Friction stir spot welding method and friction stir spot welding device

1. A method of performing friction stir spot welding of a plurality of steel plates, the method comprising the steps of:

friction stirring a spot welding portion of the plurality of steel plates by pressing a tool against the spot welding portion while rotating the tool, to plasticize the spot welding portion by friction heat;

cooling the spot welding portion to cause martensitic transformation to occur in the spot welding portion, after the step of friction stirring the spot welding portion; and

tempering the spot welding portion by the friction heat by re-pressing the tool against the spot welding portion while rotating the tool, after the step of cooling the spot welding portion.

2. The method of performing friction stir spot welding according to claim 1 , wherein in the step of cooling the spot welding portion, the spot welding portion is cooled at a cooling rate which is equal to or higher than an upper critical cooling rate of the plurality of steel plates.

3. The method of performing friction stir spot welding according to claim 1 , wherein in the step of cooling the spot welding portion, a temperature of the spot welding portion is decreased to a temperature which is equal to or lower than a martensitic transformation completion temperature of the plurality of steel plates.

4. The method of performing friction stir spot welding according to claim 1 ,

wherein the step of cooling the spot welding portion includes pulling up the tool to form a space between the tool and the spot welding portion, and

wherein the tool is pulled up so that a pull-up amount of the tool is less than a pin hole depth.

5. The method of performing friction stir spot welding according to claim 1 , wherein in the step of tempering the spot welding portion, a temperature of the spot welding portion is increased up to 550 to 650 degrees C. and then decreased.

6. The method of performing friction stir spot welding according to claim 5 , wherein in the step of tempering the spot welding portion, a rotational speed of the tool is set to be higher than a rotational speed of the tool in the step of friction stirring the spot welding portion.

7. The method of performing friction stir spot welding according to claim 1 , wherein the plurality of steel plates have a carbon content of 0.06% or more, or a tensile strength of 590 MPa or higher.

8. A device that performs friction stir spot welding of a plurality of steel plates, the device comprising:

a displacement driving unit that displaces a spot welding portion of the plurality of steel plates and a tool relative to each other;

a rotation driving unit that rotates the tool; and

a control unit including a processor and a memory, the memory storing an operation program causing the processor to:

perform a friction stirring step, in which the control unit causes the displacement driving unit and the rotation driving unit to operate to press the tool against the spot welding portion while rotating the tool to plasticize the spot welding portion of the plurality of steel plates by friction heat,

perform a cooling step that cools the spot welding portion, in which the control unit causes at least one of the displacement driving unit and the rotation driving unit to pull up the tool or reduce a rotational speed of the tool, and

perform a tempering step that tempers the spot welding portion, in which the control unit causes the displacement driving unit and the rotation driving unit to re-press the tool against the spot welding portion while simultaneously rotating the tool.