alloy steels according to the present invention and an sus 630 ( aisi standard type 630 ) stainless steel as a comparative example , the contents of component elements of which are shown in table 1 , were separately prepared . these steels were separately melted in an inert gas atmosphere , poured into a mold and solidified to form a martensitic texture , thereby producing golf club heads according to examples 1 to 17 and comparative examples 1 and 2 , which are shown in table 2 . the tensile strength , elongation , hardness and the like of these steels were controlled by changing the conditions of a heat treatment . the mechanical properties of the alloy steels according to the present invention and the sus 630 ( aisi standard type 630 ) stainless steel as a comparative example are shown in table 3 . in this case , examples 1 to 16 are embodiments of metal wood heads , and example 17 is an embodiment of an iron head (# 5 ). the metal wood heads of examples 15 and 16 are such that their faces are formed of their corresponding alloy steels , and other portions thereof are formed of the sus 630 ( aisi standard type 630 ) stainless steel . the heads according to examples 1 to 17 and comparative examples 1 and 2 were separately manufactured with the thicknesses of their faces , and the weights and volumes thereof varied . their actual - hit durability was evaluated by separately setting golf clubs obtained by attaching a shaft and a grip to the heads in a swing robot and repeatedly hitting a golf ball . the results are shown in table 2 . the durability was ranked as aa in case of &# 34 ; very good &# 34 ;, a in case of &# 34 ; good &# 34 ;, b in case of &# 34 ; somewhat poor &# 34 ;, or c in case of &# 34 ; poor &# 34 ;. in table 3 , the specific gravities of the alloy steels according to the present invention and the sus 630 ( aisi standard type 630 ) stainless steel of the comparative example are 7 . 9 - 8 . 1 , and 7 . 8 , respectively . therefore , these steels can be considered to have substantially the same specific gravity . on the other hand , the tensile strength of the sus 630 ( aisi standard type 630 ) stainless steel is 119 . 1 - 126 . 3 kgf / mm 2 , while those of the alloy steels according to the present invention are within a range of 121 . 5 - 182 . 4 kgf / mm 2 . it was therefore revealed that the alloy steels have a strength equal to or higher than the sus 630 ( aisi standard type 630 ) stainless steel . further , with respect to the elongation , the sus 630 ( aisi standard type 630 ) stainless steel is within a range of 5 . 3 - 8 . 0 % for the tensile strength of 119 . 1 kgf / mm 2 or higher , while the alloy steels of the present invention having tensile strengths of 121 . 5 kgf / mm 2 and 182 . 4 kgf / mm 2 , respectively , are 20 . 9 % and 9 . 2 %, respectively . accordingly , the alloy steels according to the present invention are higher in strength and better stretched compared with the sus 630 ( aisi standard type 630 ) stainless steel , and so they can be sufficiently thinned without impairing the durability . furthermore , with respect to the reduction of area , the sus 630 ( aisi standard type 630 ) stainless steel is within a range of 12 . 0 - 17 . 9 % for the tensile strength ranging from 119 . 2 to 126 . 3 kgf / mm 2 , while the alloy steels according to the present invention are within a range of 20 . 4 - 51 . 3 % for the tensile strength ranging from 121 . 5 to 182 . 4 kgf / mm 2 . therefore , the reduction of area is also improved . accordingly , in examples 1 and 7 shown in table 2 , the heads were able to be thinned in face thickness , and reduced in weight for the same volume as the head of comparative example 1 . in addition , very good actual - hit durability was obtained . in this case , it is considered that the reduction in the weights of the heads permits easy swinging . in addition , when examples 4 , 9 , 10 - 14 and 16 were compared with comparative example 2 , remarkable differences arose in actual - hit durability between them even if they were equal to each other in face thickness , head weight and head volume . this means that the strength of the faces in particular was improved . in the case of examples 4 , 9 , 10 - 14 and 16 , the heads were increased in volume to a significant extent compared with the head of comparative example 1 , and the actual - hit durability was also improved . in this case , the widening of sweet spot is realized . furthermore , in example 5 , the head was made thinner in face thickness and greater in volume than both heads of comparative examples 1 and 2 . even in this case , good actual - hit durability was obtained . table 1__________________________________________________________________________ sus 630 ( aisiinventive alloy steel ( wt . %) standard type 630 ) no . 1 no . 2 no . 3 no . 4 no . 5 no . 6 no . 7 no . 8 ( wt . %) __________________________________________________________________________c 0 . 02 0 . 03 0 . 03 0 . 04 0 . 007 0 . 005 0 . 006 0 . 003 0 . 04si 0 . 10 0 . 08 0 . 08 0 . 10 0 . 08 0 . 08 0 . 10 0 . 10 0 . 60mn 0 . 10 0 . 10 0 . 08 0 . 10 0 . 04 0 . 03 0 . 04 0 . 04 -- ni 4 . 00 4 . 20 4 . 30 4 . 20 4 . 50 6 . 50 3 . 10 3 . 20 4 . 00cr 15 . 00 14 . 70 14 . 70 14 . 50 10 . 00 10 . 00 12 . 00 17 . 30 16 . 50mo 4 . 00 4 . 50 4 . 50 4 . 60 3 . 80 4 . 50 7 . 40 3 . 60 -- co 15 . 00 13 . 20 14 . 20 15 . 00 16 . 40 16 . 60 16 . 30 11 . 20 -- al -- 0 . 05 0 . 06 -- -- -- -- -- -- ti -- -- 0 . 06 -- -- -- -- -- -- zr -- -- -- 0 . 03 -- -- -- -- -- cu -- -- -- -- -- -- -- -- 4 . 00nb -- -- -- -- -- -- -- -- 0 . 27fe balance balance balance balance balance balance balance balance balance__________________________________________________________________________ table 2______________________________________alloy thickness of weight volume actual - hitsteel face ( mm ) ( g ) ( cc ) durability______________________________________ex . 1 no . 1 2 . 3 192 185 aaex . 2 no . 1 2 . 5 196 185 aaex . 3 no . 1 2 . 7 200 185 aaex . 4 no . 1 2 . 5 200 250 aaex . 5 no . 1 2 . 3 200 260 aex . 6 no . 1 2 . 5 200 250 aex . 7 no . 2 2 . 3 193 185 aaex . 8 no . 2 2 . 5 200 250 aex . 9 no . 3 2 . 5 200 250 aaex . 10 no . 4 2 . 5 200 250 aaex . 11 no . 5 2 . 5 200 250 aaex . 12 no . 6 2 . 5 200 250 aaex . 13 no . 7 2 . 5 200 250 aaex . 14 no . 8 2 . 5 200 250 aaex . 15 no . 2 2 . 3 200 250 aex . 16 no . 2 2 . 5 200 250 aaex . 17 no . 2 2 . 2 265 -- acomp -- 2 . 5 195 185 bex . 1comp . -- 2 . 5 200 250 cex . 2______________________________________ table 3__________________________________________________________________________ tensile strength elongation reduction specific hardness heat treatment conditions ( kgf / mm . sup . 2 ) ( ksi ) (%) of area (%) gravity ( hrc ) __________________________________________________________________________inventivealloysteelno . 1 1040 ° c . × 1 . 5 h + 540 ° c . × 6 180 . 0 256 . 0 10 . 2 25 . 0 7 . 9 50no . 1 1040 ° c . × 1 . 5 h + 510 ° c . × 1 121 . 5 172 . 8 20 . 9 51 . 3 7 . 9 38no . 1 1040 ° c . × 1 . 5 h + 540 ° c . × 2 165 . 0 234 . 7 13 . 0 31 . 4 7 . 9 48no . 2 1040 ° c . × 1 . 5 h + 540 ° c . × 6 180 . 2 256 . 3 10 . 4 26 . 4 7 . 9 50no . 2 1040 ° c . × 1 . 5 h + 480 ° c . × 3 153 . 7 218 . 6 15 . 1 38 . 5 7 . 9 46no . 2 1040 ° c . × 1 . 5 h + 480 ° c . × 1 . 5 130 . 0 184 . 9 19 . 3 47 . 2 7 . 9 40no . 3 1040 ° c . × 1 . 5 h + 540 ° c . × 6 182 . 4 259 . 5 9 . 2 20 . 4 7 . 9 51no . 4 1040 ° c . × 1 . 5 h + 540 ° c . × 6 182 . 1 259 . 0 9 . 8 20 . 6 7 . 9 51no . 5 1040 ° c . × 1 . 5 h + 540 ° c . × 6 181 . 5 258 . 2 11 . 0 24 . 3 8 . 0 50no . 6 1040 ° c . × 1 . 5 h + 540 ° c . × 6 178 . 2 253 . 5 13 . 0 25 . 4 8 . 1 49no . 7 1040 ° c . × 1 . 5 h + 540 ° c . × 6 180 . 2 256 . 3 11 . 5 24 . 0 8 . 1 50no . 8 1040 ° c . × 1 . 5 h + 540 ° c . × 6 179 . 2 254 . 9 12 . 0 25 . 1 7 . 9 50sus 630 ( aisi 1040 ° c . × 1 . 5 h + 540 ° c . × 4 119 . 1 169 . 4 8 . 0 17 . 9 7 . 8 38standard 1040 ° c . × 1 . 5 h + 480 ° c . × 1 126 . 3 179 . 7 5 . 3 12 . 0 7 . 8 39type 630 ) __________________________________________________________________________ the golf club heads according to the present invention are made of the alloy steels each having a specific gravity almost equal to that of a stainless steel such as sus 630 ( aisi standard type 630 ) or sus 431 ( aisi standard type 431 ) and strength higher than such a stainless steel . for example , in the case of metal wood heads having the same weight , therefore , the thickness of the head according to the present invention can be thinned compared with a head made of the conventional stainless steel to increase its volume or make a portion near a sole of the head heavier than other portions . as a result , the sweet spot of the head can be widened while keeping strength required of the head to ensure that the direction of a hit ball is made stable . incidentally , if the volume of the head is adjusted to the same volume as a head made of the conventional stainless steel , the weight of the head becomes reduced , thereby facilitating swinging . even in the case of an iron head , the thickness of its face can be similarly thinned to make its peripheral portion heavier than other portions . upon the manufacturing of the above - described golf club heads by a precision casting process , the alloy steels can be melted in an inert gas atmosphere . therefore , such heads can be mass - produced with ease .
a golf club head is formed of an alloy steel which comprises at most 0 . 2 wt . % of c , 0 . 05 - 1 . 0 wt . % of si , at most 0 . 5 wt . % of mn , 3 . 0 - 8 . 0 wt . % of ni , 10 . 0 - 20 . 0 wt . % of cr , 3 . 0 - 8 . 0 wt . % of mo and 10 . 0 - 20 . 0 wt . % of co , the remainder being substantially fe , and has a metallic texture composed principally of martensite .