JPS63210235A - Manufacture of steel excellent in toughness at low temperature in welding heat affected zone - Google Patents

Abstract

PURPOSE:To improve toughness at low temp. in a welding heat-affected zone, by specifying the relations among Ti, O, and N in a low-alloy steel with a specific composition and by subjecting a slab practically free from Al to reheating at a specific temp. and then to thermomechanical treatment. CONSTITUTION:A steel having a composition which consists of, by weight, 0.01-0.15% C, <=0.5% Si, 0.5-2.0% Mn, <=0.025% P, <=0.005% S, <=0.004% Al, 0.005-0.060% Nb, 0.005-0.030% Ti, 0.0010-0.0065% N, 0.0015-0.0060% O, and the balance Fe with inevitable impurities and in which -0.010%<=[Ti]-2[O]-3.4[N]<=+0.015% is satisfied is cast continuously. The resulting steel slab is reheated at <=1,250 deg.C and then subjected to thermomechanical treatment such as controlled rolling-colling, etc. In this way, the structure in the heat-affected zone up to the vicinity of a fusion line in small-large heat input welding is refined, so that a steel excellent in toughness at low temp. in the welding heat-affected zone can be obtained.

Description

[Detailed Description of the Invention] (Field of Industrial Application) This report relates to a method for manufacturing steel with excellent low-temperature toughness in the heat affected zone (HAZ), from dwarf heat welding to large heat input welding. .

(Prior art) The HAZ toughness of low alloy steel is determined by (1) grain size, (2)
) High carbon island martensite, upper bainii) (Bu)
(3) Presence or absence of grain boundary embrittlement, (4)
) It is controlled by various metallurgical factors such as micro-segregation of elements. In particular, it is known that the grain size of the HAZ has a large effect on low-temperature toughness, and many techniques have been developed and put into practical use to refine the HAZ structure.

By finely dispersing nitrides such as TiN, which are relatively stable even at high temperatures, into the steel, the austenite (γ) of the HAZ is
The technique of suppressing grain coarsening is particularly famous. However, in the HAZ area heated above 1400°C, T
iN coarsens or dissolves, and the ability to suppress coarsening of γ grains disappears.

For this reason, the toughness deteriorates significantly near the fusion line, making it impossible to stably obtain high toughness throughout the HAZ. That is, in the Charpy test or CTOD test in which a notch is made near the fusion line, low values appear, although the frequency is low, and this is not preferable from the viewpoint of the safety of welded steel structures.

On the other hand, steel (!Pf Application No. 59-203099) in which T1 oxide (mainly Tl2O,) is finely dispersed can have a /F structure in the HAZ even near the fusion line, and the Ti
Superior low-temperature toughness can be obtained compared to N steel. but,
Even in this steel, the toughness of the high heat input welding HAZ is about -15 to -35°C, which is the Charpy transition temperature, which is not sufficient. The main reason for this is that the steel composition does not contain Nb as an essential element, as will be described later.

As described above, there is currently no technology for stably refining the HAZ structure to the vicinity of the fusion line in small to medium heat welding, and there is a strong desire to develop a new steel based on new knowledge.

(Problems to be Solved by the Invention) The present invention provides a technique for inexpensively producing steel with extremely excellent HAZ toughness in small to small heat welding. Steel produced by the method of the present invention has a finer HAZ structure even near the fusion line during welding, and exhibits excellent low-temperature toughness throughout the HAZ.

(Means for solving the problem) The gist of the present invention is that C: 0.01-0.15%, Sl: 0
．． 5 or less, Mn: 0.5-2.0%, P:
0.025 q6 or less, S: 0.005% or less, A-S: 0.004 or less.

Nb: o, oo5-o, o6o%, Ti: 0.0
05-0°030%, N:O,0OIO-0.0065
%, O: 0.0015-0.0060% as necessary v: 0°005-0.10%, Ni: 0.05-
2.0%, Cu: 0.05-1.0%, Cr:
0.05-1.0%, Mo: 0°05-0.4
0%, B: O, OOO”3-0.002.

Ca: 0.0005-0.005%, REM: 0
．． 005-0.051% of one or more kinds, and 10,010%≦[Ti)-2C○]-s,4[
N]≦+0.015%, the balance consists of iron and unavoidable impurities, and the steel is made into a slab by a continuous casting method, which is reheated at a temperature of 1250°C or less, and then processed. Heat treatment (rolling, cooling).

According to the inventors' research, HAZ toughness is determined by: l) chemical composition of #, 2) structure (crystal grain size and hardened phase distribution state)
It was thought that optimizing the steel components and thereby refining the grains were essential for increasing the toughness of HAZO. We have invented a new method to finely disperse T1 oxide in steel and thereby refine the HAZ structure.

Although T1 oxide (mainly T1203) has a small ability to suppress the coarsening of γ grains, it is possible to remove insects by radially fine acicular ferrites (AF) using Ti2O3 present in γ grains as cores during r-α transformation. , the HAZ structure is significantly refined.

Tl2O3 is stable even in the region (coarse grain region) heated to 1400° C. or higher near the melting line, and is effective in refining the structure even in this region. Also Ti, O.

When the balance of N is appropriate, fine TiN is also generated, which suppresses the coarsening of γ grains in the HAZ (sub-coarse grain region) heated to 1350° C. or lower and refines the HAZ structure.

As a result, the HAZ structure is refined over the entire area, and extremely excellent low-temperature toughness is obtained.

In order to finely disperse Tl2O3+ TiN in steel in a normal steelmaking method without limiting the amount of O and AJSi when adding T1 in steelmaking as in Japanese Patent Application No. 59-203099, it is necessary to particularly adjust the amounts of Ti, O and N. It is essential to strike a proper balance between the two. For this reason, Ti.

○, N amount, Ti: 0.005-0.03, respectively
0%, O: 0.0015-0.0060%, N: 0.
0010-0.0065%, and the balance is -0.010chi≦[T1)-z(o)-3,4[N
]≦+0.015.

The lower limit of the amount of Ti, O, and N is T'1203, which is the minimum amount required to generate TiN. The upper limit of T1 is TiC
This is to prevent deterioration of low temperature toughness due to the formation of O.
The upper limit is set to prevent deterioration of the cleanliness and toughness of the steel due to the formation of nonmetallic inclusions. In addition, the upper limit of the amount of N is
0.0065 to prevent deterioration of HAZ toughness due to
I was in charge.

However, simply limiting the amount of each element will result in fine Ti
Since it is not possible to stably obtain both 2O3 and TiN precipitates at the same time, the balance of Ti, O, and N amounts is set to -0.0.
Coefficient 10≦(Til-2[○]-3,4[N)≦+0.0
It was limited to 15%. When the amounts of Ti, O, and N are within this range, HAZ toughness is dramatically improved.

The above equation expresses the excess and deficiency of T1 from a stoichiometric perspective when it is assumed that only Ti2O3 and TiN are produced. The lower limit is set to prevent insufficient production of Ti2O3 and TiN due to insufficient Tl, and the upper limit is set to prevent TiC from being precipitated due to excess T1. However, even if Ti2O3 and TiN are finely dispersed in steel, excellent HAZ toughness cannot be obtained unless the basic components are appropriate.

This point will be explained below.

The lower limit of 0.01% of C is the minimum amount to ensure the strength of the base metal and the welded part and to exhibit these effects when adding Nb, V, etc. However, if the amount of C is too large, it will not only adversely affect the low-temperature toughness of the base metal but also deteriorate weldability and HAZ toughness, so the upper limit should be set to 0.
It was set at 15%. When the amount of C is large, high carbon island martensite and pseudo pearlite (P') are generated in the HAZ, which significantly deteriorates the low temperature toughness.

Sl is an element contained in steel for deoxidation purposes, but if too much is added, weldability and HAZ toughness deteriorate, so the upper limit is set at 0.5.
Very limited. It is possible to deoxidize steel by using Ti alone, preventing the formation of high carbon island martensite and reducing T(AZ).
From the viewpoint of improving toughness, the content is preferably 0.15% or less.

Mn is an essential element for ensuring strength and toughness, and its lower limit is 0.5%. In order to improve HAZ toughness, it is necessary to prevent coarse pro-eutectoid ferrite from forming at the γ grain boundaries, and the addition of Mn has the effect of suppressing this. However, if the amount of Mn is too large, the hardenability increases and not only the weldability and HAZfA properties are deteriorated, but also the center segregation of the Nurap is promoted, so the upper limit was set to a factor of 2.0.

91 In the steel of the present invention, the impurities p and s are each 0.0
The reason why it is set to 25% or less and 0.005% or less is to further improve the low-temperature toughness of the base metal and welded part. Reducing the amount of P tends to reduce grain boundary fracture in the HAZ, and reducing the amount of S tends to suppress the formation of grain boundary ferrite.

The most preferable amounts of p and s are o and o 1'%, respectively.
o, o O2% or less.

Al is an element that is generally included in deoxidized steel, but it is not a favorable element in the steel of the present invention, and its upper limit is set at 0.00.
It was set at 4%. This is because if Al is contained in steel, it will combine with ◯ to form Tl2O3.

Deoxidation is possible with T1 alone, and in the present invention, the smaller the amount of An, the better.

Nb is an essential element in the steel of the present invention, and Nb
It is not possible to obtain excellent HAZ strength without adding . Nb has the function of suppressing ferrite generated at the γ grain boundaries and promoting the generation of fine AF with Ti203 as the nucleus. To obtain this effect, a minimum Nb content of 0.005% is required. However, if the amount of Nb is too large, it will hinder the formation of fine AF, so the upper limit should be set at 0.
060%.

Next, V, Ni, Cu, Cr, Mo, B,
The reason for adding Ca REAl will be explained.

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

V is an element that has almost the same effect as Nb, but 0.00
If it is less than 5%, the effect is small, and the upper limit is permissible up to 0.10-.

N1 improves the strength and toughness of the base metal without adversely affecting weldability and HAZ toughness, but if it exceeds 2.0, it is unfavorable for weldability, so the upper limit was set at 2°0.

Cu has almost the same effects as N1, as well as corrosion resistance, hydrogen-induced cracking resistance, etc., but if it exceeds 1.0%, Cu-cracks will occur during hot rolling, making manufacturing difficult.

For this reason, the upper limit was set at 1.0%.

Cr increases the strength of the base metal and the welded part, but too much Cr deteriorates weldability and HAZ toughness. Its upper limit is 1.0%.

Mo is an element that improves both the strength and toughness of the base metal, but if it is in too much, it will improve the toughness and toughness of the base metal and welded joint, just like Cr.
This is undesirable as it leads to deterioration of weldability. Its upper limit is 0.40
I am in charge.

Note that the lower limit of the addition amount of Cr and Mo should be the minimum amount for obtaining the effect on the material, and both are 0.05%.

B is an element that increases the hardenability and strength of steel. The solid solution B segregated at the γ grain boundaries of the HAZ suppresses the formation of ferrite and helps the formation of fine AF from Ti2O3. Further, BN combined with N acts as a ferrite generation nucleus and refines the HAZ structure.

To obtain this kind of effect, a minimum of 0.0003
The relevant amount of B is required. However, if the amount of B is too large, Fe2
Coarse precipitates such as 3 (CB) and the like precipitate at the γ grain boundaries and deteriorate low temperature toughness. For this reason, the upper limit of the amount of B is set to 0.
It is necessary to limit it to 0.002%.

Ca and REM control the morphology of sulfides (lvnS), improve low-temperature toughness (increase Charpy absorbed energy), and are also effective in improving hydrogen-induced cracking resistance.

However, if the amount of CaB is less than 0,005%, it has no practical effect, and if it exceeds 0,005%, CaO and CaS
is produced in large quantities, resulting in large inclusions that impair not only the toughness but also the cleanliness of the steel, and also have an adverse effect on weldability. Therefore, the range of addition amount was limited to 00005% to 0.005%. REM also has the same effect as Ca, and its appropriate range is 0.005 to 0.005%.

Even if the components of the steel are limited as described above, fine T1203 + TIN cannot be dispersed in the steel before welding unless the manufacturing method is appropriate. For this reason, it is also necessary to limit the manufacturing conditions.

First, industrially, it is essential to manufacture this steel using a continuous casting method. The reason for this is that in the continuous casting method, the solidification and cooling rate of molten steel is fast and a large amount of fine Ti2O3 and TiN are obtained in the slab. In the ingot-blowing method using large steel ingots, T1□03. It is difficult to finely disperse T1N in a slab.

In the case of the 13-continuous casting method, the cooling rate varies depending on the slab thickness, but from the viewpoint of HAZ toughness, the thickness is preferably 350 degrees or less. Furthermore, the reheating temperature of the slab needs to be 1250°C or lower. If reheated at a temperature higher than this, TiN
This is because the grains become coarse and fine TiN disappears in the steel before welding, making it impossible to refine the structure in the sub-coarse grain regions of HA and Z.

In the present invention, it is not necessary to reheat the slab, and there is no problem even if hot charge rolling or direct rolling is performed.

Next, regarding the rolling method after reheating the slab, so-called processing heat treatment is essential. This is because even if excellent HAZ toughness is obtained, if the toughness of the base metal is poor, the steel material will be insufficient. In order to improve the low-temperature toughness of the base metal, it is necessary to refine the crystal grains of the steel through heat treatment.

Examples of the processing heat treatment methods include l) controlled rolling, 2) controlled rolling-accelerated cooling, and 3) rolling direct quenching-tempering, but the most preferred is a combination of controlled rolling and accelerated cooling.

=14- Note that even if the steel is repeatedly dehydrated and reheated to a temperature below the Ac transformation point for any purpose after manufacturing, the characteristics of the present invention will not be impaired.

(Example) Steel plates (thickness: 32 rMl) of various steel compositions were manufactured in a converter continuous casting-thick plate process, and HAZ toughness was investigated using a welding thermal cycle reproduction device.

The reproducible thermal cycle was performed using a test piece taken from plate thickness]/4t at a peak temperature (maximum temperature reached) of 400°C and 1
The cooling time was 192 seconds at 300°C and 800-500°C. This condition roughly corresponds to welding heat input of 200 and T/cm, and the peak temperatures of 1400°C and 11300°C are respectively in the coarse grain area of the actual welding HAZ (near the fusion line) and
Corresponds to the sub-coarse grain area.

Examples are shown in Table 1.

All of the steel plates produced by the method of the present invention (inventive steel) have good base metal properties and HAZ toughness, whereas comparative steels that are not manufactured by the method of the present invention have poor base metal properties or HAZ toughness.
Not suitable for steel plates used in harsh environments.

Among the comparative steels, Steel 16 has too large amounts of C and Si, so its toughness is particularly poor in the reproduced HAZ at a peak temperature of 1400°C. In addition, steel 17 has a high Mn content and therefore has poor toughness in the reproduced HAZ despite its low C content. Since @A18 does not contain Nb, which is an essential element of the present invention, the grain boundary ferrite is large and the reproduced HAZ toughness is extremely poor.

The reproduced HAZ toughness of 1i119 and 20 is relatively good, but steel ] 9 has a high S content, and steel 20 has a high Tl content,
Because f(Ti) is high, it is not as good as the steel of the present invention.

Steel 21.22 is a reproduction HAZ with a peak temperature of 1300'c
The toughness is sufficient. This means that in steel 21, the Tl amount is i#
This is because in No. 22, the amount of N is small and the amount of fine TiN0 is insufficient.

Steel 23 is A9. Since the amount is high and T1□03 is not generated, the reproduced HAZ toughness at a peak temperature of 1400°C is significantly inferior. Steel 24 has too much O content, so the reproduced HAZ toughness is just a step away. Further, although Steel 25 has appropriate steel components, the slab reheating temperature is too high, so the TjN becomes coarse and the reproduced HAZ toughness at the peak temperature of 1300°C is poor.

Although the present invention is most preferably applied to plate mills, it can also be applied to hot coils, shaped steel, etc.

Moreover, the thick steel plates manufactured by this method can be used for welded steel structures used in harsh environments such as offshore structures, pressure vessels, and line pipes.

(Effects of the Invention) According to the present invention, it has become possible to manufacture steel having excellent low-temperature toughness not only in the base material but also in the entire welded area in large quantities and at low cost. As a result, we were able to greatly improve the safety of welded steel structures used in harsh environments such as in extremely cold regions and at sea.

Claims (2)

[Claims] (1) C: 0.01-0.15%, Si: 0.
5% or less, Mn: 0.5%-2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.004% or less, Nb: 0.005-0.060% , Ti: 0.005-0.030%, N: 0.0010-0.0065%, O: 0.0015-0.0060%, and -0
．． 010%≦[Ti]-2[O]-3.4[N]≦+0
．． 015% and the balance is iron and unavoidable impurities and is made into a slab by a continuous casting method, which is then reheated at a temperature of 1250°C or less and then subjected to processing heat treatment. A method for manufacturing steel with excellent low-temperature toughness in the weld heat affected zone. (2) C: 0.01-0.15%, Si: 0.
5% or less, Mn: 0.5%-2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.004% or less, Nb: 0.005-0.060% , Ti: 0.005-0.030%, N: 0.0010-0.0065%, O: 0.0015-0.0060%, and V: 0.005-0.10%, Ni: 0. 05-2
．． 0%, Cu: 0.05-1.0%, Cr: 0.05-
1.0%, Mo: 0.05-0.40%, B: 0.00
03-0.002%, Ca: 0.0005-0.005
%, REM: 0.005-0.05%, and -0.010%≦[Ti]-2[O
] −3.4 [N]≦+0.015%, the balance is iron and inevitable impurities, and the steel is made into a slab by a continuous casting method and is made into a slab with 1250%
A method for producing steel with excellent low-temperature toughness in the weld heat-affected zone, which is characterized by reheating at a temperature below ℃ and then processing heat treatment.