JP2001107175A - Steel excellent in fatigue characteristic of plasma-cut part and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that steel usable as a member applied with repeated stress even as plasma-cut is not present. SOLUTION: This steel has a steel composition containing 0.01 to 0.2% C, 0.1 to 0.6% Si, 0.4 to 1.60% Mn, 0.01 to 0.6% Cr, 0.01 to 0.5% Mo, 0.01 to 0.06% Nb and/or 0.005 to 0.03% Ti and moreover has a structure in which the hardenability index DI is >=18, and austenitic grain size in the hardened layer part in the plasma-cut part is <=20 μm. The steel is excellent in the fatigue characteristics in the plasma-cut part and can be used as a member applied with repeated stress even as plasma-cut.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent fatigue characteristics of a plasma cutting portion and a method of manufacturing the same, and more specifically, to a steel material having excellent fatigue characteristics of a plasma cutting portion. The present invention relates to a steel material which can be suitably used as it is in a plasma cut for a bridge, a building, a tank, an automobile and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Steel materials used for ships, marine structures, bridges, buildings, tanks, automobiles, and the like are cut into a desired shape by an appropriate cutting method. As such a cutting method, high-speed, high-speed plasma formed by using the thermal pinch effect of a metal nozzle can be cut several times faster than gas cutting that has been used conventionally. In recent years, plasma cutting using a flow has been frequently used.
Since the plasma cutting has a considerably high cutting accuracy, the plasma-cut steel material may be used as it is.

[0003] However, for example, the "Steel Handbook 3rd Edition"
Cooling rate near the plasma cut surface by plasma cutting, as described in (16.3.4.Material change of cut part) on page 651 in “VI secondary processing / surface treatment / heat treatment / welding”. Becomes large, the tissue near the cut surface is hardened. For this reason, when a steel material that has been plasma-cut is used as a member to which stress is repeatedly applied, fatigue cracks are likely to be generated from the vicinity of the cut surface having increased hardness, and the cracks serve as starting points to reduce the fatigue life.

[0004] For this reason, in many cases, plasma-cut steel materials need to be subjected to machining such as grinding in the vicinity of the plasma-cut surface as a finishing process, resulting in an increase in processing cost and processing time.

As an invention for improving the fatigue life of a steel material, Japanese Patent Application Laid-Open No. Hei 6-271930 discloses that a steel sheet having a composite structure containing bainite and retained austenite having a volume fraction of 10% or more as a main phase is formed. An invention has been proposed in which a shot peening treatment is performed as a treatment to transform a residual austenite phase in a surface layer into a strain-induced transformation, thereby improving the fatigue properties of a steel material. According to the invention according to this proposal, it is possible to improve the fatigue characteristics of the whole steel material including the cut surface of the steel material cut by plasma.

[0006]

However, according to the present invention, it is necessary to perform post-treatment on the steel material, so that the processing cost and the processing time cannot be denied. That is, conventionally, there has been no steel material that can be used as a member to which stress is repeatedly applied even when plasma cutting is performed.

Here, an object of the present invention is to provide a steel material excellent in the fatigue characteristics of a plasma cut portion and a method of manufacturing the same. More specifically, the object of the present invention is to improve the fatigue characteristics of the plasma cutting portion, for example, a repeated stress acts on components such as ships, marine structures, bridges, buildings, tanks, and automobiles. It is an object of the present invention to provide a steel material which can be suitably used as a member to be cut even by plasma cutting and a method for manufacturing the same. Furthermore, an object of the present invention is to reduce both the processing cost and processing time of these steel materials.

[0008]

Means for Solving the Problems The present inventors have intensively studied factors affecting the improvement of the fatigue characteristics of the plasma cut portion, not the entire steel sheet, and as a result, the following new findings are listed.
Having obtained the knowledge, the present invention has been completed.

By reducing the austenite grain size in the hardened layer portion on the plasma cut surface to 20 μm or less and setting the hardenability index Dl to 18 or more, the structure of the hardened layer portion on the plasma cut surface is made to have a fine grain structure. Accordingly, generation of dislocations due to the action of repeated stress can be suppressed, and a long fatigue life of the plasma cut portion can be achieved.

Although the structure of the plasma cutting portion is austenitized by heating due to the heat input of the plasma cutting, the growth of austenite grains can be suppressed by forming a carbonitride by adding an appropriate amount of Nb or Ti, Thereby, the austenite grain size in the hardened layer portion of the plasma cut surface can be reduced to a fine grain structure of 20 μm or less.

In order to increase the hardenability index Dl to 18 or more,
Increasing both the C content and the Mn content is effective and increases the strength of the steel sheet itself, but deteriorates the toughness. Therefore, by appropriately specifying the composition of the steel sheet, it is possible to maintain the properties of both the base material and the plasma cut portion to a desired degree without deteriorating the properties of the base material.

[0012] By using the above items (1) to (4),
Even without performing post-processing such as machining on the cut surface, it is possible to obtain a steel material having high fatigue properties while plasma cutting,
This makes it possible to provide a steel material that can reduce both the processing cost and the processing time.

In the present invention, C: 0.01 to 0.2% (In the present specification, "%" means "% by mass" unless otherwise specified.
Shall mean. ), Si: 0.1-0.6%, Mn: 0.
4 to 1.60%, Cr: 0.01 to 0.6%, Mo: 0.01 to 0.5%, N
b: 0.01 to 0.06% and / or Ti: 0.005 to 0.03%,
Desirably, one or two selected from the group consisting of Cu: 0.05 to 0.6%, Ni: 0.05 to 0.6%, and V: 0.01 to 0.08%.
Or more, having a steel composition containing the following (1)
The hardenability index Dl defined by the formula is 18 or more,
A steel material excellent in fatigue characteristics of a plasma cut portion, characterized by having a structure in which an austenite grain size is 20 μm or less in a hardened layer portion of a plasma cut surface formed by plasma cutting.

D1 = D _IC × D _ISi × D _IMn × D _ICr × D _IMo (1) In the equation (1), the symbol D _IC indicates a basic value of the carbon content, and D _ISi indicates the _{hardenability} index of Si (1 + 0.7 × mass% of Si), and the symbol D _IMn is the _{hardenability} index of Mn [If the Mn content is 1.2% or less, (1 + 0.33 × Mn Mass%), and when the Mn content is more than 1.2%, it is (5 + 5.1 × [mass% of Mn−1.2])], and the sign D _ICr is the _{hardenability} index of Cr (1 + 2.16). × Cr mass%), and the sign D _IMo is Mo
Shows the hardenability index (1 + 3 × Mo% by mass) of the sample.

[0015] From another aspect, the present invention provides a method wherein C: 0.01 to 0.2.
%, Si: 0.1 to 0.6%, Mn: 0.4 to 1.60%, Cr: 0.01 to
0.6%, Mo: 0.01-0.5%, Nb: 0.01-0.06% and / or
Or Ti: 0.005 to 0.03%, desirably Cu: 0.05 to 0.6
%, Ni: 0.05 to 0.6%, and V: 0.01 to 0.08%, and has a steel composition containing at least one selected from the group consisting of: After heating a steel having an index Dl of 18 or more to a temperature range of 1200 ° C. or less, after finishing rolling at a temperature range of Ar ₃ points or more (Ar ₃ points + 50 ° C.) or less, air cooling or 30 ° C./min or less A method for producing a steel material having excellent fatigue characteristics in a plasma cut portion, wherein the steel material is cooled at a cooling rate of.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of a steel material excellent in the fatigue characteristics of a plasma cut portion and a method of manufacturing the same according to the present invention will be described in detail with reference to the accompanying drawings. In the following description, the case where the steel material is a hot-rolled steel sheet is taken as an example, but this is merely an example, and the present invention is similarly applied to steel materials other than the hot-rolled steel sheet.

First, in the manufacturing method according to the present invention,
The reason for limiting the composition of the steel used will be described.

(Steel composition) C: 0.01 to 0.2% C is a component that increases the strength and hardenability index Dl of steel by containing 0.01% or more. However, C
If the content exceeds 0.2%, it becomes difficult to secure necessary strength and toughness. Therefore, in the present invention, the C content is
Limit to 0.01% or more and 0.2% or less. In order to secure the hardenability index Dl and the strength and toughness of the base material,
It is desirable that the lower limit of the content be 0.06% and the upper limit be 0.18%.

Si: 0.1 to 0.6% Si contributes to deoxidation of steel by containing 0.1%.
However, if the Si content exceeds 0.6%, the toughness of the steel is impaired. Therefore, in the present invention, the Si content is 0.1% or more and 0.6% or more.
% Or less. From the same viewpoint, the lower limit of the Si content is
It is desirable that the upper limit is 0.15% and the upper limit is 0.4%.

Mn: 0.4 to 1.60% By containing Mn at 0.4% or more, the strength of steel can be improved and the hardenability index Dl can be secured. However, if the Mn content exceeds 1.60%, 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 1.60% or less. From a similar perspective, Mn
It is desirable that the upper limit of the content be 1.50%.

Cr: 0.01 to 0.6% Cr is a component that improves the hardenability and increases the strength by containing 0.01% or more. However, when the content exceeds 0.6%, a remarkable increase in strength is observed, but the toughness is deteriorated. Therefore, in the present invention, the Cr content is limited to 0.01% or more and 0.6% or less. From a similar perspective,
The lower limit of the Cr content is 0.03% and the upper limit is 0.5%.
Each is desirable.

Mo: 0.01 to 0.5% Mo is a component that increases the hardenability and increases the strength by containing 0.01% or more. However, the Mo content is 0.
If it exceeds 5%, a remarkable increase in strength is observed, but the toughness is deteriorated. Therefore, in the present invention, the Mo content is
Limit to 0.01% or more and 0.5% or less. From a similar perspective, Mo
It is desirable that the lower limit of the content be 0.02% and the upper limit be 0.4%.

Nb: 0.01 to 0.06 % Nb forms carbonitrides by containing 0.01% or more to suppress grain growth of ferrite and austenite,
An austenite particle size of 20 μm or less, which is the target of the present invention, is obtained, which is effective for fine-graining the structure and improving strength and toughness. However, when the Nb content exceeds 0.06%, the strength of the steel significantly increases, and the toughness is impaired. Therefore, in the present invention, the Nb content is limited to 0.01% or more and 0.06% or less. From the same viewpoint, it is preferable that the lower limit of the Nb content is 0.02% and the upper limit is 0.05%.

Ti: 0.005 to 0.03% Ti is an element having the same effect as Nb when contained at 0.005% or more. However, when the Ti content exceeds 0.03%, welding cracks are easily generated. 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, at least one of Nb and Ti described above may be contained.

In the present invention, in addition to these elements, Cu, Ni and V may be contained as optional additives in order to improve quality. Therefore, these optional elements will be described.

Cu: 0.05-0.6% When steel is used in a corrosive environment, the corrosion resistance can be improved by adding 0.05% or more as necessary. However, when 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, the lower limit of the Cu content is
It is desirable that the upper limit is 0.1% and the upper limit is 0.5%.

Ni: 0.05 to 0.6% Ni has an effect of improving corrosion resistance in a corrosive environment when added at 0.05% or more. However, the Ni content is 0.
If it exceeds 6%, these effects are saturated, and the strength of the steel is excessively increased, and the toughness is impaired. Therefore,
When adding Ni, its content should be 0.05% or more and 0.6%
It is desirable to limit to the following. From the same viewpoint, it is desirable that the lower limit of the Ni content is 0.1% and the upper limit is 0.5%.

V: 0.01 to 0.08% V, when added in an amount of 0.01% or more, makes the structure finer and contributes to an increase in the fatigue strength of the steel material. However, V
If the content exceeds 0.08%, such an effect is saturated and the strength is excessively increased, so that the toughness is impaired. Therefore, when V is added, its content is 0.01% or more and 0.1% or more.
It is desirable to limit it to 08% or less. From a similar perspective,
The lower limit of the V content is 0.02% and the upper limit is 0.06%.
Each is desirable.

Hardenability index Dl: 18 or more The hardenability index Dl is a characteristic value defined by the present inventors for indicating hardenability. That is, the hardenability index Dl
Is the limit thickness (50% of the structure at the center of the sample becomes martensite) by quenching various samples (round bars) with different thicknesses under the same conditions and microscopically examining the cross section. (Diameter). The hardenability index Dl is defined by the above equation (1).

When the hardenability index Dl is 18 mm or more, the hardened structure formed by the heat in cutting becomes martensite. For this reason, generation of dislocations due to repeated stress is suppressed, the fatigue life is extended, and the fatigue limit is improved. On the other hand, when the hardenability index Dl is less than 18 mm, the fatigue limit is at the conventional level. Therefore, in the present invention, the hardenability index Dl
Is limited to 18 mm or more.

(Steel Heating) In the present invention, steel having such a steel composition and hardenability index Dl is heated to a temperature range of 1200 ° C. or less. When the steel is heated to a high temperature exceeding 1200 ° C., the austenite grains of the steel become extremely coarse and hinder the toughness of the steel. Therefore, the steel heating temperature is limited to 1200 ° C or less.

(Rolling) For the steel thus heated,
Perform rolling. Rolling is completed in a temperature range of not less than Ar ₃ point (Ar ₃ point + 50 ° C.) or less. In order to improve the toughness of the steel, rolling in the non-recrystallized region is preferable.
This is because the steel structure becomes coarse and the toughness is deteriorated.

(Cooling) After the rolling is completed in this way, air cooling or cooling at a cooling rate of 30 ° C./min or less is performed. In the present invention, desired mechanical properties can be obtained by performing air cooling or cooling at a cooling rate of 30 ° C./min or less after completion of rolling.

Here, examples of the cooling at a cooling rate of 30 ° C./min or less include water cooling and mist cooling.

(Steel according to the present invention) Thus, the steel according to the present invention having the above-described steel composition and hardenability index D1, that is, C: 0.01-0.2%, Si: 0.1-0.6.
%, Mn: 0.4 to 1.60%, Cr: 0.01 to 0.6%, Mo: 0.01 to
0.5%, Nb: 0.01 to 0.06% and / or Ti: 0.005 to
A steel material having a steel composition containing 0.03% and having a hardenability index Dl of 18 or more defined by the equation (1) is obtained.

The steel material according to the present invention has a structure in which the austenite grain size in the hardened layer portion of the plasma cut surface formed by plasma cutting is 20 μm or less. The width of the hardened layer portion of the plasma cut surface formed by plasma cutting varies depending on cutting conditions, but according to the knowledge of the present inventors, the hardenability index Dl is
If it is 18 mm or more, a hardened layer portion is formed over the entire thickness in a range of about 2 mm from the cut surface toward the inside. In the present invention, the austenite particle size in the hardened layer portion is
Limited to 20 μm or less. Hereinafter, the reason for this limitation will be described.

An axial force fatigue test specimen having a plane shape shown in FIG. 1 was prepared by plasma cutting, and at room temperature, an axial force pulsating tensile load control having a repetition frequency of 5 Hz, a stress ratio of 0.1, and a stress amplitude of 284 to 421 N / mm ^2. According to the method, a fatigue test was performed using a 20-ton electrohydraulic fatigue tester shown in FIG. In addition,
In FIG. 2, reference numeral 1 denotes an axial force fatigue test piece, reference numeral 2 denotes a load cell for detecting a load, reference numeral 3 denotes a hydraulic cylinder that applies a load to the axial force fatigue test piece 1, and reference numeral 4 denotes a waveform generator. Reference numeral 5 denotes a load controller, reference numeral 6 denotes a servo valve, and reference numeral 7 denotes a hydraulic pressure source.

[0039] Then, in the axial force fatigue test, a stress amplitude rupture repetition number becomes 10 ⁷ times endurance limit Δσw (N /
mm ² ), and the grain size of the structure observed by corroding the structure of the hardened layer portion of the plasma cut surface of the test specimen after the test with a mixed etchant of picric acid, Lipon F and ferric chloride. The diameter was measured as the austenite particle size. The means for measuring the austenite grain size is not limited to this means, but may be measured by other means.

The results are shown graphically in FIGS. 3 and 4.
Figure 3 shows that the austenite grain size (μm) is the fatigue limit Δσw
(N / mm ² ), and FIG. 4 shows the hardenability index D
The effect of 1 (mm) on the fatigue limit Δσw (N / mm ² ) is shown.

From the graph shown in FIG. 3, it can be seen that when the austenite grain size is 20 μm or less, the fatigue limit Δσw (N / mm ² ) is significantly improved, and when the austenite grain size exceeds 20 μm, the fatigue limit Δσw (N / mm ² ) is at the same level as before. Therefore, in the present invention, the austenite particle size in the cured layer portion of the plasma cut surface formed by plasma cutting is limited to 20 μm or less. By suppressing the austenite particle size to 20 μm or less,
Generation of dislocations due to repeated stress is suppressed, and a long fatigue life is obtained.

From the graph shown in FIG. 4, the hardenability index Dl was
When it is 18 mm or more, the fatigue limit is improved, and when the hardenability index Dl is less than 18 mm, the fatigue limit is not much different from the conventional level. Therefore, in the present invention, the hardenability index Dl is limited to 18 mm or more.

As described above, according to the present invention, a steel material excellent in the fatigue characteristics of a plasma cutting portion and a method of manufacturing the same, more specifically, a ship material, a marine structure, etc. A steel material which can be suitably used as it is in a plasma, such as a bridge, a building, a tank, or a component to which a repeated stress acts among components such as an automobile, and a method of manufacturing the same. Therefore, both the processing cost and the processing time can be reduced.

[0044]

Next, the present invention will be described more specifically with reference to examples.

FIG. 1 shows specimens 1 to 40 having the steel composition and hardenability index Dl shown in Tables 1 and 2, respectively, and having a strength conforming to the SM490A standard and a plate thickness of 25 mm. After cutting the indicated axial force fatigue test piece 1 by plasma cutting under the cutting conditions shown in Table 3, an axial force fatigue test was performed using an axial force fatigue tester shown in FIG.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

[Table 3]

[0049] Then, in the axial force fatigue test, the stress amplitude rupture repetition number Nf of 10 ⁷ times fatigue limit Δσw
Was measured. Further, the structure of the cut surface hardened layer portion of the test material after cutting was corroded by picric acid etching, and the austenite particle size (mm) was measured. The measurement results are shown in Tables 1 and 2. The asterisk in Table 2 indicates that it is outside the scope of the present invention.

Samples No. 1 to No. 24 in Table 1 are all examples of the present invention satisfying the scope of the present invention, and have a high fatigue limit Δσw of 431 to 372 (N / mm ² ). Is shown.

On the other hand, sample No. 25 in Table 2
The heating temperature exceeds the upper limit of the range of the present invention, and the quenchability index Dl is lower than the lower limit of the range of the present invention, and the quenchability is insufficient. Therefore, the fatigue limit Δσw is 304 (N / mm ² )
And deteriorated.

In Sample No. 26, the austenite particle size exceeded the upper limit of the range of the present invention because the amount of Nb was below the lower limit of the range of the present invention. Therefore, the fatigue limit Δσw is 314
(N / mm ² ).

In sample No. 27, since the rolling end temperature was below the lower limit of the range of the present invention and the Ti content was above the upper limit of the range of the present invention, the austenite particle size exceeded the upper limit of the range of the present invention. Was. Therefore, the fatigue limit Δσw is 294
(N / mm ² ).

In Sample No. 28, the austenite particle size exceeded the upper limit of the range of the present invention because the amount of Nb exceeded the upper limit of the range of the present invention. Therefore, the fatigue limit Δσw is 284
(N / mm ² ).

In sample No. 29, the cooling rate was higher than the upper limit of the range of the present invention, and the Ti content was lower than the lower limit of the range of the present invention. Exceeded. Therefore, the fatigue limit Δσw
Was 284 (N / mm ² ).

In sample No. 30, since the Nb content was below the lower limit of the range of the present invention and the Ti content was higher than the upper limit of the range of the present invention, the austenite particle size was lower than the upper limit of the range of the present invention. Exceeded. Therefore, the fatigue limit Δσw is 2
Degraded to 74 (N / mm ² ).

In sample No. 31, the rolling end temperature exceeded the upper limit of the range of the present invention, the Nb content exceeded the upper limit of the range of the present invention, and the Ti content fell below the lower limit of the range of the present invention. Therefore, the austenite particle size exceeded the upper limit of the range of the present invention. Therefore, the fatigue limit Δσw is 245 (N / m
m ² ).

In sample No. 32, since the Nb content exceeded the upper limit of the range of the present invention and the Ti content exceeded the upper limit of the range of the present invention, the austenite particle size was lower than the upper limit of the range of the present invention. Exceeded. Therefore, the fatigue limit Δσw is 2
Degraded to 55 (N / mm ² ).

In Sample No. 33, the austenite particle size exceeded the upper limit of the range of the present invention because the Nb content exceeded the upper limit of the range of the present invention. Therefore, the fatigue limit Δσw is 298
(N / mm ² ).

In Sample No. 34, the austenite particle size exceeded the upper limit of the range of the present invention because the amount of Ti exceeded the upper limit of the range of the present invention. Therefore, the fatigue limit Δσw is 280
(N / mm ² ).

In sample No. 35, the austenite particle size exceeded the upper limit of the range of the present invention because the Nb content was below the lower limit of the range of the present invention. Therefore, the fatigue limit Δσw is 278
(N / mm ² ).

In sample No. 36, the austenite particle size exceeded the upper limit of the range of the present invention because the Ti content was below the lower limit of the range of the present invention. Therefore, the fatigue limit Δσw is 240
(N / mm ² ).

In sample No. 37, the austenite particle size exceeded the upper limit of the range of the present invention because the amount of Nb exceeded the upper limit of the range of the present invention. Therefore, the fatigue limit Δσw is 273
(N / mm ² ).

In sample No. 38, the austenite particle size exceeded the upper limit of the range of the present invention because the amount of Ti exceeded the upper limit of the range of the present invention. Therefore, the fatigue limit Δσw is 298
(N / mm ² ).

In sample No. 39, since the Nb content was higher than the upper limit of the range of the present invention and the Ti content was lower than the lower limit of the range of the present invention, the austenite particle size was lower than the upper limit of the range of the present invention. Exceeded. Therefore, the fatigue limit Δσw is 320 (N
/ mm ² ).

Further, in Sample No. 40, since the Nb content was below the lower limit of the range of the present invention, the austenite particle size exceeded the upper limit of the range of the present invention. Therefore, the fatigue limit Δσ
w deteriorated to 282 (N / mm ² ).

[0067]

As described in detail above, according to the present invention, since the fatigue characteristics of the plasma cutting portion are excellent, for example, among the constituent members of ships, marine structures, bridges, buildings, tanks and automobiles, etc. Thus, it has become possible to provide a steel material that can be suitably used for a member on which stress is repeatedly applied even with plasma cutting, and a method for manufacturing the same. For this reason, both the processing cost and processing time of such a steel material could be reduced.

The significance of the present invention having such an effect is extremely remarkable.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a planar shape of an axial fatigue test piece.

FIG. 2 is an explanatory view showing a 20-ton electrohydraulic fatigue tester.

Fig. 3 Austenitic grain size (μm) is the fatigue limit Δσw
6 is a graph showing the effect on (N / mm ² ).

FIG. 4 shows that the hardenability index Dl (mm) is the fatigue limit Δσw (N / mm ² )
4 is a graph showing the effect on the stiffness.

[Explanation of symbols]

 1 Axial force fatigue test piece 2 Load cell 3 Hydraulic cylinder 4 Waveform generator 5 Load controller 6 Servo valve 7 Hydraulic source

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K032 AA04 AA05 AA11 AA14 AA16 AA19 AA22 AA23 AA31 AA35 AA36 BA01 CA02 CC02 CC03 CD01 CD05

Claims (4)

[Claims] C .: 0.01 to 0.2%, Si: 0.1 to 0.6%, Mn: 0.4 to 1.60% by mass.
%, Cr: 0.01 to 0.6%, Mo: 0.01 to 0.5%, Nb: 0.01 to 0.06%, and / or Ti: 0.005 to 0.03%. Fatigue characteristics of the plasma cut portion, wherein the penetration index Dl is 18 or more, and the structure has an austenite grain size of 20 µm or less in the hardened layer portion of the plasma cut surface formed by plasma cutting. Excellent steel material. Dl = D _IC × D _ISi × D _IMn × D _ICr × D _IMo (1) where D _IC represents a basic value of carbon content and D _DSi
Indicates the _{hardenability} index of Si (1 + 0.7 × mass% of Si), and the symbol D _IMn is the _{hardenability} index of Mn [If the Mn content is 1.2% or less, (1 + 0.33 × mass% of Mn) ) And the Mn content is 1.2
%, It is (5 + 5.1 × [mass% of Mn− _1.2 ])], and the sign D _ICr is the _{hardenability} index of Cr (1 + 2.16 × C
The symbol D _IMo is the hardenability index of Mo.
(1 + 3 × Mo% by mass). 2. The composition according to claim 1, further comprising Cu in an amount of 0.05 to 0.6% by mass.
2. The steel material according to claim 1, which contains one or more selected from the group consisting of Ni: 0.05 to 0.6% and V: 0.01 to 0.08%. 3. In mass%, C: 0.01 to 0.2%, Si: 0.1 to 0.6%, Mn: 0.4 to 1.60.
%, Cr: 0.01 to 0.6%, Mo: 0.01 to 0.5%, Nb: 0.01 to 0.06%, and / or Ti: 0.005 to 0.03%. After heating the steel having the penetration index Dl of 18 or more to a temperature range of 1200 ° C. or less, after finishing rolling at a temperature range of Ar ₃ points or more (Ar ₃ points + 50 ° C.) or less, air cooling or 30 ° C. / A method for producing a steel material having excellent fatigue characteristics in a plasma cut portion, wherein the steel material is cooled at a cooling rate of not more than one minute. Dl = D _IC × D _ISi × D _IMn × D _ICr × D _IMo (1) where D _IC represents a basic value of carbon content and D _DSi
Indicates the _{hardenability} index of Si (1 + 0.7 × mass% of Si), and the symbol D _IMn is the _{hardenability} index of Mn [If the Mn content is 1.2% or less, (1 + 0.33 × mass% of Mn) ) And the Mn content is 1.2
%, It is (5 + 5.1 × [mass% of Mn− _1.2 ])], and the sign D _ICr is the _{hardenability} index of Cr (1 + 2.16 × C
The symbol D _IMo is the hardenability index of Mo.
(1 + 3 × Mo% by mass). 4. The steel further comprises, in mass%, Cu: 0.05
4. The steel material according to claim 3, wherein the steel material contains one or more selected from the group consisting of -0.6%, Ni: 0.05-0.6% and V: 0.01-0.08%. Manufacturing method.