JPH01159356A - High tension steel having superior tougeness at weld heat-affected zone - Google Patents

Abstract

PURPOSE:To obtain a high tension steel having superior toughness at the weld heat-affected zone by specifying a compsn. consisting of C, Si, Mn, P, S, Al, Ti, Zr, N, O and Fe and by incorporating specified grains of Ti-Zr compd. oxide. CONSTITUTION:Grains of Ti-Zr compd. oxide having 0.05-10mum grain size are incorporated into a steel consisting of, by weight, 0.01-0.60% C, <=0.5% Si, 0.5-2.2% Mn, <=0.025% P, <=0.010% S, <=0.005% Al, 0.003-0.020% Ti, 0.002-0.018% Zr (Ti+Zr=0.005-0.022%),<=0.0040% N, 0.0010-0.0080% O and the balance Fe with inevitable impurities or further contg. prescribed amts. of Cu, Ni, Nb, V, B, Ca, Cr, Mo, etc. The number of the grains incorporated is 3mu10<5>-1X10<10> per 1mm<3> steel. A high tension steel having superior toughness at low temp. is obtd. When the steel is welded, the structure of the weld heat- affected zone is made fine and the zone has superior toughness whether the welding is low heat input welding or high heat input welding.

Description

[Detailed description of the invention] (Industrial use! [!F) The present invention is a high tensile strength steel with excellent low-temperature toughness in the heat affected zone (I(AZ)), which can be used for everything from dwarf heat welding to large heat input welding. It relates to steel. Also, this steel can be used in welded steel structures such as pressure vessels, shipbuilding, bridges, architecture, and line pipes.

(Prior art) The HAZ toughness of low alloy steel is determined by (1) grain size, (2)
) The dispersion state of hardened phases such as high carbon island martensite (Mo) and upper bainite (Bu), (3) presence or absence of grain boundary embrittlement, and (4) various metallurgical factors such as elemental microsegregation. be controlled.

In particular, it is known that the grain size of the HAZ has a large effect on low-temperature toughness, and many techniques for refining the HAZ structure have been developed and put into practical use.

For example, a technology has been developed in which nitrides, such as TiNsZrN, which are relatively stable even at high temperatures, are made to have a fine fraction of 11 in steel, thereby suppressing the coarsening of austenite (r) grains in HAZ. However, near the melting line where the steel melts, TiN and ZrN coarsen or dissolve, and the ability to suppress coarsening of γ grains disappears.

On the other hand, according to JP-A-59-190313, molten steel is deoxidized with Ti or Ti alloy, and then Zr
By adding ZrN, the steel in which Ti oxide and ZrN are dispersed has the formation of fine radial acicular ferrite (AF) mainly composed of Ti oxide near the melting line and the formation of ZrN.
The HAZm weave can be made smaller due to the effect of rN on suppressing the coarsening of γ grains, resulting in superior low-temperature toughness compared to TiN or ZrN steel.

(Problem to be solved by the invention) However, according to the inventors' subsequent research, Ti
Steel with finely dispersed oxides has a great effect of reducing the HAZ structure in the region where one grain near the melting line becomes coarse (coarse grain region: region heated to 1400°C or higher), but TiN,
A region where some of the ZrN particles are coarsened and the γ grains are somewhat large (sub-coarse grain region: tzo).

In the region (heated to ~1400°C), the effect of Ti oxide on refining H zmm is smaller than in the coarse grain region;
+1 AZ It has been found that the toughness deteriorates.

In particular, when high heat input welding is performed, the width of the sub-coarse grain region becomes large, making it impossible to stably obtain high toughness throughout the HAZ.

The present invention proposes a high-strength steel with excellent weld heat-affected zone toughness.
The m-weave is fine throughout the entire area and has excellent low-temperature toughness.

(Means for Solving the Problems) The present invention provides c:o, oot ~ 0.20%, S
i: 0.5% or less, Mn: 0.5-2.2%, p:
.

, 025% or less, s:o, 010% or less, AIl:0.
005% or less, Ti: 0.003 to 0.020%, Z"
: o, oo2-0.018%, N: 0.0040% or less, o: 0.0010-0.0080%, 0.005
%≦Ti+Zr≦0.022%, contains the balance iron and unavoidable impurities, and has a particle size of 0.05 to 1Opm
, the number of particles is 3X10'-IXIO"ke/Tsuru 3 Ti and Z
The specified invention is a high-strength steel containing an r composite oxide and having excellent weld heat-affected zone toughness.

The present invention will be explained below.

According to the research conducted by the present inventors, HAZ toughness is determined by 1) chemical composition of steel, 2) microstructure (crystal grain size and hardened phase distribution)
), and it is thought that optimization of steel components and the resulting refinement of grain size are essential for achieving high toughness.

Therefore, Ti and Zr are simultaneously added to molten steel that does not substantially contain A1, and a fine composite oxide of Ti and Zr (Ti-
We have invented a new method for forming and dispersing Zr-0 (Zr-0 series) and thereby refining the ML weave.

The composite oxide of Ti and Zr is Ti oxide (Ti-O system)
Alternatively, since the formation temperature is higher than that of Zr1llide (Zr-0 series) and it is less affected by the solidification cooling rate, it is finely dispersed in the steel. In particular, the number of composite oxides of Ti and Zr in the center of the slab increases significantly compared to conventional oxides of Ti or Zr.

The fine composite oxide of Ti and Zr has an extremely large effect of suppressing the coarsening of γ grains, and in the region heated to 1400°C or less, the
Since grain coarsening is suppressed to a minimum compared to TiN, ZrN, TiM compound and Zr oxide, the width of the sub-coarse grain region becomes extremely small.

For this reason, the HAZ Mi weave has become finer, and T i N steel,
Extremely superior low-temperature toughness can be obtained compared to ZrN steel and steel containing Ti oxide or Zr oxide.

Steel in which composite oxides of 7'i and Z' are finely dispersed exhibits γ
-During the α transformation, the complex oxide of Ti and Z present in the γ grains forms a radial fine Ashiqui error ferrite, which significantly refines the HAZ structure.As a result, the welded area is It is made finer and has extremely excellent low-temperature toughness.

In order for the composite oxide of Ti and Z to have the effect of refining the HAZ structure, the particle size of the composite oxide of Ti and Z must first be in the range of 0.05 to 10 μm. It is.

According to the results of electron microscopy observation using extracted replicas by the present inventors, when the particle size is less than 0.05 μm, the effect of suppressing coarsening of γ grains and the effect of forming intragranular Ashkiller ferrite nuclei is extremely weak; In this case, the HAZ toughness decreases because the steel itself tends to become a point of occurrence of fracture.

In addition, if the number of particles is too small in terms of the number of oxides, the effect of suppressing the coarsening of a single grain during welding and the effect of forming intragranular Ashkiller ferrite nuclei cannot be obtained, so 3XIO
It is necessary to have more than 100 particles present. lX10
Excessive oxide exceeding "1X10" causes a decrease in the toughness or ductility of the base metal and HAZ.

In order to finely disperse the composite oxide of Ti and Zr,
It is particularly important to simultaneously add Ti and Zr to molten steel for deoxidation. As disclosed in JP-A No. 59-190313, when deoxidizing with Ti or Ti alloy and then adding Zr, Ti i2 and ZrN are produced, and a composite oxide of Ti and Zr is Not generated.

Some methods of improving HAZ toughness with oxides use Ti oxides, as disclosed in Japanese Patent Application Laid-Open No. 61-79745, but composite oxides of Ti and 2" Since the formation temperature is high, it is hardly affected by the solidification cooling rate, so it is excellent in that fine and uniform dispersion can be achieved over all 17 slabs, and furthermore, the effect of suppressing coarsening of γ grains is extremely large.

As a result, the &lI weave is refined throughout the HAZ at all thickness positions, including the center of the plate thickness, resulting in extremely excellent low-temperature toughness.

In order to finely disperse the composite oxide of Ti and Zr in steel, it is essential to optimize the balance between the amounts of Ti, Zr, and 0 and the amount of T s % Zr. For this reason, the amounts of Ti, Zr and O are respectively Ti:Q, QQ3~0.020%,
Zr: 0.002-0.018%, o: o, oo
It is necessary to limit the amount of Ti to 0.080% and to balance the amounts of Ti and Zr such that 0.005%≦Ti+Zr≦0.022%.

The lower limit of Ti1i is 0.003%, which is the minimum amount required to produce a composite oxide of Ti1i and Zr. Further, in order to prevent deterioration of low temperature toughness due to the formation of TiC, the upper limit was set to 0.020%.

The lower limit of Zr content is 0.002%, which is
This is the minimum amount necessary to generate a complex oxide of r. In addition, the upper limit was set at 0.018% to prevent deterioration of HAZ toughness.
And so.

The lower limit of 0 amount is o, ooio% is Ti and Z in HAZ.
This is the minimum amount necessary to generate a complex oxide of r. Further, in order to prevent deterioration of the cleanliness and toughness of the steel due to the formation of non-metallic inclusions, the upper limit of 0 was set to 0.0080%.

The lower limit 0.005% of T i +, Z r fft is HA
Ti in Z? ! : The required minimum value (■) for producing a Zr complex oxide. Further, in order to prevent deterioration of low temperature toughness and HAZ toughness, the upper limit was set to 0.022%.

Even if composite oxides of Ti and Zr are finely dispersed in steel, excellent HA2 toughness cannot be obtained unless the basic components are appropriate.

This point will be explained below.

The lower limit of the amount of C, 0.005%, 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 is an element that not only adversely affects the low-temperature toughness of the base metal and weld zone, but also deteriorates weldability and HAZ toughness, the upper limit was limited to 0.20%.

Si is an element contained in PIL acid coated steel, but it improves weldability,
Since it is an element that deteriorates HAZ toughness, the upper limit is set to 0.5.
%.

Mn is an essential element for ensuring strength and toughness, and its lower limit is 0.6%. However, if Mn is too large, the hardenability of the steel will increase and the weldability and HAZ toughness will deteriorate, so the upper limit was set at 2.2%.

In the steel of the present invention, the impurities P and S are each 0.0
The reason for setting it below 2% and below 0.010% is that the base material, HA
This is to further improve the low-temperature toughness of Z. Pl
The reduction in S reduces intergranular fracture in all directions at the joint, and
Reducing l tends to suppress the formation of grain boundary ferrite. Most preferred P.

The amount of S is 0.01% and 0.0050% or less, respectively.

Although Al is generally an element contained in deoxidized steel, it is an undesirable element in the present invention, and is therefore limited to 0.005% or less. This is AI! This is because if , is contained in the steel, it will combine with O and a composite oxide of Ti and Zr will not be formed.

Deoxidation is possible using only Ti and Zr, and in the present invention, the lower the AAI content, the better, and preferably 0.003% or less.

N inevitably mixes into steel and reduces the low-temperature toughness of the steel. In particular, a large amount of solid solution N tends to generate high-carbon island martensite in the HAZ, which significantly deteriorates toughness. For this reason, the upper limit of N was limited to 0.0040%.

Next, Cu, Ni%Nbb, Cr%Mo, VSB.

The reason for adding Ca will be explained.

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

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-crank occurs during hot rolling, making production difficult.

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

Ni improves the strength and toughness of the base metal without adversely affecting weldability and joint toughness, but if it exceeds 4.0%, it is unfavorable for weldability, so the upper limit was set at 4.0%.

Nb suppresses ferrite generated at one grain boundary, and Ti and Z
It has the function of promoting the generation of fine AF with r complex oxide as the core. Minimum 0.003% to obtain this effect
Nbi 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.
It was set at 06%.

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 too large, it will cause deterioration of the toughness and weldability of the base metal and joints, similar to C, which is undesirable.The upper limit is 0.4%. be.

V is an element that has almost the same effect as Nb, but 0.01
% or less, the effect is small, and an upper limit of 0.08% is permissible.

Note that the lower limit of the addition range of Cu, Ni, Cr, and Mo should be the minimum amount in order to obtain the effect on the material, and each is 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 joint suppresses the formation of ferrite, and prevents the formation of fine AF from the composite oxide of Ti and Zr.
helps generate.

Furthermore, BN combined with N acts as a ferrite generation nucleus and refines the HAZm weave. In order to obtain such an effect, a B content of at least 0.0005% is required. However, if Bl is too large, coarse precipitates such as Fezs (CB>h) will precipitate at the γ grain boundaries, deteriorating the low-temperature toughness.

Therefore, it is necessary to limit the upper limit of Bm to 0.0020%.

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, if Ca is added below 10.001%, it has no practical effect, and if it exceeds 0.005%, CaO1CaS
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 0.001 to 0.005%.

Now, industrially, this steel can be manufactured by either the continuous casting method or the ingot-blooming method using large steel ingots.
Further, it is not always necessary to reheat the slab, and there is no problem even if charge rolling or direct rolling is performed.

The rolling method after reheating the slab in the present invention is not particularly limited, but so-called working heat treatment, quenching and tempering after rolling, and normalizing treatment are suitable for ensuring strength and toughness. This is because even if excellent HAZ toughness is obtained, if the toughness of the base metal is poor, the steel is insufficient. In order to improve the low-temperature toughness of the base metal, it is necessary to refine the crystal grains of steel.

Examples of the processing heat treatment methods include 1) controlled rolling, 2) controlled rolling-accelerated cooling, and 3) rolling direct quenching and tempering. Most preferred is a combination of controlled rolling and accelerated cooling. Note that even if the product is reheated to a temperature below the AC1 transformation point for the purpose of dehydrogenation or the like after production, the features of the present invention will not be impaired.

The steel plate manufactured in this way has a composite oxide containing Ti and Z finely dispersed in the steel, and has extremely excellent low-temperature toughness over a wide range of welding heat input.In addition, the steel plate welding method Examples include submerged arc welding, electron beam welding, etc., and the characteristics of the present invention are not impaired even if welding is performed using any of the welding methods.

(Example) Steel plates (thickness 30u) of various components were manufactured in a converter-continuous casting or (ingot-blending)-thick plate process, and the HAZ toughness was improved to 21mm using a welding heat cycle reproduction device. ■Investigated by the Notschial Rupee test.

The simulated thermal cycle test uses a Charpy test piece taken from a plate with a thickness of 1/4 t, and the peak temperature (maximum temperature reached) is set to 1.
The temperature was 400°C and 1300°C, and the cooling time from 800 to 500°C was 192 seconds. ``This condition corresponds to a welding heat input of 200 KJ/e11, and simulates the thermal cycle of the coarse grain region and sub-coarse grain region near the fusion line, respectively.

Examples are shown in Table 1.

The steel plates manufactured by the method of the present invention (inventive steel) all have good base metal properties and HAZ toughness, whereas comparative steels not manufactured by the method of the present invention have poor HAZ toughness and cannot be used in harsh environments. Suitable as steel for welded structures.

Among comparative steels, Steel 19 has too much A4fi, so T
The number of composite oxides of i and Zr is insufficient, the HAZ structure is not refined, and the HAZ toughness is poor.

Steel 20 has a PT of 1400℃ because Ti oxide is generated.
Good toughness can be obtained in the coarse grain region of 271300℃.
In the sub-coarse grain region, the effect of refining the HAZ structure is small and the toughness is poor.

Although Zr oxide is produced in Steel 21, the effect of refining the HAZ structure is small and the toughness is poor. Since steel 22 contains too much Ti, TiC is generated and the toughness is poor. Steel 23 contains too much Zr, so the toughness of the base metal and HAZ is poor.

Steel 24 has a large amount of Ti+Zrl and therefore has poor HAZ toughness.

Steel 25 has poor HAZ toughness because it has a small amount of Ti+Zr and a small amount of composite oxide of Ti and Zr.

Steel 26 is a TiN steel with AA deoxidation, but PT140
The structure is not refined in the coarse grain region at 0°C, resulting in poor toughness.

Steel 27 has a small number of composite oxides of Ti and Zr, so H
The Mi weave of the AZ is not refined and the HAZ toughness is poor. Steel 28 has poor HAZ toughness because it has too many composite oxides of Ti and Zr.

Steel 29 has a small particle size of composite oxide of Ti and Zr, and has poor HAZ toughness.

[30 has a large particle size of composite oxide of Ti and Zr, and H
AZ toughness is poor. Steel 31 is Z after deoxidizing molten steel with Ti.
Due to the addition of ``Ti oxide and ZrN, the effect of refining the HAZ structure is small in the sub-coarse grain region at 271,300°C, and the toughness is poor.

What about steel 32? Since Ti is added after deoxidizing the No. 8 steel with Zr, Zr oxide and TiN are generated, and the temperature at 271,300°C
The effect of refining the HAZ structure is small in the sub-coarse grain region (poor toughness).

(The following margins continue on the next page) (Effects of the invention) The steel with excellent low-temperature toughness in the weld heat affected zone by the present invention can be used in pressure vessels, shipbuilding, bridges, architecture, etc. used in harsh environments.
The construction efficiency of welded structures such as line pipes has been significantly improved, and their safety has also been greatly improved.

Agent: Patent Attorney Tatsuo Cha Noki

Claims (1)

[Claims] (1) C: 0.01 to 0.20%, Si: 0.5% or less, Mn: 0.5% to 2.2%, P: 0.025% or less in weight%. , S: 0.010% or less, Al: 0.005% or less, Ti: 0.003 to 0.020%, Zr: 0.002 to 0.018%, N: 0.0040% or less, O: 0 .0010 to 0.0080%, 0.005%≦Ti+Zr≦0.022%, contains the balance iron and unavoidable impurities, has a particle size of 0.05 to 10 μm, and has a particle number of 3×10^5 ~1×
High tensile strength steel with excellent weld heat affected zone toughness, containing 10^1^0 ke/mm^3 composite oxide of Ti and Zr. (2) In weight%, C: 0.01 to 0.20%, Si: 0.5% or less, Mn: 0.5% to 2.2%, P: 0.025% or less, S: 0.010 % or less, Al: 0.005% or less, Ti: 0.003 to 0.020%, Zr: 0.002 to 0.018%, N: 0.0040% or less, O: 0.0010 to 0.0080 %, 0.005%≦Ti+Zr≦0.022%, Cu: 0.05-1.0%, Ni: 0.05-4.0%, Nb: 0.003-0.060% , V: 0.005-0.080%, B: 0.0003-0.0020%, Ca: 0.001-0.005%, and the balance contains iron and inevitable impurities. Contains Ti with a particle diameter of 0.05 to 10 μm and a particle number of 3 x 10^5 to 1 x 10^1^0 pieces/mm^3
A high-strength steel with excellent weld heat-affected zone toughness, containing a composite oxide of Zr and Zr. (3) C: 0.01 to 0.20% by weight, Si: 0.5% or less, Mn: 0.5% to 2.2%, P: 0.025% or less, S: 0.010 % or less, Al: 0.005% or less, Ti: 0.003 to 0.020%, Zr: 0.002 to 0.018%, N: 0.0040% or less, O: 0.0010 to 0.0080 %, 0.005%≦Ti+Zr≦0.022%, Cu: 0.05-1.0%, Ni: 0.05-4.0%, Nb: 0.003-0.060% , Cr: 0.05-1.0%, Mo: 0.05-0.4%, V: 0.005-0.080%, B: 0.0003-0.0020%, any two of these. Ti containing seeds, balance iron and unavoidable impurities, particle size 0.05 to 10 μm, and number of particles 3 x 10^5 to 1 x 10^1^0 particles/mm^3
A high-strength steel with excellent weld heat-affected zone toughness, containing a composite oxide of Zr and Zr. (4) C: 0.01 to 0.20% by weight, Si: 0.5% or less, Mn: 0.5% to 2.2%, P: 0.025% or less, S: 0.010 % or less, Al: 0.005% or less, Ti: 0.003 to 0.020%, Zr: 0.002 to 0.018%, N: 0.0040% or less, O: 0.0010 to 0.0080 %, 0.005%≦Ti+Zr≦0.022%, and contains any one of the combinations of the following three elements (a) to (f), with the remainder containing iron and inevitable impurities, and the particle size is 0.05 to 10 μm and the number of particles is 3
Ti and Z of ×10^5 to 1 × 10^1^0 pieces/mm^3
A high-strength steel with excellent weld heat-affected zone toughness that contains a composite oxide of r. (a) Cu: 0.05-1.0%, Ni: 0.05-4
．． 0%, Nb: 0.003-0.060%, (b) Cu: 0.05-1.0%, Ni: 0.05-1
．． 0%, B: 0.0003-0.0020%, (c) Ni: 0.05-4.0%, Mo: 0.05-0
．． 4%, Nb: 0.003-0.060%, (d) Ni: 0.05-1.0%, Mo: 0.05-0
．． 4%, B: 0.0003-0.0020%, (e) Ni: 0.05-4.0%, Cr: 0.05-1
．． 0%, Nb: 0.003-0.060%, (f) Ni: 0.05-4.0%, Cr: 0.05-1
．． 0%, V: 0.005-0.080%,