JP2001234242A - Method for producing high toughness and high tensile strength steel fine in crystal grain - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing method for high toughness and high tensile strength steel fine in crystal grains capable of obtaining a remarkable fine- grained structure. SOLUTION: In this producing method for high toughness and high tensile strength steel fine in crystal grains, at the time of casting a slab containing, by mass, 0.05 to 0.8% C, 0.01 to 0.5% Si, 0.5 to 5% Mn, and the balance Fe with inevitable impurities, cooling the same to <=600 deg.C after rolling or as it is without rolling and thereafter performing hot rolling, reheating and the subsequent rolling are performed at 550 deg.C to Ac1 point, and the rolling is performed for one pass in which the draft of one pass is controlled to >=30% or for continuous two or more passes in which the time between the passes is controlled to <=10 sec under the conditions that the strain rate is 0.1 to 200/sec, and the total strain quantity is 1 to 10. Preferably, in the hot rolling, the rolling for one or more passes including the final pass is performed at >Ac1 point to Ac1 point+30 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a steel product (a thin steel plate, a thick steel plate, a wire rod, a shape steel bar, a steel bar, a steel pipe, etc.) manufactured by hot rolling, which has excellent basic properties such as strength, toughness and ductility. The present invention relates to a method for producing a high-toughness high-tensile steel.

[0002]

2. Description of the Related Art In recent years, with the reduction in the weight of steel products and the severer operating conditions of steel structures, the development of tougher and more secure steel has been required. To meet such demands, rolling methods have been developed to improve the method of manufacturing steel sheets, refine crystal grains of the metal structure, and improve the strength and toughness of steel. As an example of such a method, there is a so-called controlled rolling method. As a production method combined with an accelerated cooling method, Japanese Patent Application Laid-Open Nos. 63-223124 and 63-1
No. 28117, for example.

In the controlled rolling method shown in these conventional methods, static recrystallization generated between rolling passes in a recrystallization temperature region of relatively high temperature austenite (hereinafter abbreviated as γ) is used to reduce γ grains. Finely granulate. Then, waiting for the temperature of the steel sheet to decrease, it is possible to introduce defects such as dislocations into the crystal of γ by performing rolling again in a temperature range in which recrystallization of γ does not occur (non-recrystallization temperature range). Is being done. Such a defect, when γ is transformed into ferrite or the like, becomes a nucleation site of a transformation formation structure such as ferrite like the γ grain boundary, so that a large number of crystal grains are generated at the same time upon cooling,
This is because the metal structure can be made finer. However, the grain size of ferrite obtained by such a method is at most about 5 μm even though it is small, and there is a demand for a method of making crystal grains finer. Especially 700
In the case of high-strength steels of MPa or higher, sufficient toughness could not be obtained only by the effect of grain refinement at a particle size of about 5 μm.

[0004]

SUMMARY OF THE INVENTION The present invention relates to a 720MP
a, and even in high-tensile steel exceeding 800 MPa, the average grain size can be obtained by a processing and cooling method capable of obtaining remarkable fine grains that cannot be obtained by conventional grain refining means such as controlled rolling and accelerated cooling. It is an object of the present invention to provide a method for producing high-toughness and high-strength steel that realizes fine grains having a diameter of 2 μm or less and is extremely advantageous in terms of cost.

[0005]

Accordingly, the inventors have overcome the limit of grain refinement utilizing the above-mentioned transformation, and can obtain a ferrite structure with extremely small grain size.
The present invention has completed the following manufacturing method which can sufficiently improve the toughness only by the grain refining effect even in a high-strength steel exceeding MPa. (1) In mass%, C: 0.05 to 0.8%, Si: 0.
A cast slab containing 01 to 0.5% and Mn: 0.5 to 5%, the balance being Fe and unavoidable impurities, was cast and cooled to 600 ° C. or lower without rolling or rolling. Thereafter, when hot rolling is performed, reheating and subsequent rolling are performed at a temperature of 550 ° C. to Ac ₁ point, and the rolling is performed by setting a rolling reduction of one pass to 30% or more and 1%.
The crystal grain is characterized in that it is performed in two or more consecutive passes with a pass or inter-pass time of 10 seconds or less, under the conditions that the strain rate is 0.1 to 200 / sec and the total strain amount is 1 or more and 10 or less. A method for producing fine high-toughness high-tensile steel.

(2) In the hot rolling, reheating is performed by 55
Performed at 0 ° C. to Ac ₁ point temperature, one or more passes of rolling including final pass, the crystal grains according to above, wherein the carried out at a temperature of Ac ₁ point super to Ac ₁ point + 30 ° C. (1) Production method of fine high toughness and high strength steel. (3) In the reheating of the hot rolling, the heating rate is set to 0.1.
The method for producing a high-toughness high-strength steel with fine crystal grains according to the above (1) or (2), wherein the temperature is 1 to 50 ° C./sec.

(4) Prior to reheating in hot rolling, Ac
_The method according to the above-mentioned, further comprising performing a homogenization treatment of heating to ₃ points to 1450 ° C. and performing rolling or cooling to 600 ° C. or less at a cooling rate of 0.2 to 80 ° C./sec as it is without rolling. (1) The method for producing a high-toughness high-tensile steel having fine crystal grains according to any one of (1) to (3). (5) The method according to any one of (1) to (4), wherein forcible cooling is performed at a cooling rate of 0.2 to 80 ° C./sec to 600 ° C. or less within 90 seconds after hot rolling. A method for producing high-toughness high-strength steel with fine crystal grains.

(6) After hot rolling, the steel sheet is allowed to cool to 600 ° C. or less or forcibly cool at a cooling rate of 0.2 to 80 ° C./sec, and then heat to 550 ° C. to _one point of Ac + 30 ° C. The method according to any one of (1) to (5), wherein the method is further performed. (7) Within 300 seconds after the hot rolling, a recrystallization treatment for maintaining the temperature at 550 ° C. to _one point of Ac + 30 ° C. without cooling to 600 ° C. or less is further performed.
(5) The method for producing a high-toughness high-strength steel having fine crystal grains according to any one of (5) and (5).

(8) In the recrystallization treatment, the holding temperature: 5
50 to 850 ° C, heating or cooling rate up to holding temperature:
(6) or the above (6), wherein the temperature is 0.5 to 50 ° C./sec, and the holding time is 300 seconds or less, and then the solution is allowed to cool to 600 ° C. or less or to forcibly cool at a cooling rate of 0.1 to 80 ° C./sec. (7) The method for producing a high-toughness high-tensile steel having fine crystal grains according to (7). (9) After cooling to room temperature, tempering is further performed at a temperature of 300 ° C. to _one point of Ac + 30 ° C., wherein the crystal grains according to any one of (1) to (8) are further refined. Method for producing high-toughness high-strength steel.

(10) The slab is N: 0.00 in mass%.
2 to 0.1%, Ti: 0.003 to 0.6%, Nb:
0.003 to 0.5%, V: 0.001 to 0.5% 1
Species or two or more species, and C% ≧
0.05 + 12 × (Ti% / 48 + Nb / 93% + V%
/ 23-N% / 14), wherein the method for producing a high-toughness high-strength steel with fine crystal grains according to any one of the above (1) to (9). (11) The slab is in mass%, Mo: 0.01 to 1%, N
i: 0.01 to 5%, Cr: 0.01 to 3%, Cu:
0.01 to 3%, B: 0.0001 to 0.003% 1
The method for producing a high-toughness high-strength steel with fine crystal grains according to any one of the above (1) to (10), further comprising a seed or two or more kinds. (12) The slab is REM: 0.002-0.
1%, Ca: 0.0003 to 0.003%, and further contains one or more of them. This is a method for producing a tough high strength steel.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. First, when a conventional method of grain refinement by controlled rolling is examined from a metallurgical point of view, as described above, it is considered that mainly the following effects are obtained. Γ grains are refined by utilizing static recrystallization that occurs between rolling passes in a recrystallization temperature range of relatively high austenite (hereinafter abbreviated as γ). Further, by performing rolling at a relatively low temperature range in which recrystallization of γ does not occur (non-recrystallization temperature range), a number of defects such as dislocations are introduced into the γ crystal. The defects such as the γ grain boundaries and dislocations described above serve as nucleation sites of a transformation formation structure when γ is transformed into ferrite or the like, and thus make the metal structure fine.

All of these provide a nucleation site of a transformation formation structure such as ferrite, thereby making the final ferrite crystal grain size fine. Increase the number of ferrite grains generated at the time of transformation to ferrite to achieve miniaturization. However, in the refinement utilizing such transformation, the transformation initiation temperature of ordinary steel from austenite to ferrite is relatively high, from 750 ° C. to 800 ° C., and the crystal growth is fast. I can't get it. Further, the temperature of the ferrite transformation can be forcibly reduced by forced cooling, but in such a case, the ferrite to be formed is acicular, or a structure that is not preferable from the viewpoint of toughness such as formation of bainite is obtained. .

The essential points of the technology which is the basis of the present invention are as follows. (1) Refinement by Dynamic Recrystallization of Ferrite Crystal grains can be refined to 1 μm or less by dynamic recrystallization of ferrite. At this time, in order to obtain fine and uniform ferrite grains, the following conditions regarding pretreatment, processing and cooling are necessary. First, it has been found that a mixed structure in which cementite is dispersed in ferrite is desirable as a structure before processing that causes dynamic recrystallization. this is,
Due to the difference in deformation resistance between the second phase and the ferrite, the ferrite is more easily processed, and when the ferrite is elongated and recrystallized by the processing, the second phase breaks and the pinning effect causes grain growth after recrystallization, It is thought that coalescence is also suppressed.

As such a prestructure, a mixed structure of ferrite and bainite or martensite, or a structure in which fine cementite is dispersed, such as a pearlite, bainite or tempered martensite structure, is preferable as compared with ferrite and pearlite. In such a state, processing for recrystallizing ferrite is performed at Ac ₁ ° C.
It was found that it was necessary to reheat to a temperature of 550 ° C. or higher. Prior to this reheating, the steel slab is used as it is, or once again at Ac _{3 to} 1450 ° C., and then at room temperature to 6 at a cooling rate of 0.2 to 80 ° C./sec.
It is considered that cooling at 00 ° C. acts more effectively on grain refinement by making the steel slab uniform. In particular, 60
The reason for cooling to a temperature of 0 ° C. or less is to make the structure before processing into a structure containing bainite or martensite.

Next, the structure (mixed structure of ferrite and cementite) thus obtained is reheated and processed to finely recrystallize the ferrite.
At this time, the reheating temperature was set to 2 for ferrite and cementite.
The temperature must be such that the phases coexist, and in order to stably achieve this, the heating temperature before processing must be Ac ₁ or less. The processing temperature is set to Ac ₁ from the viewpoint that cementite suppresses grain growth after dynamic recrystallization by processing.
It is desirable that: However, if the processing temperature is too low, the diffusion of atoms is significantly delayed and dynamic recrystallization cannot be stably generated. From such a viewpoint, it is necessary to process at a temperature of 550 ° C. or more. When heating to the above-mentioned temperature, it is desirable to apply rapid heating in order not to excessively coarsen cementite as the second phase particles.

Further, it is necessary to set the amount of strain and the strain rate in processing so that dynamic recrystallization can be stably generated and the crystal grain size after recrystallization can be made fine.
Dynamic recrystallization occurs only when the amount of strain due to processing is larger than a certain value, and the lower the strain rate, the more easily it is generated.
We have found that a large amount of processing of 30% or more per pass and a total amount of strain of about 1 to 10 can stably recrystallize and achieve miniaturization of 1 μm or less per pass. Regarding the strain rate, when the strain rate is low, the obtained crystal grains tend to be larger than when the strain rate is high, but when the strain rate is too high, dynamic recrystallization does not occur, and during processing, The load also becomes extremely large. In view of these characteristics,
As a result of experimentally examining the conditions for obtaining a uniform and fine metal structure by dynamic recrystallization, it was found that the temperature range was approximately 1 in the temperature range of the present invention.
The above strain is required, and the range of the appropriate strain rate is approximately 0.
It was found to be 1 to 200 / sec.

Next, whether the above processing is performed in one pass or in two or more passes, the effect is basically the same if the time between passes is short. In the temperature range of the present invention, if the processing temperature is relatively low and the recovery between passes is not so fast and the time between passes is set to 10 seconds or less, the recovery during that time is small,
This is because distortions in multiple passes almost accumulate. However, as the rolling reduction per pass is increased and the number of passes is reduced, the grain refinement effect and the effect of inducing recrystallization are greater. next,
By the above processing, an extremely fine ferrite structure can be obtained immediately after the processing.However, after the processing is completed, the cooling is promptly performed so that the crystal growth of the ferrite can be suppressed, and a finer structure can be obtained. Make it possible.

In general, recrystallization is induced in the whole sample by the above-mentioned processing, and a fine grain structure can be obtained. However, recrystallization may not be induced in some parts depending on processing conditions and components. The structure can be refined by performing a recrystallization treatment after cooling once after the end of rolling.
This recrystallization treatment can be usually carried out at a normal heating rate, but it is effective from the viewpoint of grain refining that the recrystallization treatment is carried out continuously and by rapid heating without cooling after rolling. This is because in the ordinary recrystallization treatment, excessive recrystallization occurs during the temperature raising process or during the holding, and the grain growth occurs. At the same time, the growth of the second phase particles such as cementite is also fine. This is because it was found to be unfavorable for chemical conversion. In addition, by processing and cooling to temper and soften the second phase during processing, it is possible to produce steel with extremely fine crystal grains.

(2) Suppression of grain growth by Ti In contrast to the above-mentioned grain refining method, the addition of Ti and N and the optimization of the amount of carbon according to the added amount increase the processing transition of TiN and TiC. It has been found that a remarkable grain refinement effect is exhibited by suppressing the growth of crystal grains and the like.
Conventionally, the addition of such a large amount of Ti or N has been considered to be undesirable from the viewpoint of toughness. However, it has been found that the effect is extremely small in a remarkable fine grain structure obtained in this case. The nitride of Ti exists stably up to a high temperature of 1200 ° C. or more, and contributes to austenite grain refinement. Also, Ti carbides are extremely finely precipitated in ferrite. It has been found that recrystallized ferrite grains can be formed extremely finely by dispersing both of these in an appropriate ratio in steel and securing a sufficient amount of carbon for forming cementite. Furthermore, Nb and V
It has also been found that the effect of a nitride such as Ti cannot be expected from the addition of Ti, but a certain effect can be obtained by forming carbides.

(3) In the above manufacturing process, one or more passes including the final pass of rolling are performed in the two-phase temperature range of γ and α, and the subsequent recrystallization treatment and tempering are performed in two phases of γ and α. The additional operation in the temperature range allows the high-carbon-concentration martensite phase to be finely dispersed in the metal structure after cooling due to the effect of carbon atoms concentrated from α to γ. This is known to increase work hardening in steel having a normal crystal grain size, but to significantly deteriorate toughness. However, it has been found that when having a very fine crystal grain size as in the present invention, deterioration of toughness is extremely small. As a result, it has been found that this treatment eliminates the drawback that steel with fine crystal grains has low work hardening due to high yield strength and low uniform elongation in a tensile test.

Based on the above findings, the production conditions for the fine-grained steel of the present application were clarified. In the following, each component and the reason for limiting the production conditions will be described. C is an element effective for strengthening steel, and if it is less than 0.05, sufficient strength cannot be obtained. On the other hand, if the content exceeds 0.75%,
Deteriorates weldability. Further, in order to obtain a remarkable grain refining effect, it is necessary to disperse cementite in the metal structure. From such a viewpoint, the amount of carbon to be added is C% ≧
0.05 + 12 × (Ti% / 48 + Nb% / 93 + V%
/ 23-N% / 14). Although Si is effective as a deoxidizing element and as a strengthening element for steel, it is not effective at a content of less than 0.01%. On the other hand, if it exceeds 0.5%, the surface properties of the steel are impaired.

Mn is an element effective for improving the strength and hardenability of steel and appropriately leading the structure before rolling.
If it is less than 0.5%, a sufficient effect cannot be obtained. On the other hand, if the content exceeds 5%, the workability of steel deteriorates. N
Is added within a range that does not deteriorate toughness because it forms a nitride with Ti and effectively acts on refinement of austenite and refinement of recrystallized grains of ferrite. From such a viewpoint, the upper limit is set to 0.1% and the lower limit is set to 0.002%.
Further, in order to suppress excessive nitrogen addition and maintain good toughness, the addition amount of carbon and nitrogen is C% ≧ 0.05 + 12 × (Ti%
/ 48 + Nb% / 93 + V% / 23-N% / 14).

Ti, Nb and V function effectively in terms of crystal grain refinement and precipitation strengthening. In particular, since the effect of refining the crystal grains by these is remarkable, it is used in a range where the toughness is not deteriorated. From such a viewpoint, the upper limit of the addition amount is T
i: 0.6%, Nb: 0.5%, V: 0.5%.
Further, the lower limit of the addition amount is Ti: 0.003%, Nb:
0.003% and V: 0.001% are because there is no effect below this. Further, in order to make the effect of grain refinement more remarkable, N is added and the relationship between the content of carbon and nitrogen is C% ≧ 0.05 + 12 × (Ti% / 48
+ Nb% / 93 + V% / 23-N% / 14).

Cu, Ni, Cr, Mo, and B are all elements that improve the hardenability of steel.
The addition can increase the strength of the steel. However, excessive addition impairs the toughness and weldability of the steel,
0.01% ≦ Cu ≦ 3%, 0.01% ≦ Ni ≦ 5%,
0.01% ≦ Cr ≦ 3%, 0.01% ≦ Mo ≦ 1%,
It is limited to 0.0001% ≦ B ≦ 0.003%. Cu,
The lower limits of Ni, Cr, and Mo are set to 0.01%, and the lower limit of B is set to 0.0001%.

REM and Ca are effective in detoxifying S. However, if the amount of addition is small, the effect is not obtained, and excessive addition impairs the toughness.
0.10%, 0.0003 to 0.003 for Ca
%. Note that Al is added as a deoxidizing element and is not particularly limited. However, if its content is less than 0.001%, it has no effect, and if it exceeds 0.1%, the surface properties of steel are deteriorated. It is preferably added in the range of 001 to 0.1%. In addition, the contents of inevitable impurities P and S are 0.02% or less and 0.008% or less, respectively.
%: O: 0.0005 to 0.0
02%, Zr: 0.001 to 0.3%, T
a: 0.001 to 0.3%, Hf: 0.001 to 0.3
% Is preferably added as needed.

Next, the manufacturing conditions in the present invention will be described. Since the present invention is effective for billets cast under any casting conditions, there is no need to specify the casting conditions. In addition, the processing method that is the basis of the present invention, once,
After the transformation is completed, the method is heated to 550 ° C. to Ac ₁ , and a process for causing dynamic recrystallization of ferrite in the temperature range (the method according to claim 1) and a part of the method is performed for more than Ac ₁ to Ac _There is a method performed in the range of ₁ + 30 ° C. (a method according to claim 2), in which more remarkable grain refinement can be obtained by appropriately controlling the amounts of Ti, carbon, and nitrogen, and further, before each treatment. In addition, reheating and processing for refining the structure and soaking and processing for uniformity are performed.

In the case of the method described in claim 1, claim 1
Or the steel shown in claims 10 to 12, after rolling without cooling once or without rolling as it is,
Once cooled to a temperature of 600 ° C. or less, and reheated to a temperature of Ac ₁ or less and 550 ° C. or more by ordinary heating or, if necessary, by rapid heating of 0.1 to 50 ° C./sec according to claim 3, Ac _When performing hot rolling at a temperature of _{1 to} 550 ° C. or higher, the rolling reduction of one pass in a series of hot rolling is 3
Rolling of two or more passes in a single pass or two or more passes in which the time between passes is set to 10 seconds or less at 0% or more at 550 ° C. below Ac ₁
The hot rolling is performed at the above temperature and the rolling strain rate of 0.1 to 200 / sec, and the total strain amount is 1 or more and 10 or less.

After the hot rolling, the temperature is not particularly specified, but is allowed to be from room temperature to 600 ° C. within a range of from 0.2 ° C./sec to 80 ° C./sec. And then, if necessary, a recrystallization treatment as described in claim 6, 7, or 8 after cooling or immediately after hot rolling. Further, after that, if necessary, a temperature of 300 ° C. to Ac ₁ according to claim 9 may be used.
Tempering is performed at a temperature of + 30 ° C. Further, prior to reheating at a temperature of 550 ° C. or higher below Ac ₁ performed before hot rolling, the steel slab is once cooled to a temperature of 600 ° C. or lower after casting, and then reheated to a temperature of Ac _{3 to} 1450 ° C. From a temperature of Ar ₃ or more to a temperature of 600 ° C. or less, 0.2 to 80 ° C. /
The pretreatment according to claim 4 is performed in which cooling is performed in seconds.

First, when hot working with large strain is performed at a temperature of not more than Ac _{1 and not} less than 550 ° C., after casting a slab, it is rolled once without cooling or without rolling. Once cooled to a temperature of 600 ° C. or less and then reheated to a temperature of Ac ₁ or less and 550 ° C. or more, hot working is performed directly after casting, and then hot working is not performed. It is cooled Te, again because then transformed completely metal structure is cooled once 600 ° C. or less Ac ₁
When the steel is reheated to a temperature of 550 ° C. or higher, the metal structure of the steel becomes a mixed structure of ferrite and bainite and martensite, or pearlite, bainite and tempered martensite, and becomes a metal structure in which cementite is finely dispersed in ferrite. This is because the basic requirements of the invention are satisfied. At temperatures above Ac ₁ cementite transforms into austenite, because not ferrite, a two-phase structure of the cementite. The temperature of 550 ° C. or higher is
If the temperature is lower than this, the steel is not recrystallized by the subsequent processing.

In the third aspect, the heating to the temperature of not more than Ac _{1 and not} less than 550 ° C. is performed by rapid heating at a rate of 0.1 to 50 ° C./sec. This is because the growth cannot be suppressed at a rate of less than 0.1 ° C./sec, and it is difficult to obtain a heating rate of more than 50 ° C./sec with a normal heating device. Next, processing for causing dynamic recrystallization of ferrite is performed from Ac ₁ or less.
It must be performed at 550 ° C. This is because a temperature of more than Ac _1, because not enough undergo working strain introduced accelerated and grain growth inhibiting effect on the ferrite by cementite by reverse transformation to cementite austenite. Further, the temperature exceeding the Ac ₁ point generally exceeds 700 ° C. in many cases, and the dynamically recrystallized ferrite tends to grow grains,
This is because the processing temperature tends to increase as the processing temperature increases, and the processing temperature is preferably as low as possible. However, if the processing temperature is too low, diffusion of atoms hardly occurs, and recrystallization hardly occurs. In such a case, the processed ferrite grains simply flatten, and a fine sized grain structure cannot be obtained. Therefore, in order to stably generate dynamic recrystallization of ferrite, it is necessary to perform processing in a temperature range of 550 ° C. or more.

Next, the amount of strain in the processing at Ac _{1 to} 550 ° C. requires that the entire structure be recrystallized during the processing in this temperature range and that the crystal grain size after the recrystallization be fine. is there. In order for recrystallization to occur in the entire structure, a processing amount is required to be a certain amount or more, and from such a viewpoint, a total distortion amount by a series of processings needs to be one or more. Further, as long as the distortion amount is 1 or more, the larger the distortion amount, the more preferable in terms of grain refinement. However, it is difficult to secure a distortion amount of 10 or more in ordinary processing such as rolling. Therefore, in the present invention, the upper limit of the amount of distortion to be applied is set to 10. The larger the processing amount per pass, the more effective the effect of the introduced processing does not dissipate and is effective for miniaturization. In the case of a processing amount of 30% or more, a fine grain structure of 1 μm or less is obtained. And limited.

In addition, dynamic recrystallization is more likely to occur as the strain rate during processing is lower, and less likely to occur as the strain rate is higher. On the other hand, when the strain rate is low, the reduction of dislocation during processing (dynamic recovery) is large, and as a result, the crystal grain size obtained after recrystallization is large, and the crystal grain size is small as the strain rate is high. Considering both the susceptibility of such dynamic recrystallization and the crystal grain size after recrystallization, there is an appropriate range for the strain rate. From this viewpoint, the strain rate during processing is limited to 0.1 / sec or more and 200 / sec or less. If the rate is less than 0.1 / sec, the time required for processing is too long, and the recovery of dislocations occurs during this time, so that many dislocations cannot be introduced into the ferrite, and even if dynamic recrystallization occurs, fine crystal This is because grains cannot be obtained. In addition, the reason why the strain rate at the time of processing is set to 200 / sec or less is that when the strain rate is higher than this, Ac _{1 to} 55 is used.
This is because it is difficult to cause dynamic recrystallization in a temperature range of 0 ° C. In addition, it is desirable that the above-described rolling be performed in one pass, but when performing the rolling in multiple passes, it is necessary to set the inter-pass time to within 10 seconds. This is because if the inter-pass time is longer than 10 seconds, ferrite is recovered between the passes and the effect of accumulating strain cannot be obtained.

Next, a method for performing forced cooling according to claim 5 will be described following a series of hot working operations that cause dynamic recrystallization. First, the effect of the forced cooling is to suppress that the fine ferrite structure obtained after processing grows by crystal grain growth during the subsequent cooling, and the fine structure is not damaged. In the present invention, following the hot working for producing the dynamic transformation from such a viewpoint, 90 °
It defines the effectiveness of starting forced cooling within seconds and cooling to a temperature of 600 ° C. or lower at 0.2 to 80 ° C./second. Here, the reason for starting the cooling within 90 seconds from the end of the processing is to cool down as quickly as possible in order to prevent the fine ferrite structure generated by dynamic recrystallization during the processing from becoming coarse due to grain growth. This means that cooling beyond this does not maximize its effect, it is not much different from cooling down after processing, and the effect of forced cooling does not appear significantly is there.

Next, the reason why the cooling end temperature is set to 600 ° C. or less is that if the temperature exceeds 600 ° C., the temperature is still too high, atoms easily diffuse, and a fine ferrite structure or cementite obtained by processing. This is because grain growth cannot be suppressed, and the reason why the temperature is set to room temperature or higher is that cooling to a temperature lower than room temperature cannot be easily performed by ordinary water cooling or the like. Claims 6 and 7 describe a recrystallization treatment method that is additionally applied when dynamic recrystallization is not completely obtained after cooling or forced cooling after rolling as described above. Show. Such a part where recrystallization was not completely obtained is a ferrite structure that has been worked to a high degree, and recrystallization can be induced by reheating. Such recrystallization treatment is performed at 550 ° C. to Ac ₁ +30 as described in claim 6.
Perform in the range of ° C. It is 5 to limit to such a range
Can not induce recrystallization is less than 50 ° C., Ac ₁
If the temperature exceeds + 30 ° C., recrystallization proceeds excessively and crystal grains become coarse.

It has been found that such recrystallization treatment is desirably carried out by rapid heating as defined in claim 7. This is because the rapid reheating can complete the recrystallization process in a short time, thereby avoiding unnecessary growth of crystal grains. At this time, it can be considered that the coarsening of cementite and other precipitates can be suppressed in the process of raising the temperature, and the effect of further suppressing the coarsening of crystal grains is also superimposed. The reason why the lower limit of the heating rate is set to 0.5 ° C./sec is that the effect of rapid heating cannot be obtained below this, and the upper limit is set to 50 ° C./sec. Is not easily obtained due to the current facility restrictions.
Further, the upper limit of the processing temperature can be as high as 850 ° C. because of rapid heating. In addition, the recrystallization processing time is 300
The reason for limiting the time to less than the second is that if the time is longer than this, the crystal grains become coarse during the holding, and the effect of the rapid heating disappears.

In order to carry out such a recrystallization treatment more effectively, cooling is not performed once after rolling as in claims 6 and 7, but as soon as rolling is completed, as in claim 8, At a heating rate of 0.5 to 50 ° C./sec.
It is necessary to finish at a temperature of 0 to 850 ° C. within a time of 300 seconds or less. The reason for performing the recrystallization treatment immediately after the rolling in this way is that during the cooling after the rolling or during the temperature rise of the recrystallization treatment, the coarsening of the precipitate occurs, and thus the crystal grains are coarsened during the recrystallization treatment. It is because. The reason why the lower limit of the heating rate is set to 0.5 ° C./sec is that the effect of rapid heating cannot be obtained below this, and the upper limit is set to 50 ° C./sec. Is not easily obtained due to the current facility restrictions. The upper limit of the processing temperature is 850.
This is because rapid heating is possible up to a high temperature of ° C. Also,
The reason why the recrystallization treatment time is limited to 300 seconds or less is that if it is longer than this, crystal grains become coarse during this holding, and the effect of rapid heating disappears.

Next, when forced cooling is performed in claim 5 or when forced cooling is performed after the recrystallization treatment in claims 7 and 8, carbon atoms dissolved in ferrite precipitate as cementite. However, even at room temperature, it may become supersaturated in ferrite and form a solid solution to cause significant deterioration in toughness. In such a case, by performing tempering at a temperature of 300 ° C. to Ac ₁ + 30 ° C., solid solution carbon is precipitated as cementite, and a metal structure having excellent strength toughness can be obtained. Is intended the process tempering to be carried out for this purpose, is less than 300 ° C. is because tempering not readily diffuse carbon atoms temperature is too low can not be performed in a short time, Ac ₁ + 30 ° C. or less The reason for this is that if it exceeds this, a large amount of reverse transformation will occur, destroying the fine structure that has been generated.

Finally, in the case of the method according to the first aspect, a mixed structure state of ferrite and fine cementite, which is advantageous for refining the metal structure when reheated to a temperature of 550 ° C. or lower below Ac ₁ as described above. to a, Ac ₁ or less 5
Prior to reheating to a temperature of 50 ° C. or higher, the steel slab is heated once to Ac _{3 to} 1450 ° C.
It is possible to choose from r ₃ points or more temperature at a cooling rate of 0.2 to 80 ° C. / sec. Here, the reason for setting the reheating temperature to Ac _{3 to} 1450 ° C. is to promote the diffusion of solid solution atoms, to make the metal structure uniform, to make the austenite grain size relatively large, and to cool the metal structure after cooling to ferrite and bainite. This is because a mixed structure of martensite or pearlite, particularly preferred bainite or martensite is easily formed.

By reheating this to a temperature not higher than Ac _{1 and not} lower than 550 ° C., a fine dispersion structure of ferrite and cementite which is desired as a metal structure before working can be obtained. The reason for reheating in such a temperature range is that the added element such as Ti is dissolved once and has an effect of precipitating finely in ferrite during the subsequent cooling.
Such effects heating temperature so obtained Ac ₃ above temperature is limited to Ac ₃ or more, and remarkable in particular 1150 ° C. or higher, more heating temperature is desirable. However, when heated to 1450 ° C. or more, the coarsening of the metal structure becomes remarkable, which is not preferable for the subsequent finer metal structure.

The reason why the cooling is performed at a cooling rate of 0.2 to 80 ° C./sec after the soaking treatment is to keep the atoms such as Ti diffused uniformly by the soaking treatment as much as possible. . The reason for limiting such cooling conditions is that, at a cooling rate of less than 0.2 ° C./sec, the distribution of elements such as Ti (transfer of elements generated between ferrite and austenite) and the coarsening of precipitates occur during ferrite transformation. This is because it cannot be avoided. To avoid this, it is better to use a high cooling rate, but it is difficult to obtain a cooling rate exceeding 80 ° C./sec with the current facility capacity.

Next, the case of the method described in claim 2 will be described. In the case of the method according to the second aspect, after casting the steel slab, even if the hot rolling is started without cooling without cooling, the cast slab once cooled to a temperature of 600 ° C. or less is obtained from Ac ₁ point to 5 points.
Claim 1 in that rolling is started after reheating to 50 ° C.
Is the same as In addition, if necessary, the.
Ac at 550 ° C or higher by rapid heating at 1 to 50 ° C / sec
_The point of reheating to a temperature of ₁ or less, a series of hot working after the reheating, a continuous reduction in which the rolling reduction of one pass is 30% or more and the time of one pass or the time between passes is 10 seconds or less.
The processing is performed under the condition that the strain rate of rolling is 0.1 to 200 / sec and the total strain amount is 1 or more and 10 or less, and then the cooling is performed or the hot working is completed. Within 90 seconds thereafter, it is forcibly cooled to a temperature of room temperature to 600 ° C. at a cooling rate of 0.2 ° C./sec to 80 ° C./sec or less, and if necessary, re-cooled according to claim 6, claim 7 or claim 8. Perform crystallization treatment.

Further, if necessary, the present invention may be applied to a method as set forth in claim 9.
Tempering is performed at a temperature of 0 ° C. to Ac ₁ + 30 ° C. Also,
Prior to reheating to a temperature of 550 ° C. or higher below Ac ₁ , the steel slab is once cooled to a temperature of 600 ° C. or lower and then reheated to a temperature of Ac _{3 to} 1450 ° C. again, from a temperature of Ar ₃ or higher to 600 ° C. or lower. The pretreatment described in claim 4 is performed at a rate of 0.2 to 80 ° C./sec. The method of claim 2 is different from the method of claim 1 in that one pass performed after reheating is performed at a reduction rate of 30% or more for one pass or two or more consecutive passes in which the time between passes is set to 10 seconds or less. Rolling was performed at a strain rate of 0.1 to 200 /
Sec, not only performed in a total amount of strain conditions to be 1 to 10, characterized in that at least one pass or more rolling end temperature includes a final rolling pass than Ac ₁ + 30 ° C. greater than the Ac ₁ point .

Such a temperature rise to a temperature exceeding the Ac ₁ point can be performed by a characteristic phenomenon of an increase in the heat generated during processing due to large strain rolling. That is, the strain energy applied during rolling is converted into heat energy and the temperature of the steel sheet rises. In order to make the most of this processing heat, the rolling reduction of one or more passes is set to 50% or more. Is preferred. Thus, by utilizing the process heat generation, the reheating temperature without a temperature above _a point Ac, Ac ₁ + 30 ° C. exceeds Ac ₁ point of the temperature during hot rolling
It is possible to further improve the material by setting the temperature below and introducing a hard phase by partial austenitization as described below. At least one including the final rolling pass
Ac ₁ +30 exceed the Ac ₁ point of the end of rolling temperature equal to or higher than the path
In order to reduce the temperature to below ℃, it is preferable to use the heat generated by processing as described above because special equipment and a manufacturing process are not required. However, methods other than heat generated by processing such as induction heating during hot rolling are preferable. The present invention does not particularly limit a specific method of setting the steel sheet during hot rolling to a temperature exceeding the Ac ₁ point and not more than Ac ₁ + 30 ° C.

[0044] at least one pass or more rolling end temperature includes a final rolling pass as described above exceeds the Ac ₁ point Ac ₁
+ 30 ° C. The purpose of rolling below, the crystal grains of the fine metal structure shown by the method of claim 1, Ac beyond _a point Ac ₁ +
By terminating the rolling at a temperature of 30 ° C. or less, a small amount of austenite is generated in the fine metal structure, and the subsequent cooling transforms the portion into a hard phase such as high carbon martensite or bainite. It is.

[0045] Such a hard phase improves the tensile properties of the final steel material. This is due to the following reasons. First, the tensile properties of steel having fine crystal grains are disadvantageous in that the work hardening coefficient decreases due to an increase in yield stress due to the fine crystal grains, and uniform elongation decreases. On the other hand, the hard phase introduced into the material increases the tensile strength of the material, which increases the work hardening coefficient of the material and consequently increases the uniform elongation of the material. However, it is considered that such a hard phase induces stress concentration in the material at the time of deformation and lowers toughness.

However, in the present invention, it was found that the effect of grain refinement was remarkable, and even in such a case, deterioration of toughness was hardly observed. Therefore, by the application carried out in claim 2 the temperature after processing at least one pass of the series of rolling as shown in exceeding the Ac ₁ point Ac ₁ + 30 ° C. below the temperature, which is also a fine metal structure By introducing a hard phase such as fine high-carbon martensite or bainite, it has become possible to improve the uniform elongation of fine-grained steel. Here, the reason why the temperature exceeding the Ac ₁ point is required is that below this temperature, partial gamma formation does not occur, and the hard phase as described above cannot be formed. In addition, Ac ₁ +
The reason why the temperature is set to 30 ° C. or lower is that if the temperature is higher than 30 ° C., the volume fraction of the hard phase is so high that the toughness is deteriorated and also the crystal grains grow.

[0047]

Next, the effectiveness of the present invention will be described with reference to examples of the present invention. Table 1 shows the components of the steels of the examples. In the table, the steels with the numbers shown with paint colors indicate comparative steels, and items that do not correspond to the present invention are shown with paint colors. Next, the grain size obtained for steel slabs manufactured under various manufacturing conditions using steel having such a composition,
The hardness, the yield strength (YS), the tensile strength (TS), and the ductile-brittle transition temperature (vTrs) of the Charpy impact test are shown in Tables 2 to 4 together with the production conditions. In all cases, the thickness of the steel after processing is 3 to 10 mm. In the table, the strength estimated from the hardness is shown in the tensile strength column for the steels for which only the hardness was measured.

[0048]

[Table 1]

[0049]

[Table 2]

[0050]

[Table 3]

[0051]

[Table 4]

In any case, the steel satisfying the requirements of the present invention has a very fine crystal grain size of 2 μm, and has good strength and toughness. On the other hand, steels that do not satisfy the requirements of the present invention have high crystallinity of high carbon martensite having a crystal grain size of more than 2 μm, flat grains or high hardness. In steel containing flat grains or high carbon martensite, the toughness is significantly deteriorated even if the strength is high. From the above, it is clear that it is possible to produce a steel having a very fine grain size by the method of the present invention, and it is possible to produce a high-toughness high-strength steel excellent in strength and toughness, The present invention is effective.

[0053]

As described above, according to the production method of the present invention, it is possible to produce steel having a very fine crystal grain size.
In particular, it is possible to provide a manufacturing method capable of inexpensively producing a high-toughness high-strength steel excellent in toughness even in a high-tensile steel exceeding 720 MPa.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/58 C22C 38/58 (71) Applicant 000001258 Kawasaki Steel Corp. Kitahonmachidori, Chuo-ku, Kobe-shi, Hyogo 1-1-28 (71) Applicant 000001199 Kobe Steel, Ltd. 1-3-18, Wakihama-cho, Chuo-ku, Kobe City, Hyogo Prefecture (72) Inventor Masaaki Fujioka 20-1 Shintomi, Futtsu City, Chiba Prefecture Nippon Steel Corporation (72) Inventor Yoshio Abe 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation (72) Inventor Yukito Hagiwara 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel (72) Inventor Mitsuru Sato 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Headquarters (72) Inventor Tomoyuki Yokota Chiyoda, Tokyo 1-2-2, Marunouchi-ku, Japan Nippon Kokan Co., Ltd. (72) Yoshitaka Adachi Inventor, 4-5-33 Kitahama, Chuo-ku, Osaka-shi Sumitomo Metal Industries, Ltd. (72) Inventor Akihiro Matsuzaki Chiba, Chiba 1 Kawasaki-cho, Chuo-ku, Kawasaki Steel Engineering Co., Ltd. (72) Inventor Kowa Masukura 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo F-term Kobe Steel Engineering Co., Ltd. ) 4K032 AA02 AA04 AA05 AA06 AA08 AA11 AA12 AA14 AA15 AA16 AA17 AA19 AA21 AA22 AA23 AA24 AA27 AA29 AA31 AA35 AA36 AA40 BA01 BA02 BA03 CA01 CA02 CA03 CA05 CC02 CD01 EA01 CF02 EA01 EA17 EA18 EA19 EA20 EA23 EA25 EA27 EA31 EA32 EA36 EB05 FA01 FA02 FA03 FB04 FB05 FD01 FD02 FD03 FD04 FD05 FF01 FF02 JA03 JA06

Claims (12)

[Claims] 1. A mass% of C: 0.05 to 0.8%, Si: 0.01 to 0.5%, Mn: 0.5 to 5%, the balance being Fe and unavoidable impurities. After casting a slab consisting of, and after being rolled or cooled as it is without rolling to 600 ° C. or less, when performing hot rolling, reheating and subsequent rolling are performed at a temperature of 550 ° C. to Ac ₁ point. The rolling was carried out, and the rolling reduction in one pass was 3
0% or more, with one pass or two or more consecutive passes with an inter-pass time of 10 seconds or less, at a strain rate of 0.1 to 200 /
A method for producing a high-toughness high-strength steel with fine crystal grains, wherein the method is performed under the condition that the second and the total strain amount are 1 to 10 inclusive. 2. In the hot rolling, reheating is performed at 550 ° C.
Carried out at a temperature of to Ac ₁ point, one pass or more rolling including final pass, claim 1 in crystal grains fine in wherein carried out at a temperature of Ac ₁ point super to Ac ₁ point + 30 ° C. Manufacturing method of high toughness and high strength steel. 3. The fine toughness and high tensile strength of crystal grains according to claim 1, wherein the rate of temperature rise is 0.1 to 50 ° C./sec during reheating in hot rolling. Steel production method. 4. Prior to reheating in hot rolling, the material is heated to an Ac temperature of _{3 to} 1450 ° C. and rolled or without rolling at a cooling rate of 0.2 to 80 ° C./sec.
The method for producing a high-toughness high-strength steel with fine crystal grains according to any one of claims 1 to 3, further comprising performing a homogenization treatment of cooling to 0 ° C or lower. 5. Within 90 seconds after hot rolling, 0.2-80
The method for producing a high-toughness high-strength steel with fine crystal grains according to any one of claims 1 to 4, wherein forced cooling is performed at a cooling rate of 600C / sec to 600C or less. 6. After hot rolling, the steel sheet is allowed to cool to 600 ° C. or less or to forcibly cool at a cooling rate of 0.2 to 80 ° C./sec.
The method for producing a high-toughness high-strength steel with fine crystal grains according to any one of claims 1 to 5, further comprising performing a recrystallization treatment of heating to 50 ° C to _one point of Ac + 30 ° C. 7. A recrystallization treatment is further performed within 300 seconds after hot rolling, in which the temperature is kept at 550 ° C. to _one point of Ac + 30 ° C. without cooling to 600 ° C. or less. The method for producing a high-toughness high-strength steel with fine crystal grains according to any one of the above. 8. The recrystallization treatment has a holding temperature of 550.
To 850 ° C., the rate of temperature rise or fall to the holding temperature: 0.
8. The method according to claim 6, wherein the temperature is 5 to 50 ° C./sec, the holding time is 300 seconds or less, and then the solution is allowed to cool to 600 ° C. or less or to be forcibly cooled at a cooling rate of 0.1 to 80 ° C./sec. Method for producing high-toughness high-strength steel with fine crystal grains. 9. After cooling to room temperature, 300 ° C. to Ac ₁ point +
The method for producing a high-toughness high-strength steel with fine crystal grains according to any one of claims 1 to 8, wherein tempering is further performed at a temperature of 30 ° C. 10. The cast slab is represented by mass%, N: 0.002 to 0.1%, Ti: 0.003 to 0.6%, Nb: 0.003 to 0.5%, V: 0. 0.001 to 0.5%, and C%
≧ 0.05 + 12 × (Ti% / 48 + Nb / 93% + V
% / 23-N% / 14), the method for producing a high-toughness high-strength steel having fine crystal grains according to any one of claims 1 to 9. 11. The slab is represented by mass%: Mo: 0.01 to 1%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Cu: 0.01 to 3%, B The method for producing a high-toughness high-strength steel with fine crystal grains according to any one of claims 1 to 10, further comprising one or more of 0.0001 to 0.003%. . 12. The slab further comprises one or more of REM: 0.002 to 0.1% and Ca: 0.0003 to 0.003% by mass. The method for producing a high-toughness high-strength steel having fine crystal grains according to any one of claims 1 to 11.