JP2003183768A - Steel material, steel structure, and connecting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a medium and high carbon steel material which can improve characteristics in the pressure welded part, without decreasing application efficiency and versatility, and to provide a steel structure made by connecting the steel material, and a connecting method. <P>SOLUTION: The steel material used for pressure welding comprises, by mass%, 0.1-1% C, 2% or less Si, and 3% or less Mn, and the balance Fe with unavoidable impurities, and, in addition, a microstructure in which a total volume fraction of ferrite and pearlite is 40% or less. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material, a steel structure and a joining method which are used by being pressure-welded to machines, transportation equipment, buildings and the like.

[0002]

2. Description of the Related Art Medium-high carbon steel is often used for members requiring high strength, but it is easily hardened when rapidly cooled from a high temperature during welding. For this reason, when melt welding represented by arc welding is used, weld cracks and brittle structures are likely to occur. As a countermeasure against this, pre-heat treatment and / or post-heat treatment are required at the time of welding, which reduces the work efficiency and the economical efficiency.

Of the joining methods, the pressure welding method has a smaller amount of heat input than arc welding and is capable of performing surface joining.
Excellent in economics of joining. However, when a steel material having a carbon content of 0.3 mass% (hereinafter referred to as%) or more is pressure-welded, a hardened layer is also formed at the joint interface, and the bending ductility, which is an index of the joint characteristics, especially the joint reliability, is reduced. There is a problem of deterioration.

Research on the pressure welding technique has been conducted for a long time, and as a measure for improving the bonding strength, for example, a method is known in which oxides generated at the bonding interface are discharged as burrs to obtain a normal bonding portion. It is presumed that if the measures for discharging such oxides as burrs can be applied to discharge the hardened layer as burrs, the characteristics of the pressure-welded portion of the medium-high carbon steel can be improved.

However, even if a force (upset pressure) perpendicular to the bonding interface is applied in order to discharge the portion to be the hardened layer as burr, the portion to be the hardened layer is always discharged from the bonding interface as intended. It will not be done.
For this reason, it has been considered difficult to obtain a joining strength sufficient to bear the load applied to the structure at the pressure-bonded portion of the medium-high carbon steel without practically implementing the above-mentioned measures in the pressure-bonding method. It was

In order to break this, a proposal has been made that the shape of the abutting surface forming the pressure contact interface is a male-female fitting shape (Japanese Patent Laid-Open No. 2000-15462). Improvements in the characteristics of the joint have been obtained by taking such a geometrical measure of the joint interface.

[0007]

However, depending on the member, there are structures in which the abutting surface cannot have such a male-and-female shape, and the above-mentioned measures relying on the shape of the abutting surface are not always universal. Not a solution. Also,
It takes man-hours to process the butt surfaces, which is not desirable from the viewpoint of construction efficiency and economy. Therefore, in the pressure welding of medium-high carbon steel, a general-purpose solution means for improving the characteristics of the joint without lowering the working efficiency has been desired.

The object of the present invention is to use a steel material capable of improving the characteristics of the pressure-welded portion of medium-high carbon steel without deteriorating the construction efficiency and versatility, a steel structure using the steel material and the steel material. It is to provide a joining method.

In the following description, pressure welding refers to a method of intensively heating the surfaces to be joined and applying a pressure sufficient to cause plastic deformation to join them. Therefore, no molten pool is formed in melt welding such as arc welding or laser welding. For example, flash butt welding, D
C (direct current) butt welding, spot welding, friction welding, electric resistance welding, etc. can be mentioned, but not limited to the above-exemplified joining method. For example, if the action of the present invention is exhibited by heating and pressurizing the butt portion as described above, and the portion that becomes the hardened layer of the joint interface portion is discharged as burrs, butt joint and the like are also the subject of the present invention. It is a joining method.

The hardened layer is composed of a high-carbon martensite structure or the like, which is produced by the condensing portion being heated, the carbon being concentrated in a portion heated to a melting point or a high temperature region close to it, and then being cooled. In the following description, the "cured layer portion" refers to a molten portion where carbon is concentrated as described above, or a very softened solid portion or a mixed portion thereof.

[0011]

A steel material of the present invention is a steel material used for pressure welding, and in mass%, C: 0.1% or more and 1% or less, Si: 2% or less, Mn: 3% or less. Contains
The balance consists of Fe and impurities, and further has a microstructure with a volume ratio of ferrite and pearlite of 40% or less (claim 1).

According to this structure, it is easy to discharge the portion which becomes the hardened layer in the vicinity of the bonding interface as burrs during pressure welding. The grounds for this are as follows.

(A) 40% or less in volume ratio of ferrite and pearlite combined: (a1) By simply increasing the upset pressure, not only the bonding interface but also the region heated to a high temperature (hereinafter referred to as a heat It is impossible to entirely deform a wide portion including an affected portion) and to discharge the hardened portion as a burr. Further, if the upsetting force is excessively large, the entire bonded body will be crushed. Ideally, if only the joint interface is heated and softened, and the surrounding area is not heated, the deformation due to the upset pressure is concentrated only on the softened joint interface, and the hardened layer is discharged as burr. To be done. However, in reality, the area around the bonding interface is also heated by heat conduction.

(A2) Therefore, even if the region around the joint interface is heated, the high temperature deformation resistance of the region is relatively higher than that of the joint interface during the transient period, so that the microstructure of the base metal is high. I came up with a method of adjusting the organization in advance. In other words, the idea is that even if the area around the joint interface portion is heated, if it does not soften until the pressure welding is completed, the deformation concentrates on the joint interface portion, and the portion to be the hardened layer can be discharged as burrs. Is.

(A3) Specifically, by setting the total volume ratio of ferrite and pearlite of the base material to 40% or less, it is possible to delay the softening due to heating and to discharge the portion to be the hardened layer. confirmed. In the medium-high carbon steel, when the area of ferrite and pearlite is reduced, the area of bainite, or the area of bainite and martensite is increased. Retained austenite may be present, but it remains dispersed in the bainite or martensite regions. As will be described later, since the method for measuring the volume fraction of ferrite and pearlite can be performed within the field of view of an optical microscope, a finely dispersed residual structure such as retained austenite is not detected by itself, and, for example, bainite is used. If the volume ratio of martensite is calculated, it is included in it. Since the residual austenite is rarely dispersed in ferrite or pearlite, the volume ratio of ferrite and pearlite measured by the measurement method described later does not usually include the above retained austenite. Further, the perlite may be layered perlite or pseudo perlite or a mixture thereof.

Ferrite and ferrite, which is a constituent of pearlite, have a low solid solution carbon concentration, while bainite has a high solid solution carbon concentration or carbon is contained as fine cementite particles. In the case of rapid heating, in the bainite region, the above-mentioned carbon morphology effectively contributes to securing high temperature strength, and exhibits high high temperature strength, that is, high deformation resistance in the early stage of heating. Therefore, even if the region around the bonding interface is heated, the deformation can be concentrated on the bonding interface, and the portion to be the hardened layer can be discharged. When ferrite and pearlite occupy a large amount, the amount of solid solution carbon is low at the initial stage of heating, so that the deformation resistance is low, and even if an upset pressure is applied, the deformation cannot be concentrated at the bonding interface.

(A4) By setting the volume ratio of ferrite and pearlite of the base material to 40% or less, it is possible to delay softening due to heating in the region around the joint interface portion,
It is possible to concentrate the deformation due to the upset pressure on the bonding interface. As a result, the portion to be the cured layer can be discharged as burrs, and excellent joint characteristics can be obtained. The volume ratio of the above ferrite and pearlite is
By more preferably setting it to 30% or less, and further preferably 20% or less, it is possible to further delay the softening in the initial stage of heating, and it is possible to more reliably discharge the portion to be the hardened layer as burrs.

When the volume ratio of ferrite and pearlite of the base material exceeds 40%, coarse cementite in pearlite is solid-dissolved and diffusion does not occur sufficiently, so that the high temperature strength is lowered in the initial stage of heating. Therefore, even if an upset pressure is applied, the deformation occurs in the heat-affected zone (HAZ), and the hardened layer cannot be discharged.

As described above, usually, the portion other than the ferrite and pearlite structures is mainly composed of bainite or a mixed structure of bainite and martensite. Bainite or martensite has a large amount of solute carbon and has a lath-like structure with a high dislocation density. In the bainite structure, fine cementite is dispersed in the lath structure having a high dislocation density. Therefore, these bainite and martensite have high high-temperature strength and therefore high-temperature deformation resistance.

The volume ratio of the microstructure including ferrite and pearlite can be measured by the following method.

In a field of view of 500 times of an optical microscope, the frequency of ferrite and pearlite overlapping grid points when a square grid of 20 × 20 is placed is measured for 10 fields of view. The volume ratio of the amount of ferrite and pearlite can be obtained by obtaining the ratio of the average value to all the lattice points.

(B) Chemical composition: Next, the range of the chemical composition will be described.

C: 0.1 to 1.0% C greatly affects the microstructure of steel and improves the strength such as tensile strength required for steel materials, but its content is 1
If it exceeds%, the part that becomes the hardened layer is not discharged during pressure contact,
The effect of the present invention cannot be obtained. Therefore, the upper limit of the content rate is set to 1%, but 0.8% or less is preferable. A more desirable upper limit is 0.7%. on the other hand,
In order to obtain the effect of the present invention, it is not necessary to set the lower limit of the C content in particular, but if it is less than 0.1%, there is no problem in the pressure contact property even if the volume ratio of ferrite and pearlite is not particularly limited. . In particular, the limitation of the volume ratio of the ferrite and the pearlite of the present invention works effectively because C
Is 0.2% or more.

Si: 2% or less Si is used to obtain a deoxidizing action during refining of molten steel and to improve castability. However, the content rate is 2%
If it exceeds, the pressure contact property deteriorates, so the upper limit is made 2%.
The lower the Si content is, the more preferable from the viewpoint of pressure contact property, but it is preferable that the Si content is 0.05% or more in view of the above-mentioned deoxidizing action and improvement of castability.

Mn: 3% or less Mn is used to obtain a deoxidizing action during refining of molten steel, and to secure the strength and toughness of the steel material. But,
If its content exceeds 3%, it becomes rather brittle, so its upper limit is made 3%. From the viewpoint of pressure contact, the lower the Mn content is, the more preferable.

The above chemical components are contained in order to secure the characteristics of the steel, but in addition to the above, Al may be added as a deoxidizing agent during refining of molten steel. When Al is added in excess of the amount that bonds with (oxygen), it remains in the steel as Al. This Al combines with N (nitrogen) inevitably contained in the steel to form AlN, or exists as solid solution Al. The steel material of the present invention may of course contain such Al and N. The Al deoxidation as described above is used when the C content is relatively low, but may be used when the C content is high. N
Is about 0.0005% when melting steel in a converter.
It is about 0.02%, and in the case of melting in an electric furnace, it is about 0.002% to 0.03%. However, since it is possible to control the increase / decrease by sealing with nitrogen gas or evacuation during casting, it cannot be decided unconditionally. After deoxidation, the Al content remaining in the steel is usually 0.1%.
It is the following.

Those contained as inevitable impurities include
There are P and S. Since these impurities usually adversely affect the properties of steel, they are intentionally reduced in refining, but the production cost greatly varies depending on the reduction level. In the present invention, it is desirable to reduce the P content and the S content according to the following criteria.

P: 0.04% or less P is the lower the content of P to secure the characteristics of the steel material, but it is desirable to remove it, but refining costs are high. Is desirable.
From the viewpoint of pressure contact property, if it exceeds 0.04%, it becomes difficult to secure the bonding strength, so it is desirable to set it to 0.04% or less.

S: 0.01% or less Since S secures the characteristics of the steel material, it is desirable that its content is as low as possible, but refining cost is required for its removal, so 0.001% is more desirable from the economical point of view, and more desirably. It is desirable to set it to 0.003% or more, and more desirably 0.005% or more. From the viewpoint of pressure contact property, if the content exceeds 0.01%, it becomes difficult to secure the bonding strength. Therefore, the content is preferably 0.01% or less.

Further, especially in the case of the electric furnace molten steel making, and also in the case of the converter molten steel making, Cr, Ni, C are used as impurities.
u, Mo, Ti, Nb, V, etc. are included, but 0.1%
If the amount is less than the range, the function of the present invention is not impaired and the properties of the base material are considered to be substantially the same as those of the steel of the present invention.

The steel of the present invention may further contain impurities, such as As and Sn, which are inevitably contained in addition to the above, in a range contained in ordinary steel materials.

The steel structure of the present invention is provided with a joint portion in which at least one steel material to be joined is abutted with the steel material of the present invention and heated and pressed to join (claim 2).

With this structure, since the steel structure includes the pressure-bonded joint portion that does not include the hardened layer, the steel structure having excellent characteristics can be obtained. This steel structure is
For example, it may be a member that constitutes a building, and may be a member that has a joint portion formed by joining steel materials. Further, only one of the steel materials sandwiching the joint may be the steel material according to any one of the above inventions.

In the joining method of the present invention, when the steel materials to be joined are butted to each other, at least one of the steel materials is butted with the above steel material of the present invention, and a step of heating and pressurizing the butted parts. And (claim 3).

By this joining method, it becomes possible to easily obtain a joining portion having no hardened layer by pressure welding without increasing the processing man-hours and the like and without impairing the economical efficiency.

[0036]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating a pressure welding method according to an embodiment of the present invention. FIG. 1A shows a steel material having a small volume ratio of ferrite and pearlite of 40% or less (invention example: solid line) and a steel material having a large volume ratio of ferrite and pearlite and more than 40% (comparative example: broken line). of,
It is a figure which shows the time transition of the deformation resistance at the time of press contact. Further, FIG. 1B is a diagram showing point A where the deformation resistance in FIG. 1A is estimated.

According to these figures, in the comparative example, since the volume ratio of ferrite and pearlite is large, the softening proceeds at the point A at the initial stage of heating, and the deformation resistance at the point A is greatly reduced.
For this reason, it is difficult for the portion including the heat-affected zone to be entirely deformed to become the hardened layer and be discharged as burrs.

With further passage of time, the diffusion resistance of carbon should advance and the deformation resistance should rise, but at the same time, since the temperature at point A rises, the deformation resistance does not rise, but rather falls.

On the other hand, in the example of the present invention, the deformation resistance at the point A, which is the representative point of the region around the pressure contact interface portion, decreases with the temperature rise during the pressure contact period, but the decrease is small. For this reason, the point A of the example of the present invention has a remarkably high deformation resistance as compared with the joint interface portion which is heated to the melting point or a temperature close to the melting point and the deformation resistance is greatly reduced. Therefore, the upset pressure is concentrated on the bonding interface portion, and the portion to be the hardened layer is easily discharged as burr.

Next, description will be made on the joint portion that verifies the above. FIG. 2 is a cross-sectional view of a pressure-welded portion of the steel material of the present invention having a volume ratio of ferrite and pearlite of 40% or less. The feature of this press contact portion is that the bulging portion 4 obviously includes a burr, and the burr portion is largely pushed outward. It can be seen that there is no hardened layer at the bonding interface portion 3, and the portion that will become the hardened layer during pressure welding was extruded as burrs to the outside. The region around the bonding interface is the heat-affected zone 2 and is continuous with the base material 1. FIG. 3 is a speculative diagram showing the characteristics of the heat affected zone (HAZ) 2 in the initial stage of the pressure welding process of the steel material of the present invention example according to the distance from the joint interface. As shown in FIG. 3, the high-temperature strength of the heat-affected zone is high in the region near the bonding interface, reflecting the high solid solution C, and then decreases with a gentle gradient. However, it shows that the deformation resistance of the region around the bonding interface is sufficiently high.
As a result, it is considered that the deformation due to the upset pressure was concentrated on the joint interface portion and the portion to be the hardened layer was discharged as burr.

On the other hand, FIG. 4 is a sectional view of a pressure contact portion of a steel material of a comparative example. The feature of this press contact portion is that the deformation includes the bonding interface portion over the entire HAZ, and the bulging portion 4 contains almost no burr. Therefore, the hardened layer 5 is recognized in the bonding interface portion 3. FIG. 5 is an estimated diagram showing the characteristics of the heat-affected zone 2 in the initial stage of the pressure welding process of the steel material of the comparative example according to the distance from the joint interface. As shown in FIG. 5, the high-temperature strength of the heat-affected zone falls steeply from the bonding interface, reflecting the low solid solution C in the region near the bonding interface. Therefore, it is considered that the deformation due to the upset pressure was distributed over the bonding interface and the heat-affected zone, and the portion to be the hardened layer was not discharged as burr.

The above-mentioned steel in which the volume ratio of ferrite and pearlite is 40% or less can be obtained by controlling the cooling rate from the austenite temperature range.
It can also be obtained by cooling from the austenite temperature range to the ferrite transformation temperature range, holding for a predetermined time, and then cooling. Further, it can be obtained by adjusting the cooling method from the austenite temperature range. In the case of a hot rolled coil, the above microstructure can be obtained by adjusting the finishing temperature, cooling rate and winding temperature. Further, in the case of a thick steel plate, it can be obtained by adjusting the hot rolling finish temperature and applying controlled cooling. When the above microstructure is obtained by isothermal transformation, a TTT diagram (Time Temperature Transfor
The temperature obtained by subtracting about 50 ° C. from the temperature of the ferrite precipitation nose may be maintained at a temperature which does not exceed the temperature of the ferrite nose. Specifically, for example, a steel material having the above microstructure can be manufactured under the following conditions. (A) One example of welded steel pipe: 950 after electric resistance welding production pipe
Heat to 5 ° C for 5 minutes, quench to 500 ° C, hold at that temperature for 3 minutes, then air cool. (B) One example of hot-rolled steel sheet (hot coil): After finishing rolling at 950 ° C., it is wound in a temperature range of 400 ° C. to 500 ° C.

[0043]

EXAMPLES Steels P1 to P3 having the chemical compositions shown in Table 1 were each heated to 50 k
6mm after melting in a vacuum high-frequency furnace, then casting and hot rolling
It was a thick steel plate. Then, in order to evaluate the pressure contact property, Table 2
-Joint joints were produced by various pressure welding methods shown in Table 4 and evaluation tests were carried out.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

[0047]

[Table 4]

(A) Flash butt welding and DC butt welding The above steel sheet obtained by hot rolling was heated at 880 ° C. for 10 minutes and air-cooled, and then at one temperature (holding temperature) in the temperature range of 400 to 650 ° C. After being held for 10 minutes, a heat treatment of air cooling was performed to fabricate test materials having the microstructure shown in Table 5. That is, for the sample steel P1, by setting the holding temperature to 530 ° C. or lower in the above temperature range, a microstructure in which the volume ratio of ferrite and pearlite is 40% or less in total is obtained,
Further, by setting the holding temperature to a temperature higher than 530 ° C., a microstructure having a volume ratio of more than 40% was obtained. For the sample steels P2 and P3, the microstructure having a volume ratio of 40% or less was obtained by setting the holding temperature to 510 ° C. or lower in the above temperature range.

The end surface of the test material was machined into an I-groove,
Butt the 6 mm x 200 mm rectangular cross sections, and
Alternatively, welding was performed under the conditions shown in Table 3 to produce a bonded joint. (B) The above steel sheet obtained by friction welding hot rolling was formed into a tubular shape with a roll, and electric resistance welding was performed to produce a welded tube having an outer diameter of 35 mm.
This welded tube was subjected to the above heat treatment to produce a tube having a microstructure shown in Table 5. That is, for the test steel P1,
By setting the holding temperature to 530 ° C. or lower in the above temperature range, the volume ratio of ferrite and pearlite is 40 in total.
% Microstructure is obtained, and the holding temperature is 530 ° C.
By setting the temperature to exceed, a microstructure having the above volume ratio of more than 40% was obtained.

The end faces of the obtained pipe were machined into I-grooves, the end faces were abutted against each other, and friction welded under the conditions shown in Table 4 to obtain a joint.

From the above-mentioned bonded joint, five side-bending test pieces having a thickness of 6 mm, a width of 10 mm, and a length of 200 mm and having a bonded portion in the center in the direction orthogonal to the welding line were sampled from each bonded joint. This side bending test piece has a bending radius of 12 mm
It was bent 180 degrees. With respect to this bent test piece, the presence or absence of cracks at the joint interface portion of the bending surface was examined with a 50 times magnified field of view, and the presence or absence of cracks at the joint portion was examined. In addition, in addition, a Vickers hardness test was performed on the cross section of the joint with a load of 1 kgf, and the hardened layer was evaluated at the joint interface. The test body in which no crack was generated in the joint was evaluated based on the criterion that the joint strength was good. The evaluation results are shown in Table 5.

[0052]

[Table 5]

The volume ratio of ferrite and pearlite is 40.
%, The Vickers hardness of the joint interface is 350 or less in any joint using any joining method, as in the alternatives A1 to A15. No cracks occurred in the. Such good pressure contact property is achieved only when the volume ratio of ferrite and pearlite is 40% or less for all the steel materials P1 to P3 having different chemical compositions.

On the other hand, as in the substitutes B1 to B6, B10 and B11, for steel materials having a total volume ratio of ferrite and pearlite of the base material of more than 40%, the upset pressure can be increased by any pressure welding method. However, the Vickers hardness of the joint interface exceeds 380. Therefore, cracks occurred in the bending test body at the joint.

From the above, it was found that it is possible to obtain a pressure-welded joint having excellent joining characteristics by using a steel material satisfying the requirements of the present invention.

Although the embodiments of the present invention have been described above, the embodiments of the present invention disclosed above are merely examples, and the scope of the present invention is not limited to these embodiments. Not limited. For example, the joining method of the present invention is not limited to the flash butt welding method and the friction welding method. Any welding method may be used as long as it is a joining method in which the joined portions are butted and heated and pressed to extrude the portion to be the hardened layer. The scope of the present invention is shown by the description of the claims, and further includes meanings equivalent to the claims and all modifications within the scope.

[0057]

EFFECTS OF THE INVENTION By using the steel material and the joining method of the present invention, the portion to be the hardened layer can be extruded and a joined joint having excellent joining characteristics can be obtained.

[Brief description of drawings]

FIG. 1A is a diagram for explaining deformation resistance in a region around a joint interface portion of a steel material of the present invention, and FIG. 1B is a diagram showing a joint portion.

FIG. 2 is a cross-sectional view of a joint portion where the steel material of the present invention is pressure-welded.

FIG. 3 is an estimated view of deformation resistance and the like at the initial stage of pressure welding of the joint portion in FIG.

FIG. 4 is a cross-sectional view of a joint portion where a steel material of a comparative example is pressure-welded.

5 is an estimated diagram of deformation resistance and the like at the initial stage of pressure welding of the joint portion of FIG.

[Explanation of symbols]

1 base material, 2 heat affected zone (HAZ), 3 joint interface,
4 bulging part of joint part, 5 hardened layer.

Claims (3)

[Claims] 1. A steel material used for pressure welding, which comprises% by mass.
C: 0.1% or more and 1% or less, Si: 2% or less, M
A steel material containing n: 3% or less, the balance being Fe and impurities, and further having a microstructure having a volume ratio of ferrite and pearlite of 40% or less. 2. A steel structure comprising a joint portion in which at least one of the steel materials to be joined is abutted with the steel material according to claim 1, heated, pressurized and joined. 3. When abutting steel materials to be joined together, a step of abutting at least one of the steel materials with the steel material according to claim 1, and a step of heating and pressing the abutted portion A joining method characterized by the above.