JPH09202919A - Production of high tensile strength steel material excellent in toughness at low temperature - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high tensile strength steel material excellent in toughness at low temp. by successively applying heating, roughing, cooling, and finish rolling under respectively specified conditions to a slab of a steel in which composition and oxide grain content are respectively specified. SOLUTION: A slab of a steel, having a composition which consists of, excluding oxide grains, 0.01-0.20%, by weight, C, 0.03-1.0% Si, 0.30-2.0% Mn, 0.001-0.1% Al, 0.001-0.01% N, and the balance Fe with inevitable impurities and in which the contents of P and S as impurities are limited to <=0.015% and <=0.010%, respectively, and further containing oxide grains of 0.1-1μm grain size by 0.02-0.5 pieces per μm<2> of area of base material, is heated to a temp. between the Ac3 transformation point and 1250 deg.C. After roughing in the austenitic region at 10-70% cumulative draft, cooling is performed at a rate of (0.1 to 10) deg.C/sec until finish rolling is started. Then, from the state of 50-90% ferrite proportion, finish rolling of 30-90% cumulative draft is finished at 650-750 deg.C. By this method, the high tensile strength steel material, having <= about 3μm average α-grain size and also having ultra-fine grained α-structure, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength steel material used for a structural member that requires low temperature toughness. The steel material manufactured by this method can be generally used for welded steel structures such as marine structures, pressure vessels, shipbuilding, bridges, buildings, line pipes, etc., but particularly marine structures requiring low temperature toughness, shipbuilding It is useful as a structural steel material. Although the form of the steel material is not particularly limited, it is particularly useful as a steel plate used as a structural member and requiring low temperature toughness, particularly a thick plate, a steel pipe material, or a shaped steel.

[0002]

2. Description of the Related Art Since high-strength steel materials are often used as structural steels, low temperature toughness is required from the viewpoint of ensuring the safety of structures. Although various methods for improving low temperature toughness have been proposed for high-strength steel materials, as a method for improving low temperature toughness without causing deterioration of other characteristics without using an expensive alloying element such as Ni, ferrite ( The particle size is typically reduced.

Various methods have heretofore been proposed as a method for reducing the α particle size. Typical methods include, for example, Japanese Patent Publication No. 49-7291 and Japanese Patent Publication No. 57-2100.
As disclosed in Japanese Patent Publication No. 7 and Japanese Patent Publication No. 59-14535, etc., controlled rolling is performed in the unrecrystallized temperature range of austenite (hereinafter referred to as γ), and then γ to α by performing accelerated cooling. There is a method of refining α during the transformation into.

In the methods utilizing the transformation from γ to α as described above, when γ is coarse, the γ / α conversion ratio (γ grain size before transformation / α grain after transformation) is effectively utilized by effectively utilizing the non-recrystallization zone rolling. It is possible to make α smaller by increasing the (particle size), but when the γ particle size is small, the γ / α conversion ratio approaches 1, so α
The miniaturization of is saturated. Therefore, in the method of refining α through the transformation from γ to α, the degree thereof is regulated by the degree of refining of γ, and thus the drastic miniaturization of the α particle size cannot be expected.

In order to solve this problem, there has been proposed a technique for improving strength and toughness by so-called two-phase region rolling in which the temperature range of controlled rolling is expanded to the γ / α two-phase region. For example, as disclosed in Japanese Patent Publication No. 58-5967, there has been proposed a technique for improving toughness by rolling in a two-phase region by devising the composition and rolling conditions. However, in these conventional two-phase rolling techniques, the α grain size is comparable to the α grain size obtained by controlled rolling, and it is essentially parallel to the steel plate surface at the time of fracture due to the texture called separation, which is mainly due to the texture. The toughness is improved by using the effect of reducing the triaxial stress due to the occurrence of the layered cracks that occur in the. However, although separation is effective in reducing the fracture transition temperature in the Charpy test,
There is a limit to its use because it causes a decrease in absorbed energy.

There is also disclosed a method of reducing the α grain size by heat treatment without using hot working such as rolling. For example, [Iron and Steel, 1977, No. 1, 1991, 171
, Pp. 178], V and N are added in a larger amount than usual to make γ finer and the γ / α conversion ratio at the time of transformation is increased to perform normalizing treatment. A method of making a fine α-structure has been developed. But,
To obtain a fine α-structure by this method, V is 0.1%.
As described above, it is necessary to add N by 0.01% or more, and the attainable α particle size is about 5 μm.

Furthermore, [Materials and Processes, Third Year, Sixth Year]
No., 1990, p. 1796], the grain size was 3 μm due to complicated thermomechanical treatment including repeated γ / α transformation.
The following methods for obtaining ultra-fine grain steel are shown. In this method, after controlled rolling, accelerated cooling is performed, accelerated cooling is stopped at about 500 ° C, and then 900 ° C without cooling to room temperature.
It is reheated to, and hot-rolled at a predetermined temperature to obtain ultra-fine grain steel. However, the α particle size is strongly affected by the cooling stop temperature, and the cooling stop temperature is 50
Ultrafine particles α having a particle size of 3 μm or less have not been obtained except in the very vicinity of 0 ° C., and it is considered difficult to industrially stably manufacture them.

Therefore, in all of the above-mentioned conventional methods, high cost is inevitable due to deterioration of productivity, increase of heat treatment steps, and increase of alloy elements. Further, the α particle size that can be stably obtained is about 10 μm except for some experimental methods, and the limit is about 5 μm even by a complicated process that is strictly controlled. Has not been realized.

[0009]

SUMMARY OF THE INVENTION The present invention has an average α particle diameter of 3 μm or less, without adding expensive alloying elements or complicated hot working or heat treatment step with poor productivity.
Further, the present invention provides a method for producing a high-strength steel material having excellent low-temperature toughness, which has an ultrafine-grained α-structure having a uniform mixed grain size.

[0010]

Since the γ / α transformation, which is a conventional typical grain refining method, has limitations, attention is paid to a method utilizing the recovery / recrystallization of α by hot working. The present invention has been completed by finding out means for ultra-fine graining of α through detailed investigation of hot working behavior.

That is, the gist of the present invention is as follows.
It is as described. (1) As a component excluding oxide particles, in weight% (hereinafter
Same), C: 0.01 to 0.20%, Si: 0.03 to
1.0%, Mn: 0.30 to 2.0%, Al: 0.00
1-0.1%, containing N: 0.001-0.01%,
P and S as impurities are P ≦ 0.015%, S ≦ 0.
Acid with a particle size of 0.1 to 1 μm.
Base material area of 1 μm^Two0.02-0.5 per
A steel slab containing the balance Fe and unavoidable impurities
c _ThreeIn the austenite range, heated to the transformation point to 1250 ° C
After rough rolling with cumulative reduction of 10 to 70%, start finish rolling
To 0.1-10 ° C / sec until the ferrite content is
From the state of 50 to 90%, the cumulative rolling reduction is 30 to 90%
Finishing rolling is finished at a temperature of 650 to 750 ° C.
Of high strength steel with excellent low temperature toughness
Law.

(2) Further, Cr: 0.01 by weight%
.About.0.50%, Ni: 0.01 to 3.0%, Mo: 0.
01 to 0.50%, Cu: 0.01 to 1.5%, Ti:
0.003-0.10%, V: 0.005-0.20
%, Nb: 0.003 to 0.05%, B: 0.0003
~ 0.0020%, Ta: 0.01-1.0%, W:
0.01-1.0% of 1 type (s) or 2 or more types are contained, The manufacturing method of the high-strength steel material excellent in the low temperature toughness of the said (1) description characterized by the above-mentioned.

(3) Further, in weight%, Ca: 0.00
05-0.005%, REM: 0.0005-0.02
% Of 1 type or 2 types, The manufacturing method of the high strength steel material excellent in the low temperature toughness as described in said (1) or (2) characterized by the above-mentioned.

(4) After finishing rolling, 5 to 5
The method for producing a high-tensile steel material excellent in low-temperature toughness according to any one of (1) to (3) above, which comprises accelerating cooling to 550 to 20 ° C at 0 ° C / sec. (5) The method for producing a high-strength steel material excellent in low-temperature toughness as described in (4) above, which comprises performing accelerated cooling and then tempering at 400 to 650 ° C.

Here, the oxide particles mean α in the processing temperature range.
Refers to thermally stable oxide particles that are harder than the phases. The type of the oxide particles is not limited, but an oxide that is uniform and easily dispersed in a large amount is preferable. As a result of examination by experiments, preferable oxides from the viewpoint of the dispersed state are Ta oxide, Nb oxide, Ti oxide, Mg oxide, and a composite oxide containing one or more of the above oxide-forming elements, and Ti. Al complex oxide, complex oxide consisting of Ti, Al and Mg, Ti as the main component, and Al, Ca, La, Ce and Y as 1
There are complex oxides containing one or more species. Furthermore, oxides containing a small amount of Si, Mn, and Fe in the above-mentioned oxides also have the same effect.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The present invention is characterized in that a method by recovery / recrystallization of processed α is used as a means for making α finer than the level achieved conventionally. That is, in the conventional two-phase region rolling technology, the strain introduced into α by the two-phase region rolling works for the development and strengthening of the texture, but it is positively used for the refinement of α. On the other hand, in the present invention, the processing strain introduced in the γ / α two-phase rolling allows the processing α to be recovered and recrystallized to the utmost, thereby achieving the ultrafine graining of α. It is a thing. From the viewpoint of producing uniform fine grains without impairing productivity, recovery and recrystallization of α after processing is not a method like conventional reheating heat treatment, but is preferably performed during cooling after rolling, preferably during rolling. It is more advantageous to generate it during or immediately after rolling.

The present inventors examined the conditions for making α into ultra-fine grains by rolling, set the γ grain size before transformation to 50 μm or less, and ensured the ratio of α in the two-phase region rolling to 50% or more. By doing so, it was found that an ultrafine grain structure having an average grain size of 3 μm or less can be achieved (Japanese Patent Application No. 6-198829). However, in order to reduce the γ grain size, it was necessary to increase the cumulative reduction ratio in the γ region and perform rolling in the non-recrystallized region, which sometimes caused some restrictions on the plate thickness and productivity. Therefore, as a result of investigating another new method of ultra-fine-graining of α, when α is directly processed and ultra-fine-grained by recovery / recrystallization, the γ-phase is slightly smaller than that of α-single phase processing. It was found that the existing particles are uniformly finely divided. This is because the dispersal of the hard γ phase makes the introduction of dislocations into α more uniform during processing. However, it is difficult to disperse the γ phase in the transformation sufficiently uniformly, and even if the γ phase is harder than the α phase, it can be deformed in the rolling temperature range, so that the uniform introduction of dislocations into α is introduced. The effect on is saturated. Therefore, even if the variance of γ is used as much as possible, it has a limit. Therefore, if the present inventors can finely disperse another hard phase more than the γ phase in addition to the dispersion of the γ phase, the introduction of dislocations into α can be made more uniform and ultrafine graining cannot be facilitated. Therefore, as a result of various studies, it was found that by appropriately dispersing fine oxide particles, ultrafine graining of α by the two-phase region processing can be achieved more easily.

The oxide particles may be of any type as long as they are harder than the α phase in the processing temperature range. However, an oxide that is uniform and easily dispersed in a large amount is preferable. As a result of examination by experiments, as a preferable oxide from the viewpoint of the dispersed state,
Ta oxide, Nb oxide, Ti oxide, Mg oxide, and a composite oxide containing one or more of the above oxide-forming elements,
Further, a complex oxide of Ti and Al, a complex oxide of Ti, Al and Mg, and Al, Ca and La mainly containing Ti,
There are complex oxides containing one or more of Ce and Y.
In addition, trace amounts of Si, Mn, and Fe are added to the oxides listed above.
Oxides containing are also effective.

The particle size of the oxide particles is required to be 0.1 μm or more. If the particle size is less than 0.1 μm, the function as an obstacle for uniforming the deformation of the α phase is small, and rather the recovery / recrystallization may be delayed in some cases. The larger the particle size is, the more effective it is in homogenizing α, but 1 μm
If it is over, the effect will be saturated. Therefore, the oxide particles having a diameter in the range of 0.1 to 1 μm are effective for ultrafine graining of α through uniform deformation of α.

In order that the oxide particles having a particle size in the range of 0.1 to 1 μm are substantially effective for the ultrafine graining of α, the number of oxide particles must be a certain number or more. Based on the findings based on detailed experiments, the required number of particles is at least 0.02 per 1 μm ^2.
Individual. Similarly, the upper limit of the number of particles is 1 from the experiment.
0.5 is suitable per μm ² . This is a conclusion derived from the viewpoint that even if the number of particles is increased unnecessarily, the effect is saturated and the material properties such as ductility and toughness are particularly deteriorated from the viewpoint of uniform deformation of α. In the present invention, the upper limit of the number of oxide particles having a particle diameter of 0.1 to 1 μm is 0.5 per 1 μm ² from the viewpoint of maximizing the ultrafine graining of α without inhibiting ductility and toughness. And

Although it is preferable that the number of particles deviating from the particle diameter range of the present invention is small, the particle diameter is 0.1 μm as a range which does not substantially show the adverse effect on the refinement of α and the material.
less than 0.5 per 1 μm ² for particles less than m,
0.0 per 1 μm ² for particles with a diameter of more than 1 μm
It may contain less than 005 pieces. The oxide particle size and number in the present invention are measured directly from a photograph by observing an extracted replica of the steel material with an electron microscope. At this time, select an appropriate magnification from 1000 to 50000 times and 3
100 or more particles are measured in the field of view or more. When the shape of the particle is not a circle, the equivalent circle diameter obtained from the area of the particle is the particle size.

With respect to the type of oxide particles, an effect is produced if the strength difference between α and γ phase is larger than the strength difference between α phase and γ phase in the temperature range where processing to α is performed. From the viewpoint of producing a sufficiently stable effect, it is preferable that the hardness is 3 times or more harder than α in the working temperature range. In the present invention, the method of dispersing the oxide particles in the steel is arbitrary, as a specific method, by adding an oxide-forming element to the molten steel to bond with oxygen in the molten steel, Primary deoxidation product or 2
The method of dispersing as the next deoxidation product, or if the melting point of the oxide particles to be dispersed is sufficiently higher than that of the molten steel, direct addition of fine powder of oxide particles into the molten steel increases the plate thickness and material size. It is an industrially preferable method for structural high-strength steel materials that need to be processed.

The above are the most important requirements for oxide particles in the present invention. In the present invention, the production conditions and chemical components are determined for the following reasons, assuming that the oxide particles specified in the present invention are dispersed. First, in the present invention, the heating temperature of the steel slab is set in a range from the Ac ₃ transformation point to 1250 ° C. If the heating temperature is lower than the Ac ₃ transformation point, solution treatment is not sufficiently carried out.
If the heating temperature is higher than 50 ° C., the heating γ grain size becomes extremely coarse, and subsequent rolling causes α before the start of the two-phase rolling.
This is because it may be difficult to reduce the grain size.

Since it is necessary to refine the α grain size before processing at the time of performing the two-phase region rolling, rough rolling is performed in the γ region prior to the two-phase region rolling. In the present invention, the presence of oxide particles is present. As a result, the reduction condition can be greatly relaxed compared to the conventional one, and the cumulative reduction ratio in rough rolling may be in the range of 10 to 70%. However, even if the required amount of oxide particles are dispersed, if the cumulative rolling reduction in the γ region is less than 10%, it is not possible to sufficiently miniaturize the final structure. Also, the larger the cumulative rolling reduction in the γ region, the more advantageous it is to refine the α grain size before entering the two-phase region rolling. However, if it exceeds 70%, the effect saturates, and the cumulative rolling reduction in the γ region If the reduction ratio is too large, the reduction ratio of the two-phase region rolling as finish rolling cannot be sufficiently obtained, which is disadvantageous for ultra-fine graining, and the plate thickness range that can be manufactured is restricted. Therefore, the upper limit is 70%. Restricted to.

After rolling in the γ region and before finishing rolling in the two-phase region, it is necessary to regulate the cooling rate in order to generate a required amount of fine α. That is, if the cooling rate during this period is less than 0.1 ° C./second, α is easily generated, but α that is generated becomes coarse, and it becomes difficult to refine the final structure after the two-phase region rolling. On the other hand, the cooling rate during this period is 10
If the temperature exceeds ℃ / sec, α formation will be suppressed, and if an attempt is made to secure the required α amount, the temperature of finish rolling will inevitably decrease, and recovery and recrystallization after processing will be insufficient, resulting in ultra-fine grain formation. Graining and reduction of dislocation density of matrix are not sufficiently achieved, and improvement of material cannot be expected. Therefore, the cooling rate until the start of the two-phase region rolling as finish rolling needs to be in the range of 0.1 to 10 ° C./sec.

Further, the α fraction at the stage of entering finish rolling in the two-phase region is also important. Considering the induced transformation during the two-phase region rolling, if the α fraction before the start of rolling can be secured at 50% or more. In the final structure, α can be uniformly fine-grained. The upper limit of the α fraction is specified to 90%, but this is because the slightly remaining hard γ phase is effective in uniformizing the α processing and finely sizing, and exerts its effect. Γ is 10
This is because it is preferable that the content is at least%. Further, incidentally, when cooling is performed until α becomes 90% or more, the temperature becomes substantially lower than the lower limit temperature at which α can be recovered and recrystallized. Therefore, the α fraction at the time of entering the two-phase region rolling is 50 to 90%.

Regarding the cumulative rolling reduction of the two-phase rolling, α becomes finer as the cumulative rolling reduction of the two-phase rolling increases.
In order for recovery / recrystallization to take place in a short time after rolling, and for subsequent ultrafine graining to be sufficiently achieved, it is necessary to keep the cumulative rolling reduction of the two-phase region rolling above a certain level. In the present invention, since the introduction of dislocations into α is made uniform by dispersing the oxide particles, the required reduction ratio of the two-phase region rolling may be smaller than that in the conventional technique. According to the experiment, the required cumulative rolling reduction in the two-phase region rolling is 30% or more. In the case of two-phase rolling, the rolling temperature is inevitably low and recrystallization does not occur in a short time during rolling, so it suffices to specify only the cumulative reduction ratio. Time does not matter. On the other hand, the larger the cumulative rolling reduction of the two-phase rolling, the more effective it is for grain refinement. However, even if the rolling reduction exceeds 90%, the effect saturates, and the rolling time becomes long and the finishing temperature is substantially secured. Therefore, the upper limit is 90% in consideration of economic efficiency.

However, if the end temperature of the two-phase region rolling is too low, no matter how the α grain size before the two-phase region rolling is refined, the α recovery and recrystallization after the rolling will not proceed sufficiently, and thus the ultra-fine grain size will be too high. Since the grain refinement and the reduction of dislocations in the α matrix are insufficient, the lower limit temperature at which recovery / recrystallization proceeds under the condition that the cumulative rolling reduction of the two-phase region rolling is 30% or more is determined based on the experimental results. The two-phase region rolling end temperature is 650 ° C. or higher. The upper limit of the end temperature of the two-phase region rolling is 750 ° C. In the two-phase rolling, the end temperature may be higher than the start of rolling due to heat generated during processing. In this case, if the temperature rises excessively, the obtained ultrafine particles α grow and become coarse and mixed particles, so as an end temperature sufficient to prevent this, based on the experimental results, the upper limit of the end temperature is 750 ° C.

Further, as described below, as the cooling condition after the two-phase region rolling, it is possible to perform accelerated cooling according to the desired characteristics. If the time until the start of cooling is extremely short, there is concern that recovery and recrystallization will not proceed sufficiently. According to the actual manufacturing results, recovery and recrystallization sufficiently proceed within the transportation time from the end of rolling in the actual steel plate manufacturing to the cooling equipment for accelerated cooling. It is preferable to secure a time for this recovery / recrystallization of 20 seconds or more.

As for the heat history of the steel sheet after the completion of the two-phase rolling, various heat histories can be received in order to obtain desired mechanical properties within the range in which the structure produced at the end of rolling is preserved. Is. That is, after rolling, it may be allowed to cool as it is, or after rolling, it may be subjected to accelerated cooling, or after accelerated cooling, it may be subjected to tempering treatment. However,
In the case of accelerated cooling, the cooling rate needs to be 5 ° C./second or more in order to exert the effect of accelerated cooling. However, even if the cooling rate exceeds 50 ° C./sec, the effect of improving the structure control and mechanical properties is saturated, so the cooling rate range in accelerated cooling is set to 5 to 50 ° C./sec. Further, for the same reason, accelerated cooling needs to be performed up to 550 ° C. or lower, but the metallurgical factor affecting mechanical properties changes substantially at room temperature, so the lower limit of the cooling stop temperature is 20 ° C
And

When tempering is performed after the accelerated cooling to adjust the strength, improve the toughness, etc., the tempering temperature is 650 because the ultrafine grain structure obtained by rolling needs to be preserved.
It must be limited to below ℃. However, the components of the present invention,
In the structural range, tempering is expected to improve mechanical properties from 400 ° C or higher, so the tempering temperature range is 4
The temperature is set to 00 to 650 ° C.

The above are the reasons for limiting the present invention with respect to the production conditions. In order to secure desired strength and low temperature toughness, not only the production method but also the chemical composition must be within an appropriate range. The reasons for limiting the chemical components in the present invention are described below. C is added as an effective component for improving the strength of steel. If it is less than 0.01%, it is difficult to secure the strength required for structural steel, and if it is added in excess of 0.20%, the toughness and The range of 0.01 to 0.20% is set because the weld crack resistance and the like are significantly reduced.

Si is an element effective as a deoxidizing element and for ensuring the strength of the base material. Addition of less than 0.03% results in insufficient deoxidation and is disadvantageous in securing strength. Conversely, 1.
Excessive addition of more than 0% forms a coarse oxide and causes deterioration of ductility and toughness. Therefore, the range of Si is 0.03
.About.1.0%. Mn is an element necessary to secure the strength and toughness of the base metal, and it is necessary to add at least 0.30%, but the upper limit is 2.0% within the allowable range of the material such as the toughness and crackability of the weld. And

In addition to its role as a deoxidizing element, Al is Al
It is an element effective in forming N and making the γ grain size finer, and in order to exert its effect, the content of elements other than oxides must be 0.001% or more. If added in excess of 1%, coarse precipitates are formed and ductility is extremely deteriorated, so it is necessary to limit the content to 0.001 to 0.1%.

N combines with Al and Ti and works effectively for γ grain refinement, but in order to clarify the effect, N is 0.00
While it is necessary to contain 1% or more, if added in excess, the amount of solute N increases and the toughness is adversely affected. As an allowable range, the upper limit of N is 0.01%. The above are the basic components of the steel of the present invention, but depending on the desired strength level, Cr, Ni, Mo, Cu, Ti, V, if necessary, for the purpose of increasing the base metal strength and ensuring toughness, Nb, B, T
One, two or more of a, W, Ca and REM can be contained.

Cr and Mo are both effective elements for improving the strength of the base material, but in order to produce a clear effect, 0.01% or more of each must be added, while each of them is 0%. If added in excess of 0.50%, the toughness tends to deteriorate, so the addition amount range of each of them is set to 0.01 to 0.50%. Ni is a very effective element because it can improve the strength and toughness of the base material at the same time, but it is necessary to contain Ni in an amount of 0.01% or more in order to exert the effect. When the content is high, the strength and toughness are improved, but 3.
And the effect is saturated even if added over 0%, Ar ₃
The transformation point is extremely lowered, the α amount before the two-phase region rolling which is the condition of the present invention is 50% or more, and the two-phase region rolling end temperature is 650 ° C.
Since the above cannot be satisfied at the same time, the upper limit is set to 3.0% in consideration of economic efficiency.

If Cu is added in an amount of 0.01% or more, it has almost the same effect as Ni. However, if it exceeds 1.5%, a problem occurs in hot workability. It is limited to the range of. Ti contributes to the improvement of the strength of the base material by precipitation strengthening, and is an element effective for γ grain refinement due to the formation of TiN.
It is necessary to add 3% or more. On the other hand, if it exceeds 0.10%, similarly to Al, a coarse oxide is formed to deteriorate toughness and ductility, so the upper limit is made 0.10%.

Both V and Nb mainly contribute to improving the strength of the base material by precipitation strengthening, but if added excessively, the toughness deteriorates. Therefore, V is 0.005 to 0.20 as a range in which the effect is exhibited without deteriorating the toughness.
% And Nb are 0.003 to 0.05%. B is 0.0
Addition of a very small amount of 003% or more is very effective in increasing the hardenability of steel materials and increasing the strength, but if added in excess, BN
Therefore, the upper limit is set to 0.0020% in order to reduce the hardenability and significantly deteriorate the toughness.

Ta is effective for improving the strength of the base material mainly by precipitation strengthening. 0.0 to exert its effect
It is necessary to contain 1% or more. However, if it exceeds 1.0%, the toughness is significantly deteriorated, so 0.01 to 1.0
%. W has an effect similar to that of Mo on strength and toughness, and it is necessary to contain W in an amount of 0.01% or more in order to surely exhibit the effect. On the other hand, if the content exceeds 1.0%, the toughness is impaired, so the upper limit of W is 1.0
%.

Both Ca and REM are effective elements for improving the anisotropy of mechanical properties and improving the lamella tear resistance. In the case of Ca, if the content is less than 0.0005%, the effect is not clear, and if it exceeds 0.005%, inclusions become coarse and the toughness and ductility may be adversely affected.
The range is from 05 to 0.005%. In the case of REM,
If less than 0.0005%, the effect is not clear and 0.02%
If it is over 0.00, inclusions may become coarse and adversely affect toughness and ductility as in the case of Ca, so 0.0005 to 0.
The range is 02%.

Since P and S as impurity elements significantly reduce toughness and ductility, it is preferable to reduce them as much as possible, but the reduction of P and S is due to careful selection of raw materials, ingenuity in steel making, strict management, etc. Since the load on the manufacturing process and the manufacturing cost are increased, the upper limit values of P are 0.015% and S are 0.010% as the tolerable amounts of the toughness and ductility.

Next, the effects of the present invention will be described more specifically by way of examples.

[0043]

[Examples] Table 1, Table 2 (Continued-1 of Table 1), Table 3 (Continued-2 of Table 1) and Table 4 show the chemical composition and the state of oxide particles of the sample steel used in the Examples. (Continued from Table 1-3). Each of the test steels is made into a slab by ingot rolling, slab rolling, or continuous casting. In Tables 1 to 4, Steel Nos. 1 to 15 satisfy the chemical composition range of the present invention and the dispersed state of oxide particles, and Steel Nos. 16 to 21 are the chemical composition range of the present invention and oxide particles. Either or both of the dispersed states are not satisfied.

Steel pieces having the chemical composition shown in Tables 1 to 4 are shown in Tables 5 and 6.
Steel plates were manufactured under the conditions shown in Table 1 (continued-1), Table 7 (table-2 continued-2), and Table 8 (table-5 continued-3), and the strength, Charpy impact characteristics, and DWTT characteristics were investigated. . All the test pieces were taken from the center of the sheet thickness at a right angle to the rolling direction (C direction). The Charpy impact property was evaluated at 50% fracture transition temperature (vTrs), and the DWTT property was evaluated at 85% ductile fracture transition temperature (85% FATT). The test results of strength and toughness are also shown in Tables 5 to 8.

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

[Table 3]

[0048]

[Table 4]

[0049]

[Table 5]

[0050]

[Table 6]

[0051]

[Table 7]

[0052]

[Table 8]

In Tables 5 to 8, the test No. A1-A
No. 20 is a steel sheet produced by producing a steel slab having the chemical composition of the present invention according to the conditions of the present invention, and all α structures finally obtained are sized and the average grain size is 2.2 μm or less. A stable ultrafine grain structure having an average grain size of 3 μm or less is obtained. Further, the toughness value is vTrs of −120 ° C. or lower, D
A temperature of −90 ° C. or lower is achieved by 80% FATT in the WTT test.

On the other hand, the test No. B1 to B9 are comparative examples, and since any of the conditions is out of the limited range of the present invention, both Charpy impact characteristics and DWTT characteristics are far inferior to those of the present invention. That is, the test No. B1 is C
Therefore, both the Charpy characteristic and the DWTT characteristic are inferior. Test No. Since B2 has an excessive amount of Mn, good Charpy characteristics and DWTT characteristics are not obtained. Test N
o. B3 has an excessively low transformation point because Cr, Mo, and Ni are excessive, and although the finish rolling start temperature is as low as 710 ° C., the α transformation has not yet occurred at the start of finish rolling, and is therefore achieved. The α particle size is also coarse, and toughness has not been improved.

Test No. In B4 to B6, the state of the oxide particles, which is a requirement of the present invention, does not satisfy the range of the present invention, so that the ultrafine graining of α is not sufficient as compared with the steel produced by the present invention, and the Charpy property is , DWTT characteristics are inferior. That is, the test No. The diameter of B4 and B5 is 0.1 to 1
Since the number of oxide particles of μm is too small, the average α particle size is 3 μm or less, but the mixed particle size is large, and the obtained Charpy property and DWTT property level are slightly inferior to the steel manufactured by the present invention. . On the other hand, Test No. Since the number of oxide particles of B6 is excessive, the recovery and recrystallization of α are suppressed, and the Charpy characteristic and DWTT characteristic are not improved.

Test No. B7 to B9 satisfy the chemical composition and the dispersion state of the oxide particles of the present invention, but the good Charpy characteristics and DWTT characteristics are not obtained because the manufacturing conditions do not comply with the present invention. That is, the test No. In B7, since the starting temperature of finish rolling is too high, α does not exist at the start of finish rolling, and as a result, ultrafine graining of the final structure is not achieved, and therefore Charpy characteristics and DWTT characteristics are inferior. Test No. In B8, since the cooling rate from the rough rolling to the finish rolling is too high, α does not exist at the start of the finish rolling, and thus ultrafine graining is not achieved. Test No. Since the finishing temperature of finish rolling of B9 is too high, grain growth of α once refined occurs, the mixed grain size becomes large, the average grain size exceeds 3 μm, and the Charpy property and DWTT property are poor.

From the above examples, it is clear that the present invention stably achieves an ultrafine grain structure, and thereby very good low temperature toughness is obtained.

[0058]

INDUSTRIAL APPLICABILITY The present invention has an average α particle size of 3 μm or less and a small mixed particle size without requiring the addition of expensive alloying elements and complicated hot working or heat treatment step with poor productivity. It is an epoch-making invention that can produce thick steel plate with good low temperature toughness by obtaining ultra-fine grain α structure, and has industrial advantages such as reduction of production cost and improvement of safety as a structure. Extremely large.

Claims (5)

[Claims] 1. As a component excluding oxide particles, weight%
(Same below), C: 0.01 to 0.20%, Si: 0.03 to 1.0%, Mn: 0.30 to 2.0%, Al: 0.001 to 0.1%, N: 0.001-0.01% is contained, and as an impurity
P and S are limited to P ≦ 0.015% and S ≦ 0.010%, and the particle size is 0.1 to 1
A base material area of 1 μm for oxide particles of μm^TwoPer 0.02
0.5 contained, balance Fe and unavoidable impurities
Steel billet Ac _ThreeAustena by heating to transformation point ~ 1250 ℃
Finish after rough rolling with cumulative reduction of 10 to 70%
Ferrite cooled to 0.1-10 ° C / sec until the start of rolling
Is 50 to 90%, the cumulative rolling reduction is 30
~ 90% finish rolling at a temperature of 650-750 ° C
Of high-strength steel with excellent low temperature toughness
Production method. 2. Further, by weight%, Cr: 0.01 to 0.50%, Ni: 0.01 to 3.0%, Mo: 0.01 to 0.50%, Cu: 0.01 to 1.5%, Ti: 0.003 to 0.10%, V: 0.005 to 0.20%, Nb: 0.003 to 0.05%, B: 0.0003 to 0.0020%, Ta : 0.01 to 1.0%, W: 0.01 to 1.0%, or a combination of two or more thereof. The production of a high-strength steel material having excellent low temperature toughness according to claim 1. Method. 3. The composition according to claim 1, further comprising one or two of Ca: 0.0005 to 0.005% and REM: 0.0005 to 0.02% by weight. 2. A method for producing a high-strength steel material having excellent low temperature toughness as described in 2. 4. After finishing rolling, 5 to 50 ° C. continuously.
The method for producing a high-strength steel material having excellent low-temperature toughness according to any one of claims 1 to 3, wherein accelerated cooling is performed at 550 to 20 ° C per second. 5. The method for producing a high-strength steel material excellent in low-temperature toughness according to claim 4, wherein the material is tempered at 400 to 650 ° C. after accelerated cooling.