JP2002363644A - Method for manufacturing high-tensile steel with excellent toughness and fatigue strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for attaining an ultrafine-grained structure of 1-3 μm average ferrite grain size over the whole thickness of steel and obtaining high-tensile steel combinedly having extremely excellent toughness, brittle crack arrest property, and fatigue characteristics of welded joint and having about >=490 MPa class tensile strength. SOLUTION: A manufacturing method wherein, as a basic requirement, the temperature of the steel during rolling stands within the range from (Ac1 transformation point - 50 deg.C) to (Ac3 transformation point - 50 deg.C) is applied to the steel having a microstructure with a prescribed component composition when the steel is heated to a temperature between 300 deg.C and rolling temperature at (0.1 to 50) deg.C/s temperature-rise rate and rolling is started in the course of temperature raise from the Ac1 transformation point to (Ac3 transformation point - 100 deg.C) or after holding at this temperature for 1-60 s and then rolling of 50-90% cumulative draft is applied. By this method, the ultrafine-grained structure of 1-3 μm minimal in the degree of duplex grain can be obtained, and extremely excellent toughness, brittle crack arrest property, and fatigue characteristics at welded joint can be attained.

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 requiring low-temperature toughness and / or fatigue strength. Steel products manufactured in this way can be used, for example, in offshore structures, pressure vessels, shipbuilding,
It can be used in general for welded steel structures such as bridges, buildings, line pipes, etc., but is particularly useful as steel plates for structures such as marine structures, ships, bridges, etc., which require low-temperature toughness and / or fatigue strength. In addition, the present invention can be applied to a steel pipe material or a shaped steel which is used as a structural member and requires low-temperature toughness and / or fatigue strength.

[0002]

2. Description of the Related Art Since high-strength steels represented by thick steel plates are used as structural members, low-temperature toughness is required from the viewpoint of ensuring the safety of structures. Various methods have been proposed for improving the low-temperature toughness of high-strength steel materials. However, the only method for improving the low-temperature toughness without deteriorating the other properties is to refine the crystal grain size.

The present inventors have already set the ferrite grain size to 3 μm.
Japanese Patent Application Laid-Open Nos. Hei 8-60239 and Hei 10-109
No. 121132 discloses this. All of the requirements for this technology are to refine the transformed ferrite by optimizing the heating and hot rolling conditions in the austenite region and the austenite / ferrite two-phase region, and then recrystallize the ferrite by processing to obtain an ultrafine structure. To get the grains. This technique is actually a means suitable for achieving ultra-fine graining of about 1 to 3 μm, and has a fracture surface transition temperature (vTrs) of −100 ° C. in a 2 mm V notch Charpy impact test.
This is a technique sufficient to stably obtain a toughness of about -120 ° C, but a new technique is required to achieve further low temperature toughness.

As one means for further improving the low temperature toughness, as disclosed in Japanese Patent Application Laid-Open No. 2000-8123, the average α grain size is controlled by precise control of the rolling conditions including the rolling reduction per pass. Is further reduced by one step. However, in the case of such a method, precise and strict control of the rolling conditions is required, and not only the cumulative rolling reduction but also the rolling reduction per pass needs to be reduced at a low temperature to a large degree. There are also problems such as an increase in load on the device and a decrease in productivity.

As another method of further improving toughness, there is a method of minimizing variation in grain size due to a portion of a steel material, which is difficult to achieve by the existing method, that is, a method of achieving a more uniform ultrafine grain structure. Conceivable. According to this method, 1
Achieving a toughness exceeding the level that can be achieved by the conventional ultrafine graining method using ferrite processing and recrystallization, even if the average grain size is about 1 to 3 µm, without having an extremely ultrafine grain structure of less than μm. It is considered possible to do this, but JP-A-8-60239 and JP-A-10-121
No. 132, etc., the average particle size is 1 to
In the conventional method for obtaining a ferrite grain size of about 3 μm, even if the structure before processing is refined to some extent, there is a difference in the difficulty of recrystallization depending on the crystal orientation. Was inevitable in the particle size of the particles.

[0006]

SUMMARY OF THE INVENTION The present invention requires a complicated hot working or heat treatment step by further developing the ultra-fine graining technique by working and recrystallization already disclosed by the present inventors. Without having to limit the chemical composition to a specific narrow range, an ultrafine grain structure with an extremely small mixed grain size and an average ferrite grain size of 1 to 3 μm or less is achieved over the entire thickness of the steel material. An object of the present invention is to provide a method for producing a high-tensile steel material having mechanical properties and a tensile strength of about 490 MPa or more.

[0007]

According to the results of the research conducted by the present inventors, the processing of ferrite in the hot rolling stage has been described.
The dominant effects on the crystal grain size obtained by recrystallization are the cumulative rolling reduction in the austenite / ferrite dual phase region to the ferrite single phase region and the fineness of the structure before rolling. In particular, when an appropriate amount of austenite is present in the ferrite phase, recrystallization of ferrite occurs more easily. If the austenite fraction is the same, individual austenite grains are finer, and the more uniformly dispersed, the more recrystallization occurs and the smaller the ferrite grain size. In the detailed study of the mechanism of accelerating the recrystallization of steel, it is possible to achieve an efficient fine and uniform dispersion of austenite and consequently a uniform ultrafine ferrite structure by rolling during and after heating. I found it for the first time.

[0008] As means for fine dispersion of austenite,
As disclosed in Japanese Patent Application Laid-Open No. Hei 7-126797, the structure before the austenite / ferrite dual phase region processing is changed to a single phase structure of martensite or bainite, a mixed structure of both structures, or a ratio of the structure of ferrite to the structure. In the present invention, ultrafine graining is facilitated by using a mixed structure containing ferrite with a limited grain size and a mixed structure with ferrite single phase or martensite, bainite, or pearlite having a specified ferrite grain size. A more effective method is to heat a certain microstructure to the two-phase region at an appropriate heating rate, and then roll or work immediately or within a certain holding time. Since austenite is generated, the introduction of strain into ferrite becomes more uniform and the ferrite / austenite interface Is significantly increased, the recrystallization of the ferrite becomes very uniform, and the average ferrite grain size stably ultra fine grain structure 1~3μm is obtained.

Investigations were made on various mechanical properties of a uniform ultrafine-grained structure having a small mixed grain size, which were achieved by the method of the present invention utilizing the processing and recrystallization of ferrite. Compared with steel manufactured by hot rolling, normalizing, or quenching / tempering, the fatigue crack propagation speed of the base metal is delayed to about 1/5 to 1/10, and the fatigue life of the turning welded joint It was found that it was possible to make it approximately ほ ぼ or less even when evaluated by. Therefore, the steel produced according to the present invention can satisfy both low temperature toughness and high fatigue strength at the same time, and is extremely useful for applications requiring low temperature toughness and / or fatigue strength.

The present invention has been made based on the above-mentioned new findings, and the gist thereof is as follows. (1) In mass%, C: 0.01 to 0.2% Si: 0.01 to 1% Mn: 0.1 to 2% Al: 0.001 to 0.1% N: 0.001 to 0 0.01% and as impurities
A steel containing P: 0.02% or less, S: 0.01% or less, the balance being iron and unavoidable impurities, and having any of the following structures:
Heating is performed at a heating rate of 0 ° _C./s , and rolling is started while the temperature is increasing from the A _c1 transformation point to the (A _c3 transformation point−100 ° C.). The temperature of the steel during rolling is (A _c1 transformation point−50 ° C.) to (A _c3 transformation point−50 ° C.)
A method for producing a high-strength steel excellent in toughness and fatigue strength, which is within the range described above. Martensite single-phase structure, bainite single-phase structure, or a mixed structure of both The ferrite content is less than 20%, and ferrite and martensite, or ferrite and bainite, or a mixture of ferrite, martensite, and bainite Microstructure The ferrite content is 20% or more and the average grain size is 2%.
0 μm or less, and a ferrite single phase structure, or a mixed structure of ferrite and one or more of martensite, bainite, and pearlite

(2) In mass%, C: 0.01-0.2% Si: 0.01-1% Mn: 0.1-2% Al: 0.001-0.1% N: 0. 001-0.01%, as impurities
A steel containing P: 0.02% or less, S: 0.01% or less, the balance being iron and unavoidable impurities, and having any of the following structures:
Rolling is started at a heating rate of 0 ° C./s, after holding at a temperature of A _c1 transformation point to (A _c3 transformation point−100 ° C.) for 1 to 600 s, rolling is started, and a rolling reduction of 50% to 90% is achieved. When performing the rolling, the temperature of the steel during rolling is (A _c1 transformation point−50 ° C.) to (A
_c3 transformation point −50 ° C.). A method for producing a high-strength steel excellent in toughness and fatigue strength, which is within the range of ( _c3 transformation point −50 ° C.). Martensite single-phase structure, bainite single-phase structure, or a mixed structure of both The ferrite content is less than 20%, and ferrite and martensite, or ferrite and bainite, or a mixture of ferrite, martensite, and bainite Microstructure The ferrite content is 20% or more and the average grain size is 2%.
0 μm or less, and a ferrite single phase structure, or a mixed structure of ferrite and one or more of martensite, bainite, and pearlite

(3) After rolling is completed, accelerated cooling is performed at a cooling rate of 2 to 100 ° C. to 20 ° C. to 650 ° C. The toughness, fatigue strength and toughness described in (1) or (2) above. Method for producing high-strength steel excellent in quality. (4) After the rolling, tempering at a heating temperature of 300 ° C. or more and an A _c1 transformation point is further performed.
(3) The method for producing a high-tensile steel having excellent toughness and fatigue strength according to any one of (3) and (4).

(5) Further, in mass%, Ni: 0.1 to
5% Cu: 0.1 to 1.5% Cr: 0.01 to 2% Mo: 0.01 to 2% W: 0.01 to 2% Ti: 0.003 to 0.1% V: 0. 005-0.5% Nb: 0.003-0.2% Zr: 0.003-0.1% Ta: 0.005-0.2% B: 0.0002-0.005% The method for producing a high-tensile steel having excellent toughness and fatigue strength according to any one of the above (1) to (4), comprising two or more kinds.

(6) Further, in mass%, Mg: 0.00
(1) to (5), wherein one or more of Ca: 0.0005 to 0.01% Y: 0.005 to 0.1% are contained. The method for producing a high-tensile steel having excellent toughness and fatigue strength according to any one of the above.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. The present invention relates to the processing and processing of ferrite in the hot rolling stage.
The basic technology is to make the ferrite grain size ultra-fine by recrystallization. For easier and uniform ultrafine grain refinement, it is preferable to perform rolling in the two-phase region where ferrite and austenite coexist. The basic requirement of the present invention is to make effective use of austenite as a hard phase. I have. That is, in order to make ferrite ultra-fine, it is premised that at the stage of processing into ferrite, the structure before processing is refined and an appropriate amount of austenite is present. However, ferrite processing
In order to obtain an ultrafine grained structure having a ferrite grain size of about 1 to 3 μm by recrystallization more easily and uniformly than the conventional technique, in addition to the basic requirements,
Furthermore, it is important to disperse the individual austenite particles as finely and densely as possible.

According to the present invention, in order to finely disperse austenite particles, a steel having an appropriate structure before working is subjected to a heating process or a temperature-rolling process, which is performed during heating or holding within a certain range. It is based on the new finding that it is effective to generate austenite from a non-equilibrium state. That is, concretely, a steel having an appropriate chemical composition and a structure corresponding to the following (1) is prepared at a temperature of 300 ° C to a rolling temperature of 0.1 to 50 ° C / s.
At a heating rate of A _c1 transformation point to (A _c3 transformation point −10).
0 ° C.), or after holding at this temperature for 1 to 600 s, rolling is started, and when rolling is performed with a cumulative rolling reduction of 50 to 90%, the temperature of the steel during rolling becomes (A _c1 transformation point). -5
(0 ° C.) to ( _{Ac 3} transformation point −50 ° C.). Martensite single-phase structure, bainite single-phase structure, or a mixed structure of both The ferrite content is less than 20%, and ferrite and martensite, or ferrite and bainite, or a mixture of ferrite, martensite, and bainite Microstructure The ferrite content is 20% or more and the average grain size is 2%.
0 μm or less, and a ferrite single phase structure, or a mixed structure of ferrite and one or more of martensite, bainite, and pearlite

In the present invention, the heating is performed at a heating rate of 0.1 to 50 ° C./s from 300 ° C. to the rolling temperature, and while the temperature is rising from the A _c1 transformation point to the (A _c3 transformation point−100 ° C.), Or,
After holding at this temperature for 1 to 600 s, rolling is started, and when rolling at a cumulative reduction of 50 to 90%, the temperature of the steel during rolling is (A _c1 transformation point−50 ° C.) to (A _c3 transformation point). -50
(° C), the hot-rolling in the two-phase region is mainly characterized by ultra-fine graining of the processed and recrystallized structure of ferrite, but the structure before entering the hot-rolling was optimized. There is a need to. That is, in order to easily and uniformly generate recrystallization of ferrite by rolling, it is necessary to have a uniform and large number of recrystallization nucleation sites and austenite generation sites at the time of elevated temperature rolling, which is preferable. The structure of the structure satisfies the above conditions.

The preferred conditions of the tissue morphology were determined by detailed experiments and will be described below. In the martensitic single-phase structure, bainite single-phase structure, or a mixed structure of the two, not only the former austenite grain boundaries but also the fine lath boundaries function as recrystallization nucleation sites and austenite formation sites. Is a preferred tissue form for easily and uniformly generating the following.

On the other hand, since ferrite hardly has a substructure in grains, recrystallization nucleus generation sites and austenite generation sites are generally only at ferrite grain boundaries, making recrystallization difficult to occur and non-uniform. Easy and not preferred. Therefore, when the presence of ferrite is inevitable due to the composition of the steel, it is necessary to limit the ratio of ferrite to the structure to a certain level or less, or to reduce the ferrite grain size. If the proportion of ferrite is less than 20% and the balance is martensite or bainite or both, the ferrite grain size may be coarse. This is because if the proportion of ferrite is less than 20%, the processing concentrates on the ferrite, which is a soft phase, so that recrystallization and austenite nucleation can occur sufficiently uniformly even with ferrite.

On the other hand, when the proportion of ferrite is 20% or more, coarse ferrite cannot avoid recrystallization or non-uniform nucleation sites of austenite, so that it is necessary to reduce the ferrite grain size. . If the ferrite grain size in the structure before rolling is 20 μm or less, recrystallization easily and uniformly occurs regardless of the structure form other than ferrite, and as a result, a uniform ultrafine grain structure is finally achieved. Therefore, in the present invention, when the proportion of ferrite is 20% or more, the particle size of the ferrite is limited to 20 μm or less. If the ferrite grain size is more than 20 μm, it is difficult to achieve a uniform ultrafine grain structure with a small mixed grain size even if austenite is finely dispersed by elevated temperature rolling. The above is the requirement of the structure before the temperature-increasing rolling. Regarding the structure requirement, as long as the chemical composition satisfies the present invention, there is no limitation on the means for achieving the structure.

Next, the limiting conditions of the temperature raising processing conditions will be described. The steel having the appropriate structure of the above is subjected to warming rolling during the heating or after holding in the appropriate range. The heating rate until reaching the predetermined rolling temperature is 0 on average from 300 ° C to the rolling temperature. 0.1 to 50 ° C./s.
This is a condition for suppressing the generation of austenite during the temperature rise, and generating fine austenite from a new site for each rolling pass of the temperature raising rolling. If the temperature is less than, an austenite phase near equilibrium is formed at each temperature during the heating, and there is almost no austenite induced by rolling, and fine dispersion of austenite becomes difficult. I can't. On the other hand, as the heating rate increases, the degree of non-equilibrium increases, and austenite is generated more easily during the heating process, which is preferable for fine dispersion of the austenite phase.
If the temperature exceeds ℃ / s, the effect is saturated, but it is difficult to uniformly raise the temperature of the entire steel industrially. Therefore, in the present invention, the upper limit of the temperature rising rate is set to 50 ° C / s.

After the temperature is raised at the above-mentioned heating rate, the rolling start temperature is set to A _c1 transformation point to (A _c3 transformation point −100 ° C.) immediately after the temperature rise or 1 to 600 s at the rolling start temperature.
After the holding, rolling is performed at a cumulative reduction of 50 to 90%. In the rolling step, a large amount of dislocations are introduced into the ferrite due to processing, which results in recrystallization and the formation of an ultrafine grain structure, but fine and high-density austenite is formed before and during rolling. As a result, recrystallization occurs uniformly, and a uniform ultrafine grain structure with a small mixed particle size is achieved. The reason for setting the lower limit of the rolling start temperature to the A _c1 transformation point is that it is necessary to form austenite before and during rolling, so if the starting temperature is lower than the A _c1 transformation point, the temperature is maintained at that temperature. However, the formation of sufficient austenite cannot be expected, and the amount of austenite generated by processing during rolling is also small, so that uniform ultrafine graining cannot be expected.

On the other hand, in the present invention, the upper limit of the rolling start temperature is (A _c3 transformation point−100 ° C.). This is because, in the present invention, in which recrystallization of ferrite is the basis of ultrafine grain refinement, it is necessary to ensure a sufficient ferrite phase before rolling, so that the rolling start temperature is (A _c3 transformation point −100 ° C.) or less. Then, in the present invention, an austenite phase does not excessively increase during rolling, and a substantially uniform ultrafine recrystallized ferrite can be obtained. When the rolling start temperature exceeds (A _c3 transformation point −100 ° C.), the austenite phase before rolling becomes excessive, and the ferrite grains formed by transformation from the austenite become coarser than recrystallized ferrite, and the mixed grain size increases. Adversely affects both toughness and fatigue properties.

It is the most important requirement of the present invention to form austenite finely and at a high density before or during the temperature raising rolling. In addition to limiting to the appropriate range,
When the temperature is maintained during the heating or before rolling, the holding time is limited to 1 to 600 s. When the temperature is raised at an average rate of 0.1 to 50 ° C./s from 300 ° C. to the rolling temperature, austenite formation is greatly suppressed as compared with the equilibrium state. By starting rolling from a state in which austenite is suppressed, fine austenite is generated at high density from various sites, and the presence of hard austenite allows uniform deformation of ferrite in the subsequent rolling process. Introduced and the austenite functions as a recrystallization site, so that a uniform ultrafine graining of ferrite is achieved.

In order to keep the non-equilibrium state at the start of rolling or continuously during rolling and to suppress the formation of austenite phase, it is necessary to start rolling immediately during the temperature rise or to keep the rolling time at 600 s or less even if it is maintained. There is. If the holding time is longer than 600 s, austenite is generated to a ratio close to the equilibrium state during the holding, and in this case, the austenite becomes coarser than in the present invention, so that uniform ultrafine graining becomes insufficient. The holding time may be short, but as a condition for reliably controlling the holding step, in the present invention, the lower limit of the holding time when holding is performed before the temperature-increasing rolling is 1 s.

From 300 ° C. to the rolling temperature, 0.1 to 50 ° C. /
a heating rate of s, A _c1 transformation point ~ (A _c3 transformation point -1
(00 ° C) or before the start of rolling.
After the holding, rolling is started, and the cumulative rolling reduction of the rolling is set to 50 to 90%. If the cumulative rolling reduction is less than 50%, even if the other requirements satisfy the present invention, a recrystallized structure over the entire surface cannot be obtained, and uniform ultrafine graining cannot be expected. It is preferable that the cumulative rolling reduction is as large as possible. However, if the cumulative rolling reduction exceeds 90%, the effect is saturated, and if the cumulative rolling reduction is excessive, the thickness of the sheet that can be produced is limited, and the shape and productivity are disadvantageous. From a practical point of view, in the present invention, the upper limit of the cumulative rolling reduction is set to 90%.

In the rolling temperature range of the present invention, the applied processing strain is substantially accumulated, so that the rolling reduction of each pass does not matter. However, the requirement for the temperature history during rolling described below must be satisfied. Unnecessarily long intervals between the rolling passes are not preferable because the recovery of the strain occurs, and the processing strain introduced by the bending is not effectively used for refining the structure.
The spacing between each pass should preferably be within 30s.

As described above, in the present invention, the heating rate, the holding time before rolling, the rolling start temperature,
In addition to limiting the rolling reduction temperature, in addition to limiting the rolling reduction temperature to ensure uniform ultra-fine graining,
At any stage during rolling, the temperature of the steel (A _c1 transformation point -5)
(0 ° C.) to ( _{Ac 3} transformation point −50 ° C.). Rolling start temperature is from A _c1 transformation point to (A
_{If the c3} transformation point is -100 ° C), austenite is generated in a fine and high density before and during rolling and effectively works for ultra-fine graining. However, the present invention is premised on multi-pass rolling. In order for each pass to work effectively for ultra-fine graining, and for the other reasons described below, the rolling temperature of each pass after the start of rolling is set to (A _c1 transformation point −50 ° C.) to (A _c3 transformation). Point-5
(0 ° C.).

That is, if the temperature during rolling is lower than (A _c1 transformation point −50 ° C.), it is not preferable because the anisotropy of the material increases and the shape of the steel deteriorates. In addition, the effect of the cumulative reduction is accumulated for ultra-fine graining.
Although a part of the rolling pass may be less than (A _c1 transformation point −50 ° C.), if most of the rolling passes are less than (A _c1 transformation point −50 ° C.), recrystallization may not easily occur. Because
In the present invention, the lower limit of the temperature during rolling is set to (A _c1 transformation point −50).
℃) or higher.

On the other hand, the temperature during rolling is (A _c3 transformation point −50).
℃) directly affects the ultra-fine graining,
Not preferred. That is, during rolling, the (A _c3 transformation point −50)
If the temperature exceeds (° C), the austenite phase excessively increases and the processed structure is eliminated. In addition, since the grain growth of the processed ferrite cannot be ignored, there is a concern that ultra-fine graining may be insufficient. Elimination of the processed structure has a particularly bad influence on the fatigue strength.

The above are the basic requirements for the manufacturing method of the present invention. After rolling, accelerated cooling and / or tempering can be performed after the rolling, if necessary, for adjusting the strength. 2 to 100 ° C for accelerated cooling
Accelerated cooling to 20 ° C. to 650 ° C. at a cooling rate of / s.
If the cooling rate of the accelerated cooling is less than 2 ° C./s, the effect of controlling the structure by the accelerated cooling is not sufficient, whereas if it exceeds 100 ° C./s, the effect of the accelerated cooling is saturated and the residual stress Is undesirably large or the flatness of the steel sheet deteriorates. Therefore, in the present invention, the cooling rate of the accelerated cooling is limited to 2 to 100 ° C./s.

When tempering is performed after rolling or after further accelerated cooling after rolling, the heating temperature is limited to 300 ° C. or higher and A _c1 transformation point or lower. Heating temperature for tempering is 3
If the temperature is lower than 00 ° C., the effect of tempering on the material is not sufficient, and the significance of tempering is lost. On the other hand, when the heating temperature is A
_{Exceeding the c1} transformation point is not preferable because an austenite phase is formed and the ultrafine grain structure is eliminated. Therefore, in the present invention, the heating temperature in tempering is limited to 300 ° C. to the A _c1 transformation point. The effect on the material of the tempering holding time and cooling conditions is very small compared to the heating temperature.
Although it is not necessary to specify this in a practical condition range, in order to suppress the coarsening of the structure, it is more preferable to use a cooling method of cooling at a cooling rate higher than or equal to cooling, and the holding time is set at the heating temperature. Is less than 500h, 48h or less, 500
When the temperature exceeds ℃, it is preferably 4 hours or less. In order to ensure that the fatigue life of the lap welded joint is less than half that of a normal structure steel with emphasis on fatigue characteristics, accelerated cooling after rolling and tempering should be performed. When performing tempering, the heating temperature is more preferably set to 600 ° C. or lower.

The above is the reason for limiting the requirements relating to the production method for obtaining an ultrafine grain structure having an extremely small mixed grain size and an average ferrite grain size of 1 to 3 μm or less, which is the object of the present invention. In order to obtain this structure, and to achieve extremely excellent toughness and brittle crack propagation arrestability in high-tensile steel with a tensile strength of about 490 MPa or higher, and at the same time, to achieve good weld joint fatigue properties. In addition to the organizational requirements, it is necessary to limit the chemical composition at the same time.

The reasons for limiting the chemical composition are described below. First, C is contained as an effective component for improving the strength of steel. If it is less than 0.01%, it is difficult to secure the strength necessary for structural steel, but if it is more than 0.20%, it is excessive. In order to reduce the toughness and weld crack resistance of the base metal and the welded portion, the content is set in the range of 0.01 to 0.2%. Next, Si
As a deoxidizing element, it is an element effective for securing the strength of the base material. However, if the content is less than 0.01%, deoxidation becomes insufficient and disadvantageous for securing the strength. Conversely, an excessive content exceeding 1% forms a coarse oxide and causes deterioration of ductility and toughness. Therefore, the range of Si is set to 0.01 to 1%. Further, Mn is an element necessary for securing the strength and toughness of the base material, and it is necessary to contain at least 0.1% or more. Since the material toughness, the toughness of the welded part, and the weld cracking property are deteriorated, the upper limit is set to 2% as far as the material allows.

Al is an element effective for deoxidation, grain refinement of the heated austenite grain size, etc., but it is necessary to contain 0.001% or more in order to exhibit the effect. On the other hand, 0.1%
If it is contained in excess of more than 0.001%, a coarse oxide is formed and ductility is extremely deteriorated, so it is necessary to limit the content to the range of 0.001% to 0.1%. N is effective in refining austenite grains in combination with Al and Ti, so that a small amount of N is effective in improving mechanical properties. Further, it is impossible to industrially completely remove N in steel, and it is not preferable to reduce N more than necessary because an excessive load is applied to a manufacturing process. Therefore, the lower limit is set to 0.001% as a range in which industrial control is possible and load on the manufacturing process can be tolerated. If it is contained excessively, the amount of dissolved N increases, which may adversely affect ductility and toughness. Therefore, the upper limit is set to 0.01% as an allowable range.

P is an impurity element, and is harmful to various properties of steel. Therefore, it is preferable to reduce P as much as possible. However, in the present invention, the upper limit is set to 0.02% as a practically allowable amount. And S is also basically an impurity element, and particularly has a large adverse effect on the ductility, toughness and fatigue characteristics of steel. The upper limit is limited to 0.01% as a practically allowable amount. Where S
In the trace amount range, fine sulfides are formed to contribute to the improvement of the weld heat affected zone (HAZ) toughness. Therefore, when considering the HAZ toughness, it is preferable to add 0.0005 to 0.005% in the range. .

The above is the reason for limiting the basic components of the steel material of the present invention. In the present invention, Ni, Cu, Cr, Mo, W, T
One, two or more of i, V, Nb, Zr, Ta, and B can be contained.

Ni is a very effective element that can improve the strength and toughness of the base material at the same time, but it is necessary to add 0.1% or more to exhibit its effect. As the amount of Ni increases, the strength and toughness of the base material improve. However, an excessive addition of more than 5% saturates the effect, but may cause deterioration of HAZ toughness and weldability. Since it is an expensive element, in the present invention, N
The upper limit of i is set to 5%. Cu is an element having almost the same effect as Ni, but in order to exhibit the effect, 0.1%
% Or more is required, and if added over 1.5%, there is a problem in hot workability and HAZ toughness. Therefore, in the present invention, the content is limited to the range of 0.1 to 1.5%.

Cr is an element effective for improving the strength by solid solution strengthening and precipitation strengthening.
However, if Cr is added excessively, the base metal and HAZ are increased through the increase of quenching hardness, formation of coarse precipitates, etc.
Since the toughness of the steel is adversely affected, the upper limit is limited to 2% as an allowable range. Mo and W are similar to Cr,
These elements are effective for increasing the strength by solid solution strengthening and precipitation strengthening. However, Mo and W are both 0.01 to 2 as far as they can exert their effects and do not adversely affect other properties.
%.

Since Ti forms stable TiN in austenite and contributes to refinement of the heated austenite grain size of the HAZ as well as the base material, Ti is an element effective for improving the toughness as well as the strength. However, in order to exhibit the effect, it is necessary to contain 0.003% or more.
If it is contained in excess of 0.1%, coarse TiN is formed and toughness is adversely deteriorated. Therefore, in the present invention, the content is limited to the range of 0.003 to 0.1%. V is an element effective for improving the strength of the base material by precipitation strengthening, but in order to exhibit the effect, 0.005% or more is necessary. As the amount of addition increases, the amount of reinforcement also increases.
Since the base material toughness and the HAZ toughness are deteriorated, and the precipitates tend to be coarse and the effect of strengthening tends to be saturated, the upper limit is set to 0.5% as a range where the toughness deterioration is small with respect to the strengthening amount.

Nb is an element that is effective in increasing the strength in a trace amount by precipitation strengthening and transformation strengthening, and also has a significant effect on the processing and recrystallization behavior of austenite, and is therefore effective in improving the base material toughness. In order to achieve the effect,
003% or more is necessary. However, if added in excess of 0.2%, the toughness will be extremely deteriorated. Therefore, in the present invention, the content is limited to the range of 0.003 to 0.2%. Zr is also an element that is effective for improving the strength mainly by precipitation strengthening, but is required to be 0.003% or more to exhibit the effect. On the other hand, if it is added in excess of 0.1%, coarse precipitates are formed and the toughness is adversely affected.
The upper limit is set to 0.1%. Ta also has the same effect as Nb, and when added in an appropriate amount, contributes to improvement in strength and toughness.
If the addition exceeds 2%, toughness deterioration due to coarse precipitates becomes remarkable, so the range is 0.005 to 0.2%.
And

B is an element that enhances hardenability in a very small amount.
It is an effective element for increasing strength. B is segregated at the austenite grain boundaries in a solid solution state to enhance hardenability by increasing the hardenability. Therefore, even a very small amount is effective, but if it is less than 0.0002%, the amount of segregation at the grain boundaries cannot be sufficiently ensured. This is not preferable because the effect of improving the performance is likely to be insufficient or the effect tends to vary. On the other hand, if added in excess of 0.005%, coarse precipitates are often formed during the production of the slab or during the reheating stage, so that the effect of improving hardenability becomes insufficient, and The risk of toughness degradation due to precipitates also increases. Therefore, in the present invention, the range of B is set to 0.0002 to 0.005%.

Further, in the present invention, improvement of ductility,
To improve the joint toughness, Mg, Ca,
One or more kinds of Y can be contained. M
All of g, Ca, and Y are effective for suppressing ductile expansion of sulfide during hot rolling and improving ductility. It works effectively to improve the joint toughness by making the oxide finer. The lower limit contents for exhibiting the effect are 0.0005% of Mg,
Is 0.0005% and Y is 0.005%. On the other hand, if it is contained excessively, sulfides and oxides are coarsened and the ductility, toughness and fatigue properties are deteriorated. Therefore, the upper limits are respectively 0.01% for Mg and Ca and 0.1% for Y. And

[0044]

Next, the effects of the present invention will be described more specifically with reference to examples. Tables 1 and 2 show the chemical compositions of the test steels used in the examples. Each test steel was made into a slab by ingot slab, by slab rolling, or by continuous casting. Table 1,
Among 2, the billet numbers 1 to 10 satisfy the chemical composition range of the present invention, and the billet numbers 11 to 13 do not satisfy the chemical composition range of the present invention.

[0045]

[Table 1]

[0046]

[Table 2]

The steel slabs having the chemical compositions shown in Tables 1 and 2 are shown in Tables 3, 4 and 5
The steel plate was manufactured into a steel plate having a thickness of 25 mm under the conditions shown in (1), and the tensile characteristics at room temperature, the 2 mm V notch Charpy impact characteristics, ESSO characteristics as brittle crack propagation stopping characteristics, and the fatigue characteristics of welded joints were investigated. The structure (structure fraction, ferrite grain size) at the slab stage before hot rolling was adjusted as cast or by normalizing and quenching and tempering as appropriate.

[0048]

[Table 3]

[0049]

[Table 4]

[0050]

[Table 5]

The measurement of the structure fraction at the billet stage was performed under the surface below
An optical microscope structure photograph taken continuously from mm to the center of the plate thickness was taken by an image analyzer. Further, the ferrite grain size was obtained by a cutting method at 1 mm below the surface, 1/4 of the plate thickness, and the center of the plate thickness, and the average value was used. Tensile test specimens and Charpy impact test specimens were taken from the center of the sheet thickness at right angles to the rolling direction (C direction). 50% Charpy impact properties
Evaluation was made based on the fracture surface transition temperature (vTrs). The propagation arrestability of brittle cracks was measured by a temperature gradient type ESSO test for all thicknesses.
ca value 400kgf · mm ^-3/2 becomes temperature (T _Kca400)
Was evaluated.

The fatigue test was performed on the circular welded joint shown in FIG. 1 in order to evaluate the fatigue characteristics when a fatigue crack was generated from the weld toe of the structure and propagated through the base material.
A test piece was sampled from a steel plate at a length of 300 mm in the longitudinal direction of the steel sheet, a length of 80 mm in the width direction, a thickness of 25 mm (all thickness), and a width of 10 mm, a length of 30 mm,
Height: Carbon dioxide welding of 30mm rib plate (CO ₂ welding)
And welded by turning around the center of the test plate. In this case, the carbon dioxide welding was performed with a chemical composition of C: 0.06 mass.
%, Si: 0.5 mass%, Mn: 1.4 mass
%, Using a 1.4 mm diameter welding wire, the current:
270A, voltage: 30V, welding speed: 20cm / min
I went in. In the fatigue test, the span of the load fulcrum was 70 mm for the lower span and 220 mm for the upper span, and the maximum load (Pmax) was 5500 kgf and the stress ratio (R) was 0.1.
, And the fatigue life was measured. Table 6 shows the mechanical properties of each steel sheet.

[0053]

[Table 6]

In Tables 3, 4, 5, and 6, steel plate numbers A1 to A1
A14 is a slab number 1 to 10 having the chemical composition of the present invention.
A steel sheet that has been manufactured by the manufacturing method of the present invention and that satisfies all of the requirements of the present invention, and has good joint fatigue properties, strength, toughness (2 mmV notch Charpy impact properties), and brittle cracks. It is clear that the propagation stop characteristics (ESSO test characteristics) are simultaneously achieved.

On the other hand, steel sheet numbers B1 to B10 are comparative steel sheets manufactured by a method not satisfying any of the requirements of the present invention, and have higher toughness and brittleness than the steel sheets manufactured according to the present invention. It is clear that some or all of the crack arrest characteristics and joint fatigue characteristics are inferior. Steel sheet numbers B1 to B3 are examples in which good properties could not be achieved even when manufactured by the method of the present invention because their chemical compositions did not satisfy the present invention.

That is, in the steel sheet number B1, although ultra-fine graining was achieved due to the excessive amount of C, the toughness (vT
rs), brittle crack propagation arresting properties, and joint fatigue properties are all significantly inferior to those of the present invention. Steel plate number B2
Although ultra-fine graining was achieved due to the excessive amount of Mn, the brittle crack propagation arresting property was greatly deteriorated.
The toughness and joint fatigue properties are also clearly inferior to those of the present invention. In steel sheet No. B3, although ultra-fine graining was achieved due to excessive P content as an impurity,
All of the toughness, brittle crack propagation arresting properties and joint fatigue properties are significantly inferior to those of the present invention.

Although the steel sheets Nos. B4 to B10 satisfy the present invention in chemical composition, good properties are achieved because some of the production methods relating to Tables 3, 4, and 5 do not satisfy the present invention. This is an example that could not be done. That is, steel plate number B4
Is because the microstructure requirement at the slab stage before the temperature raising rolling does not satisfy the present invention, that is, in the microstructure in which ferrite is present in an amount of 20% or more, the grain size of the ferrite is over 20 μm and coarse However, ultra-fine graining was not sufficient, and all of the toughness, brittle crack propagation arrestability, and joint fatigue properties were significantly inferior to those of the present invention. Since the steel sheet number B5 has an excessively high rolling temperature and a starting temperature exceeding the A _c3 transformation point, austenite is hot-rolled, and the ferrite processing and recrystallization of the present invention was used. Compared with the case, the final ferrite grain size is remarkably coarse. Therefore, all of the toughness, brittle crack propagation arrestability, and joint fatigue properties are significantly inferior to those of the present invention.

For the steel sheet No. B6, the rolling temperature is too low, and the processing in the ferrite single-phase region has been started all the time. No final ferrite grain size is obtained. Therefore, the toughness and brittle crack propagation arresting properties are remarkably inferior. In steel sheet No. B7, the austenite is formed in a state close to the equilibrium state during the temperature rise because the heating rate until rolling is too low, so that fine and high-density formation during rolling does not occur. As a result, ultrafine graining was not sufficient, and all of the toughness, brittle crack propagation arrestability, and joint fatigue properties were inferior to those of the present invention.

The steel sheet No. B8 did not sufficiently recrystallize because of the insufficient rolling reduction in rolling, and as a result, the average grain size was coarse. The drop is large. Steel sheet number B9 is an example in which the temperature of the steel sheet was excessively high during rolling. Since ultra-fine graining was not sufficient, all of the toughness, brittle crack propagation arrestability, and joint fatigue properties were lower than those of the present invention. Inferior. Steel plate number B
No. 10 is an example in which the temperature of the steel sheet became too low during rolling, and the ferrite grain size was slightly coarse due to insufficient recrystallized steel, especially toughness and brittle crack arrestability. However, it is significantly inferior to the present invention.

From the above examples, it is clear that according to the present invention, it is possible to obtain excellent joint fatigue characteristics while having extremely excellent low-temperature toughness and brittle crack propagation stopping characteristics. .

[0061]

The present invention is an effective means for simultaneously imparting extremely good toughness, brittle crack propagation arresting properties and joint fatigue properties to a high tensile steel material having a tensile strength of about 490 MPa class or more. It is possible to manufacture steel materials for welded steel structures such as marine structures, pressure vessels, shipbuilding, bridges, buildings, line pipes, etc., where safety is important, without incurring a significant cost increase. The industrial utility is extremely large.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a turning welding four-point bending test method for evaluating the fatigue characteristics of a welded joint in an example.

Continuing from the front page (72) Inventor Hiroyuki Shirahata Oita, Oita, Oita, Nishi-no-Shi 1 F-term in Nippon Steel Corporation Oita Works (reference) 4K032 AA01 AA02 AA05 AA11 AA12 AA14 AA15 AA16 AA19 AA20 AA21 AA22 AA23 AA24 AA27 AA29 AA31 AA35 AA36 AA39 BA01 BA02 CA01 CD02 CD03

Claims (6)

[Claims] 1. In mass%, C: 0.01 to 0.2% Si: 0.01 to 1% Mn: 0.1 to 2% Al: 0.001 to 0.1% N: 0.001 ~ 0.01%, as impurities
A steel containing P: 0.02% or less, S: 0.01% or less, the balance being iron and unavoidable impurities, and having any of the following structures:
Heating is performed at a heating rate of 0 ° C / s. Rolling is started while the temperature is rising from the A _c1 transformation point to the (A _c3 transformation point −100 ° C.). The temperature of the steel during rolling is (A _c1 transformation point−50 ° C.) to (A _c3 transformation point−50 ° C.)
A method for producing a high-strength steel excellent in toughness and fatigue strength, which is within the range described above. Martensite single phase structure, bainite single phase structure, or a mixed structure of both. A ferrite content of less than 20%, and a mixed structure of ferrite and martensite, or ferrite and bainite, or ferrite, martensite, and bainite. Ferrite content is 20% or more and its average particle size is 2%
0 μm or less and a ferrite single phase structure, or a mixed structure composed of one or more of ferrite and martensite, bainite, and pearlite. 2. In mass%, C: 0.01 to 0.2% Si: 0.01 to 1% Mn: 0.1 to 2% Al: 0.001 to 0.1% N: 0.001 ~ 0.01%, as impurities
A steel containing P: 0.02% or less, S: 0.01% or less, the balance being iron and unavoidable impurities, and having any of the following structures:
Rolling is started at a heating rate of 0 ° C./s, after holding at a temperature of A _c1 transformation point to (A _c3 transformation point−100 ° C.) for 1 to 600 s, rolling is started, and a rolling reduction of 50% to 90% is achieved. When performing the rolling, the temperature of the steel during rolling is (A _c1 transformation point−50 ° C.) to (A
_c3 transformation point −50 ° C.). A method for producing a high-strength steel excellent in toughness and fatigue strength, which is within the range of ( _c3 transformation point −50 ° C.). Martensite single phase structure, bainite single phase structure, or a mixed structure of both. A ferrite content of less than 20%, and a mixed structure of ferrite and martensite, or ferrite and bainite, or ferrite, martensite, and bainite. Ferrite content is 20% or more and its average particle size is 2%
0 μm or less, and a ferrite single phase structure, or a mixed structure of ferrite and one or more of martensite, bainite, and pearlite 3. The toughness and fatigue strength according to claim 1 or 2, wherein after completion of rolling, the steel sheet is accelerated and cooled at a cooling rate of 2 to 100 ° C./s to 20 ° C. to 650 ° C. Excellent method for producing high tensile steel. 4. A high temperature steel with excellent toughness and fatigue strength according to claim 1, further comprising, after rolling, tempering at a heating temperature of 300 ° C. or more and an A _c1 transformation point or less. Manufacturing method for tensile steel. 5. Ni: 0.1 to 5% Cu: 0.1 to 1.5% Cr: 0.01 to 2% Mo: 0.01 to 2% W: 0.01 by mass% ２2% Ti: 0.003 to 0.1% V: 0.005 to 0.5% Nb: 0.003 to 0.2% Zr: 0.003 to 0.1% Ta: 0.005 to 0% High strength steel excellent in toughness and fatigue strength according to any one of claims 1 to 4, wherein the steel contains one or more of 0.2% B: 0.0002 to 0.005%. Manufacturing method. 6. Mg: 0.0005 in mass%.
6 to 0.01% Ca: 0.0005 to 0.01% Y: 0.005 to 0.1% A method for producing a high-tensile steel having excellent toughness and fatigue strength as described.