JPH0569902B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention provides a low-temperature high-toughness welding material that has good low-temperature toughness of the weld heat-affected zone even in welding with a wide range of heat inputs, including large heat input welding with a welding heat input of about 200 kJ/cm. It is related to the method of manufacturing steel. (Prior Art) In recent years, requirements for materials for large structures such as offshore structures and ships have become increasingly strict from the standpoint of ensuring safety. In particular, it is desired to improve the low-temperature toughness of the weld heat-affected zone, which tends to deteriorate in quality compared to the base metal. Generally, when steel materials are subjected to automatic welding with a large welding heat input such as submerged arc welding or electroslag welding, the weld heat affected zone (hereinafter referred to as HAZ)
By coarsening the austenite grains of
The structure of the HAZ becomes coarse and the HAZ toughness decreases significantly. In order to improve HAZ toughness, HAZ, especially the fusion line (hereinafter FL) exposed to high temperatures, must be
There is a need to refine the HAZ structure in the vicinity (referred to as . For example, in "Tetsu to Hagane" Vol. 65, No. 8, p. 1232, published in June 1978, the austenite grains in HAZ were refined by finely precipitating TiN, and 50Kgf/mm grade ² high-strength steel was produced. During large heat input welding
Although techniques for improving HAZ toughness have been disclosed,
Most of TiN melts during welding in the vicinity of the FL, and due to the coarsening of austenite and the increase in solute N,
There is a drawback that deterioration of HAZ toughness is unavoidable. Very recently, as seen in JP-A-61-117213, for example, by generating intragranular ferrite without refining austenite,
Techniques have been developed to refine the HAZ structure.
Some of the present inventors believe that Ti oxide is effective as a generation nucleus for intragranular ferrite, that Ti oxide does not dissolve even when exposed to high temperatures, and that it acts as a nucleus for intragranular ferrite even in the vicinity of the FL. It is possible to refine the structure, and compared to steels using TiN etc.
For example, it was shown in JP-A-61-117245 that HAZ toughness can be significantly improved. (Problem to be solved by the invention) When Ti oxide is used as the transformation nucleus of intragranular ferrite, it has better high-temperature stability than when TiN or other composite mononitrides are used as the nucleus. , since it is an oxide, its dispersion state is determined during solidification,
It is difficult to control the dispersion state compared to mononitrides,
Furthermore, there are problems in that the number of Ti oxides itself is very small compared to TiN etc. in the current steel manufacturing and solidification methods. Furthermore, coarse Ti oxides tend to form, and in that case, the oxide itself becomes the starting point for brittle fracture.
This leads to deterioration of HAZ toughness. Therefore, steel materials using oxides ensure stable HAZ toughness and further improve HAZ
In order to improve toughness, a method is required that can uniformly distribute oxides that can form the nucleus of intragranular ferrite in a fine, large amount and uniformly. The present invention provides a method for producing a low-temperature, high-toughness steel for welding that is highly safe under severe usage conditions. (Means for Solving the Problems) The present invention was developed as a result of various studies on oxides that are stably and finely dispersed in steel and that serve as nuclei for the formation of intragranular ferrite. Where C: 0.02 to 0.18%, Si: 0.5% or less,
Mn: 0.4-2.0%, S: 0.001-0.01%, N: 0.006
% or less, Al: 0.006% or less as a basic component, and if necessary further added: Cr: 1.0% or less, Ni: 3.0% or less, Mo: 0.5% or less, V: 0.1% or less, Nb: 0.05
% or less, Cu: contains one or more of 1.5% or less, P: 0.015% or less as impurities, the remainder is
The amount of O in molten steel consisting of Fe and unavoidable impurities
0.0030~0.015%, Ti and Y: 0.005~0.020
%, Y: in the range of 0.0010 to 0.010%, and then solidified. (Function) The present invention effectively functions as a generation nucleus for intragranular ferrite among oxides that can stably exist even in the HAZ exposed to high temperatures near the FL, and
More stable in steel than steel using Ti oxide,
The invention was developed as a result of various studies on steel containing oxides that can be uniformly and finely dispersed in large amounts and significantly improve HAZ toughness. Hereinafter, the gist of the present invention will be explained in detail based on experimental results. The present inventors found that C: 0.06%, Si: 0.1%, Mn: 1.4
%, Ni: 0.3%, Cu: 0.3%, Nb: 0.015%, P:
Steel having a chemical composition of about 0.005%, S: 0.0030%, N: 0.003% is melted in a vacuum melting furnace, and the purpose of dispersing Ti oxide and various oxides in the steel is to melt the steel before tapping. Approximately 0.015% Ti for 5-10 minutes
Add one type of element listed below.
After adding it in a range of 0.0050 to 0.010%, it was cast into a mold to form an ingot weighing 25 kg. La, Ca, Ce, and Y were selected as additive elements other than Ti. Each ingot was hot rolled and subjected to quenching and tempering treatment to obtain a material. A welding reproduction thermal cycle was applied to the test piece taken from the material, simulating the thermal history experienced by the HAZ near the FL. The welding reproduction thermal cycle conditions are a heating temperature of 1400℃, a holding time of 1 second, and a cooling time (Δ _8/5 ) from 800 to 500℃ of 40 seconds, which is equivalent to medium heat input welding of submerged arc (SAW) welding. Equivalent to heat input welding
There are two types of 161 seconds. After repeated thermal cycling, the oxide and 2 mm V notch characteristics were investigated. First, we investigated oxides by observing the extracted replica with an electron microscope, and found that in addition to Ti, there were La, Ca,
It has been found that in steels with combined addition of Ce and Y, the main oxides contained in the steel are all composite oxides of Ti and another element added. The distribution state of the composite oxide is as shown in Figure 1.
Compared to other composite oxides, Ti and Y composite oxides are
There are many fine oxides with a particle size of 2.0μm or less,
The average size is also small. That is, when compared under the same dissolution and solidification conditions,
It has been found that the composite oxide of Ti and Y can be dispersed more finely than the oxide of Ti alone, which has traditionally been used to improve HAZ toughness. Figures 2 and 3 show the number of oxides with a particle size of 0.1 to 2.0 μm and the shear pi properties (50% fracture surface transition temperature:
vTrs). The number of oxides with a particle size of 0.1 to 2.0 μm, except for composite oxides of Ti and Ce, including oxides of Ti alone.
vTrs has an approximately linear relationship, and as the number of oxides increases, vTrs improves, and because the oxides are finely dispersed, the composite oxide of Ti and Y improves the HAZ toughness even more than the single oxide of Ti. can be achieved.
Although the composite oxide of Ti and Ce is dispersed in a relatively large amount and finely, the formation of intragranular ferrite is not observed in the thermal cycle structure, and the toughness is low. That is, the composite oxide of Ti and Ce does not have the ability to generate intragranular ferrite, and is therefore not effective. All of the others have the ability to generate intragranular ferrite, but Ti and La
Since the number of multiple participants is very small, Figure 2,
Under the thermal cycle conditions shown in FIG. 3, the amount of intragranular ferrite produced is small, and the toughness is significantly inferior. Based on the above experimental results, adding Ti and Y to molten steel is a method to ensure stable HAZ toughness in steel materials using oxides and to further improve HAZ toughness than steels using Ti oxides. By this,
It was concluded that it is extremely effective to form a composite oxide of Ti and Y that serves as the core of intragranular ferrite and can be distributed finely, in large quantities, and uniformly. In addition, the composite oxide of Ti and Y in the present invention is an oxide whose main elements are Ti, Y, and O, and this oxide contains other elements brought in from impurities in the steel and raw materials. may be contained in trace amounts, but these elements are included in the composite oxide of Ti and Y as long as they have the ability to form ferrite. Specifically, trace amounts of Fe, Mn, Ca, Al, etc.
It is often contained in addition to Y and O, but both have sufficient ability to generate intragranular ferrite. For the reasons described below, the amount of Ti and Y added is 0.005% to 0.020% based on the weight of molten steel.
Y: The range is preferably 0.0010% to 0.010%. First, if Ti is less than 0.005%, the necessary oxide amount cannot be secured, so 0.005% or more is necessary.
In addition to forming a composite oxide of Ti and Y, Ti
By forming TiN, the heated austenite in the base metal becomes finer, and the austenite becomes finer in the region far from the FL of the HAZ.
Since it is effective in improving the toughness of the HAZ, it is possible to contain it in an amount exceeding the amount that forms oxides.
If it exceeds 0.020%, coarse TiN may be formed,
0.005 to 0.020 as there is a risk of precipitation embrittlement.
% range. Next, Y is an element that enables uniform and fine dispersion of the oxide by forming a composite oxide with Ti, and although it is not necessary to add a large amount, sufficient effects cannot be obtained unless it is added in an amount of 0.0010% or more. I can't. on the other hand,
Addition of more than 0.010% produces coarse oxides,
Since it is not effective for fine dispersion of oxides and is also unfavorable in terms of toughness, it is set in the range of 0.0010 to 0.010%. The means for adding Ti and Y may be a method in which pure titanium or a titanium alloy and pure yttrium or a master alloy containing yttrium are added to molten steel simultaneously or sequentially and then cast and solidified, or by adjusting the particle size in advance within the above range. A composite oxide of Ti and Y may be added to molten steel by injection, and the molten steel may be directly cast and solidified. The order of addition of Ti and Y does not matter. In addition, since composite oxides of Ti and Y are formed by adding Ti and Y to molten steel, Ti
The O concentration when adding Y and Y is important. According to the study results of the present inventors, when O is less than 0.0030%, it is difficult to form a composite oxide of Ti and Y sufficient for intragranular ferrite formation. On the other hand, Ti in molten steel with an O concentration exceeding 0.015%
When Y is added, coarse oxides are formed, the number of composite oxides decreases, and toughness deteriorates as the coarse oxides become a starting point for fracture. Therefore, when adding Ti and Y, the O concentration in the molten steel needs to be 0.005 to 0.015%. After solidification, the steel material is shaped into the desired shape by some means, and the steel material may be as-rolled, controlled-rolled, controlled-cooled and tempered, or quenched, tempered, or tempered. Even if the oxide is used or a combination of the two is used, the effect of the oxide is not affected in any way. Next, the reason for limiting the basic component range of the steel of the present invention will be described. First of all, C is added as an effective component to improve the strength of steel; if it is less than 0.02%, it is difficult to secure the strength required for structural molten steel, and if it is added in excess of 0.18%, it may reduce the weld cracking resistance. Because it significantly reduces the Next, although Si is an effective element for ensuring the strength of the base metal, excessive addition of more than 0.5% will generate high carbon island martensite in the HAZ and reduce toughness, so the upper limit was set at 0.5%. . In addition, Mn is an element necessary to ensure the strength and toughness of the base metal, and must be added at least 0.4%, but the upper limit was set at 2.0% as long as it was acceptable for the toughness and crackability of the welded part. On the other hand, P has a negative effect on the toughness of the base metal and weld, so it should be reduced as much as possible, and the upper limit should be
It was set as 0.015%. S is an element that forms MnS and promotes intragranular ferrite formation, so 0.001% or more is required, but excessive addition of more than 0.01% may form coarse A-based inclusions and reduce the ductility of the base metal. S should be avoided because it causes a decrease in toughness and an increase in anisotropy of mechanical properties. Therefore, S is set in the range of 0.001 to 0.01%. N contributes to grain refinement of the base material as TiN, but
In particular, during high heat input welding, high carbon island martensite is generated and toughness is reduced, so the upper limit has been set to 0.006%.
And so. Al is an element necessary for deoxidation and grain refinement of the base material in ordinary steel, but even when added to the normal level of aluminum kill, it significantly reduces the amount of oxygen in molten steel and forms a composite oxide of Ti and Y that becomes the nucleus for ferrite formation. The upper limit was set at 0.006%. The above are the basic components of the steel of the present invention, but one or more of Cr, Ni, Mo, V, Nb, and Cu may be added as necessary for the purpose of increasing base metal strength and improving base metal and HAZ toughness. can contain. First of all, Ni is an extremely effective element that can simultaneously improve the strength and toughness of the base metal and the toughness of the HAZ.
If excessive addition exceeds 3.0%, the hardenability will increase and the formation of intragranular ferrite, which is necessary for the steel of the present invention, will be suppressed and the HAZ toughness will deteriorate, so the upper limit is set at 3.0%.
And so. Next, Cu is an effective element in that it does not increase the hardness of HAZ even though it increases the strength of the base metal.
The upper limit was set at 1.5% due to the risk of hardening of the HAZ and deterioration of toughness due to stress relief annealing. Furthermore, Cr, Mo, V, and Nb are effective elements for increasing the strength of the base metal by improving hardenability and precipitation hardening, but addition exceeding the upper limit causes deterioration of HAZ toughness, so Cr, Mo, , Nb were set at 1.0%, 0.5%, 0.1%, and 0.05%, respectively. Next, the effects of the present invention will be described in more detail with reference to Examples. (Example) Table 1 shows the chemical composition of steel prototyped according to the present invention and comparative steel, types of oxide-forming elements such as Ti and Y,
Indicates the amount added, the size and number of oxides, the toughness of the weld, etc. Here, No. 1 to No. 10 are the steels of the present invention, and No. 11 to No. 10 are the steels of the present invention.
No.16 is the comparison steel. Both the invention steel and comparative steel are 20mm and 30mm by rolling.
steel plate. For 20mm material, X groove, current 700A, voltage
Double-sided, single-layer, single-electrode latent arc welding (submerged arc welding) was performed at 32V, welding speed of 30cm/min, and heat input of 45kJ/cm. For 30mm material, Y groove, current 1380A (L
pole), 1150A (T1 pole), 1040A (T2 pole), voltage 36V
(L pole), 42V (T1 pole), 46V (T2 pole), welding speed 45
Single-sided, single-layer, three-electrode submerged arc welding with a heat input of 194 kJ/cm and a heat input of 194 kJ/cm was performed, and a 2 mm V notched pylon impact test piece was placed at the center of the test piece at a position 7 mm from the plate surface, and the boundary between the weld metal and the HAZ ( A sample was taken so that the notch position was 1 mm from the fusion part (FL) to the HAZ side, and the test was conducted at 60°C.

【table】

【table】

[Table] As is clear from Table 1, the steel of the present invention has superior HAZ toughness compared to comparative steels, and absorbs enough energy in the Charpy test to ensure the safety of structures even at temperatures as low as -60°C. I understand what is shown. That is, in all of the steels of the present invention, by adding Ti and Y to molten steel containing 0.0030 to 0.015% O, a large amount of fine composite oxides of Ti and Y with a diameter of 0.1 to 2.0 μm is dispersed in the steel. As a result, it shows extremely excellent shear strength properties not only in double-sided, single-layer welding with a heat input of 40 kJ/cm, but also in single-sided, single-layer large heat input welding with a heat input of 194 kJ/cm. On the other hand, among comparative steels, No. 11 and No. 12 are steels in which Ti oxide alone is dispersed, but No. 11 does not have a sufficient number of oxides, and No. 12 has a 2.0 μm In both cases, the HAZ toughness is slightly inferior to that of the steel of the present invention due to the presence of oxides exceeding the above. Comparative steel No.113~No.
15 is a steel containing a composite oxide, but because the composite oxide is not a composite oxide of Ti and Y, the number of oxides is insufficient, or the oxide itself does not have the ability to generate intragranular ferrite. The toughness value is extremely low. Comparison steel No. 16 is a steel that contains Al, has little Ti oxide, and has a finer structure with TiN, but the TiN melts near the FL and coarsens the austenio grain size. , the HAZ toughness near the FL is inferior to that of the steel of the present invention due to the increased amount of solid solute N. From the above embodiments, according to the present invention, 200kJ/cm
It is clear that extremely excellent HAZ toughness, which shows sufficient absorbed energy even at -60°C, can be obtained up to moderately high heat input welding. (Effects of the invention) The technology that uses Ti oxide to generate intragranular ferrite in the HAZ structure to refine the structure is as follows:
This is an excellent technology for improving HAZ toughness. It is clear from the above examples that according to the present invention, by finely dispersing the composite oxide of Ti and Y, the HAZ toughness can be further improved than by using Ti oxide. Therefore, the present invention makes it possible to provide a low-temperature, high-toughness steel for welded structures that is highly safe even under harsher usage conditions, and its effects are extremely significant.

[Brief explanation of drawings]

Figure 1 is a chart showing the relationship between the particle size and number of particles of various composite oxides contained in steel, and Figure 2 is a graph showing the relationship between the number of oxide particles and the simulated welding thermal cycle of Δ _8/5 = 40 seconds. Chart showing the relationship between the Charpey characteristics when
The figure is a chart showing the relationship between the number of oxide particles and the shear pie characteristics when a welding reproduction thermal cycle of Δ _8/5 = 161 seconds is applied.

Claims (1)

[Claims] 1 In terms of weight%, C: 0.02 to 0.18% Si: 0.5% or less Mn: 0.4 to 2.0% S: 0.001 to 0.01% N: 0.006% or less Al: 0.006% or less as basic components, and as impurities P: 0.015% or less,
The remainder is O in molten steel consisting of Fe and unavoidable impurities.
The amount is 0.0030-0.015%, and Ti and Y are Ti: 0.005-0.005%.
After adding 0.020%, Y: in the range of 0.0010 to 0.010%,
A method for producing a low-temperature high-toughness steel for welding, characterized by solidifying it. 2. A patent characterized by containing one or more of the following in weight%: Cr: 1.0% or less Ni: 3.0% Mo: 0.5% or less V: 0.1% or less Nb: 0.05% or less Cu: 1.5% or less A method for producing a low-temperature high-toughness steel for welding according to claim 1.