JP6725020B2 - Valve plate and method for manufacturing valve plate - Google Patents

Description

The present invention relates to an angle steel used as a reinforcing material for shipbuilding, a reinforcing material for a crane garter, and the like, and particularly to an angle steel having a yield stress (YS) of 460 MPa or more. The present invention also relates to a method for manufacturing the angle steel. Here, the “angle iron” includes any of unequal side unequal thick angle angle steel, equilateral angle angle steel, valve plate (spherical flat steel), and channel (channel steel).

When building various ships such as large ships in a shipyard, thick steel plates cut into predetermined dimensions are welded to build up the outer plate of the hull. The outer plate of the hull has a structure in which longitudinal members of various shapes such as T-shaped and L-shaped are welded as a reinforcing member in the longitudinal direction on the back side to reinforce the outer plate. In recent years, in order to increase the efficiency of marine transportation, various types of vessels have been increasing in size, and the outer panels and reinforcing materials tend to be thicker and larger.

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

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

On the other hand, angle steel is another member used as a reinforcing material. Since the angle steel is manufactured by hot rolling, it has an advantage that it is less labor-intensive and costly than the T-shape steel manufactured by welding, and has a great merit such as a shortened construction period and cost reduction for the consumer.

As angle steel, mild steel, strength grades up to YS325, YS360MPa class are mainly used, but in recent years, in order to use it for cranes installed in ships that are becoming increasingly huge and docks that manufacture it, Angle steel having higher strength and excellent weldability is required. Specifically, angle steel having high strength such as yield stress (YS): 460 MPa or more and tensile strength (TS): 570 MPa or more is required.

Regarding the manufacturing technology of angle steel, for example, in Patent Document 1, in the manufacturing of unequal thickness non-thick angle steel (NAB), the temperature of the thin side (long side) and the thick side (short side) in finish rolling is set to a predetermined value. It is proposed to control the range.

Further, in Patent Document 2, it is proposed to perform forced cooling during and after rolling so that the temperatures of the thick wall portion and the thin wall portion become equal in the production of the unequal thickness unequal thick angle section steel.

Patent Document 3 proposes an angle steel in which the strength of the flange (short side) is higher than the strength of the web (long side). In Patent Document 3, in order to manufacture the angle steel, the flange portion is controlled and cooled while restraining the rolled material for each pass by the side guides on the front and rear surfaces of the intermediate rolling mill arranged in the preceding stage of the finishing rolling mill. 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 due to non-uniformity of temperature, and aimed at improving the strength of the angle steel. Not a thing. Moreover, in the angle steel described in Patent Document 3, although the strength of the flange portion and the web portion is controlled, the strength is still insufficient.

It is difficult to adopt the same controlled rolling/accelerated cooling process (TMCP) as that of thick steel plates, because the cross-sectional shape of the angle steel, especially the unequal-thickness non-equal thick angle steel is complicated. In particular, since it is necessary to build the material in consideration of bending and warpage during rolling, in order to obtain a high-strength shaped steel with a yield stress YS of 460 MPa or more, we examined the original manufacturing method of angle steel. There is a need to. It is also necessary to give due consideration to weldability.

The present invention has been made in view of the above circumstances, and stably provides an angle steel having excellent mechanical properties such as 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 part of the valve plate has a large cross section, and the center part is a part where rolling and cooling effects due to thermomechanical treatment are difficult to obtain, and even in such parts excellent mechanical properties (high strength and high toughness) are achieved. The purpose is to meet.

As a result of intensive studies to solve the above problems, the inventors controlled the component composition and manufacturing conditions within a specific range, and optimized the microstructure to obtain a chevron steel having the above mechanical properties. We have found that it can be manufactured.

The present invention has been made on the basis of the above findings, and its gist is as follows.

1. In 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: 0.005-0.07%,
N: 0.006 to 0.018%, and one or both of V: 0.01 to 0.12% and Nb: 0.001 to 0.05%, the balance consisting of 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 20% or more of worked ferrite in area fraction,
A valve plate having mechanical characteristics of YS: 460 MPa or more, TS: 570 MPa or more, and Charpy absorbed energy at -5°C: 47 J or more.
Note Ceq (mass %)=C+Mn/6+(Cu+Ni)/15+(Mo+V)/5 (1)

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

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

4. The above component composition is mass%,
B: The valve plate according to any one of 1 to 3 above, further containing 0.0002 to 0.003%.

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

6. A method for producing a valve plate by preparing a steel material having the component composition according to any one of 1 to 5 above, heating it to 1150 to 1350° C., and then hot rolling it.
The hot rolling is performed under the conditions of a cumulative rolling reduction at an Ar3 temperature or lower: 20 to 80% and a finishing temperature: (Ar3-50) to (Ar3-120)°C.
After the hot rolling, accelerated cooling is performed under the conditions of a cooling start temperature: (Ar3-50) to (Ar3-120)°C and a cooling stop temperature: 650 to 500°C.
The valve plate is
Has a microstructure containing 20% or more of worked ferrite in area fraction,
A method for producing a valve plate having mechanical characteristics of YS: 460 MPa or more, TS: 570 MPa or more, and Charpy absorbed energy at -5°C: 47 J or more.

Further, the gist configuration of another embodiment of the present invention is as follows.

1. In 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: 0.005-0.07%,
N: 0.006 to 0.018%, and one or both of V: 0.01 to 0.12% and Nb: 0.001 to 0.05%, the balance consisting of 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,
An angle steel having mechanical properties of YS: 460 MPa or more, TS: 570 MPa or more, and Charpy absorbed energy at -5°C: 47 J or more.
Note Ceq (mass %)=C+Mn/6+(Cu+Ni)/15+(Mo+V)/5 (1)

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

3. The above component composition is mass%,
Angle iron 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 above component composition is mass%,
B: The angle steel according to any one of 1 to 3, further containing 0.0002 to 0.003%.

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

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

ADVANTAGE OF THE INVENTION According to this invention, although it is cheap as compared with the T-section steel manufactured by welding a thick steel plate, and the T-section steel manufactured by cutting the web of shaped steels, such as H-section steel, although strength is low, It is possible to provide an angle steel having excellent toughness and weldability.

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 the present invention, it is important that the angle steel and the steel material used for producing the angle steel have the above-described composition. Therefore, 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 the content of each component means "mass%" unless otherwise specified.

C: 0.03 to 0.18%
C is an element effective in increasing the strength of steel, and in the present invention, the C content needs to be 0.03% or more in order to obtain the desired strength. On the other hand, the addition of C exceeding 0.18% reduces 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 that the C content be 0.05 to 0.15% from the viewpoint of achieving both strength and toughness by the processed ferrite described later.

Si: 0.05 to 0.50%
Si is an element added as a deoxidizing agent and for increasing the strength of steel. In the present invention, the Si content is 0.05% or more. On the other hand, addition of Si in excess of 0.50% lowers the HAZ toughness, so the Si content should be 0.50% or less.

Mn: 0.1 to 1.8%
Mn is an element that has the effect of increasing the strength of steel, and is added in an amount of 0.1% or more. However, the addition of Mn in excess of 1.8% deteriorates the toughness and weldability of the steel, so the Mn content is made 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 base material toughness and the weld zone toughness decrease significantly. Therefore, the P content is 0.030% or less. The P content is preferably 0.020% or less.

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

Al: 0.005-0.07%
Al is an element added as a deoxidizer, and it is necessary to add 0.005% or more. However, if added in excess of 0.07%, coarse oxide-based inclusions will be present in the steel, and the toughness will rather deteriorate. Therefore, the Al content is 0.07% or less.

N: 0.006 to 0.018%
N is an element having an effect of improving strength by combining with V and Nb described later to form V(C,N) and Nb(C,N). In the present invention, in order to obtain the above effect, the N content is set to 0.006% or more. On the other hand, free N is harmful to the toughness and promotes surface cracking during continuous casting. Therefore, the N content is 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 or nitrogen to form V(C,N) or Nb(C,N). And is an element that contributes to the enhancement of strength. In order to expect the above effect, V is required to be 0.01% or more and Nb is 0.001% or more. On the other hand, if the content of V exceeds 0.12%, the HAZ toughness decreases, so the content of V is set to 0.12% or less. Similarly, if Nb is also contained in an amount of more than 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, and the balance Fe and unavoidable impurities. The term "comprising the balance Fe and unavoidable impurities" means that those containing other trace elements including unavoidable impurities are included in the scope of the present invention as long as the action and effect of the present invention are not impaired. To do.

In another embodiment of the present invention, the above 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 the effect of forming a solid solution in iron to enhance the strength of steel, but addition of less than 0.05% is insufficient. On the other hand, if added in excess of 0.50%, toughness decreases. Therefore, when Cu is added, its content is set to 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 above effect is expected, but if it is less than 0.05%, the effect is small. On the other hand, Ni is an extremely expensive element, and the addition of Ni over 0.25% causes a cost increase. Therefore, when Ni is added, its content is set to 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 it is necessary to add 0.01% or more to obtain the effect. On the other hand, Mo is an expensive element like Ni, and addition of more than 0.50% causes an increase in cost and reduces weldability. Therefore, when Mo is added, its content is set to 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 contained in the above component composition for the purpose of further improving strength and toughness. Can be included.

Ti: 0.001-0.1%
Zr: 0.001-0.1%
Ti and Zr are both elements that improve the HAZ toughness of steel, and one or both of them can be added arbitrarily. To obtain the effect of improving the HAZ toughness, Ti must be added in an amount of 0.001% or more. On the other hand, if Ti is added in excess of 0.1%, TiC is excessively formed and becomes rather brittle, so the Ti content is set to 0.1% or less. Similarly, Zr must be added in an amount of 0.001% or more to obtain the effect of improving the HAZ toughness. On the other hand, addition of Zr in excess of 0.1% rather reduces toughness, so the Zr content should be 0.1% or less.

In another embodiment of the present invention, the above 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. To obtain the above effect, 0.0002% or more must be added. On the other hand, if added in excess of 0.003%, the toughness is rather reduced. Therefore, when B is contained, its content is set to 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%
Both Ca and REM (rare earth metal) are elements effective in improving the toughness of the weld heat affected zone, and one or both of them can be added arbitrarily. The above effect is obtained by adding Ca: 0.0002% or more and REM: 0.0002% or more. On the other hand, if Ca is added in excess of 0.01% and REM in excess of 0.015%, the toughness is rather lowered. Therefore, when Ca is added, its content is 0.0002 to 0.01%, and when REM is added, its content is. It is 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 above 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). In addition, all the element symbols “C”, “Mn”, “Cu”, “Ni”, “Mo”, and “V” in the formula (1) include each element expressed in “mass%” unit. It means the amount, and is set to “0” when the element is not added.
Ceq (mass %)=C+Mn/6+(Cu+Ni)/15+(Mo+V)/5 (1)

If Ceq is less than 0.36%, it is not possible to obtain the target sufficient strength as an angle steel. On the other hand, if Ceq exceeds 0.44%, low-temperature cracking at the time of welding may occur, so preheating is required, leading to deterioration in welding workability. Therefore, Ceq is set to a range of 0.36 to 0.44%. Preferably, it is in the range of 0.36 to 0.43%.

[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 contains the worked ferrite. When the microstructure does not include worked 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 (austenite+ferrite) two-phase region below the Ar3 transformation point. In the present invention, ferrite in which the ratio of the major axis to the minor axis (aspect ratio) is 2.0 or more is defined as processed ferrite.

The area fraction of the worked ferrite is not particularly limited, but from the viewpoint of improving the mechanical strength, it is preferably 15% or more, and more preferably 20% or more. In addition, the microstructure is the half width part of the short side (flange) and the plate thickness 1/4 t position in the case of unequal angle angle steel, and the half width part of each side, the plate thickness in the case of equilateral angle steel. The 1/4t position is the observation position, and in the case of the valve plate, the center of the thickness of the sphere 1/2 width portion is the observation position.

In the present invention, it is acceptable for the microstructure to contain a structure other than worked ferrite. The remaining structure other than the processed ferrite is not particularly limited and may be any structure, but for example, it may be one or more selected from the group consisting of pearlite and bainite which is not processed ferrite. Particularly, from the viewpoint of further improving the strength, it is preferable that the second phase structure is bainite.

Next, a method for manufacturing the angle steel in the embodiment of the present invention will be described. The angle steel of the present invention can be produced by preparing a steel material having the above-described composition, heating it to 1150 to 1350° C., and then hot rolling it. And at that time, the hot rolling is performed under the conditions of a cumulative reduction of 20 to 80% at a temperature of Ar3 or lower and a finishing temperature of (Ar3-50)°C to (Ar3-120)°C, and the hot rolling is performed. After that, 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. The reasons for limiting the conditions in each step will be described below.

[heating]
Heating temperature: 1150 to 1350°C
It is necessary to heat the steel material at a sufficiently high temperature in order to reduce the deformation resistance in hot rolling, homogenize the solidification structure, and once form a solid solution of Nb and V precipitates. If the heating temperature is less than 1150°C, these sufficient effects cannot be recognized. Therefore, the heating temperature before hot rolling is set to 1150°C or higher. On the other hand, if the heating temperature exceeds 1350° C., problems such as a decrease in yield and a decrease in productivity due to scale loss occur. Therefore, the heating temperature is 1350° C. or lower. The heating temperature is preferably 1180 to 1320°C.

[Hot rolling]
Cumulative rolling reduction at an Ar3 temperature or lower: 20 to 80%
If the cumulative rolling reduction at the 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. Further, the deformation resistance becomes high, and the modeling becomes difficult. Therefore, the cumulative reduction rate at the Ar3 temperature or lower during hot rolling is set to 20 to 80%. The value of Ar3 point can be calculated by the following formula.
Ar3(°C)=910-273C+25Si-74Mn-56Ni-9Mo-5Cu
However, the element symbol in the above formula represents the content (mass %) of each element, and is set to zero when the element is not contained.

Finishing temperature: (Ar3-50) to (Ar3-120)°C
When the finishing temperature is higher than (Ar3-50)°C, the amount of processed ferrite decreases and the strength decreases. On the other hand, when the finishing temperature is lower than (Ar3-120)°C, the rolling load is high and the formability is deteriorated. Therefore, the rolling finishing temperature is set to (Ar3-50)°C or higher and (Ar3-120)°C or lower.

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

The method of accelerated cooling is not particularly limited, but water cooling such as spraying is preferable. Further, from the viewpoint of further improving the strength, it is preferable to start accelerated cooling immediately after completion of hot rolling, and specifically, it is preferable to start within 5 seconds after completion of hot rolling. By performing accelerated cooling immediately after the completion of rolling, the second phase structure can be transformed into bainite and the strength can be further improved. If it takes time from the end of hot rolling to the start of accelerated cooling, pearlite transformation occurs from 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 less than 500° C., the residual stress generated during cooling causes twisting, bending, warping, etc., and the shape cannot be maintained. Therefore, the cooling stop temperature is set to 650 to 500°C. The cooling stop temperature is preferably 650°C to 550°C.

<Example 1>
Steel having the chemical composition shown in Table 1 is melted in a vacuum melting furnace or a converter to form a bloom, and the bloom is charged into a heating furnace, heated, and then subjected to hot rolling and accelerated cooling to produce a bloom in Table 2. Non-equal unequal thick angle section steels having the cross-sectional dimensions shown were manufactured. The conditions of heating before hot rolling, hot rolling, and accelerated cooling after hot rolling were as shown in Table 2. Regarding the rolling temperature, radiation thermometers were installed on the inlet side and the outlet side of the mill, and the temperature on the short side was measured. The finishing temperature in Table 2 is the inlet temperature at the time of final rolling. Then, for each of the obtained angle steels, the microstructure and the mechanical properties in the flange portion and the weld bond portion were evaluated. The measurement results are shown in Table 3. The evaluation methods were as follows.

(Microstructure)
The microstructure at the short side (flange) 1/2 width portion and the plate thickness 1/4 t position of the unequal side unequal thick angle steel was observed with an optical microscope. Observation was performed at 3 magnifications or more at a magnification of 500, and a ferrite portion was traced as a metallographic structure. After that, the minor axis and major axis of the ferrite grains and the aspect ratio were determined by image analysis software. A processed ferrite having an aspect ratio of 2.0 or more was defined, the area of each processed ferrite was calculated, and the area fraction of the processed ferrite in the metal structure was calculated.

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

As can be seen from Tables 1 to 3, the angle steels of the invention examples satisfying the conditions of the present invention have excellent strengths of YS: 460 MPa or more and TS: 570 MPa or more, and also have excellent toughness in the base material and the welded portion. It was On the other hand, in Comparative Examples that do not satisfy the conditions of the present invention, any of YS, TS, and toughness was inferior. In addition, No. In No. 21, the torsional warpage and bending after cooling were excessive, and normal molding could not be performed. Therefore, evaluation of mechanical properties was not performed. In addition, No. In No. 22, since the rolling load was excessive and hot rolling could not be normally performed, rolling was stopped halfway.

<Example 2>
Steel having the composition shown in Table 4 is melted in a vacuum melting furnace or a converter to form a bloom, and the bloom is charged into a heating furnace and heated, followed by hot rolling and accelerated cooling, A valve plate having the cross-sectional shape and dimensions shown in Table 5 was manufactured. The conditions of heating before hot rolling, hot rolling, and accelerated cooling after hot rolling were as shown in Table 5. As the rolling temperature, radiation thermometers were installed on the inlet side and the outlet side of the mill, and the temperature of the long side (web) was measured. The finishing temperature in Table 5 is the inlet temperature at the time of final rolling. Then, for each of the obtained valve plates, the microstructure and mechanical properties, and the Charpy impact property of the weld bond were evaluated. The measurement results are shown in Table 6. The evaluation methods were as follows.

(Microstructure)
The microstructure at the plate thickness center position 1 (center position of the plate thickness t ₂ ) of the sphere half width portion of the valve plate shown in FIG. 2 was observed with an optical microscope. Observation was performed at 3 magnifications or more at a magnification of 500, and a ferrite portion was traced as a metallographic structure. After that, the minor axis and major axis of the ferrite grains and the aspect ratio were determined by image analysis software. A processed ferrite having an aspect ratio of 2.0 or more was defined, the area of each processed ferrite was calculated, and the area fraction of the processed ferrite in the metal structure was calculated.

In addition, as shown in FIG. 2, a JIS 1A tensile test piece was sampled from the web 1/3 width portion 2 of the valve plate 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, from the plate thickness center position 1 of the sphere 1/2 width portion, a round bar micro-tensile test piece (parallel portion diameter) whose tensile direction is the length direction (rolling direction) of the valve plate. 6 mmφ, GL25 mm) was sampled and the tensile properties (yield stress YS, tensile strength TS, elongation El) were measured. Regarding toughness, 2 mm V notch Charpy impact test pieces described in JIS Z 2242 were taken from the plate thickness center position 1 of the web 1/3 width portion 2 and the spherical portion 1/2 width portion, respectively, and the toughness at -5°C was obtained. Was evaluated. Moreover, after performing multi-layer welding with a heat input of 2 kJ/mm, the toughness of the weld bond portion at −5° C. was also evaluated.

As can be seen from Tables 4 to 6, the valve plates of the invention examples satisfying the conditions of the present invention have high strength of YS: 460 MPa or more and TS: 570 MPa or more even in the center of the spherical portion having a large cross-sectional area, and the welding The toughness including the parts was also excellent. On the other hand, in Comparative Examples that do not satisfy the conditions of the present invention, any of YS, TS, and toughness was inferior. In addition, some data showed that the strength of the sphere was low or the toughness was low even if the strength of the web portion was sufficiently satisfied. These show that the controlled rolling effect in the ball portion is lower than that in the web.

According to the present invention, high-strength and high-toughness-provided YS: 460 MPa or more, TS: 570 MPa or more, Charpy absorbed energy at -5°C: 47 J or more angle iron steel, according to the present invention. Can be manufactured with high productivity and at low cost.

1 Center position of the thickness of the ball 1/2 width part 2 Web 1/3 width part

Claims (6)

In 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: 0.005-0.07%,
N: 0.006 to 0.018%, and one or both of V: 0.01 to 0.12% and Nb: 0.001 to 0.05%, the balance consisting of 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 20% or more of worked ferrite in area fraction,
A valve plate having mechanical characteristics of YS: 460 MPa or more, TS: 570 MPa or more, and Charpy absorbed energy at -5°C: 47 J or more.
Note Ceq (mass %)=C+Mn/6+(Cu+Ni)/15+(Mo+V)/5 (1) The above component composition is mass%,
Cu: 0.05 to 0.50%,
The valve plate 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 above component composition is mass%,
The valve plate according to claim 1 or 2, further containing one or both of Ti: 0.001 to 0.1% and Zr: 0.001 to 0.1%. The above component composition is mass%,
B: The valve plate according to any one of claims 1 to 3, further containing 0.0002 to 0.003%. The above component composition is mass%,
The valve plate 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 method for producing a valve plate by preparing a steel material having the composition according to any one of claims 1 to 5, heating it to 1150 to 1350°C, and then hot rolling it.
The hot rolling is performed under the conditions of a cumulative rolling reduction at an Ar3 temperature or lower: 20 to 80% and a finishing temperature: (Ar3-50) to (Ar3-120)°C.
After the hot rolling, accelerated cooling is performed under the conditions of a cooling start temperature: (Ar3-50) to (Ar3-120)°C and a cooling stop temperature: 650 to 500°C.
The valve plate is
Has a microstructure containing 20% or more of worked ferrite in area fraction,
A method for producing a valve plate having mechanical characteristics of YS: 460 MPa or more, TS: 570 MPa or more, and Charpy absorbed energy at -5°C: 47 J or more.

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