JP3225788B2 - Method for producing steel with excellent toughness in weld heat affected zone - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing steel requiring toughness in a heat affected zone such as a line pipe.

[0002]

2. Description of the Related Art In recent years, large heat input of welding has been performed for the purpose of streamlining the welding process. Generally, when welding steel materials, crystal grains of a base material side heat affected zone (hereinafter, referred to as "HAZ zone") are formed. Is known to be coarse and the toughness is reduced. On the other hand, if fine particles such as appropriate oxides and nitrides are dispersed, the structure becomes finer, which is effective in improving the toughness of the HAZ.

Therefore, in a series of patents including Japanese Patent Publication No. 5-17300, the amount of Al in steel is reduced, and fine Ti-based oxides are precipitated and dispersed by adding Ti to change the structure of the HAZ portion. A method for improving toughness by miniaturization has been proposed. In order to further refine the precipitated particles,
No. 7467 proposes a method of controlling the cooling rate during solidification, and Japanese Patent Application Laid-Open No. 4-6243 proposes a method of defining the time from the addition of Ti to the tapping. Also, JP-A-4-27
No. 13 and JP-A-3-177535 further disclose Zr, Y
It is described that the addition of the above is effective for achieving fine and dispersed precipitate particles.

As described above, a method for improving HAZ toughness by dispersing fine particles in steel is known, and there are many disclosures that Ti-based oxides are suitable as fine dispersed particles. However, it is a fact that steels in which these Ti-based oxides are dispersed cannot sufficiently meet the performance requirements of materials, which have been increasing year by year.
A material capable of improving the Z toughness and a manufacturing method thereof have been required.

Also, in order to improve the HAZ toughness, it is considered important to disperse MnS in the steel, and JP-A-5-271864 and JP-A-5-271864 disclose a method of defining an oxide which is likely to be a nucleus of precipitation. It is disclosed in JP-A-5-255801. These include, as specific oxides, Si
O ₂ -MnO or Al ₂ O ₃ -MnO has been proposed.

[0006] On the other hand, the present applicant has disclosed Japanese Patent Application Laid-Open No. Hei 3-162592.
Proposes a steel in which particles having an Al-Mn oxide phase and an unavoidably coexisting oxide phase are dispersed. In this steel, dispersed particles having a diameter of 0.2 to 20 μm have a diameter of 1 in the cross section of the steel material.
^{4 to} 1000 particles per mm ² are dispersed, and (Al + Mn) is 4 in atomic ratio as an oxide phase constituting dispersed particles.
Oxide-dispersed steel having an Al-Mn oxide phase as a component having a characteristic of 0% or more and an Al: Mn ratio of 1: 1 to 5: 1, which is dramatically higher than conventional steel. Toughness is obtained. In this steel, the oxide itself is intended to be the nucleus for the formation of the intragranular ferrite phase, and is essentially different from the oxide-dispersed steel in which the aforementioned oxide is intended to be the precipitation nucleus of MnS. Things.

Further, as a method for producing an oxide-dispersed steel having such an Al-Mn oxide phase as a constituent element,
In the specification and drawings of Japanese Patent Application No. 6-141960, the present applicant describes a method for adding an alloy containing Al,
In the specification and drawings of Japanese Patent Application No. 6-141961,
A method of adding an oxide capable of controlling the oxygen potential has been proposed. These methods are excellent in that an oxide having an Al-Mn oxide phase as a constituent element can be obtained stably, but since the amount of Ti added is not properly defined, depending on the amount of Ti added, There was a problem that the target oxide was not generated.

On the other hand, as a method for controlling the content of components for controlling such finely dispersed particles in steel, there is a method described in JP-A-2-175815. In this method, from the stoichiometric point of view, the amount of Ti, N, and O is defined in a predetermined range so as to be balanced, and at the time of solidification, the cooling rate after solidification is specified to make the dispersed particles finer. Met. However, this method does not attempt to control the morphology of the oxide, and the generation of TiN is essential to ensure good toughness.

[0009]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and is intended to provide an oxide capable of obtaining high HAZ toughness by controlling the form of the oxide in steel. It is an object of the present invention to provide a method for stably producing a steel material in which is dispersed.

[0010]

Means for Solving the Problems The present inventors conducted a series of studies on the correlation between the morphology of finely dispersed particles and the HAZ toughness by varying the conditions of preliminary deoxidation, the amount of dissolved oxygen, and the amount of deoxidizing element added. It has been found that the HAZ toughness is dramatically improved if finely dispersed particles having a more appropriate form are dispersed in steel. Furthermore, it has been found that it is important to appropriately define the chemical composition of the steel and the form of the finely dispersed particles in the smelting process in order to form the finely dispersed particles in this form.

[0011] The steel material of the present invention having excellent toughness in the heat affected zone is
The manufacturing method is based on the above findings.
Ri, in weight%, C: 0.01~0.25% Si: 0.05~0.6% Mn: 0.3~3.0% P: 0.03% or less S: 0.01% or less Ti: 0.005 to 0.03% O: 0.0010 to 0.0070%, Cr: 1.0% or less, Mo: 0.7% or less, Cu: 2.0% or less, Ni: 2.0% or less, Nb: 0.0
8% or less, V: 0.1% or less, B: 0.002% or less, and Si and Mn are added in advance to produce a carbon steel containing Fe and unavoidable impurities. Pre-deoxidation, and dissolved oxygen concentration 20 ~ 100ppm
And the Al concentration in the molten steel is 0.0001 to 0.003.
After controlling to 0%, Ti is added to obtain a steel having a composition satisfying the relationship of 0.5 <[Ti mass%] / [O mass%] <4.0, and Al-Mn oxide phase and Ti The particles containing the oxide phase are dispersed in the steel.

[0012]

The present invention will be described below in detail. The present inventors smelt steel in which the pre-deoxidation conditions, dissolved oxygen amount, and deoxidized element addition amount are variously changed, and investigate the dispersion state and form of finely dispersed particles in the steel, and further, the actual joint performance. A series of experiments were performed. As a result, it was found that the actual joint performance was correlated with the dispersion state and form of the finely dispersed particles in the steel, and that the dispersion state and form of the finely dispersed particles in the steel were changed by controlling the steel composition. Furthermore, finely dispersing the oxide in the appropriate form can provide a steel material with extremely good actual joint performance, and this oxide can be obtained stably by appropriately controlling the molten steel component and the Ti addition amount before adding Ti I found that. The present invention has been made based on these findings.

As described above, it is effective to disperse particles of an appropriate form in steel in order to refine the HAZ structure and improve toughness. As described above, Ti-based oxides are effective as finely dispersed particles. Some are well known. further,
As described above, JP-A-3-162592 discloses that a steel in which particles having an Al-Mn oxide phase and an unavoidably coexisting oxide phase are dispersed significantly improves the performance of a steel in which a Ti-based oxide is dispersed. It is disclosed that superior toughness can be obtained. Specifically, the oxide particles have a diameter of 0.2 to 20 μm, a metal element molar fraction of (Al + Mn) of 40% or more, and an Al: Mn ratio of 1: 1 to 5: refers to those containing Al-Mn oxide phase has a feature that 1 as a component, such particles are 1 mm ² per 4 to the steel section
It is necessary that 1000 are dispersed.

The inventors of the present invention have conducted a series of experiments to finely disperse particles having an Al-Mn oxide phase and an unavoidably coexisting oxide phase in steel. It has been found that it is important to control the morphology of oxides dissolved in water. That is, Mn,
It is necessary to adjust the total oxygen concentration by performing preliminary deoxidation by adding Si, and to generate MnO—SiO ₂ as a deoxidation product. Part of this MnO—SiO ₂ floats and separates, but fine ones remain in the molten steel. Thereafter, the morphology of this oxide is changed by the addition of an appropriate amount of a stronger deoxidizing element. By adding an appropriate amount of an appropriate deoxidizing element, particles having an Al-Mn oxide phase and an unavoidable oxide phase can be finely dispersed in steel.

In the present invention, Si added at the time of preliminary deoxidation
And the concentration of Mn need to be defined as follows from the viewpoint of control of the deoxidation product form and performance of the obtained steel. In other words, the upper limit is 0.6% because excessive addition of Si promotes the formation of island martensite in the HAZ. On the other hand, Mn is an element necessary for securing the strength of the base material and the HAZ portion, so the lower limit is set to 0.3%. However, since excessive addition lowers the toughness of the HAZ portion, it is 3.0%. Is the upper limit. Further, in order to set the dissolved oxygen concentration to 20 to 100 ppm, and to form MnO-SiO ₂ which is advantageous for efficiently preliminarily deoxidizing the oxide form and forming a core for forming an Al-Mn oxide phase, And the addition amount of Mn is expressed by weight ratio of Si: 0.
0.5-0.6% and Mn: 0.3-3.0% are required. The dissolved oxygen concentration after preliminary deoxidation is 20pp
If it is less than m, the amount of oxide nuclei to be dispersed becomes insufficient,
If it exceeds 100 ppm, the cleanliness of the molten steel becomes insufficient.

Further, regarding the amount of Mn added,
Reliably contain a Mn rather than including SiO ₂ and MnO-SiO _2,
In order to ensure the dissolved oxygen concentration of 20 to 100 ppm, it is more preferable to control the concentration to 0.8 to 2%. As for Si, when the Si concentration in steel exceeds 0.2%, the low-temperature toughness is deteriorated.

When a stronger deoxidizing element is added to the molten steel that has been subjected to such preliminary deoxidation, Mn and Si in the deoxidized product are replaced by the deoxidizing element and become particles that are finely dispersed in the steel. .

Therefore, in order to obtain particles in the form of Mn remaining in the oxide phase, Mn in all the deoxidized products must not be replaced at the stage where the deoxidizing element is added. In things
It is necessary to leave Mn. That is, O in molten steel is Ti
It only has to be present in excess with respect to the amount. Therefore,
As will be described later, as a result of examining the dissolved oxygen content, the amount of the deoxidizing element added, and the composition of the deoxidation product, if the value of [Ti mass%] / [O mass%] is less than 4.0, the deoxidation product It was found that Mn was contained therein. Ti ₂ O ₃ in the steel oxides of Ti in stoichiometric
In order to make O excessive with respect to the amount of Ti, it is necessary that [Ti mass%] / [O mass%] be 2.0 or less. It is considered that Mn is contained in the deoxidized product if it is less than 4.0 because of its solid solubility limit and interaction. On the other hand, as for the lower limit, when the amount of Ti added is small, Si of MnO—SiO ₂ generated during preliminary deoxidation is not sufficiently replaced by Ti, and Si remains in the dispersed particles in the steel ingot. Such particles do not provide sufficient toughness and result in insufficient deoxidation of the molten steel. Therefore, it was found that if it is larger than 0.5, dispersed particles of an appropriate form can be obtained.

Since Al is a very strong deoxidizing element, it is preferentially contained in the deoxidized product to become Al2O3. Therefore , Al
By defining the concentration to be 0.0001 to 0.0030% by weight, the proportion of fine particles mainly composed of Al ₂ O ₃ and those mainly composed of Ti ₂ O ₃ is reduced, and Al-Mn Since particles having an oxide phase and an unavoidably coexisting oxide phase are stably formed, better toughness can be obtained. As described above, since Al also forms an oxide in steel, the Al content should be originally considered in the relational expression defining the Ti / O ratio, but in this experiment, Al = 0 It has been found that there is no need to consider it in the component region of less than 0.01%.

In addition, since Al is transferred from the added elements, slag, various refractories, etc. into the molten steel during the refining period, a small amount is inevitably contained in the molten steel even if it is not particularly added. Japanese Patent Application No. Hei 6-14
Description and drawings of 1960, or Japanese Patent Application No. 6-141
No. 961 discloses and discloses that it is effective to intentionally add an alloy or oxide containing Al.

In the prior art, it is often stated that good toughness can be obtained when MnS is present around a Ti-based oxide. However, in the method of the present invention, contribution by a finely dispersed form is obtained. Overwhelmingly large, adhered to oxide
MnS is not required.

Next, the reasons for limiting the content of other elements in the present invention will be described. C is an element necessary to secure the strength of the base material and the HAZ portion, and about 0.01% is necessary to obtain these effects when Nb or V is added, so that the lower limit is set. . However, an excessive amount adversely affects toughness, so the upper limit was made 0.25%.

P is an impurity inevitably contained in steel and is an element that segregates at the grain boundary and causes cracking. Therefore, the upper limit is set to 0.03%. S is also an impurity unavoidably contained in steel, and if present in excess, causes welding cracks, so the upper limit was made 0.01%.

Cr, Mo, Cu, Ni, Nb, and V are elements that can obtain good strength and toughness by adding one or more of these, but when any of them is added in excess, Deteriorates the toughness of the HAZ part,
% Or less, Mo is 0.7% or less, Cu is 2.0% or less, Ni is 2.0% or less, Nb is 0.08% or less, and V is 0.1% or less. Since these elements do not affect the dispersion of fine particles that affect toughness, they are appropriately added in accordance with the requirements of the material properties.

B is usually an element effective for improving the strength of the base material, but has an adverse effect on the toughness of the HAZ.
However, in the case of the steel in which the oxide is dispersed as in the present invention, the addition of a small amount can improve the toughness of the HAZ. For this purpose, an appropriate addition amount is 0.00005 to 0.5.
Although it is 0004%, if the purpose is to secure the strength of the base material in addition, 0.0004 to 0.002% is appropriate.

Even if the contents of Cr, Mo, Cu, Ni, Nb, V, B, etc. are changed, these elements do not affect the morphology of the oxide contributing to the improvement of the toughness. It goes without saying that the method is effective.

The method of the present invention is characterized in that [Ti mass%] / [O mas
By controlling the value of [s%], it is intended to control the form of oxides present in the steel, and it goes without saying that it is effective even if the production equipment and scale are changed. In addition, it is clear that the particle size changes depending on the cooling rate during solidification of steel, and the finer the cooling rate is, the higher the cooling rate is. However, in the case of the method of the present invention, the contribution of the oxide form to the low-temperature toughness is large. It goes without saying that good results can be obtained in any of the usual manufacturing processes such as continuous casting and static casting, and in other manufacturing processes.

[0028]

EXAMPLES The effects of the method for producing a steel material according to the present invention will be described below based on examples and comparative examples. We have 150
Steels of various compositions were smelted using a high-frequency vacuum melting furnace of kg, and preliminary deoxidation conditions, dissolved oxygen content, deoxidized element content and composition of deoxidized products were examined. The initial oxygen concentration is 0.04
~ 0.07% molten steel at 1550-1650 ° C
Preliminary deoxidation was performed by adding Mn and Si, and after confirming the total oxygen concentration in the molten steel, a predetermined amount of Ti was added. Vacuum degassing or addition of oxide powder was performed as necessary to adjust the total oxygen concentration in the molten steel. Further, in order to control the Al content, a small amount of Al was added, an oxide powder was added, or an alloy element containing Al was added as necessary. At the time of smelting these steels, samples were appropriately collected and the form, size, etc. of the oxides present in the molten steel were investigated.

After the obtained ingot is rolled, the HA is reproduced.
A Z test was performed, a JIS No. 4 Charpy test piece was prepared, and an impact test was performed. Tests were conducted at various heat input and test temperatures, but a typical value was obtained for the absorbed energy when the HAZ test was reproduced at a heat input equivalent to SAW = 20 kJ / cm and the impact test was performed at -30 ° C. Therefore, evaluation was made based on the absorbed energy under these conditions.

The number of dispersed particles in the steel was counted with an optical microscope, and the composition was examined with an energy dispersive X-ray microanalyzer. In the analysis of the composition, the amount of Fe considered to be the influence of the base material was subtracted from the analysis value of the metal element contained in the oxide, and the other metal elements were converted as if they existed as an oxide.

[Examples] [Timass%] / [Omass%] obtained from the pre-deoxidation conditions and the component analysis values of the steel ingot in Examples of the present invention.
Is shown in Table 1, and the form of the oxide, the results of the impact test and the like are shown in Table 2. Table 3 shows [Ti mass%] / [O mass%] obtained from the preliminary deoxidation conditions and the component analysis values of the steel ingot in Comparative Example.
Table 4 shows the form of the oxide, the results of the impact test, and the like.

[0032]

[Table 1]

[0033]

[Table 2] Note) Regarding the oxide form, ○ indicates that the particles have an Al-Mn oxide phase and an oxide phase that inevitably coexists. Regarding the number of particles, ○ indicates that there are 4 to 1000 particles having a diameter of 0.2 to 20 μm per 1 mm ^{2 of the} cross section of the slab.

[0034]

[Table 3]

[0035]

[Table 4] Note) Regarding the oxide form, ○ indicates that the particles have an Al-Mn oxide phase and an oxide phase that inevitably coexists. Regarding the number of particles, ○ indicates that there are 4 to 1000 particles having a diameter of 0.2 to 20 μm per 1 mm ^{2 of the} cross section of the slab.

In Comparative Examples 1 and 2 in which [Ti mass%] / [O mass%] was 4.0 or more, Mn was scarcely contained in the oxide. Was the center, and in Comparative Example 2, the Ti-based oxide was the center. In addition, [Ti ma
In Comparative Example 3 in which [ss%] / [O mass%] was 0.5 or less, Si was also present in the oxide .

[0037] controls the Al content in strictly, the Al-Mn oxide phases and Ti particles containing oxide phase dispersed in the steel in Example 1, 2, oxide forms of the intended It was dispersed in the steel, and very good performance was obtained in terms of joint toughness.

FIG. 1 shows the results of an investigation of the joint toughness of steel under the conditions that are substantially equal to the pre-deoxidation conditions, including these results.
When 0.5 <[Ti mass%] / [O mass%] <4.0, it is clear that toughness is significantly improved. The morphology of the oxide in the steel is clearly correlated with [Ti mass%] / [O mass%], and when it is less than 0.5, Si is present in the oxide, and 4.0. In the above case, Mn does not exist in the oxide, and the oxide becomes an oxide of Ti or an oxide of a metal element inevitably contained with Ti-Al. Further, as is apparent from FIG. 1, remarkably high performance is obtained in the range of 1.5 <[Ti mass%] / [O mass%] <3.5.

Regarding the form of the oxides present in the molten steel after the pre-deoxidation, MnO-SiO ₂ was present in all cases except when the pre-deoxidation was performed using only Si and when only Mn was used. . In Comparative Example 4 in which preliminary deoxidation was performed only with Si, MnO-Si was added before adding Ti.
Only SiO ₂ was generated instead of O ₂ , and even after Ti addition
Sufficient performance could not be obtained because Ti, Al, and Si oxides were generated. On the other hand, in Comparative Example 5 in which preliminary deoxidation was performed only with Mn, deoxidation was insufficient, and good performance could not be obtained due to the presence of bubbles in the steel ingot. Therefore, in the method of the present invention, it is essential to carry out preliminary deoxidation by adding both Si and Mn.

In Example 6 in which the content of Si was 0.41%, sufficient absorption energy was obtained, but the value was slightly lower. In Example 7 in which Mn was 0.52%, the proportion of Mn in the fine oxide in the steel was slightly lower, and the value of the absorbed energy was correspondingly slightly lower.

Regarding the dissolved oxygen content after the pre-deoxidation, almost no oxide was found in the steel in Comparative Example 6 in which the dissolved oxygen content was less than 20 ppm, while in Comparative Example 7 in which the dissolved oxygen content exceeded 100 ppm, the molten steel was not cleaned. The properties were insufficient, and large inclusions of 10 μm or more were present in the steel in large quantities. Therefore, in order to maintain the cleanliness of the steel and to disperse fine oxides, it is clear that the dissolved oxygen concentration needs to be 20 to 100 ppm as in the present invention.

As described above, a method of finely dispersing an appropriate oxide to finely disperse MnS is also known. Therefore, a steel in which the precipitation ratio of MnS on the oxide was changed by changing the S concentration was intentionally manufactured, and the same investigation was conducted.
In other Examples and Comparative Examples, S = 0.001 to 0.005
%, Whereas in Example 8 where S was reduced to 0.0005%, almost no MnS was precipitated on the oxide, but the same toughness as in the other examples could be obtained. Therefore, it was found that the number of MnS particles hardly affected the performance of the steel material in the method of the present invention.

In Comparative Example 8 containing 0.01% or more of Al, fine oxidation in steel was performed despite pre-deoxidation conditions and [Ti mass%] / [Omass%] being controlled within a predetermined range. Most of the materials were Al 2 O 3 , and sufficient toughness was not obtained.

It is known that the low-temperature toughness of the heat affected zone is affected by the N content, and the effect of N was investigated. In Example 9 , N = 20 to 40 pp for all other steel materials
m, but increased to 55 ppm, but the same performance as in the other examples was obtained, and it was found that the N content did not affect the toughness in the method of the present invention.

[0045]

As described in the foregoing, in the manufacturing method of the steel of the present invention, in producing a defined steel in a predetermined content such as, A preliminary deoxidation by adding previously Si and Mn, soluble After controlling the oxygen concentration to 20 to 100 ppm and the Al concentration in the molten steel to 0.0001 to 0.0030%, Ti is added, and 0.5 <[Ti mass%] / [O mass %] <a 4.0 composition of the steel satisfies the relationship, is et to Al as the dispersed particles
-Since the particles containing the Mn oxide phase and the Ti oxide phase are dispersed in the steel, it is possible to stably produce a steel in which the oxide having the Al-Mn oxide phase in which high HAZ toughness is obtained is dispersed. it can.

[Brief description of the drawings]

FIG. 1 shows [Ti mass%] in Examples and Comparative Examples of the present invention.
It is a figure which shows the result of having investigated the relationship between / [O mass%] and joint toughness (absorbed energy).

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-100924 (JP, A) JP-A-6-293936 (JP, A) JP-A-6-293937 (JP, A) JP-A-7-1997 242985 (JP, A) (58) Fields surveyed (Int. Cl. ⁷ , DB name) C22C 1/00-49/14 C21C ⁷ /00-7/06

Claims (1)

(57) [Claims] C: 0.01 to 0.25% Si: 0.05 to 0.6% Mn: 0.3 to 3.0% P: 0.03% or less S: 0. 01% or less Ti: 0.005 to 0.03% O: 0.0010 to 0.0070%, Cr: 1.0% or less, Mo: 0.7% or less, Cu: 2.0 % Or less, Ni: 2.0% or less, Nb: 0.0
8% or less, V: 0.1% or less, B: 0.002% or less, with the balance being Fe and unavoidable impurities. To carry out preliminary deoxidation to bring the dissolved oxygen concentration to 20 to 100 ppm and the Al concentration in the molten steel to 0.00
After controlling to 0.01 to 0.0030%, Ti is added to obtain a steel having a composition satisfying the relationship of 0.5 <[Ti mass%] / [O mass%] <4.0, and Al-Mn oxide is used as dispersed particles. A method for producing a steel material having excellent weld heat affected zone toughness, characterized in that particles containing a material phase and a Ti oxide phase are dispersed in steel.