JPH0499248A - Steel product having superior toughness of weld heat-affected zone and its production - Google Patents

Abstract

PURPOSE:To industrially and stably mass-produce steel products each having superior toughness of the base metal, superior toughness of a deposited metal and superior toughness of the weld heat-affected zone by regulating the amts. of dissolved oxygen and nitrogen in molten steel by predeoxidation, deoxidizing the molten steel with Ti and immediately carrying out continuous casting. CONSTITUTION:Molten steel is predeoxidized to regulate the amt. of dissolved oxygen to 20-80ppm and that of dissolved nitrogen to 30-80ppm, the predeoxidized molten steel is deoxidized with Ti and the resulting molten steel having a compsn. contg., by weight, 0.03-0.20% C, 0.05-0.60% Si, 0.40-2.0O% Mn, 0.003-0.050% Ti, 0.0030-0.0080% N and 0.005-O.0060% O is immediately subjected to continuous casting. Steel products each having superior toughness as mentioned above and a structure contg. 0.001-0.100% oxide type inclusion having a fine Ti (O, N) type composite crystal phase of <1mum grain diameter dispersed in the matrix are stably produced.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a steel material with excellent weld heat-affected zone toughness and a method for manufacturing the same.

<Prior art and its challenges> In recent years, there have been remarkable technological innovations in the production and construction of steel materials, and as a result, steel plates with excellent low-temperature toughness even with a small amount of alloying elements added have not been manufactured. As a result, the performance of various steel structures such as offshore structures, various low-temperature storage containers, ice-water vessels, and line pipes has improved significantly.

However, even with these low-temperature steel materials, excellent toughness at low temperatures can generally be stably ensured only in the base metal, and only in the base metal, where the material changes due to heat effects during welding. (referred to as “welding heat affected zone 1”)
However, it is difficult to say that the low-temperature toughness can always be stably maintained at a good level. For example, thick steel plates for offshore structures tend to have high carbon equivalent values in order to ensure strength, but for this reason, the weld heat affected zone tends to change to a bainite structure, making it necessary to ensure good toughness. is difficult. In addition, in recent years, due to the rationalization of welding work, high heat input welding work with a small number of welding buses has become popular, but in this case, the weld heat affected zone is kept at a high temperature for a long time and is extremely Since it is cooled at a slow rate, the structure becomes coarse and the toughness decreases.

In particular, the embrittlement of the weld heat-affected zone is generally noticeable in the case of extremely high heat input welding of thick steel plates as mentioned above, and in the case of dwarf heat welding. The main cause of embrittlement is coarsening of austenite grains in the weld heat-affected zone near the weld metal, whereas in the latter case, the weld heat-affected zone hardened by welding is altered by the subsequent welding heat. Island martensite or M-A Con5ti
This is said to be due to the formation of a structure called tuent, which is unfavorable in terms of toughness.

In this way, no matter how high the base metal toughness can be obtained through improvements in manufacturing processes such as controlled rolling and accelerated cooling, the weld heat affected zone toughness is determined for each steel material regardless of the base metal toughness. Conventional low-temperature steel materials have not been fully satisfactory for use in welded assembled structures, as they typically fall to a low level of 100%.

Therefore, many steel processing methods have been studied to improve the toughness of the weld heat-affected zone.

For example, in order to prevent deterioration of the toughness of the weld heat-affected zone when performing dwarf heat welding, there has been a proposal to keep the Si content low in order to suppress the formation of island-shaped martensite (Japanese Patent Application Laid-Open No. No. 54-43116, Japanese Patent Publication No. 54-4311
No. 7). This method takes advantage of the fact that since St does not dissolve in cementite, when the amount of Si increases, the formation of cementite is suppressed and it is easy to turn into martensite.
This is an attempt to suppress martensitic transformation by t-oxidation. However, this reduction in St was not very effective in the case of high heat input welding.

Apart from this, N, O, and P in steel.

There is also a proposal aimed at improving the toughness of the weld heat-affected zone by reducing harmful elements such as S as much as possible to improve the toughness of the microstructure itself (Japanese Patent Laid-Open No. 52-5461).
No. 1, JP-A-52-54612, JP-A-61-237
However, although this method is known to be a desirable method from the viewpoint of preventing martensitic formation and is effective in improving the toughness of the weld heat affected zone, the degree of improvement in toughness is not so significant. I couldn't say that it was.

On the other hand, focusing on high heat input welding in particular, there has been a long-standing idea that precipitates, which are stable even in high-temperature ranges, are finely dispersed in the steel to suppress the coarsening of austenite grains in the weld heat-affected zone. , you can see a number of proposals that follow this idea. For example, Tokuko Sho 55-261
No. 64, JP 56-11742, JP 52-73
No. 19.

Special Publication No. 60-10105, Special Publication No. 54-4.3970
No., Special Publication No. 55-31819, Special Publication No. 55-3182
No. 0, Special Publication No. 58-52007, Special Publication No. 59-184
No. 65, Special Publication No. 59-39494 or Special Publication No. 1-1
Proposals related to No. 76016 etc. include finely dispersing stable nitride particles such as ZrN and TiN in steel, and REM (
The aim is to improve the toughness of the weld heat-affected zone by finely dispersing oxides, sulfides, or nitrides such as rare earth elements) in the steel. However, when the dispersed particles are nitrides, most of them are dissolved in areas where the maximum temperature of the weld heat-affected zone exceeds 1350°C, so the effect of preventing crystal grain coarsening cannot be said to be sufficient. . In addition, REM oxides and sulfides are so stable that they come off as slag during welding of steel materials, or even if they remain, they become rather coarse precipitates, so it is necessary to prevent austenite grains from coarsening during welding. could not bring out the full effect.

By the way, the nitrides used in the above proposal are those "generated in a high temperature range close to the melting temperature of the steel or in a molten state," but there are also nitrides that precipitate in austenite at relatively low temperatures. There are also proposals to use particles to suppress deterioration of the toughness of the weld heat affected zone. For example, JP 59-2733, JP 5B-1184, JP 59-45747, JP 60-30724, JP 59-3537, or JP 61-270354.
In order to suppress coarsening of austenite grains during welding, ``BN precipitates that form in the relatively low-temperature region of austenite'' are utilized.

BN is finely precipitated in austenite in the temperature range of 1000 to 1100°C, and this precipitate becomes a ferrite transformation nucleus, promotes ferrite transformation, and has the effect of suppressing the formation of a bainite structure that is harmful to toughness. However,
When the cooling rate is slow, BN mainly precipitates at the austenite grain boundaries, but when the cooling rate is relatively fast, such as during welding, it precipitates finely within the austenite grains, leading to the formation of ferrite from within the austenite grains. encourage. Therefore,
Even if the austenite grain size becomes coarse due to welding heat, a large number of fine ferrites precipitate within the austenite grains during the γ→α transformation, and the structure after transformation becomes finer and at the same time the amount of hardened structure decreases, resulting in the welding heat being affected. This contributes to improving the toughness of the parts.

However, although the use of BN particles is effective in improving the toughness of the weld heat-affected zone, the element B (boron) has the effect of increasing the hardenability of the steel and hardening the hardened structure when dissolved in the steel. It can be said that the addition of B carries the risk of deteriorating toughness. Therefore, when using B, along with the amount of B added, C, + N
It is also necessary to control the optimum amounts of additive elements such as + ri and A1 within a narrow range, and a fairly high degree of manufacturing control is required during mass production.

In this way, conventional weld heat-affected zone toughness improvement technology that utilizes nitrides generally requires detailed control of elements other than nitride-forming elements; A technology that does not require management is desired. The above-mentioned problems pointed out in nitride utilization technology are due to the fact that nitrides tend to dissolve in part or in whole during welding. If a means to finely disperse "precipitates, which are more stable than solids" in steel, could be developed, it would be ideal for suppressing the growth of austenite grains during welding and for refining and softening the structure after T-α transformation. It is thought that.

Compounds that are more difficult to dissolve than nitrides are sulfides and oxides, but these compounds are generally very thermally stable, as represented by oxides and sulfides in REM, so they escape as slag in molten steel. Even if it remains in the steel, it becomes a large inclusion. Therefore, it is very difficult to achieve desired effects with these sulfides and oxides. However, among oxides, TiO has a relatively low stability, so there is a possibility that it will be finely dispersed in steel, and therefore several proposals have been made regarding the use of TiO (Japanese Unexamined Patent Application Publication No. 1983-1972). 190313 No. 2 JP-A-60-245768,
JP-A-61-79745, JP-A-63-210235
No. etc.).

All of these proposals aim to suppress deterioration of the weld heat affected zone toughness by using Ti as a deoxidizing agent when deoxidizing molten steel, thereby finely dispersing Ti0 particles in the steel.
Although relatively good effects could be obtained, in reality there is a limit to the fine dispersion of Ti1, and it is extremely difficult to generate fine TiO as desired that can bring out sufficient effects. It was true.

Therefore, the purpose of the present invention is to exhibit better weld heat-affected zone toughness than conventional steels even when high heat input welding is performed, and at the same time exhibit superior weld heat-affected zone toughness compared to conventional steels. The objective was to stably provide low-temperature steel materials that were as easy to manufacture as possible.

<Means for Solving the Problems> The present invention has been completed based on the results of experiments and research conducted from various viewpoints in order to achieve the above-mentioned objects. , C: 0.03 to 0.20% (hereinafter, percentages representing component proportions are weight percentages).

Si: 0.05-0.60%, Mn: 0
．． 40-2.00%.

Ti: 0.003-0.050%, N: 0
．． 0030~o, ooso%.

0: Contains 0.0005 to 0.0060%,
Alternatively, Cu: 0.5% or less, Ni: 1.0% or less.

Cr: 0.5% or less, Mo: 0.5% or less.

v: o, io% or less, Nb: 0.003
~0.050%.

Ca: It has a chemical composition that contains one or more types of Ca: 0.0050% or less, and the balance is Pe and unavoidable impurities, and has o, ooi to o,
By adopting a structure in which oxide-based inclusions having a grain size of IQQ10Ti (O, N) composite crystal phase are dispersed at a ratio of ioo%, not only sufficient base metal and weld metal toughness can be achieved, but also It is characterized by ensuring excellent weld heat-affected zone toughness, and furthermore, the amount of dissolved oxygen is adjusted to 20 to 80 ppm through preliminary deoxidation, and the amount of dissolved nitrogen is adjusted to 30 to soppm. By continuously casting molten steel by deoxidizing Ti immediately before casting, it is possible to industrially and stably mass-produce steel materials that have excellent base metal toughness, weld metal part toughness, and weld heat-affected zone toughness. It is also characterized by point J.

As mentioned above, the present invention provides Aj! The amount of dissolved oxygen and nitrogen in molten steel should be controlled within a specified range by preliminary deoxidation using St, etc., and immediately before pouring in continuous casting, Tt should be added in a tundish, etc. to deoxidize, and then casting should be performed. A specific material with excellent base metal toughness, weld metal toughness, and weld heat-affected zone toughness is obtained by using Ti oxide as a main component and having fine inclusions of a predetermined grain size dispersed in the base material at a high density. The present invention is characterized by being able to stably provide a steel material with a chemical composition, and the mechanism of improving the toughness of the weld metal part in the steel material of the present invention will be explained below.

When welding steel materials, the weld heat affected zone (HA) near the weld metal
Since Z) is heated by welding heat to just below the melting point of the steel, the austenite grains in general steel become extremely coarse. On the other hand, there is a situation in which the subsequent cooling rate in this part is extremely fast. In this way, in the weld heat affected zone, the hardenability increases due to the large austenite grains, and at the same time the cooling rate is fast, so martensitic transformation or bainite transformation dominates in this area, and generally a hard and coarse structure is formed. This results in a decrease in toughness.

However, in the steel material according to the present invention, fine grains of Ti-based composite oxynitride that do not melt and disappear even with welding heat are dispersed in the matrix at a predetermined high density, and the grain size is regulated to a specific grain size. Oxynitrides change the weld heat-affected zone structure through two actions:

(i) a Fine oxynitride suppresses the growth of austenite grains and prevents their coarsening, resulting in haynite transformation.

By preventing martensitic transformation from easily occurring, the weld heat-affected zone structure is slightly refined and softened.

(ii) During r-α transformation, dispersed oxynitrides act as nuclei and promote ferrite formation, changing the structure of the weld heat-affected zone to a “Widmannian-like structure consisting mainly of ferrite side plates” or “ferrite-pearlite.” change to a state in which the main character is

Therefore, even if high heat input welding is performed, the deterioration of the toughness of the welded heat affected zone assembly is very small, and good welded heat affected zone toughness is maintained.

On the other hand, in the steel material according to the present invention, the content ratio of each chemical component constituting it is also regulated in a well-balanced manner, so that the base metal and weld metal part exhibits excellent toughness comparable to conventional steel. becomes.

It is generally said that the effect shown in item (i) above is most effective if the diameter of the precipitate is 0.02 m or less. However, when the precipitate is an oxide, the particle size: I
It is difficult to make even precipitates of R or less remain in the steel.

Therefore, in the present invention, Ti oxide (Ti
e) At the time of generation, a predetermined amount of N was added at the same time, so that the crystal phase generated was a Ti-〇-N double gold phase. In this case, these composite phases are more difficult to form than TiO and have a slower growth rate, resulting in more finely dispersed precipitation than in the case of TiO alone, and even if the average particle size is 1 m or less. It was found that the effect shown in item (i) above can be utilized, leading to the completion of the present invention.

In addition, the precipitated Ti-based composite oxynitride has a particle size of 0.05
~1 Lord has the highest ferrite nucleation ability, and in order to achieve the effect shown in item (ii) above, the average particle size of the Ti-based composite oxynitride dispersed in the steel matrix must be reduced to 1. The research conducted by the present inventors has revealed that it is necessary to reduce the brightness to below.

Moreover, the adverse effect on the toughness of the base material, which becomes noticeable when the oxide diameter exceeds 3p, is not observed at all in the case of Ti-based composite oxynitride precipitates having an average particle diameter of IQ or less.

Next, the reasons for limiting the composition and manufacturing conditions of the steel materials according to the present invention as described above will be explained in detail.

(A) Chemical composition ratio of steel materials (a) C C is a necessary component to ensure the strength of steel materials, but if its content is less than 0.03%, it will cause softening of the weld heat affected zone and damage the weld metal. Dilution reduces its hardenability, prevents the formation of acicular ferrite, and deteriorates the toughness of the weld metal. On the other hand, if C is contained in an amount exceeding 0.20%, the weld heat-affected zone structure will be hardened, weldability will be deteriorated, and weld cracking will easily occur. Therefore, the C content is 0.0
It was limited to 3-0.20%.

(b) 5i Si is a component added to ensure the desired strength of steel materials in addition to acting as a deoxidizing agent during steel melting, but if its content is less than 0.05%, it will not have the above effects. On the other hand, if Si content exceeds 0.60%, weldability deteriorates, so the Si content was set at 0.05 to 0.60%.

(Cl Mn Mn is a necessary component to improve the strength of steel materials and to ensure the toughness of steel through rolling, but if its content is less than 0.40%, the desired effect due to the above action cannot be obtained. On the other hand, if the Mn content exceeds 2.00%, it will cause hardening of the weld heat affected zone and deteriorate weldability.
The content was determined to be 0.40 to 2.00%.

(0) Ti, O, and N Ti, 0 and N are important components that characterize the weld heat affected zone structure of the steel material according to the present invention. That is, Ti is added to the molten steel.
When adding Ti, Ti oxides will be formed if Ti is added when the amount of dissolved oxygen in the molten steel is relatively large, but the Ti oxides formed at this time are generally coarse and few in number. Therefore, the effect of significantly improving the toughness of the weld heat-affected zone, which is the objective of the present invention, cannot be expected. In order to form "a structure in which fine Ti-based composite oxynitrides are dispersed and generated in steel at high density", which fully exhibits the above-mentioned effects aimed at by the present invention, it is necessary to simultaneously dissolve dissolved oxygen in molten steel. It is necessary to deoxidize Ti and continuously cast molten steel in which the amount of nitrogen is adjusted to be high within a predetermined range immediately before pouring. Only by taking such measures will the generated oxide become a Ti(O,N)-based composite crystal phase, and will be dispersed in the steel in an extremely fine form and at a high density.

For that purpose, it is necessary to ensure a Ti content of at least 0.003%, and if the Ti content is less than this, the desired fine Ti-based composite oxynitride cannot be obtained. On the other hand, if Ti is contained in an amount exceeding 0.050%, there is a concern that excessive oxynitrides will be formed and coarsened, and the toughness of the base metal and weld metal will deteriorate. Therefore, Ti
The content was determined to be 0.003% to 0.050%.

Also, for 0 and N, each content is 0.0
If the content of O and N is less than 0.005% or 0.0030%, the desired fine Ti-based composite oxynitride cannot be obtained.
If the N content exceeds 0.0005% to 0.0060%, it will result in the formation of excessive oxynitrides, their coarsening, and deterioration of the toughness of the base metal and weld metal. The content was limited to 0.0030 to 0.0080%.

As mentioned above, a predetermined amount of Ti and O are essential components for generating Ti oxide in steel, which forms the basis of the steel material of the present invention, while a predetermined amount of N converts this Ti oxide into a Ti(
O, N) These are essential components for the fine dispersion effect ensured by changing to a composite crystal phase. The oxide-based inclusions having the Ti(O,N) composite crystal phase promote the formation of ferrite from within the grains independently of the austenite grain boundaries during the cooling process from the austenite of the steel, and As already mentioned, it prevents the weld heat-affected zone structure from becoming only coarse bainite and contributes to improving the toughness of the weld heat-affected zone.

In addition, if the average grain size of these Ti-based composite oxynitrides exceeds 1 mm, the growth suppressing effect of austenite grains and the ferrite forming effect will not be sufficiently exerted, and the desired effect of improving the toughness of the weld heat affected zone will not be stabilized. As mentioned above, it is not possible to secure the weld heat-affected zone toughness, but the quantity density of this Ti-based composite oxynitride must be relatively high to ensure the effect of improving the weld heat-affected zone toughness because the number of ferrite nuclei generated is small. Can not. Therefore, a quantity density of approximately 5×10 3 cells/fi 3 or more is required. As the number of Ti-based composite oxynitride particles increases, the obtained ferrite becomes finer and the weld heat affected zone toughness improves, but the number increases to 108 particles/
If it exceeds l13, the toughness and ductility of the base metal tend to decrease. The quantitative density of such a Ti-based composite oxynitride corresponds to about 0.001 to 0.100 when converted to the content ratio in the matrix assuming a particle size of IQ or less.

(dl The above are the basic components of the steel material according to the present invention, but in order to provide steel materials that meet the needs of various structures by changing the properties such as strength and toughness related to the base material, Cu, Ni + Cr+ Mo, Nb, V and C
It is also possible to selectively add one or more of a.

In addition, Cu and N are added as necessary because they have the effect of simultaneously increasing the strength and toughness of the steel material without adversely affecting the low-temperature toughness of the weld heat affected zone, but in this case, C
C
The U content was determined to be 0.5% or less, and the Ni content was determined to be 160% or less.

Red-grained Cr and Mo are effective in increasing the strength of the base metal through their action of improving hardenability, and at the same time have the effect of suppressing strength loss at high temperatures, so they are added as necessary, but each If the content exceeds 0.5%, it will cause hardening of the weld heat-affected zone, deteriorating the toughness, and making welding cold cracks more likely to occur, so the content of each was set at 0.5% or less. .

C) Anti-degradation M Nb and V form carbonitrides in austenite during steel rolling, promote the effect of controlled rolling, and increase the strength and strength of the base metal.
Since it has the effect of improving toughness, it is added as necessary. However, although the above effect is observed in a very small amount of Nb, the above effect is achieved by containing 0.003% or more of Nb. The effect becomes noticeable. On the other hand, if Nb is contained in an amount exceeding o or oso%, and if V is contained in an amount exceeding 0.10%, the toughness of the weld heat affected zone deteriorates, so the Nb content is 0.003 to 0.050%. (2) The content was limited to 0.10% or less.

Ca Ca may be added to control the shape of MnS inclusions, but if it is contained in an amount exceeding 0.0050%, the Ti-based oxynitride will be reduced and the effects of the present invention will be impaired. Therefore, the Ca content was determined to be 0.0050% or less.

(B) Manufacturing conditions In the steel material of the present invention, the γ-α transformation during cooling is controlled in the coarse-grained region of the weld heat-affected zone, and a large number of ferrites are generated from within the grains to ensure good weld heat-affected zone toughness. In order to
A major feature is that the Ti-based composite oxynitride is finely dispersed, but in order to disperse the Ti-based composite oxynitride with a specific particle size in the matrix at a predetermined density, special measures are required. processing is required. That is, in order to generate a suitable Ti-based composite oxynitride, the amount of dissolved nitrogen in molten steel should be 30 to 80 p.
pm, and the amount of dissolved oxygen in molten steel is Af! , S i
The molten steel adjusted in this way is heated to T
It is a process of deoxidizing and continuous casting. In this case, some of the dissolved oxygen escapes as inclusions before it becomes the final product, resulting in an O content of 0.0005 to 0.006.
The appropriate range is 0%.

In the slab solidified in such a continuous casting process, grain size: IJ! Mainly Ti (O, N
) A structure with a large number of finely dispersed oxide inclusions with a complex crystal phase is stably obtained, resulting in a steel material with excellent weld heat-affected zone toughness, but before final deoxidation by adding Ti. If the amount of dissolved oxygen or dissolved nitrogen in the molten steel is outside the above range, finely dispersed Ti-based composite oxynitrides cannot be formed as desired, and the desired weld heat-affected zone toughness cannot be achieved. Stable manufacturing of steel materials is not possible.

By the way, in the above treatment, it is preferable to cover the steel bath surface with an inert gas such as Ar gas to prevent oxidation from the atmosphere before and after Ti deoxidation. In addition, if the Ti deoxidation time is too early or the temperature of the steel bath is too high, the Ti oxide will become coarse.
i It is better to carry out deoxidation as late as possible and close to continuous casting.

Furthermore, in order to efficiently perform fine dispersion of the Ti-based composite oxynitride, Ti deoxidation may be performed in two steps.

Note that when casting is performed using the ingot method instead of the continuous casting method, even if Ti is deoxidized immediately before casting, the oxide will become coarse due to the slow cooling rate until solidification, making it difficult to obtain the desired effect. It is not possible.

After continuous casting, regardless of the type of treatment, whether the slab is rolled or subjected to controlled cooling, quenching/tempering, normalizing, etc., the above-mentioned excellent performance can be achieved regardless of the type of treatment. A steel material having the following properties is obtained.

Next, the effects of the present invention will be explained in more detail with reference to Examples.

<Example> First, it contains the necessary components and is pre-deoxidized so that the amount of dissolved oxygen is 20 to 80 ppm and the amount of dissolved nitrogen is 30 to 80 ppm.
The molten steel adjusted within the range is continuously cast by deoxidizing Ti immediately before casting in a tundish (however, in the case of comparative steels P and Q, Ti deoxidizing was performed somewhat earlier than casting), A slab having the chemical composition shown in Table 1 was obtained.

Next, these slabs were subjected to treatments such as hot rolling to produce steel plates (plate thickness: 25N). A portion of the hot-rolled steel sheet was subjected to controlled rolling, normalizing, or quenching/tempering.

Next, each of the obtained steel plates was
Layer welding was performed.

At this time, all welding materials used are the same low-temperature steel materials,
Two-electrode submerged arc welding (welding heat input: 140 kJ/ca+) under the following conditions was applied.

Poisonous smallpox Current: 1050A.

Voltage = 32■.

Jukyu Toenen Current near 50A.

Voltage: 40■.

Next, Charpy impact test specimens were taken from this welded material, with the notch positions set at the "center of the weld metal,""the weld bond," and "1 meter from the weld bond toward the HAZ side." The toughness level of the Ti-based oxynitride was investigated, and the results are shown in Table 2 along with the investigation results of the average particle diameter and particle density of the Ti-based oxynitride and the base material performance.

From the results shown in Table 2, it is clear that according to the present invention, all of the base metal toughness, weld metal part toughness, and weld heat-affected zone toughness are fully satisfied, and the performance is extremely well-balanced for low-temperature use. It can be seen that steel materials can be stably obtained.

On the other hand, in Comparative Steel N, the N content is lower than the range specified by the present invention, so the Ti-based oxynitrides become slightly coarser, and the weld heat affected zone toughness is lower than that of the Inventive Steel. ing.

In addition, it can be seen that in comparative steel O, both the N content and the Ti content are lower than the range defined by the present invention, and therefore the number of Ti-based oxynitrides is small and does not satisfy the desired characteristics.

Furthermore, in comparative steels P and Q, the Ti deoxidation treatment was performed earlier than the time of pouring into the molten steel.
As the oxynitrides become coarser, their number also decreases, and sufficient toughness of the weld heat-affected zone is not ensured.

<Summary of Effects> As explained above, according to the present invention, it is possible to produce industrially stable steel materials that not only have good base metal toughness and weld metal part toughness, but also exhibit extremely excellent weld heat affected zone toughness. This brings about extremely useful effects industrially, such as making it possible to manufacture the product.

Claims (3)

[Claims] (1) C: 0.03-0.20%, Si: 0.05-0.60 in weight percentage
%, Mn: 0.40-2.00%, Ti: 0.003-
0.050%, N: 0.0030-0.0080%, O
: Contains 0.0005-0.0060% and the remainder is F
An oxide having a chemical composition that is e and unavoidable impurities, and having a Ti(O,N) composite crystal phase with a grain size of 1 dia or less in a proportion of 0.001 to 0.100% by weight in the matrix. A steel material with excellent weld heat-affected zone toughness, characterized by dispersed system inclusions. (2) C: 0.03-0.20%, Si: 0.05-0.60 in weight percentage
%, Mn: 0.40-2.00%, Ti: 0.003-
0.050%, N: 0.0030-0.0080%, O
:0.0005 to 0.0060%, and Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.
5% or less, Mo: 0.5% or less, V: 0.10% or less,
Nb: 0.003-0.050%. It has a chemical composition that contains at least one type of Ca: 0.0050% or less, and the balance is Fe and inevitable impurities, and the base material contains 0.001 to 0.0% Ca.
Ti(O,N) with a particle size of 1 μm or less at a ratio of 100% by weight
) A steel material with excellent weld heat-affected zone toughness, characterized by being composed of dispersed oxide inclusions having a composite crystal phase. (3) Molten steel whose dissolved oxygen content has been adjusted to 20 to 80 ppm through preliminary deoxidation and whose dissolved nitrogen content has been adjusted to 30 to 80 ppm is continuously cast by deoxidizing Ti immediately before casting. The method for producing a steel material having excellent weld heat affected zone toughness according to claim 1 or 2.