Method of production of brazed joint and such a brazed joint

A method of production of a brazed joint comprising:

• a heating step of heating a sheet combination having a plurality of steel sheets superposed so that the sheet surfaces face each other and a second brazing filler metal placed in a predetermined region between the surfaces of two mutually adjoining steel sheets among the plurality of steel sheets by a heating temperature of an Ac3 point of the steel sheets or more, and
• a hot stamping step of quenching and shaping the sheet combination heated by the heating step,
• wherein the second brazing filler metal has a solidus temperature over the heating temperature in the heating step and less than the solidus temperature of the steel sheets,
• wherein before the start of quenching in the hot stamping step, the Ar3 point in the region near the brazing filler metal at the steel sheets is higher than the Ar3 point of the steel sheets, and
• wherein the quenching start temperature of the sheet combination at the hot stamping step is made a temperature of the Ar3 point of the steel sheet or less, and
• said method further comprising a brazing step of heating the sheet combination to a temperature over a liquidus temperature of the second brazing filler metal and less than the solidus temperature of the steel sheets before the heating step, then cooling down to less than the solidus temperature of the second brazing filler metal to braze the steel sheets.

A brazed joint comprising:

• a plurality of steel sheets overlaid so that their surfaces face each other, and
• a brazing filler metal placed in a predetermined region between the surfaces of two mutually adjoining steel sheets in the plurality of steel sheets,
• the two steel sheets being joined by the brazing filler metal,
• in which brazed joint, a relationship between an average hardness of positions near the brazing filler metal in the steel sheets contacting the brazing filler metal and an average hardness of representative positions of a base metal region in the steel sheets satisfies the following (A):
• Average hardness of positions near brazing filler metal (HV)+50 HV<average hardness of representative positions of base metal region (HV) (A)
• wherein, in (A), the positions near the brazing filler metal are positions separated by exactly 10 μm from the positions of interfaces between the two steel sheets contacting the brazing filler metal and the brazing filler metal to the steel sheet sides along the direction of thickness of the steel sheets,
• the representative positions of the base metal region are positions at the two steel sheets contacting the brazing filler metal and separated by lengths of exactly ¼ of the thicknesses of the steel sheets from the surfaces of sides of the steel sheets not contacting the brazing filler metal in the two surfaces of the sheets to the steel sheet sides along the direction of thickness of the steel sheets,
• the average hardness of positions near the brazing filler metal is an arithmetic average of Vicker's hardnesses at three positions near the brazing filler metal,
• the average hardness of representative positions of the base metal region is an arithmetic average of Vicker's hardnesses at representative positions of the base metal region, and
• the intervals of the three locations have lengths of 3 times the arithmetic average of the lengths of two diagonals of an indentation first formed for measurement of the Vicker's hardnesses.