JP6475839B2 - Structural heavy steel with excellent brittle crack propagation resistance and method for producing the same - Google Patents

Description

The present invention relates to a structural ultra-thick steel material excellent in brittle crack propagation resistance and a method for producing the same, and more particularly, including Ni, Nb, Ti, ferrite, bainite single phase structure, ferrite and bainite, ferrite and pearlite. Further, the present invention relates to a structural extra heavy steel material excellent in brittle crack propagation resistance having a microstructure including one structure selected from the group consisting of a composite structure of ferrite, bainite and pearlite, and a method for producing the same.

In recent years, the development of extra heavy steel having high strength characteristics has been demanded in designing structures used in the fields of ships, oceans, architecture, and civil engineering in Japan and overseas.
The use of high-strength steel when designing structures not only provides economic benefits because the weight of the structure can be reduced, but also reduces the thickness of the steel sheet, making it easier to work and weld. Can be secured.
In general, high-strength steel has a lower total rolling reduction during the production of extra-thick materials and cannot give sufficient deformation to the center, resulting in a coarse structure in the center, resulting in increased hardenability. A low temperature transformation phase such as bainite is generated.
Moreover, it becomes difficult to ensure the impact toughness of the central portion due to the coarsened structure.

In particular, in the case of brittle crack propagation resistance, which indicates the stability of the structure, there are an increasing number of cases seeking guarantees when applied to main structures such as ships, but when a low-temperature transformation phase is generated in the center, Since a phenomenon occurs in which the brittle crack propagation resistance is greatly lowered, it is very difficult to improve the brittle crack propagation resistance of the ultra-thick high-strength steel material.
On the other hand, in the case of high-strength steel with a yield strength of 350 MPa or more, in order to improve brittle crack propagation resistance, surface cooling is applied during finish rolling in order to refine the grain size of the surface layer, or bending stress is applied during rolling. Various techniques have been introduced, such as adjusting the grain size or refining the surface layer by two-phase rolling.

However, in the case of the above technique, although it is advantageous for refining the structure of the surface layer part, the problem that the impact toughness decreases due to the coarsening of the structure of the central part cannot be solved, so a fundamental measure for brittle crack propagation resistance Is hard to say.
In addition, it can be said that the commercial application is impossible because the productivity is expected to decrease greatly in the mass production system.
Furthermore, it is possible to improve the resistance to brittle crack propagation by adding a large amount of elements such as Ni that are useful for improving toughness. However, since Ni is an expensive element, it is difficult to commercialize it in terms of manufacturing cost. It is.

Korean Published Patent No. 2009-0069818 Korean Published Patent No. 2002-0091844

The object of the present invention is to provide a structural heavy steel material excellent in brittle crack propagation resistance.
Another object of the present invention is to provide a manufacturing method for controlling the alloy composition and the microstructure in manufacturing a structural heavy steel material excellent in brittle crack propagation resistance.

In the present invention, by mass , C: 0.02 to 0.10%, Mn: 0.8 to 2.5%, Ni: 0.05 to 1.5%, Nb: 0.005 to 0.1 %, Ti: 0.005 to 0.1%, with the balance being iron (Fe) and other inevitable impurities, ferrite single phase structure, bainite single phase structure, ferrite and bainite composite structure, ferrite and pearlite complex structure, and the ferrite, the microstructure consisting of bainite and pearlite composite structure, one of the tissue selected from the group consisting possess,
The ferrite is an acicular ferrite or a polygonal ferrite, and the bainite is a granular bainite,
The structural ultra-thick steel material has a grain size of 15 μm or less having a high inclination boundary of 15 ° or more measured by the ESBD method at the center of the plate thickness,
The ultra-thick structural steel material has a yield strength of 350 MPa or more and an impact transition temperature at the center of −60 ° C. or less .

The structural heavy steel material has a thickness of 10 to 100 mm.

Moreover, this invention is the mass %, C: 0.02-0.1%, Mn: 0.8-2.5%, Ni: 0.05-1.5%, Nb: 0.005-0 .10%, Ti: 0.005 to 0.1%, and the remainder comprising iron (Fe) and other inevitable impurities is reheated to 950 to 1100 ° C. and then roughened at a temperature of 1100 to 900 ° C. Rolling the rough rolled bar at a temperature of Ar3 or higher to obtain a steel plate, and cooling the steel plate to a temperature of 700 ° C. or lower. The temperature difference between the central part in the thickness direction of the slab or bar before rolling and the outer surface of the slab or bar is 100 ° C. or more, and a ferrite single phase structure, a bainite single phase structure, and a composite of ferrite and bainite Structure, composite structure of ferrite and pearlite, and Ferrite, having a microstructure composed of one structure selected from the group consisting of a composite structure of bainite and pearlite, wherein the ferrite is acicular ferrite or polygonal ferrite, bainite is It is granular bainite, and the structural heavy steel material has a grain size having a high inclination boundary of 15 degrees or more measured by the ESBD method at the center of the plate thickness of 15 μm or less. The steel material is characterized by producing a steel material having a yield strength of 350 MPa or more and an impact transition temperature at the center of −60 ° C. or less .

The temperature difference between the central portion in the thickness direction of the slab or bar and the outer surface of the slab or bar is 100 to 300 ° C.

The temperature difference between the central portion in the thickness direction of the slab or bar and the outer surface of the slab or bar is the surface temperature of the slab or bar measured immediately before rough rolling, the cooling conditions, and the slab or bar immediately before rough rolling. It is a difference from the center temperature calculated in consideration of the thickness.

The temperature difference between the central portion in the thickness direction of the slab or bar and the outer surface of the slab or bar is obtained by cooling the slab or bar using a cooling device.

The cooling medium of the cooling device is at least one of water, air, a liquid phase coolant, and a gas phase coolant.

The total cumulative rolling reduction during the rough rolling is 40% or more.

The steel sheet is cooled at a cooling rate at a central portion of 2 ° C./s or more.

The steel sheet is cooled at an average cooling rate of 3 to 300 ° C./s.

According to the present invention, it is possible to obtain a structural extra heavy steel material having excellent brittle crack propagation resistance having excellent yield strength and impact transition temperature at the center.

It is the photograph which observed the thickness center part of the steel 1 by this invention with the optical microscope.

The inventors of the present invention have solved the conventional problems, and have conducted research in order to secure a structural ultra-thick steel material having yield strength and impact transition temperature at the center that is superior to conventional structures. It was recognized that the brittle crack propagation resistance of structural heavy-thick steel can be improved by appropriately controlling the alloy design and microstructure of the structural heavy-thick steel, and the present invention has been completed based on this.

Hereinafter, the structural heavy steel material excellent in brittle crack propagation resistance of the present invention will be described in detail.
The structural thick steel material excellent in brittle crack propagation resistance of the present invention is mass %, C: 0.02 to 0.10%, Mn: 0.8 to 2.5%, Ni: 0.05 to 1.5%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1%, and the balance is composed of iron (Fe) and other inevitable impurities. It has a microstructure including one structure selected from the group consisting of a phase structure, a composite structure of ferrite and bainite, a composite structure of ferrite and pearlite, and a composite structure of ferrite, bainite and pearlite.
Such a structural thick steel material can have a thickness of 10 to 100 mm, and preferably has a thickness of 50 to 100 mm.

Hereinafter, the steel components and component ranges of the present invention will be described.
C (carbon): 0.02 to 0.10% (hereinafter, the content of each component means mass %)
Since C is the most important element for ensuring basic strength, it is necessary to be contained in the steel within an appropriate range. It is preferable to add 02% or more.
However, if the C content exceeds 0.10%, the low temperature toughness is lowered due to the large amount of island martensite and the high strength of the ferrite itself, so the C content is 0.02 to 0.10. % Is preferable.

Mn (manganese): 0.8 to 2.5%
Since Mn is a useful element that improves the strength by solid solution strengthening and improves the curability so that a low-temperature transformation phase is generated, it is preferably added in an amount of 0.8% or more.
However, if the Mn content exceeds 2.5%, the hardenability will increase too much, and the formation of upper bainite and martensite will be promoted to reduce impact toughness and brittle crack propagation resistance. Therefore, the Mn content is preferably limited to 0.8 to 2.5%.

Ni (nickel): 0.05 to 1.5%
Ni is an important element that improves strength by facilitating cross slip of dislocations at low temperatures and improving impact toughness and hardenability, in order to improve impact toughness and brittle crack propagation resistance. It is preferable to add 0.05% or more. However, when Ni is added in an amount of 1.5% or more, the curability is excessively increased and a low-temperature transformation phase is generated. As a result, the toughness may be reduced and the manufacturing cost may be increased. The upper limit of the Ni content is preferably limited to 1.5%.

Nb (Niobium): 0.005-0.1%
Nb precipitates in the form of NbC or NbCN and improves the strength of the base material.
Further, when reheated to a high temperature, the dissolved Nb precipitates very finely in the form of NbC during rolling, and has the effect of refining the structure by suppressing recrystallization of austenite.
Therefore, Nb is preferably added in an amount of 0.005% or more, but if added in excess, it may cause brittle cracks in the corners of the steel, so the lower limit of the Nb content should be limited to 0.1%. Is preferred.

Ti (titanium): 0.005 to 0.1%
Ti is a component that precipitates as TiN during reheating and greatly improves low-temperature toughness by suppressing the growth of crystal grains in the base material and the weld heat affected zone, and in order to obtain such an addition effect, It is preferable to add 0.005% or more.
However, if Ti is added in excess of 0.1%, the low temperature toughness may decrease due to clogging of the continuous casting nozzle or crystallization of the central portion, so the Ti content is 0.005 to 0.00. It is preferable to limit to 1%.

The balance of the present invention is iron (Fe).
However, in an ordinary manufacturing process, unintended impurities may be inevitably mixed depending on the raw material or the surrounding environment, and thus cannot be excluded.
Since such an impurity can be understood by any ordinary engineer, all contents are not specifically mentioned in this specification.

The steel material of the present invention has a single structure selected from the group consisting of a ferrite single phase structure, a bainite single phase structure, a composite structure of ferrite and bainite, a composite structure of ferrite and pearlite, and a composite structure of ferrite, bainite and pearlite. It has a fine structure.
In the composite structure of ferrite, bainite and pearlite, the pearlite ratio is preferably limited to 30% by volume or less.
The ferrite is preferably acicular ferrite and the bainite is preferably granular bainite. At this time, as the ferrite, polygonal ferrite can be used as necessary.

As the contents of Mn and Ni increase, the fraction of acicular ferrite or polygonal ferrite and granular bainite increases. This also increases the strength.
The steel material preferably has a particle size of 15 μm or less having a high inclination boundary of 15 degrees or more measured by the EBSD method at the center of the plate thickness.
Moreover, it is preferable that the yield strength is 350 MPa or more and the impact transition temperature at the center is −60 ° C. or less.

The manufacturing method of the structural ultra-thick steel material having excellent brittle crack propagation resistance according to the present invention is mass %, C: 0.02 to 0.1%, Mn: 0.8 to 2.5%, Ni: 0. 0.05 to 1.5%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.1%,
After the slab consisting of iron (Fe) and other inevitable impurities is reheated to 950 to 1100 ° C., rough rolling is performed at a temperature of 1100 to 900 ° C., and the rough rolled bar is set to Ar 3 or more. A step of obtaining a steel sheet by finish rolling at a temperature of, and a step of cooling the steel sheet to a temperature of 700 ° C. or less, the center part in the thickness direction of the slab or bar before rolling during the rough rolling and the above The temperature difference from the outer surface of the slab or bar is set to 100 ° C. or more.

Reheating temperature of slab: 950-1100 ° C
The reheating temperature of the slab is preferably 950 ° C. or higher. This is to dissolve the Ti and / or Nb carbonitride formed during casting. Further, in order to sufficiently dissolve Ti and / or Nb carbonitride, it is more preferable to heat to 1000 ° C. or higher. However, if the reheating is performed at a temperature that is too high, the austenite may be coarsened, so the upper limit of the reheating temperature is preferably 1100 ° C.

Rough rolling temperature: 1100 to 900 ° C., and temperature difference between the central portion in the thickness direction of the slab or bar before rough rolling and the outer surface of the slab or bar: rough rolling of the slab reheated at 100 ° C. or more The rolling temperature is preferably equal to or higher than the temperature (Tnr) at which recrystallization of austenite stops. An effect of destroying a cast structure such as dendrite formed during casting by rolling and reducing the size of austenite can also be obtained. In order to obtain such an effect, it is preferable to limit the rough rolling temperature to 1100 to 900 ° C.
In the present invention, the temperature difference between the center portion in the thickness direction of the slab or bar immediately before rolling during rough rolling and the outer surface of the slab or bar is set to 100 ° C. or more.

Such a temperature difference between the central portion and the outer surface can be obtained, for example, by cooling a heated slab or bar using a cooling device. The cooling device is not particularly limited, and examples of the cooling medium include at least one of water, air, a liquid phase coolant, and a gas phase coolant.
Thus, by providing a temperature difference between the center portion in the thickness direction of the slab or bar during rough rolling and the outer surface of the slab or bar, the outer surface of the slab or bar maintains a lower temperature than the center portion. However, when rolling is performed in a state where such a temperature difference exists, more deformation occurs in the center portion having a relatively higher temperature than in the surface portion having a relatively lower temperature, and the grain size in the center portion is further finer. It becomes. The average particle size at the center is preferably maintained at 15 μm or less.

This is a technology that takes advantage of the phenomenon that the surface portion having a relatively low temperature has higher strength than the center portion having a relatively high temperature, and more deformation occurs in the center portion having a relatively low temperature. is there. In order to effectively impart more deformation to the central portion, the temperature difference between the central portion and the outer surface is preferably 100 ° C. or more, and more preferably 100 to 300 ° C.
Here, the temperature difference between the central portion in the thickness direction of the slab or bar and the outer surface of the slab or bar is the surface temperature of the slab or bar measured immediately before rough rolling, the cooling conditions, and the slab immediately before rough rolling. Or the difference with the center temperature calculated in consideration of the thickness of the bar is meant.

The surface temperature and thickness of the slab are measured before the first rough rolling, and the bar surface temperature and thickness are measured from the second rough rolling to before the rough rolling.
And when performing rough rolling two or more passes, the temperature difference of the center part in the thickness direction of a slab or bar and the outer surface of the said slab or bar measures the temperature difference of each pass (pass) in rough rolling. This means that the temperature difference obtained by calculating the average value of the whole is 100 ° C. or more.
In the present invention, in order to refine the structure at the center during rough rolling, the total cumulative rolling reduction during rough rolling is preferably 40% or more.

Finish rolling temperature: Ar3 (ferrite transformation start temperature) or higher.
The rough-rolled bar is finish-rolled with Ar3 or more to obtain a steel plate.
During finish rolling, the austenite structure is deformed.
Cooling after rolling: Cooling below 700 ° C.
After finish rolling, the steel sheet is cooled to 700 ° C. or lower.
When the cooling end temperature exceeds 700 ° C., the microstructure is not formed properly, and the yield strength may be 350 Mpa or less.
The steel sheet is cooled at a cooling rate at the center of 2 ° C./s or more. If the cooling rate of the central part of the steel sheet is less than 2 ° C./s, the fine structure is not properly formed, and the yield strength may be 350 Mpa or less.
The steel sheet is cooled at an average cooling rate of 3 to 300 ° C./s.

Hereinafter, the present invention will be described more specifically with reference to examples.
The steel slab having the composition shown in Table 1 was reheated to a temperature of 1070 ° C., and then rough rolled at a temperature of 1050 ° C. Table 2 shows the average temperature difference between the center portion and the outer surface during rough rolling of the slab. The cumulative rolling reduction was 50%.
The average temperature difference between the center and the surface during rough rolling in Table 2 is the surface temperature of the slab or bar measured immediately before rough rolling, the amount of water sprayed on the bar, and the thickness of the slab immediately before rough rolling. The difference with the center temperature calculated in consideration is shown, the temperature difference of each pass in rough rolling is measured, and the average value of the whole is calculated.
After rough rolling, finish rolling was performed at a finishing rolling temperature of 780 ° C. to obtain a steel sheet having the thickness shown in Table 2, and then cooled to a temperature of 700 ° C. or lower at a cooling rate of 5 ° C./sec.

＊ＰＦ：多角形フェライト（Ｐｏｌｙｇｏｎａｌ ｆｅｒｒｉｔｅ）、Ｐ：パーライト（Ｐｅａｒｌｉｔｅ）、ＡＦ：針状フェライト（Ａｃｉｃｕｌａｒ ｆｅｒｒｉｔｅ）、グラニュラーベイナイト（ＧＢ：Ｇｒａｎｕｌａｒ ｂａｉｎｉｔｅ）、上部ベイナイト（ＵＢ：Ｕｐｐｅｒ ｂａｉｎｉｔｅ）ここで、製品厚さは厚物材を対象として評価したことを表す。

* PF: Polygonal ferrite, P: Pearlite, AF: Acicular ferrite, GB (Granular bainite), Upper bainite (UB: Upper bainite) This represents that the evaluation was made on a thick material.

As shown in Table 2, in Comparative Steels 1 and 2, the average temperature difference between the central portion and the outer surface in the thickness direction during rough rolling presented in the present invention is controlled to be less than 100 ° C. It can be seen that sufficient deformation is not transmitted to the central portion of and the particle size of the central portion is 25.3 μm and 29.6 μm, respectively, so that the impact transition temperature of the central portion is less than −60 ° C. Therefore, it can be seen that the Kca value measured at −10 ° C. does not exceed 6000, which is required in general steel for shipbuilding.
Moreover, although the comparative steels 3 and 5 have a value higher than the upper limit of C and Mn presented in the present invention, despite the refinement of the austenite grain size in the center by cooling during rough rolling, Since the upper bainite is formed, the final microstructure has a grain size of 32 μm or more and 38 μm or more, respectively. Further, since the upper bainite, which is easily brittle, is included as a base structure, the impact transition temperature in the center is It turns out that it is -60 ° C or more.
Therefore, it can be seen that the Kca value also has a value of 6000 or less at −10 ° C.

The comparative steel 4 has a value higher than the upper limit of the Ni content presented in the present invention, and it can be seen that the microstructure of the base material is granular bainite and granular bainite due to high hardening ability. .
Although the grain size of the austenite at the center is refined by cooling during rough rolling, the grain size of the final microstructure is 26 μm, and the upper bainite, which tends to be brittle, is used as the base structure. It can be seen that the transition temperature is −60 ° C. or higher.
Therefore, the Kca value also has a value of 6000 or less at -10 ° C.

On the other hand, the inventive steels 1 to 6 satisfying the component range of the present invention and having the central austenite grain size refined by cooling during rough rolling satisfy a yield strength of 350 MPa or more and a central grain size of 15 μm or less. It can be seen that the microstructure has a ferrite and pearlite structure, a single structure of acicular ferrite, a composite structure of acicular ferrite or polygonal ferrite and granular bainite, or a composite structure of acicular ferrite, pearlite and granular bainite.
The impact transition temperature at the center is 60 ° C. or lower, and the Kca value also satisfies a value of 6000 or higher at −10 ° C.
As can be seen from FIG. 1 showing a photograph of the thickness center of the steel 1 of the present invention observed with an optical microscope, the structure of the center of the steel 1 of the present invention is refined.

As mentioned above, although the preferable Example regarding this invention was described, this invention is not limited to the said embodiment, All the changes in the range which does not deviate from the technical field to which this invention belongs are included.

Claims (10)

In mass %, C: 0.02-0.10%, Mn: 0.8-2.5%, Ni: 0.05-1.5%, Nb: 0.005-0.1%, Ti: Including 0.005 to 0.1%, the balance being iron (Fe) and other inevitable impurities, ferrite single phase structure, bainite single phase structure, ferrite and bainite composite structure, ferrite and pearlite composite structure, and ferrite, the microstructure consisting of bainite and pearlite composite structure, one of the tissue selected from the group consisting possess,
The ferrite is an acicular ferrite or a polygonal ferrite, and the bainite is a granular bainite,
The structural heavy steel material has a particle size of 15 μm or less having a high inclination boundary of 15 ° or more measured by the ESBD method at the center of the plate thickness,
The structural ultra-thick steel material has a yield strength of 350 MPa or more and an impact transition temperature at a central portion of −60 ° C. or less, and the structural ultra-thick steel material having excellent brittle crack propagation resistance. The structural heavy steel material having excellent brittle crack propagation resistance according to claim 1, wherein the structural heavy steel material has a thickness of 10 to 100 mm. In mass %, C: 0.02-0.1%, Mn: 0.8-2.5%, Ni: 0.05-1.5%, Nb: 0.005-0.10%, Ti: A step of re-rolling a slab containing 0.005 to 0.1%, the balance of iron (Fe) and other inevitable impurities to 950 to 1100 ° C., and then roughly rolling at a temperature of 1100 to 900 ° C .; Including a step of finish rolling a roughly rolled bar at a temperature of Ar3 or higher to obtain a steel plate, and a step of cooling the steel plate to a temperature of 700 ° C. or lower, before rolling during the rough rolling. The temperature difference between the central portion in the thickness direction of the slab or bar and the outer surface of the slab or bar is 100 ° C. or more, and a ferrite single phase structure, a bainite single phase structure, a composite structure of ferrite and bainite, Composite structure, ferrite, A ferrite having a microstructure selected from the group consisting of a composite structure of knight and pearlite, wherein the ferrite is acicular ferrite or polygonal ferrite, and bainite is granular bainite. The structural thick steel material has a grain size of 15 μm or less having a high inclination boundary of 15 degrees or more measured by the ESBD method at the center of the plate thickness, and the structural heavy steel material is: A method for producing a structural thick steel having excellent brittle crack propagation resistance, characterized in that a steel having a yield strength of 350 MPa or more and an impact transition temperature at a central portion of −60 ° C. or less is produced. The structure excellent in brittle crack propagation resistance according to claim 3 , wherein the temperature difference between the central portion in the thickness direction of the slab or bar and the outer surface of the slab or bar is 100 to 300 ° C. Method for manufacturing ultra-thick steel. The temperature difference between the central portion in the thickness direction of the slab or bar and the outer surface of the slab or bar is the surface temperature of the slab or bar measured immediately before rough rolling, the cooling conditions, and the slab or bar immediately before rough rolling. The method of manufacturing a heavy steel for structural use having excellent brittle crack propagation resistance according to claim 3 , wherein the difference is a difference from the center temperature calculated in consideration of the thickness of the steel. The temperature difference between the central portion and the slab or bar of the outer surface in the thickness direction of the slab or bar, in claim 3, characterized in that it is obtained by cooling the slab or bar by using a cooling device The manufacturing method of the structural thick steel material excellent in the brittle crack propagation resistance of description. The structural medium having excellent brittle crack propagation resistance according to claim 6 , wherein the cooling medium of the cooling device is at least one of water, air, a liquid phase coolant, and a gas phase coolant. Manufacturing method for extra heavy steel. The method for producing an extra heavy steel for structural use having excellent brittle crack propagation resistance according to claim 3 , wherein a total cumulative rolling reduction during the rough rolling is 40% or more. The method for producing a structural heavy steel with excellent brittle crack propagation resistance according to claim 3 , wherein the steel sheet is cooled at a cooling rate at a central portion of 2 ° C./s or more. The method for producing a structural heavy steel with excellent brittle crack propagation resistance according to claim 3 , wherein the steel plate is cooled at an average cooling rate of 3 to 300 ° C./s.

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