CA1190835A

CA1190835A - Method of producing as rolled high toughness steel

Info

Publication number: CA1190835A
Application number: CA000390950A
Authority: CA
Inventors: Hiroo Mazuda; Hiroshi Tamehiro; Rikio Chigiiwa; Hiroaki Masui; Minoru Matsunaka; Seiji Isoda; Hozumi Nishida
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1980-11-27
Filing date: 1981-11-26
Publication date: 1985-07-23
Also published as: JPS5792129A; DE3146950A1; DE3146950C2; JPS613370B2; IT1172106B; IT8149774A0

Abstract

ABSTRACT OF THE DISCLOSURE
A method has been developed for making high strength and high toughness steel having superior toughness even at heat affected zones due to welding.
The method consists of preparing a cast steel of strictly controlled chemical composition by an oxygen converter followed by continuous casting, heating the steel at a considerably lower temperature of 950° - 1050°C
and subjecting the heated steel to controlled rolling so that the steel can be rolled at a temperature within an austenite single phase region.
The controlled chemical composition of the steel is maintained to have 0.01 - 0.15% C, not more than 0.6% Si, 0.8 - 2.0% Mn, 0.01 - 0.08% Al, not more than 0.008% S, 0.008 - 0.025% Ti, 0.001 - 0.007% N all being by weight and the balance Fe and incidental impurities.
By virtue of the lowered content of S and small amount of added Ti and N combined with rapid cooling rate obtained through the continuous casting method and through the controlled rolling the rolled steel plate has very fine grained micro structure together with minimum anisotropy in mechanical property even in the direction of the thickness of the plate.
Due to these features, the steel obtained by this method is particularly suitable for weld construction members used at low temperature such as line pipes, ship building and so on.
The steel may further contain one or more of other alloying elements such as Nb, V, Ni, Cu, Cr and Mo.

Description

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1 BACKGROUN~ OF THE INVEN~IOM
This invention relates to a method of producing non-heat trea'ced as rolled hi.gh toughness steel having high toughness at the base metal and the weld zone~
A controlled rolling process has been widely in use as a method of producing line pipe material used at low temperature or extremely low temperature. However, novel controlled rolling methods have in recent years been developed and have come to attract attention. They include, in combination with the conventional controlled rolling~ a lower temperature slab heating and controlled rolling process in which the heating temperature is lowered to a level immediately above Ar3 point and a rolling process, what is generally referred to as (r-d) two phase region rolling between Ar3 and Arl pointsO ~hese novel processes offer the advan~ages that they cause remarlcable grain refining (including subgral.ns) and an increase in separation density 7 with the result that the transition temperature from brittle to ductile fracture in Charpy impact test and down weight tear test (DW~) which serves as an index for stopping brittle fracture is markedly increased and that it is possible to maintain balance between st.rength and ductility (~racture transi-tion temperature) when ~wo phase region rolling ls carried out t.o increase the strength. ~he steel as rolled produced 3~
1 by these processes has the possibilities of being used for making pressure vessels and the like, in addition to being used as line plpe material.
United States Patent NoO 3,673~oo7 discloses a method for manufacturing non heat treating type high toughness steel starting from ingot making ~ollowed by hot rolling or forging, however, it does nct disclose properties requlred for welding particularly toughness at heat effected zone due to weldlng.
With regard to chemical composition, ~he U.S.P. 3673007 specifies~ in addition to the elements for fandamental low carbon steel~ minor amount of at least one element selected from the group consisting of upto 0.20% Nb, 0.20~ V, upto 0.15% ~i and upto 0.30% Ta all being by weight.
The present invention, on the other hand critically sets forth chemical composition, especially, allowable upper limit for S and strictly restricted ranges for Ti and N, for the purpose of retarding formation of MnS and to ensure formation of uni~ormly distributed fine grains of TiN which are effective for refîning crystal grains in the rolled steel.
With regard to the condition of slab making~
the present invention also dif~ers ~rom that of the U.S.P. 3 that is, the present invention directly performs hot rolling of the cast slab prepared by continuous casting in order to ensure rapid cooling necessary to obtain sufficient amount of fine particles of ~iN~ while the ~ 3~ ~ ~

1 method of the U.S.P. relies on ordinary ingot making method.
As to the condition of rolling reduction, the U.S.P. merely defines thickness reduction of more than 10% and that the rolling shall be completed in the vicinity of Ar3 point.
Differrirlg greatly from the conditions defined by U.S.P., the present invention specifies more strict conditions of rolling, namely~ mlnimum rolling reduction Of 40% at t;he temperature not more than 850C and the temperature for final rolling to be hept withln a range between Ar3 plus 10C and not more than 800C.
These strict requirements have been established base on the knowledge obtained by the present inventors' finding that at least 40% of rolling reduction of austenite at comparatively lower temperature range of not more than 850C is indispensable to obtain refined microstructure of the finally transformed steel stock necessary for obtaining good mechanical property in the direction of the thickness of the rolled steel.
Distinguishable feature of the present invention resides in the strictly limited ranges for Ti and N and the strictly limited controlled rolling necessary for obtaining greatly improved toughness at heat affect;ed ~one of the steel as well as the base metal.
However7 some disadvantages are associated with these methods. The steel produced by them has the follow-ing defects. (1) The steel has increased anisotropy and t~

its mechanical properties in the directlon of plate thickness become wrong, while energy ab~orptl~n in Charpy and DWTT tests decreases (or ~he brittle fracture prevention characteristic is reduced) as a reaction to 5 lncrea~ed separatlon densi~y. (2) Even i~ the base metal may h~ve superior low temperature toughness (brlttle rupture preventioll charac~eristic) " ~he ~oughness of a heat a~ected zone (herelnaf'~er re~erred to as HAZ ) in weld constructlon is not compatlble with that o~ the base metal. Thus the steel produced by these methods i~; still limited in use and have no~ yet come to be us ed wide ~y .
The inventors of the present invention have invented a method of producing steel having both high energy absorption characteristic in impact testing such as Charpy test and low anisotropy, for which a patent application has been filed in Japan, published as Japanese Laid-Open Patent Publication No. 131125/80 (Japanese Post-Exam Patent Publication No. 14848/83 dated March 22, 1983), the applicant of this publication being Nippon Steel Corporation, t.he inventors of this publication being Hiroo Mazuda, Hiroshi Tamehiror Mamoru Oohashi, and Ho Nakasugi. This method, however, suffers a disadvantage, that although anisotropy is reduced within the plate surface, no fundamental solution has been provided in improving the characteristics in the plate thickness direction although they have been improved to a certain exten-t as compared with those steels of the prior art.

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SUMMARY OF T~E INVENTION
This invention has been developed fox the purpose of obviating the aforesaid drawbacks of the prior art.

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l Accordir~ly~ the invention has as its objec~ the provislon of a method of producing steel of very high toughness and strength having balanced strength and toughness, which is low in anisotropy, high in energy absorption in Charpy and DW~T testsg high in the toughness of the weld zone thereby can be used as an entirely novel method of pro~
duction of steel for weld construction.
The diskinguishable feature of the in~ention resides in that the steel having a low S content added with small amounts of critically controlled Ti and A
is made into a slab of a thickness below 300 mm by a continuous casting process which is heated at low temperature to effect controlled rolling at a temperature above Ar3 point.
The invention has been developed based on the concept that steel of low separation and high plate thickness direction characteristics can be produced without lowering;the features of the inventors' prior application9 by paying due attention to the chemical composition of steel and to the heating and rolling conditions.
BRIEF DESCRIPTION OF ~HE DRAWING
The sole draw~ng a graph showing the finishing temperature in relation to the transition temperature in the directions of plate thickness direction and the rolling.

~ he method according to the present invention enables anisotropy of the steel plate to be markedly reduced and allows e~ery absorption in Charpy impact test and the like to be increased, while markedly increas-ing toughness in HAZ. A reduction in energy absorption in Charpy test and the like is accounted for the ~act that separation occurs at the impact frackure and is caused mainly by elongated MnS, non-recrystallized austenite region and the formation of a (100) texture parallel to the.plate surface created by the rolling in (y-a) two phase region. In the present invention, the S content of the steel is reduced and rolling is termi-nated within the single pha~e region o. austenite to reduce the texture9 to thereby improve the plate thickness directîon characteristics.
It is believed that rolling at low temperature in the vicinity o~ the Ar3 point and the (y-~) region is lndispensable for providing improvements in low temper~-ture toughness (transition temperat.ure). We have found,however, that by effecting thorough grain refining of t~le austenite grains at initial stages by the combination of low temperature heating and fine TiN particles, it is possible to obtain enough low temperature toughness even i~ low temperature rolling is reducted to some exten~.
Meanwhile the end of improving the toughness of the weld zone has been attained according to the l invention by forming a steel of~ low Ti and N contents into a strand by means of a continuous castin~ process for effecting high cooling rate and subjecting the strand to low temperature rolling at 950-lG50C. This is because the continuous casting process gives a higher cooling rate and enables a formation of large amount of f'ine TiN grains (less than 0.05 ~) in the rolled slab when the cast strand is rolled.
The reason why the thickness of the billet has been set at a value less than and inclusive of 300 mm is that, if~ this level is exceeded the cooling speed is reduced and sufflcient amount of flne grains of' TiN are unobtainable. As f'or a cooling rate, it is most desirable that the average cooling rate at the temperature level f'rom immediately below the liquids line of molken steel to 1100C be kept over 60C/min in the center of a billet. However, even if fine grains o~ TiN are obtained in large amount in the billet, it would be impossible to obtain a large amount of flne grains of ~iN in the rolled product if they are coarsened in the course of heating and rolling steps, thereby rnaking it impossible to obtain a f`ine structure at HAZ.
In view of the foregoing, the temperature at which the billet is heated has been limited to the range between 950 and 1050C. It has been ~ound that by setting this limit it is possible to provide marked improvements to the toughness of the HAZ as compared with the t~oughness obtained by high temperature heating 1 ac~ording to the prior art. The upper limit of the heating temperature range should be such that the fine grains of TiN in the slab are prevented from being coarsened by such heating and the lower limit thereof is such that heating under the lower temperature limit would not produce products acceptable for specifications due to deterioration of the inner quality of the steel caused by insufficient solutionizing o~ the slab in the austenite region. Heating at a temperature over 950C
enables the inner quality of the steel to be thoroughly imoroved because of ~ts S content having been reduced.
The fine grains of TiN which are not coarsened at the time of heating help;refining of austenite grains at the time of heating, recrystallized grains during rolling and of the rolled structure as a whole~ thereby improving the toughness o~ the base metal.
The rolling process according to the invention will now be described. In order to obtain sufficient strength and toughne~s~ it is essential to carry out controlled rolling. rrO this end 7 the rolling conditions have been set according to the invention, that is 3 rolling reduction over 40% at a temperature below 850C~ and the finishing temperature of rolling over Ar3 point plu~
10C but below 800C. The Ar3 point during rolling can be empirically set forth by the following formula:

Ar3 point = 880 - 400 (C%) - 70 (Mn%) + 25 (Si%) - 35 (Ni%) - 20 (Cu%) - 25 (Cr%) + 30 (mo%) ~3~3~

1 By carrying out rolling under the aforesaid conditions, the steel can display greatly increased strength and toughness. The reasons why the rolling conditions have been limited as described hereinabove will now be described.
When the rolling reduction is kept over 40% at a temperatu~e below 850C, the grains of the steel are markedly reduced in slze and the strength and koughness o~ the steel can be greatiy increased. ~rnen the rolling reduction is below 40%~ however, it is impossible to obtain high strength and superb toughness. Meanwhile, even if the rolling reduction is above 40% at a tempera-ture below 850C, it is impossible to produce steel of high strength and superb toughness if the finishing temperature exceeds 800C, due to insu~ficient refining of the grainsO By setting the finishing temperature at a level below 800C, refining of the grains is enhanced~
thereby making it possible to increase both the strength and toughness of steel or at least to increase its stren~th without lowerin~ its toughness.
According to the present invention, rolling within the ~errite-austenite region is not carried out.
~his is because i~ the lower limit o~ the finishing temperature is below the Ar3 point~ separation occurs in the impact ~racture, with the result that energy ab~orption is reduced and the plate thickness direction characteristics show deterioration (see the drawing)O
~hus the finishing temperature has been limited to the 3~

l range of over Ar3 point plus 10C and below 800C including allowance. The desired finishing temperatxre for achieving best propertles in the plate thickness direc-tion lles in the range between 750 and 800Co No particuler limitation is set to the cooling subsequent to rolling, however~ the range of 0.2 and 10C/sec is preferred. Heating of the steel to a temperature below the Acl ~ransformation ~oint ~or the purpose of effecting dehydrogenation, for example~
does not impair the features of the invention.
The steel produced by the method accor~ing to the invention offers a superb weld zone property of base metal as compared wi~h steel produced by any prior art methods and further has the characteristics equa] to normallzed or quenched and tempered steel, so that the steel produced ~y the method of the present invention is applicable to any other practicable use ranging from formation of line pipes for sour gas and for use in regions of extreme coldness, pressure vessels~ marine contruction, ship building industry, etc.
The reasons for limiting the components of the steel according to the invention will be described. The steel claimed in claim l of the application contains 0.01 - 0.15% C, not more than 0.6% Si, 0.8 - 2.0% Mn~
0.01 - o.08% Al~ not more khan 0.008% S~ o.oo8 - 0.025%
Ti~ and 0.001 - 0.007% N.
The lower limit 0.01% of C is a minimum essential level for ensuring that the base metal and 1 weld zone have satisfactory strength and that carbide forming ele~ents, such as Nb and V, can satisfactorily their effects. When ~he amount o~ C is too largeg however, coarse grains of fainite or island-like martensite might be rormed in large amounts in the base metal and HAZ, which unfavorably affecting -toughness and considerably reducing weldability. Thus the upper limit is set at 0.15%.
Si which is inevitably contained in steel due to its addition for deoxidizing purposes is not desirable for improving weldability and increasing the toughness of the HAZ. Thus the upper limit of Si is set at o.6%.
Steel can be deoxidized by Al alone, so that the content of Si can pre~erably be main-tained not more than 0.2%.
Mn is an important element which lowers the transformation point of the steel and enables the effects of improving the quality of steel by controlled rolling according to the invention, thereby enabling strength and toughness to be simultaneously increased. When the Mn content is less than 0.8~ the strength and toughness of the steel are lowered, so that its lower limit is set at 0.8%. However5 when Mn is too large in amount~
the hardenability of the steel increases and coarse grains o~ bainite or islant-like martensite are formed in large quantities, thereby reducing the toughness of both the base metal an~ HAZ. Thus the upper limit is set at

2.0%.
Since Al is used as a deoxidizing agent~ it is ~ .

l inevitably contained in this type of killed s'ceel.
However, when it is less than 0.01% in amount, deoxidiz~
ing cannot be satisfactorily effected and the base metal has its toughness reduced, so that the lower limit is set at 0.01%. Meanwhile when Al exceeds o.o8%9 the cleanliness of the steel and the HAZ toughness are reduced, so that the upper limit is set at 0.08%.
The reason why S as an impurity is limited not more than o.008% is that the anisotropy of the base metal should be reduced and energy absorption should be increas ed. In the method according to the invention, rolling is carried out at a temperature below Ar3 point.
Howeverg the steel heated at low temperature wouid have increased isotropy and energy absorption in Charpy impact testing would be reduced as compared with ordinary cold rolled material even when rolling is carried out at a temperature above Ar3 point.
This is accounted for by the fact that, as set forth hereinabove, the presence of MnS in the steel and rolling of austenite ln the non-recrystallized region form texture. Limitations are placed on the amount of S in order to reduce the absolute amount of MnS. 3y setting S at a level not more than o.oo8%, it is possible to markedly increase the toughness of the steel. In this case~ the lower the S content of steel is, the higher is the troughness thereof. By setting the S content at a level not more than 0.0015%9 it is possible to greatly increase the toughness of the steel.

1 However, it would be impossible to entirely eliminate MnS no matter how small the content of S in the steel may be made. Thus the present invention controlls the formation of textured structure by finishing rolling at a temperature above Ar3 pointO
Ti and N are added for the purpose of increasing the toughness of the HAZ by dispersing minute grains of TiN in the steel, as set forth hereinabove. ~o this end 7 it is effective to have fine grains of ~iN distributed as many as possible in the slab~ However, when the amounts of Ti and N are too large~ ~iN could be coarsened while the molten steel is being cooled and solidified even if a continuous ca~ting process is used. Thus the upper limits of Ti and N are set at 0.025% and 0.007%, respec-tively. Meanwhile when the amounts of Ti and N are toosmall, no marked effects can be achieved in improving the toughness of HAZ, so that the lower limits of Ti and N are set at 0.008% and 0.001%, respectively.
The steel according to the invention comprises P as an impurity. ~he amount of this element is usually not more than 0.030~, and the lower the P content is, the higher the toughness of the weld zone and the more improved the weldability. ~he amount of this element is preferably not more than 0.015~ to improve welding characteristics. Oxygen content of the steel aceording to the invention is not more than o.oo8%. In order to - achieve improved cleanliness and toughness of the steel, the amount of this element is preferably as small as - 13 ~

33~i 1 possible.
In the invention as claimed in claim 2, the steel as claimed ~urther conta~n at least one element selected ~rom the group consisting of not more than o.o8% Nb, not more than 0.10% V, not more than 2.0% Ni, not more than 1.0% Cu, not more than 1.0% Cr and not more than 0.4% MOg in addition to the elements contained in the steel claimed in claim 1.
The reason why these elemen~s are additionally corltained in the steel according to the invention is that it is desired to increase strength3 toughness and to expand a thickness range of a steel to be produced~
so that the amounts of the elements should be naturally limited.
Nb is contained in the steel o~ the invention to achieve both grain refining and precipitation hardening o~ the rolled structure. It is an important element for increasing both strength and toughness. However, when the amount o~ Nb exceeds o.o8%. It has harmful effects in weldability and in increasing the toughness of the HAZ.
~hus the upper limit is set at 0.08%.
V achleves substantially the same effects as Nb.
Its upper lim~t can be as hl~h as 0.10%.
Ni increases the strength and toughness of the b~se m~tal without adversely affecting the hardenability and toughness of the HAZ. However, when the amount exceeds 2.0%, the hardenability and toughness of the HAZ
are ad~rersely affected, so that the upper limit is set l a~ 2.0%.
Like Ni, Cu has the effect of increasing the corrosion resistance of steelO However, when the amount exceeds 1.0%, Cu cracks might initiate druing rolling when rolling is carried out at low temperature heating as done in the present invention, and making production difficult. Thus ~he upper limit is set at 1.0%.
Cr increases the strengkh of the base metal and weld zone and has the effect of preventing h~dro~en induced cracking. However~ if the amount of this element is too greatg the hardenability of the HAZ would be increased and the toughness and weldability thereof would be reduced. Thus the upper limit is set at 1.0~.
Mo is an element effective for inc~easing both strength and tou~hness of the base metal, however~ excessive amount of MOg like excessive Cr in the steel, liable to excessively increase hardenability of the steel which gives rise to lower toughness at ~AZ and weldability, Accordingly~ upper limit of Mo is speci~ied as 0.4%.
The lower limits of these additive elements are desired to be essential minimum value for achieving excel-lent results in improving the quality of the steel. The lower limits of Nb, V, Ni, Cu, Cr and Mo are 0.01%, 0.01%, 0~1%g 0.1%, 0.1% and 0.05%, respectively.
The steel and the production method caIimed in claims 3 and 4 respectively contain 0.0005 - 0.005%
Ca in addition to the components of the steel as claimed in claims l and 2, respectively.

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l Ca is added for the purpose of increasing energy absorption and improving the characteristics in the plate thickness direction by effecting morphological control of sulfides (MnS). The reason why the amount o~
Ca is limited to the range ~etween 0.0005 and 0.005%
is as follows. When it is less than 0.0005% 3 the addi-tion of this element can achieve no practical e~ect;
when it exceeds 0.005%, it has harmful effects in increasing the toughness of steel and its cleanliness because of the production of large amounts of non~- -metallic inclusion, such as Ca-O~S. AlSOg an increase in the amount of this element gives rise to a problem with regard to operability of welding principally in carrying out C02 gas arc welding.
An embodiment of the invention will now be described. Strands o~ a variety o~ chemical composition were produced by an oxygen converter-continuous casting processg and then they were rolled down as plates within the thickness range of 18 - 35 mm under varying heating and rolling conditions. The mechanical properties of the base metal and the weld zone are shown in Tables l and 2.

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Table l . _ . . . . _ . _ .
Speci- Type Chemical Composition (~) men of No. Steel C Sl Mn A1 S Ca Tl __ __ ~ . _ 1 A O .14 O . 20 1. 37 O . 022 O . o()4 _ O . 009 ._ ____ _ _ ........... .. ... _ _ _ _ 2 B ,. ,. " ,. ,. _ "
__ ___ __ __ _ _ _ _ 3 A 0 . 10 0 . 17 1 . 58 0 ~ TO 35 . 002 _ 0 . 015 _

4 B ~ ~ ~t II II = ~l _ _ _ ~ ___._ . T _ _ _ _ _ A 0 . 04 0 . 24 1. 66 0 . 027 0 . 006 _ 0 . 022 ~ . . . __ _ _ ~ _ 6 B ~ .. n I n ~- _ ____ ~ ._L_ T _ _ __ _ 7 A 0 .13 0 . 25 1. 06 l O . 027 0 . 005 _ 0 . 017 __ . . . ,~ _ _ _ 8 B " ,. " I ,. ,. _ ..
.__ _ _ . _ ~ _ .
9 A 0 . 08 0 . 28 1 . 46 0 . 015 0 . 001 0 . 0019 0 . 011 _ __ . . __ __ __ _ _ __ , _ B ,. ,. ., ,. ,. " "
. ~ - - - -- _ _ _ .. .__ ~ _ 11 A 0 . 13 0 . 28 1 . 50 0 . 035 0 . 002 0 . 0016 0 . 014 _ _ .. ., . _ 12 B ,. ,. ,. ,- ,. ,. ,.
_ .. ,. ._ ~ "",, ~_._ __ _ _ 13 A 0 ~ o6 0 . o6 1. 05 0 . 033 0 . 003 0 . 0042 0 . 016 _ __ . , _ . . . _ _ 14 B ,. ,. ,~ ,. ,. .l "
. . _ . . ~ .... . .... _ _ A 0 . 07 0 . 16 1 . 10 0 . 0 32 0 . 001 0 . 0017 0 . 013 1 6 B __ _ _~ ~ ~ ____ - Cont'd -Note: A: Inventive Steel, B: Steel ~or Comparison - 17 ~

- Table 1 (Cont'd) Production Conditions _. ,_ ___ _ . _ . _ Slab Heating Rolling Other Thick- Tempera- Reduction N Elements Ar3 ness ture below 850C
(mm) (C) (%) _ _ _ 0.0026 _ 733 250 95 65 ... . ...... __ __.__ _ _ ll ~ " ,. 1150 "
. . . ., ~ . ___ ~
o.oo48 _ 734 210 1000 70 .. ~ . _ _ _ _ _ _ ,. _, " .. ,. ,.
_ . _. . . _ .. _ 0.0065 Nl 0 32743 ,- 950 55 ,- _ _ . _ . _ __ __ _ 0.0046 Nb- 0 030 756 " 950 7o _ ... ,.. ~ _ ..
,~ _" ~ " " 30 0.003~ V : 0.020 753 .. 1050 60 . .... ~ .. . _ " " " ,. " 30 . _ _ 0.0042 Nb. 0.010 73 1000 50 _ ll ,. ,- ,. ,. "
_ ~ ., ._ _ .. _ 0.0050 Cu 0 25 720 ,- 950 5o _ _ _ _ . ~ .. ~ _ _ _ _ _ . ~ . ~, _ 1~50 _ 0.0044 Mo~ 0;10 730 160 1000 65 _ .. . __ . . . .. __ __ . _ ____ _ ~t ~I . ~ . .__.. . . __ - Cont~d r ~' Table 1 (Cont 'd~

Finishing Plate TemperaThickness Remarks t ure ( C ) (n~n) .. ..... _. ~ _. .

_ . . . _ _ _ ~__ ... _ _ . . .. _.
695 "
_ ____._. .. , _.
~ 1~
........ . _~

. . . _ _ _ - - -~
~ ~1 _ _ Air Cooling for I u~ LU 20 Min. at 600Co _ .._. _ _. .

_ _ 710 I~
~ ~ ~ __ ~

._ _ I~ 1 26 ,, I

__ _ ~ _ 1~

83~i Table 2 ~ . . _ .. ~__ Speci- Type Base Metal Characteristics men of No. Steel Yield Tensile 2vE-60C vTrS _ (kg/mm2) Strength (kg-m) (C) _ _ _ ___ _ 1 A 40,4 52.7 20.6 - 95 _ _ __ _~
2 B 36.5 50.4 11.4 - 60 . . . _ ~
3 A 33.7 5.5 30.1 - 95 _ 4 B 45.3 55~6 6.3 -105 _ . . __ A 37.2 48.1 34.6 -120 _ B 35.8 47.8 22.5 -105 _ 7 A 39.4 48.9 25.8 -115 _ . . _ _ _ _ , ~_ . . .__ _ _ 8 B 34 . 2 45.8 20.4 - 75 __ ~ . .. _ ~ . . _ __ 9 A 32.6 45.2 25.6-105 ... ___ _ _ _ ~ , _ _ B 30.2 44.6 9.0 - 75 ~ _ ~ . . _., _ 11 A 43.2 56.o 33.3 -115 _ . . __ . . ~ _ 12 48.6 57.3 6.5 -120 _ 3 37.6 48.3 35.8 -160 _ 14 B 34.7 47~6 30.7 130 __ . ~ _ _ ~ _ _ A 42.4 51.3 31.9 ~150 - .~ _ . . _,.~. _. . . ~_, _ 16 B 48.9 53.4 10.4 -160 - Cont'd ;~ - 20 -, ~i j ~able 2 (Cont'd) Toughness of Weld Zone _ _ _ . . . __ . ___ Thickness 2vE-600C
Direc~ion (kg-m) _~
_ 60 14.3 - 40 4~7 _ - 85 17.7 --~ 20 15.9 - 65 25.0 - 55 3.1 _ 65 14.6 __ . .. ..
- 65 ~3.5 - 95 16.3 .

~ . . . _ . _ -100 15.3 ~ . _ _ __ _ ~ 30 14.6 .. .. __ _ . .
-120 29.7 ~ 95 lg.3 ., ..., . _ .
-1~0 25.6 ~ .. .~_ _ _ . .
O ~
-3~i l The slabs of steel pro~uced by the method accord-ing to the invention have high ~oughness at a base metal and at weld zone. However, steel slabs produced by a method of the prior do not have satisfactory toughness eigher at base metal or at a weld zone. Thus the steel slabs of the prior art lack necessary balance that makes them suitable for use as weld constructions.
The steel slabs presented for testing will now be reviewed. Comparison of specimens l and 2 of the same composltion shows that spechmen 2 is inferior in the toughness at a base metal and at a weld zone because its heating temperature is high.
Specimens 3 and 4 are of the same composition, but specimen 4 is inferior in the toughness at a base metal because o~ ~ts finishing temperature being low.
Particularly~ specimen 4 is markedly low in toughness in the directlon o~ thickness.
Specimens 5 and 6 a.re substantially o~ the same composition~ but specimen 6 is infericr to specimen 5 in the toughness at the base metal and at the weld zone because no Ti is added, in spite o~ the ~act that other production conditions are similar.
Specimens 7 and 8 are of the same composition, but specimen 8 is inferior to specimen 7 in the toughness at the base metal due to low rolling reduction rate at a temperatxre below 8500C.
Speclmens 9 and lO are of the same composition7 but specimen lO is inferior to specimen 9 due to low 3~;

1 rolling reduction rate at a temperature below 850C.
Specimen 9 is su~vected to hea~ treatment after rolling by heating at 600C ~or 20 minutes followed by air cooling.
Specimen 9 has good toughness at the base rnetal and at the weld zone and this indicates that the characteristics of the steel according to the invention are not impaired by such heat treatment.
Specimens 11 and 12 are of the same composition but specimen 12 is inferior to specimen 11 ln the tough-ness of at the base metal due to its 10W finishing tempera~ture o~ rolling. Particularly its ~hickness direction toughness is extremely low.
Specimens 13 and 14 are of the same cornposition, but specimen 14 is inferior to specimen 13 in the toughness at the base metal because of its high heating temperature.
Specimens 15 and 16 are o~ the same composition but specimen 16 is inferior in -the toughness of a base metal due to the low finishing temperatxre. Particularly its thickness direction toughness is remarkably low.

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Claims

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

1. A method of producing as rolled steel of reduced anisotropy and having high toughness at both base metal and heat affected zone of welded portion comprising the steps of:
making by continous casting a steel slab of not more than 300 mm thickness consisting essentially all by weight of 0.01 - 0.15% C, not more than 0.6% Si, 0.8 - 2.0%
mn, 0.01 - 0.08% Al, not more than 0.008 S, 0.008 - 0.025% Ti, 0.001 - 0.007% N and the balance being Fe and incidental impurities;
heating the slab at a temperature between 950 -1050°C, and rolling down the heated steel under such a condition that reduction rate of at least 40% is applied to the steel not higher than 850°C and the finishing temperature of the rolling being maintained between Ar3 + 10°C and 800°C.

2. A method of producing as rolled steel of reduced anisotropy and having high toughness at both base metal and heat affected zone of welded portion comprising the steps of:
making by continuous casting a steel slab of not more than 300 mm thickness consisting essentially all by weight of 0.01 - 0.15% C, not more than 0.6% Si, 0.8 - 2.0% Mn, 0.01 - 0.08% Al, not more than 0.008 S, 0.008 - 0.025% Ti, 0.001 - 0.007% N, at least one element selected from the group consisting of not more than 0.08% Nb, not more than 2.0% Ni, not more than 1.0% Cu, not more than 1.0% Cr, and the balance being Fe and incidental impurities;
heating the steel slab at a temperature between 950 - 1050°C, and rolling down the heated steel slab under such a condition that reduction rate of at least 40% is applied to the steel at a temperature not higher than 850°C and the finishing temperature of the rolling being maintained between Ar3 plus 10°C and 800°C.

3. A method of producing as rolled steel of reduced anisotropy and having high toughness at both base metal and heat affected zone of welded portion comprising the steps of:
making by continuous casting a steel slab of not more that 300 mm thickness consisting essentially all by weight of 0.01 - 0.15% C, not more than 0.6% Si, 0.8 - 2.0% Mn, 0.01 - 0.08% Al, not more than 0.008 S, 0.008 - 0.025% Ti, 0.001 - 0.007% N, 0.0005 - 0.005% Ca and the balance being Fe and incidental impurities;
heating the steel slab at a temperature between 950 - 1050°C, and rolling down the heated steel under such a condition that reduction rate of at least 40%
is applied to the steel at a temperature not higher than 850°C and the finishing temperature of the rolling being maintained between Ar3 plus 10°C and 800°C.

4. A method of producing as rolled steel of reduced anisotropy and having high toughness at both base metal and heat affected zone of welded portion comprising the steps of:
making by continuous casting a steel slab of not more than 300 mm thickness consisting essentially all by weight of 0.01 - 0.15% C, not more than 0.6% Si, 0.8 -2.0% Mn, 0.01 - 0.08% Al, not more than 0.008 S, 0.008 -0.025% Ti, 0.001 - 0.007% N, 0.0005 - 0.005% Ca, at least one element selected from the group consisting of not more than 0.08% Nb, not more than 2.0% Ni, not more than 1.0% Cu, not more than 1.0% Cr and the balance being Fe and incidental impurities;
heating the slab at a temperature between 950 -1050°C, and rolling down the heated steel under such a condition that reduction rate of at least 40% is applied to the steel at a temperature not higher than 850° and the finishing temperature of the rolling being maintained between Ar3 plus 10°C and 800°C.

5. The process of claim 1 wherein the finishing temperature is between 750° and 850°C.

6. The process of claim 2 wherein the finishing temperature is between 750° and 800°C.

7. The process of claim 3 wherein the finishing temperature is between 750° and 800°C.

8. The process of claim 4 wherein the finishing temperature is between 750° and 800°C.

9. The process of claim 1 wherein after the rolling, the steel is cooled at a rate of 0.2° to 10°C
per second.

10. The method of claim 2 wherein after the rolling, the steel is cooled at a rate of 0.2° to 10°C per second.

11. The method of claim 3 wherein after the rolling, the steel is cooled at a rate of 0.2° to 10°C per second.

12. The method of claim 4 wherein after the rolling, the steel is cooled at a rate of 0.2° to 10°C per second.