part of a cut surface or the hardened layer part of a weld heat affected zone is 20 micrometers or less Steel material excellent in the fatigue characteristic characterized by the above-mentioned. Mn: When 1.2% or less DI = 0.311 × √C × (1 + 0.7 × Si) × (1 + 3.33 × Mn) × (1 + 2.16 × Cr) × (1 + 3 × Mo) × 25.4 Mn: When greater than 1.2% DI = 0.311 × √C × (1 + 0.7 × Si) × (5 + 5.1 × (Mn-1.2)) × (1 + 2.16 × Cr) × (1 + 3 × Mo) × 25.4 The method according to claim 1, Steel material whose said steel composition further contains at least 1 sort (s) of element chosen from the following (1)-(3) group by mass%: (1) one or two of Cu: 0.05 to 0.6% and Ni: 0.05 to 0.6%; (2) V: 0.005% to 0.08%; (3) One or two or more of Mo: 0.01% to 0.5%, B: 0.0003% to 0.0030%, and W: 0.05% to 0.50%. The steel piece which has the steel composition of Claim 1 or 2 was heated and rolled to the temperature range of 1200 degrees C or less, and after completion of this rolling in the temperature range of Ar ₃ or more, the average cooling rate between 650-500 degreeC is 5 Accelerated cooling of -50 ° C / s is carried out, and the accelerated cooling is stopped at 450 ° C or lower, wherein the steel is excellent in fatigue characteristics. The steel piece having the chemical composition according to claim 1 or 2 is heated to a temperature range of 1200 ° C. or lower, and rolled. After the rolling is completed in a temperature range of at least ₃ Ar, the sheet is reheated to at least ₁ Ac. Cooling which makes an average cooling rate between 500 degreeC into 5 degrees C / s or more, and stops this cooling at 500 degrees C or less, The manufacturing method of the steel material excellent in the fatigue characteristic. The method according to claim 3, Subsequent to the cooling, the method is further heated to 450 ° C. or lower to perform annealing. The structure consisting of the steel materials of Claim 1 or 2. The structure made of the steel according to claim 1 or 2, which is subjected to at least one of plasma cutting, laser cutting, and high heat input welding. delete

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