JPH02267220A - Production of steel sheet for flash butt welding excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To easily obtain a steel sheet for flash butt welding excellent in toughness at low temp. in large quantities by subjecting a slab of a steel in which respective contents of C, Si, Mn, P, S, Al, N, O, Nb, Ti, and Zr are specified to reheating at the prescribed temp. CONSTITUTION:A steel which has a composition consisting of, by weight, 0.03-0.1% C, <=0.5% Si, 1.4-2% Mn, <=0.025% P, <=0.005% S, <=0.004% Al, 0.001-0.0035% N, 0.0015-0.005% O, 0.03-0.06% Nb, 0.005-0.025% Ti and/or 0.005-0.025% Zr so that the total content of Ti and Zr is regulated to 0.005-0.025%, and the balance Fe is refined. This steel is formed into a slab by a continuous casting method, reheated at <=1250 deg.C, and then formed into a steel sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for producing steel particularly suitable for flash butt welding (FBW).

(Prior Art) Recently, examples of applying FBW on a large scale (for example, line pipes) are rapidly increasing from the viewpoint of improving welding efficiency. However, there has been little development of steel suitable for FBW, and ensuring low-temperature toughness of joints has become a major problem.

In FBW, the steel itself (base material) is melted and
This is because it is difficult to adjust the components (structure control) of the joint because it is solidified and joined. To date, welding heat affected zone (HAZ)
), many steels with improved low-temperature toughness have been developed, but with these steels it is impossible to improve the toughness of the FBW part.

For example, a technology has been developed to finely disperse nitrides such as TiN, which are relatively stable even at high temperatures, into steel, thereby suppressing the coarsening of austenite (γ) grains in the HAZ. In this section, TiN is also completely dissolved,
Totally useless.

In contrast, TI oxide (mainly T l 20s )
Steel in which finely dispersed T
Superior low-temperature toughness can be obtained compared to iN steel.

However, this steel was not developed with the intention of FBW, and is not sufficient for controlling the structure of FBW joints. In conventional steel, the toughness of the FBW part is extremely poor, with Charpy absorbed energy of less than 1 kg-m even at 0℃, and heat treatment after FBW is required to ensure toughness, which poses a major problem in terms of welding efficiency. There is a strong desire to develop new steels based on new knowledge.

(Problems to be Solved by the Invention) The present invention provides a technique for manufacturing steel suitable for FBW at low cost. The steel produced by the method of the present invention has a finer structure in the welded zone (joint zone, heat affected zone), and exhibits excellent low-temperature toughness throughout the entire region even without post-heat treatment.

(Means for Solving the Problems) The gist of the present invention is that, in weight%, C: 0.03 to 0.10
%, Si: 0.5% or less, Mn: 1.4-2.0%, P
: 0.025% or less, S: 0.005% or less, Al
: 0.004% or less, N: 0.0010 to 0.0
035%, O: 0.0015-0.0050%, N
b: o, 03~o, oa%, Ti+0.005~0
．． 025%, Zr: 0.005≦TI for either or both of 0.005 to 0.025% +Zr≦0.0
25%, and the balance is iron and inevitable impurities, which is substantially free of Ai, or this steel is further supplemented with V: 0.005~o, oao%, Ni: 0.05~
2.0%, Cu: 0.05-1.0%, Cr: 0.05
~1.0%, Mo: 0.05~0.40%, B: 0
．． 0003-0.0015%, Ca: 0.0005-0
．． Substantially A containing one or more of 005% and the remainder consisting of iron and inevitable impurities.
The purpose of this method is to form a slab of steel that does not contain l by a continuous casting method, reheat the slab at a temperature of 1250° C. or lower, and then manufacture steel.

(Function) According to the inventors' research, the toughness of the FBW joint is as follows: 1)
The chemical composition of steel is highly dependent on the 2) group ta (crystal grain size and hardening phase distribution!), and it is believed that optimization of steel composition and the resulting refinement of grain size are essential for achieving high toughness. It was done.

Therefore, Ti or Zr oxides or Tl-
We have invented a new method for finely dispersing a Zr complex oxide and thereby refining its structure.

Since the steel melts in FBW joints, even these oxides dissolve, but they re-precipitate during solidification cooling. Then, T i OZ re-precipitated within the γ grains during the γ-α transformation.
Since fine acicular ferrite (AF) is generated radially with rO2 or (Ti゜2 3'Z'')0 as the nucleus, the structure of the joint becomes extremely fine.

Further, these oxides are stable without being dissolved in the region heated to high temperatures near the melting line (coarse grain region HAZ), and are effective in refining the structure even in this region. the result,
The joint is made finer over the entire area, resulting in extremely excellent low-temperature toughness.

As mentioned above, the addition of Ti or Z is stable even at high temperatures, and Tl, which is reprecipitated during the solidification process even in the molten zone,
Zr is essential for realizing microstructure using Zr alone or as a composite oxide, but care must be taken to determine the appropriate amount of Zr added.

That is, it is essential to add 0.005% or more of Ti or Zr as a lower limit to ensure effective oxide production (0.005%≦Tt+Zr). Further, since addition of an appropriate amount of Ti or Zr promotes the formation of TIC or ZrC and causes deterioration of toughness, it is necessary to regulate the upper limit thereof. According to the inventors' research, the upper limit of the addition of rt and zr alone or in combination without causing deterioration of toughness is 0.025%, and TI + Zr≦0.025%.

In addition, the addition of TI and Zr not only refines the structure of the joint due to the formation of oxides, but also refines the HAZ sub-coarse grain region (at 1350°C) due to the formation of nitrides (TiN or ZrN).
The effect of suppressing the coarsening of γ grains in the heated HAZ) can also be expected.

For this purpose, it is necessary to ensure an appropriate balance of Ti, Zr, O, and N.

For N and Ojl, 'ri2o3゜Z as an oxide
rO2, (Ti.Zr)0 or T as nitride
In order to ensure the generation of iN and ZrN, it is necessary to set the lower limits to N≧0.0010% and 0≧0.0015%, respectively. In addition, the upper limit of N is set to N≦0.0035% to prevent deterioration of toughness due to solid solution N, and the upper limit of 0 is set to O to prevent deterioration of cleanliness and toughness of steel due to the formation of nonmetallic inclusions. ≦0.0050%.

However, even if TI, Zr, N, and Oi are properly controlled, Ti. Even if Zr oxide or nitride is finely dispersed, excellent toughness cannot be obtained unless the basic components are appropriate.

This point will be explained below.

The lower limit of 0.03% 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,
Since it not only adversely affects the low-temperature toughness of the joint but also deteriorates the toughness and weldability of the base material, the upper limit was set at 0.10%. When Cff1 is abundant, island-like martensite (M
n) Pseudo pearlite (P') is generated and the low temperature toughness is significantly deteriorated.

Sl is an element contained in steel for deoxidation purposes, but if too much is added, weldability and toughness of joints will deteriorate, so the upper limit should be set at 0.
It was limited to 5%. It is possible to deoxidize steel by using only Ti, and from the viewpoint of preventing the formation of Mn and improving toughness, it is possible to deoxidize steel with 0.
15% or less is desirable.

Mn is an essential element for ensuring strength and toughness.

In order to improve the toughness of FBW joints, it is necessary to prevent coarse pro-eutectoid ferrite from forming at the γ grain boundaries.
Addition has the effect of suppressing this.

The cooling rate of the FBW joint (after welding) is relatively slow, and the lower limit of Mn to prevent the formation of pro-eutectoid ferrite and promote the formation of AF is 1.5%. However, if the amount of Mn is too large, the hardenability not only increases and the weldability and toughness of the joint are deteriorated, but also promotes segregation at the center of the slab, so the upper limit was set at 2.0%.

In the steel of the present invention, the impurities P and S are each 0.0
The reason why the content is 25% or less and 0.005% or less is to further improve the low-temperature toughness of the base material and the joint. Reducing the amount of P reduces the tendency for intergranular fracture at the joint, 5
Reducing ffi tends to suppress the formation of grain boundary ferrite. The most preferable P and Sl are each 0.010%
, 0.0020% or less.

AJ is an element that is generally included in deoxidized steel, but it is an element that is not preferred in the steel of the present invention, and its upper limit is set to 0.0.
04%. This is because if A and IJ are contained in steel, they combine with oxygen to form Ti2O3°TZrO2 or a composite oxide of TI and Zr. Deoxidation is Ti. In the present invention, it is possible to use Zr alone.
The smaller the amount, the better, and preferably 0.002% or less.

Nb is an important element in the steel of the present invention, and it is difficult to obtain excellent joint toughness in high-strength steel without adding Nb. Nb suppresses ferrite generated at the γ grain boundaries, and reduces Ti2O3゜Z r O2 or TI
, has the function of promoting the generation of fine AF with the composite oxide of Zr as the core.

In FBW, minimum 0.03 to obtain this effect.
% Nb addition is required. However, if the amount of Nb is too large, the formation of fine AF will be hindered, so the upper limit was set at 0.06%.

Next, V, Ni, Cu, Cr, Mo, and B.

The reason for adding Ca will be explained.

The main purpose of adding these elements to the basic ingredients 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 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 an upper limit of 0.060% is permissible.

Ni improves the strength and toughness of the base metal without adversely affecting weldability and joint 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 Ni, 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 joint 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 present in too much, it causes deterioration of the toughness and weldability of the base metal and joints, similar to Cr, and is therefore undesirable. Its upper limit is 0.40%.

Note that the lower limit of the amount of Cr and Mo added should be the minimum amount in order to obtain the effect on the material, and both are 0.05%.

B is an element that increases hardenability and strength. The solid solution B segregated at the γ grain boundaries of the joint suppresses the formation of ferrite and helps the formation of fine AF from Tl2O3, ZrO2 or the composite oxide of Tl and Zr. In addition, BN combined with N acts as a ferrite generation nucleus and HA
Refine the Z structure.

To obtain this kind of effect, a minimum of 0.0003
% B amount is required. However, if there are too many Bfl, F
Coarse precipitates such as e (CB)e precipitate at the γ grain boundaries and deteriorate low temperature toughness. Therefore, the upper limit of the amount of B is set to 0.0
It is necessary to limit it to 0.015%.

Ca controls the morphology of sulfide (MnS), improves low-temperature toughness (increases Charpy absorbed energy), and is also effective in improving hydrogen-induced cracking resistance. However, C
If the a content is less than 0.0005%, there is no practical effect, and
If added in excess of 0.005%, a large amount of Cab and CaS will be generated and become large inclusions, which will impair not only the toughness but also the cleanliness of the steel, and will also have an adverse effect on the weldability. For this reason, the amount added was limited to 0.0005 to 0.005%.

Even if the composition of the steel is limited as above, if the manufacturing method is not appropriate, fine particles of T l 203°Z r may be present in the steel before welding.
It is not possible to disperse composite oxides of 02, TI, and Zr. For this reason, it is also necessary to limit the manufacturing conditions. First, this steel must be manufactured using a continuous casting method. The reason for this is that in the continuous casting method, the solidification rate of molten steel is fast and a large amount of fine TiOZ r O2 or Ti, Zr2 3 ' composite oxide is obtained in the slab.

In the ingot-blooming method using large steel ingots, Ti2O3゜Z'0
It is difficult to finely disperse composite oxides of 2, TI, and Zr in a slab. In the case of continuous casting, the cooling rate varies depending on the thickness of the slab, but from the viewpoint of joint toughness, the thickness is preferably 350 mm or less.

Furthermore, the reheating temperature of the slab needs to be 1250°C or lower. If reheated at a temperature higher than this, TfN, ZrN
This is because the fine TiN and ZrN are lost in the shell before welding, making it impossible to create a fine structure at the boundary of the joint and in the HAZ. Further, the lower limit of the reheating temperature is not particularly specified, but it is preferably set to 0.degree. C. or higher.

In the present invention, it is not necessary to reheat the slab for C in the present invention, and hot charge rolling or direct rolling from a normal slab or a thin slab (slab thickness of 100 mm or less) may be performed without any problem.

In the present invention, the rolling method after reheating the slab is not particularly limited, but so-called processing heat treatment, quenching and tempering, and normalizing treatment are appropriate for ensuring the strength and toughness of the base material. This is because even if excellent joint toughness is obtained, if the toughness of the base material is poor, the steel is insufficient.

In order to improve the low-temperature toughness of the base material, it is necessary to refine the grains of steel. As a method of processing heat treatment, l)
Examples include 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.

Note that after manufacturing this steel, A c 1 is used for purposes such as dehydrogenation.
Reheating to a temperature below the transformation point does not impair the features of the present invention.

Furthermore, although wood chips are steel intended to obtain excellent low-temperature toughness with FBW as is, there is no problem in performing heat treatment after FBW in the same manner as conventional steel.

The present invention is most preferably applied to plate mills, but can also be applied to hot coils, shaped steel, wire rods, etc. In addition, the thick steel plates manufactured using this method are used for pressure vessels, offshore structures,
It can be used for welded steel structures such as line pipes that are used in harsh environments.

(Example) Steel plates of various steel compositions are produced in the converter-continuous casting-thick plate process, and the toughness of the joint is reduced to 2mmV by flash butt welding.
It was investigated by the Notch Charpy test.

The test piece was taken from the l/2 position, and the notch position was set at the center of the joint. Examples are shown in Tables 1 and 2.

m2-8 indicates an example in which selected elements are la, m9-33c2PHJ, and all steel plates manufactured using the steel of the present invention (steel of the present invention) have good base material properties and joint toughness.

Comparative steel 34, which is not based on the method of the present invention, has Ag of 0.0010
%, the generation of TI and Zr-based oxides is insufficient and F
The toughness of the BW joint is poor. Comparative Steel 35 has low Mn and Nb contents, and Comparative Steel 36 has TI.

Toughness is poor due to excessive addition of ZrgS. Comparative steel 37゜3
8 has appropriate ingredients, but the former uses an agglomeration method, and the latter has a too high heating temperature of 1300°C, resulting in F.
The toughness of the BW joint is poor.

(Effects of the Invention) According to the present invention, it has become possible to manufacture steel in large quantities and at low cost, which has excellent low-temperature toughness not only in the base material but also in the entire area of the flash-butt welded joint. the result,
The construction efficiency of line pipes or welded structures has been significantly improved, as well as their safety.

-11,3-

Claims (1)

[Claims] 1. C: 0.03 to 0.10%, Si: 0.5% or less, Mn: 1.4 to 2.0%, P: 0.025% or less, S : 0.0050% or less, Al: 0.004% or less, N: 0.0010 to 0.0035%, O: 0.0015 to 0.0050%, Nb: 0.03 to 0.06%, and Ti: 0.005 to 0.025%, Zr: 0.005 to 0.025%, or both 0.005≦Ti+Zr≦0.0
Low-temperature toughness characterized by producing a steel plate by forming a slab by a continuous casting method into a steel containing 25% Al and the balance consisting of Fe and unavoidable impurities, and then reheating the slab at a temperature of 1250°C or less. A method of manufacturing excellent steel plates for flash butt welding. 2. V: 0.005-0.060%, Ni: 0.05-2.0%, Cu: 0.05-1.0%, Cr: 0.05-1.0%, Mo : 0.05 to 0.40%, B: 0.0003 to 0.0015%, Ca: 0.0005 to 0.005%. 2. The method for producing a flash butt welding steel plate having excellent low-temperature toughness according to claim 1, wherein the steel is substantially free of Al, which is an unavoidable impurity.