JP2001020033A - Non-heattreated high tensile strength steel excellent in toughness of base material and weld heat-affected zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the toughness of the base material and weld heat-affected zone, by dispersing oxide inclusions contg. specified amounts of Ti oxide, Ca oxide, REM oxide and Al2O3 into steel having a specified compsn. in a specified state. SOLUTION: This steel contains, by weight, 0.01 to 0.18% C, 0.02 to 0.60% Si, 0.60 to 2.00% Mn, <=0.03% P, <=0.015% S, 0.005 to 0.08% Ti, 0.0020 to 0.0100% N, 0.0010 to 0.0200% REM, 0.0010 to 0.0200% Ca and Al:Ti/<=5, and the value of Ceq defined by the formula is 0.36 to 0.45%. The content of each element in the formula is denoted by weight %. Then, into the steel, oxide inclusions composed of, by weight, <=90% Ti oxide, Ca oxide and REM oxide by 5 to 50% in total and <=70% Al2O3 are dispersed by 1×103 to <1×105 pieces/mm2 by the number of the ones having the size equivalent to a circle of >=200 nm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat treated high-strength steel material used for building structures, marine structures, pipes, ships, storage tanks, civil engineering structures, construction machines, etc., and more particularly to a base material and welding heat. It relates to a steel material with excellent toughness in the affected zone. The steel material in the present invention includes a thick steel plate, a steel strip, a shaped steel, and a steel bar.

[0002]

2. Description of the Related Art As an excellent method for producing a thick steel plate having both high strength, high toughness, and high weldability, TMCP (Thermo
Mechanical Control Process) is known. However, in the thick steel plate manufactured by the above-mentioned TMCP method, the toughness of the weld heat affected zone is not sufficient, and therefore, there is a problem in using such a steel material for a welded structure used at a low temperature.

Further, in order to improve the toughness of the heat affected zone (HAZ), a method of uniformly dispersing an oxide or nitride in steel to refine the HAZ structure and improve toughness has been studied. Have been done. For example, Japanese Patent Laid-Open No. 2-12
No. 5812 discloses that a slab containing 5 × 10 ³ to 1 × 10 ⁶ particles / mm ^{2 of} an oxide mainly composed of Ti having a particle diameter of 0.05 to 10 μm in steel is reheated at 900 to 1100 ° C. Rolling at a rolling reduction temperature of 700 to 850 ° C, cooling at a cumulative reduction of 30 to 90% below 900 ° C, cooling, and then aging at a temperature of 500 ° C to the Ac ₁ transformation point, HAZ
A method for producing a Cu-added steel having excellent toughness has been proposed.

Japanese Patent Application Laid-Open No. 4-48048 discloses that C:
Contains 0.03 to 0.20 wt%, Nb and Ti, and 0.001 to 0.
There has been proposed a steel material having excellent toughness of a heat affected zone in which oxide inclusions having a (Ti, Nb) (O, N) composite crystal phase having a grain size of 0.5 μm or less of 100 wt% are dispersed therein.

[0005]

However, in the techniques described in JP-A-2-125812 and JP-A-4-48048, although the toughness of the weld heat-affected zone is improved, the base metal toughness is low. However, in order to improve the toughness of the base material, it is necessary to further improve the existing state of the oxide-based inclusions.

[0006] Furthermore, if an amount of Ti oxide effective for controlling the structure of the base metal and the weld heat affected zone is to be present in steel, nozzle clogging is likely to occur during casting, resulting in poor productivity. Was. The present invention advantageously solves the above-mentioned problems of the prior art, and proposes a non-refined high-strength steel material having both base metal toughness and weld heat-affected zone toughness, and capable of being manufactured stably with high productivity without nozzle clogging. The purpose is to do.

[0007]

Means for Solving the Problems The present inventors have further studied a method of uniformly and finely dispersing oxide-based inclusions in a steel material without nozzle clogging. And found that the composition of the oxide-based inclusions needs to be within the optimum range. Next, the results of a study performed by the present inventors on the optimum composition range of the oxide-based inclusion will be described.

The present inventors added an alloy for adjusting the composition of inclusions to the molten steel after the deoxidization treatment to control the composition of the inclusions,
Ti ₂ O ₃ , CaO + REM oxide, Al ₂ O ₃ in inclusions in molten steel
A steel material for rolling with a varied concentration was cast, and the steel material for rolling was hot-rolled to produce a steel sheet. First, the occurrence of nozzle clogging during casting of a rolled material was investigated. Regarding the nozzle clogging, when the immersion nozzle was clogged in the continuous casting process, it was determined that the nozzle was clogged.

Further, the present inventors have found that the inclusion of CaO and REM oxides in the inclusions affects the rust resistance of the steel material. did. Regarding the rusting of the steel sheet, the steel sheet after rolling and cooling was allowed to stand in the air for 10 days and then macroscopically observed. The results are summarized in FIG.

When the concentration of Ti ₂ O ₃ in the inclusion is more than 90% by weight or the concentration of CaO + REM oxide is less than 5% by weight, the melting point of the inclusion is high and the inclusion on the inner surface of the nozzle during casting is high. An object is likely to adhere, causing nozzle blockage. If the Al ₂ O ₃ concentration exceeds 70% by weight, large cluster inclusions are easily formed, and the wettability with molten steel decreases.
Nozzle blockage becomes significant. In addition, CaO + REM in inclusions
If the oxide concentration exceeds 50% by weight, the inclusions tend to contain sulfur in a liquid phase. As a result, when the liquid phase inclusion solidifies, CaS and REM sulfide are generated around the inclusion. For this reason, the inclusions are coarsened, and rust on the steel sheet becomes remarkable.

That is, in order to uniformly and finely disperse the oxide-based inclusions, it is necessary to improve the wettability between the deoxidized inclusions and the molten steel. To this end, the Al ₂ O ₃ concentration in the inclusions is required. To 70% by weight or less, and the CaO and REM oxide concentration in the oxide-based inclusions to 50% by weight or less. It is necessary to lower the melting point of the material. To achieve this, the CaO or REM oxide concentration must be at least 5% by weight or more by Ca treatment or REM treatment, and the Al ₂ O ₃ concentration must be 70% by weight or less. , Ti oxide concentration 90% by weight
It has been found that it is important to reduce the content of CaO + REM oxide in inclusions to 50 wt% or less in order to prevent rusting.

Based on these findings, the present inventors have found that the optimum composition range of the oxide-based inclusions is, as shown in FIG. 1, a Ti oxide: 90% by weight or less, a total of CaO and REM oxides. : 5-50 wt%, Al ₂ O _3: was is 70 wt% or less. By controlling the composition of the oxide-based inclusions to fall within the range shown in FIG. 1, the ability to suppress grain coarsening of the inclusions (pinning effect) without causing nozzle clogging and generation of harmful inclusion clusters Can be effectively used, the structure of the weld heat affected zone can be improved, and the weld heat affected zone toughness can be effectively improved.

Further, the present inventors have studied diligently about further improvement in the toughness of the base metal and the weld heat affected zone. As a result, it has been found that the base material toughness can be further improved by optimally dispersing oxide-based inclusions in the number having the above-mentioned composition and having a circle equivalent diameter of 200 nm or more. The present invention has been made based on the above findings.

That is, in the present invention, C: 0.01% by weight.
~ 0.18%, Si: 0.02 ~ 0.60%, Mn: 0.60 ~ 2.00%, P:
0.030% or less, S: 0.015% or less, Ti: 0.005 to 0.08
%, N: 0.0020-0.0100%, REM: 0.0010-0.0200%,
Ca: 0.0010 to 0.0200%, Al: (Ti%) / 5 or less, the balance being Fe and unavoidable impurities, and the following equation (1): Ceq (%) = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1) (where Ceq defined by C, Si, Mn, Ni, Cr, Mo, and V: the content (% by weight) of each element) is 0.36 to 0.45%. It has a steel composition and, in weight percent, Ti oxides: 90% or less,
Total of Ca oxide and REM oxide: 5 to 50%, Al ₂ O ₃ :
Oxide-based inclusions having an inclusion composition of 70% or less are counted as 1 ×
A base material characterized by being dispersed in a range of 10 ³ pieces / mm ² or more and less than 1 × 10 ⁵ pieces / mm ^2, and a non-heat-treated high-strength steel material excellent in weld heat-affected zone toughness. In addition to the steel composition, it is preferable to further contain V: 0.03 to 0.15% by weight, and in the present invention, Cu: 0.02 to 1.5% by weight in addition to the above steel compositions. , Ni: 0.02 ~
0.06%, Cr: 0.05 to 0.50%, Mo: 0.02 to 0.50%, Nb: 0.
It is preferable to contain one or more of 003 to 0.020%. In the present invention, in addition to the above steel compositions, it is preferable that B: 0.0002 to 0.0020% by weight% is further contained. preferable.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the present invention will be described below. First, the reasons for limiting the composition of the steel material of the present invention will be described. Hereinafter,% for the composition means% by weight. C: 0.01 to 0.18% C is an element that increases the strength of steel, and requires 0.01% or more of C in order to secure desired strength.
%, The base material toughness and the weldability decrease. For this reason, C was limited to 0.01 to 0.18%. In practice, it is preferably 0.08 to 0.16%.

Si: 0.02 to 0.60% Si is an effective element for increasing the strength of steel by solid solution strengthening. However, if the content is less than 0.02%, the effect is small, while on the other hand, it exceeds 0.60%. If it is contained, the HAZ toughness is significantly deteriorated. For this reason, Si was limited to 0.02 to 0.60%. Mn: 0.60 to 2.00% Mn is an element that increases the strength of steel and is effective in increasing the strength.
To secure the desired strength, the content of 0.60% or more is required. However, when the content exceeds 2.00%, the structure air-cooled after rolling becomes a ferrite + bainite structure, and the base material toughness decreases. For this reason, Mn was limited to the range of 0.60 to 2.00%. In addition, it is preferably 1.00 to 1.70%.

P: 0.030% or less Since P segregates at the grain boundaries and deteriorates the toughness of the steel, it is desirable to reduce P as much as possible. However, since P can be tolerated up to 0.030%, it is limited to 0.030% or less. S: 0.015% or less S is mainly formed in steel by forming MnS and has an effect of making the structure after rolling and cooling fine. However, if it exceeds 0.015%, the toughness in the thickness direction is reduced. Reduces ductility. Therefore, S is limited to 0.015% or less. In order to obtain a grain refining effect as MnS, S is desirably in the range of 0.004 to 0.010%.

Ti: 0.005 to 0.08% Ti is one of the important elements in the present invention. Effective utilization of the oxide generated by Ti deoxidation is one important element of the present invention. Oxide-based inclusions mainly composed of Ti oxide uniformly and finely dispersed in steel have an effect of suppressing austenite grain growth by a crystal grain pinning effect. Further, oxide-based inclusions of 200 nm or more are uniformly dispersed, and the base material structure and the weld heat affected zone structure are refined as ferrite generation nuclei. Further, Ti remaining after deoxidation combines with N in the subsequent cooling process to form TiN. TiN contributes to suppressing the coarsening of austenite grains in the heat affected zone and improves the toughness of the heat affected zone. Further, it acts as a ferrite generation nucleus, refines the structure, and improves the base material toughness.

In order to exert these effects, Ti is added in an amount of 0.00
Requires a content of 5% or more. Further, if the content exceeds 0.08%, the cleanliness of the steel is deteriorated, and in addition, the increase in solid solution Ti or the precipitation of Ti carbides deteriorates the toughness. For this reason,
Ti is limited to 0.005 to 0.08%. Incidentally, preferably 0.01
0 to 0.025%. N: 0.0020% to 0.0100% N combines with Ti to form TiN, suppresses the growth of γ grains when the rolled material is heated, and further, becomes a precipitation nucleus of ferrite during rolling, and refines the crystal grains. Has the effect of
It greatly contributes to improving toughness. In order to exert these effects effectively, 0.0020% or more is required,
If the content exceeds 0.0100%, the base material toughness and weldability are greatly impaired, so N was limited to the range of 0.0020 to 0.0100%.

Al: (Ti%) / 5 or less Al acts as a deoxidizing agent. In the present invention, Al can be used as a preliminary deoxidizing agent for adjusting the O concentration before Ti deoxidizing. In the present invention, the Al content is limited to 1/5 or less of the Ti content (% by weight) in order not to generate Al ₂ O ₃ clusters. If the Al content exceeds 1/5 of the Ti content, Ti-Al
From -O equilibrium, Al ₂ O ₃ clusters are generated, can not be uniformly and finely dispersed in the oxide inclusions. Preferably, Al is (Ti%) / 6 or less.

Ca: 0.0010-0.0200%, REM: 0.0010-0.
0200% Ca, REM oxide, REM oxide and CaO in inclusions
Although the concentration of the oxide increases, the melting point of the inclusions decreases, and the adhesion to the nozzle inner surface during casting can be suppressed,
Nozzle blockage can be avoided. In addition, Ca and REM are elements that contribute to the improvement of wettability and are indispensable for achieving fine and uniform dispersion of deoxidized products. REM and Ca form stable oxides even at high temperatures and finely disperse, thereby suppressing γ grain growth. Further, it has the effect of reducing the ferrite grain size after rolling, and is also effective in improving the HAZ toughness. In order to obtain these effects, the content of each is required to be 0.0010% or more. On the other hand, if the content exceeds 0.0200%, the cleanliness of steel is reduced and the base material toughness is impaired. For this reason, Ca and REM were each limited to the range of 0.0010 to 0.0200%. It should be noted that REM and Ca need to be simultaneously contained in the present invention, since adding REM and Ca alone has little effect on avoiding nozzle clogging of inclusions.

V: 0.03 to 0.15% V precipitates as VN in γ grains during rolling and cooling, and ferrite precipitates as nuclei, thereby effectively acting to refine crystal grains and greatly contributing to improvement in toughness. I do. In addition, VN is precipitated in the ferrite even after the ferrite transformation, and the base material strength can be increased without performing strong water cooling during cooling. It is effective in ensuring uniformity of properties in the thickness direction and reducing residual stress and distortion, and can be contained as necessary.

In order to exhibit these effects effectively, it is preferable to contain V in an amount of 0.03% or more. However, if the content exceeds 0.15%, the base material toughness and weldability are greatly impaired, so V is preferably set in the range of 0.04 to 0.15%. Cu: 0.02 to 1.5%, Ni: 0.02 to 0.60%, Cr: 0.05 to 0.50
%, Mo: 0.02 to 0.50%, Nb: 0.003 to 0.020% One or more of them Cu, Ni, Nb, Cr, and Mo are all effective elements for improving hardenability. One or two or more can be selected and contained. The inclusion of Cu, Ni, Nb, Cr, and Mo lowers the Ar ₃ point and makes the ferrite grains finer, contributing to an improvement in toughness.

When the contents of Cu, Ni, Nb, Cr and Mo are too large, the Ar ₃ point is too low, the bainite transformation is mainly carried out, the grain size of ferrite becomes insufficient, and the strength increases. In addition, grain refinement of ferrite becomes insufficient. From these facts, Cu, Ni, Nb, Cr, and Mo are 0.02% and 0.02%, respectively.
%, 0.05%, 0.02%, 0.003% or more. In addition, when adding Cu, it is preferable to add approximately the same amount of Ni as Cu in order to compensate for the reduction in hot workability due to Cu. However, since the addition of a large amount of Ni increases the production cost, the upper limits of Cu and Ni are preferably set to 0.6%. Nb, Cr, and Mo are 0.020%, 0.50%, and 0.2%, respectively.
If it exceeds 50%, the weldability and toughness are impaired, so it is preferable to make these the upper limits.

B: 0.0002% to 0.0020% B has the effect of segregating at the grain boundaries to suppress the formation of coarse grain boundary ferrite and to make the ferrite grains after rolling finer.
It can be contained as needed. This effect is recognized at a content of 0.0002% or more. On the other hand, if the content exceeds 0.0020%, the toughness decreases. For this reason, B is preferably limited to the range of 0.0002 to 0.0020%.

Ceq: 0.36 to 0.45 wt% Ceq is defined by the following equation (1). Ceq (%) = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1) where, C, Si, Mn, Ni, Cr, Mo, V: content of each element (weight) %) When Ceq exceeds 0.45%, weld crack susceptibility increases,
HAZ toughness decreases. On the other hand, if Ceq is less than 0.36%, it becomes difficult to secure strength in the base material and the HAZ softened portion. For this reason, Ceq is limited to the range of 0.36 to 0.45%.

The balance other than the above components is Fe and inevitable impurities. O: 0.0100 as inevitable impurities
% Or less, N: 0.0100% or less is acceptable. Next, a preferred method for producing the steel material of the present invention will be described. The molten steel having the above composition is melted by deoxidizing Ti. It goes without saying that preliminary deoxidation with Al may be performed. The smelting method is
Although not particularly limited, any commonly known melting method such as a converter, an electric furnace, and a vacuum melting furnace can be suitably used. By using Ti deoxidation as the deoxidation method, deoxidation products
It becomes an inclusion mainly composed of Ti oxide.

The molten steel is then suitably cast by any of the conventionally known casting methods such as a continuous casting method and an ingot casting method, and is cast into a rolling steel material such as a slab. Next, the reasons for limiting the composition of the oxide-based inclusions that are finely dispersed in the steel material will be described. Oxide inclusions that are finely dispersed, as a main component Ti oxides, by weight%, Ti oxide: 90% or less, Al ₂ 0 _3: 70% or less, the total of Ca oxide and REM oxides: 5 It has an inclusion composition of ~ 50%.

Ti oxide: 90% or less Ti oxide has a crystal grain pinning effect of suppressing coarsening of austenite grains in the heat affected zone. For this reason,
In the present invention, the oxide-based inclusion has a composition mainly composed of Ti oxide. However, when the concentration of the Ti oxide in the oxide-based inclusions exceeds 90%, the melting point of the oxide-based inclusions becomes high, and the inclusions of the inclusions easily adhere to the immersion nozzle wall,
Nozzle clogging is likely to occur. For this reason, the concentration of Ti oxide in the oxide-based inclusions is limited to 90% or less. When the concentration of Ti oxide in the oxide-based inclusions is less than 50%, the effect of pinning the crystal grains is reduced, so that the concentration of Ti oxide in the oxide-based inclusions is preferably set to 50% or more. . The Ti oxide referred to in the present invention is preferably TiO ₂ , Ti ₂ O ₃ or the like, and among them, Ti ₂ O ₃ is particularly preferred.

Al ₂ O ₃ : 70% or less Al ₂ O ₃ easily forms large cluster inclusions and hinders uniform and fine dispersion of oxide-based inclusions. Therefore, in the present invention, it is preferable to reduce the Al ₂ O ₃ concentration in the oxide-based inclusions as much as possible. Al ₂ O ₃ concentration in oxide-based inclusions
When it exceeds 70%, the wettability of inclusions with molten steel is reduced,
Furthermore, nozzle clogging becomes remarkable. For these reasons, the Al ₂ O ₃ concentration in the oxide-based inclusions is set to 70% or less.

The total of Ca oxide and REM oxide: 5 to 50% In the present invention, in order to lower the melting point of oxide-based inclusions,
A total of 5% or more of Ca oxide (CaO) + REM oxide is contained in the oxide-based inclusions. If the Ca oxide (CaO 2) + REM oxide concentration is less than 5%, the melting point of the inclusions is high, and the inclusions tend to adhere to the inner surface of the nozzle during casting, causing nozzle blockage. In addition, since Ca and REM are easily bonded to S to form sulfides, Ca oxide (CaO) + REM in oxide-based inclusions
If the oxide concentration exceeds 50%, the inclusions tend to contain S in the liquid state, and CaS, REM
Sulfides are formed. For this reason, the inclusions are coarsened, the crystal pinning ability of the oxide-based inclusions is reduced, and the rusting of the steel material becomes remarkable. Also, since the specific gravity of REM oxide is larger than other oxides, if this REM oxide exceeds 50 wt%, the levitation of inclusions in molten steel will deteriorate, and the total oxygen concentration in the steel will decrease. It becomes high and deteriorates the cleanliness of the steel sheet. For these reasons, the total content of Ca oxide and REM oxide in the oxide-based inclusions is limited to the range of 5 to 50%.

In the present invention, the amount of inclusions is determined by a cleanness test using an optical microscope or the amount of extraction residue is determined. The composition of the inclusions is determined by a scanning electron microscope (SE).
M), and measurement is performed by a procedure called quantitative analysis using EDX. To adjust the composition of the oxide-based inclusions to the above range, after deoxidation using Ti or Ti alloy, Fe, Al, Si, contains at least one of Ti, and , Ca less than 10wt%, REM of Ce, La etc.
An alloy for adjusting the composition of inclusions contained in a range of less than wt% may be added.

According to the present invention, there is provided the above-mentioned inclusion composition,
In addition, an appropriate number of oxide-based inclusions
Disperse. In the present invention, Ti oxide having an equivalent circle diameter of 200 nm or more is used.
Object 1 × 10^ThreePieces / mm^TwoMore than 1 × 10 ^FivePieces / mm^TwoLess dispersion
Let it. Oxide-based inclusions in rolling and welding processes
In addition, it acts as a ferrite formation nucleus,
In order to improve the toughness of the heat affected zone, the equivalent
A size of 0 nm or more is required, and the equivalent circle diameter is 200 nm
Make sure that oxide inclusions of the above size are dispersed in appropriate amounts.
Is essential.

When the number of oxide-based inclusions having an equivalent circle diameter of 200 nm or more is less than 1 × 10 ³ / mm ² , the number of generated ferrite is small, and the ferrite grains in the base material and the weld heat-affected zone are small. If the effect of miniaturization is insufficient and the number of oxide-based inclusions having an equivalent circle diameter of 200 nm or more is 1 × 10 ⁵ / mm ² or more, the toughness in the base metal and the weld heat affected zone is reduced. Deteriorates. For this reason, in the present invention, the circle equivalent diameter 20
Oxide inclusions of 0 nm or more 1 × 10 ³ / mm ² or more 1 ×
Disperse less than 10 ⁵ pieces / mm ² .

The identification of oxide-based inclusions and the equivalent circle diameter thereof are measured by analysis and observation with a transmission electron microscope. The dispersion form of the oxide-based inclusions described above can be obtained by controlling the amounts of Ti and O, and adjusting the amounts of Ca and REM. The steel material adjusted as described above is then preferably reheated to a temperature range of 1000 to 1250 ° C., or is subjected to hot rolling without reheating to obtain a steel plate or the like.

Heating temperature: 1000-1250 ° C. If the heating temperature is lower than 1000 ° C., the austenite may not be completely formed, so that the effect of homogenization by reheating cannot be sufficiently obtained. On the other hand, when the heating temperature exceeds 1250 ° C., the austenite grains become extremely coarse, the structure after rolling becomes coarse, and the toughness decreases. Therefore, the heating temperature is
Preferably, it is limited to the range of 1000 to 1250 ° C. In addition, preferably it is 1050-1200 degreeC. When rolling without reheating, after casting, it is necessary to start rolling before the temperature of the steel material is excessively lowered, and at least 900
It is preferred to have a temperature of at least ℃.

The hot rolling is preferably performed under the following conditions. Cumulative reduction in temperature range below 1000 ℃: 30% or more Increase in reduction in temperature range below 1000 ℃ is due to refinement of ferrite grains by introducing strain into austenite grains.
Improve the mechanical properties of the base material. These effects are 10
When the cumulative rolling reduction in the temperature range of 00 ° C. or less is 30% or more, it becomes remarkable according to the cumulative rolling reduction. For this reason, it is preferable to limit the cumulative rolling reduction in the temperature range of 1000 ° C. or less to 30% or more.

Hot rolling end temperature: 700 ° C. or more As the hot rolling end temperature becomes lower, the rate at which strains (dislocations) introduced into austenite grains by rolling are accumulated in the grains increases. As a result, the amount of dislocation transferred to the structure after transformation is significantly increased, and the strength is increased. However, even if the hot-rolling end temperature is less than 700 ° C., the tendency of the increase in strength is saturated, and the rolling efficiency decreases due to an increase in deformation resistance. Therefore, in the present invention, it is preferable to limit the rolling end temperature to 700 ° C. or higher.

In the present invention, after hot rolling, air cooling or accelerated cooling may be performed. By performing accelerated cooling after hot rolling, the resulting structure is refined, and the toughness can be further improved. The accelerated cooling conditions are as follows:
It is preferable that the cooling temperature is 1 to 30 ° C / s and the cooling stop temperature is 650 ° C or less.

If the cooling rate is less than 1 ° C./s, the structure cannot be further refined, and the effect of accelerated cooling is small. Further, a cooling rate exceeding 30 ° C./s is difficult to realize industrially. Therefore, the cooling rate of accelerated cooling is 1 to 30 ° C.
/ S. Also, the cooling stop temperature
If it exceeds 650 ° C., the effect of accelerated cooling is small and the effect of improving toughness is small. For this reason, the cooling stop temperature is preferably set to 650 ° C. or lower.

[0041]

EXAMPLES Steel having the composition shown in Table 1 was melted in an electric furnace. The composition of the oxide inclusions was adjusted mainly by changing the balance of Ti / Al and the addition amounts of Ca and REM. Also,
Molten steel was injected into the mold from a ladle using a nozzle to obtain a steel material. Regarding the state of adhesion of inclusions in the nozzle during casting,
After the casting, the inside of the nozzle was visually observed to check for the presence of inclusions. In addition, oxide oxides with a circle equivalent diameter of 200 nm or more
The dispersion number was adjusted by controlling the Ti amount and the O amount.

As a comparative example, to make the composition of the oxide-based inclusions out of the scope of the present invention and increase the amount of Ti oxide,
In order not to deoxidize Al, increase Ti / Al, and increase CaO and REM oxides, increase the amount of Ca or REM added and increase the amount of Al ₂ O
₃ was increased by reducing Ti / Al.
These steel materials were subjected to hot rolling under the conditions shown in Table 2 and cooling after rolling to obtain thick steel plates.

[0043]

[Table 1]

[0044]

[Table 2]

With respect to these thick steel plates, a structure investigation, a base metal characteristic, and a characteristic of a weld heat affected zone were examined. As a microstructure investigation, the ferrite grain size of the base metal, the inclusion composition of oxide-based inclusions in the thick steel plate, and the number of oxide-based inclusions having a circle equivalent diameter of 200 nm or more were investigated. Identification of oxide inclusions,
The number and the equivalent circle diameter were determined by analysis and observation with a transmission electron microscope. The composition of the oxide-based inclusions was analyzed by a scanning electron microscope (SEM) and a quantitative analysis by EDX attached to the SEM.

As a base material characteristic, a tensile test specimen and a Charpy impact test specimen were sampled from a quarter of the thickness of each thick steel plate.
The tensile properties and toughness (Charpy absorbed energy) of the base material were evaluated. Further, as a heat affected zone (HAZ) characteristic, a reproducible heat cycle test was performed and evaluated. In the reproducible heat cycle test, a 12 mm thick x 75 mm x 80 mm test piece was sampled from a quarter of each thick steel plate in a direction perpendicular to the rolling direction, and submerged arc welding with a heat input of 100 kJ / cm was performed using a high-frequency heating device. A thermal cycle (maximum heating temperature of 1400 ° C.) simulating the thermal cycle received by the coarse grain area HAZ was applied, a Charpy impact test specimen was collected, and the Charpy absorbed energy at −40 ° C. (vE _-40 ) was determined.

Table 3 shows the results.

[0048]

[Table 3]

The example of the present invention can be manufactured without occurrence of nozzle clogging at the time of casting, and the base material has a tensile strength TS of 500 MPa.
High strength and a high toughness with a Charpy absorbed energy vE _-40 at −40 ° C. of 300 J or more.
AZ has Charpy absorbed energy at -40 ° C of vE- ₄₀ of 15
It has excellent HAZ toughness of 0 J or more. On the other hand, in the comparative examples out of the range of the present invention, either the base metal toughness or the weld heat affected zone toughness is reduced, or nozzle clogging occurs.

[0050]

According to the present invention, a non-heat treated high-strength steel material having both base metal toughness and weld toughness can be stably manufactured without occurrence of nozzle clogging at the time of casting. Play.

[Brief description of the drawings]

FIG. 1 is a ternary phase diagram showing a preferred range of an oxide-based inclusion composition.

 ────────────────────────────────────────────────── ─── Continued on the front page F-term (reference)

Claims (4)

[Claims] 1. In weight%, C: 0.01 to 0.18%, Si: 0.02 to 0.60%, Mn: 0.60 to 2.00%, P: 0.030% or less, S: 0.015% or less, Ti: 0.005 to 0.08%, REM : 0.0010-0.0200%, Ca: 0.0010-0.0200%, Al: (Ti%) / 5 or less, with the balance being Fe and unavoidable impurities, and the Ceq defined by the following formula (1) being 0.36-0.45% Inclusion composition having a steel composition of: 90% or less by weight, Ti oxide: 90% or less, total of Ca oxide and REM oxide: 5 to 50%, Al ₂ O ₃ : 70% or less by weight% Oxide-based inclusions with
1 × 10 ³ pieces / mm with the above circle equivalent diameter
^A base material and a non-heat-treated, high-strength steel material excellent in toughness of a heat-affected zone of a weld, characterized by being dispersed in ² or more and less than 1 × 10 ⁵ / mm ² . Ceq (%) = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1) where, C, Si, Mn, Ni, Cr, Mo, V: content of each element (weight) %) 2. In addition to the steel composition, further in weight%
V: 0.03 to 0.15% is contained.
A non-heat-treated high-strength steel excellent in toughness of the base metal and the weld heat-affected zone according to the above. 3. In addition to the steel composition, in addition to the steel composition,
u: 0.02 to 1.5%, Ni: 0.02 to 0.06%, Cr: 0.05 to 0.50
%, Mo: 0.02 to 0.50%, Nb: 0.003 to 0.020%, one or two or more of which are excellent in the toughness of the base metal and the weld heat affected zone according to claim 1 or 2. Non-heat treated high tensile steel. 4. In addition to the steel composition, further in weight%
B: A non-heat-treated, high-strength steel material having excellent toughness in a heat affected zone and a base material according to any one of claims 1 to 3, wherein the steel material contains 0.0002 to 0.0020%.