JPH05179344A - Production of high tensile steel having excellent low-temperature toughness of multilayer weld zone - Google Patents

Abstract

PURPOSE:To easily produce the high tensile steel having the low-temp. toughness of a multilayer weld zone by subjecting a continuously cast slab contg. specific ratios of C, Si, Mn, S, Cu, Ni, Al and Ca to a specific heat treatment and hot rolling. CONSTITUTION:The continuously cast slab contg. 0.01 to 0.15wt.% C, <=0.15% Si, 0.5 to 2.0% Mn, <=0.005% S, 0.5 to 2.0% Cu, 0.1 to 1.5% Ni, 0.005 to 0.1% Al, 0.005 to 0.005% Ca and contg. further >=1 kinds of <=0.07% Nb, <=0.1% V, <=0.2% Mo and <=0.02% Ti is heated to 1050 to 1250 deg.C. This slab is thereafter subjected to the hot rolling to attain >=25% cumulative draft at Ar3 to 900 deg.C and if necessary, to attain 10 to 45% cumulating draft at (Ar3-500 deg.C) to Ar3. The steel product is then subjected to air cooling or force cooling then further to reheating to 400 to 650 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength steel used for marine structures, line pipes, etc. .

[0002]

2. Description of the Related Art In recent years, marine structures and pipelines have become large in size, and the thickness of steel sheets used has tended to become thicker, and extremely high levels of low temperature toughness are required at the welded portions. The high-strength steel sheet utilizing the precipitation hardening mechanism of Cu is characterized by low hardenability in the weld heat affected zone (HAZ) and excellent weldability, especially cold cracking susceptibility. No. 3692514. However, it has been pointed out that the low temperature toughness is impaired if the strength is increased by utilizing the precipitation hardening mechanism of Cu. In order to improve such a defect of the Cu precipitation hardening type steel sheet, for example, JP-A-60-59018, JP-A-61-149430, and JP-A-62-1.
Many manufacturing methods have been proposed as disclosed in Japanese Patent Publication No. 49845.

However, although the Charpy impact test has been mainly used as a conventional evaluation method of low temperature toughness, in recent years, the CTOD test prescribed in British Standard BS5762 (1979) is often required. This test evaluates the brittle fracture initiation resistance of an extremely small portion by causing a fatigue pre-crack in the evaluation portion. -Although welding of thick steel plates is performed on-site with a large number of multi-layer welding,
In such a construction, a local embrittlement zone that has undergone a complicated heat history is generated in the welding heat affected zone (HAZ). Especially 1350
Coarse grain region of crystal grains heated to a high temperature of ℃ or more (hereinafter CGH
(Abbreviated as AZ) is the part where the toughness deteriorates most.

In the American Petroleum Institute, APIPI 2Z (1
987), detailed rules for processing are incorporated so that CGHAZ exists at a certain ratio or more at the tip of the precrack. In this way, when the strict evaluation of brittle fracture initiation characteristics is made, the required brittle fracture initiation resistance may not be obtained even if high toughness is obtained in the Charpy impact test by the above-mentioned prior art. In this regard, Japanese Patent Laid-Open No. 25517/1990 discloses a method for producing a Cu precipitation hardening type high-strength steel excellent in weldability and low temperature toughness corresponding to the CTOD test. However, in recent years, the CTOD value has been required to be even higher, and the demand for improving the resistance to brittle fracture occurrence has become even stronger.

[0005]

The present invention has been made in view of the current state of the art as described above, and in particular, new production of Cu precipitation hardening type high strength steel excellent in low temperature toughness of multi-layer welds. The challenge is to provide a method.

[0006]

Means for Solving the Problems The inventors of the present invention have made extensive studies and experiments in order to effectively solve the above problems, and as a result, have obtained the following findings. That is, a) The heating temperature of the continuously cast steel billet is 1050 ° C. or higher, 125
By setting the temperature to 0 ° C. or less, the chemical composition inside the steel sheet is made uniform, and the generation of an abnormal structure is prevented. b) The addition amount of Si is set to 0.15% by weight (hereinafter, all chemical compositions are abbreviated as%) or less to suppress the formation of island martensite formed in ICCGHAZ. c) S is 0.005% or less and Ca is 0.0005 to
M in steel by adding in the range of 0.005%
Prevent the generation of nS. d) A fine grain structure is formed by performing controlled rolling on steel having the above-described chemical composition characteristics and then air cooling or forced cooling. e) Further, by subjecting the obtained steel sheet having high toughness to Cu precipitation hardening treatment, high strength is achieved without causing deterioration of toughness. Only when all of the above are satisfied, a multi-layer weld HA with excellent low temperature toughness that can support brittle fracture resistance evaluation in CTOD test.
The present invention has been completed by finding that it becomes possible to manufacture a steel sheet for forming Z.

That is, the present invention is, in terms of weight ratio,
C: 0.01 to 0.15%, Si: 0.15% or less,
Mn: 0.5 to 2.0%, S: 0.005% or less,
Cu: 0.5-2.0%, Ni: 0.1-1.5
%, Al: 0.005-0.1%, Ca: 0.00
A continuously cast steel slab containing 05-0.005% was
Heating to a temperature of 0 to 1250 ° C., then Ar _{3 to} 900
After hot rolling with a cumulative reduction of 25% or more in the temperature range of ° C, air cooling or forced cooling, 400 to 6
A method for producing a high-strength steel excellent in low-temperature toughness of a multi-layer weld, characterized by reheating to 50 ° C.,

Also, in a weight ratio, C: 0.01-
0.15%, Si: 0.15% or less, Mn: 0.5
~ 2.0%, S: 0.005% or less, Cu: 0.5
.About.2.0%, Ni: 0.1 to 1.5%, Al: 0.
005-0.1%, Ca: 0.0005-0.005
% Of the continuous cast steel billet is heated to a temperature of 1050 to 1250 ° C., and thereafter, a rolling reduction is 25% or more in a temperature range of Ar _{3 to} 900 ° C., and a temperature of (Ar ₃ −50 ° C.) to Ar ₃ . High tensile strength excellent in low-temperature toughness of a multi-layer welded portion, which is characterized by performing hot rolling with a cumulative rolling reduction of 10 to 45% in the range, air-cooling or forced cooling, and then reheating to 400 to 650 ° C. A method for producing steel, wherein the weight ratio of the continuously cast steel slab is C: 0.01 to 0.15%, S
i: 0.15% or less, Mn: 0.5 to 2.0%,
S: 0.005% or less, Cu: 0.5 to 2.0%, N
i: 0.1 to 1.5%, Al: 0.005 to 0.1
%, Ca: 0.0005 to 0.005%, Nb: 0.07% or less, V: 0.1% or less,
Mo: 0.2% or less, Ti: 0.02% or less, a high-strength steel excellent in low-temperature toughness in a multi-layer weld, which is a continuously cast steel slab containing one or more kinds. Is a manufacturing method.

[0009]

The reason for limiting the chemical composition and manufacturing conditions such as rolling will be described in detail below together with its function. C: C is limited to 0.01 to 0.15% in order to improve the weldability and HAZ toughness of multilayer welding. Generally, small heat input welds are easily hardened and various cracks are likely to occur. In order to prevent these obstacles, it is effective and necessary to keep the hardness of steel low. Further, in order to suppress the formation of island martensite formed in the HAZ of the multi-layer weld, it is desirable that C be low. For this reason, the upper limit of the C content is set to 0.15%. If this value is exceeded, the effect of lowering Si as described in the next section will not be exhibited. Also,
On the other hand, C is one of the important strengthening elements of steel, and extreme reduction is disadvantageous in terms of strength, so its lower limit is 0.01%.
And The characteristics of the present invention are sufficiently exhibited in the above chemical composition range, but a particularly preferable range is 0.02 to 0.08.
%.

Si: Si is one of the most important elements for exerting the characteristics of the present invention. Si is an element that is inevitably contained as a deoxidizing element in the steelmaking process, but is also generally used as a strengthening element. However, it is said that there is an adverse effect on the toughness of the welded portion, and this effect is also observed particularly in the CTOD test even at the 0.2% level which is considered to be the low Si region. Therefore, in the present invention, Si is strictly limited to 0.15% or less. S
When i exceeds 0.15%, not only the effect of suppressing the formation of island martensite is reduced, but also the tempering effect due to the laminating pass peculiar to multilayer welding is not obtained, and the HAZ of the welded part is not obtained.
It cannot be expected to improve the toughness of the locally embrittled region. The amount and morphology of the island-like martensite in the locally embrittled region produced in the multi-layer weld HAZ depend on the structure before reheating. Therefore, even if Si is reduced, the effect cannot be fully enjoyed when the preceding structure is rough and the hardenability is high. In addition, of course, if there is an inclusion such as MnS, the effect of reducing Si does not appear due to deterioration in toughness. Therefore, the low S of the present invention
In order to maximize the effect of i-conversion, it is necessary to combine the control of the anterior tissue and the control of inclusions. According to the Ca-added steel, in addition to the effect of controlling the morphology of MnS, which will be described later, a synergistic effect is obtained because the effect of refining the pre-structure and suppressing the hardenability works.

Mn: If Mn is less than 0.5%, sufficient hardenability of the steel cannot be secured, and the toughness of the base metal is deteriorated. On the other hand, if it exceeds 2.0%, the HAZ of the weld zone is hardened. As a result, the cold cracking susceptibility increases, which impairs the local welding workability. Therefore, Mn is limited to the range of 0.5 to 2.0%. S: S is a harmful impurity and should be reduced as much as possible, but due to the effect of low Si and the effect of Ca described later, if it is 0.005% or less, the effect of the present invention is not impaired. Cu: Cu is an element that dramatically improves the strength of steel by precipitation hardening, and is an important element in the present invention. Cu
In order to effectively carry out precipitation hardening by means of at least 0.
It is necessary to add 5% or more. On the other hand, if it exceeds 2.0%, the HAZ hardenability is increased and the weld crack susceptibility is increased.
Therefore, Cu is limited to the range of 0.5 to 2.0%. Ni: Ni is an element that improves the strength and toughness of the base material without impairing the weldability. Further, addition of about 50% of Cu is effective in order to prevent Cu cracking during hot rolling.
However, if not added at 0.1%, strengthening by Ni,
The toughening effect cannot be obtained. On the other hand, if it exceeds 1.5%, H
It enhances the hardenability of AZ to increase the hardenability and the weld crack susceptibility. Therefore, Ni is limited to the range of 0.1 to 1.5%.

Al: Al is an element that is inevitably contained in this type of Al-killed steel as a deoxidizing element like Si.
Acid soluble Al is increased Al ₂ 0 ₃ based inclusions exceeds 0.1%, impair toughness. On the other hand, if it is less than 0.005%, sufficient deoxidation cannot be performed and the toughness of the base material deteriorates. Therefore, Al is limited to the range of 0.005 to 0.1%. Ca: Ca is an essential component for a steel having severe demands for toughness as in the present invention, and particularly in the steel of the present invention having low Si, it is effective in significantly improving the toughness of a welded portion. If the amount added is less than 0.0005%, Mn
There is no morphology control effect of S, and if added in excess of 0.005%, it remains as an oxide in steel and deteriorates toughness. Therefore, Ca is limited to the range of 0.0005 to 0.005%. Nb: Nb is Nb in the austenite region during rolling.
Since it is precipitated as (C, N) and has the effect of pinning the austenite grain boundaries, it is an element effective in preventing coarsening of recrystallized grains, and an element effective in finally obtaining a fine structure. is there. In addition, it is a useful element that has both a transformation strengthening by solid solution and a strengthening function by precipitation in ferrite. However, if the addition amount is too large, the hardenability of the HAZ is increased and the weld crack susceptibility is deteriorated, so the upper limit of the Nb content was set to 0.07%. V: V is an element that strengthens steel by solid solution in ferrite, and is an element effective in improving the strength and toughness of steel. The points become brittle due to precipitation. Therefore, the V content is 0.1% or less.

Mo: Mo accelerates the formation of bainite, but at the same time promotes the formation of martensite, so HAZ
Enhances the hardenability and deteriorates the toughness. In particular, in multi-layer welding, carbide of Mo precipitates in the reheated portion and deteriorates toughness. Therefore, the content of Mo is set to 0.2% or less where such adverse effects are not observed. Ti: Ti is an element that is precipitation strengthened by carbides, and at the same time is an element that has an effect of suppressing coarsening of CGHAZ by nitrides and suppressing deterioration of toughness. But,
If it exceeds 0.02%, the precipitation is excessive and the toughness is significantly deteriorated. Therefore, the Ti content is 0.02% or less.

The above Nb, V, Mo and Ti are optional components, and it is necessary to contain one or more of them. REM: REM itself may easily form an oxide and may be added to prevent coarsening of γ grains in a high temperature (1350 ° C. or higher) heating region near the melting line, but as in the present invention, The coexistence of Ca results in binding with Ca to form coarse inclusions and reducing the effect of added Ca. Therefore, even if added, it should be limited to 0.0003% or less.

Although the chemical composition has been mainly described above, the action and effect of the present invention cannot be sufficiently obtained only by limiting the chemical composition. In order to improve the low temperature toughness of the multi-layer welded HAZ that can meet the high CTOD value requirement, which is the object of the present invention, it is necessary to maintain high toughness of the base metal. For that purpose, it is essential to limit manufacturing conditions such as rolling conditions as described below. That is, in the present invention, a continuously cast steel piece that can be efficiently produced is used as a base material, but in the continuously cast steel piece, segregation occurs in the center part of the plate thickness, so after rolling, this portion is hard and brittle and has an abnormal structure. And significantly impairs toughness. In order to prevent the occurrence of this abnormal structure, it is necessary to make the components in the steel uniform. For that purpose, a heating temperature of 1050 ° C. or higher is necessary. However, the heating temperature is 1250 ° C
If it exceeds, the austenite grains are coarsened, and the effect of refining by the subsequent rolling cannot be sufficiently exerted, and the toughness deteriorates. In addition, when Nb is added, it is necessary to form a solid solution of Nb to some extent in order to obtain its effect, and for this purpose, a heating temperature of 1050 ° C. or higher is necessary. But,
When heated above 1250 ° C., all Nb is solid-dissolved, the pinning effect of the austenite grain boundaries described above is lost, and coarse grains are formed. Therefore, the heating temperature is 1050-1
Limit to the range of 250 ° C.

In the present invention, rolling is performed using a steel slab heated to the above temperature range, and the rolling reduction is 25% or more in the temperature range of Ar ₃ or more and 900 ° C. or less. A particularly preferable range of effective rolling reduction is 25 to 60%. here,
The Ar ₃ temperature is calculated by the following formula. _{Ar 3 (℃) = 910-273C (} %) - 74Mn (%)
-56Ni (%)-16Cr (%)-9Mo (%)-5
Cu (%) If the rolling reduction in this temperature range is less than 25%, the rolling reduction in the non-recrystallized region is small, and sufficient austenite refining and subsequent ferrite grain refinement cannot be obtained. Therefore, the toughness cannot be maintained at a high level. Further, if the rolling temperature range is below the Ar ₃ temperature or exceeds 900 ° C., the rolling reduction in the effective unrecrystallized region decreases. Further, after this rolling, rolling may be carried out within a temperature range of Ar _{3 to} 50 ° C. or higher and Ar ₃ or lower and a cumulative rolling reduction of 10% or more and 45% or less. By this rolling, the ferrite can be further refined, but if rolling exceeding 45% is performed, the accumulation of rolling strain becomes too large, so that the work hardening conversely significantly deteriorates the toughness.

According to the above rolling method, the effect of the present invention can be sufficiently obtained regardless of whether the cooling after rolling is performed by air cooling or forced cooling. The cooling method may be selected according to the intended strength of the steel sheet. After cooling, 400 ℃ or more,
Reheating is performed in a temperature range of 650 ° C. or lower to increase the strength by utilizing the precipitation hardening of Cu. A particularly preferred reheating temperature range is between 450 ° C and 550 ° C. At a reheating temperature of 400 ° C or less, precipitation is insufficient and sufficient precipitation hardening cannot be obtained. On the other hand, if the temperature is 650 ° C. or higher, not only will the precipitation be excessive, or the particles will aggregate and the effect of precipitation hardening will not be sufficiently exerted, but also the toughness will be lowered by the precipitates. The most appropriate temperature range is 450 ° C or higher and 550 ° C or lower.

[0018]

EXAMPLES Table 1 shows the chemical compositions of the steels of Examples of the present invention and the steels of Comparative Examples. All chemical compositions are slab ladle analysis values. Further, the calculated value of the Ar ₃ temperature and the heating temperature according to the above-mentioned calculation formula are also shown. Table 2 also shows the rolling conditions such as the rolling reduction and the reheating temperature, and the test results such as the tensile properties and impact properties of the base material and the toughness values of the welded parts. Tensile test was performed on a tensile test piece having a parallel portion of 10 mmφ according to JIS. In the toughness evaluation of the welded portion, a K-shaped groove joint was prepared by sub-marque welding with an input of 5 kJ / mm, and the vicinity of the substantially linear fusion line generated in the plate thickness direction was evaluated. No preheating was performed during welding. FIG. 1 shows a sampling position of a CTOD test piece. CTOD test is British standard BS5762 (1979)
In accordance with the above, notch processing was performed on the entire thickness in the plate thickness direction, and the rolling direction was performed at -10 ° C with the notch direction. The dimensions of each part in FIG. 1 are as follows. L = 250 mm W = 50 mm B = 25 mm
a = 25 mm

Table 1 shows the chemical compositions of Examples and Comparative Examples of the present invention and the Ar ₃ temperature determined by calculation, and Table 2 shows the rolling conditions, mechanical properties and joints of steel sheets of steel types 1 to 30. Shows toughness. Steel sheets of steel type numbers 1 to 15 are steel sheets of Examples satisfying both the chemical composition of the present invention and rolling conditions. On the other hand, the steel sheets of steel type numbers 16, 17, and 23 are cases where the rolling conditions are satisfied but the chemical compositions are not satisfied.
For example, the steel plates of steel type numbers 16 and 17 have a large amount of Si, and the steel plates of steel type number 23 have too high Nb. Also, steel type number 1
The steel sheets of 8 to 22 and 24 and 25 do not satisfy both the chemical composition condition and the rolling condition. Further steel type number 26-30
The steel sheet of No. 1 satisfies the chemical composition but does not satisfy the rolling conditions. For example, the steel plates of steel types Nos. 27 and 28 have heating temperatures that are too low and too high, respectively. Further, the steel sheets of steel type Nos. 26 and 30 have reheating conditions that are either too low or too high, respectively, compared to the conditions of the present invention. The steel plate of steel type number 29 has a reduction ratio of Ar _{3 to} 900 ° smaller than the condition required by the present invention. When the joint toughness is evaluated by the CTOD value, the steel sheets of the examples of the present invention having steel type numbers 1 to 15 have extremely excellent properties,
It is clear that excellent toughness is not obtained in the steel sheets of Comparative Examples of the present invention of Steel Grade Nos. 16 to 30 in which both the chemical composition and the rolling conditions do not satisfy the conditions of the present invention.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

As described above, according to the present invention, it is possible to easily manufacture a high-strength steel sheet having a high CTOD value in a multi-layer weld portion as compared with conventional steel and having extremely excellent low temperature toughness.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a method for collecting a test piece in a CTOD test of an example.

[Explanation of symbols]

 t plate thickness

Claims (3)

[Claims] 1. A weight ratio of C: 0.01 to 0.15.
%, Si: 0.15% or less, Mn: 0.5 to 2.0
%, S: 0.005% or less, Cu: 0.5 to 2.0
%, Ni: 0.1-1.5%, Al: 0.005-
A continuously cast steel slab containing 0.1% and Ca: 0.0005 to 0.005% is heated to a temperature of 1050 to 1250 ° C., and then Ar ₃ to.
In the temperature range of 900 ° C., hot rolling with a cumulative reduction of 25% or more is performed, and after air cooling or forced cooling, further 40
A method for producing a high-strength steel excellent in low-temperature toughness of a multi-layer weld, characterized by reheating to 0 to 650 ° C. 2. A weight ratio of C: 0.01 to 0.15.
%, Si: 0.15% or less, Mn: 0.5 to 2.0
%, S: 0.005% or less, Cu: 0.5 to 2.0
%, Ni: 0.1-1.5%, Al: 0.005-
A continuously cast steel slab containing 0.1% and Ca: 0.0005 to 0.005% is heated to a temperature of 1050 to 1250 ° C., and then Ar ₃ to.
Rolling reduction of 25% or more in a temperature range of 900 ℃, (Ar ₃ -5
(0 ° C.) to Ar ₃ hot rolling with a cumulative rolling reduction of 10 to 45%, air cooling or forced cooling, and then reheating to 400 to 650 ° C. Method for producing high strength steel excellent in low temperature toughness. 3. The continuously cast steel billets have a weight ratio of C: 0.0.
1 to 0.15%, Si: 0.15% or less, Mn: 0.
5 to 2.0%, S: 0.005% or less, Cu: 0.5
.About.2.0%, Ni: 0.1 to 1.5%, Al: 0.
005-0.1%, Ca: 0.0005-0.005
%, Nb: 0.07% or less, V: 0.1% or less,
The multi-layer welded part according to claim 1 or 2, which is a continuously cast steel slab containing one or more of Mo: 0.2% or less and Ti: 0.02% or less. Method for producing high strength steel excellent in low temperature toughness.