JPH11302771A - Steel material excellent in strength and toughness and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve strength and toughness of a steel material without requiring a process of forming a microstructure such as controlled rolling which decreases the productivity, by specifying the amts. of C, Si, Mn, P, S, Al, Ti and N and controlling the metal structure and precipitated old austenite grains. SOLUTION: This steel material contains, by weight, 0.02 to <0.15% C; <=1% Si; 0.3 to 2.5% Mn; <=0.05% P; <0.004% S; 0.001 to 0.1% sol.Al; <=0.02% Ti and <=0.009% N, relating to this material, the metal structure of the steel is controlled to a structure containing one or both of martensite and bainite or its tempered structure, and the average aspect ratio and the average minor diameter dr of the old austenite grains are controlled to <=1.5 and 60 to 700 μm, respectively. When Ti/N<3.4, dr satisfies formula I, and when Ti/N>=3.4, dr satisfies formula II. The steel material is obtd. by controlling the hot processing temp. so as to obtain 60 to 200 μm minor diameter of the austenite grains after the hot processing is completed, and then directly quenching.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel having excellent strength and toughness and a method for producing the same.

[0002]

2. Description of the Related Art In many cases, steel materials, particularly structural steel materials, are required to have both strength and toughness. In order to satisfy the above requirements without adding an expensive element such as Ni, various methods for refining the structure by temper treatment, controlled rolling, and the like have been proposed and adopted.

[0003] For example, Japanese Patent Publication No. 55-30050 discloses a method for producing a tough steel. This method
A method in which AlN is finely dispersed in steel by defining the chemical composition, slab casting conditions, and slab heating conditions during hot rolling, and the growth of austenite grains is suppressed with the AlN to form a fine grain structure. It is.

According to this method, it is possible to obtain a fine-grained structure, but AlN is a precipitate that causes lateral cracking of the slab during continuous casting, and the efficiency of continuous casting is high. Application of production methods becomes significantly more difficult.

JP-A-57-131320 discloses a method for producing a high-tensile steel sheet having excellent low-temperature toughness. This manufacturing method is a method in which a rolling end temperature and a subsequent cooling rate are specified. However, this method requires rolling at a temperature from the austenite non-recrystallized region to the two-phase region, so that the rolling efficiency is significantly reduced. In addition, although the fracture surface transition temperature is improved, separation tends to occur, so that the absorbed energy tends to be small. For this reason, it is not an effective method when the absorption energy of a certain value or more is required in the Charpy impact value.

[0006] JP-A-7-258730 and JP-A-7-258731 disclose a method of manufacturing a structural steel plate having excellent toughness and low acoustic anisotropy. In these methods, in order to secure toughness while reducing acoustic anisotropy, non-recrystallized region rolling is performed by utilizing grain refinement by recrystallization of austenite as much as possible.

However, although this method does not utilize the controlled rolling effect, in order to obtain practically satisfactory toughness, the rolling end temperature must be controlled to around 900 ° C. or lower. However, a decrease in productivity due to low-temperature rolling is inevitable.

[0008] As described above, it is possible to secure the required toughness by finely refining the structure through a subtle combination of thermomechanical treatment, cooling, and reheating. Is low. .

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a steel material having excellent strength and toughness which can be manufactured without requiring a fine structure by controlled rolling or the like which lowers productivity, and a method for manufacturing the same. Is to do.

[0010]

The gist of the present invention relating to a steel material having both strength and toughness and a method for producing the same is as follows.

1) C: 0.02 to 0.15% by weight
, Si: 1% or less, Mn: 0.3-2.5%, P:
0.05% or less, S: less than 0.004%, sol. Al:
0.001-0.1%, Ti: 0.02% or less, N:
0.009% or less, and the metal structure is a structure containing one or both of martensite and bainite, or a tempered structure thereof, and the average value of the aspect ratio of the prior austenite grains is 1.5 or less; Has an average minor axis of 60 to 700 μm, and Ti, N, S
And a steel material having excellent strength and toughness, characterized in that the former austenite grain minor diameter dγ satisfies the following formula (1) or (2).

[0012]

(Equation 1)

[0013]

(Equation 2)

2) C: 0.02% to 0.15% by weight
%, Si: 1% or less, Mn: 0.3 to 2.5%,
P: 0.05% or less, S: less than 0.004%, sol.A
l: 0.001 to 0.1%, Ti: 0.004 to 0.0
2%, N: 0.001 to 0.009%, Ti / N
Is 0.4 to 4, the metal structure is a structure containing one or both of martensite and bainite, or a tempered structure thereof, and the average value of the aspect ratio of the prior austenite grains is 1.5 or less; Has an average minor axis of 60 to 700 μm, and the minor axis dγ of Ti, N, S and prior austenite grain satisfies the following formulas (3) and (4). Excellent steel material for large heat input welding.

[0015]

(Equation 3)

[0016]

(Equation 4)

3) When hot working a steel having the chemical composition described in 1) or 2) above, the hot working temperature is controlled so that the minor axis of austenite grains at the end of hot working is 60 to 700 μm. A method for producing a steel material having excellent strength and toughness, wherein hot working is completed and quenching is performed directly.

Here, the diameter of austenite grains (hereinafter referred to as γ grains) refers to the former γ grain size in the metal structure of a steel material obtained by cooling after hot working. The old γ grain boundary can be easily revealed by etching in steel containing martensite and bainite, and can be distinguished by an optical microscope and the particle size can be measured. Further, the steel material may have any shape, and typical examples include a steel plate, a steel pipe, a shaped steel and the like.

The present inventors, at the laboratory level, have developed a steel material that does not need to be refined in the production process, has a tensile strength of 400 MPa or more, and has excellent toughness and a method for producing the same. Using a small rolling mill and a heat treatment furnace for a prototype test, tests were performed under various conditions, and the strength and toughness of the obtained steel slab were investigated.

At the start of the study, the aim is to increase the production efficiency, and a grain refining method relying on controlled rolling to reduce the rolling efficiency, or a reheating step after rolling is required.
It was assumed that the grain refinement method by reheating and quenching was not used. Furthermore, the goal was to increase the finishing temperature of hot working as much as possible to improve productivity.

However, if the raw material heating temperature is raised and the rolling finishing temperature is raised without using controlled rolling, the γ grains at the end of rolling must be coarse.

If controlled rolling is used, it is possible to refine the final structure and increase toughness even with relatively coarse γ grains, but since controlled rolling is not used, γ grains can be reduced to sufficiently fine grains. It is difficult to secure toughness if it is not changed. γ
In order to reduce the size of the grains, it is necessary to continue rolling to a low temperature of 900 ° C. or less, but this cannot achieve the above goal.

In fact, when a rolling test is performed under the condition of no controlled rolling, the impact fracture surface transition temperature in the Charpy test is −50.
In order to maintain the temperature below ℃, it was necessary to reduce the γ grains to 40 μm or less. For this purpose, if the rolling is completed at 900 ° C. or lower, or reheated from the α region to reverse transformation, and the grain growth is pinned with AlN or NbC, it can be relatively easily heated at about 950 ° C. Miniaturization with a particle size of 40 μm or less was achieved. However, if the temperature exceeds 1000 ° C., the pinned particles are dissolved in solid solution and are lost, so that coarsening occurs and remarkable deterioration of toughness is caused.

The present inventors have repeatedly studied to develop a steel material having strength and toughness that can be manufactured without the need for grain refinement of γ grains in a production process. Knowledge was obtained.

(1) The steel material in which the old γ grains are coarse is deteriorated in toughness, but when S is reduced and the precipitation of MnS is suppressed in the coarse state, the transition temperature and the absorbed energy are remarkably improved. . However, when the γ grains are fine, this effect cannot be expected so much.

(2) TiN in steel has a similar adverse effect on toughness. The transition temperature is improved by reducing the amount of TiN by reducing N or Ti. However, when the γ grains are fine, the improvement is not achieved.

(3) MnS according to the above (1) and (2),
The effect of improving the toughness due to cleaning by reducing the amount of TiN precipitation can hardly be obtained when the metal structure of the steel material does not include bainite, martensite, and the tempered structure thereof.

(4) Inclusion-forming elements such as Ca and REM are also preferably reduced because coarse gamma grains similarly have an adverse effect on the transition temperature. However, the adverse effect is smaller than that of MnS or TiN, and the reduction is not as important as MnS or TiN in terms of toughness.

(5) If the amounts of precipitated MnS and TiN are sufficiently reduced, the transition temperature rise is negligible even if the γ particle size exceeds 60 μm. Further, even if a portion having a particle size of more than 100 μm is generated, a rise in the transition temperature is slight.

(6) Under conditions in which the amount of MnS and TiN precipitation is limited, hardening can be increased by increasing the size of the γ grains, thereby increasing the strength.
m or more, high strength steel can be obtained at low production cost. In addition, since there is no need for controlled rolling, γ
Since the grains can be transformed from a completely recrystallized state and the structure can be made uniform, a good product can be stably manufactured. As a guide for the recrystallization state,
The particles may be manufactured so that the average aspect ratio of the γ grains is appropriate and this value is 1.5 or less.

(7) When the γ grains are made coarse, it is necessary to appropriately reduce MnS and TiN in accordance with the minor diameter dγ of the γ grains. High toughness can be ensured within the range. However, when the γ particle size exceeds 700 μm, the adverse effect on toughness due to coarsening cannot be ignored.

The present invention has been made based on these findings, and it is not necessary to make the structure finer by controlled rolling or tempering, so that productivity can be increased.

FIG. 1 is a diagram showing the relationship between the γ grain size and toughness. Steel containing 0.019% of Ti, 0.0041% of S and 0.0057% of N (square);
06%, S: 0.0009% and N: 0.0015%
This is a result obtained by performing hot rolling at various rolling temperatures using steel (■) containing, and examining an impact fracture surface transition temperature by a Charpy test.

As can be seen from FIG. 1, the toughness decreases as the crystal grain size increases, regardless of whether the content of Ti, S and N is large or small, but as shown by the black squares in the figure,
When the N content and especially the S content are reduced, the toughness is significantly improved. Also, it can be seen that the effect of improving toughness is remarkable when the γ particle size is large.

[0035]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition of a steel material according to the present invention will be described below (hereinafter,% is expressed by weight%
Is shown).

C is necessary to secure the strength, and if it is less than 0.02%, the required strength cannot be secured. On the other hand, if it exceeds 0.15%, the toughness of both the weld heat affected zone and the base metal deteriorates when welding is performed. Therefore, the content of C is set to 0.02 to 0.15%.

Si Si has a deoxidizing effect and also contributes to an increase in the strength of the steel sheet. However, if the content exceeds 1%, the toughness is reduced, so 1% is made the upper limit. In addition, as long as it does not hinder deoxidation of steel, Si has at least no problem.

Mn Mn has an effect of improving hardenability and is an effective component for securing strength. If the content is less than 0.3%, strength and toughness cannot be obtained due to insufficient hardenability. On the other hand, 2.5%
If it is contained in excess of, the segregation increases and the hardenability is too high, and the toughness of both the weld heat affected zone and the base metal decreases during welding. Therefore, the content of Mn is 0.3 to
2.5%.

PP is inevitably present in steel as an impurity. 0.0
If the content exceeds 5%, not only segregation at the grain boundaries causes a decrease in toughness, but also hot cracking at the time of welding is caused.

SS combines with Mn, Ca and REM to form oxysulfide, and is present in steel as inclusions. If the strength of the steel is low, or if the structure is sufficiently fine, these do not have a significant adverse effect on toughness, but if the structure is coarse to some extent, the content may satisfy the formula described below. Must be restricted to However, even if the formula is satisfied, if the content is 0.004% or more, an adverse effect on toughness cannot be avoided. More preferably, it is less than 0.003%.

Sol.Al Al is an essential element for deoxidation, and unless 0.001% or more of sol.Al is contained, insufficient deoxidation causes deterioration of steel quality. However, if the content exceeds 0.1%, the toughness of the base material is deteriorated and the toughness of the welded portion is reduced, which is not preferable. Therefore, the upper limit is 0.1%.

Ti Ti is usually contained as an element for fixing N in steel and improving high-temperature ductility. However, since TiN causes a decrease in toughness, it is desirable not to add Ti as much as possible, and the allowable range in terms of toughness is limited by the following formula. However, even if the formula is satisfied, if the content exceeds 0.02%, deterioration of toughness is inevitable.

For steel materials subjected to high heat input welding, excessive cleaning may cause excessive coarsening of γ grains and deterioration of toughness, so that Ti is contained at 0.004% or more. Further, it is preferable to control the ratio of Ti / N to a range of 0.4 or more and 4.0 or less.

NN is an impurity that causes a decrease in high-temperature ductility, and usually avoids adverse effects by adding Ti and fixing in the form of TiN. However, in the present invention, it is necessary to suppress the formation of TiN because TiN itself causes deterioration of toughness. Therefore, N itself is reduced or Ti content is reduced.

The range of the N content for obtaining excellent toughness needs to satisfy the following formula, but even if the formula is satisfied, if N exceeds 0.009%, the toughness of TiN The adverse effect on the toughness due to the decrease or the solid solution of N that is not sufficiently fixed cannot be ignored. Also, N is set to 0.
When the content is 001% or less, the growth of γ grains becomes very easy under the condition that S is reduced and MnS hardly exists.
For this reason, by using a submerged arc welding method, etc.
When welding is performed with a large heat input of about 0 kJ / cm, γ grains may locally become coarse in the heat affected zone.

The steel material of the present invention has the property that the toughness does not easily deteriorate due to the coarsening of γ grains, but the hardness is distributed in the heat-affected zone of the large heat input welding, and the size of the crystal grains is also small. Since non-uniformity occurs, the upper limit of the γ grain size allowed from the toughness side is about 300 μm. For this reason, when large heat input welding is presupposed, TiN having an effect of inhibiting the growth of γ grains must be contained to some extent, and it is preferable that N is contained at 0.001% or more. Of Ti is also preferably contained.

On the other hand, a steel material that does not require welding,
For steel materials that only perform small heat input welding of / cm or less,
N may be reduced as much as is economically permissible.

The steel material of the present invention may contain the following elements as necessary in order to improve hardenability, strength and the like, in addition to the above elements.

Cr Cr is an element useful for improving hardenability. The minimum necessary hardenability is secured by only the above-mentioned essential elements, but when the steel material is a thick steel pipe or the like, it is used to secure further hardenability. When the Cr content is 0.02% or more, the effect of increasing the tempering softening resistance in addition to the hardenability can be obtained. Therefore, it is preferable to set the Cr content to 0.02% or more. But,
If it exceeds 1.5%, the toughness of the weld cannot be avoided.

In the case where the Mo steel material is a thick steel pipe or the like, it is preferable to include the Mo steel material in order to further enhance hardenability and temper softening resistance.
If the content is less than 0.02%, these effects cannot be obtained. Therefore, the content is preferably set to 0.02% or more. But,
If it exceeds 1%, the toughness of the welded portion deteriorates significantly, so the upper limit is preferably 1%.

BB particularly enhances the hardenability of the γ grain boundary and contributes to an increase in strength. The content is preferably set to 0.003% or less.

Nb Nb is an indispensable element in steel materials produced by so-called controlled rolling, but is not an essential element in the present invention because controlled rolling is not basically used.
However, although effective to further increase the strength, if it is contained in a large amount, when rolling is terminated at a high temperature of 1000 ° C. or more, the toughness is significantly impaired through precipitation strengthening. for that reason,
The content must be less than 0.015%. More preferably, it is 0.01% or less.

VV has the effect of increasing the strength by precipitation strengthening, has relatively little adverse effect on toughness, and is effective for increasing the strength. When the content is 0.01% or more, the effect of improving the hardenability as well as the tempering softening resistance can be obtained.
It is desirable that the content be 0.01% or more. However, 0.15
%, The toughness is greatly deteriorated.
It is good to make it 5% or less.

Cu Since Cu is effective for increasing the strength and improving the corrosion resistance, it is preferable to include Cu when further higher yield strength and higher corrosion resistance are required. When the content is 0.05% or more, the hardenability in direct quenching is increased, so that it is desirable to set the content to 0.05% or more. However, even if it exceeds 1.5%,
Since there is no improvement in performance corresponding to the cost increase, the upper limit is preferably 1.5% or less.

Ni Ni has the effect of increasing the toughness of the steel matrix (matrix) in a solid solution state, and is preferably contained when it is necessary to stably obtain more excellent toughness. If the content is 0.05% or more, the effect of improving hardenability can be obtained, so it is preferable to set the content to 0.05% or more. But 4
%, The toughness cannot be improved in proportion to the increase in alloy cost. Therefore, the upper limit is preferably 4%.

Ca Ca reacts with S in steel to form sulfate in molten steel. This sulfate, unlike MnS or the like, does not elongate in the rolling direction by rolling, and remains spherical after rolling. Therefore, welding cracks or hydrogen-induced cracks (HIC: Hydrogen I
nduced cracking) to prevent welding cracks or H
It is desirable to include it when suppressing IC. When the content is 0.0002% or more, the effect of improving the toughness of the welded portion is also effective. Therefore, the content is desirably 0.0002% or more. However, when the content exceeds 0.004%, the base material toughness decreases due to a decrease in cleanliness.

REM REM contributes to the refinement of the structure of the weld heat affected zone and the fixation of S, but reduces the cleanliness as inclusions. However, inclusions formed by the addition of REM have relatively little effect on toughness degradation, so that if they are contained at 0.004% or less, a decrease in the toughness of the base material can be tolerated.

Next, the metal structure and the prior austenite grains will be described.

1) Metallographic Structure The metallic structure of the steel material is a structure containing one or both of bainite and martensite formed by transformation at a low temperature, or a tempered structure thereof, in order to increase the tensile strength to 450 MPa or more. And a structure containing ferrite and pearlite. Such a structure is obtained by performing quenching from the γ region after hot rolling and, if necessary, performing tempering.

2) Aspect Ratio of Old γ Grains The reason why the average value of the aspect ratios of old γ grains is 1.5 or less is to reduce anisotropy and prevent a decrease in strength. In the γ grains containing dislocations inside after being processed, the α phase nucleates from the dislocations in the grains, so that the hardenability is reduced and the strength is reduced. In order to prevent this, the γ grains are sufficiently recrystallized (the recrystallized γ grains have an aspect ratio of 1).
To get rid of). If the average value of the aspect ratio of the old γ grains is 1.5 or less, it is possible to prevent the strength from decreasing.

The average value of the aspect ratio is determined by selecting a direction in which the surface where γ grains are most elongated can be observed, cutting out a sample for an optical microscope, and revealing a microstructure.
This is a value obtained by measuring old γ grains by image processing and averaging the ratio of the major axis to the minor axis when each γ grain is approximated by an ellipse.

3) Average minor diameter of old γ grains In the present invention, old γ grains are relatively coarse because low temperature processing is not performed to refine the structure in order to increase production efficiency. In addition, the effect of reducing Ti, N and S on toughness and strength becomes remarkable by making the grains coarse. If the average short diameter of the former γ grains is less than 60 μm, the desired strength and toughness cannot be obtained. On the other hand, if it exceeds 700 μm, the grains become too coarse and the toughness deteriorates.

4) Relational expression between Ti, N, S and minor axis of old γ grains When the Ti / N content ratio Ti / N is less than 3.4, that is, the N content is larger than the Ti content. In this case, if the following expression (1) is not satisfied, the contents of Ti and S become too large, so that TiN and MnS become large and the toughness deteriorates. This equation was obtained from many experiments, and
The amount of Ti and S is determined according to the minor diameter of the grain.

[0064]

(Equation 1)

The Ti / N content ratio Ti / N is 3.4.
If the above is satisfied, that is, if the N content is smaller than the Ti content, unless the following formula (2) is satisfied, the N and S contents become too large, the TiN and MnS become too large, and the toughness deteriorates.

Next, when the steel material of the present invention is used for large heat input welding, the Ti / N should be in the range of 0.4 to 4 and the following formulas (3) and (4) must not be satisfied. In the heat-affected zone of the weld, the gamma grains become too coarse and the toughness of that portion deteriorates. That is, when performing large heat input welding, it is necessary to suppress the grain growth in the heat affected zone by slightly increasing the amounts of Ti and N to precipitate TiN and MnS to some extent.

[0067]

(Equation 3)

[0068]

(Equation 4)

Hereinafter, the method for producing a steel material of the present invention will be described.

When hot-working steel having the above-mentioned chemical composition, the minor axis of austenite grains at the end of hot-working is 60 to 60%.
The hot working temperature is controlled so as to be 700 μm, the hot working is completed, and direct quenching provides a steel material having excellent strength and toughness.

The hot working temperature at which the minor axis of the austenite grains at the end of the hot working is 60 to 700 μm differs depending on the chemical composition and the working degree at the time of hot working.
As a guide, it is recommended to keep the temperature above ℃. As long as the old γ particle size does not exceed 700 μm, good performance can be obtained even if the processing finish temperature is high, but it is difficult to secure a processing finish temperature exceeding 1150 ° C. in an actual production line. Further, at such a high temperature, loss of steel material due to generation of scale increases. From such a viewpoint, the working finish temperature is about 1150 ° C. as a practical upper limit.

It is not necessary to perform hot rolling to a low temperature for the purpose of securing toughness. However, if the rolling is performed by 30% or more in a temperature range of 900 ° C. or less, the effects of finer γ grains and controlled rolling appear. ,
The strength may be greatly reduced. Such a property is
When mass-producing steel materials, it is necessary to avoid processing at a low temperature because it causes variation in quality and is not preferable.
In order to avoid this adverse effect, the hot-rolling termination temperature must be controlled so that the old γ grain size does not fall below 60 μm, and so that the γ grains are water-cooled from a state where they are not work hardened. .

Cooling for quenching after processing is not necessarily water-cooled, but at least the structure after transformation is bainite or martensite.
Desirably, it occupies an area of 0% or more, and such a cooling rate is required. Such cooling condition is C
It can be estimated from the CT diagram.

[0074]

EXAMPLES In a vacuum melting furnace, 150 kg round ingots having 16 kinds of chemical compositions shown in Table 1 were melted. Also,
In an actual 250t converter, steels having ten kinds of chemical compositions shown in Table 2 were melted and continuously cast to a thickness of 150 to 300 m.
m slab.

[0075]

[Table 1]

[0076]

[Table 2]

As shown in Table 3, the ingot was formed into a thick plate having a thickness of 120 to 170 mm by forging.
After heating to 70 ° C., a hot-rolled steel sheet having a thickness of 25 to 50 mm was formed by hot rolling.

Further, the continuously cast slab was heated to 1200 to 1250 ° C. as shown in Table 4, and then hot-rolled to obtain a hot-rolled steel sheet having a thickness of 25 to 40 mm. Each of these hot-rolled steel sheets was subjected to a water quenching treatment as shown in Tables 3 and 4, and a part was subjected to a heat treatment of quenching and tempering.

[0079]

[Table 3]

[0080]

[Table 4]

A JIS No. 4 Charpy test piece and a round bar tensile test piece were sampled from each heat-treated hot-rolled steel sheet and subjected to a Charpy impact test and a tensile test, respectively.

For the hot-rolled steel sheets 17 to 26, welded joints of submerged arc welding were prepared and subjected to a Charpy impact test. Welding is done on both sides of V groove, and welding heat input is 70kj / c for 30t or less.
m and 30 t, it was 100 kj / cm.

FIG. 2 is a diagram showing a sampling position of a Charpy impact test piece. As shown in the figure, the weld metal part 1 and the weld heat affected zone 2 on the fracture surface (notch portion) of the test piece 1 after the test.
Was almost evenly split.

The results of each test are shown in Tables 3 and 4. As is apparent from Table 3, despite the fact that the γ grains are coarse as a result of finishing the rolling at a high temperature ranging from 990 ° C. to 1100 ° C., in the example of the present invention, It shows sufficient toughness that can be used for applications. In addition, since the Nb content is low or the component does not contain Nb at all, it is disadvantageous for securing the strength, but the hardenability is increased by coarsening the γ grains, so that 400 to 500 MPa Yield point strength exceeding.

Table 4 shows the results of the reproducible heat cycle test in addition to the mechanical properties of the base material.

[0086] Regarding the reference numerals 17 to 21, the toughness of the weld heat affected zone is also good. However, the symbols of the comparative examples, 22, 2
In the cases of Nos. 4 and 25, the toughness after the heat treatment was good, but the toughness of the heat-affected zone was significantly deteriorated due to excessive cleaning of the steel.

As for the forged materials having the chemical components indicated by symbols 1 to 8 shown in Table 1, hot rolling was performed at a relatively low temperature of 900 ° C. or less. Table 5 shows the rolling conditions and heat treatment conditions. Each test similar to the above was performed from the heat-treated steel sheet. The results are shown in Table 5.

[0088]

[Table 5]

Although the grains were refined by low-temperature rolling and the toughness was slightly improved, as can be seen from Table 3, the strength was greatly reduced. Considering that sufficient toughness was already secured at the stage of Table 3, such low-temperature rolling finish only impairs strength and has no merit. Rather, Table 4
In consideration of the fact that the steel reheated and quenched to a high temperature exhibited good strength and toughness, the steel whose temperature was too low during the rolling as shown in Table 5 was re-cooled before water cooling. Good performance can be obtained by heating to 1000 ° C. or higher in a heating furnace to sufficiently recrystallize and coarsen γ grains and then cooling with water.

[0090]

According to the present invention, a steel material having high strength and excellent toughness can be produced with a smaller alloy addition amount while maintaining high productivity. This means that it is possible to increase the production of high-performance steel materials without adding new facilities, which is extremely useful in steel production.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a crystal grain size and toughness.

FIG. 2 is a diagram showing a sampling position of a Charpy impact test specimen.

Claims (3)

[Claims] C .: 0.02 to less than 0.15%, Si: 1% or less, Mn: 0.3 to 2.5%, P:
0.05% or less, S: less than 0.004%, sol. Al:
0.001-0.1%, Ti: 0.02% or less, N:
0.009% or less, the metal structure is a structure containing one or both of martensite and bainite, or a tempered structure thereof, and the average value of the aspect ratio of the prior austenite grains is 1.5 or less; Has an average minor axis of 60 to 700 μm, and Ti, N, S
And a steel material having excellent strength and toughness, characterized in that the former austenite grain minor diameter dγ satisfies the following formula (1) or (2). (Equation 1) (Equation 2) 2. In% by weight, C: 0.02% to less than 0.15%, Si: 1% or less, Mn: 0.3 to 2.5%, P:
0.05% or less, S: less than 0.004%, sol. Al:
0.001-0.1%, Ti: 0.004-0.02
%, N: 0.001 to 0.009%, Ti / N is 0.4 to 4, and the metal structure is a structure containing one or both of martensite and bainite, or a tempered structure thereof. The average value of the aspect ratio of the prior-austenite grains is 1.5 or less, the average value of the minor axes of the prior-austenite grains is 60 to 700 μm, and the minor axis dγ of Ti, N, S and the prior-austenite grains is represented by the following formula (3). A large heat input welding steel excellent in strength and toughness characterized by satisfying (4). (Equation 3) (Equation 4) 3. The hot working temperature of the steel having the chemical composition according to claim 1 or 2 is controlled such that the minor axis of the austenite grains at the end of the hot working is 60 to 700 μm. A method for producing a steel material having excellent strength and toughness, wherein hot working is completed and direct quenching is performed.