JPH0757886B2 - Process for producing Cu-added steel with excellent weld heat-affected zone toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a method for producing a high-strength steel excellent in low temperature toughness of a heat affected zone (HAZ) from small heat input welding to large heat input welding. In the steel industry, it is most preferable to apply it to thick plate mills, but it is also applicable to hot coils, shaped steels, and the like. Further, the thick steel plate manufactured by this method can be used for a welded steel structure used in a severe environment such as a pressure vessel, an offshore structure, and a line pipe.

(Prior art) HZA toughness of low alloy steel is (1) grain size, (2)
High carbon island martensite (M *), dispersed state of hardened phase such as upper bainite (Bu), (3) Presence or absence of grain boundary embrittlement,
(4) Controlled by various metallurgical factors such as elemental microsegregation. In particular, it is known that the size of HAZ crystal grains greatly affects low temperature toughness, and many technologies have been developed and put to practical use for refining the HAZ structure.

Recently, fine Ti oxides (mainly Ti ₂ O ₃ ) have been dispersed in steel, and during cooling after welding, radial formation of acicular ferrite (AF) with Ti oxides as nuclei has made the HAZ structure finer. A method for doing so has been developed (Japanese Patent Application Laid-Open No. 61-79745, Japanese Patent Application No. 62-168577, etc.).

With this technology, since the Ti oxide is stable at high temperatures of 1400 ° C or higher, it is possible to control the HAZ structure up to near the melting line.
Excellent low temperature toughness compared to steel. But,
HAZ toughness is, of course, greatly affected by the basic constituents even in the steel in which Ti oxide is dispersed.

In other words, the conventional Ti oxide-containing steel has a tensile strength of 55 kgf / mm.
It was difficult to obtain high strength steel of ² or more, high heat input welding, and stable HAZ toughness at low temperatures.

(Problems to be solved by the invention) The present invention finds an optimal basic component for a Ti oxide-containing steel,
It has succeeded in remarkably improving its strength and low temperature toughness, and provides a technique for inexpensively producing high-strength steel excellent in HAZ toughness in small to large heat input welding. The steel manufactured by the method of the present invention is a welded part (joint part, HAZ).
Has a finer structure and exhibits excellent low temperature toughness over the entire area.

(Means for Solving the Problems) The gist of the present invention is C: 0.03 to 0.12% and Si: 0.6% by weight.
Below, Mn: 0.8-2.0%, P: 0.025% or less, S: 0.005% or less, Cu: 0.8-1.5%, Al: 0.003% or less, Ti: 0.005-0.025
%, N: 0.001 to 0.0065%, O: 0.001 to 0.006%, particle size
An oxide containing 0.05 to 10 μm of Ti as the main component is added to 5 × 10 ³ to 1
× 10 ⁶ pieces / mm ³ , containing the balance of iron and unavoidable impurities, or Ni, 0.05 to 1.0
%, Nb: 0.005 to 0.05%, Cr: 0.05 to 0.30%, Mo: 0.05 to 0.
30%, 1 type of Ca: 0.001 to 0.005% or Ni: 0.05 to 1.0
%, Nb: 0.005 to 0.05%, Mo: 0.05 to 0.30%, Ca: 0.001 to
Steel containing 2 kinds of 0.005% is made into a slab by continuous casting method, reheated in the temperature range of 900 to 1100 ° C, and cumulative rolling reduction of 900 ° C or less 30 to 90%, rolling is completed. Roll at a temperature of 700 to 850 ℃, let cool or water cool, then 50
The aging treatment is performed at a temperature of 0 ° C to ₁ point of Ac.

(Operation) Hereinafter, the present invention will be described.

According to the research conducted by the inventors, the HAZ toughness largely depends on 1) chemical composition of steel, 2) structure (size of crystal grains and distribution state of hardening phase), and optimization of steel components and fine graining of resulting grains Was considered to be essential for high toughness.

As a method of refining the HAZ structure, a technique of finely dispersing Ti oxide (mainly Ti ₂ O ₃ ) in steel has already been developed (Japanese Patent Application No. 59-203099 and Japanese Patent Application No. 62-168577).
No.).

According to this method, finely acicular ferrite (AF) is radially generated with Ti ₂ O ₃ reprecipitated in the γ grains during the γ-α transformation of HAZ as a nucleus, and the HAZ structure becomes extremely fine.

Further, Ti ₂ O ₃ is stable without melting even in a region (coarse grain region HAZ) heated to a high temperature near the melting line, and the structure becomes fine in this region as well. Furthermore, if Ti, O, and N are properly balanced, fine TiN is also produced, which is generated by HAZ heated to 1350 ° C or lower.
(Sub-coarse grain region) The HAZ structure is refined by suppressing the coarsening of γ grains. As a result, the welded part is refined over the entire area, and excellent low temperature toughness is obtained.

In order to obtain the effect of refining the HAZ structure by the Ti oxide as described above, it is necessary to uniformly disperse the oxide of appropriate size in the steel. Particle size of Ti oxide is 0.05-10μ
m is suitable. If the diameter is 0.05 μm or less, the AF generation ability becomes weak, and if the diameter is 10 μm or more, the Ti oxide itself becomes the starting point of brittle cracking and deteriorates the HAZ toughness.

On the other hand, in order to uniformly refine the HAZ structure, the number of Ti oxides needs to be 5 × 10 ³ to 1 × 10 ⁶ / mm ³ . If the number of particles is small, a uniform and fine HAZ structure cannot be obtained, and if the number of particles is too large, the cleanliness of the steel deteriorates and not only the HAZ,
The low temperature toughness of the base material also deteriorates.

In order to finely disperse Ti ₂ O ₃ and TiN in steel in the ordinary steelmaking method, it is essential to optimize the amounts of Ti, O, and N in particular. Therefore, the Ti, O, and N contents are Ti: 0.005 to 0.025% and N: 0.001 to 0.
0065%, O: 0.001 to 0.006% must be limited.

The lower limit of the amount of Ti, O, N is the minimum amount necessary for producing Ti ₂ O ₃ , TiN. This is to prevent the HAZ toughness from deteriorating due to the formation of TiC with the upper limit of Ti amount, and the upper limit of the N amount is HAZ due to the solid solution N.
This is to prevent deterioration of toughness. The upper limit of the amount of O is to prevent the cleanliness and toughness of steel from being deteriorated due to the formation of non-metallic inclusions.

However, even if Ti ₂ O ₃ and TiN are finely dispersed in the steel, excellent toughness cannot be obtained if the basic components are not appropriate. This point will be described below.

As a result of the inventors' research, especially high strength steel (tensile strength 55 kg
f / mm ² or more) and large heat input welding with heat input of 50 kJ / cm or more,
It was found that in order to obtain excellent HAZ toughness in the cryogenic temperature range of -60 ° C or higher, strict and proper selection of basic components is essential.

That is, if the basic components are not appropriate, excellent HAZ toughness cannot be obtained even if fine AF is formed with Ti ₂ O ₃ as the nucleus. It was also found that AF is difficult to generate with Ti ₂ O ₃ as a nucleus.

Therefore, the present inventors set out to develop a new steel capable of dramatically improving the cryogenic toughness of the high-strength steel and the high heat input welded HAZ,
We found that the use of Cu precipitation hardening is effective as a method that can completely solve these problems.

By utilizing Cu precipitation hardening, the hardenability of the basic components can be greatly reduced, but it becomes extremely easy to generate fine AF from Ti ₂ O ₃ . Also, if Cu is added at about 1%,
It was clarified that even if the hardenability of steel is slightly increased, it does not produce a hardened structure detrimental to HAZ toughness.

In order to obtain such an excellent effect of Cu addition, it is necessary to add 0.8 to 1.5%. The upper limit of Cu content is to prevent deterioration of HAZ toughness due to coarsening of Cu precipitates and increase in hardenability. The lower limit is the minimum amount for obtaining sufficient precipitation hardening.

The reasons for limiting the other basic components will be described below.

The lower limit of the C content is 0.03%, which is the minimum amount for ensuring the strength of the base material and the welded portion and exerting these effects when Nb is added. However, if the amount of C is too large, not only the low temperature toughness of HAZ is adversely affected but also the base metal toughness and weldability are deteriorated, so the upper limit was made 0.12%. When the amount of C is large, M * and pseudo pearlite (P ') are generated in the welded portion, and the low temperature toughness is significantly deteriorated.

Si is an element contained in steel for deoxidation, but if added in a large amount, the weldability and toughness of the weld will deteriorate, so the upper limit is 0.6%.
Limited to. Deoxidation of steel is sufficiently possible with Ti alone.
From the viewpoint of preventing the formation of * and improving toughness, 0.15% or less is desirable.

Mn is an essential element for ensuring strength and toughness, and its lower limit is 0.8%. In order to improve the HAZ toughness, it is necessary to prevent the coarse proeutectoid ferrite generated at the γ grain boundary, but the addition of Mn has the effect of suppressing this. However, if the Mn content is too large, not only the hardenability increases, the weldability and HAZ toughness deteriorate, but also the center segregation of the slab is promoted, so the upper limit was made 2.0%.

The reason that the impurities P and S in the steel of the present invention are 0.025% or less and 0.005% or less, respectively, is to further improve the low temperature toughness of the base material and HAZ. A decrease in the amount of P tends to reduce the grain boundary fracture tendency in the HAZ, and a decrease in the amount of S tends to suppress the production of grain boundary ferrite. Most preferred P,
The S content is 0.010% and 0.0020% or less, respectively.

Al is an element generally contained in the deoxidized upper steel, but it is an unfavorable element in the steel of the present invention, and its upper limit was set to 0.003%. This is because when Al is contained in the steel, it combines with O to form Ti
_This is because ₂ O ₃ cannot be done. Deoxidation is also possible with Ti alone. In the present invention, the smaller the Al content, the better, and 0.002% or less is desirable.

Next, the reason for adding any one of Ni, Nb, Cr, Mo and Ca and any two or three of Ni, Nb, Mo and Ca will be described. The main purpose of adding these elements to the basic composition 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 is of a nature that should be limited by itself.

Ni is an important element that improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness.
Is also effective in preventing cracks during hot rolling. However, if the addition amount exceeds 1.0%, the weldability is not preferable, so the upper limit was made 1.0%.

Nb is an important element in the present invention, and it is difficult to obtain excellent joint toughness in high strength steel without adding Nb. Nb has a function of suppressing ferrite generated at the γ grain boundary and promoting generation of fine AF having Ti ₂ O ₃ as a nucleus. To obtain this effect, a minimum Nb content of 0.005% is required. However, if the amount of Nb is too large, the formation of fine AF is adversely affected, so the upper limit is 0.05%.

Cr increases the strength of the base material and the weld, but if it is too much, it deteriorates the weldability and the toughness of the weld. The upper limit is 0.30%.

Mo is an element that improves the strength and toughness of the base metal, but if it is too large, it causes deterioration of the toughness and weldability of the base metal and welded portion like Cr, which is not preferable. The upper limit is 0.30%.

The lower limit of the addition amount of Ni, Nb, Cr, Mo should be the minimum amount to obtain the effect on the material, Ni, Cr, Mo is 0.05%,
Nb is 0.005%.

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, the amount of Ca is 0.
If it is less than 001%, it has no practical effect, and if it is added in excess of 0.005%, a large amount of CaO and CaS are generated and become large inclusions, which not only impairs the toughness of steel but also the cleanliness, and also adversely affects weldability give. Therefore, the range of addition amount is 0.001 to 0.005.
Limited to%.

Even if the components of steel are limited as described above, fine Ti ₂ O ₃ and TiN cannot be dispersed in the steel before welding unless the manufacturing method is appropriate, and the base metal suitable for HAZ properties is used. The characteristics cannot be obtained. Therefore, it is necessary to limit the manufacturing conditions.

First, it is essential that this steel be industrially manufactured by a continuous casting method. The reason for this is that in the continuous casting method, the solidification cooling rate of molten steel is fast and a large amount of fine Ti ₂ O ₃ and TiN is obtained in the slab. In the ingot-segmentation method using a large steel ingot, Ti
It is difficult to finely disperse ₂ O ₃ and TiN in a slab.

In the case of the continuous casting method, the cooling rate differs depending on the slab thickness, but the thickness is preferably 350 mm or less from the viewpoint of weld zone toughness. Furthermore, the reheating temperature of the slab must be 900 to 1100 ° C or lower. When reheated at a temperature higher than this, TiN becomes coarse, and fine TiN disappears in the steel before welding, making it impossible to refine the structure at the weld boundary and HAZ. Base metal toughness cannot be obtained either.

In the present invention, there is no problem even if hot charge rolling or direct rolling is performed.

Next is a rolling method, but proper controlled rolling is essential from the viewpoint of strength and toughness. For this purpose, rolling is performed at a rolling reduction of 900 to ℃ 30 to 90% and a rolling finish temperature of 700 to 850 ℃. If the cumulative rolling reduction is 30% or less, the refinement of the γ structure is insufficient and excellent low temperature toughness cannot be obtained. Generally, the low temperature toughness is improved as the cumulative rolling reduction is increased, but it is impossible to actually roll the rolling over 90%. This is because when the rolling reduction reaches 90% or more, the temperature decrease becomes remarkable with the extension of rolling time.
This is because an appropriate rolling end temperature cannot be secured.

The rolling end temperature of 700 ° C or higher is due to excessive (γ-α)
This is to prevent deterioration of low temperature toughness and anisotropy of material due to zone rolling. However, if the rolling end temperature is too high, sufficient strength and toughness cannot be obtained, so the upper limit was made 850 ° C.

In the present invention, the cooling method after rolling is not particularly limited. This is because it does not significantly affect the properties of the steel of the present invention.
In addition to standing cooling, accelerated cooling and quenching after rolling are also possible.

Next, in the present invention, in order to obtain the Cu precipitation effect, it is necessary to perform aging treatment in the temperature range of 500 ° C. to Ac ₁ point. The reason for limiting the temperature in this way is that sufficient Cu precipitation hardening cannot be obtained at 500 ° C. or lower, and at the Ac ₁ point or higher, transformation starts and the characteristics of the steel of the present invention are lost (age aging). A suitable holding time at temperature is 0 to 60 minutes).

Note that the aging treatment may be performed, for example, by rolling a steel sheet and then working it into a steel pipe, and performing it in a steel pipe state. Further, even if the steel is subjected to heat treatment such as quenching after rolling and aging treatment, the characteristics of the present invention are not impaired.

(Example) Steel plates of various steel components (thickness in the converter-continuous casting-thick plate process
15-60 mm) was manufactured, and the HAZ toughness was investigated by a 2 mm V notch Charpy test after performing latent arc welding under various conditions.

The test piece was taken from the 1 / 4t position, and the notch position was HAZ near the fusion line.

Examples are shown in Table 1 and Table 2.

The steel sheets produced by the method of the present invention (invention steel) all have good base material properties and joint toughness, whereas the comparative steels not produced by the method of the present invention are inferior in base material properties or HAZ toughness and have a severe environment. Not suitable as a steel plate used below.

(Effects of the Invention) According to the present invention, it is possible to manufacture a high-strength steel and a steel having excellent cryogenic toughness throughout the HAZ even in high heat input welding in a large amount at low cost. As a result, the construction efficiency of the welded structure was remarkably improved and its safety was greatly improved.

Claims (4)

[Claims] 1. By weight%, C: 0.03 to 0.12%, Si: 0.6% or less, Mn: 0.8 to 2.0%, P: 0.025% or less, S: 0.005% or less, Cu: 0.8 to 1.5%, Al: 0.003% or less, Ti: 0.005 to 0.025%, N: 0.001 to 0.0065%, O: 0.001 to 0.006%, 5x10 oxides containing Ti with a particle size of 0.05 to 10 μm as the main component
³ to 1 × 10 ⁶ pieces / mm ³ , steel with balance of iron and unavoidable impurities is made into a slab by continuous casting method, reheated in the temperature range of 900 to 1100 ℃, and then cumulative reduction of 900 ℃ or less Amount of 30 to 90%, rolling at the finishing temperature of 700 to 850 ℃, and after leaving to cool or water cooling, 500
A method for producing a Cu-added steel having excellent weld heat-affected zone toughness, which is characterized by performing aging treatment at a temperature of ℃ to Ac ₁ point. 2. By weight%, C: 0.03 to 0.12%, Si: 0.6% or less, Mn: 0.8 to 2.0%, P: 0.025% or less, S: 0.005% or less, Cu: 0.8 to 1.5%, Al: 0.003% or less, Ti: 0.005 to 0.025%, N: 0.001 to 0.0065%, O: 0.001 to 0.006%, 5x10 oxides containing Ti with a particle size of 0.05 to 10 μm as the main component
³ to 1 × 10 ⁶ pieces / mm ³ , Ni: 0.05 to 1.0%, Nb: 0.005 to 0.05%, Cr: 0.05 to 0.30%, Mo: 0.05 to 0.30%, Ca: 0.001 to 0.005% The method for producing a Cu-added steel excellent in weld heat-affected zone toughness according to claim 1, wherein one type of steel, the balance of which is iron and inevitable impurities, is used. 3. In weight%, C: 0.03 to 0.12%, Si: 0.6% or less, Mn: 0.8 to 2.0%, P: 0.025% or less, S: 0.005% or less, Cu: 0.8 to 1.5%, Al: 0.003% or less, Ti: 0.005 to 0.025%, N: 0.001 to 0.0065%, O: 0.001 to 0.006%, 5x10 oxides containing Ti with a particle size of 0.05 to 10 μm as the main component
³ to 1 × 10 ⁶ pieces / mm ³ , Ni: 0.05 to 1.0%, Nb: 0.005 to 0.05%, Mo: 0.05 to 0.30%, Ca: 0.001 to 0.005%, any two types, the balance is iron The method for producing a Cu-added steel having excellent weld heat-affected zone toughness according to claim 1, wherein steel consisting of unavoidable impurities is used. 4. By weight%, C: 0.03 to 0.12%, Si: 0.6% or less, Mn: 0.8 to 2.0%, P: 0.025% or less, S: 0.005% or less, Cu: 0.8 to 1.5%, Al: 0.003% or less, Ti: 0.005 to 0.025%, N: 0.001 to 0.0065%, O: 0.001 to 0.006%, 5x10 oxides containing Ti with a particle size of 0.05 to 10 μm as the main component
³ to 1 × 10 ⁶ pieces / mm ³ , Ni: 0.05 to 1.0%, Nb: 0.005 to 0.05%, Mo: 0.05 to 0.30%, Ca: 0.001 to 0.005%, any three types, and the balance is iron. The method for producing a Cu-added steel having excellent weld heat-affected zone toughness according to claim 1, wherein steel consisting of unavoidable impurities is used.