JP2017137576A - Angle steel and production method of angle steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an angle steel having excellent mechanical properties, namely, yield stress (YS):460 MPa or more and tensile strength (TS):570 MPa or more.SOLUTION: The angle steel is provided that has a component composition containing, by mass%, C:0.03 to 0.18%, Si:0.05 to 0.50%, Mn:0.1 to 1.8%, P:0.030% or less, S:0.010% or less, Al:0.005 to 0.07%, N:0.006 to 0.018% and one or both of V:0.01 to 0.12% and Nb:0.001 to 0.05% and the balance Fe with inevitable impurities and having carbon equivalent Ceq defined by the following formula (1), and that has a micro structure including deformed ferrite of 0.36 to 0.44% and has mechanical properties, namely, YS:460 MPa or more, TS:570 MPa or more and Charpy absorption energy at -5°C:47 J or more. Ceq (mass%)=C+Mn/6+(Cu+Ni)/15+(Mo+V)/5... (1).SELECTED DRAWING: None

Description

  The present invention relates to angle irons used for shipbuilding reinforcements, crane garter reinforcements, and the like, and particularly relates to angle steels having a yield stress (YS) of 460 MPa or more. The present invention also relates to a method for producing the angle steel. Here, the “angle steel” includes any of unequal side unequal thickness angle steel, equilateral angle steel, valve plate (spherical flat steel), and channel (grooved steel).

  When building various types of ships such as large ships at a shipyard, a steel plate cut to a predetermined size is welded to build up the outer plate of the hull. The outer plate of the hull is structured to reinforce the outer plate by welding a long material of various shapes such as T-type and L-type as a reinforcing material in the longitudinal direction on the back side. In recent years, in order to increase the efficiency of marine transportation, various ships have been increased in size, and outer plates and reinforcing materials tend to be thicker and larger.

  Reinforcing materials are also used in the garters of cranes that lift and carry heavy objects at docks for harbors and shipbuilding. In recent years, this reinforcing material also tends to become thicker and larger as the span increases and the load weight increases.

  A T-shaped longi material used as a reinforcing material is manufactured by welding a material from a steel plate such as a thick plate. This T-type longe material has a high section modulus, and has various advantages that the dimensions and shapes can be arbitrarily selected and the strength of the flange and web can be freely combined as required. There is a problem that it takes time and cost to form the mold.

  On the other hand, another member used as a reinforcing material is angle steel. Since angle steel is manufactured by hot rolling, it has the advantage that it does not require labor and cost compared to T-shaped steel manufactured by welding, and there are significant advantages such as shortening the construction period and cost reduction for consumers.

  As the angle steel, those of mild steel, strength grades up to YS325, YS360MPa class are mainly used, but in recent years, for use in ships that are becoming increasingly large and cranes installed in the docks that produce them, There is a need for an angle steel that has higher strength and better weldability. Specifically, there is a demand for angle steel having high strength such as yield stress (YS): 460 MPa or more and tensile strength (TS): 570 MPa or more.

  With regard to the manufacturing technology of angle steel, for example, in Patent Document 1, the temperature of the thin wall side (long side) and the thick wall side (short side) in finish rolling is set to a predetermined value in the manufacture of unequal side unequal thick angle steel (NAB). It has been proposed to control to a range.

  In addition, Patent Document 2 proposes that forced cooling is performed during and after rolling so that the temperatures of the thick wall portion and the thin wall portion are equal in the manufacture of unequal side unequal thick angle steel.

  Patent Document 3 proposes an angle steel characterized in that the strength of the flange (short side) portion is higher than the strength of the web (long side) portion. In Patent Document 3, in order to manufacture the angle steel, the flange portion is controlled and cooled on the front and rear surfaces of the intermediate rolling mill arranged in the front stage of the finish rolling mill while restraining the rolled material with a side guide for each pass. Has been shown to do.

JP-A-53-040670 JP-A 53-055458 JP 2007-216251 A

  However, the methods described in Patent Documents 1 and 2 are for preventing large bending and web waves caused by temperature non-uniformity, and aimed to improve the strength of the angle steel. It is not a thing. Further, in the angle steel described in Patent Document 3, the strength of the flange portion and the web portion is controlled, but the strength is still not sufficient.

  In angle irons, in particular, inhomogeneous unequal thickness angle steels, the cross-sectional shape is complicated, and therefore it is difficult to employ a controlled rolling / accelerated cooling process (TMCP) similar to that for thick steel sheets. In particular, because it is necessary to build the material while considering bending and warping during rolling, in order to obtain a high-strength section steel with a yield stress YS of 460 MPa or more, an original manufacturing method for angle steel is studied. There is a need to. In addition, sufficient consideration must be given to weldability.

  This invention is made | formed in view of the said actual condition, and provides stably the angle iron which has the outstanding mechanical characteristics of yield stress (YS): 460 MPa or more and tensile strength (TS): 570 MPa or more. With the goal. In particular, the center of the bulb plate sphere has a large cross-section, and the center is difficult to obtain rolling and cooling effects by thermomechanical treatment. Even in these areas, excellent mechanical properties (high strength and high toughness) are obtained. The purpose is to satisfy.

  As a result of intensive studies to solve the above problems, the inventors have controlled the component composition and manufacturing conditions to a specific range, and optimized the microstructure to obtain an angle steel having the above mechanical characteristics. It was found that it can be manufactured.

This invention is made | formed based on the said knowledge, The summary structure is as follows.
1. % By mass
C: 0.03-0.18%,
Si: 0.05 to 0.50%,
Mn: 0.1 to 1.8%,
P: 0.030% or less,
S: 0.010% or less,
Al: 0.005 to 0.07%,
N: 0.006 to 0.018%, and V: 0.01 to 0.12% and Nb: 0.001 to 0.05% or both of which contain Fe and inevitable impurities, The carbon equivalent Ceq defined by the following formula (1) has a component composition of 0.36 to 0.44%,
Has a microstructure containing processed ferrite,
Angle steel with mechanical properties of YS: 460 MPa or more, TS: 570 MPa or more, Charpy absorbed energy at −5 ° C .: 47 J or more.
Ceq (mass%) = C + Mn / 6 + (Cu + Ni) / 15 + (Mo + V) / 5 (1)

2. The component composition is mass%,
Cu: 0.05 to 0.50%,
The angle steel according to the above 1, further comprising at least one selected from the group consisting of Ni: 0.05 to 0.25% and Mo: 0.01 to 0.50%.

3. The component composition is mass%,
The angle steel according to 1 or 2, further containing one or both of Ti: 0.001 to 0.1% and Zr: 0.001 to 0.1%.

4). The component composition is mass%,
B: Angle iron as described in any one of said 1-3 which further contains 0.0002 to 0.003%.

5. The component composition is mass%,
The angle steel according to any one of the above 1 to 4, further containing one or both of Ca: 0.0002 to 0.01% and REM: 0.0002 to 0.015%.

6). A steel material having the component composition according to any one of 1 to 5 above is prepared, heated to 1150 to 1350 ° C., and then hot rolled to produce an angle steel,
The hot rolling is performed under the conditions of the cumulative rolling reduction at Ar3 temperature or lower: 20 to 80%, the finishing temperature: (Ar3-50) to (Ar3-120) ° C.
A method for producing angle iron, in which, after the hot rolling, accelerated cooling is performed under conditions of a cooling start temperature: (Ar3-50) to (Ar3-120) ° C. and a cooling stop temperature: 650 to 500 ° C.

  According to the present invention, while being cheaper than a T-shaped steel manufactured by welding thick steel plates and a T-shaped steel manufactured by cutting a shaped steel web such as an H-shaped steel, the strength, An angle steel excellent in toughness and weldability can be provided.

It is sectional drawing which shows the cross-sectional shape of a valve plate. It is a figure which shows the test piece collection position from a valve plate.

[Ingredient composition]
Next, a method for carrying out the present invention will be specifically described. In this invention, it is important that the steel raw material used when manufacturing angle iron and angle steel has the said component composition. First, the reason why the component composition is limited as described above in the present invention will be described. In the following description, “%” used as a unit of content of each component means “% by mass” unless otherwise specified.

C: 0.03-0.18%
C is an element effective for increasing the strength of steel. In the present invention, the C content needs to be 0.03% or more in order to obtain a desired strength. On the other hand, the addition of C exceeding 0.18% lowers the weldability and the toughness of the weld heat affected zone (HAZ). Therefore, the C content is 0.03 to 0.18%. In addition, it is preferable to make C content 0.05 to 0.15% from a viewpoint of making intensity | strength and toughness compatible by the processed ferrite mentioned later.

Si: 0.05 to 0.50%
Si is an element added as a deoxidizer and to increase the strength of steel. In the present invention, the Si content is 0.05% or more. On the other hand, the addition of Si exceeding 0.50% lowers the HAZ toughness, so the Si content is 0.50% or less.

Mn: 0.1 to 1.8%
Mn is an element that has an effect of increasing the strength of steel, and is added in an amount of 0.1% or more. However, addition of Mn exceeding 1.8% decreases the toughness and weldability of the steel, so the Mn content is set to 1.8% or less. The Mn content is preferably 0.5 to 1.6%.

P: 0.030% or less P is a harmful element that lowers the base metal toughness, weldability, and weld zone toughness of steel, so it is desirable to reduce it as much as possible. In particular, when the P content exceeds 0.030%, the deterioration of the base metal toughness and weld zone toughness becomes large. Therefore, the P content is 0.030% or less. The P content is preferably 0.020% or less.

S: 0.010% or less Since S is a harmful element that lowers the toughness and weldability of steel, it is desirable to reduce it as much as possible. In the present invention, the S content is 0.010% or less.

Al: 0.005 to 0.07%
Al is an element added as a deoxidizer, and it is necessary to add 0.005% or more. However, if added over 0.07%, coarse oxide inclusions are present in the steel, so that the toughness is reduced. Therefore, the Al content is set to 0.07% or less.

N: 0.006 to 0.018%
N is an element having an effect of improving the strength by forming V (C, N) or Nb (C, N) by combining with V or Nb described later. In this invention, in order to acquire the said effect, N content shall be 0.006% or more. On the other hand, free N is harmful to toughness and promotes surface cracking during continuous casting. Therefore, the N content is set to 0.018% or less. The N content is preferably 0.008 to 0.015%.

One or both of V: 0.01 to 0.12% and Nb: 0.001 to 0.05% V and Nb combine with carbon and nitrogen to form V (C, N) or Nb (C, N) Is an element that contributes to higher strength. In order to expect the effect, V needs to be 0.01% or more and Nb needs to be 0.001% or more. On the other hand, if the V content exceeds 0.12%, the HAZ toughness decreases, so the V content is set to 0.12% or less. Similarly, if Nb is contained in excess of 0.05%, the HAZ toughness decreases, so the Nb content is set to 0.05% or less. When V and Nb are added in combination, the total amount is preferably 0.10% or less.

  The component composition of steel in one embodiment of the present invention is composed of the above components, the remainder Fe and inevitable impurities. In addition, “consisting of remaining Fe and inevitable impurities” means that the elements containing other trace elements including inevitable impurities are included in the scope of the present invention as long as the effects and effects of the present invention are not impaired. To do.

  In another embodiment of the present invention, the component composition further comprises Cu: 0.05 to 0.50%, Ni: 0.05 to 0.25%, and Mo: 0.01 to 0.50%. At least one selected from the group consisting of:

Cu: 0.05 to 0.50%
Cu is an element having an effect of increasing the strength of steel by solid solution in iron, but addition of less than 0.05% is insufficient. On the other hand, if added over 0.50%, toughness decreases. Therefore, when adding Cu, the content is made 0.05 to 0.50%.

Ni: 0.05-0.25%
Ni is an element having an effect of improving low-temperature toughness, and can be added when the effect is expected, but if it is less than 0.05%, the effect is small. On the other hand, Ni is a very expensive element, and addition exceeding 0.25% causes an increase in cost. Therefore, when adding Ni, the content is made 0.05 to 0.25%.

Mo: 0.01 to 0.50%
Mo is an element having an effect of improving hardenability and high temperature strength, and in order to obtain the effect, addition of 0.01% or more is necessary. On the other hand, Mo is an expensive element like Ni, and addition over 0.50% causes an increase in cost and lowers weldability. Therefore, when adding Mo, the content is made 0.01 to 0.50%.

  In another embodiment of the present invention, one or both of Ti: 0.001 to 0.1% and Zr: 0.001 to 0.1% is used for the purpose of further improving the strength and toughness. Can be contained.

Ti: 0.001 to 0.1%
Zr: 0.001 to 0.1%
Ti and Zr are both elements that improve the HAZ toughness of the steel, and one or both of them can be optionally added. In order to obtain the effect of improving the HAZ toughness, Ti needs to be added by 0.001% or more. On the other hand, if Ti is added in excess of 0.1%, TiC is excessively formed and embrittles on the contrary, so the Ti content is set to 0.1% or less. Similarly, Zr needs to be added in an amount of 0.001% or more in order to obtain the effect of improving the HAZ toughness. On the other hand, the addition of Zr exceeding 0.1% rather reduces the toughness, so the Zr content is 0.1% or less.

  In another embodiment of the present invention, the component composition may further contain B: 0.0002 to 0.003%.

B: 0.0002 to 0.003%
B is an element having an effect of increasing the strength of steel, and can be arbitrarily contained. In order to acquire the said effect, 0.0002% or more needs to be added. On the other hand, if added over 0.003%, the toughness is rather lowered. Therefore, when it contains B, the content shall be 0.0002 to 0.003%.

  In another embodiment of the present invention, the component composition may further contain one or both of Ca: 0.0002 to 0.01% and REM: 0.0002 to 0.015%.

Ca: 0.0002 to 0.01%
REM: 0.0002 to 0.015%
Ca and REM (rare earth metal) are both elements that are effective in improving the toughness of the weld heat affected zone, and one or both of them can be arbitrarily added. The said effect is acquired by addition of Ca: 0.0002% or more and REM: 0.0002% or more. On the other hand, when Ca is added in excess of 0.01% and REM: 0.015%, the toughness is reduced. Therefore, when adding Ca, the content shall be 0.0002 to 0.01%, and when adding REM, the content. 0.0002 to 0.015%.

[Carbon equivalent Ceq]
Next, the reason for limiting the carbon equivalent will be described. In the present invention, it is important that the component composition further satisfies the condition of Ceq: 0.36 to 0.44%. Here, Ceq is a carbon equivalent defined by the following formula (1). It should be noted that the element symbols “C”, “Mn”, “Cu”, “Ni”, “Mo”, and “V” in the formula (1) are all contained in units of “mass%”. This means an amount, and is “0” when the element is not added.
Ceq (mass%) = C + Mn / 6 + (Cu + Ni) / 15 + (Mo + V) / 5 (1)

  When Ceq is less than 0.36%, it is not possible to obtain sufficient strength as an angle steel. On the other hand, if Ceq exceeds 0.44%, there is a concern about cold cracking during welding, so preheating is required, leading to a decrease in welding workability. Therefore, Ceq is set in the range of 0.36 to 0.44%. Preferably, it is 0.36 to 0.43% of range.

[Microstructure]
Next, the reason for limiting the microstructure in the present invention will be described. In the present invention, it is important that the microstructure of the angle steel includes processed ferrite. When the microstructure does not contain processed ferrite, it is difficult to obtain sufficient strength and low temperature toughness. The processed ferrite is a flattened ferrite formed by hot rolling in the two-phase region (austenite + ferrite) below the Ar3 transformation point. In the present invention, a ferrite having a ratio of the major axis to the minor axis (aspect ratio) of 2.0 or more is defined as processed ferrite.

  The area fraction of the processed ferrite is not particularly limited, but is preferably 15% or more, and more preferably 20% or more from the viewpoint of improving the mechanical strength. In addition, in the case of an unequal angle iron, the microstructure is a ½ width part of the short side (flange) and a thickness of 1/4 t, and in the case of an equilateral angle iron, a ½ width part of each side and a thickness. The ¼t position is the observation position in the case of a valve plate, and the center of the thickness of the sphere part ½ width part.

  In the present invention, the microstructure is allowed to contain a structure other than the processed ferrite. The remaining structure other than the processed ferrite is not particularly limited and may be an arbitrary structure. For example, the remaining structure may be one or more selected from the group consisting of pearlite and bainite that are not processed ferrite. In particular, from the viewpoint of further improving the strength, the second phase structure is preferably bainite.

  Next, the manufacturing method of the angle iron in one Embodiment of this invention is demonstrated. The angle steel of the present invention can be manufactured by preparing a steel material having the above composition and heating it to 1150 to 1350 ° C., followed by hot rolling. And in that case, the said hot rolling is performed on the conditions of the cumulative reduction rate in Ar3 temperature or less: 20-80%, finishing temperature: (Ar3-50) degreeC-(Ar3-120) degreeC, The said hot rolling Later, it is important to perform accelerated cooling under the conditions of the cooling start temperature: (Ar3-50) to (Ar3-120) ° C. and the cooling stop temperature: 650 to 500 ° C. Hereinafter, the reasons for limiting the conditions in each step will be described.

[heating]
Heating temperature: 1150-1350 ° C
In order to reduce the deformation resistance in hot rolling of a steel material, to homogenize the solidified structure, and to once dissolve Nb and V precipitates, it is necessary to heat to a sufficiently high temperature. When the heating temperature is less than 1150 ° C., these sufficient effects are not recognized. Therefore, the heating temperature before hot rolling is 1150 ° C. or higher. On the other hand, when the heating temperature exceeds 1350 ° C., problems such as a decrease in yield due to scale loss and a decrease in productivity occur. Therefore, heating temperature shall be 1350 degrees C or less. In addition, it is preferable that heating temperature shall be 1180-1320 degreeC.

[Hot rolling]
Cumulative rolling reduction below Ar3 temperature: 20-80%
If the cumulative rolling reduction at an Ar3 temperature or lower is less than 20%, the ferrite is not sufficiently strengthened. On the other hand, when the cumulative rolling reduction at the Ar3 temperature or lower exceeds 80%, the anisotropy in the rolling direction and the width direction becomes strong, and the Charpy absorbed energy decreases. In addition, the deformation resistance becomes high and modeling becomes difficult. Therefore, the cumulative rolling reduction at the Ar3 temperature or lower during hot rolling is set to 20 to 80%. In addition, the value of Ar3 point can be calculated | required by the following formula.
Ar3 (° C.) = 910-273C + 25Si-74Mn-56Ni-9Mo-5Cu
However, the element symbol in the above formula represents the content (% by mass) of each element, and is zero when the element is not contained.

Finishing temperature: (Ar3-50) to (Ar3-120) ° C
When the finishing temperature exceeds (Ar3-50) ° C., the amount of processed ferrite decreases, and the strength decreases. On the other hand, if the finishing temperature is less than (Ar3-120) ° C., the rolling load is high and the formability is lowered. Therefore, rolling finishing temperature shall be (Ar3-50) degreeC or more and (Ar3-120) degreeC or less.

[Accelerated cooling]
Cooling start temperature: (Ar3-50) to (Ar3-120) ° C
After hot rolling is completed, accelerated cooling is performed. The cooling start temperature in that case shall be (Ar3-50)-(Ar3-120) ° C. By setting the cooling start temperature within the above range, the strength can be improved.

  The accelerated cooling method is not particularly limited, but water cooling by spraying or the like is preferable. Further, from the viewpoint of further improving the strength, it is preferable to start accelerated cooling immediately after the end of hot rolling, and specifically, it is preferable to start within 5 seconds after the end of hot rolling. Immediately after the end of rolling, accelerated cooling can transform the second phase structure to bainite and further improve the strength. When time is required from the end of hot rolling to the start of accelerated cooling, pearlite transformation occurs from some austenite and the strength decreases.

Cooling stop temperature: 650-500 ° C
If cooling is stopped at a temperature higher than 650 ° C., sufficient strength cannot be obtained. On the other hand, when cooled to below 500 ° C., the residual stress generated during cooling causes twisting, bending, warping, etc., and the shape cannot be maintained. Therefore, cooling stop temperature shall be 650-500 degreeC. The cooling stop temperature is preferably 650 ° C to 550 ° C.

<Example 1>
Steel having the component composition shown in Table 1 is melted in a vacuum melting furnace or converter to form a bloom. After the bloom is charged into a heating furnace and heated, it is subjected to hot rolling and accelerated cooling. An unequal unequal thick angle steel with the indicated cross-sectional dimensions was produced. Conditions for heating before hot rolling, hot rolling, and accelerated cooling after hot rolling were as shown in Table 2. In addition, as rolling temperature, the radiation thermometer was installed in the entrance side of the mill, and the exit side, and the temperature of the short side was measured. The finishing temperature in Table 2 is the entry side temperature at the time of final rolling. Then, about each of the obtained angle steel, the microstructure and the mechanical characteristic in a flange part and a weld bond part were evaluated. Table 3 shows the measurement results. The evaluation methods were as follows.

(Micro structure)
The microstructure at the short side (flange) 1/2 width part and the plate thickness 1 / 4t position of the unequal side unequal thickness angle steel was observed with an optical microscope. While observing at least three fields of view at a magnification of 500, the ferrite portion was traced as a metal structure. Thereafter, the minor axis, major axis, and aspect ratio of the ferrite grains were determined by image analysis software. A ferrite having an aspect ratio of 2.0 or more was defined as a processed ferrite, the area of each processed ferrite was calculated, and the area fraction of the processed ferrite in the metal structure was calculated.

(Mechanical properties)
From a short side of the angle steel, a JIS No. 1A tensile test piece was collected and measured for tensile properties (yield stress YS, tensile strength TS, elongation El). As for toughness, 2 mmV notch Charpy impact test pieces described in JIS Z 2242 were similarly collected from the short side, and the toughness at −5 ° C. was evaluated. Further, after multi-layer welding with a heat input of 2 kJ / mm, the toughness at −5 ° C. of the weld bond portion was also evaluated.

  As can be seen from Tables 1 to 3, the angle steel of the inventive example that satisfies the conditions of the present invention has excellent strength such as YS: 460 MPa or more, TS: 570 MPa or more, and is excellent in toughness in the base material and the welded portion. It was. On the other hand, in the comparative example which does not satisfy the conditions of the present invention, any of YS, TS, and toughness was inferior. In addition, No. In No. 21, the torsional warping and bending after cooling were excessive and could not be molded normally, so the mechanical properties were not evaluated. No. In No. 22, the rolling load was excessive and the hot rolling could not be performed normally, so the rolling was stopped halfway.

<Example 2>
A steel having the composition shown in Table 4 is melted in a vacuum melting furnace or converter to form a bloom. The bloom is charged into a heating furnace and heated, and then subjected to hot rolling and accelerated cooling. And the valve plate of the cross-sectional shape and dimension shown in Table 5 was manufactured. Conditions for heating before hot rolling, hot rolling, and accelerated cooling after hot rolling were as shown in Table 5. In addition, as rolling temperature, the radiation thermometer was installed in the entrance side of the mill, and the exit side, and the temperature of the long side (web) was measured. The finishing temperature in Table 5 is the entry temperature at the time of final rolling. Thereafter, for each of the obtained valve plates, the microstructure and mechanical properties and the Charpy impact properties of the weld bond were evaluated. Table 6 shows the measurement results. The evaluation methods were as follows.

(Micro structure)
2, was observed microstructure in the sheet thickness center position 1 of the sphere part 1/2 width portion of the valve plate (center position of the sheet thickness t ₂₎ with an optical microscope. While observing at least three fields of view at a magnification of 500, the ferrite portion was traced as a metal structure. Thereafter, the minor axis, major axis, and aspect ratio of the ferrite grains were determined by image analysis software. A ferrite having an aspect ratio of 2.0 or more was defined as a processed ferrite, the area of each processed ferrite was calculated, and the area fraction of the processed ferrite in the metal structure was calculated.

  Further, as shown in FIG. 2, a JIS No. 1A tensile test piece was taken from the web plate 1/3 width portion 2 so that the tensile direction was parallel to the length direction (rolling direction) of the valve plate, Tensile properties (yield stress YS, tensile strength TS, elongation El) were measured. Furthermore, as shown in FIG. 2, the micro tensile test piece (parallel part diameter) of a round bar whose tensile direction is the length direction (rolling direction) of the valve plate from the plate thickness center position 1 of the sphere part 1/2 width part. 6 mmφ, GL25 mm) was sampled, and tensile properties (yield stress YS, tensile strength TS, elongation El) were measured. As for toughness, 2 mm V notch Charpy impact test pieces described in JIS Z 2242 were sampled from the plate thickness center positions 1 of the web 1/3 width portion 2 and the sphere width 1/2 width portion, respectively, and the toughness at −5 ° C. Evaluated. Further, after multi-layer welding with a heat input of 2 kJ / mm, the toughness at −5 ° C. of the weld bond portion was also evaluated.

  As can be seen from Tables 4 to 6, the valve plate of the invention example that satisfies the conditions of the present invention has high strength at the center of a sphere having a large cross-sectional area, such as YS: 460 MPa or more and TS: 570 MPa or more, and welding. Excellent toughness including the part. On the other hand, in the comparative example which does not satisfy the conditions of the present invention, any of YS, TS, and toughness was inferior. In addition, even when the web part strength was sufficiently satisfied, data on the strength of the sphere part was low or the toughness was low. These represent that the control rolling effect is lower in the ball portion than in the web.

  According to the present invention, by optimizing the component composition and manufacturing conditions, angle steel with high strength and high toughness, YS: 460 MPa or more, TS: 570 MPa or more, Charpy absorbed energy at −5 ° C .: 47 J or more Can be manufactured at low cost with good productivity.

1 Thickness center position of sphere part 1/2 width part 2 Web 1/3 width part

Claims (6)

% By mass
C: 0.03-0.18%,
Si: 0.05 to 0.50%,
Mn: 0.1 to 1.8%,
P: 0.030% or less,
S: 0.010% or less,
Al: 0.005 to 0.07%,
N: 0.006 to 0.018%, and V: 0.01 to 0.12% and Nb: 0.001 to 0.05% or both of which contain Fe and inevitable impurities, The carbon equivalent Ceq defined by the following formula (1) has a component composition of 0.36 to 0.44%,
Has a microstructure containing processed ferrite,
Angle steel with mechanical properties of YS: 460 MPa or more, TS: 570 MPa or more, Charpy absorbed energy at −5 ° C .: 47 J or more.
Ceq (mass%) = C + Mn / 6 + (Cu + Ni) / 15 + (Mo + V) / 5 (1) The component composition is mass%,
Cu: 0.05 to 0.50%,
The angle steel according to claim 1, further comprising at least one selected from the group consisting of Ni: 0.05 to 0.25% and Mo: 0.01 to 0.50%. The component composition is mass%,
The angle steel according to claim 1 or 2, further comprising one or both of Ti: 0.001 to 0.1% and Zr: 0.001 to 0.1%. The component composition is mass%,
B: Angle iron as described in any one of Claims 1-3 which further contains 0.0002 to 0.003%. The component composition is mass%,
The angle steel according to any one of claims 1 to 4, further containing one or both of Ca: 0.0002 to 0.01% and REM: 0.0002 to 0.015%. A steel material having the composition according to any one of claims 1 to 5 is prepared, heated to 1150 to 1350 ° C, and then hot rolled to produce an angle steel,
The hot rolling is performed under the conditions of the cumulative rolling reduction at Ar3 temperature or lower: 20 to 80%, the finishing temperature: (Ar3-50) to (Ar3-120) ° C.
A method for producing angle iron, in which, after the hot rolling, accelerated cooling is performed under conditions of a cooling start temperature: (Ar3-50) to (Ar3-120) ° C. and a cooling stop temperature: 650 to 500 ° C.

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