High strength bolt

A high strength bolt made of a steel having a specifically defined chemical composition, i.e., by weight C: 0.30-0.50%; Si: not more than 0.15%; Mn: not more than 0.40%; Cr: 0.30-1.50%; Mo: 0.10-0.70%; and V: 0.15-0.40%, the balance being Fe and inevitable impurities such as P, S, etc. in trace amount. The manufacturing method therefor is featured in a strictly controlled heat treatment in respect to the temperature range such as: hardening by quenching from 940.degree..+-.10.degree. C. and tempering 575.degree..+-.25.degree. C.

BACKGROUND OF THE INVENTION 
Field of the Art 
The present invention relates to a high strength bolt and a method of 
manufacturing the same, and more particularly to a high strength bolt 
having a specific chemical composition and a manufacturing method therefor 
featured in heat treatment. 
Related Art Statement 
In recent years, remarkable tendency of lightening weight of automotive 
structural parts for the purpose of reducing fuel consumption naturally 
caused the necessity, also in the field of fastening bolts for fastening 
parts, to pursue high strength while demanding light weight. 
When for example automotive parts or components become compact and of high 
strength, fastening bolts, such as connecting rod bolts and cylinder head 
bolts, for fastening those parts or components are necessarily required to 
be compact. It is quite natural that a small-sized bolt must be of high 
strength for maintaining its fastening capability. 
Bolts of 12.9 class in the strength level, according to the ISO standard, 
have traditionally been utilized for such automotive-assembly use. 
Required strength standard conditions for such bolts of 12.9 class are: 
EQU tensile strength=120-140 kgf/mm.sup.2 ; 
and 
EQU 0.2% proof stress.gtoreq.0.9.times.tensile strength. 
Since the parts, which have been in harmony with bolts of just mentioned 
standard strength conditions, are now required to be more and more 
compact, bolts also have to catch up with the new demand for becoming 
smaller in size and greater in strength. This trend of the day demands 
appearance of higher strength bolts satisfying the conditions of ISO 14.9 
class, that is to say: 
EQU tensile strength=140-160 kgf/mm.sup.2 ; 
and 
EQU 0.2% proof stress.gtoreq.0.9.times.tensile strength. 
Although there is stipulated, in JIS (Japanese Industrial Standard) as well 
as in ISO standard a high strenght bolt of 14.9 class in strength level, 
development of steel satisfying the necessary conditions for such a high 
strength bolt can not be said completed. That is to say, progress of the 
material for such a high strength bolt does not, as a matter of fact, 
satisfactorily follow the necessity of the present day. 
Traditionally used bolt steel belongs to, as for its material quality, a 
Cr-Mo type steel such as JIS SCM440. It is well known that such a steel is 
remarkably deteriorated in the resistance to delayed fracture, when the 
tensile strength exceeds 120 kgf/mm.sup.2. This resistance to delayed 
fracture is in fact a key condition required for the bolts in automotive 
use, which must be improved by all means today. Steel which has been 
improved to a somewhat required level in the tensile strength, can not be 
practically used in places where the tensile strength of 140-160 
kgf/mm.sup.2 level is actually applied, due to the deterioration of the 
resistance to delayed fracture. 
An ideal steel, which is excellent in the resistance to delayed fracture 
and parallelly characterized in possessing features of high resistance to 
fatigue as well as high tensile strength, i.e., essential requirements to 
high strength bolts, has so far not been found. 
SUMMARY OF THE INVENTION 
The present invention was made in view of the above described situation in 
the art. It is accordingly an primary object of the present invention to 
provide high strength bolts, for pursuing the demand of the day, i.e., 
being compact and of high strength in compliance with the miniaturizing 
trend in parts, having unique chemical compositions for satisfying 
required standard conditions such as: 
tensile strength within 140-160 kgf/mm.sup.2 ; and additionally, resistance 
to delayed fracture as well as fatigue. 
It is another object of the invention to provide a novel method of 
manufacturing such high strength bolts, being featured in the heat 
treatment thereof. 
It has traditionally been ascertained that the delayed fracture takes 
place, in the Cr-Mo type steel of high strength used for bolts, along the 
prior austenite grain boundaries. 
The inventors made various strenuous studies and experiments for finding 
out the influence of the microstructure, the alloying elements, and the 
impurity elements to the occurring mechanism of the delayed fracture. 
Essential points observed in the course of the study are summarized as 
follows (1)-(3): 
(1) It is particularly preferable to choose a tempering temperature as high 
as possible. Since in the third stage of the tempering, wherein cementite 
precipitates, the cementite precipitated into the grain boundaries tends 
to embrittle the grain boundaries themselves, it is recommended to exclude 
this temperature range of cementite precipitation for obtaining steel of 
high tensile strength like 140-160 kgf/mm.sup.2, i.e., it is preferable to 
choose a higher temperature for the tempering. 
(2) Impurities such as P and S tend to segregate into austenite grain 
boundaries in the course of austenitization, so as to embrittle the grain 
boundaries, it is therefore advisable to hold down content of impurities 
to the lowest possible level. 
(3) Since oxidation of the grain boundaries in the course of heat treatment 
such as hardening and tempering greatly degrades the strength of the grain 
boundaries, which deteriorates in turn the resistance to delayed fracture, 
it is preferable to reduce content of such elements as Mn, Si, etc. which 
are liable to oxidize the grain boundaries, to the minimum. 
Among the above three findings, (3) is a unique and original discovery by 
the inventors, because there having been no such referring so far to the 
relation between the resistance to delayed fracture and the oxidation in 
the grain boundaries. 
It is also another unique finding by the inventors that heat treatment 
conditions, above all the temperature range for the tempering, must be 
minutely controlled for parallelly satisfying both required conditions, 
that is, the tensile strength and the resistance to delayed fracture. 
After having carefully studied and checked the chemical compositions and 
the heat treatment conditions necessitated for a special bolt steel of 
high strength, the inventors invented a bolt of high strength made of iron 
base alloy or steel with a specific chemical composition and a 
manufacturing method therefor including a specific heat treatment. 
The gist of the present invention can be summarized into two sorts of high 
strength bolt made of steel consisting essentially of the composition of 
(I) and (II), and a manufacturing method for those two sorts of bolt. 
The first chemical composition (I) of the invented high strength bolt 
consists essentially of: 
0.30-0.50% by weight of C; not more than 0.15% by weight of Si; not more 
than 0.40% by weight of Mn; 0.30-1.50% by weight of Cr; 0.10-0.70% by 
weight of Mo; and 0.15-0.40% by weight of V, the balance being Fe and 
inevitable impurities such as P not exceeding 0.015% and S not exceeding 
0.010%. 
The second composition (II) thereof is permitted to additionally include 
one or more elements of the group consisting of 0.05-0.15% by weight of 
Nb; 0.05-0.15% by weight of Ti; and 0.05-0.15% by weight of Zr. 
The method invention is specified, as to the manufacturing of the above 
defined high strength bolts of (I) and (II) composition, in the hardening 
by quenching the steel heated at a temperature of 
940.degree..+-.10.degree. C. and the tempering thereafter at a temperature 
of 575.degree..+-.25.degree. C. In other words, the method according to 
the present invention comprises the steps of: (a) preparing a steel 
material of an iron base alloy consisting essentially of 0.30-0.50% by 
weight of carbon, not more than 0.15% by weight of silicon, not more than 
0.40% by weight of manganese, 0.30-1.50% by weight of chromiun, 0.10-0.70% 
by weight of molybdenum, and 0.15-0.40% by weight of vanadium, the balance 
being composed of iron and, as inevitable impurities, not more than 0.015% 
by weight of phosphorus and not more than 0.010% by weight of sulphur; (b) 
hardening by quenching said steel material heated at a temperature of 
940.degree..+-.10.degree. C.; and (c) tempering said hardened material at 
a temperature of 575.degree..+-.25.degree. C. 
The invention has thus succeeded in providing bolts of high strength which 
can not only fully satisfy the demands of the day requiring parallelly the 
high tensile strength of 140-160 kgf/mm.sup.2 and the enhancement of 0.2% 
proof stress, but also possess excellent resistance to delayed fracture 
and fatigue. The invented bolts are of great effect, being likewise usable 
in the traditional strength level with equal or more performance, and 
further usable in a wider sphere, for example as bolts resistable in a 
high temperature place.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention aims to improve the material steel for high strength 
bolts, considering the insufficiency of the traditional Cr-Mo type steel 
for answering the demand of the day to require higher and higher strength, 
by means of respectively limiting the content of elements to a specified 
ratio and minutely controlling the conditions of the heat treatment as 
follows. 
Carbon (C) is an essential element for increasing the tensile strength, and 
the lower limit of its content for ensuring the tensile strength of 
140-160 kgf/mm.sup.2 is 0.30% by weight. When however the content thereof 
exceeds 0.50% by weight, it deteriorates not only toughness but also 
resistance to delayed fracture, obliging the upper limit to 0.50% by 
weight. For particularly enhancing the resistance to delayed fracture, in 
respect to relation with other elements, it is desired to keep the C 
content within the range of 0.40-0.50% by weight. 
Silicon (Si) must be held down to as low content as possible, because it 
tends to promote internal oxidation and subsequently bring about the 
delayed fracture. Considering however its effect as a deoxidation element, 
only the upper limit of the content thereof is defined as 0.15% by weight. 
It is however preferable to keep its content below 0.10% by weight, for 
preventing deterioration in the resistance to delayed fracture by means of 
more effectively deterring the oxidation in the grain boundaries. 
Manganese (Mn) is, like Si, preferable to be held down to the lowest 
possible content because of its inclination to promote undesirable 
oxidation of the grain boundaries. Considering however its role to make 
sure the tempering, the upper limit of the content alone being defined 
here as 0.40% by weight. 
Phosphorus (P) must be reduced to the possible extreme limit so far as the 
refining technology permits, being consequently defined to 0.015% by 
weight or less, because it tends to embrittle the grain boundaries by 
segregating to the austenite grain boundaries in the course of 
austenization. It is more preferable to reduce it less than 0.010% by 
weight. 
Sulphur (S) is, like P, preferable to be held down to the possible lowest 
limit so far as the refining technology permits, because of its 
inclination to deteriorate the resistance to delayed fracture due to its 
segregation to the grain boundaries and its coexistence with Mn as MnS. It 
is defined to less than 0.010% by weight, being preferable to be further 
confined to less than 0.005% by weight. 
Chromium (Cr) is a necessary element for ensuring the resistance to 
softening of the invented steel. It is required to be contained, at the 
lowest, at the rate of 0.30% by weight so as to ensure a tempering 
temperature exceeding a certain temperature zone, wherein cementite is 
precipitated to the prior austenite grain boundaries, i.e., tempering 
temperature above approximately 500.degree. C. in the present invention. 
Cr tends to lower, when its amount is increased, hardness of the steel in 
the temperature zone for high temperature tempering, consequently 
hindering to get a stable tensile strength not less than 140 kgf/mm.sup.2. 
Its upper limit is fixed at 1.50% by weight, because of its liability to 
promote, like Si and Mn, the oxidation of the grain boundaries. It is 
however preferable to add it within a sphere of 0.90-1.10% by weight for 
stably obtaining a required tensile strength, preventing deterioration of 
the resistance to delayed fracture, and ensuring more effectively the 
hardenability and a temperature for the high temperature tempering. 
Molybdenum (Mo) must be added, at the least, at 0.10% by weight for getting 
the tensile strength, at a tempering temperature not less than 500.degree. 
C., within the scope of 140-160 kgf/mm.sup.2. Adding Mo superabundantly 
exceeding 0.70% by weight is utterly useless because of saturation of the 
effect caused thereby. Another reason for limiting the highest content of 
0.70% by weight is the expensiveness of the Mo element. It is however 
desirable to add Mo within the sphere of 0.45-0.65% by weight for ensuring 
a high tensile strength at a high temperature tempering. 
Vanadium (V) is effective, forming a carbide, for refining austenite 
grains, and consequently contributes not only to enhancing the proof 
stress but also to improving the toughness. It is, similarly to Mo, 
helpful in increasing resistance to softening by its secondary hardening 
phenomenon, through being precipitated as a carbide in the course of a 
high temperature tempering process. It is required to add if for this 
purpose at a rate not less than 0.15% by weight, more preferably not less 
than 0.25% by weight. Superabundant addition thereof is also useless 
because of saturation of the effect. It is necessary on the contrary to 
fix the upper limit of its content not exceeding 0.40% by weight and 
preferably not exceeding 0.35% by weight, because too much addition is 
even harmful due to degradation of the toughness through formation of 
coarse carbide (primary carbide) during the process of ingot casting or 
billet formation. 
Niobium (Nb), titanium (Ti), and zirconium (Zr) are respectively a useful 
element for making the crystal grains finer, indicating similar effect to 
V, and one or more of them may be optionally added, when necessary, 
because V is already added as the essential element. For each of them the 
content ratio is limited to within the sphere of 0.50-0.15% by weight. 
Addition thereof less than 0.05% by weight does not bring about the 
above-mentioned effect, and that exceeding 0.15% by weight uselessly 
saturates the effect because of the essentiality of V element addition. 
In regard to the heat treatment conditions applied on steels having the 
earlier mentioned specific compositions, for simply satisfying the 
strength standard 14.9 in the ISO classification a considerably wide range 
of hardening temperature, i.e. temperature of steel to be quenched for 
hardening, like 900.degree.-980.degree. C., and of tempering temperature, 
i.e. temperature of heated steel for tempering, like 
500.degree.-650.degree. C. is permissible. It has been discovered however 
in the experiments made by the inventors that application of the limited 
heat treatment conditions according to the invention on steels having 
compositions specified to the preferable range established by this 
invention remarkably improves the resistance to delayed fracture. Strict 
controlling of the hardening temperature within the range of 
940.degree..+-.10.degree. C. and the tempering temperature within the 
range of 575.degree..+-.25.degree. C. is therefore essential for 
parallelly ensuring both the excellent tensile strength and resistance to 
delayed fracture. 
EXAMPLE 1 
Steels respectively having the composition indicated in Table 1 were rolled 
into bars of 8.0 mm.phi.. Samples extracted from rolled bars were hardened 
from 940.degree. C. and tempered at 575.degree. C. Only the specimen L for 
comparison was hardened from 850.degree. C. and tempered at 450.degree. C. 
Each of the rolled bars was formed into m8 bolts, having been heat treated 
so as to have the tensile strength class of 140-160 kgf/mm.sup.2. The 
quality of the formed bolts body and the material bar was respectively 
checked. 
First of all, specimens or test pieces (FIG. 3) were made, according to JIS 
14A standard, out of the formed M8 bolts for executing the tensile 
strength test. The results are indicated in Table 2, wherein all of the 
invention steels A-J fully satisfied the ISO strength standard 14.9, i.e., 
tensile strength and 0.2% proof stress. In each of the groups of the 
invention steels, D-F and I-J, wherein one or more out of the three 
elements Nb, Ti, and Zr was added to make the structure finer, an 
individual specimen showed a higher 0.2% proof stress in comparison with 
any specimen out of the groups A-C and G-H of the invention steels, 
wherein none of the three elements was added. On the other hand, 
comparative steels K (AMS 6304D) and L (JIS SCM440) had both the required 
tensile strength, while the comparative steel L did not reach the standard 
0.2% proof stress. 
On the bolt body the resistance to delayed fracture was executed. In 
particular, a bolt body, on which a stress was loaded by means of 
fastening it up as high as 0.2% proof stress, was thereafter immersed in a 
test solution of 0.1N HCl for as long as two hundred hours. Number of 
bolts fractured during the test was checked out of the twenty test bolts 
for figuring out the percentage thereof. The results were shown in FIG. 1, 
by means of plotting them on a graph, wherein the tempering temperatures 
were put on the abscissa as a criterion so as to fix each plotting 
position within the range of tensile strength 140-160 kgf/mm.sup.2. As the 
comparative steel AMS 6304D was adapted to plot the result thereof on the 
same graph. 
As can be seen in the test results of delayed fracture executed on bolt 
bodies, the temperature range in which none of the twenty bolt bodies were 
fractured was as wide as between 550.degree. C. and 600.degree. C. in case 
of the invented steels (4) and (5), while that in case of the comparative 
steel AMS 6304D was 600.degree.-625.degree. C., being somewhat narrow. 
From the material bars of 8 mm.phi. bending type test pieces illustrated in 
FIG. 4 were made for executing delayed fracture test (bending type 
accelerated test). The adapted test method was as undermentioned. The 
bending moment was applied by the dead weight sustained at the extended 
end of the test piece in a cantilever type testing device. The test 
solution of 0.1N HCl, was dropped on the notched part of the specimen. The 
delayed fracture curve was described as the ratio of bending moment vs 
time to fracture. Based on this curve the stress at 30 hr: .sigma..sub.30 
hr (the stress at which fracture occurs after the holding time of 30 
hours) and the static bending stress: .sigma..sub.SB (the stress at the 
zero time of the bending moment application) were determined, so as to 
define the ratio: .sigma..sub.30 hr/.sigma..sub.SB as the delayed fracture 
ratio. The resistance to delayed fracture was numerically evaluated based 
on this ratio. In FIG. 2 relation between the delayed fracture strength 
ratio and the tensile strength is indicated, by taking the former on the 
ordinate and the latter on the abscissa. On the graph, data of the 
comparative steels JIS SCM440, which is commonly used as equivalent to ISO 
12.8 class, and AMS 6304D, which shows relatively high resistance to 
delayed fracture, are also indicated. 
In FIG. 2, superiority of the invention steels to the comparative steels, 
in respect to the resistance to delayed fracture, can be evidently 
observed. Particularly the invention steels (4) and (5), wherein chemical 
components are limited within a preferable range of content, indicate 
remarkably high delayed fracture strength ratio. On the other hand, the 
comparative steel JIS SCM440 indicates, even in the range of low tensile 
strength of 120-140 kgf/mm.sup.2, a gradual degradation of the delayed 
fracture strength ratio as the tensile strength rises upwards, while the 
invention steels indicate equal or higher ratio to the above-mentioned 
comparative steel even in such a high strength range. 
TABLE 1 
__________________________________________________________________________ 
Chemical composition of test steels 
(wt. %) 
Test steel 
C Si Mn P S Cr Mo V Nb Ti Zr 
__________________________________________________________________________ 
Invention 
A 0.32 
0.04 
0.36 
0.010 
0.006 
1.34 
0.20 
0.36 
-- -- -- 
steel B 0.41 
0.07 
0.15 
0.013 
0.008 
0.55 
0.63 
0.23 
-- -- -- 
(1) C 0.47 
0.11 
0.28 
0.009 
0.007 
0.38 
0.48 
0.17 
-- -- -- 
Invention 
D 0.38 
0.13 
0.37 
0.011 
0.005 
0.75 
0.16 
0.18 
0.13 
-- -- 
steel E 0.42 
0.08 
0.24 
0.010 
0.006 
0.47 
0.43 
0.30 
-- 0.11 
-- 
(2, 3) F 0.46 
0.11 
0.16 
0.008 
0.007 
1.22 
0.28 
0.26 
0.08 
-- 0.09 
Invention 
G 0.48 
0.05 
0.30 
0.004 
0.003 
0.93 
0.58 
0.33 
-- -- -- 
steel (4) 
H 0.42 
0.06 
0.25 
0.002 
0.002 
1.06 
0.62 
0.28 
-- -- -- 
Invention 
I 0.46 
0.04 
0.22 
0.007 
0.004 
1.05 
0.41 
0.32 
-- 0.06 
0.08 
steel (5) 
J 0.43 
0.05 
0.28 
0.003 
0.001 
0.92 
0.53 
0.26 
0.07 
0.11 
-- 
Comparative 
K 0.44 
0.28 
0.55 
0.024 
0.025 
1.04 
0.52 
0.29 
-- -- -- 
steel 
AMS 6304D 
Comparative 
L 0.40 
0.26 
0.74 
0.018 
0.027 
0.99 
0.21 
-- -- -- -- 
steel 
JIS SCM440 
__________________________________________________________________________ 
TABLE 2 
______________________________________ 
Results of tensile strength test 
Tensile 0.2% proof 
Elon- Reduction 
strength stress gation 
of area 
Test steel (kgf/mm.sup.2) 
(kgf/mm.sup.2) 
(%) (%) 
______________________________________ 
Invention 
A 143 130 15 55 
steel B 148 135 14 50 
(1) C 157 143 13 48 
Invention 
D 144 135 15 54 
steel E 150 140 13 49 
(2, 3) F 151 141 13 48 
Invention 
G 155 141 13 48 
steel (4) 
H 151 140 13 50 
Invention 
I 150 142 13 52 
steel (5) 
J 152 143 14 53 
Comparative 
K 147 138 13 50 
steel 
AMS 6304D 
Comparative 
L 150 121 11 52 
steel 
JIS SCM440 
______________________________________ 
EXAMPLE 2 
For studying and checking the influence of the heat treatment conditions, 
particularly that of the tempering temperature, to the resistance to 
delayed fracture, bolts were made under the same conditions as in the 
Example 1, however with the variable hardening temperature. In this 
experiment tensile strength test was executed along with a checking of the 
delayed fracture strength ratio performed partially with regard to the 
material steel. The results are indicated in Table 3. What has been found 
from this experiment is that a slight deviation of the hardening 
temperature from the predetermined range 940.degree..+-.10.degree. C., 
upwardly or downardly, does not affect the maintenance of the tensile 
strength at not lower than 140 kgf/mm.sup.2 level, but deteriorates the 
resistance to delayed fracture. 
TABLE 3 
______________________________________ 
Heat treatment conditions and strength 
Hardening Tempering Delayed 
temper- temper- Tensile fraction 
Test Classi- ature ature strength 
strength 
steel 
fication (.degree.C.) 
(.degree.C.) 
(kgf/mm.sup.2) 
ratio* 
______________________________________ 
G The 940 575 151 0.68 
invention 
Com- 935 500 156 0.55 
parative 
example 
I The 940 600 149 0.71 
invention 
Com- 960 575 150 0.60 
parative 
example 
______________________________________ 
*.sup..sigma. 30 hr/.sup..sigma. SB 
EXAMPLE 3 
Bolts must be, for being utilized as high strength bolts, high not only in 
the resistance to delayed fracture but also in the resistance to fatigue. 
As a means for enhancing resistance or strength against fatigue, it seems 
to be recommendable to divide the roll threading process into two stages, 
i.e., one half prior to the heat treatment and another half after the heat 
treatment, so as to raise the compressive residual stress after the heat 
treatment. It is appropriate, in this regard of division, to do the roll 
threading from 50 to 95% prior to the heat treatment, so as to leave from 
50 to 5% thereof after the heat treatment. 
For the purpose of ascertaining this theory, roll threading test was 
executed on a bolt body of the invention steel H, which was obtained in 
Example 1, under the conditions of roll threading indicated in Table 4. 
The test was concerned to fatigue of the bolt, conditions and results 
thereof being indicated in the Table 4. What was found from the experiment 
is that the strength against fatigue can be raised, in the bolts of the 
invention steel, without deteriorating the resistance to delayed fracture, 
which is originally the strong point of the invention steel. Further 
raising of the strength against fatigue can be expected in the division of 
the roll threading before and after the heat treatment. 
It was ascertained in another experiment that raising of the compressive 
stress, in ordinary steel for bolts, i.e., raising of the strength is 
liable to deteriorate or sacrifice the resistance to delayed fracture. 
TABLE 4 
______________________________________ 
Alternating fatigue test 
Test Tensile Fatigue strength 
steel strength Roll threading 
at 2 .times. 10.sup.6 cycles 
______________________________________ 
H 153 Before heat 11 kgf/mm.sup.2 
kgf/mm.sup.2 
treatment 80% 
After heat 
treatment 20% 
Before heat 9 kgf/mm.sup.2 
treatment 100% 
______________________________________ 
Test condition: Average stress 81 kgf/mm.sup.2 
The steel according to this invention was developed aiming at the use in a 
class of strength 140-160 kgf/mm.sup.2, but it can of course be used, as 
is evidently cleared in the Examples, at a lower strength with the 
expectation of equal or higher performance than the conventional steel. 
Furthermore, the invented high strength bolt can be used not only 
undernormal room temperature, but also under high temperature. 
It must be understood that various slight alterations and variations can be 
thought of by those skilled in the art, and that this invention is not 
limited to the disclosed examples and what was described herein, but 
include all of those modifications so far as they do not deviate from the 
spirit and scope of this invention stated herein and appended claims.