JPH11264048A - High-strength steel plate excellent in toughness of welded zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel plate of high strength and excellent in toughness of a heat affected zone by containing a combined grain among a fine TiN precipitate with a Mg oxide, the oxide essentially made of Ti-Mg-REM and MnS in a low C-Ti-Mg-REM-low Al steel of a specified composition. SOLUTION: This high strength steel plate contains, by weight, 0.01-0.10% C, <=0.6% Si, 0.8-2.5% Mn, <=0.015% P, 0.001-0.005% S, 0.01-0.05% Nb, 0.005-0.030% Ti, 0.0003-0.0020% Mg, 0.0003-0.0010% REM, <=0.01% Al, 0.001-0.006% N, 0.001-0.006% O, further a prescribed quantity of Ni, Cu, etc., and the balance Fe, further satisfying P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+ Mo+V=2.5-3.5. Further, in addition to the above, 0.1-5.0 μm Ti, Mg, >=5 each/mm<2> a mainly REM oxide-MnS combined body, >=10000 each/mm<2> a Mg oxide containing TiN are contained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the American Petroleum Institute (A)
PI) X100 or higher (Yield strength: about 689 N / m)
high strength and excellent heat affected zone of the m ² or more) (relates steel sheet having a HAZ) toughness.

[0002]

2. Description of the Related Art Line pipes used for pipelines for transporting crude oil and natural gas over long distances are required to (1) improve transport efficiency by increasing pressure and (2) improve welding efficiency on site by reducing wall thickness. The tension tends to be higher. Until now, line pipes up to X80 have been put to practical use in accordance with the API standard, but there is a need for higher strength line pipes. At present, ultra-high-strength line pipes of X100 or more are manufactured using X80 class line pipes (NKK technical report
No. 138 (1992), pp24-31 and Th.
e 7th Offshore Machines
and Arctic Engineering (19
88), Volume V, pp 179-185), but these line pipes have many problems in terms of low-temperature toughness, particularly HAZ toughness, etc. High strength steel sheets are desired.

[0003] The HAZ toughness of a low alloy steel is as follows: (1) crystal grain size, (2) dispersed state of hardened phase such as high carbon island martensite (M ^* ), upper bainite (Bu), and (3)
It is governed by various metallurgical factors such as the presence or absence of grain boundary embrittlement and microsegregation of element (4). Above all, it is known that the size of the crystal grains of HAZ has a great effect on the low-temperature toughness, and a number of techniques for refining the HAZ structure have been developed and put to practical use. For example, TiN is finely dispersed and 490
Means for improving the HAZ toughness of N / mm ² class high strength steel during large heat input welding has been disclosed (Iron and Steel, Vol. 65, No. 8, page 1232, issued June 1979). However, since these precipitates are exposed to a high temperature of 1400 ° C. or more near the melting line, most of them are coarsened or dissolved, and HA
There is a disadvantage that the Z structure is coarsened and the HAZ toughness is deteriorated.

To solve this problem, Ti oxide is finely dispersed in steel, and intragranular acicular in HAZ at the time of welding.
It is disclosed in JP-A-63-210235, JP-A-1-15321 and the like that the formation of ferrite (hereinafter referred to as IGF) makes the HAZ structure near the melting line finer and improves HAZ toughness. Have been. However, when the strength becomes higher than X100, the production of IGF from the Ti oxide is suppressed, and the HAZ toughness is deteriorated. Therefore, it is strongly desired to improve the HAZ toughness of the high-strength steel sheet of X100 or more.

[0005]

The present invention provides a good HAZ.
It is intended to provide a high-strength steel sheet of X100 or more having toughness.

[0006]

Means for Solving the Problems The gist of the present invention is that the weight%
And C: 0.01 to 0.10%, Si: 0.6% or less,
Mn: 0.8-2.5%, P: 0.015% or less, S:
0.001 to 0.005%, Nb: 0.01 to 0.05
%, Ti: 0.005 to 0.030%, Mg: 0.00
03-0.0020%, REM: 0.0003-0.0
010%, Al: 0.01% or less, N: 0.001 to
0.006%, O: 0.001 to 0.006%, and if necessary, Ni: 0.1 to 1.0%, Cu: 0.
1 to 1.2%, Cr: 0.1 to 1.0%, Mo: 0.1
1.0%, V: 0.01 to 0.10%, Ca: 0.0
One or two or more kinds of P-2.7C + are contained, and the balance consists of iron and unavoidable impurities.
0.4Si + Mn + 0.8Cr + 0.45 (Ni + C
u) The P value defined by the formula of + Mo + V is 2.5 to 3.5.
And a particle diameter of 0.1 to 5.0 μm.
A composite of an oxide mainly composed of i, Mg, and REM and MnS in a cross section perpendicular to the casting direction at 5 particles / mm ² or more, and a particle diameter of 0.1 μm containing an Mg-based oxide
The following TiN is contained at least 10,000 / mm ² .

Hereinafter, the high strength steel sheet of the present invention will be described in detail. According to the study of the inventors, the HAZ toughness largely depends on (1) the chemical composition of the steel, and (2) the structure (the size of the crystal grains and the dispersed state of the hardened phase). It is considered that miniaturization is indispensable for increasing toughness.
The feature of the present invention is that, in low C-Ti-Mg-REM-low Al-based steel, (1) fine Ti containing 0.1 μm or less of Mg-based oxide which is not coarse and does not melt even near the melting line.
(2) an oxide mainly composed of Ti-Mg-REM of 0.1 µm or more, which has excellent ability to generate IGF even in relatively small austenite (γ) grains of 100 µm or less; By containing a complex with MnS,
An object of the present invention is to improve the HAZ toughness by refining the structure over the entire HAZ.

Conventionally, in a steel in which TiN is finely dispersed, coarsening of γ grains is suppressed up to a region where the steel is reheated to 1350 ° C., and this is effective in refining the HAZ structure. But 14
In the vicinity of the melting line exposed to a high temperature of 00 ° C. or more, the TiN particles are coarse and dissolved, so that the effect of suppressing γ particles almost disappears. Therefore, as a result of intensive studies on precipitates that are stable even at a high temperature of 1400 ° C. or higher, TiN particles generated by using fine Mg-based oxides as nuclei hardly become coarse and dissolve even at a high temperature of 1400 ° C. or higher. It has been found that there is a great effect on suppressing coarsening of grains. In the conventional steel, the γ particle diameter in the vicinity of the melting line is 200 μm or more, whereas in the steel of the present invention, the content of TiN particles generated by using oxides of 0.1 μm or less as nuclei is 10000 particles / mm ² or more. The γ particle size can be reduced to 100 μm or less.

Further, it has not been possible to generate IGF from within the γ grains suppressed to about 100 μm conventionally. This is because the temperature at which ferrite is generated at the γ grain boundary is higher than the temperature at which IGF is generated, and the ferrite generated from the γ grain boundary covers the inside of the γ grain, making it difficult to generate IGF. Therefore, as a result of intensive studies on the type of oxide that generates IGF even within relatively small γ grains, T
An oxide mainly composed of i, Mg, and REM (the ratio of Ti + Mg + REM in the oxide is 50% or more) promotes precipitation of MnS on the oxide, and a complex of this oxide and MnS is formed of IGF. It has been found that IGF is remarkably formed even in small γ grains by containing 5 μm / mm ² or more of a composite of oxide and MnS having a size of 0.1 μm or more, which is excellent in generating ability.

That is, the gist of the present invention is that Ti, Mg,
By limiting the amounts of REM, Al, S, and N, (1)
Containing TiN-based fine precipitates containing Mg-based oxides of 0.1 μm or less to suppress the coarsening of γ grains even in the vicinity of the melting line; (2) Ti, Mg, 0.1 μm or more,
A complex of an oxide mainly composed of REM and MnS is contained to generate IGF from relatively small γ grains, and HA
The purpose is to improve the HAZ toughness by refining the structure over the entire Z region.

In order to contain the above oxides and precipitates in steel, it is important to optimize the amounts of Ti, Mg, REM, Al, S and N. For this purpose, Ti, Mg, REM, Al,
The amounts of S and N are respectively Ti: 0.005 to 0.030%,
Mg: 0.0003-0.0020%, REM: 0.0
003 to 0.0010%, Al: 0.01% or less, S:
0.001 to 0.005%, N: 0.001 to 0.00
It must be limited to 6%.

The lower limits of the amounts of Ti, Mg, REM, S and N are T
This is the minimum amount required to generate a composite of oxide and MnS mainly composed of i, Mg, and REM and TiN particles containing Mg oxide. The upper limit of the amount of Ti is to prevent HAZ toughness from deteriorating due to generation of TiC. Mg and R
Since the addition of a large amount of EM deteriorates the HAZ toughness, the upper limit values are set to 0.0020% and 0.0010%, respectively. The upper limit of the amount of N is to prevent deterioration of HAZ toughness due to slab surface flaws and solid solution N.

In order to generate an oxide mainly composed of Ti, Mg and REM, the upper limit of the Al content is set to 0.01%. If the amount of Al exceeds 0.01%, the oxide contains a large amount of Al, so that excellent IGF-forming ability cannot be obtained. The upper limit of S is for preventing deterioration of the base material toughness due to the large amount of MnS. However, even if T
Even if a composite of an oxide mainly composed of i, Mg, and REM and MnS and TiN particles containing an Mg-based oxide can be dispersed in steel, excellent HAZ toughness can be obtained if the basic components are not appropriate. Absent.

The reasons for limiting other basic components will be described below. The lower limit of 0.01% of C is a minimum amount for securing the strength and low-temperature toughness of the base material and the welded portion, and for exhibiting the effects of precipitation hardening and crystal grain refinement by adding Nb and V. However, if the C content is too large, the low-temperature toughness and the on-site weldability are remarkably deteriorated, so the upper limit is set to 0.10%. S
i is an element added for deoxidation and strength improvement, but if added too much, it deteriorates on-site weldability and HAZ toughness.
The upper limit was set to 0.6%. Deoxidation of steel is sufficient with Ti alone, and Si need not always be added.

Mn is an indispensable element for securing strength and low-temperature toughness, and its lower limit is 0.8%. But M
If n is too large, the hardenability of the steel increases and the weldability on site, HA
Not only deteriorates Z toughness but also promotes center segregation of continuous cast steel slab and lowers low temperature toughness, so the upper limit is 2.5%
And In the present invention, the amount of P, which is an unavoidable impurity element, is set to 0.015% or less. The main reason for this is to further improve the low-temperature toughness of the base material and HAZ.
Reduction of P content reduces center segregation of continuous cast slab,
Prevents grain boundary fracture and improves low temperature toughness.

Nb suppresses ferrite generated at the γ grain boundary, and an oxide mainly composed of Ti, Mg, REM and MnS
Has the function of promoting the production of IGF whose complex is the nucleus. To obtain this effect, a minimum amount of 0.01% of Nb is required. However, if the Nb content is too large, HAZ
Since the toughness deteriorates, the upper limit value is set to 0.05%. O is required to be 0.001% or more to generate an oxide mainly composed of Ti, Mg, and REM. However, when the content is too large, the cleanliness of the steel and the low-temperature toughness of the base material are deteriorated due to the formation of nonmetallic inclusions. Therefore, the upper limit value is set to 0.006%.

Next, Ni, Cu, Cr, Mo, V, Ca
The reason for adding is described. The main purpose of adding these elements to the basic components is to improve properties such as strength and low-temperature toughness without impairing the features of the steel of the present invention. Therefore, the amount of addition is of a nature that should be restricted by itself. Ni improves the strength and low-temperature toughness of the base material without adversely affecting the weldability and the HAZ toughness. However, the effect is small at 0.1% or less, and the addition of 1.0% or more is not preferable for the weldability. 1.0% of the upper limit value
And

Cu has almost the same effect as Ni, and also has an effect on corrosion resistance, resistance to hydrogen-induced cracking, and the like.
It is necessary to add 0.1% or more. However, when added in excess, precipitation hardening causes deterioration in the toughness of the base material and HAZ and Cu-cracks during hot rolling, so the upper limit value was set to 1.2%. Cr has the effect of increasing the strength of the base material and the welded portion, and it is necessary to add 0.1% or more. However, if it is too much, the on-site weldability and HAZ toughness are remarkably deteriorated. Therefore, the upper limit of the Cr content is set to 1.0%.

Mo is an element that increases the strength of the base material and the welded portion, but if it exceeds 1.0%, the toughness and weldability of the base material and the HAZ are deteriorated like Cr. Also 0.
Addition of 1% or less has little effect. V has almost the same effect as Nb, but the effect is much weaker than Nb. In order to exert the effect, it is necessary to add 0.01% or more. Further, the upper limit is allowable up to 0.10% from the viewpoint of on-site weldability and HAZ toughness.

Ca controls the form of sulfide (MnS),
In addition to improving the low-temperature toughness (such as an increase in the absorbed energy in the Charpy test), it also has a remarkable effect on sour resistance. When the content is less than 0.001%, the effect is weak, and when it exceeds 0.005%, CaO—Ca
S is generated in large amounts to form clusters and large inclusions, which not only impairs the cleanliness of steel but also adversely affects on-site weldability. For this reason, the amount of Ca added is 0.001 to 0.00.
Limited to 5%.

In the present invention, in addition to the limitation of the individual additive elements described above, P = 2.7C + 0.4Si + Mn + 0.
It is necessary to limit the value represented by 8Cr + 0.45 (Ni + Cu) + Mo + V to 2.5 ≦ P ≦ 3.5. This is HA
This is to achieve the desired balance between strength and low-temperature toughness without impairing Z toughness and on-site weldability. The lower limit of the P value is set to 2.5 in order to obtain a strength of X100 or more and excellent low-temperature toughness. The upper limit of the P value is set at 3.5 in order to maintain excellent HAZ toughness and on-site weldability.

The method for measuring fine particles according to the present invention will be described below. Fine Mg of 0.1μm or less
Oxide-containing TiN precipitates are measured by transmission electron microscopy (TEM)
Is used to observe an area of at least 1000 μm ² at a magnification of 10,000 to 50,000 times to determine the number thereof. Regarding the presence or absence of Mg oxide, composition analysis by energy dispersive X-ray spectroscopy (EDS) and structural analysis by X-ray diffraction are desirable. The number of oxides contributing to the production of IGF of 0.1 μm or more was 1000
The measurement was performed at a magnification of 1 and a visual field of 4 mm ² . Then EPM
With A, the state of precipitation of MnS on the oxide is observed.

[0023]

An embodiment of the present invention will be described. A steel plate was manufactured from steel slabs of various steel components by a converter-continuous casting method, and a steel plate was manufactured by an accelerated cooling method, and various properties were investigated. The properties of the welded portion were evaluated using a Charpy test specimen sampled from a 1 / 2t portion of the steel sheet after each steel sheet was subjected to one layer of inner and outer surfaces by SAW (submerged-arc welding). The notch position was the point where the weld metal of the inner surface weld and the outer surface weld intersect. Example 1
3 are shown. The steel sheet of the present invention has excellent strength / low temperature toughness and HAZ toughness. On the other hand, the comparative steel is not suitable for the type and number of chemical components and oxides, and is inferior in either property.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

Since the steel 11 has too much C content, the base material and the HAZ toughness are inferior. Steel 12 is inferior in base metal and HAZ toughness because Ti is not added. Steel 13 has an inferior HAZ toughness because the amount of Ti is too large. Steel 14 has no HAZ toughness because Mg is not added. Steel 15 has inferior HAZ toughness because the amount of Mg is too large. Steel 16 is inferior in HAZ toughness because no REM is added. Steel 17 is RE
Since the amount of M is too large, the HAZ toughness is poor. Steel 18 is Al
Since the amount is too large, the HAZ toughness is poor. Steel 19 has a P value that is too low, so that the strength as X100 cannot be obtained.

Since the steel 20 has too high a P value, the HAZ toughness is poor. In the steel 21, the elements mainly contained in the oxide of 0.1 μm or more are Al, Mg, REM, Ti, M
g, REM content is 50% or less.
Poor AZ toughness. In the steel 22, the elements mainly contained in the oxide of 0.1 μm or more are Ti and Mg.
g, the content of REM is 50% or less.
Poor AZ toughness. Steel 23 has an oxide of 0.1 μm or more
Since nS is not precipitated, the HAZ toughness is poor. Steel 24
Is less than 0.1 μm because the number of oxides is too small.
Poor AZ toughness. Since the steel 25 does not contain a Mg-based oxide in TiN precipitates of 0.1 μm or less, the HAZ toughness is poor. Steel 26 has an inferior HAZ toughness because the number of TiN precipitates of 0.1 μm or less is too small.

[0029]

According to the present invention, an ultra-high-strength steel sheet (API standard X100 or more) excellent in low-temperature toughness, particularly HAZ toughness, can be stably manufactured. As a result, the safety of welded structures and pipelines is significantly improved,
Construction efficiency has been dramatically improved.

Claims (2)

[Claims] C: 0.01 to 0.10%, Si: 0.6% or less, Mn: 0.8 to 2.5%, P: 0.015% or less, S: 0 0.001 to 0.005%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.030%, Mg: 0.0003 to 0.0020%, REM: 0.0003 to 0.0010 %, Al: 0.01% or less, N: 0.001 to 0.006%, O: 0.001 to 0.006%, with the balance being iron and unavoidable impurities, defined by the following formula: P value is in the range of 2.5 to 3.5, and Ti particle diameter of 0.1 to 5.0 [mu] m, Mg, a complex oxide and MnS mainly of REM 5 pieces / mm ² or more Containing at least 10000 particles / mm ² of TiN containing Mg-based oxide and having a particle diameter of 0.1 μm or less. High strength steel sheet with excellent weld toughness. P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.4
5 (Ni + Cu) + Mo + V 2. In% by weight, C: 0.01 to 0.10%, Si: 0.6% or less, Mn: 0.8 to 2.5%, P: 0.015% or less, S: 0 0.001 to 0.005%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.030%, Mg: 0.0003 to 0.0020%, REM: 0.0003 to 0.0010 %, Al: 0.01% or less, N: 0.001 to 0.006%, O: 0.001 to 0.006%, Ni: 0.1 to 1.0% as required. Cu: 0.1 to 1.2%, Cr: 0.1 to 1.0%, Mo: 0.1 to 1.0%, V: 0.01 to 0.10%, Ca: 0.001 to One or more of 0.005%, the balance being iron and unavoidable impurities, and having a P value defined by the following formula of 2.5
~ 3.5 and the particle size is 0.1 ~ 5.0μ
m of Ti, Mg, a complex oxide and MnS mainly of REM containing 5 / mm ² or more and 10,000 particle diameter 0.1μm or less of TiN containing a Mg-based oxides / mm ² A high-strength steel sheet having excellent weld toughness characterized by containing the above. P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.4
5 (Ni + Cu) + Mo + V