JPH01180948A - High-tensile steel for low temperature use excellent in toughness in weld zone - Google Patents

Abstract

PURPOSE:To improve toughness in a weld heat-affected zone in a low-temp. environment by limiting respective contents of Al and P in a low-alloy steel with a specific composition, also limiting respective contents of N and Si, and incorporating Ti oxides and combined compounds while regulating the numbers of respective grains to specific ranges. CONSTITUTION:A high-tensile steel has a composition consisting of, by weight, 0.02-0.18% C, 0.4-2.0% Mn, 0.0007-00.0060% S, 0.005-0.030% Ti, <0.003% Al, <0.015% P, 0.0010-0.0040% N, 0.03-0.25% Si, and the balance Fe with inevitable impurities. Moreover, the sum of Ti oxide, such as Ti2O3 and Ti3O5, in which several percent of Mn mainly of 0.1-3.0mum grain size are allowed to enter into solid solution and composite bodies of the above oxides and TiN and MnS and a composite body of TiN+MnS are simultaneously incorporated while regulating the numbers of grains to 3X10<3>-1X10<6>pieces/mm<3>, respectively. Further, limited amounts of Ni, Cu, Nb, V, Cr, Mo, and B are incorporated to the above steel, if necessary. This steel stock is applicable to a structure used under an extremely low-temp. environment and capable of securing its stability, and further, weldability, etc., can be improved by using this steel.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a strong, high-strength steel with excellent weldability, and particularly to a steel material with excellent low-temperature toughness in a weld heat affected zone (hereinafter referred to as HAZ). It is related to.

[Prior art] Weld HAZ toughness of low alloy steel is determined by (1) effective grain size (austenite grain size, microstructure), (2) grain size and volume fraction of hardened phase (carbide, high carbon marten (3) hardness and toughness of the matrix (solid solution C, N in ferrite). Among these, HAZ
A simple method is to refine the structure and refine the effective crystal grains, and various methods have been proposed that utilize various precipitates that are stable at high temperatures.

For example, if TiN is finely dispersed, 50 kg I f/-
Measures have been taken to improve the HAZ toughness during high heat input welding of high tensile strength steel ("Tetsu to Hagane" Vol. 65 No. 8, June 1970, p. 1232). However, most of these precipitates are dissolved in high heat input welding, resulting in coarse graining of the HAZ structure and increase in solute N, which has the disadvantage of degrading the HAZ toughness.

On the other hand, some of the inventors of the present invention have proposed a T1 method to replace AΩ deoxidation of molten iron.
By deoxidizing, T1 oxide is finely dispersed in the steel, and in the HAZ area during welding, an intragranular ferrite transformation structure (hereinafter referred to as I
JP-A-80-245768, JP-A-Sho Go-79745, and JP-A-Sho [il-11] have shown that the HAZ toughness can be significantly improved by developing a
No. 7245 and Japanese Unexamined Patent Application Publication No. 62-1842.

However, when we later investigated the relationship between the HAZ structure and toughness in detail, we found that even in steel in which T1 oxide was finely dispersed in the steel through T1 deoxidation, the HAZ region could not be completely covered by the IFP structure. In order to guarantee the weld toughness of low-temperature steel used in extremely low-temperature environments such as Arctic waters and LPG tanks, it has been found that it is necessary to introduce a technological philosophy that dramatically improves HAZ toughness. .

[Problems to be Solved by the Invention] Based on the above-mentioned current situation, the present inventors have investigated the formation of high carbon martensite (hereinafter referred to as M*), which is the starting point of brittle fracture that affects HAZ toughness, and the ferrite matrix. Focusing on the toughness of the steel, we conducted intensive studies on how to control them, and obtained the following results.

Even in steel in which T1 oxide particles are uniformly and finely dispersed, low-temperature brittle fracture of steel may occur and propagate from relatively coarse hardened phases, carbides, and M* generated near grain boundaries. In particular, it has been found that when alloying is added to increase the strength and thickness of steel, the formation of M* becomes noticeable and the toughness decreases.

It has become clear that among these additive alloys, the influence of Si addition is significant. In addition, components that do not generate M*,
It was also revealed that the presence of solid solution N deteriorates the toughness of the matrix even under cooling conditions.

However, Sl is an element useful as a strengthening and deoxidizing element, and N, as a nitride of Ti, Zr, etc., is sometimes effective in strengthening the base material and refining austenite grains, so the content of both is effective. We considered the appropriate amount range and improved the low-temperature toughness of HAZ for marine structures, ships, storage tanks, and other materials with excellent weldability.
We came to the conclusion that it is possible to develop steel for structures such as transportation pipes, and created the present invention.

[Means for Solving the Problems] The present invention has been made based on the above findings, and
The gist is, in weight%, C: 0.02-0.18%,
Mn: 0.4-2.0%, S: 0.000
7-0.0060%.

Contains Ti: 0.005 to 0.030%, Aj! <
0.003%.

Limited to P < 0.015%, especially i:N: 0.0
010-0.0040%, Si:0.03-0.
25%, and further add Ni<
3.0%, Cu<1.5%.

Nb<0.05%, V<0.1%, Cr<1.0%.

One or two of Mo<0.5% and B<0.002%
titanium oxides of Tl2O3 and Ti3O5 in which a few percent of Mn is dissolved in solid solution, and these oxides and TiN, with the remainder consisting of Fe and unavoidable impurities. Total complex of MnS and Ti
This is a strong, high-strength steel with excellent weld toughness, characterized in that it simultaneously contains a composite of N+MnS in a particle number range of 3×10 to 1×10”/mni, respectively.

The present invention will be explained in detail below.

First, the reason for limiting the basic component range of the steel of the present invention will be described.

First, C is added as an effective component to improve the strength of steel, and if it is less than 0.02%, the strength required for welded structural steel cannot be obtained, and if it is added in excess of 0.18%, it is , weld cracking resistance, HAZ toughness, etc. are significantly reduced, so the upper limit was set at 0.18%.

Si is necessary to ensure the strength of the base metal and to preliminarily deoxidize molten steel, but if it exceeds 0.25%, it will generate M* in the HAZ and
Significantly reduces Z toughness. Further, if it is 0.03% or less, M* is not generated but HAZ toughness is decreased. Therefore, the Sj content was limited to this range.

When the content of N exceeds 0.0040%, it embrittles the matrix even in the absence of M* and reduces HAZ toughness.

Further, when N is 0.0010% or less, nitrides are not generated in the HAZ, the amount of IFP structure generated is reduced, and the HAZ toughness is decreased.

Although it is necessary to add Mn in an amount of 0.4% or more to ensure the strength and toughness of the base metal, the upper limit was set to 2.0% within an allowable range such as the toughness and crackability of the welded part.

Regarding S, in order to precipitate MnS of the complex, 0.
0.0007% or more is necessary, but excessive addition of more than 0.0060% will form coarse sulfide inclusions, leading to a decrease in ductility and an increase in anisotropy of the base material, so 0.0007 to 0. .0
060%.

Ti is an essential element for the formation of T1 oxide and T1 nitride, or addition of more than 0.03% causes excessive precipitation of T1 carbide, increases HAZ hardness, and reduces toughness. , 003% or less.

P should be reduced as much as possible to prevent deterioration of weld cracking properties and toughness due to solidification segregation, and the upper limit should be set at 0015.
%.

AΩ is a strong deoxidizing element, and by adding 0.003% or more, Ti oxide formed by Ti deoxidation will not be formed, IFP will not be formed in the HAZ, and the HAZ toughness will decrease. It was limited to 0.003% or less.

The above are the basic components of the steel of the present invention.
For the purpose of improving the toughness of the matrix and HAZ, Ni, Cu
, Nb, V, Cr, Mo, and B.

First, N1 is an extremely effective element that simultaneously increases the toughness of the base metal and the HAZ, but adding more than 3.0% increases hardenability and inhibits the formation of the IFP structure. , M*, which results in a decrease in HAZ toughness, the upper limit was set at 30%.

Although Cu strengthens the base material, it hardens the HAZ less.
Although it is an effective element, the upper limit was set at 1.5% in consideration of temper embrittlement caused by stress relief annealing, weld cracking resistance, etc.

Nb and V are effective in improving HAZ toughness by strengthening the base material and suppressing the formation of grain boundary ferrite, but excessive addition exceeding the upper limit of each component is harmful from the viewpoint of toughness and hardenability. Because, Nb.

For each of (2), the upper limits were set to 0.005% and 0.1%.

Cr and Mo are effective in strengthening the matrix by improving hardenability and precipitation hardening. Furthermore, adding an appropriate process such as TMCP is effective in improving the low-temperature toughness of the base material.

However, excessive addition exceeding the upper limit of each component is harmful from the viewpoint of HAZ toughness and hardenability, so Cr, Mo
For each, the upper limit is 1.0%.

It was set to 0.5%.

B is expected to increase the strength of the base material by improving hardenability and improve HAZ toughness by suppressing the growth of grain boundary ferrite, but addition of more than 0.002%
) 6 precipitation leads to a decrease in toughness and hardening of the HAZ.
The upper limit was set to 0.002%.

Next, the IFP nuclear precipitates that form the basis of IFP generation in the HAZ, refinement of the structure, and improvement of HAZ toughness will be described below.

IFP mainly has a particle size of 0.1 to 3. A few percent in Otlm
Ti2O3, Ti3O5 titanium oxides containing Mn as a solid solution, and these oxides and TiN.

It is produced from a MnS complex and a TiN+MnS complex.

If the particle size is less than 0.1 μm, the IFP generation effect is extremely weak, and if the particle size exceeds 3.0 μm, although it has IFP generation ability, it tends to become a point where destruction occurs, and H
This results in a decrease in AZ toughness.

Regarding the number of particles, Ti oxide and TiN+Mn
If the number of particles of the S complex is small, sufficient IFP cannot be generated in the HAZ, so it is necessary to have 3 x 103/- or more of each particle.

As the number of particles increases, the number of IFPs also increases,
Excessive presence of more than 1 x 106 particles/mm for each of these particle numbers tends to cause a decrease in the ductility of the base metal and weld zone, so the upper limit of the particle number is 1 x 106 particles/+n.
Must be m.

The above method for producing Ti oxides in steel is to pre-deoxidize the dissolved oxygen concentration of molten steel to 10 to 60 ppm, then perform T1 deoxidation, and cool the molten steel at a cooling rate of 20 to 400°C/m during solidification.
It is obtained by casting in. Also, TiN+Mn
The S composite was heated at 1100-800° during cooling after casting.
C range at a cooling rate of 2° C./sec or less.

If these formation conditions are deviated from, for example, if the [○] concentration before T1 deoxidation exceeds 60 ppm, even if other conditions are met, Ti oxides will become coarse grains and become the starting point of brittle fracture, resulting in poor toughness. does not improve.

Further, in a steel ingot manufactured under such limited conditions, the effect of the compound is not affected by the thermal history during the subsequent normal steel manufacturing process.

[Example] Table 1 shows the chemical composition values and the degree of dispersion of Ti oxide and composite compounds of the prototype steel.
We have even produced prototypes of 0 kg class steel. The prototype material is 30++ by rolling.
+ m steel plate, plate thickness 1/4 t to 12 x 12 x 60 m
A test piece of m was taken, and H
AZ toughness was evaluated.

In the welding reproduction thermal cycle test, the central part of the test piece was rapidly heated to 1400°C by high-frequency induction heating, and then cooled from 800°C to 500°C for a cooling time of 161 seconds.

This condition corresponds to a welding heat input of 130 KJ/cm, and a heating temperature of 1400° C. corresponds to the heating area near the actual melting edge of the HAZ.

Furthermore, the toughness was evaluated by processing this test piece into a 2 mm V-notch Charpy and determining the impact fracture transition temperature (vTrs).

Table 2 shows the mechanical properties of the steel materials.

Steels 1 to 6 have a hardening phase of M even in the welding simulated thermal cycle.
* indicates components that are hardly produced. Also, T1 oxide,
The density of the composite compound fully satisfies the range of the present invention, and this steel material was prototyped to determine the appropriate range of N.

゛ As is clear from Figure 1, when the N content is 10
Steel 1, a comparative steel with a ppm or less, has low solute N but contains TiN.
vTr
s increases and HAZ toughness deteriorates.

In addition, in steel 6 where the N content exceeds 40 pprtl, TiN
Although TiN is present, there is also a large amount of TiN that decomposes, and the toughness of the matrix decreases due to the increase in solid solution N, resulting in deterioration of HAZ toughness. In comparison, Steels 2 to 5, each of which has a N content within the range of the present invention, exhibit high toughness.

Steels 7 to 12 have N content, Ti oxide, and composite compound density within the requirements of the present invention, but Cu.

N1 is added and is a component that generates the hardening phase M* in the welding reproduction thermal cycle. In particular, since the addition of Si tends to generate M*, its influence was studied and an appropriate range was determined.

As shown in Figure 2, steel 7 with extremely reduced Si is M
Although * decreases, HAZ toughness deteriorates. The cause of this is thought to be due to the interaction between solid solution C1N and Si, which is related to the toughness of the matrix, but it is unclear.

In Steel 11.12, in which Si is outside the range of the requirements of the present invention, M* increases as the Si amount increases, and the HAZ toughness significantly decreases.

Furthermore, in order to clarify the characteristics of the steel of the present invention, a comparative steel 1 whose N content is within the range of the requirements of the present invention, but the density of T1 oxide and composite compound is low and outside the range of the requirements of the present invention.
3 to 17 steels were used to investigate the influence of Si.

This is shown in Fig. 2, superimposed on the invented steel.

As is clear from the figure, N in the comparative steel.

Even when both the Si contents are within the requirements of the present invention, vTrs is high<HAZ toughness is decreased. This is IFP
The density of the core Ti oxide and composite compound is extremely low.
This is because no FP tissue is generated.

That is, when all three requirements of the steel of the present invention are satisfied, -80'C (vT rs = -85
This makes it possible to produce low-temperature steel materials with excellent weld toughness that can be used at temperatures up to 30°F (°C).

[Effects of the Invention] The present invention makes it possible to manufacture steel with excellent welded HAZ low-temperature toughness, making it suitable for steel materials used in extremely low-temperature environments such as the North Sea, such as marine structures, line pipes, and low-temperature vessels. Can be applied. As a result, industrial effects such as ensuring the safety of structures and economic effects due to improved welding performance are extremely significant.

[Brief explanation of the drawing]

Figure 1 is a diagram of N content and vTrs('C) value, Figure 2
The figure is a chart of S1 content and vTrs('C) value.

Claims (1)

[Claims] 1. C: 0.02-0.18%, Mn: 0.4-2.0%, S: 0.0007-0.0060%, Ti: 0.005-0.005% by weight. 0.030%, limited to Al < 0.003%, P < 0.015%, further N: 0.0010 to 0.0040%, Si: 0.03
~0.25%, the remainder consists of Fe and unavoidable impurities, mainly Ti_ with a solid solution of several percent Mn with a particle size of 0.1 to 3.0 μm.
The sum of titanium oxides of 2O_3, Ti_3O_5 and complexes of these oxides with TiN and MnS, and TiN+
MnS complex and 3×10^3 to 1×10^6, respectively.
A strong, high-strength steel with excellent weld toughness, characterized by simultaneously containing a number of particles in the number range of 0.035 mm/mm^3. 2. C: 0.02-0.18%, Mn: 0.4-2.0%, S: 0.0007-0.0060%, Ti: 0.005-0.030% in weight%. Contains, limited to Al < 0.003%, P < 0.015%, especially N: 0.0010~0.0040%, Si: 0.03~
In addition to these, Ni < 3.0%, Cu < 1.5%, Nb < 0.05%, V < 0.1%, Cr < 1.0%, Mo < 0. .5%, B < 0.002%, the remainder consists of Fe and unavoidable impurities, and contains several % of Mn with a particle size of mainly 0.1 to 3.0 μm. Solid-dissolved Ti_
TiN
+ MnS complex, respectively, 3 × 10^3 ~ 1 × 10^
A strong, high-strength steel with excellent weld toughness, characterized by simultaneously containing particles in a number range of 6 particles/mm^3.