CA1084310A - High tension steel sheet product - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

High tension steel sheet having excellent low-tempera-ture toughness and a yield point not lower than 40 kg/mm2 is produced by heating a steel ingot or slab to a temperature not higher than 1150°C, and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temp-erature range not higher than 930°C and a finishing temperature not higher than 830°C, the steel ingot or slab contains 0.01 to 0.13% C, 0.1 to 1.0% si, 0.7 to 2.0% Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.0009% total N, 0.01 to 0.10% Nb, one or both of 0.01 to 0.15% V, 0.05 to 0.40% Mo, with the bal-ance being unavoidable impurities and Fe, in which (Nb%) x (C%) ? 5 x 10-3 and TiN is not larger than 0.02µ is not less than 0.004%; the steel sheet is useful in the manufacture of steel pipes, fittings and other components to be employed in cold regions.

Description

31~ ~
I'he present invention rela-tes to a process for produc-ing a high tension steel product, such as plates, sheets and strips (herein called sheets) having an excellent low~temperature toughness with a yield point of 40 kg/mm2 or higher.
The steel products according to the present invention - are useful as hot rolled or as heated at a temperature ranging from 300 to 750C. after the hot rolling.
-; Conventionally, steel sheets such as ~or pipe lines in cold regions which are required to have high strength and tough- ;
ness in the "as rolled" condition have been produced by a method ;
~ called "controlled rolling" (hereinafter abridged as CR), and -~ mainly Nb-containing steels have been used for this purpose.
In general, CR method is composed of two steps, the first step is a heating step and the second step is a rolling step (cooling), if largely classified. And the following con-siderations must be made in these steps respectively.
(1) In the heating step, it is required to dissolve - elements such as Nb and V enough for refinement of the struc~
ture and precipita~ion hardening, and it is required to maintain ~i 20 the austenite grains during the heating (heated y grains) as -, . ..
flne as possible.

(2) In the rolling step, it is necessary to recrystal-lize the heated y grains repeatedly by the rolling to obtain re-- fined rolled austenite grains (rolled y grains), and it is neces-sary -to elongate the rolled y grains and reduce their thickness by rolling in their non-recrystallization zone so as to obtain refinement of~the rolled structure.
However, in case of Nb-containing steels as commonly used, Nb(CN) is stable at h gh temperatures and it is difficult to ressolve Nb(CN) consistently and satisfactorily even by a long time heating i~ heating temperature is not higher than 1150C.

'~1 , ~ . .

3~L~

If the heat:ing temperature is raised, it is possible to attain satisfactory solid solution of Nb(CN), but on the other hand, the heated y grains grow e~cessively, thus resulting in considerable deterioration of the toughness of the rolled steel.
Therefore, in the CR method, it is necessary to lower the heating temperature and maint:ain the heated y grains smaller when severe low-temperature toughness requirement is satisfied.
On the other hand, when the heating temperature is lowered, the amount of Nb in solid solution increases or decreases depending ,on a slight change in the heating temperature and time in case of the commonly used Nb-containing steels, and even under the same rolling condition, resultant strength fluctuates in a wide -range depending on the change in the amount of the solid solu-tion ~b, and a high strength, if obtained is accompanied with deterioration of toughness. Thus, it is difficult to obtain a stable balance between strength and toughness. ^me above difficulties can be attributed to the facts that toughness lowers in proportion to the increase of strength, and the increase of strength corresponds to the increase of the .,., ~ - .
amount of Nb(CN) ln solid solution and the coarsening of the heated ~ grains, so that the steel structure will be of coarse '~ grains and~mixed grains.
However, in the conventional CR method, proper consid~
eration has not been given to the fact that the heated y grains ' coarsen when enough Nb(CN) is dissolved in solid solution during ~, ~ the heating step, and thus toughness is deteriorated. -As described above, it is necessary to prevent the ~;
growth of the heated y grains by means of the precipitation in order to maintain fine heated y~grains and improve toughness~
For thls purpose, it is required~to lower the heating temperature and keep the precipitates such as Nb(CN) from solid ;, ~
`~ solution during the heating. On the other hand, in order to ; maintain the strength, it is necessary to dissolve Nb(CN) into :j 3~L~

solid solution as much as possib:le duriny the heatiny so as to precipitate it during the cooling after the rolling to strengthen ` the steel. For this purpose, it is desirable to maintain the heating temperature as high as possible. :
Therefore, one of the objec-ts of the present invention is to solve the completely contradictory problems as mentioned above, and provide a steel sheet having remarkably smaller heat-.~ ed y grains than those of conventional steels in spite of Nb(CN) in solid solution for strength, and showing a stable and excel-lent balance between strength and toughness if appropriate roll-:` ing conditions are applied thereto. .
The features of the present invention may be summarized as below.
(1) In order to attain satisfactory and stable Nb(CN) ~:: in solid solution, the carbon content is lowered to an extreme : degree as understood from the solubility product relation. ~
` (2) The growth of the heated y grains due to ~b(CN) .. ~-in solid solution is prevented by TiN which is stronger than .: Nb(CN) for prevention of the growth of the heated y grains, and :` ;
20 . (3) Optimum rolling conditions are selected.
- By the above features in combination, it is possible .
; to utilize Nb(CN) and TiN separately for different purposes~
the former for strengthening the steel and the latter for pre- . ::
.~ venting the growth of the heated ~ grains, and thus the problem :~
in the heating step can be solved. .
.~; .
~ Starting from the fine heated y~grains, a rolled struc- ~ .
..~. ture having~still finer grains can be obtained by rolling under .
proper conditions and remarkable strength and toughness can be obtained through the decrease in the pearlite proportion attain- .
ed by the lowered carbon content as well through the grain refine-. ment.
' Regarding improvement of the steel toughness by refine-:. - 3 -`'~ "

. ~ ~ . . . - .

343~
ment of the heated y grains, the present inventors have pre-viously disclosed a method therefor, and the present inventors have conducted further various extensive ~tudies on production of a high tension steel having excellent toughness at low temperatures, and have found that the toughness can be stabilized and improved remarkab].y according to the production process of the present invention~
The production process according to the present inven- ;
tion is characterized in that a steel ingot or slab containing ` 10 not le~s than 0.004% of TiN not larger than 0.02~ is heated to a temperature not higher than 1150C. and rolled, and growth of the y grains during this heating and rolling step is prevented ;~
by TiN to improve the toughness. --.~
Thus in one aspect the invention contemplates a process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than ; 40 kg/mm which comprises heating a steel ingot or slab to `~ a temperature not higher than 1150C., and rolling the steel ` ingot or slab thus heated with a total reduction amount not ~ 20 less than 50/O in a temperature range not higher than 930C.
-` and a finishing temperature not higher than 830C., said steel ingot or slab containing in weight % 0.01 to 0~13% C
0.1 to 1~0% Si~ 0.7 to 2.Co/o Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.00~/O total N, 0.01 to , . . .
. 0~1~/o Nb, and 0.05 to 0.40/O Mo, 0 to 0.03/0 rare earth metal ~i ,. : .
, (REM), 0 to 0.03/O Ca, 0 to 0.15% V, 0 to 0.6% Cr, 0 to l~C% ~ -Cu, 0 to 4.0% Ni, 0 to 0.02% S, with the balance being ;,, , :, unavoidable impurities and Fe, in which (Nb%) x ~/Oj 5 x 10 3 and the content of TiN having a particle size not larger than 0.02~ being not less than 0.004% by weight and provided that when a rare earth metal (REM) and sulphur are present REM/S is 1.0 to 6~0 and when one or more of Cu, ~,:

Ni, Cr ~nd Mo is present (Cu -~ Ni)/5 + Cr ~ Mo ~ 0.90/O .
- In another aspect of the invention there is provided a high tension steel sheet having excellent low : temperature toughness with a yield point not lower than 40 kg/mm2, and having a composition as defined in the previous paragraph.
In a particular embodiment of the invention there is provided steel pipe formed from the steel sheet of the invention.
The present invention will now be described with ~ ~
reference to the accompanying drawings which show a preferred ;~ . -form theréof and wherein: .
.~ Figure 1 shows the relation between the heated y grain ; :~
size and the content of TiN (%) not larger than 0.02~, when ~ : .
- heated to 1150C. and held at the temperature for 60 ; 20 minutes' ~
Figure 2 shows the relation between the heating temp- ~ .
; eratures and the heated y grain size when the steel No. 2 . : :
,. in Table 1 according to the present invention is heated to .
`. various temperatures and held at the various temperatures ., :~
for 60 minutes. ;~
Figure 3 shows the relation between the ratio of NaS .
: .~ TiN/N (marked ~y 0) and the heating temperature when the ;: steel No. 1 in Table 1 according to the present invention ~.
is heated to various temperatures and hald at the various . , .,..:
: 30 temperatures for 120 minutes and rapidly cooled in water, . and the relation between the content (%) of TiN (marked by ) not larger than 0.02~ when the same steel is heated to ~ :
~. , :
: . ;:
,,, :~:

, : - .

` - 4a -.. ..
:' ~

1150C. and held at the temperature for 120 minutes;
Figure 4 shows -the relation between the average cooling rate and -the content (%) of TiN not larger than 0.02~ when the steel No. 1 in Table 1 according to the present inven-tion is cast at various solidification rates, -~ Figure 5 shows the relation between the amount of solid solution Nb and the carbon content when steels with differ-ent carbon contents are heated to various temperatures and held at the various temperatures for 30 minutes;
Figure 6 shows the relation between the heating temp-erature and the product of (solid solution Nb%) x (solid ;, solution C%) when the steel according to the present inven- -; tion is heated to various temperatures and held at the various temperatures for 60 minutes;
Figure 7 shows the relation between the reduction - amount at temperatures not higher than 930C. and the yield . . ., ~
strength YS as well as vTrs in the steel No. 2 in Table 1 according to the present invention, and ~ Figure 8 shows the relation between the finlshing temp~
;~- 20 erature;and YS as well as vTrs in the steel No. 2 in Table ^;
-~ 1 according to the present invention.
Regarding the definition of TiN not larger than 0.02~
it also includes Ti and N which are present in solid solution in the steel and TiN which is present in the form of precipitate and has a size not larger than 0.02~. Ti and N which are present ;in solid solution in the steel precipitate as TiN not larger than - 0.02~ during the subsequent heating and effectively prevent the ;~ ;
coarsening of the heated y grains. In this case, according to the studies made by the present inventors, there is~acorrelation 30 between the heated ~ grain size and the heating rate, and when the heating rate starting from 800C. to a predetermined tempera- ~-ture is excessively high, Ti and N do not precipitate ~ully, thus _ 5 _ ',: :
, I .

31(~
failin~ to obtain a satisfactory reLiillement o~ the heated y grains. Therefore, in order -to r~fine the heated y grains, it is necessary to decrease the heating rate to some degree, and i-t is preferable to control the heating rate starting from 800C. to a predetermined temperature to a rate not larger than 6C/min.
In Figure 1 which shows the amount of TiN not larger than 0.02~ and the heated y grain size, when the steel is heated to 1150C. and held at the temperature for 60 minutes, it is clear that unless TiN not larger than 0.02~ is present in an amount of 0.004% or more, no satisfactory refinement of the heated y grains can be expected. Therefore, it is necessary that TiN not larger than 0.02~ is present in an amount not less ~ ~
than 0.004% in the steel before the heating. However, even when ~ ;
this condition is satisfied, prevention effect on the coarsening of the heated y grains by TiN becomes unstable if the heating temperature is excessively high. -;-As is understood from Figure 2 showing the relation be~
tween the heating temperature and the heated y grain size, it is necessary to maintain the heating temperature at 1150C. or lower, preferably in a range from 900 to 1150C. in order to obtain fine heated y grains (not lower than No. 3 o~ ASTM).
As described above, satisfactory refinement of the heat-ed y grains can be obtained under appropriate condition when TiN
not larger than 0.02~ is present in an amount not less than 0.004%.
Hereinbelow descriptions will be made on the method of ~ -introducing lnto the steel not less than 0.004% of TiN not larger than 0.02~ in connection with the ingot-making method and the continuous casting method respectively.
In the ingot-making method, the coarse TiN which has - precipitated during the solidification step of the molten metal , are dissolved in solid solution in an amount not less than 0.004%

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: during the ingot heatiny (soaking) s-tep, and part of the solid solution TiN i~ precipita-ted during the brea]cdown rolling step and the cooling step to maintain not less than 0.004% of TiN, not larger than 0.02~ in the steel slab before the heating. In this case, if the Ti content is excessive, it is diffi.cult to maintain not less than 0.004% of TiN in solid solution during the ordinary ingot heating step, because TiN precipitates in an excessively coarse form during the solidification step. Even in ~ this case, the solid dissolution of TiN depends on the heating - 10 temperature and the holding time, but if the heating temperature is too high, there is caused the burning phenomenon and thus . there is a certain maximum heating temperature inherent to the .steel. Therefore, on the basis of the present steel making techniquesit is necessaxy that the content of Ti is maintained ~ not larger than 0.03% and the amount of Ti required for the -~
minimum amount of 0.004% for TiN not larger than 0.02~ is 0.004% ~ ~-on a comntercial production taking into consideration the amount of Ti consumed for formation of oxides etc. Therefore, the :
content of Ti should be in a range from 0.004 to 0.03%.
.
: 20 Next, detailed explanations will be made hereinbelow on the llmitations of the heating temperature for a solid dissolu-. tion of TiN which has precipitated during the solidification of.
.
: the molten steel as well as the limitations of the N and TiN
l contents.
: For economical and stable solid dissolution of TiN
during the heating step on a comntercial level, it is effective to llmit not only the Ti content, but also the N content. The : reason for setting the lower limit of the total N content at ,~-. 0.001%, is that it is the minumum amount required for the lower .i 30 :limit of 0.004% of TiN which must be dissolved in solid solution . during the heating step. Further, in order to maintain an enough . amount of TiN which is dissolved in solid solution during the .
:~ - 7 -.:

.. .. .

heating step, i-t is not favourab]e that the upper limit of the total N content exceeds the upper limit of the Ti content.
Therefore the upper limi-t oE the to-tal N content is set at 0.009% which corresporlds -to 0.03% of Ti. On the other hand, i~
the TiN content exceeds 0.04%, the toughness of the steel sheet is deteriorated and thus it is necessary to set the upper limit of the TiN content at 0.04%, but so far as the Ti and total N
contents are within the ranges defined above, the TiN content does not exceed 0.04%.
; lO When the Ti and N content are within the ranges defined in the present invention, the lower limit of the heating tempera-,,, ~, ture for dissolving not less than 0.004% of TiN into solid solu-tion may be 1250C. as shown in Figure 3 and confirmed by ex-periments, while the upper limit is set at 1400C. as a practic-ally feasible temperature in spite of a partial burning of the iron oxide on the steel surface.
In the continuous casting method, where the steel slabs are made directly from the molten steel, if the Ti and N
contents are excessive, coarse TiN precipitates during the solid-ification so that it is impossible to maintain not less than 0~004% of TiN not larger than 0.02~, therefore, just as in the ingot-making method, it is necessary to limit the Ti and N con-tents respectively to 0.004 to 0.03% Ti and 0.001 to 0.009% N.
Even when the Ti and N contents are within thase ranges, it is :: -- impossible to obtain not less than 0.004% of TiN not larger than - ~0.02~, if the solidification cooling rate is too slow. Therefore, ~i it is desirable the cooling rate at the center portion of the ~ steel slab is not less than 8C/min. in average from the molten -, steel temperature at the time of casting to 1100C. When the :.1 ".: .
' 30 cooling rate is below 8C~min., it is difficult to attain not less than 0.004% of TiN not larger than 0.02~ in the steel slab as shown in Figure 4 and no effective prevention of -the coarsening . ~ .

~ ~ 8 -.-, , of the heated y grairls can be assured.
The basic features of -the present invention have been described hereinabove.
It has been further found by the present inventors that the hot rolled steel material obtained by the above production process is reheated to a tempera-ture ranging from 300 to 750C., ; part of the fine carbides or the solid solution carbon coagu-lates into carbides of favourable size so that the toughness is improved due to the relief of stress by the precipitation harden-.~ 10 ing of the matrlx, and the arrest property as represen-ted by ~: B~WTT (Battele Drop Wear Tear Test), as well as the yield strength are still remarkably improved.
. Explanations will be made on the limitation on the ~-~ steel compositions defined in the present invention. `
.
The base steel composition applicable to the present :

invention comprises 0.01 to 0.13% C, 0.1 to 1.0% Si, 0.7 to ~ :
.. ` 2.0% Mn, not more than 0.10% total Al, 0.004 to 0.03% Ti, 0.001 ~: to 0O009% total ~, 0.01 to 0.10% Nb, one or more of 0.01 to -~ 0.15% V and 0.05 to 0.4% Mo and satisfying the condition of ;;~ `

(Nb%) x (C/O) ~ 5 x 10 3 ~-~ Now the lower limit of 0.01% for the carbon content is set because it is a minimum amount for assuring the grain refine-~, ment of the steel material and strength of the weld joints as -j well as full development of effects of carbide forming elements ,~j . , ~ such as Nb and V. On the other hand, when the carbon content is ~ .
, ~. excessive, the amount of Nb in solid solution readily increases .,,,. ~
.t~: .
~ or decreases depending on even slight changes in the heating ;;
.~. :
:', conditions as mentioned hereinbefore, and thus the strength--toughness balance becomes unstabl~e. Therefore, it is effective . 30 to define an upper limit for the carbon content for assuring a . . : stable solid solution of Nb(CN) in the steel slab to maintain desired strength and toughness even in cases where the heating . , .
.j _ 9 _ .. ; .

temperature is below 1150C.
In Figure 5 showing the rela-tion between the amount of the solid solution Nb and the heating temperature in connection with various carbon contents, i-t is clearly shown that when the carbon content is lowered, the arnount of the solid solution Nb (at a cons-tant total Nb content of 0.05%) increases, and when the carbon content is not higher than 0.13%, Nb is completely dissolv-ed in solid solution at 1150C.
The reason for defining the total Nb content of 0.05%, is that this amount is enough for obtaining desired strength and toughness in case of 0.13% C. Thus the upper limit of the carbon ~ ~
content is set at 0.13%. In cases where the Nb content is large ~' or the heating temperature is below 1150C., it is necessary to further lower the carbon content in order to assure a stable and enough Nb(CN) in solid solution. But for this purpose it is '.:
favourable to limit not only the carbon content by itself, but ~ `
also the carbon content in relation with the Nb content. .. ,~
Figure 6 shows the experimental results concerning the relation between the heating temperature and (solid solution Nb%)~

; .- :
'.' 20 x (solid solution C/O)~ and it is shown that Nb(CN) can be stably dissolved in solid solution when (~/O) x (Nb%) ~ ~solid solution : .. ..
Nb%) x Isolid solution C%). :

'., Withln the preferable heating temperature range from :~

' 1050 to 1150C. according to the present invention, it is prefer~ .: ,;

. ~ abLe to define as below despite some fluctuation in the data.

(C%) x (Nb%) < 5.0 x 10 ,~

~f For the reasons set above, the upper limit of the car-`, bon content is set at 0.13% and the carbon content is further ~ ;

~: limited in relation with the Nb content as (C%) x ,(Nb%) < 5 x 10 ~

,, Silicon is an element which comes into the steel un- ' .

avoidably during the deoxidation step ! but less than 0.1% silicon ..

' - 10 -~8~3~L~

causes deterioration of the tou~hness. Therefore, the lower limit of the silicon cont~nt is set at 0 1%. On the other hand, when the silicon con-tent is excessive it damages the cleanness of the steel. Thus, the upper limit of the silicon content is set at 1.0%.
Manganese is an important element for assuring the desired strength and toughness of the low-carbon steel applicable to the present invention, and with manganese contents less than 0.7% the strength and toughness are low. Thus the lower limit of the manganese content is set at 0.7%. On the other hand, when the manganese content is excessive, the toughness o~ HA2 (heat affected zone) deteriorates. Thus the upper limit is set at 2.0%.
Aluminum is contained in a killed steel unavoidably from the deoxidation step. However, when the total Al content exceeds 0.1%, not only the toughness of HAZ but also the tough-ness of t~e weld metal are remarkably deteriorated. Thus, the upper limit of the total Al content is set at 0.1%.
Regarding the Ti and total N contents, they are limited to 0~004 to 0.03% Ti and 0.001 to 0.009% total N respectively as mentioned hereinbe~ore. So far as Ti and N are within these ;;
ranges the TiN content does not exceed 0.04%. ~ `~
Niobium lS added for improving the toughness of the steel material and expanding the feasible range of the plate - thickness as well as for assuring the joint strength of the welded portion. The lower limit of the Nb content is set at ;
0.01% for -the reason that with Nb contents less than 0.01%, the desired refinement of the strucbure and the precipitation strength~ning by Nb cannot be attained, and thus it is difficult to~obtain the desired strength and toughness. However, Nb addi-tion beyond 0.10% causes difficulty in attainlng stable and enough solid solution Nb with a heating temperature not higher `
than 1150C., and causes HAZ toughness deterioration.

~84;3~(~
Van~dium, similar a~ niobium, may be con-tained up to 0.15%.
Molybdenum, similar as niobium and vanadium, increases hardening of X~Z and lowers ~Z toughness and cracking resis-tance, if present in an excessive amount. Therefor~, the upper limit of the molybdenum is set at 0.40%. The lower limits of V
and Mo are set at 0.01% and 0.05% respectively, because these amounts are minimum amounts for development of full effectiveness of these elements.
The steel applicable to the present invention contains phosphorus and sulfur as impurities. Regarding the phosphorus content, it is usually not more than 0.03% and phosphorus is not intentionally added, and a lower phosphorus assures improvement of toughness. Regarding the sulfur content, it is usually not more than 0.02%, and it is possibLe to lower the sulfur content to about 0.0005% by the present level of the techniques, and ;~
thereby the toughness of the steel sheet is improved. In the present invention, sulfur is not added lntentionally.
According to one modification of the present invention, -~
one-or more of 0.001 to 0.03% REM (mainly Ce, La, Pr) and 0.0005 -~
- to 0.03%, preferably 0.0005 to 0.003% Ca is added under the -;
., condition of REM/S = l.0 to 6.0 With this modification, the toughness of the steel product ob-tained by the present invention is still fur-ther improved as shown in TabIe 2O ;, REM contents less than 0.001% produce no practical improvement of toughness, while REM contents exceeding 0.03% `
cause increase not only in size but also in amount of REM-oxy-sulfides, so that large inclusions are formed, which damage remarkably the toughness as well as the cleanness of the steel product.

, , - ., .

1~334~

ThereEore, the REM content is limited to the ranye from 0.001 to 0.03%. Meanwhile REM is effec-tive to improve and stabilize the toughness of -the steel sheet in correlation with the sulfur content, and the optimum range for this purpose is 1.O to 6.0 of REM/S. Calcium has similar effects as REM and is limited to the range from 0.0005 to 0.03%, preferably 0.0005 to 0.003%.
According to another modi~ication o~ the present inven-tion, one or more of not more than 0.6% Cr, not more than 1.0%
Cu and not more than 4.0% Ni is added under the condition of (Cu + Ni)/5 ~ Cr + Mo < 0.90%.
The main object of addition of these elements is to improve the strength and toughness of the steel product and to expand the feasible plate thickness range. Naturally, the addi-tion of these elements has limita-tion in their amounts, but in the low-carbon steel applicable to the present invention, their upper limits may be higher than those in an ordinary carbon steel.
Regarding chromium, an excessive chromium content increases the hardenability of HAZ and lowers the toughness and ;
cracking resistance. Therefore, the upper limit of the chromium ;
content is 0.6%.
Nickel is effective to improve the strength and tough-ness of the steel product without adverse effect on the harden-ability and toughness of ~Z, but nickel contents exceeding 4.0%
are not favourable on the hardenability and toughness of HAZ even in case of a low-carbon steel as used in the present invention.
Therefore, the upper limit of t~e nickel content is set at 4.0%.
Copper has almost similar effects as nickel and fur-ther improves the hydrogen-induced cracking resistance, but copper contents beyond 1.0% cause the copper-cracking during the rolling.
Therefore, the upper limit of the copper content is set at 1.0%.

Further, the above addition elements are not added independently within their respective ranges, but they are added ~84~

under the condi-tion of (cu ~ Ni)/5 -~ Cr ~ Mo < 0.90%
Otherwise the hardness oE HAZ is remarkably higher so that HAZ is susceptible to cracking during a small heat--input welding, and thus the steel cannot be used for welding.
In a still further modif:ication of the present inven-tion where the steel product after hot rolling is reheated in a temperature range from 300 to 750C., the basic steel composi-tion should be limited.
First of all, when the carbon content is more than 0.10%, the amount of Nb, V or Mo which is dissolved in solid ;
solution during the slab heating decreases so that the amount of the fine carbide precipitates of Nb, V or Mo during the re~
heating which is favourable for the strength, particularly the tensile strength, decreases. ~'~
Further, in the reheating step after the hot rolling, the fine carbides are coagulated into suitable sizé~so as to im-prove the toughness. For this purpose, carbon contents less than ;~;~
0.08% are remarkably effective without formation of excessively large coagulated carbides.
Regarding the aluminum content, deoxidation of the `~
;,. . .
molten stee} by aluminum is particularly necessary for assuring enough precipitates of fine carbides of Nb, V or Mo during the réheating, which are required for the desired strength. There-fore, aluminum lS present in an amount of 0.005% at~least.
The sulfur content should be limited 0.010% or lower so as to fully develop the toughness improvement by the reheat-ing Descriptions have been made on the limitations of the '~
various elements of the steel composition used in the present invent1on. It has been further found that it is difficult to produce a steel sheet having an excellent iow-temperature tough-, '. , ness and hicJh strenyth of not lower than ~0 kg/mm2 yield poin-t by rolling the steel of -the defined composition within the de-fined range merely in an ordinary ~ay. There~ore, in the present invention, the final rolling condi1;ions are limited.
As the basic feature of the present invention, the rolling condition has been defined as below.
The~total reduction amount in the temperature range not higher than 930C. is not less than 50% and the finishing temp-erature is not higher than 830C. Under this rolling condition, the strength and toughness of the steel product are improved remarkably.
Explanations will be made on the limitations of the rolling condition.
:
When the total reduction amount at 930C. or lower is not less than 50%, the yield point and toughness are remark-ably improved as shown in Figure 7, but if the total reduction -amount in the temperature range is less than 50%, it is impos- ~-sible to obtain a yield point not lower than 40 kg/mm2 and an excellent toughness. However, even when the total reduction amount in the temperature range is not less than 50%, the desired strength and toughness cannot be obtained if the finishing temp-erature is higher than 830C. as shown in Figure 8.
Regarding the finishing temperature or the rolling temperature in several reductions prior to the finishing, satis-factory low-temperature toughness~is obtained even when the temp-erature is partially below the Ar3 transformation point if the steel composition being treated-is within the range defined in the presen-t invention and the rolling is done as defined. There-fore, some dual phase (y - ~) rolling is within the scope of the present invention. However, it is desirable the temperature is not lower than 650C. from the aspect of toughness.

In case where a continuous casting slab is used, the - 15 ~

3~

slab is introduced directly -to the hot rolling step, for example into a heating furnace for a thick plate mill, and rolled un~er the condition that the total red~lction amount in the ternperature range not higher than 930C. is not less than 50% and the finish-ing rolling temperature is not higher than 830C.
Meanwhile, in case where steel ingo-ts made by the ingot-making process are used, the steel ingot is charged in ~ ~
a heating furnace in the break-down rolling step where it is ~;
heated to a temperature ranging from 1250 to 1400C. to obtain not less than 0.004% TiN in solid solution, and broken down, then subjected to the reprecipitation heating not higher than 1150C.
in a heating furnace of the subsequent hot rolling step, and rolled under the condition that the total reduction amount at 930C. or lower is not less than 50% and the finishing rolling temperature is not higher than 830C.
Regarding the cooling rate after the break-down rolling, a higher rate is better, and the effect Or the cooling is more remarkable with a less titanium content. For the subse- ~;
quent hot rolling step, a plate rolllng mill is desirable, but `~
the present invention is not limited thereto and applicable to production of a hot steel strip and steel wire.
The basic rolling condition in the present invention has been described above, but this basic rolling condition should ~
be further limlted as below when the reheating is added accord- ~;
ing to the modification of the present invention. First, the total reduction amount should be limited as below. Thus, the total reduction ambunt at 900C~ or lower should be 60% or more.
Less than 60%, the amount of the fine precipitates Nb, V or Mo which are required for remarkably increasing the strength and the toughness after the reheating is not enough, and thus the result- ;`
ant strength and toughness are not satisfactory. On the other hand, the total reduction amount at 900C. or lower is more than - . : : ~: . . .~. , ~

.. , .. . , - . ~ ..

34~

95%, N~, v or Mo precipitates are coarse so that it ;ls difficult to obtain the desirecl fine carbicles, and it is di~ficult to main-tain the desired strength, particularly the desired strength after the reheating.
Regarding the finishing rolling temperature, it should be further limited to 800C. or lower. Otherwise the amount of the fine precipitates is not enough and the resultant strength and toughness are not satisfactory. On the other hand, when the finishing temperature is below 500C., it causes deterioration of toughness due to intermittent workings and excessive precip-itation of the fine carbides of ~b, V or Mo which coagulate into coarse form during the reheating step so that satisfactory strength cannot be maintained.
When the finishing temperature is low, the rolling is ~ -done in a ferrite-predominant zone so that the precipitates of fine carbides of ~, V or Mo are excessively formed in the worked ferrite matrix. This is rather unfavourable for the strength-; toughness balance. Therefore the finishing rolling temperature should be preferably not lower than 700C. On the other hand, when particularly excellent toughness is to be obtained, coarseprecipitates of carbides of Nb, V or Mo are promoted by excessive - working in the austenite zone of higher temperatures, and the coarse precipitates coagulate excessively in the reheating step and produce adverse effects on the toughness. Thus, in this case, it is preferable to maintain the finishing temperature not higher than 780C. ~ Therefore, in respect of both the strength and the toughness, the most preferable finishing temperature ran~e is from 700 to 780C.
Regarding the heating step after the hot rolling step, this step is required for uniformly and appropriately coarsening the fine carbides of ~b, V and Mo, thus relieving the stress of the matrix due to the precipitation hardening and improving the .

. .: . . . ................ . .
- . ~ .: . . . :, . - :~

84~

toughness. For -this purpose, a minimum temperature of 30~C. is enough. On the other hand, when the reheating temperature is higher than 750C., the above fine carbides become coarse excess-ively, thus lowering the strength considerabiy. The most prefer-able reheating temperature range for both the strength and the toughness is from 500 to 700C. Meanwhile, regarding the holding ~ ;s time in the reheating step, it should be at least one mi~ute for uniformly and appropriately coarsening the fine carbides, thus relieving the stress of the matrix due to the precipitation hard-ening and improving the toughness.
On the other hand, if the holding time is longer than i10 hours, the fine carbides become excessively coarse, thus lower- ;
ing the strength considerably. The most preferable holding time range is from 10 minutes to 2 hours for both the strength and ~
the toughness. ~ -The reheating step as defined above may be done before ; ;~
the hot rolled steel sheet cools down near the ordinary tempera- ;~ ~:
ture. In this case, the reheating has also the effect of hydro-gen removal.
The limitations of the production conditions in the case where the reheating step is added have been explained before.
The steel~produc-ts obtained by this modification have been found to have also an excellent resistance~against the hydrogen-induced cracking. ;
Although it has not been fully clarified, the hydrogen-induced cracking resistance may be attributed to the fact that . .
the carbon content is low wlth less segregation, that formation of coarse carbides is prevented by the formation of fine carbides of Nb, V or Mo, and that the stress of the matrix is relieved by the uniform coarsening of the fine carbides during the reheating step.

The present invention will be more clearly understood `

~343~

~rom the examples showll in the tables.
Tables 1 to 3 show examples according to the basic process of the presen-t invention.
TabLe 4 shows examples according to the modification of the present invention. In these examples, various steel com-positions as shown (G: electric furnace steel, Cl, C2, C3: refin-ed in converter and with special phosphorus treatment) were made into slabs (L, M: continuous casting) and hot rolled. The con-ditions of slab making and hot rolling are shown in Table 4.
Thickness of the products and tensile strength (API ;~
test piece) in the direction at right angle to the rolling, 2 mmV
Charpy impact property, B, DWTT 85% SATT property, and 2mmV Charpy ~ ;~
impact values of 50% bond portion of submerged arc welding joints welded with 30 KG/cm input are shown in Table 4.
Further Table 4 shows the number of cross~sectional ;~
.
crackings (per 5mm thickness) of the test pieces (ground 1 mm -; ;
on both sides) after immersion in 100% H2S saturated aqueous solution (25C.) for 96 hours.
~ ~ As clearly shown in Table 4 the steels Al, B1, Cl, M
and N accordlng to the present inventlon show excellent tensile ~;~ strength property and toughness, particularly DWTT-property, as well as excellent weld toughness and hydrogen-induced cracking resistance.
The steels A2, A3, B2, B3, C2 and C3 having the steel composition within the range defined in the present invention . . .
but outside the scope of the present invention in respect of ;~
, the rolling condition and the r~heating condition show inferior ~ -~

properties. ~ ~ , . .
As clearly understood from the examples, the steel product according~to the present invention has excellent strength , toughness and additionally excellent weldability and hydrogen- ;

induced cracking resistance.

: ' ~

~8~3~(~

The steel procluct accorcling to the present invention ;
is mos-t suitable for procluction o~ steel pipes and also is use-ful for fitting tank structural components, ship building materials frame members of various machine and apparatus for cold regions, etc. where the arrest property is required.

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Claims (28)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm2 which comprises heating a steel ingot or slab to a temperature not higher than 1150°C., and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930°C. and a finishing temperature not higher than 830°C., said steel ingot or slab containing in weight % 0.01 to 0.13%
C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, and 0.05 to 0.40% Mo, 0 to 0.03% rare earth metal (REM), 0 to 0.03% Ca, 0 to 0.15% V, 0 to 0.6% Cr, 0 to 1.0% Cu, 0 to 4.0%
Ni, 0 to 0.02% S, with the balance being unavoidable impurities and Fe, in which (Nb%) x (C%) ? 5 x 10 3 and the content of TiN having a particle size not larger than 0.02µ being not less than 0.004% by weight, and provided that when a rare earth metal (REM) and sulphur are present REM/S is 1.0 to 6.0 and when one or more of Cu, Ni, Cr and Mo is present (Cu + Ni)/5 + Cr + Mo ? 0.90%.

2. A process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm2 which comprises heating a steel ingot or slab to a temperature not higher than 1150°C., and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930°C, and a finishing temperature not higher than 830°C., said steel ingot or slab containing in weight % 0.01 to 0.13%
C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, 0.05 to 0.40% Mo, 0 to 0.15% V, with the balance being unavoid-able impurities and Fe, in which (Nb%) x (C%) ? 5 x 10-3 and the content of TiN having a particle size not larger than 0.02µ
being not less than 0.004% by weight,

3. A process according to claim 2, wherein said ingot or slab contains 0.01 to 0.15% V.

4, A process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm2 which comprises heating a steel ingot or slab to a temperature not higher than 1150°C., and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930°C. and a finishing temperature not higher than 830°C., said steel ingot or slab containing in weight % 0.01 to 0.13%
C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, not higher than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10%
Nb, 0.05 to 0.40% Mo, 0 to 0.02% S, and at least one of a rare earth metal (REM) and Ca in an amount of 0.001 to 0. 03%
REM, 0.0005 to 0.03% Ca, with the balance being unavoidable impurities and Fe, in which (Nb%) x (C%) < 5 x 10 3, and the content of TiN having a particle size not larger than 0.02µ
being not less than 0.004% by weight, and provided that when a rare earth metal (REM) and S are present REM/S is 1.0 to 6Ø

5. A process for producing a high tension steel sheet having excellent low-temperature thoughness with a yield point not lower than 40 kg/mm2 which comprises heating a steel ingot or slab to a temperature not higher than 1150°C., and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930°C.

and a finishing temperature not higher than 830°C., said steel ingot or slab containing in weight % 0.01 to 0.13% C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, and 0.05 to 0.40% Mo, and further containing at least one of V, Cr, Cu and Ni in an amount of 0.01 to 0.15% V, not more than 0.6%
Cr, not more than 1.0% Cu, and not more than 4.0% Ni with the balance being unavoidable impurities and Fe in which (Nb%) x (C%) ? 5 x 10 , (Cu + Ni)/5 + Cr + Mo < 0.90%, and the content of TiN having a particle size not larger than 0.02µ
is not less than 0.004% by weight.

6. A process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower. than 40 kg/mm2 which comprises heating a steel ingot or slab to a temperature not higher than 1150°C., and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930°C. and a finishing temperature not higher than 830°C. said steel ingot or slab containing in weight % 0.01 to 0.13% C, 0.1 to 1.0% Si, 0.7 to 2.0%Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, 0.05 to 0.40% Mo, 0 to 0.02% S, and further containing at least one of a rare earth metal (REM) and Ca in an amount of 0.001 to 0.03% REM and 0.0005 to 0.03% Ca, and at least one of V, Cr, Cu and Ni in an mount of 0.01 to 0.15%V, not more than 0.6%
Cr, not more than 1.0% Cu and not more than 4.0% Ni with the balance being unavoidable impurities and Fe in which (Nb%) x (C%) ? 5 x 10-3 the content of TiN having a particle size not larger than 0.02µ being not less than 0.004% by weight and provided that when a rare earth metal (REM) and S are present REM/S is 1.0 to 6.0 and (Cu + Ni)/5 + Cr + Mo ? 0.90%.

7. A process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm2 which comprises heating a steel ingot or slab to a temperature not higher than 1150°C., hot rolling the steel ingot or slab thus heated with a total reduction amount from 60 to 95% in a temperature range not higher than 900°C. and a finishing temperature from 800 to 500°C., and reheating the hot rolled ingot or slab in a temperature range from 300 to 750°C., said steel ingot or slab containing in weight % 0.01 to 0.10% C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, 0.005 to 0.1% Al, not more than 0. 0.010% Si, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, 0.05 to 0.40% Mo, 0 to 0.15% V, with the balance being unavoidable impurities and Fe in which (Nb%) x (C%) < 5 x 10 3 and the content of TiN
having a particle size not larger than 0.02µ is not less than 0.004% by weight.

8. A process according to claim 7, wherein said ingot or slab contains 0.01 to 0.15% V.

9. A process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm which comprises heating a steel ingot or slab to a temperature not higher than 1150°C., hot rolling the steel ingot or slab thus heated with a total reduction amount from 60 to 95% in a temperature range not higher than 900°C. and a finishing temperature from 800 to 500°C., and re-heating the hot rolled ingot or slab in a temperature range from 300 to 750°C., said steel ingot or slab containing in weight %
0.01 to 0.10% C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, 0.005 to 0.1%
Al, not more than 0.010% S, 0.004 to 0.03% Ti, 0.001 to 0.009%
total N, 0.01 to 0.10% Nb, 0.05 to 0.40% Mo, 0 to 0.15% V, and at least one of a rare earth metal (REM) and Ca in an amount of 0.001 to 0.03% REM and 0.0005 to 0.03% Ca with the balance being unavoidable impurities and Fe in which (Nb%) x (C%) ?
5 x 10-3 the content of TiN having a particle size not larger than 0.02µ being not less than 0.004% by weight and provided that when a rare earth metal (REM) is present REM/S is 1.0 to 6Ø

10. A process according to claim 9, wherein said ingot or slab contains 0.01 to 0.15% V.

11. A process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm2 which comprises heating a steel ingot or slab to a temperature not higher than 1150°C., hot rolling the steel ingot or slab thus heated with a total reduction amount from 60 to 95% in a temperature range not higher than 900°C and a finishing temperature from 800 to 500°C., and reheating the hot rolled ingot or slab in a temperature range from 300 to 750°C. said steel ingot or slab containing in weight % 0.01 to 0.10% C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, 0.005 to 0.1% total Al, not more than 0.010% S, 0.004 to 0.03%
Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, 0.05 to 0.40%
Mo, 0 to 0.15% V, and at least one of Cr, Cu and Ni in an amount of not more than 0.6% Cr, not more than 1.0% Cu and not more than 4.0% Ni, with the balance being unavoidable impurities and Fe in which (Nb%) x (C%) ? 5 x 10-3 (Cu + Ni)/5 + Cr +
Mo ? 0.90% and the content of TiN having a particle size not larger than 0.02µ being not less than 0.004% by weight.

12. A process according to claim 11, wherein said ingot or slab contains 0.01 to 0.15% V.

13. A process for producing a high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm2 which comprises heating a steel ingot or slab to a temperature not higher than 1150°C., hot rolling the steel ingot or slab thus heated with a total reduction amount from 60 to 95% in a temperature range not higher than 900°C. and a finishing temperature from 800 to 500°C., and re-heating the hot rolled ingot or slab in a temperature range from 300 to 750°C., said steel ingot or slab containing in weight % 0.01 to 0.10% C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, 0.005 to 0.1% Al, not more than 0.010% S, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, 0.05 to 0.40% Mo, 0 to 0.15% V, at least one of a rare earth metal (REM) and Ca in an amount of 0.001 to 0.03% REM, 0.0005 to 0.003% Ca, and at least one of Cr, Cu and Ni in an amount of not more than 0.6% Cr, not more than 1.0% Cu, and not more than 4.0% Ni with the balance being unavoidable impurities and Fe in which (Nb%) x (C%) ? 5 x 10-3 and the content of TiN having a particle size not larger than 0.02µ is not less than 0.004% by weight, provided that when a rare earth metal is present REM/S is 1.0 to 6.0 and (Cu + Ni)/5 + Cr + Mo ? 0.90%.

14. A process according to claim 13, wherein said ingot or slab contains 0.01 to 0.15% V.

15. A process according to claim 1, 2 or 3 wherein said steel ingot or slab is formed by casting, the cast ingot or slab being cooled at an average cooling rate of not less than 8°C/min. from the temperature at casting to 1100°C.

16. A process according to claim 4, 5 or 6 wherein said steel ingot or slab is formed by casting, the cast ingot or slab being cooled at an average cooling rate of not less than 8°C/min. from the temperature at casting to 1100°C.

17. A process according to claim 7 or 8 wherein said steel ingot or slab is formed by casting, the cast ingot or slab being cooled at an average cooling rate of not less than 8°C/min. from the temperature at casting to 1100°C.

18. A process according to claim 9 or 10 wherein said steel ingot or slab is formed by casting" the cast ingot or slab being cooled at an average cooling rate of not less than 8°C/min. from the temperature at casting to 1100°C.

19. A process according to claim 11 or 12 wherein said steel ingot or slab is formed by casting, the cast ingot or slab being cooled at an average cooling rate of not less than 8°C/min. from the temperature at casting to 1100°C.

20. A process according to claim 13 or 14 wherein said steel ingot or slab is formed by casting, the cast ingot or slab being cooled at an average cooling rate of not less than 8°C/min. from the temperature at casting to 1100°C.

21. A high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm containing in weight % 0.01 to 0.13% C, 0.1 to 1. G%
Si, 0.7 to 2.0% Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, 0.05 to 0.40 Mo, 0 to 0.15% V, 0 to 0.03% rare earth metal (REM), 0 to 0.03%
Ca, 0 to 0.6% Cr, 0 to 1.0% Cu, 0 to 4.0% Ni, 0 to 0.02%S, with the balance being unavoidable impurities and Fe, in which (Nb%) x (C%) < 5 x 10 3 and the content of TiN having a particle size not larger than 0.02µ being not less than 0.004%
by weight, and provided that when a rare earth metal (REM) and S are present REM/S is 1.0 to 6.0, and when one or more of Cu, Ni, Cr and Mo is present (Cu + Ni)/5 + Cr + Mo ? 0.90%, and produced by heating a steel ingot or slab of said com-position to a temperature not higher than 1150°C., and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930°C. and a finishing temperature not higher than 830°C.

22. A high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm containing in weight % 0.01 to 0.13% C, 0.1 to 1.0%
Si, 0.7 to 2.0% Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, and 0.05 to 0.40% Mo, with the balance being unavoidable impurities and Fe, in which (Nb%) x (C%) ? 5 x 10-3 and the content of TiN having a particle size not larger than 0.02µ being not less than 0.004% by weight, produced by heating a steel ingot or slab of said composition to a temperature not higher than 1150°C., and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930°C. and a finishing temperature not higher than 830°C.

23. A high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm2 containing weight % 0.01 to 0.13% C, 0.1 to 1.0%
Si, 0.7 to 2.0% Mn, not higher than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, and 0.05 to 0.40% Mo, 0 to 0.02% S and at least one of a rare earth metal (REM) and Ca in an amount of 0.001 to 0.03% REM, 0.0005 to 0.03% Ca, with the balance being unavoidable impurities and Fe in which (Nb%) x (C%) ? 5 x 10-3, and the content of TiN

having a particle size not larger than 0.02µ being not less than 0.004% by weight and provided that when a rare earth metal (REM) and S are present REM/S is 1.0 to 6.0, and produced by heating a steel ingot or slab to a temperature not higher than 1150°C., and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930°C. and a finishing temperature not higher than 830°C.

24. A high tension steel sheet having excellent low-temperature toughness with a yield point not lower than 40 kg/mm containing in weight % 0.01 to 0.13% C, 0.1 to 1.0% Si, 0.7 to 2.0% Mn, not more than 0.1% total Al, 0.004 to 0.03% Ti, 0.001 to 0.009% total N, 0.01 to 0.10% Nb, and 0.05 to 0.40% Mo, and at least one of V, Cr, Cu and Ni in an amount of 0.01 to 0.15% V, not more than 0.6% Cr, not more than 1.0% Cu, and not more than 4.0% Ni, with the balance being unavoidable impurities and Fe in which (Nb%) x (C%) ? 5 x 10 (Cu + Ni)/5 + Cr + Mo ? 0.90% and the content of TiN having a particle size not larger than 0.02µ is not less than 0.004% by weight, produced by heating a steel ingot or slab to a temperature not higher than 1150°C., and rolling the steel ingot or slab thus heated with a total reduction amount not less than 50% in a temperature range not higher than 930°C.
and a finishing temperature not higher than 830°C.

25. A high tension steel sheet according to claim 21 or 22, wherein said steel ingot or slab is formed by casting, the cast ingot or slab being cooled at an average cooling rate of not less than 8°C/min from the temperature at casting to 1100°C.

26. A high tension steel sheet according to claim 23 or 24, wherein said steel ingot or slab is formed by casting, the cast ingot or slab being cooled at an average cooling rate of not less than 8°C/min from the temperature at casting to 1100°C.

27. A steel pipe formed from a steel sheet as defined in claim 21 or 22.

28. A steel pipe formed from a steel sheet as defined in claim 23 or 24.