Galvannealed steel sheet with distinguished anti-powdering and anti-flaking properties and process for producing the same

A galvannealed steel sheet with distinguished anti-powdering property and anti-flaking property, which comprises

• a matrix steel sheet;
• a double plating layer composed of the upper plating layer and the lower plating layer,
• the lower plating layer, which is a galvannealed coating layer having a coating weight of 45 to 90 g/m.sup.2, composed of 8 to 12 wt. % Fe and 0.05 to 0.35 wt. % Al, the balance being Zn, provided on at least one side of the matrix steel sheet,
• the surface region of the galvannealed coating layer being substantially composed of a .delta..sub.1 phase or the surface region thereof having a structure that when at least one of .eta. and .zeta. phases exist in the surface region of the galvannealed coating layer, the total amount of the at least one of .eta. and .zeta. phases in the galvannealed coating layer is not more than 1.5 g/m.sup.2 and
• the upper plating layer, which is an alloy plating layer having a coating weight of 1 to 10 g/m.sup.2, composed of not less than 60 wt. % Fe, the balance being Zn, provided on the surface of at least one of the galvannealed coating layer;
• and
• a .GAMMA. phase having a thickness of not more than 1.3 .mu.m, formed in the region at the boundary to the matrix steel sheet.

A galvannealed steel sheet with distinguished anti-powdering property and anti-flaking property, which comprises

• a matrix steel sheet;
• a galvannealed coating layer having a coating weight of 45 to 90 g/m.sup.2, composed of 8 to 12 wt. % Fe and 0.05 to 0.35 wt. % Al, the balance being Zn, provided on at least one side of the matrix steel sheet,
• the surface region of the galvannealed coating layer being substantially composed of a .delta..sub.1 phase or the surface region thereof having a structure that when at least one of .eta. and .zeta. phases exist in the surface region of the galvannealed coating layer, the total amount of the at least one of .eta. and .zeta. phases in the galvannealed coating layer is not more than 1.5 g/m.sup.2 ;
• and
• a .GAMMA. phase having a thickness of not more than 1.3 .mu.m, formed in the region at the boundary to the matrix steel sheet therebetween;
• and
• a lubricating layer provided on the surface of the galvannealed coating layer.

A galvannealed steel sheet with distinguished anti-powering property and anti-flaking property, which comprises

• a matrix steel sheet;
• a double plating layer composed of the upper plating layer and the lower plating layer,
• the lower plating layer, which is a galvannealed coating layer having a coating weight of 45 to 90 g/m.sub.2, composed of 8 to 12 wt. % Fe and 0.05 to 0.35 wt. % Al, the balance being Zn, provided on at least one side of the matrix steel sheet,
• the surface region of the galvannealed coating layer being substantially composed of a .delta..sub.1 phase or the surface region thereof having a structure that when at least one of .eta. and .zeta. phases exist in the surface region of the galvannealed coating layer, the total amount of the at least one of .eta. and .zeta. phases in the galvannealed coating layer is not more than 1.5 9/m.sup.2, and
• the upper plating layer, which is an alloy plating layer having a coating weight of 1 to 10 g/m.sup.2, composed of not less than 60 wt. % Fe, the balance being Zn, provided on the surface of at least one of the galvannealed coating layer;
• a .GAMMA. phase having a thickness of not more than 1.3 .mu.m, formed in the region at the boundary to the matrix steel sheet;
• and
• a lubricating layer provided on the surface of the upper plating layer.

A process for producing a galvannealed steel sheet with distinguished anti-powdering property and anti-flaking property, which comprises

• plating a matrix steel sheet in a hot dip zinc plating bath containing 0.04 to 0.12 wt. % Al, the balance being Zn and inevitable impurities, then
• controlling a coating weight of the plating layer and
• heating the plated steel sheet in an alloying furnace, thereby conducting a heating-alloying treatment of the plating layer,
• where after the coating weight control the plated steel sheet is subjected to rapid heating within a time of not more than 2.0 seconds until the plated steel sheet reaches a temperature of at least 470.degree. C. in the heating step in the alloying furnace and then
• subjected to quenching within a time of not more than 2 seconds until the plated steel sheet reaches a temperature of not more than 420.degree. C. in a cooling step successive to the completion of alloying.

A process for producing a galvannealed steel sheet with distinguished anti-powdering property and anti-flaking property, which comprises the following steps:

• (1) a first step for producing a galvannealed coating layer on the surface of at least one side of a steel sheet by
• plating a matrix steel sheet in a hot dip zinc plating bath containing 0.04 to 0.12 wt. % Al, the balance being Zn and inevitable impurities, then
• controlling a coating weight of the plating layer and
• heating the plated steel sheet in an alloying furnace, thereby conducting a heating-alloying treatment of the plating layer,
• where after the coating weight control, the plated steel sheet is subjected to rapid heating within a time of not more than 2.0 seconds until the plated steel sheet reaches a temperature of at least 470.degree. C. in the heating step in the alloying furnace and then
• subjected to quenching within a time of not more than 2 seconds until the plated steel sheet reaches a temperature of not more than 420.degree. C. in a cooling step successive to the completion of alloying;
• and then
• (2) a second step for applying an electroplating layer having a coating weight of 1 to 10 g/m.sup.2, composed of not less than 60 wt. % Fe, the balance being zinc, on the surface of at least one of the galvannealed coating layer.

A process for producing a galvannealed steel sheet with distinguished anti-powdering property and anti-flaking property, which comprises the following steps:

• (1) a first step of
• plating a matrix steel sheet in a hot dip zinc plating bath containing 0.04 to 0.12 wt. % Al, the balance being Zn and inevitable impurities, then
• controlling a coating weight of the plating layer and
• heating the plated steel sheet in an alloying furnace, thereby conducting a heating-alloying treatment of the plating layer,
• where after the coating weight control, the plated steel sheet is subjected to rapid heating within a time of not more than 2.0 seconds until the plated steel sheet reaches a temperature of at least 470.degree. C. in the heating step in the alloying furnace and then
• subjected to quenching within a time of not more than 2 seconds until the plated steel sheet reaches a temperature of not more than 420.degree. C. in a cooling step successive to the completion of alloying;
• and then
• (2) a second step of applying a lubricant to the surface of at least one of the alloyed, hot dip plating layer.

A process for producing a galvannealed steel sheet with distinguished anti-powdering property and anti-flaking property, which comprises the following steps:

• (1) a first step for producing a galvannealed coating layer on the surface of at least one side of a steel sheet by
• plating a matrix steel sheet in a hot dip zinc plating bath containing 0.04 to 0.12 wt. % Al, the balance being Zn and inevitable impurities, then
• controlling a coating weight of the plating layer and
• heating the plated steel sheet in an alloying furnace, thereby conducting a heating-alloying treatment of the plating layer,
• where after the unit coating control, the plated steel sheet is subjected to rapid heating within a time of not more than 2.0 seconds until the plated steel sheet reaches a temperature of at least 470.degree. C. in the heating step in the alloying furnace and then
• subjected to quenching within a time of not more than 2 seconds until the plated steel sheet reaches a temperature of not more than 420.degree. C. in a cooling step successive to the completion of alloying;
• (2) a second step for applying an electroplating layer having a coating weight of 1 to 10 g/m.sup.2, composed of not less than 60 wt. % Fe, the balance being zinc, on the surface of at least one of the galvannealed coating layer;
• and
• (3) a third step of applying a lubricant to the surface of at least one of the electroplating layer.