JP2002003987A - Low yield ratio high strength steel excellent in weldability and low temperature toughness and method for producing the same - Google Patents

Abstract

(57) [Abstract] (Modified) [Problem] Low yield ratio high tensile strength steel with excellent weldability and low-temperature toughness, such as construction steel that requires seismic performance and steel for various tanks loaded with different types of liquefied gases. And a method for producing the same. SOLUTION: The steel structure at the cross section 1/4 thickness position in the thickness direction is
Contains a certain amount of martensite or a mixed martensite-austenite phase, and at least 90% of each phase has an equivalent circle diameter of 4 µm or less and an aspect ratio of 4 or more, and no yield point is obtained in a tensile test. In order to realize this, C, Si, Mn, P, S, Nb,
Cu, Ni, Cr, M containing Ti, Al
contain o the like, and a cast slab or steel strip than 0.25 percent P _CM, heated to a temperature of 1000 to 1250 ° C., the cumulative reduction ratio of austenite non-recrystallization temperature range as 30% or more After finishing hot rolling at a temperature of 720 points or more, 6
Control cooling from temperatures above 80 ° C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a construction steel which is required to have high toughness and a low yield ratio from the viewpoint of seismic resistance, and a steel for various tanks. The present invention relates to a high-strength steel requiring low-temperature toughness and a low yield ratio at the same time as the corresponding composite properties, and a method for producing the same.

[0002]

2. Description of the Related Art With the shift from elastic design (allowable stress design) to ultimate strength design based on a new seismic design standard implemented in June 1981, low yield ratios are required for building steel materials. I have. In order to achieve a low yield ratio, generally, a dual phase (dual phase) of the steel structure is formed, that is, a soft phase (usually ferrite) that controls yield and a hard phase (pearlite, bainite) for securing tensile strength. , Martensite, etc.) are widely used. Specifically, the steel after hot rolling or quenching, including controlled rolling, is reheated to the two-phase temperature of ferrite and austenite to form austenite in which ferrite and C are concentrated, and then air-cooled. Japanese Patent Laid-Open No. 2-266378 discloses a method of cooling at the above-described cooling speed (and further tempering thereafter).
No., for example. At this time,
The higher the C content, the easier the two-phase organization becomes,
The hard phase is more hardened, and the yield ratio can be easily reduced. However, there is a problem that increasing the C content is disadvantageous for weldability and low-temperature toughness. On the other hand, in order to improve low-temperature toughness, low C and controlled rolling are effective, but since the yield ratio increases in both cases, improvement in low-temperature toughness and low yield ratio are incompatible with each other. It was extremely difficult. Conventionally, in building applications, the required level of toughness is low, and high C, which is advantageous for lowering the yield ratio
Although there was no particular problem with steel, the demands for seismic performance in recent years following the Great Hanshin Earthquake have been increasing.
There was a problem that it was not always possible to respond sufficiently.

[0003] Further, in the steel material used for the tank for the liquefied gas storage tank, the liquefaction temperature of the gas is generally low at normal pressure (-48 in the case of LPG, though it depends on the type of liquefied gas).
° C), excellent low-temperature toughness is required not only in the base material but also in the welded joint. In contrast, Japanese Patent Application Laid-Open
Japanese Patent Application Laid-Open No. 290246/1990 discloses a method of adding 6.5 to 12.0% Ni, and Japanese Patent Application Laid-Open No. 58-153730 discloses a method of quenching and tempering a steel having a specific composition to provide tempered martensite and bainite. A method utilizing toughness is disclosed. On the other hand, liquid ammonia is known to cause stress corrosion cracking (SCC) of steel, and IGC C
ODE 17.13 (International C
ode for the construction
and Equipment of Ships Ca
rrying LiquidifiedGames in
Bulk) regulates operating conditions during storage, such as oxygen partial pressure and temperature, and restricts the Ni content of steel to 5% or less and the actual yield strength to 440 N / mm ² or less. Stipulates. For this reason, Japanese Unexamined Patent Publication No.
No. 7613 discloses a steel sheet in which only the surface layer is softened, and Japanese Unexamined Patent Publication No. 57-139493 discloses a tank manufacturing method using mild steel clad steel and a soft welding final layer.

However, the tank in which the LPG and the liquid ammonia are mixed must naturally satisfy the specifications required for both. On the other hand, large-capacity tanks and high-tension tanks of this type, which are often mounted on ships, are required to have high tensile strength, and are subject to superior low-temperature toughness from LPG and the upper limit of yield strength from liquid ammonia. At the same time, achieving a low yield ratio at the same time has been a major issue.

[0005] Furthermore, martensite or a martensite-austenite mixed phase (MA consitite)
is harmful to low-temperature toughness because it is hard and brittle, and it is generally accepted that steel components and production conditions are limited so as not to be formed as much as possible, or when formed, they are decomposed by heat treatment such as tempering. And aggressively martensite or a martensite-austenite mixed phase (M
-A constituents) were not used. The martensite or martensite-austenite mixed phase (M-Ac) defined in the present invention.
The term “instruments” is also referred to as island-like martensite or M *, and the appearance method (etching method) for identifying its phase (structure) will be described later.

[0006]

DISCLOSURE OF THE INVENTION The present invention aims at achieving excellent weldability and low-temperature toughness as well as high strength and a low yield ratio.
Martensite or martensite-austenite mixed phase (MA constituent) in steel structure
The present invention is characterized in that the existence form such as the structure fraction and size in s) is limited, a yield point is not generated in a tensile test, and steel components and manufacturing conditions are limited in order to realize this.

According to the present invention, it is possible to supply a large amount and inexpensively of steel for construction having excellent seismic resistance and steel having both low-temperature toughness and a low yield ratio for a mixed loading tank of liquid ammonia and LPG. Since high strength can be achieved, the tank can be easily mounted on a ship.

[0008]

The basic idea of the present invention is to provide a martensite or martensite-austenite mixed phase (M-A co
(Nstudents), by defining the existence form such as the fraction, size, shape, etc., and by preventing the yield point from appearing during the tensile test, it is possible to reduce the yield ratio without impairing the low-temperature toughness.
For this purpose, the steel of a specific component containing Nb is subjected to controlled rolling-accelerated cooling to refine the structure and secure strength and toughness, and the accelerated cooling is stopped at a relatively low temperature to obtain martensite. Or a martensite-austenite mixed phase (MA constituents)
Is generated finely.

The gist of the present invention is as follows.

(1) In the steel structure at the 1/4 thickness position in the cross section in the plate thickness direction, martensite or a martensite-austenite mixed phase (MA-constituent)
s) contains 1 to 10% by area fraction of the observed cross section, and 90% or more of each phase has a circle equivalent diameter of 4 µm or less and an aspect ratio of 4 or more, and has a yield point in a tensile test. Low-yield-ratio high-strength steel with excellent weldability and low-temperature toughness characterized by the absence of cracks.

(2) Steel component in mass%, C: 0.03
0.15%, Si: 0.4% or less, Mn: 1.0-
2.0%, P: 0.02% or less, S: 0.01% or less,
Nb: 0.005 to 0.05%, Ti: 0.005 to
0.025%, Al: 0.06% or less, N: 0.001
0.005%, and _{P CM = C + Si / 30} + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.25% or less, with the balance being iron and unavoidable impurities, the low yield ratio high tensile strength steel excellent in weldability and low temperature toughness according to the above (1).

(3) Cu: 0.05-0.
5%, Ni: 0.05 to 1.0%, Cr: 0.05 to
0.5%, Mo: 0.05 to 0.5%, V: 0.01 to
0.05% B: 0.0002-0.003%, Mg:
The low yield ratio high tensile strength steel excellent in weldability and low temperature toughness according to the above (2), further comprising one or more kinds in the range of 0.0002 to 0.005%.

(4) Ca: 0.0005 to 5% by mass
Low yielding excellent in weldability and low-temperature toughness according to the above (2) or (3), further containing any one of 0.004% and REM: 0.0005 to 0.004%. High tensile steel.

(5) Any one of the above (2) to (4)
The slab or the slab consisting of the steel composition according to
After heating to a temperature of １２1250 ° C. and finishing the hot rolling at a temperature of 720 or more with the cumulative reduction in the austenite non-recrystallization temperature range of 30% or more, accelerated cooling is started from a temperature of 680 ° C. or more. In the steel structure at a 1/4 thickness position in a cross section in the thickness direction, martensite or a mixed phase of martensite-austenite (MA) is characterized in that accelerated cooling is stopped at a temperature of 150 to 350 ° C and then left to cool. c
on the basis of the area fraction of the observed cross section.
Weldability and low temperature characterized in that at least 90% of each phase contains 4% or less in equivalent circle diameter and has an aspect ratio of 4 or more and has no yield point in a tensile test. Method for producing high yield strength low tensile strength steel with excellent toughness.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the present invention has limited the steel structure, the steel composition and the manufacturing method as described in the claims will be described.

[0016] The steel structure is characterized in that it contains martensite or a martensite-austenite mixed phase (MA-constituents) in an area fraction of 1 to 10% of the observed cross section at a 1/4 thickness position in the cross section in the plate thickness direction. One component. This martensite or a martensite-austenite mixed phase (MA-constiti)
ents) has a high dislocation density and is a very hard phase enriched with C. Due to the presence of this phase, dislocations start to move at a low stress during a tensile test, and there is no yield point on the stress-strain curve. Draw a round curve. Martensite or martensite-austenite mixed phase (M-Ac)
Even when the structural fraction of the components is within the above-mentioned limited range, a yield point may appear depending on the structure of the matrix (for example, a ferrite structure). One. This is, specifically, in the tensile test, the load-
It means that the elongation curve draws a round curve, and it is essential that a yield point does not appear in order to achieve both high tension and low yield ratio. Round load-no yield point
In the elongation curve, 0.2% proof stress is generally adopted as the yield strength, and the yield strength is lower than that of steel having the same tensile strength at which the yield point is generated, resulting in a lower yield ratio.

Defining the structure of the matrix as a condition for not producing a yield point in a tensile test also depends on the hardness and constituent fraction of martensite or a martensite-austenite mixed phase (MA-constituents). Not only can't say in general,
For reasons such as ambiguous descriptions of those organizations (it is impossible to accurately describe a wide variety of organizations)
It was determined that it was inappropriate as a component of the invention. Martensite or martensite-austenite mixed phase (M
The lower limit of 1% of the constituent fraction (area fraction of the observed cross section) of the -A constituents is the minimum amount necessary for the dislocation to start to move with low stress during the tensile test, and 10% of the upper limit.
Is a critical amount that does not deteriorate the toughness more than necessary. However, from the viewpoint of low-temperature toughness, martensite or a martensite-austenite mixed phase (MA cons) is used.
composition) (area fraction of observed cross section)
It is not sufficient to limit the above as described above. When present in a large unit (size) in a lump, it acts as a fracture starting point, and the toughness is deteriorated. Phase (MA)
90% or more of each of the constituents had an equivalent circle diameter of 4 μm or less, and the ratio of the major axis to the minor axis that defines the shape = aspect ratio was limited to 4 or more. These figures are based on the experimental facts of the inventors. In short, even if only the fraction and the size of the martensite or the martensite-austenite mixed phase (MA constituents) are specified, if the shape is not appropriate, the low-temperature toughness will be deteriorated. Martensite or martensite-austenite mixed phase (MA Co
In the case of the same size (equivalent circle diameter), it is more advantageous for the low-temperature toughness to exist in a state where the aspect ratio is larger than that of the bulk, and ultimately, a film having almost no thickness is most preferable. ,Substantially,
When the aspect ratio is 4 or more, deterioration in low-temperature toughness is small. In addition, martensite or a martensite-austenite mixed phase (MA-constituent)
s) The tissue manifestation method for identification is an etching method developed by LePera (Journal of
Metals, March, 1980, p. The method based on 38) is optimal, and this etching allows
The martensite or martensite-austenite mixed phases (MA-constituents) appear white.

Next, in order to obtain a limited martensite or martensite-austenite mixed phase (MA-constituents) as in the present invention, and to prevent a yield point from appearing in a tensile test, an optimum steel component is used. Will be described.

C is most remarkably effective on the properties of steel materials.
The lower limit of 0.03% is a minimum amount for ensuring strength and preventing the heat-affected zone such as welding from softening more than necessary. However, if the C content is too large, the hardenability increases more than necessary, and the steel material has an adverse effect on the inherent strength, toughness balance, weldability, etc., so the upper limit was made 0.15%.

[0020] Si is an element contained in the deoxidized upper steel,
If a large amount is added, the weldability and the HAZ toughness deteriorate, so the upper limit is limited to 0.4%. Deoxidation of steel is sufficiently possible only with Ti and Al, and the lower the better, from the viewpoint of HAZ toughness, hardenability and the like, the more preferable, and it is not always necessary to add.

Mn is an indispensable element for securing strength and toughness, and its lower limit is 1.0%. However, Mn
If the amount is too large, the hardenability increases and not only deteriorates the weldability and HAZ toughness, but also promotes the center segregation of the continuous cast slab, so the upper limit was made 2.0%.

P is an impurity in the steel of the present invention.
Since a reduction in the amount tends to reduce grain boundary fracture in the HAZ, a smaller amount is preferable. If the content is high, the base material,
The upper limit is set to 0.02% in order to deteriorate the low-temperature toughness of the weld.

S, like P, is an impurity in the steel of the present invention, and is preferably as small as possible from the viewpoint of the low-temperature toughness of the base material. If the content is large, the low-temperature toughness of the base material and the welded portion is deteriorated, so the upper limit was made 0.01%.

Nb is an essential element for increasing the unrecrystallization temperature of austenite and maximizing the effect of controlled rolling during hot rolling, and at least 0.005% must be added. It also contributes to the refinement of heated austenite during quenching. Further, it has an effect of improving strength as precipitation hardening. However, excessive addition causes toughness degradation of the welded portion, so the upper limit was made 0.05%.

Ti is essential for improving the base material and HAZ toughness. Because, when the amount of Al is small (for example, 0.003% or less), Ti combines with O to form a precipitate containing Ti ₂ O ₃ as a main component, becomes a nucleus for the formation of intragranular transformed ferrite, and improves the HAZ toughness. Let it. Further, Ti is combined with N and finely precipitated as TiN in the slab, and γ during heating is obtained.
This is because it suppresses coarsening of the grains and is effective in reducing the grain size of the rolled structure, and the fine TiN present in the steel sheet refines the HAZ structure during welding. To get these effects,
Ti must be at least 0.005%. However, if too much, TiC is formed and the low-temperature toughness and weldability are deteriorated.
The upper limit is 0.025%.

Al is an element generally contained in the deoxidized upper steel, but Si or Ti alone is sufficient for deoxidation, and the lower limit is not limited in the steel of the present invention. But A
When the amount of l increases, not only does the cleanliness of the steel deteriorate, but also the toughness of the weld metal deteriorates, so the upper limit was made 0.06%.

N, which is contained in steel as an unavoidable impurity, combines with Nb to form a carbonitride to increase the strength, and forms TiN to form a steel as described above. Enhance the nature. Therefore, the amount of N is at least 0.001.
%is necessary. However, an increase in the amount of N is extremely harmful to HAZ toughness and weldability, and the upper limit of the steel of the present invention is 0.005%.

Next, C which can be optionally contained
The reason for adding u, Ni, Cr, Mo, V, B, and Mg will be described.

The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be naturally restricted.

Cu exhibits almost the same effects and phenomena as Ni, with 0.5% of the upper limit not only deteriorating the weldability, but excessive addition is regulated because Cu-cracks occur during hot rolling and production becomes difficult. You. The lower limit should be 0.05%, which should be the minimum for a substantial effect to be obtained. This is C
The same applies to r and Mo.

If Ni is not added excessively, weldability, H
Improves the strength and toughness of the base material without adversely affecting AZ toughness. In order to exert these effects, it is essential to add at least 0.05% or more. On the other hand, excessive addition is not only expensive but also unfavorable for weldability. In addition, it has been pointed out that adding a large amount of Ni may induce stress corrosion cracking (SCC) in liquid ammonia. According to the experiments of the inventors, the addition of up to 1% does not significantly deteriorate the weldability and SCC in liquid ammonia, and has a greater effect of improving the strength and toughness. Therefore, the upper limit is set to 1.0%.

Cr and Mo are each required to be 0.05% or more in order to improve both the strength and the toughness of the base material.
However, if the addition amount is too large, the toughness and weldability of the base material and the welded portion are deteriorated, and the control of the structure described later becomes difficult.

V has almost the same effect as Nb, but its effect is smaller than that of Nb. Further, V also affects the hardenability, and is added from the viewpoint of controlling the structure as in the case of the above-mentioned elements. The effect similar to Nb is 0.
If it is less than 01%, the effect is small, and the upper limit can be tolerated up to 0.05%.

B segregates at austenite grain boundaries and suppresses the formation of ferrite, thereby improving hardenability and contributing to strength improvement. To enjoy this effect, at least 0.0002% is required. However, too much addition not only saturates the effect of improving hardenability but also may form B precipitates that are harmful to toughness, so the upper limit was made 0.003%. In addition, in cases where stress corrosion cracking is a concern as steel for tanks, reduction of the hardness of the base metal and the weld heat affected zone is often the point (for example, to prevent sulfide stress corrosion cracking (SCC)). HR for
C ≦ 22 (HV ≦ 248) is essential. In such a case, the addition of B which increases the hardenability is not preferable.

Mg has the effect of suppressing the growth of austenite grains in the heat affected zone and reducing the size of the austenitic grains, thereby increasing the toughness of the weld. In order to enjoy such effects, Mg needs to be 0.0002% or more. On the other hand, as the addition amount increases, the effect cost on the addition amount decreases,
The upper limit is set to 0.005% because it is not advantageous in terms of cost.

Further, Ca and REM control the morphology of MnS and improve the low-temperature toughness of the base material, and also cause hydrogen-induced cracking (HIC, SSC, SOC) in a wet hydrogen sulfide environment.
HIC) reduces susceptibility. To achieve these effects, a minimum of 0.0005% is required. However, too much addition will conversely increase the cleanliness of the steel, increase base metal toughness and hydrogen-induced cracking in wet hydrogen sulfide environments (HIC, SSC, SSC).
OHIC) To increase the sensitivity, the upper limit of the amount added is 0.0
Limited to 04%. Since Ca and REM have almost the same effect, one of them may be added in the above range.

Even if the individual components of the steel are limited, excellent properties cannot be obtained unless the entire component system is appropriate. Therefore, P
Limit the value of _CM to 0.25% or less. The P _CM defined by the following formula in an index representing the weldability, the lower the weldability is good. In the present invention steel, as long as P _CM is 0.25% or less, it is possible to ensure excellent weldability. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + 5B

While ensuring excellent weldability and low-temperature toughness, in the tensile test as described above, the load-elongation curve draws a round curve and does not produce a yield point, so that the production conditions are limited according to the present invention. It is extremely effective to do so.
Hereinafter, the reason will be described.

The heating temperature prior to rolling is 1000 to 125
The reason for limiting the temperature to 0 ° C. is to keep the austenite grains small during heating and to make the rolling structure finer. 125
0 ° C. is an upper limit temperature at which austenite during heating does not become extremely coarse. If the heating temperature exceeds this temperature, austenite grains are coarsely mixed and the structure after transformation is also coarse, so that the toughness of steel is significantly deteriorated. On the other hand, if the heating temperature is too low, it is difficult not only to secure a rolling end temperature (Ar ₃ point or more) described later, but also to raise the austenite non-recrystallization temperature, thereby reducing the effect of controlled rolling during hot rolling. The lower limit was set to 1000 ° C. from the viewpoint of solutionizing Nb to maximize its effect and to develop precipitation hardening. The slab or slab heated under the above-mentioned conditions is subjected to a cumulative rolling reduction of 30% or more in the austenite non-recrystallization temperature range, at 720 ° C.
After finishing the hot rolling as described above, accelerated cooling is performed from a temperature of 680 ° C. or more.

Rolling in the austenite non-recrystallization temperature range significantly reduces the size of austenite grains, so a cumulative reduction of at least 30% or more is required. If the rolling end temperature is lower than 720 ° C., the ferrite transforms and precipitates, and the ferrite may be processed (rolled), which is not preferable in terms of lowering the yield ratio and ensuring low-temperature toughness. For this reason, the rolling end temperature is limited to 720 ° C. or higher.

After the completion of hot rolling at 720 ° C. or higher, 6
The reason why the accelerated cooling is started from a temperature of 80 ° C. or higher is to make the structure finer by increasing the cooling speed in the transformation region and simultaneously improve the strength and toughness. Further, the refinement of the structure is indispensable for finely forming the martensite-austenite mixed phase (MA-constituents) as the C-enriched phase as in the present invention. Organization 68
When the temperature is lower than 0 ° C., coarse ferrite starts to precipitate, and the strength is reduced and the toughness is deteriorated. This accelerated cooling must stop at a temperature of 150-350 ° C. At a temperature higher than 350 ° C., the cooling after stopping the accelerated cooling is substantially tempering, and the strength is reduced, and the martensite-austenite mixed phase (MA-constituents) is decomposed, so that the yield point is obtained. And a low yield ratio cannot be achieved. On the other hand, when the accelerated cooling stop temperature is lower than 150 ° C., in addition to a high possibility that a martensite-austenite mixed phase (MA-constituents) is generated more than necessary, softening due to heat influences such as welding and gas cutting occurs. It is not preferable in use performance because it becomes remarkable. For this reason, the lower limit temperature of the accelerated cooling stop temperature was set to 150 ° C.

The cooling rate during the accelerated cooling cannot be unconditionally determined because it varies depending on the steel composition, the intended yield ratio, and the low-temperature toughness level. It is desirable that the average cooling rate is up to at least 3 ° C./sec.

[0043]

EXAMPLE A steel plate (thickness: 15 to 80 mm) of various steel components is manufactured in a converter-continuous casting-thick plate process, and its strength, yield ratio (YR), toughness, and weldability (oblique y-shaped weld cracking) Test) was investigated.

Table 1 shows the steel composition of the steel of the present invention together with the comparative steel, and Table 2 shows the manufacturing conditions and various characteristics of the steel sheet.

[0045]

[Table 1]

[0046]

[Table 2]

The steel sheets produced according to the method of the invention (steel of the invention) all have good properties. On the other hand, the comparative steel not according to the present invention is inferior in any of the properties.

The comparative steel 11 has a low C content and has a low Nb, T
In addition to the fact that i is not added, since the cumulative reduction in the γ non-recrystallization temperature range is also small, the weldability is good, but the microstructure is insufficiently refined, the strength is low, and Since the martensite or martensite-austenite mixed phase (MA constituents) ratio is high, the toughness is also poor. Although the comparative steel 12 is within the scope of the present invention in terms of composition, since the water cooling stop temperature is high, martensite or a martensite-austenite mixed phase (MA-constituents) is not generated, and as a result, a yield point appears. And the yield strength is high, and the yield ratio is high. Comparative Steel 13, although the addition amount of each element is within the scope of the present invention, inferior in weldability due to high P _CM. Further, coarse or massive martensite or a martensite-austenite mixed phase (MA)
(constituents) and poor toughness. Comparative steel 14 has a high Ti content, a low rolling temperature under the manufacturing conditions,
Since the water cooling start temperature is also low, a yield point appears, yield strength,
Both the yield ratio is high and the low temperature toughness is poor. Note that Ti
It has been confirmed that HAZ toughness is inferior in the comparative steel 14 having a high amount, which is not preferable in use performance. Comparative steel 15
High C content, P _CM also inferior in high order weldability, also there accelerated cooling may stop temperature is low, martensite or martensite - austenite mixed phase (M-A cons
(Titsents) fraction is high, and the ratio of coarse and bulky ones is high, so that toughness is inferior. Furthermore, it has been confirmed that the comparative steel 15 having a low accelerated cooling stop temperature has remarkable HAZ softening during welding, which is not preferable in terms of use performance.

[0049]

According to the present invention, it has become possible to produce a low-yield-ratio high-tensile steel excellent in weldability and low-temperature toughness. as a result,
For building with excellent seismic performance, or liquid ammonia and L
It has become possible to supply inexpensively a large amount of steel having excellent weldability for use in a tank mixed with PG or the like. In particular, since the strength can be increased, the tank can be easily mounted on a ship.

Continued on the front page F term (reference) 4K032 AA00 AA01 AA02 AA04 AA05 AA08 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA27 AA29 AA31 AA35 AA36 AA40 BA01 CA02 CA03 CC02 CC03 CD05

Claims (5)

[Claims] In a steel structure at a 1/4 thickness position in a cross section in a thickness direction, martensite or a martensite-austenite mixed phase (MA-constituents) contains 1 to 10% by area fraction of an observed cross section. 90% or more of each phase has a circle equivalent diameter of 4 μm or less and an aspect ratio of 4 or more, and has no yield point in a tensile test, and has low weldability and excellent low-temperature toughness. High tensile steel. 2. The steel component in mass%, C: 0.03-0.
15%, Si: 0.4% or less, Mn: 1.0 to 2.0
%, P: 0.02% or less, S: 0.01% or less, Nb:
0.005 to 0.05%, Ti: 0.005 to 0.02
5%, Al: 0.06% or less, N: 0.001 to 0.0
05%, and P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
The low yield ratio high tensile strength steel having excellent weldability and low temperature toughness according to claim 1, characterized in that / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.25% or less and the balance consists of iron and inevitable impurities. 3. Cu: 0.05 to 0.5% by mass%;
Ni: 0.05 to 1.0%, Cr: 0.05 to 0.5
%, Mo: 0.05 to 0.5%, V: 0.01 to 0.0
5% B: 0.0002-0.003%, Mg: 0.00
The low-yield-ratio high-tensile steel having excellent weldability and low-temperature toughness according to claim 2, further comprising one or more types in the range of 02 to 0.005%. 4. Ca: 0.0005 to 0.0% by mass
The low-yield-ratio high-tensile steel having excellent weldability and low-temperature toughness according to claim 2 or 3, further comprising any one of 0.4% and REM: 0.0005 to 0.004%. 5. A slab or a slab made of the steel composition according to any one of claims 2 to 4, which is subjected to a temperature of 1000 to 1250 ° C.
After the hot rolling is completed at a temperature of 720 points or more with the cumulative reduction in the austenite non-recrystallization temperature range being 30% or more, accelerated cooling is started from a temperature of 680 ° C. or more. In the steel structure at a 1/4 thickness position in the thickness direction in the sheet thickness direction, the accelerated cooling is stopped at a temperature of 350 ° C., and then the steel is allowed to cool.
ents) in an area fraction of the observed cross section of 1 to 10%, and 90% or more of each of the phases has a circle equivalent diameter of 4 μm.
A method for producing a low-yield-ratio high-strength steel excellent in weldability and low-temperature toughness, characterized in that an aspect ratio is 4 or more and a yield point does not appear in a tensile test.