JP7490250B2 - Steel for solid-state bonding, steel material for solid-state bonding, solid-state bonding joints and solid-state bonding structures - Google Patents

Description

The present invention relates to steel for solid-state bonding and steel material for solid-state bonding that can be suitably used for solid-state bonding, as well as solid-state bonded joints and solid-state bonded structures made of the steel for solid-state bonding.

Solid-state joining methods that can reduce the decrease in strength of the joint compared to conventional fusion welding have attracted attention, and in particular, solid-state joining methods that utilize the phenomenon of frictional heating and the plastic deformation of metallic materials have been actively studied. Examples of such solid-state joining methods include "friction stir welding (FSW)", in which a cylindrical tool rotating at high speed is pressed into the workpieces to join them, "friction welding", in which a rotating cylindrical workpiece is brought into contact with a fixed workpiece to join them, and "linear friction welding", in which the workpieces are joined by moving them back and forth while in contact.

Conventional steel materials are often designed with alloys that assume the use of fusion welding, but in recent years, research has been conducted on steel materials that are suitable for friction welding methods. For example, Patent Document 1 (JP 2008-31494 A) discloses a low-alloy structural steel for friction stir welding, which is characterized in that the sum of the temperature range in which the steel becomes a single ferrite phase and the temperature range in which the steel becomes two phases, austenite and ferrite, in an equilibrium state of 600°C or higher is 200°C or higher.

In the low-alloy structural steel described in Patent Document 1, the deformation resistance of the steel during friction stir welding is significantly reduced by expanding the ferrite single phase region and the austenite-ferrite two-phase region near the temperature reached by the joint, which results in improved durability of the rotating tool and mitigated restrictions on welding conditions such as welding speed. In addition, the frequency of tool replacement due to wear and tear and breakage is reduced, and welding time is shortened, improving construction efficiency.

Additionally, research is also underway into steels suitable for the friction stir process, a surface modification technology that utilizes the principles of friction stir welding. For example, Patent Document 2 (JP 2014-162971 A) discloses a steel for friction stir process that is composed, by mass, of 0.40-1.50% C, 0.15-2.00% Si, 0.30-2.00% Mn, 0.50-3.00% Cr, with the balance being Fe and unavoidable impurities.

The steel for friction stir processing described in Patent Document 2 above is said to achieve excellent surface hardening by applying the friction stir process.

Furthermore, the present inventor also discloses in Patent Document 3 (JP 2018-16866 A) a steel for friction stir welding characterized in that the steel composition contains, in mass%, C: 0.20 to 0.45% and Cr: 1.00 to 3.50%, and the carbon equivalent CE defined by formula A is 0.40 to 1.00 mass%. CE = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 ... The element symbols described in formula (A) indicate the content of each component in the steel for friction stir welding in unit mass%.

The steel for friction stir welding described in Patent Document 3 above is a steel that can obtain joint characteristics (such as tensile strength and fracture toughness of the stir part) equivalent to or better than those of conventional high-tensile steel through friction stir welding, and it is said that it is possible to provide a steel with a minimum of added relatively inexpensive alloy elements, as well as a friction stir welding method in which the steel is used as the welded material.

JP 2008-31494 A JP 2014-162971 A JP 2018-16866 A

However, the low-alloy structural steel disclosed in the above-mentioned Patent Document 1 facilitates the application of friction stir welding to steel by reducing the deformation resistance of the steel during the process, and little consideration is given to the mechanical properties of the weld (stirred part) or the cost and availability of the elements added to the steel.

In addition, the steel for friction stir processing disclosed in Patent Document 2 has a composition optimized for surface hardening using frictional heat, and its design guidelines are completely different from those of steel materials intended to ensure the mechanical properties of the joint.

Furthermore, although the steel for friction stir welding disclosed in Patent Document 3 can obtain joint properties equal to or better than those of general high tensile steel, chromium (Cr) is used as a main additive element, which is relatively expensive, and further improvement in the mechanical properties of the base material and joints is required. In recent years, there has been an increase in demand for ultra-high tensile steel with a tensile strength of 1000 MPa or more, but the tensile properties of the steel for friction stir welding disclosed in Patent Document 3 do not reach the level of ultra-high tensile steel.

In view of the problems in the prior art as described above, the object of the present invention is to provide a steel for solid-state bonding and a steel material for solid-state bonding in which the tensile properties of the base material and the joint obtained by solid-state bonding are both equal to or greater than ultra-high tensile steel, and in which only relatively inexpensive alloy elements are added in a minimum amount. Another object of the present invention is to provide a solid-state bonded joint made of a steel for solid-state bonding in which only relatively inexpensive alloy elements are added in a minimum amount, and in which the tensile properties of the base material and the joint are both equal to or greater than ultra-high tensile steel, and a solid-state bonded structure having said joint.

In order to achieve the above-mentioned objective, the inventors conducted extensive research into the relationship between the composition and mechanical properties of steel and the structure and mechanical properties of the joint obtained by solid-state welding. As a result, they discovered that adding appropriate amounts of Si and Mn to a carbon steel base is extremely effective, and thus arrived at the present invention.

That is, the present invention provides:
The steel composition is, in mass%,
C: 0.20 to 2.14%,
Sum of Si and Al: 1.00 to 3.00%,
Mn: 2.00 to 4.00%;
The balance is composed of only Fe and inevitable impurities.
The present invention provides a steel for solid-state bonding, characterized by:

The steel for solid-phase bonding of the present invention contains a relatively large amount of carbon to increase its strength, while solid-phase bonding, which has a lower bonding temperature and a slower cooling rate compared to fusion welding, is used to obtain a good bond free of cracks and defects. By making the carbon content 0.20% by mass or more, the strength of the steel and the solid-phase bond can be sufficiently improved, and by making it 2.14% by mass or less, the deterioration of tensile properties due to the dispersion of graphite can be suppressed.

The carbon content is preferably 0.20 to 0.45 mass%, more preferably 0.20 to 0.30 mass%, and most preferably 0.20 to 0.25 mass%. In the steel for solid-phase bonding of the present invention, the tensile strength of the base material and the solid-phase bonded portion is 1000 MPa or more at a carbon content of 0.20 mass%, which is the lower limit. Therefore, in order to ensure good ductility, it is preferable to set the upper limit to a small value as long as the desired tensile strength is obtained.

In addition, Si and Al have similar effects, and Si and/or Al are added mainly to ensure the ductility of the base material and the solid-phase welded joint. By making the total content of Si and Al 1.00 mass% or more, it is possible to suppress the formation of cementite that reduces ductility and stabilize austenite. On the other hand, adding 3.00 mass% or more does not improve the effect, so the upper limit of the amount added is set to 3.00 mass%. The total content of Si and Al is preferably 1.50 to 2.50 mass%, and more preferably 1.75 to 2.25 mass%. Note that Si and Al are inexpensive alloy elements and are abundant elements. In addition, it is not necessary to add Si and Al together, and either one may be added.

The addition of Mn also contributes to improving the ductility of the base material and the solid-phase welded joint. In addition to the addition of Si, the Mn content is set to 2.00 mass% or more, and the ductility can be expected to be improved by the TRIP (martensitic transformation induced plasticity) effect due to austenite stabilization. In addition, although it depends on the conditions of solid-phase welding, adding 2.00 mass% or more of Mn maintains the hardenability and refines the austenite grain size of the solid-phase welded joint, and as a result, a fine lath-shaped martensite structure can be formed from the austenite. As a result, a solid-phase welded joint that is high in strength and has sufficient ductility can be obtained. On the other hand, since the effect is hardly improved even if the Mn content is set to 5.00 mass% or more, the upper limit of the Mn content is set to 5.00 mass%. Here, when emphasis is placed on strength, the Mn content is preferably set to 3.00 to 5.00 mass%, and when emphasis is placed on ductility, it is preferably set to 2.00 mass% or more and less than 3.00 mass%.

The solid-state welding steel of the present invention is composed of only Fe and inevitable impurities, other than the above-mentioned C, Si, Al, and Mn. By using carbon steel as the base and refraining from adding rare metals, it is possible to reduce manufacturing costs and ensure the sustainability of production (reducing the risk of uneven distribution). Note that the solid-state welding steel of the present invention is premised on solid-state welding, but the method of the solid-state welding is not particularly limited as long as it does not impair the effects of the present invention, and various conventionally known solid-state welding methods can be used. Representative solid-state welding methods include friction stir welding, friction welding, and linear friction welding.

The solid-phase bonding steel of the present invention may further contain, by mass%, Cr: 1.00 to 3.50%. By containing an appropriate amount of Cr, the strength and toughness of the solid-phase bond can be improved. Since toughness is a kind of product of strength and ductility, the addition of Cr increases both strength and ductility, resulting in improved toughness.

The present invention also provides a solid-state welded joint, characterized in that at least one of the materials to be joined is a steel material for solid-state welding made of the above-mentioned steel for solid-state welding of the present invention, and the solid-state welded portion of the steel material for solid-state welding has a fine lath-shaped martensite structure.

In the solid-phase welded joint of the present invention, at least one of the welded materials is the solid-phase welded steel of the present invention, and in addition to being inexpensive as it does not contain expensive alloying elements, it also has high strength and ductility. Furthermore, the structure of the weld is lath-shaped martensite, and the carbon content of this region is 0.20 to 2.14%, so the weld has extremely high tensile strength. In addition, the martensite is refined by the addition of Mn, and depending on the welding conditions, the TRIP effect due to retained austenite can be expected, so good ductility can be expressed.

In addition, in the solid-phase welded joint of the present invention, it is preferable that the average grain size of the prior austenite in the martensite structure is 20 μm or less. By refining the prior austenite structure, the lath-shaped martensite structure is refined, and high strength and ductility can be imparted to the solid-phase welded part. Here, the average grain size of the prior austenite is more preferably 15 μm or less, and most preferably 5 μm or less. The method for determining the average grain size of the prior austenite is not particularly limited and various conventionally known methods can be used. For example, a structure that has been appropriately etched may be observed, or prior austenite may be reconstructed from martensite based on the orientation relationship between martensite and prior austenite, and the grain size may be determined from the reconstructed prior austenite structure.

In addition, the solid-phase welded joint of the present invention is preferably such that the tensile strength of the solid-phase welded steel material at room temperature is 1000 MPa or more and the elongation is 20% or more, and the tensile strength of the solid-phase welded part at room temperature is 1500 MPa or more and the elongation is 20% or more. When the solid-phase welded steel material and the solid-phase welded part have these tensile properties, they can be fully used as a joint having mechanical properties equal to or greater than those of ultra-high tensile steel in applications requiring high strength and reliability. Here, the results of the tensile test may be affected by the size and shape of the tensile test piece, but in this specification, the values obtained using a relatively small tensile test piece with a parallel part length of about 4 mm and width of about 2 mm are used as the basis.

In addition, it is preferable that the solid-state welded joint of the present invention is a friction stir welded part. The solid-state welded part may be formed by friction welding or linear friction welding, but by using a friction stir welded part, it is possible to form a solid-state welded part in any region of the steel plate, and it is also possible to manufacture large structures.

The present invention also provides a solid-state welded structure characterized by having the solid-state welded joint of the present invention. The material, shape, and size of the structural parts other than the solid-state welded joint are not particularly limited, and may be any of various conventionally known structures.

Furthermore, the present invention also provides a steel material for solid-phase bonding, which is made of the steel for solid-phase bonding of the present invention, has a lath-shaped martensite structure, and is characterized in that the average grain size of prior austenite in the martensite structure is 20 μm or less. The steel material for solid-phase bonding having this composition and structure has a tensile strength of 1000 MPa or more and an elongation of 20% or more at room temperature, and can be fully used as a steel material with mechanical properties equal to or greater than those of ultra-high tensile steel in applications requiring high strength and reliability.

According to the present invention, it is possible to provide a steel for solid-state bonding and a steel material for solid-state bonding, in which the tensile properties of the base material and the joint obtained by solid-state bonding are both equal to or greater than ultra-high tensile steel, and in which only relatively inexpensive alloy elements are added in a minimum amount. In addition, according to the present invention, it is also possible to provide a solid-state bonded joint, which is made of a steel for solid-state bonding, in which only relatively inexpensive alloy elements are added in a minimum amount, and in which the tensile properties of the base material and the joint are both equal to or greater than ultra-high tensile steel, and a solid-state bonded structure having said joint.

FIG. 1 is a schematic diagram of a solid-state welded joint having a solid-state weld formed by friction stir welding. FIG. 1 is a schematic diagram of a solid-state welded joint having a solid-state weld formed by friction welding. FIG. 1 is a schematic diagram of a solid state welded joint having a solid state weld formed by linear friction welding. FIG. 1 is a schematic diagram of a tensile test piece of a steel material (base material). FIG. 1 is a schematic diagram of a stir zone tensile test specimen. 1 shows microstructure photographs of practical solid-state bonding steels 1 to 3 and comparative solid-state bonding steels 1 and 2. FIG. 2 is a stress-strain diagram of the practical solid-state bonding steels 1 to 4 and the comparative solid-state bonding steels 1 and 2. 1 is a photograph of a fracture surface of the example solid-state bonding steel 3. 1 shows cross-sectional macro photographs of the joints of the practical solid-state bonding steels 1 to 3 and the comparative solid-state bonding steels 1 and 2. 1 shows structural photographs of stir zones of experimental solid-state bonding steels 1 to 3 and comparative solid-state bonding steels 1 and 2. FIG. 2 is a stress-strain curve of the stir zone of the practical solid-state bonding steels 1 to 4 and the comparative solid-state bonding steels 1 and 2. 1 is a photograph of a fracture surface of a stir zone of the example solid-state bonding steel 3. 1 shows crystal orientation map images of the stir zones of the experimental solid-state bonding steels 1 to 3 and the comparative solid-state bonding steels 1 and 2. 1 is a phase map of the base material of the practical solid-state bonding steels 1 to 3 and the comparative solid-state bonding steels 1 and 2. 1 is a graph showing the tensile properties of the base material and stir zone of Example Solid-State Bonding Steels 1 to 4 and Comparative Solid-State Bonding Steels 1 and 2. This is the result of reconstruction of the prior austenite structure.

Representative embodiments of the steel for solid-state bonding, the steel material for solid-state bonding, the solid-state bonded joint, and the solid-state bonded structure of the present invention will be described in detail below, but the present invention is not limited to these. Note that in the following description, duplicated descriptions may be omitted.

(1) Steel for solid-state bonding The steel for solid-state bonding of the present invention is
The steel composition is, in mass%,
C: 0.20 to 2.14%,
Sum of Si and Al: 1.00 to 3.00%,
Mn: 2.00 to 5.00%;
The balance is composed of only Fe and inevitable impurities.
It is a steel for solid-state joining characterized by the above.

There is a strong demand for improving the specific strength of structural materials, and various high-tensile steels that incorporate rare metals have been proposed, but rare metals have a high risk of being unevenly distributed, which poses problems from the standpoints of cost and production stability. On the other hand, because the strength of steel generally increases with an increase in carbon content, the use of rare metals could be reduced if carbon steel could be actively utilized.

Here, medium and high carbon steels, which have a high carbon content, are considered to be extremely difficult to weld because cracks occur during fusion welding, but the solid-state welding steel of the present invention overcomes this problem by assuming the use of solid-state welding. In addition, the addition of appropriate amounts of Si, Al, and Mn gives good ductility to the base material and welded parts. Each component will be explained in detail below.

1. Essential additive elements C: 0.20 to 2.14 mass%
By setting the carbon content to 0.20 mass% or more, the strength of the steel and the solid-phase welded portion can be sufficiently improved, and by setting the carbon content to 2.14 mass% or less, the deterioration of tensile properties due to the dispersion of graphite can be suppressed. The carbon content is preferably 0.20 to 0.45 mass%, more preferably 0.20 to 0.30 mass%, and most preferably 0.20 to 0.25 mass%. In the steel for solid-phase bonding of the present invention, the tensile strength of the base material and the solid-phase welded portion is 1000 MPa or more at the lower limit of the carbon content of 0.20 mass%, so that the upper limit is preferably set to a small value as long as the desired tensile strength is obtained in order to ensure good ductility.

Sum of Si and Al: 1.00 to 3.00 mass%
By setting the total content of Si and/or Al to 1.00 mass% or more, it is possible to suppress the formation of cementite that reduces ductility. On the other hand, the effect is not improved even if the content is added to 3.00 mass% or more, so the upper limit of the addition amount is set to 3.00 mass%. The total content of Si and/or Al is preferably 1.50 to 2.50 mass%, and more preferably 1.75 to 2.25 mass%.

Mn: 2.00 to 5.00 mass%
In addition to the addition of Si, by making the Mn content 2.00 mass% or more, it is expected that the ductility will be improved by the TRIP (martensitic transformation induced plasticity) effect due to austenite stabilization. In addition, although it depends on the solid-state joining conditions, by adding 2.00 mass% or more of Mn, a fine martensite structure can be formed in the solid-state joining portion. As a result, a solid-state joining portion having high strength and sufficient ductility can be obtained. On the other hand, since the effect is hardly improved even if the Mn content is 5.00 mass% or more, the upper limit of the Mn content is set to 5.00 mass%. Here, when the strength is important, the Mn content is preferably 3.00 to 5.00 mass%, and when the ductility is important, it is preferably 2.00 mass% or more and less than 3.00 mass%.

2. Optional additive elements Cr: 1.00 to 3.50 mass%
By including an appropriate amount of Cr, the strength and toughness of the solid-phase bonded joint can be improved. Since toughness is a kind of product of strength and ductility, the addition of Cr increases both strength and ductility, resulting in improved toughness. Although the mechanism by which the addition of Cr improves the properties of the solid-phase bonded joint is not necessarily clear, it is believed that the addition of Cr to carbon steel suppresses the generation of pro-eutectoid ferrite during solid-phase bonding (the cooling process from austenite), thereby increasing the strength of the resulting solid-phase bonded joint and improving the ductility of martensite (or bainite). The more preferable Cr content is 1.50 to 3.00 mass%.

Cu: 3.0% by mass or less Cu is a useful element for ensuring the strength of the base material. However, if it is contained in excess of 3.0% by mass, the base material and the HAZ will harden. Therefore, it is preferable to keep the Cu content at 3.0% by mass or less.

Ni: 5.0% by mass or less Ni is an element that improves the strength and toughness of the base material, but if it is contained in excess of 5.0% by mass, the HAZ hardens, so it is preferable to set the content to 5.0% by mass or less. In addition, since Ni is expensive, it is preferable to set the content to 5.0% by mass or less.

Mo: 1.0% by mass or less Mo is an element useful for improving the strength of the base material, but if it exceeds 1.0% by mass, it has a negative effect on toughness, so it is preferable to set it to 1.0% by mass or less. In addition, since Mo is expensive, it is preferable to set it to 1.0% by mass or less.

Nb: 0.1% by mass or less Nb is a useful element for ensuring the strength and toughness of the base material and the HAZ, but if it exceeds 0.1% by mass, it has a negative effect on the toughness, so it is preferable to set it to 0.1% by mass or less. In addition, since Nb is expensive, it is preferable to set it to 0.1% by mass or less. It is considered that Nb has the effect of refining austenite, and as a result, forming a fine lath-shaped martensite structure.

Ti: 0.1% by mass or less Ti is a useful element for ensuring the strength and toughness of the base material and the HAZ, but if it exceeds 0.1%, it has a negative effect on toughness, so it is preferable to keep it at 0.1% by mass or less. Note that Ti is thought to have the effect of refining austenite, and as a result, forming a fine lath-shaped martensite structure.

V: 0.1% by mass or less
V is an element useful for increasing the strength of the base metal, but if its content exceeds 0.1 mass%, it deteriorates the toughness, so it is preferable to keep it at 0.1 mass% or less. Also, since V is expensive, it is preferable to keep it at 0.1 mass% or less.

B: 0.0040% by mass or less B has the effect of segregating to the austenite grain boundaries during rolling to improve hardenability, but if it exceeds 0.0040% by mass, it deteriorates the toughness of the HAZ portion, so it is preferable to make it 0.0040% by mass or less.

Other impurities include N, which, if present in large quantities, forms nitrides and reduces toughness, so it is preferable to keep the amount of N contained to 0.010 mass% or less.

(2) Solid-state welded joint With regard to the solid-state welded joint of the present invention, one embodiment having a solid-state weld formed by friction stir welding is shown in Fig. 1, one embodiment having a solid-state weld formed by friction welding is shown in Fig. 2, and one embodiment having a solid-state weld formed by linear friction welding is shown in Fig. 3. When friction welding or linear friction welding is used, a solid-state weld 6 is formed on the friction surfaces of the workpieces (2, 4), and when friction stir welding is used, the solid-state weld 6 is formed in the area where a friction stir welding tool has passed.

Friction stir welding includes the following four modes and combinations: (1) joining in which the ends of metal sheets are butted together to form a joint, and a rotating tool is moved while rotating along the longitudinal direction of the processed portion to join the metal sheets; (2) spot joining in which the ends of metal sheets are butted together to form a joint, and the rotating tool is rotated without moving at the joint to join the sheets; (3) spot joining in which metal sheets are overlapped at the joint, a rotating tool is inserted into the joint, and the rotating tool is rotated at that spot to join the metal sheets without moving; and (4) joining in which metal sheets are overlapped at the joint, a rotating tool is inserted into the joint, and the rotating tool is moved while rotating along the longitudinal direction of the joint to join the metal sheets.

In the solid-phase welded joint 1 of the present invention, at least one of the welded materials (2, 4) is a steel material for solid-phase welding made of the steel for solid-phase welding of the present invention. In addition, the structure of the solid-phase welded portion 6 has a lath-shaped martensite structure. Here, by making most of the region of the solid-phase welded portion 6 a lath-shaped martensite structure, extremely high strength can be obtained. On the other hand, it is difficult to achieve sufficient ductility with a general martensite structure, but as a result of the inventor of the present application intensively investigating the relationship between the martensite structure and the tensile properties, it has become clear that good ductility can be imparted by refining the lath-shaped martensite structure by adding Mn.

In the solid-phase welded portion 6, it is preferable that the average grain size of the prior austenite in the martensite structure is 20 μm or less. By refining the prior austenite structure, the lath-shaped martensite structure is refined, and high strength and ductility can be imparted to the solid-phase welded portion. Here, the average grain size of the prior austenite is more preferably 15 μm or less, and most preferably 5 μm or less. The method for determining the average grain size of the prior austenite is not particularly limited and various conventionally known methods can be used. For example, a structure that has been appropriately etched may be observed, or prior austenite may be reconstructed from martensite based on the orientation relationship between martensite and prior austenite, and the grain size may be determined from the reconstructed prior austenite structure.

It is preferable that the joined material (2, 4) made of the steel for solid-phase bonding of the present invention has a tensile strength of 1000 MPa or more and an elongation of 20% or more, and the solid-phase bonded portion 6 has a tensile strength of 1500 MPa or more and an elongation of 20% or more at room temperature. When the joined material (2, 4) and the solid-phase bonded portion 6 have these tensile properties, they can be fully used as a joint having mechanical properties equal to or greater than ultra-high tensile steel in applications requiring high strength and reliability. In addition, if the other joined material (2, 4) has a lower strength than the steel for solid-phase bonding of the present invention, the solid-phase bonded joint 1 will break from the area with the lowest strength, and the tensile strength and elongation will be values that largely reflect the mechanical properties of the fractured area.

The solid-state welded portion 6 of the solid-state welded joint 1 may be formed by friction welding or linear friction welding, but by using a friction stir welded portion, the solid-state welded portion 6 can be formed in any region of the steel plate, making it possible to manufacture large structures as well.

(3) Solid-state welded structure The solid-state welded structure of the present invention is characterized by having a solid-state welded joint 1. The material, shape, and size of the structural parts other than the solid-state welded joint 1 are not particularly limited, and may be any of various conventionally known structures.

As long as the effects of the present invention are not impaired, the solid-phase welded structure is not particularly limited, but examples include structural parts of transportation equipment such as automobiles, ships, and railway vehicles, various architectural structures, bridges, iron pipes, etc.

(4) Steel for solid-state bonding The steel for solid-state bonding of the present invention is made of the steel for solid-state bonding of the present invention, has a lath-shaped martensite structure, and is characterized in that the average grain size of prior austenite in the martensite structure is 20 μm or less. The steel for solid-state bonding having the composition and structure has a tensile strength of 1000 MPa or more and an elongation of 20% or more at room temperature, and can be fully used as a steel having mechanical properties equal to or greater than those of ultra-high tensile steel in applications requiring high strength and reliability.

The above describes representative embodiments of the present invention, but the present invention is not limited to these, and various design modifications are possible, all of which are within the technical scope of the present invention.

The following examples further explain the solid-state bonding steel, solid-state bonding steel material, solid-state bonded joints, and solid-state bonded structures of the present invention, but the present invention is not limited to these examples in any way.

Example 1
A steel ingot (φ35×20-25h) having the composition shown in Table 1 was produced by high-frequency melting, and the plate thickness was adjusted to 3 mm by hot rolling at 950° C., and then a soaking diffusion treatment was performed at 950° C. for 10 minutes to obtain a steel plate (Steel 1 for practical solid-state bonding). The values shown in Table 1 are in mass %.

Friction stir welding was performed on the obtained steel sheets using a cemented carbide tool (probe without a screw) with a shoulder diameter of 15 mm, probe diameter of 6 mm, and probe length of 2.9 mm under the following conditions: tool rotation speed: 400 rpm, joining speed: 150 mm/min, joining load: 2.5 tons, tool advance angle: 3°, and joining atmosphere: Ar.

Example 2
A steel plate (Steel 2 for practical solid-state bonding) was obtained in the same manner as in Example 1, except that the composition of Example 2 shown in Table 1 was used. Friction stir welding was also performed in the same manner as in Example 1.

Example 3
A steel plate (Steel 3 for practical solid-state bonding) was obtained in the same manner as in Example 1, except that the composition of Example 3 shown in Table 1 was used. Friction stir welding was also performed in the same manner as in Example 1.

Example 4
A steel plate (Steel 4 for practical solid-state bonding) was obtained in the same manner as in Example 1, except that the composition of Example 4 shown in Table 1 was used. Friction stir welding was also performed in the same manner as in Example 1.

Comparative Example 1
A steel plate (comparative solid-state bonding steel 1) was obtained in the same manner as in Example 1, except that the composition of Comparative Example 1 shown in Table 1 was used. Friction stir welding was also performed in the same manner as in Example 1.

Comparative Example 2
A steel plate (comparative solid-state bonding steel 2) was obtained in the same manner as in Example 1, except that the composition of Comparative Example 2 shown in Table 1 was used. Friction stir welding was also performed in the same manner as in Example 1.

[Evaluation test]
(1) Structure observation: The region including the stir zone was cut out perpendicular to the friction stir welding direction, and the cross section was polished and corroded (4% nital), after which the structure was observed using an optical microscope. For polishing, emery paper (#600 to #3000) and diamond paste (grain sizes 3 μm and 1 μm) were used. Samples for base material observation were also prepared in the same manner.

(2) Tensile test The tensile properties of the steel material (base material) and the stir zone obtained in the above examples and comparative examples were evaluated. For the steel material, the test piece shown in FIG. 4 was prepared, and for the stir zone, the test piece of the size shown in FIG. 5 was prepared. The test pieces were cut out using an electric discharge machine, and the tensile axis was perpendicular to the joining direction. Using a tensile tester (SHIMADZU Autograph AGS-X 10kN), the tensile strength of the joint was measured at a crosshead speed of 0.06mm/min.

(3) EBSD measurement In order to confirm the size of the structure in the stir zone of the friction stir welded joint in detail, a crystal orientation map image was obtained by EBSD measurement. The amount of retained austenite in the base material was also evaluated. For the EBSD measurement, an FE-SEM (JSM-7001FA manufactured by JEOL Ltd.) and OIM data Collection ver. 5.31 manufactured by TSL were used.

Figure 6 shows the structure photographs of the practical solid-state bonding steels 1 to 3 and the comparative solid-state bonding steels 1 and 2. When no Mn is contained, the structure is made up of ferrite and pearlite, when the Mn content is 1 mass%, the structure is made up of ferrite and bainite, and when the Mn content is 2 mass% or more, the structure is made up of martensite.

The tensile properties of practical solid-state bonding steels 1 to 4 and comparative solid-state bonding steels 1 and 2 are shown in Figure 7. The tensile strength increases with increasing Mn content, and in particular, when the Mn content is 2 mass% or more, the value is 1000 MPa or more. They also have good ductility, and even practical solid-state bonding steel 3 (Mn content: 4 mass%) and practical solid-state bonding steel 4 (Mn content: 5 mass%), which have extremely high tensile strengths of approximately 1400 MPa, show elongation of 20% or more. A photograph of the fracture surface of practical solid-state bonding steel 3 is shown in Figure 8, and dimples due to ductile fracture can be observed.

Figure 9 shows cross-sectional macro photographs of the joints of the experimental solid-state bonding steels 1 to 3 and the comparative solid-state bonding steels 1 and 2. In all cases, a defect-free stir zone was obtained, demonstrating that good joints can be obtained by friction stir welding.

Figure 10 shows the structure photographs of the stir zones of the practical solid-state bonding steels 1 to 3 and the comparative solid-state bonding steels 1 and 2. When no Mn is contained, the structure is made up of ferrite and bainite, when the Mn content is 1 mass%, the structure is made up of ferrite and martensite, and when the Mn content is 2 mass% or more, the structure is made up of martensite.

Figure 11 shows the tensile properties of the stir zone of practical solid-state bonding steels 1 to 4 and comparative solid-state bonding steels 1 and 2. The tensile strength increases with increasing Mn content, and in particular, when the Mn content is 2 mass% or more, the value is 1500 MPa or more. In addition, it has good ductility, and even practical solid-state bonding steel 2 (Mn content: 3 mass%), practical solid-state bonding steel 3 (Mn content: 4 mass%), and practical solid-state bonding steel 4 (Mn content: 5 mass%), which have extremely high tensile strengths of about 1700 MPa, show elongation of 25% or more. Figure 12 shows a photograph of the fracture surface of the stir zone of practical solid-state bonding steel 3, and dimples due to ductile fracture are observed.

Figure 13 shows crystal orientation map images of the stir zones of the practical solid-state bonding steels 1 to 3 and the comparative solid-state bonding steels 1 and 2. The martensite structure becomes finer with an increase in Mn content, and this refinement of the martensite structure is considered to be the main reason why the stir zones exhibit good ductility.

Figure 14 shows phase maps of the base materials of practical solid-state bonding steels 1-3 and comparative solid-state bonding steels 1 and 2. Retained austenite is present when the Si content is 2 mass% and the Mn content is 2 mass% or more, and the amount is particularly high (7.6%) at 2 mass%. As shown in Figure 7, practical solid-state bonding steel 1 with a Mn content of 2 mass% has high tensile strength (1000 MPa) but also elongation of 35% or more, which suggests the possibility of the TRIP effect being manifested.

The tensile properties of the base material and stir zone of the practical solid-state bonding steels 1 to 4 and the comparative solid-state bonding steels 1 and 2 are summarized in Figure 15. In Figure 15, the vertical axis is elongation and the horizontal axis is tensile strength, and when the Mn content is 2 mass% or more, both the base material and the stir zone are plotted in the upper right region, showing an excellent strength-ductility balance. Note that when the Mn content is 1 mass%, the strength of the base material does not increase sufficiently.

Figure 16 shows the results of reconstructing prior austenite from martensite based on the orientation relationship between martensite and prior austenite using the results of EBSD measurement (scale bar in the figure is 15 μm). The average grain size of prior austenite in practical solid-state bonding steel 1, practical solid-state bonding steel 2, and practical solid-state bonding steel 3 is 15.1 μm, 11.7 μm, and 11.9 μm, respectively. In all cases, the prior austenite grains are fine, and while they become finer with increasing Mn addition, this effect is thought to saturate at an Mn addition of about 3 to 4 mass%.

1...Solid-state welded joint,
2, 4 ... material to be joined,
6...Solid-state joint.

Claims (4)

At least one of the workpieces is a steel material for solid-state bonding made of a steel for solid-state bonding,
The steel for solid-state bonding has a steel composition, in mass%,
C: 0.20 to 0.45%,
Si: 1.00 to 3.00%,
Mn: 2.00 to 5.00%;
The balance is composed of only Fe and inevitable impurities,
The solid-state welded portion of the steel material for solid-state bonding has a lath-shaped martensite structure,
The tensile strength of the steel material for solid-phase bonding at room temperature is 1000 MPa or more and the elongation is 20% or more,
The tensile strength of the solid-phase bonded joint at room temperature is 1500 MPa or more and the elongation is 20% or more;
A solid -state welded joint characterized by: The average grain size of prior austenite in the martensite structure is 20 μm or less.
The solid-state welded joint according to claim 1 , The solid-state welded portion is a friction stir welded portion;
3. A solid-state welded joint according to claim 1 or 2, characterized in that Having the solid-state welded joint according to claim 1 or 2 ;
A solid-phase bonded structure characterized by: