Steel products excellent in machinability and machined steel parts

The present invention is directed to steel products which exhibit excellent machinability and are suitable for steel stocks of structural steel parts for a variety of machinery, such as transportation machinery including automobiles, machinery for industrial use, construction machinery, and the like; as well as to a variety of machined structual steel parts for machinery such as crankshafts, connecting rods, gears, and the like. The steel products of the present invention are endowed with excellent machinability and have the following composition based on % by weight: C: 0.05% to 0.6%; S: 0.002% to 0.2%; Ti: 0.04% to 1.0%; N: 0.008% or less; Nd: 0% to 0.1%; Se: 0% to 0.5%; Te: 0% to 0.05%; Ca: 0% to 0.01%; Pb: 0% to 0.5%; and Bi: 0% to 0.4%; wherein the maximum diameter of titanium carbosulfide contained in the steel is not greater than 10 .mu.m, and its amount expressed in the index of cleanliness of the steel is equal to or more than 0.05%. The machined parts, according to the present inventions are manufactured by subjecting the steel products of the invention to a machining process, and are useful as structural steel parts for a variety of machinery, such as transportation machinery including automobiles, machinery for industrial use, construction machinery, and the like.

TECHNICAL FIELD 
The entire disclosure of the International application No. PCT/JP97/04297, 
filed on Nov. 25, 1997 including specification, claims and summary, are 
incorporated herein by reference in its entirety. 
The present invention relates to steel products which exhibit excellent 
machinability, as well as to machined steel parts. More particularly the 
invention relates to steel products which exhibit excellent machinability 
and are suitable for steel stocks of structural steel parts for a variety 
of machinery such as transportation machinery including automobiles, 
machinery for industrial use, construction machinery, and the like, and to 
a variety of machined structural steel parts for machinery, such as 
crankshafts, connecting rods, gears, and the like. 
TECHNICAL BACKGROUND 
In conventional manufacture of structural steel parts for a variety of 
machinery such as transportation machinery, machinery for industrial use, 
construction machinery, and the like, such steel parts are generally 
either (a) formed roughly into predetermined shapes through hot working, 
then formed into desired shapes through machining, followed by thermal 
refining through quenching and tempering, or (b) subjected to hot workings 
and then quenching and tempering, followed by machining. 
However, as structural parts for machinery have been improved to be of high 
strength, the cost for machining has been increased accordingly. 
Therefore, for ease of machining and for lowering the costs there is an 
increased demand for free cutting steel having excellent machinability. 
It is well known that the machinability of steel is improved through 
addition of free-cutting elements (machinability-improving elements) such 
as Pb, Te, Bi, Ca and S, singly or in combination. For this reason, in 
order eggs to improve machinability of steels such as steels for machine 
structural use, there has been employed the method of incorporating the 
above free-cutting elements into the steels. However, when the 
free-cutting elements are merely incorporated into steels for machine 
structural use and the like, in many cases the desired mechanical 
properties (for example, toughness and fatigue strength) cannot be 
secured. 
Under these circumstances, a technique comprising hot working and then 
machining, followed by quenching and tempering, as described in (a) above 
is disclosed in Patent Application Laid-open (Kokai) Nos. 2-111842 and 
6-279849. This technique involves "hot rolled steel products endowed with 
excellent machinability and hardenability" in which C is present in steel 
as graphite and the machinability of the steel is improved through 
utilization of the notch effect and lubrication effect of graphite; as 
well as the "method of manufacturing steels for machine structural use 
with excellent machinability." 
However, in the steel products disclosed in Patent Application Laid-open 
(Kokai) No. 2-111842, it is essential that B be incorporated into the 
steel so that boron nitride particles (BN) serve as nuclei for 
precipitation, to thereby facilitate graphitization, and thus the steel 
becomes susceptible to cracks when solidified. In contrast, in the method 
disclosed in Patent Application Laid-Open (Kokai) No. 6-279849, 
graphitization in steel is accelerated under the as-hot-rolled condition, 
through addition of Al and through limitation of O (oxygen) content in 
steel to a low level. This method requires more than five hours for 
treatment of graphitization after hot rolling, and thus is not very 
economical. 
In contrast, a technique comprising hot working, and then quenching and 
tempering, followed by machining, as described in (b) above is disclosed, 
for example, in Patent Application Laid-open (Kokai) No. 6-212347. This 
involves "hot forged steel products having high fatigue strength and a 
method of manufacturing the same" in which steel having a specific 
chemical structure is quenched immediately after hot forging, followed by 
tempering, to thereby precipitate TiC. However, in the hot forged steel 
products obtained by this method, the ratio of N to Ti (N/Ti) is merely 
specified as less than 0.1, and therefore excellent machinability cannot 
always be secured. Briefly, if the content of N in steel containing 0.01 
to 0.20 wt. % of Ti is merely specified such that N/Ti is less than 0.1, 
hard TiN may often be formed in a great amount, causing degradation of 
machinability, and further causing degradation of toughness. 
In TETSU-TO-HAGANE (vol. 57 (1971) S484), it is reported that machinability 
may be improved through incorporation of Ti into deoxidation-adjusted 
free-cutting steel. However, this publication also describes that 
incorporation of a great amount of Ti produces a great amount of TiN, 
resulting in incresed wear of tools and disadvantages in terms of 
machinability. For example, the life of the drill to a steel having the 
following composition based on % by weight, C: 0.45%; Si: 0.29%; Mn: 
0.78%; P: 0.017%; S: 0.041%; Al: 0.006%; N: 0.0087%; Ti: 0.228%; O: 
0.004%; and Ca: 0.001%, is adversely short, and therefore, machinability 
of above-mentioned steel is poor. Consequently, it is concluded that 
machinability of steel is not improved through simple addition of Ti. 
SUMMARY OF THE INVENTION 
In view of the, foregoing, an object of the present invention is to provide 
steel products which have excellent machinability and thus are suitable 
for steel stocks of structural steel parts for a variety of machinery such 
as transportation machinery including automobiles, machinery for 
industrial use, construction machinery, and the like, and to provide a 
variety of machined structural steel parts for machinery, such as 
crankshafts, connecting rods, gears, and the like. 
The gist of the present invention will be summarized below. 
(I) A steel product which exhibits excellent machinability and which has 
the following chemical composition based on % by weights C: 0.05% to 0.6%; 
S: 0.002% to 0.2%; Ti: 0.04% to 1.0%; N: 0.008% or less; Nd: 0% to 0.1%; 
Se: 0% to 0.5%; Te: 0% to 0.05%; Ca: 0% to 0.01%; Pb: 0% to 0.5%; and Bi: 
0% to 0.4%; wherein the maximum diameter of titanium carbosulfide 
contained in the steel is not greater than 10 .mu.m and its amount 
expressed in the index of cleanliness of the steel is equal to or more 
than 0.05%. 
(II) A non-heat-treated type steel product, according to (I) above, which 
has the following chemical composition based on % by weights C: 0.2% to 
0.6%; Si: 0.05% to 1.5%; Mn: 0.01% to 2.0%; P: 0.07% or less; S: 0.01% to 
0.2%; Al: 0.002% to 0.05%; Cu: 0% to 1.0%; Ni: 0% to 2.0%; Cr: 0% to 2.0%; 
Mo: 0% to 0.5%; V: 0% to 0.3%; Nb: 0% to 0.1%; and the balance: Fe and 
unavoidable impurities, wherein at least 90% of the microstructure of the 
steel is constituted by ferrite and pearlite. 
(III) A non-heat-treated type steel product, according to (I) above, which 
has the following chemical composition based on % by weight, C: 0.05% to 
0.3%; Si: 0.05% to 15%; Al: 0.002% to 0.05%; Cu: 0% to 1.0%; Mo: 0% to 
0.5%; V: 0% to 0.30%; Nb: 0% to 0.1%; B: 0% to 0.02%; and the balance: Fe 
and unavoidable impurities, wherein fn3, expressed by the following 
equations has a value of 2.5% to 4.5%; and at least 90% of the 
microstructure of the steel is constituted by bainite, or ferrite and 
bainite: 
EQU fn3=0.5Si(%)+Mn(%)+1.13Cr(%)+1.98Ni(%). 
(IV) A heat-treated type steel product, according to (I) above, which has 
the following chemical composition based on % by weight, C: 0.1% to 0.6%; 
Si: 0.05% to 1.5%; Mn: 0.4% to 2.0%; Al: 0.002% to 0.05%; Cu: 0% to 1.0%; 
Ni: 0% to 2.0%; Cr: 0% to 2.0%; Mo: 0% to 0.5%; V: 0% to 0.3%; Nb: 0% to 
0.1%; B: 0% to 0.02%; and the balances Fe and unavoidable impurities, 
wherein at least 50% of the microstructure of the steel is constituted by 
martensite. 
(V) A machined steel part made of the steel product as described in (I) 
above. 
(VI) A machined steel part made of the non-heat-treated type steel product 
as described in (II) above. 
(VII) A machined steel part made of the non-heat-treated type steel product 
as described in (III) above. 
(VIII) A machined steel part made of the heat-treated type steel product as 
described in (IV) above. 
The expression "titanium carbosulfide" as used herein encompasses titanium 
sulfide. 
The expression "maximum diameter (of titanium carbosulfide)" as used herein 
refers to "the longest diameter among the diameters of respective titanium 
carbosulfide entities." 
The index of cleanliness of the steel is determined by "the microscopic 
testing method for the non-metallic inclusions in steel" prescribed in JIS 
G 0555, and performed by means of an optical microscope at .times.400 
magnification and 60 visual fields. 
The term "non-heat-treated type steel product" as used herein refers to a 
steel product manufactured without "quenching and tempering" which are 
so-called "thermal refining," and includes "steel which may be used under 
the as-cooled condition after hot working" as well as "steel obtained 
through aging corresponding to tempering after hot working and cooling." 
The term "heat-treated type steel product" refers to steel products 
obtained through "quenching and tempering". 
Ratios referred to in terms of microstructure denote those observed under a 
microscope, i.e. area percentage. 
In (II) above, "at least 90% of the microstructure of the steel is 
constituted by ferrite and pearlite" means that the total of the 
respective contents of ferrite and pearlite in the microstructure where 
ferrite and pearlite coexist is at least 90%. 
In (III) above, "at least 90% of the microstructure of the steel is 
constituted by bainite" means that the bainite content in the 
microstructure where no ferrite exists is at least 90%, and "at least 90% 
of the microstructure of the steel is constituted by ferrite and bainite" 
means that the total of the respective contents of ferrite and bainite in 
the microstructure where ferrite and bainite coexist is at least 90%. 
In (IV) above, "at least 50% of the microstructure of the steel is 
constituted by martensite" means that the martensite content in the 
microstructure is at least 50%. In additions the above (IV) is directed to 
a "heat-treated type steel product" which has undergone quenching and 
tempering. Likewise, the above mentioned martensite refers to martensite 
which has undergone tempering, i.e. "tempered martensite," and will 
hereinafter be referred to simply as "martensite". 
BEST MODE FOR CARRYING OUT THE INVENTION 
The present inventors conducted various experiments to investigate the 
effects of the chemical composition and the microstructure of steel 
products on machinability and mechanical properties. 
As a results the present inventors found that machinability of a steel 
product is improved by (a) addition of a proper amount of Ti to the steel, 
(b) transformation of sulfides to titanium carbosulfides for controlling 
inclusions in the steel, and (c) minute dispersion of the titanium 
carbosulfides in the steel. 
The present inventors continued the studies to find the facts (d) to (p) as 
follows: 
(d) Titanium carbosulfide is formed in steel when Ti is intentionally added 
to steel containing an adequate amount of S. 
(e) The formation of titanium carbosulfide in the steel decreases the 
amount of production of MnS. 
(f) If the S content in steel is constants titanium carbosulfides are 
superior to MnS in terms of effect of improving machinability. This is 
because the titanium carbosulfide has a melting point lower than that of 
MnS and thus achieves an increased lubrication effect on tool faces during 
machining. 
(g) In order to cause titanium carbosulfide to fully exert its 
machinability-improving effect, it is important to restrict the N content 
to as low as 0.008% or less in order to suppress precipitation of TiN. 
(h) The restriction of the N (nitrogen) content leads to a decrease in the 
TiN content in steel. Therefore, it becomes possible to improve, among 
mechanical properties, especially the toughness. 
(i) In order to improve machinability by making use of titanium 
carbosulfides, it is important to optimize the size of titanium 
carbosulfides and their amounts expressed in the index of cleanliness of 
the steel (hereinafter referred to simply as "index of cleanliness"). 
(j) Titanium carbosulfide produced during steelmaking is not solubule in 
the steel matrix at heating temperatures for ordinary hot working or for 
ordinary quenching in a thermal refining process. For this reasons 
titanium carbosulfides exert a so-called "pinning effect" in the austenite 
region, which is effective in preventing the enlargement of austenite 
grains. Needless to say, titanium carbosulfides are not solubule in the 
steel matrix at heating temperatures for ordinary tempering in a thermal 
refining process, or hot working, or for aging process corresponding to 
tempering. 
(k) Steel products containing at least 90% of ferrite and pearlite in the 
microstructure, very rarely suffers an occurrence of bend due to 
transformation-induced strain and residual stress. 
(l) Steel containing at least 90% of bainite and ferrite exclusively, or 
ferrite and bainite in the microstructure, exhibits an excellent balance 
between strength and toughness. 
(m) Steel containing at least 50% of martensite in the microstructure 
exhibits an extremely excellent balance between strength and toughness. 
(n) In non-heat-treated type steel products, having a certain chemical 
composition and containing at least 90% of ferrite and pearlite in the 
microstructure; an excellent balance between strength and toughness can be 
obtained, if the steel satisfies any of the following: (1) ferrite 
accounts for 20% to 70% based on area percentage, (2) ferrite grain size 
of at least 5 according to JIS grain size number, or (3) the average 
lamellar spacing of pearlite is 0.2 .mu.m or less. 
(o) If the value of fn1 expressed by equation (1) below is greater than 0%, 
and/or the value of fn2 expressed by equation (2) below is greater than 2, 
the machinability-improving effect of titanium carbosulfides is improved. 
In additions if the value of fn2, expressed by equation (2) below, is 
greater than 2, the pinning effect of titanium carbosulfide is improved 
and excellent strength and toughness is obtained. 
EQU fn1=Ti(%)-1.2S(%) (1) 
EQU fn2=Ti(%)/S(%) (2). 
(p) The value of fn3, expressed by equation (3) below, governs a certain 
relationship between the microstructure and toughness of non-heat-treated 
type steel which has a certain chemical composition. If this value is 
within a certain ranges at least 90% of the microstructure is bainite 
exclusively, or ferrite and bainite. 
EQU fn3=0.05Si(%)+Mn(%)+1.13Cr(%)+1.98Ni(%) (3). 
The present invention has been accomplished based on the above findings. 
Requirements of the present invention will now be described in detail. The 
symbol "%" indicative of the content of each element means "% by weight." 
(A) Chemical Composition of Steel Products 
C: 
C binds to Ti together with S to form titanium carbosulfide and to have an 
effect of improving machinability. Also, C is an element effective for 
securing strength. However, if the carbon content is less than 0.05%, 
these effects cannot be obtained. On the other hand, if the carbon content 
is in excess of 0.6%, toughness will be impaired. Therefore, the carbon 
content shall be from 0.05% to 0.6%. 
In non-heat-treated type steel containing at least 90% of ferrite and 
pearlite in the microstructure of the steel (hereinafter referred to as 
"steel products under Condition X" for purpose of simplicity), the carbon 
content shall be, desirably, from 0.2% to 0.6%, more desirably, from 0.25% 
to 0.5%. 
In non-heat-treated type steel containing at least 90% of bainite 
exclusively, or ferrite and bainite in the microstructure (hereinafter 
referred to as "steel products under Condition Y" for purpose of 
simplicity), the carbon content shall be, desirably, from 0.05% to 0.3%, 
more desirably, from 0.1% to 0.24%. 
In heat-treated type steel containing at least 50% of martensite in the 
microstructure (hereinafter referred to as "steel products under Condition 
Z" for purpose of simplicity), the carbon content shall be, desirably, 
from 0.1% to 0.6%. 
S: 
S binds to Ti together with C to form titanium carbosulfide and to have an 
effect of improving machinability. However, if the sulfur content is less 
than 0.002%, the effect cannot be obtained. 
Conventionally, S has been incorporated into free-cutting steel in order 
that the machinability is improved by forming MnS. According to the 
studies of the present inventors the above-mentioned 
machinability-improving effect of MnS relies on the effect of improving 
lubrication between the chips and the face of a tool during machining. To 
make matters worse, MnS may become large and cause a large 
macro-streak-flaw for steel products, resulting in a defect. 
In the present invention, the machinability-improving effect of S is 
obtained by forming titanium carbosulfide through incorporation of 
adequate amounts of C and Ti. Therefore, as mentioned above, the sulfur 
content is required to be not less than 0.002%. By contrast, if the sulfur 
content is in excess of 0.2%, although no effect is provided for 
machinability, coarse MnS is produced in the steel again, which leads to 
problems such as a macro-streak-flaw. In additions since hot workability 
is considerably impaired, plastic working becomes difficult and toughness 
may be impaired. Therefore, the sulfur content shall be from 0.002% to 
0.2%. 
In "steel products under Condition X," the sulfur content shall be, 
desirably, from 0.01% to 0.2%, more desirably, from 0.02% to 0.17%. 
In "steel products under Condition Y," the sulfur content shall be, 
desirably, from 0.005% to 0.17%. 
Ti: 
In the present invention, Ti is an important alloy element to control 
inclusions. If the titanium content is less than 0.04%; S is not fully 
incorporated into the titanium carbosulfide and thus improved 
machinability is not obtained. By contrast, if the titanium content is in 
excess of 1.0%, not only the cost increases as the machinability-improving 
effect saturates, but also the toughness and hot-workability decrease 
excessively. Therefore, the titanium content shall be from 0.04% to 1.0%. 
In "steel products under Condition X," the titanium content shall be, 
desirably, from 0.08% to 0.8%. 
In "steel products under Condition Y," the titanium content shall be, 
desirably, from 0.06% to 0.8%. 
In "steel products under Condition Z," the titanium content shall be, 
desirably, from 0.06% to 0.8%. 
N: 0.008% or less 
In the present invention, it is very important to restrict the nitrogen 
content to a low level. Briefly, N, having strong affinity with Ti, easily 
binds to Ti to form TiN, thereby immobilizing Ti. Therefore, the addition 
of a great amount of N impedes the full exertion of the above-mentioned 
machinability-improving effect of titanium carbosulfide. Moreover, coarse 
TiN impairs toughness and machinability. Therefore, the nitrogen content 
shall be 0.008% or less. In order to enhance the effect of titanium 
carbosulfide, the upper limit of the nitrogen content shall be, desirably, 
0.006%. 
Nd: 
Nd may be omitted. Nd, if added, becomes Nd.sub.2 S.sub.3 serving as a chip 
breaker to have an effect of improving machinability. Further, since 
Nd.sub.2 S.sub.3 is finely produced in molten steel in a dispersing manner 
at relatively high temperatures, the growth of austenite grains, due to 
heat, is restricted during hot working or quenching in the subsequent 
process and thus the microstructure becomes fine, resulting in high 
strength and toughness of steel. To reliably obtain this effect, the 
neodymium content shall be, desirably, not less than 0.005%. However, if 
the neodymium content is in excess of 0.1% Nd.sub.2 S.sub.3 becomes coarse 
and could impair toughness. Therefore, the neodymium content shall be from 
0% to 0.1%. Desirably, the upper limit of the neodymium content shall be 
0.08%. 
Se: 
Se may be omitted. Se, if added, has an effect of further improving the 
machinability of steel. To reliably obtain this effect, the selenium 
content shall be, desirably, not less than 0.1%. However, when the 
selenium content is in excess of 0.5%, not only the above-mentioned effect 
saturates, but also fatigue strength and/or toughness decrease as coarse 
inclusions are produced. Therefore, the selenium content shall be from 0% 
to 0.5%. 
Te: 
Te may be omitted. Te, if added, has an effect of further improving 
machinability of steel. To reliably obtain this effect the tellurium 
content shall be, desirably not less than 0.005%. However when the 
tellurium content is in excess of 0.05%, not only the above-mentioned 
effect saturates, but also fatigue strength and/or toughness of the steel 
decrease as coarse inclusions are produced. Further, addition of a great 
amount of Te leads to decreased hot-workability. Specifically, if the 
tellurium content is in excess of 0.05%, scratches are formed in the 
surfaces of steel products which have undergone hot working. Therefore, 
the tellurium content shall be from 0% to 0.05%. 
Ca: 
Ca may be omitted. Ca, if added, has an effect of remarkably improving 
machinability of steel. To reliably obtain this effect, the calcium 
content shall be, desirably not less than 0.001%. However, when the 
calcium content is in excess of 0.01%, not only the above-mentioned effect 
saturates, but also fatigue strength and/or toughness decrease as coarse 
inclusions are produced. Therefore, the calcium content shall be from 0% 
to 0.01%. 
Pb: 
Pb may be omitted. Pb, if added, has an effect of further improving the 
machinability of steel. To reliably obtain this effect, the lead content 
shall be, desirably, not less than 0.05%. However, when the lead content 
is in excess of 0.5%, not only the above-mentioned effect saturates, but 
also fatigue strength and/or toughness decrease as coarse inclusions are 
produced. Further, addition of a great amount of Pb leads to decreased 
hot-workability. Specifically, if the lead content is in excess of 0.5%, 
scratches are formed in the surfaces of steel products which have 
undergone hot working. Therefore, the lead content shall be from 0% to 
0.5%. 
Bi: 
Bi may be omitted. Bi, if added, has an effect of further improving the 
machinability of steel. To reliably obtain this effects the bismuth 
content shall be, desirably, not less than 0.05%. However, when the 
bismuth content is in excess of 0.4%, not only the above-mentioned effect 
saturates, but also fatigue strength and/or toughness decrease as coarse 
inclusions are produced. Further, addition of a great amount of Bi leads 
to decreased hot-workability, resulting in scratches which are formed in 
the surfaces of steel products which have undergone hot working. 
Therefore, the bismuth content shall be from 0% to 0.4%. 
As far as machinability is concerned, no particular restriction is imposed 
on any elements other than C, S, Ti, N, Nd, Se, Te, Ca, Pb and Bi used for 
"steel products excellent in machinability" in the present invention. 
However, there are often requirements for other properties in addition to 
machinability. These requirements include rare occurrence of bend or 
residual stress due to transformation-induced strain, excellent balance 
between strength and toughness, and so on. In such cases, the requirements 
are satisfied by determining the chemical composition of the 
above-mentioned elements other than C, S, Ti, N, Nd, Se, Te, Ca, Pb and 
Bi, in relation to the microstructures of steel products. 
The chemical composition of the elements other than C, S, Ti, N, Nd, Se, 
Te, Ca, Pb and Bi will next be described for each case of the 
above-mentioned "steel products under Condition X", "steel products under 
Condition Y" and "steel products under Condition Z". 
(A-1) In the case of non-heat-treated type steel products containing at 
least 90% of ferrite and pearlite in the microstructure ("steel products 
under Condition X") 
Si: 
Si is an element effective for deoxidizing a steel and for strengthening 
the ferrite phase. Further, the increased silicon content improves 
lubrication on the surface of the chips during machining and thus the 
service life of the tool is extended, resulting in improved machinability. 
However, if the silicon content is less than 0.05%, the effect of the 
addition is insignificant, whereas if the silicon content is in excess of 
1.5%, not only the above-mentioned effect saturates, but also toughness is 
impaired. Therefore, the silicon content shall be, desirably, from 0.05% 
to 1.5%, more desirably, from 0.3% to 1.3%, most desirably, from 0.5% to 
1.3%. 
Mn: 
Mn is an element effective for improving fatigue strength through 
solid-solution strengthening. However, if the manganese content is less 
than 0.1%, the effect is difficult to obtains whereas if the manganese 
content is in excess of 2.0%, in the case of "steel products under 
Condition X", endurance ratio (fatigue strength/tensile strength) and 
yield ratio (yield strength/tensile strength) may be impaired. Therefore, 
the manganese content shall be, desirably, from 0.1% to 2.0%, more 
desirably, from 0.4% to 2.0%, and most desirably, from 0.5% to 1.7%. 
P: 
P may be intentionally added. This is because P has an effect of improving 
tensile strength and fatigue strength in "steel products under Condition 
X". In order to reliably obtain this effect, the phosphorus content shall 
be, desirably, not less than 0.01%. However, if the phosphorus content is 
in excess of 0.07%, toughness decreases remarkably and hot-workability is 
impaired. Therefore, the phosphorus content shall be, desirably, not 
greater than 0.07%. If P is added intentionally, the phosphorus content 
shall be, desirably, from 0.015% to 0.05%. 
Al: 
Al is an element effective for deoxidizing a steel. However, if the 
aluminum content is less than 0.002%, the desired effect is difficult to 
obtain, whereas if the aluminum content is in excess of 0.05%, the effect 
is saturated and machinability is also impaired. Therefore, the aluminum 
content shall be, desirably, from 0.002% to 0.05%, more desirably, from 
0.005% to 0.03%. 
Cu: 
Cu may be omitted. Cue if added, has an effect of improving strengths 
especially fatigue strength of a steel, through precipitation 
strengthening. To reliably obtain this effects the copper content shall 
be, desirably, not less than 0.2%. However, when the copper content is in 
excess of 1.0%, hot-workability is impaired, and moreover as precipitates 
become coarse, the above-mentioned effect saturates or decreases. In 
addition, the cost increases. Therefore, the copper content shall be, 
desirably, from 0% to 1.0%. 
Ni: 
Ni may be omitted. Ni, if added, has an effect of improving strength. To 
reliably obtain this effects the nickel content shall be, desirably, not 
less than 0.02%. However, when the nickel content is in excess of 2.0%, 
this effect saturates and thus the cost increases. Therefore, the nickel 
content shall be, desirably, from 0% to 2.0%. 
Cr: 
Cr may be omitted. Cr, if added, has an effect of improving fatigue 
strength through solid-solution strengthening. To reliably obtain this 
effect, the chromium content shall be, desirably, not less than 0.02%. 
However, if the chromium content is in excess of 2.0%, in "steel products 
under Condition X", endurance ratio and yield ratio may be impaired. 
Therefore, the chromium content shall be, desirably, from 0% to 2.0%. In 
the case where Cr is added, the chromium content shall be, desirably, from 
0.05% to 1.5%. 
Mo: 
Mo may be omitted. Mo, if added, has an effect of improving strength, 
especially fatigue strength of a steel, since the microstructure composed 
of ferrite and pearlite becomes fine. To reliably obtain this effect, the 
molybdenum content shall be, desirably, not less than 0.05%. However, when 
the molybdenum content is in excess of 0.5%, the microstructure through 
hot working becomes abnormally coarse, resulting in lowered fatigue 
strength. For that reason, the molybdenum content shall be, desirably, 
from 0% to 0.5%. 
V: 
V may be omitted. V, if added, has an effect of improving strength, 
especially fatigue strength of a steel, since V precipitates as fine 
nitride or carbonitride. To reliably obtain this effect, the vanadium 
content shall be, desirably, not less than 0.05%. However, when the 
vanadium content is in excess of 0.3%, the precipitates become coarse, 
resulting in saturation, or even impairment, of the above-mentioned 
effect. In addition, the material costs increase. Therefore, the vanadium 
content shall be, desirably, from 0% to 0.3%. 
Nb: 
Nb may be omitted. Nb, if added, has an effect of preventing coarsening of 
austenite grains, to thereby enhance strength, especially fatigue strength 
of a steel, since Nb precipitates as fine nitride or carbonitride. To 
reliably obtain this effect, the niobium content shall be, desirably, not 
less than 0.005%. However, when the niobium content is in excess of 0.1%, 
not only does the above-mentioned effect saturate, but also coarse hard 
carbonitride may be produced to damage tools, resulting in lowered 
machinability. Therefore, the niobium content shall be, desirably, from 0% 
to 0.1%. More desirably, the upper limit of niobium content shall be 
0.05%. 
fn1, fn2: 
As mentioned above, if the value of fn1 expressed by the equation (1) is 
greater than 0%, and/or the value of fn2 expressed by the equation (2) is 
greater than 2, the machinability-improving effect of titanium 
carbosulfides is enhanced. In addition, if the value of fn2, expressed by 
the equation (2), is greater than 2, the pinning effect of titanium 
carbosulfides is enhanced, to thereby improve tensile strength and fatigue 
strength. Therefore, it is desired that the value of fn1 shall be greater 
than 0%, or alternatively, the value of fn2 shall be greater than 2. No 
particular limitation is imposed on the upper limits of the values of fn1 
and fn2, and they may be determined so as to comply with compositional 
requirements. 
Incidentally, O (oxygen) as an impurity element forms hard oxide-type 
inclusions, by which the machine tool may be damaged, resulting in lowered 
machinability. In particular, the oxygen content in excess of 0.015% may 
considerably impair machinability. Consequently, in order to maintain 
excellent machinability, the amount of O as an impurity element shall be, 
desirably, 0.015% or less. More desirably, the oxygen content shall be 
0.01% or less. 
(A-2) In the case of non-heat-treated type steel products in which bainite 
or a combination of ferrite and bainite accounts for at least 90% of the 
microstructure of the steel ("steel products under Condition Y") 
Si: 
Si has an effect of deoxidizing a steel and improving hardenability. 
Furthermore, in "steel products under Condition Y", the increased silicon 
content improves lubrication on the surface of the chips during machining 
and thus the service life of the tool is extended, resulting in improved 
machinability. However, when the silicon content is less than 0.05%, the 
above-mentioned effects are poor, whereas if the silicon content is in 
excess of 1.5%, not only do the above-mentioned effects saturate, but also 
toughness is impaired. Therefore, the silicon content shall be, desirably, 
from 0.05% to 1.5%. More desirably, the silicon content shall be from 0.5% 
to 1.3%. 
Al: 
Al is an element having powerful deoxidizing effect on a steel. To secure 
this effects the aluminum content shall be, desirably, not less than 
0.002%. However, when the aluminum content is in excess of 0.05%, the 
effect saturates and the only result is increased cost. Therefore, the 
aluminum content shall be, desirably from 0.002% to 0.05%, more desirably 
from 0.005% to 0.004%. 
Cu: 
Cu may be omitted. Cu, if added, has an effect of improving machinability 
as well as strength of the steel without lowering toughness To reliably 
obtain this effect, the copper content shall be, desirably, not less than 
0.2%. However, when the copper content is in excess of 1.0%, not only is 
hot workability impaired, but also precipitates may become coarse, 
resulting in saturation of the above-mentioned effect or lowered 
toughness. In addition, the cost increases. Therefore, the copper content 
shall be, desirably, from 0% to 10%. 
Mo: 
Mo may be omitted. Mo, if added, has an effect of improving hardenability 
and strength of a steel by rendering the microstructure of the steel very 
fine. To reliably obtain this effect, the molybdenum content shall be, 
desirably, not less than 0.05%. However, when the molybdenum content is in 
excess of 0.5%, the microstructure obtained through hot working becomes 
abnormally coarse, resulting in lowered toughness. For this reason, the 
molybdenum content shall be, desirably, from 0% to 0.5%. 
V: 
V may be omitted. V, if added, has an effect of improving strength, since V 
precipitates as fine nitride or carbonitride, and moreover, has an effect 
of improving lubrication on the surface of the chips during machining. To 
reliably obtain these effects, the vanadium content shall be, desirably, 
not less than 0.05%. However, when the vanadium content is in excess of 
0.30%, as the precipitates become coarse, the above-mentioned effect may 
saturate or toughness may decrease. In addition, the cost increases. 
Therefore, the vanadium content shall be, desirably, from 0% to 0.30%. 
Nb: 
Nb may be omitted. Nb, if added, has an effect of preventing coarsening of 
austenite grains and improving strength and toughness of the steel, since 
Nb precipitates as fine nitride or carbonitride. To reliably obtain this 
effect, the niobium content shall be, desirably, not less than 0.005%. 
However, when the niobium content is in excess of 0.1%, not only does the 
above-mentioned effect saturate, but also coarse hard carbonitride may be 
produced to damage tools, inviting degraded machinability. Therefore, the 
niobium content shall be, desirably, from 0% to 0.1%. 
B: 
B may be omitted. B, if added, has an effect of improving strength and 
toughness of a steel due to increased hardenability. To secure this 
effect, the boron content shall bee desirably, not less than 0.0003%. 
However, when the boron content is in excess of 0.02%, not only may the 
above-mentioned effect saturate, but also toughness may decrease. 
Therefore, the boron content shall be, desirably, from 0% to 0.02%. 
fn3: 
As described above, the value of fn3, expressed by the aforementioned 
equation (3), is correlated to the microstructure and toughness of a 
non-heat-treated type steel product having a certain chemical composition. 
When the value is in the range of 2.5-4.5%, the primary microstructure of 
the non-heat-treated type steel product comes to be bainite, or a 
combination of ferrite and bainite, thus achieving well-balanced strength 
and toughness. 
Si, Mn, Cr and Ni, which form the terms of the equation for fn3, have the 
effect of enhancing hardenability of the steel. When the value of fn3 is 
less than 2.5%, intended improvement in hardenability cannot be obtained, 
with toughness being sometimes degraded. In contrasts the values of fn3 in 
excess of 4.5% result in excessive hardenability, which may in turn 
degrade toughness. Therefore, it is desired that the value of fn3 
expressed by the equation (3) shall be from 2.5% to 4.5%. In this 
connection, the contents of the respective elements other than Si are not 
particularly limited, so long as the above-mentioned fn3 falls within the 
range of 2.5-4.5%. However, desirably, Mn, Cr and Ni shall be contained in 
amounts of 0.4-3.5%, 3.0% or less, and 2.0% or less, respectively. 
In the case of "steel products under Condition Y", as mentioned above, the 
machinability-improving effect of titanium carbosulfides is enhanced when 
the value of fn1 expressed by the equation (1) is greater than 0%, and/or 
the value of fn2 expressed by the equation (2) is greater than 2. 
Furthermore, when the value of fn2, expressed by the equation (2), is 
greater than 2, the pinning effect of titanium carbosulfides increases as 
well, to thereby improve tensile strength and fatigue strength. Therefore, 
it is desired that the value of fn1 shall be greater than 0%, or 
alternatively, the value of fn2 shall be greater than 2. The upper limits 
of the values of fn1 and fn2 are not particularly limited, and they may be 
determined based on compositional requirements. 
Incidentally, O (oxygen) as an impurity element forms hard oxide-type 
inclusions, by which the machine tool may be damaged, resulting in lowered 
machinability. In particular, the oxygen content in excess of 0.015% may 
invite significant degradation in machinability. Therefore, even in the 
case of "steel products under Condition Y", in order to maintain excellent 
machinability, the amount of O as an impurity element shall be, desirably, 
0.015% or less. More desirably, the oxygen content shall be 0.01% or less. 
Moreover, from the viewpoint of securing toughness of the steel, phosphorus 
(P) as an impurity element shall be, desirably, suppressed to 0.05% or 
less. 
(A-3) In the case of heat-treated type steel products in which martensite 
accounts for at least 50% of the microstructure of the steel ("steel 
products under Condition Z") 
Si: 
Si has an effect of deoxidizing a steel and improving hardenability. 
Furthermore, in the case of "steel products under Condition Z", increased 
silicon content improves lubrication on the surface of the chips during 
machining and thus the service life of the tool is extended, resulting in 
improved machinability. However, if the silicon content is less than 
0.05%, the above-mentioned effects are poor, whereas if the silicon 
content is in excess of 1.5%, not only the above-mentioned effects 
saturate, but also toughness is impaired. Therefore, the silicon content 
shall be, desirably, from 0.05% to 1.5%. 
Mn: 
Mn improves hardenability of a steel and improves fatigue strength through 
solid-solution strengthening. However, if the manganese content is less 
than 0.4%, these effects are difficult to obtain, whereas if the manganese 
content is in excess of 2.0%, not merely these effects saturate, but also 
the steel becomes excessively hard to cause degradation in toughness. 
Accordingly, the manganese content shall be, desirably, from 0.4% to 2.0%. 
Al: 
Al is an element having strong deoxidizing effect on a steel. In order to 
secure this effect, the aluminum content shall be, desirably, not less 
than 0.002%. However, if the aluminum content is in excess of 0.05%, the 
effect saturates and the only result is increased costs. Therefore, the 
aluminum content shall be, desirably, from 0.002% to 0.05%, more 
desirably, from 0.005% to 0.04%. 
Cu: 
Cu may be omitted. Cu, if added, has an effect of improving strength 
without lowering toughness, and in addition, enhances machinability. To 
secure these effects, the copper content shall be, desirably, not less 
than 0.2%. However, when the copper content is in excess of 1.0%, hot 
workability is impaired and precipitates become coarse, resulting in 
saturating the above-mentioned effect or even impairing the effects In 
addition, the cost increases. Therefore, the copper content shall be, 
desirably, from 0% to 1.0%. 
Ni: 
Ni may be omitted. Ni, if added, has an effect of improving hardenability 
of a steel. To secure this effect, the nickel content shall be, desirably, 
not less than 0.02%. However, when the nickel content is in excess of 
2.0%, this effect saturates and thus the cost increases. Therefore, the 
nickel content shall be, desirably, from 0% to 2.0%. 
Cr: 
Cr may be omitted. Cr, if added, has an effect of enhancing hardenability 
of a steel, and also improves fatigue strength through solid-solution 
strengthening. To reliably obtain these effects, the chromium content 
shall be, desirably, not less than 0.03%. However, when the chromium 
content is in excess of 2.0% not only do the above-mentioned effects 
saturate, but also the steel becomes excessively hard, resulting in 
lowered toughness. Therefore, the chromium content shall be, desirably, 
from 0% to 2.0%. 
Mo: 
Mo may be omitted. Mo, if added, has an effect of improving hardenability 
of a steel. To reliably obtain this effect, the molybdenum content shall 
be, desirably, not less than 0.05%. However, when the molybdenum content 
is in excess of 0.5%, not only does the above-mentioned effect saturate 
but also the steel becomes excessively hard, resulting in lowered 
toughness and increased cost. For this reason, the molybdenum content 
shall be, desirably, from 0% to 0.5%. 
V: 
V may be omitted. V, if added, has an effect of improving strength, 
especially fatigue strength of a steel, since V precipitates as fine 
nitride or carbonitride. To reliably obtain this effect, the vanadium 
content shall be, desirably, not less than 0.05%. However, when the 
vanadium content is in excess of 0.3%, the precipitates become coarse, 
resulting in saturation, or even impairment, of the above-mentioned 
effect. In addition, the material costs increase. Therefore, the vanadium 
content shall be, desirably, from 0% to 0.3%. 
Nb: 
Nb may be omitted. Nb, if added, has an effect of preventing coarsening of 
austenite grains, to thereby enhance strength, especially fatigue strength 
and toughness of a steel, since Nb precipitates as fine nitride or 
carbonitride. To reliably obtain these effects, the niobium content shall 
be, desirably, not less than 0.005%. However, when the niobium content is 
in excess of 0.1%, not only do the above-mentioned effects saturate, but 
also coarse hard carbonitride may be produced to damage tools, resulting 
in lowered machinability. Therefore, the niobium content shall be, 
desirably, from 0% to 0.1%. More desirably, the upper limit of niobium 
content shall be 0.05%. 
B: 
B may be omitted. B, if added, has an effect of improving strength and 
toughness of a steel due to increased hardenability. To secure this 
effect, the boron content shall be, desirably, not less than 0.0003%. 
However, when the boron content is in excess of 0.02%, not only may the 
above-mentioned effect saturate, but also toughness may be lowered. 
Therefore, the boron content shall be, desirably, from 0% to 0.02%. 
fn1, fn2: 
Also in "steel products under Condition Z", as aforementioned, if the value 
of fn1 expressed by the equation (1) is greater than 0%, and/or the value 
of fn2 expressed by the equation (2) is greater than 2, the 
machinability-improving effect of titanium carbosulfides is enhanced. In 
additions if the value of fn2, expressed by the equation (2), is greater 
than 2, the pinning effect of titanium carbosulfides is enhanced, to 
thereby improve tensile strength and fatigue strength. Therefore, it is 
desired that the value of fn1 shall be greater than 0%, or alternatively, 
the value of fn2 shall be greater than 2. No particular limitation is 
imposed on the upper limits of the values of fn1 and fn2, and they may be 
determined so as to comply with compositional requirements. 
Incidentally, O (oxygen) as an impurity element forms hard oxide-type 
inclusions, by which the machine tool may be damaged, resulting in lowered 
machinability. In particular, the oxygen content in excess of 0.015% may 
considerably impair machinability Consequently, also in "steel products 
under Condition Z", in order to maintain excellent machinability, the 
amount of O as an impurity element shall be, desirably, 0.015% or less. 
More desirably, the oxygen content shall be 0.01% or less. 
Moreover, from the point of securing toughness of the steel, P (phosphorus) 
as an impurity element shall be, desirably, suppressed to 0.05% or less. 
(B) The size and the index of cleanliness in terms of titanium 
carbosulfides 
In order to improve machinability of steel products having chemical 
compositions described in (A) above through use of titanium carbosulfides, 
it is important that the size and the index of cleanliness in terms of 
titanium carbosulfides be optimized. As described herein above, the 
expression "titanium carbosulfides" encompasses titanium sulfides. 
In the case in which the amount expressed by the index of cleanliness in 
terms of titanium carbosulfide having a maximum diameter of not more than 
10 .mu.m is less than 0.05%, titanium carbosulfides cannot exhibit their 
machinability-improving effect. The above-mentioned index of cleanliness 
shall be, desirably, not less than 0.08%. When the above-mentioned index 
of cleanliness in terms of titanium carbosulfides is excessively large, 
fatigue strength may sometimes be degraded. Therefore, the upper limit of 
the above-mentioned index of cleanliness in terms of titanium 
carbosulfides shall be, desirably, approximately 2.0%. 
The reason why the size of titanium carbosulfide is limited--i.e., why the 
maximum diameter of titanium carbosulfide is set to 10 .mu.m--is that 
sizes in excess of 10 .mu.m reduce fatigue strength and/or toughness. 
Desirably, the maximum diameter of titanium carbosulfide shall be 7 .mu.m. 
However, in view that too small a maximum diameter of titanium 
carbosulfides provides insignificant machinability-improving effect, the 
lower limit of the maximum diameter of titanium carbosulfide shall be, 
desirably, about 0.5 .mu.m. 
The form of titanium carbosulfide is basically determined by the amounts of 
Ti, S and N contained in the steel. In order to bring the size and the 
index of cleanliness in terms of titanium carbosulfides within the 
predetermined ranges, it is important to prevent overproduction of 
titanium oxides. To this end, according to a preferred steelmaking 
process, steel is first sufficiently deoxidized with Si and Al, then Ti is 
added; since in some cases, satisfaction of the compositional requirements 
for the steel mentioned in (A) is not sufficient by itself. 
Titanium carbosulfides can be discerned from other inclusions based on 
their color and shape through mirror-like polishing of test pieces cut 
from steel products and through observation of the polished surface under 
an optical microscope at .times.400 or higher multiplication. That is, 
titanium carbosulfides have a very pale gray color and a granular 
(spherical) shape corresponding to B-type inclusions according to JIS 
(Japanese Industrial Standards). Detailed determination of titanium 
carbosulfides may also be performed through observation of the 
aforementioned mirror-like-polished surface under an electron microscope 
equipped with an analytical device such as EDX (energy dispersive X-ray 
spectrometer). 
The index of cleanliness in terms of titanium carbosulfides is determined 
as described hereinabove; i.e., in accordance with "the microscopic 
testing method for the non-metallic inclusions in steel" prescribed in JIS 
G 0555, and performed by means of an optical microscope at .times.400 
magnification and 60 visual fields. 
(C) Microstructure of steel products 
So far as machinability is concerned, "steel products excellent in 
machinability" of the present invention can be obtained by simply 
prescribing the amounts of C, S, Ti, N, Nd, Se, Te, Ca, Pb and Bi as 
described in (A) above and also prescribing the size and the index of 
cleanliness in terms of titanium carbosulfide as described in (B) above. 
However, when the steel is required to meet other characteristics in 
addition to machinability, the microstructure of steel products may be 
additionally prescribed as well. 
First, in the case in which not less than 90% of the microstructure of a 
steel product is constituted by ferrite and pearlite, occurrence of bend 
and residual stress attributed to transformation-induced strain does not 
raise a critical issue. Therefore, if not less than 90% of the 
microstructure of a steel product is made to be constituted by ferrite and 
pearlite, reformation (straightening step) as a finish step can be 
eliminated, leading to reduced costs. Moreover, in the case in which the 
steel product is a non-heat-treated type steel product, there can be saved 
considerable energy and cost which would otherwise be required for thermal 
refining. 
In order to make not less than 90% of the microstructure of a 
non-heat-treated type steel product to be constituted by ferrite and 
pearlite, a semi-finished product having a chemical composition described 
in (II) above may first be heated to 1050-1300.degree. C., then subjected 
to hot working such as hot forging to finish at a temperature not lower 
than 900.degree. C., and subsequently subjected to air cooling or 
atmospheric cooling at a cooling rate of not more than 60.degree. C./min 
for at least a period until the temperature reaches 500.degree. C. In the 
present specifications the expression "cooling rate" refers to the cooling 
rate as measured on the surface of the steel product. 
In the case of non-heat-treated type steel products having the above 
microstructure, well-balanced excellent strength and toughness can be 
obtained when at least one of the following conditions are met: ferrite 
accounts for 20-70% in terms of the area percentages ferrite grain size is 
5 or more as expressed by the JIS grain size number; the average lamellar 
spacing of pearlite is 0.2 .mu.m or less. 
Next, in the case of steel products in which not less than 90% of the 
microstructure is constituted by bainite or a combination of ferrite and 
bainite, well-balanced strength and toughness are appreciable. Therefore, 
if well-balanced strength and toughness are required, not less than 90% of 
the microstructure of a steel product should be made to be constituted by 
bainite, or a combination of ferrite and bainite. Moreover, in the case in 
which the steel product is a non-heat-treated type steel product, there 
can be saved considerable energy and cost which would otherwise be 
required for thermal refining. 
In order to make not less than 90% of the microstructure of a 
non-heat-treated type steel product to be constituted by bainite, or by a 
combination of ferrite and bainite, a semi-finished product having a 
chemical composition described in (III) above may first be heated to 
1050-1300.degree. C., then subjected to hot working such as hot forging to 
finish at a temperature not lower than 900.degree. C., and subsequently 
subjected to air cooling or atmospheric cooling at a cooling rate of not 
more than 60.degree. C./min for at least a period until the temperature 
reaches 300.degree. C. 
In the case of non-heat-treated type steel products, the greater the 
working ratio of the steel products during hot working, the finer the 
microstructure of the steel products, thus exhibiting a better balance 
between strength and toughness. Therefore, the working ratio during hot 
working shall be, desirably, not less than 1.5. The expression "working 
ratio" is used to refer to the ratio A.sub.0 /A where A.sub.0 represents a 
sectional area before working and A represents a sectional area after 
working. 
When the prior austenite grain size in the microstructure is 4 or more as 
expressed by the JIS grain size number, a non-heat-treated type steel 
product in which not less than 90% of the microstructure is constituted by 
bainite or a combination of ferrite and bainite (i.e., a "steel product 
under Condition Y") can be consistently imparted with well-balanced 
strength and toughness. As used herein, the expression "prior austenite 
grains" in a non-heat-treated type steel product refers to austenite 
grains right before bainite or ferrite is generated therefrom as a result 
of transformation under heat and hot working. Prior austenite grains in a 
non-heat-treated type steel product in which not less than 90% of the 
microstructure is constituted by bainite or a combination of ferrite and 
bainite can be readily determined through corrosion with nital and 
observation under an optical microscope. 
When aging treatment is performed by the application of heat under 
conditions of 200-700.degree. C. for 20-150 minutes following hot working 
and cooling, a particularly excellent balance between strength and 
toughness can be obtained. 
Finally, in the case of a steel product in which not less than 50% of the 
microstructure is constituted by martensite, balance between strength and 
toughness becomes more excellent. Therefore, when more excellent balance 
between strength and toughness is required, not less than 50% of the 
microstructure should be made to be constituted by martensite. Moreover, 
in the case in which the steel product is a heat-treated type steel 
product, remarkably excellent balance between strength and toughness can 
be obtained. 
In order to make not less than 50% of the microstructure of a heat-treated 
type steel product to be constituted by martensite, a semi-finished 
product having a chemical composition described in (IV) above may be 
treated as follows. Briefly, the semi-finished product is first heated to 
1050-1300.degree. C., then subjected to hot working such as hot forging at 
a working ratio of 1.5 or more and to finishing at a temperature not lower 
than 900.degree. C. Subsequently the finished steel material is subjected 
to air cooling or atmospheric cooling at a cooling rate of not more than 
60.degree. C./min for at least a period until the temperature reaches 
300.degree. C. Subsequently, the steel product is heated to a temperature 
range of 800-950.degree. C., maintained for 20-150 minutes, then quenched 
by use of a cooling medium such as water or oils followed by heating to 
400-700.degree. C., maintained for 20-150 minutes, and then subjected to 
air cooling, atmospheric cooling, or alternatively, depending on cases, 
water cooling or oil cooling followed by tempering. The quenching 
treatment may be performed by way of so-called "direct quenching," in 
which steel products are quenched directly from the austenite region or 
austenite-ferrite dual phase region after hot working. 
In order for a heat-treated type steel product to secure remarkably 
excellent strength and toughness in a well balanced manner, it is 
preferred that not less than 80% of the microstructure be made martensite. 
The remaining portion of the microstructure other than martensite is 
constituted by microstructure resulting from tempering of ferrite, 
pearlite or bainite in the case in which an austenite region undergoes 
quenching, microstructure resulting from tempering of ferrite in the case 
in which an austenite-ferrite dual-phase region undergoes quenching, or 
microstructure resulting from temperering of austenite which has remained 
untransformed even when quenching was performed (so-called retained 
austenite). Substantially 100% of the microstructure may represent 
martensite. 
When the prior austenite grain size is not less than 5 according to the JIS 
grain size number, a heat-treated type steel product in which not less 
than 50% of the microstructure is constituted by martensite (i.e., a 
"steel product under Condition Z") can be consistently imparted with 
extremely well-balanced strength and toughness. As used herein, the 
expression "prior austenite grains" in a heat-treated type steel product 
refers to austenite grains right before being subjected to quenching. 
Prior austenite grains in a heat-treated type steel product in which not 
less than 50% of the microstructure is constituted by martensite can be 
readily identified as follows, for example. A steel product is quenched or 
is quenched and then tempered, and a sample steel piece is cut out. The 
test piece is etched with aqueous solution of picric acid to which a 
surfactant has been added. The etched surface of the test piece is 
observed under an optical microscope.