JPH1121655A - Steel with superfine structure, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel having a superfine structure and combining high strength with high toughness, and its production. SOLUTION: In this production of the steel with superfine structure, a hypo- eutectoid steel containing >=0.02 wt.% C and having <=40 μm average grain size is heated up to the ferrite-austenite two-phase temp. region where the fraction of austenitic structure becorries 20 to <80%, hot-rolled, and cooled and the above treatment is performed one or more times and then cooling is applied, and this steel has a structure of <=2 μm average grain size. In this steel, the principal structure is composed of a two phase structure of bainite.martensite and worked ferrite. Furthermore, >0.5 wt.% Ni is added to the above. Tempering is carried out at a temp. not higher than the Ac1 transformation point. By this treatment, the steel with superfine structure which cannot possibly be obtained by the conventional methods can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel having an ultrafine structure which can be widely used for a hull of a high-speed ship, a steel pipe for high-pressure gas transport, a pressure vessel, an industrial machine, an aircraft, and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Refinement of the structure is important as a method for obtaining the properties of high-strength, high-toughness steel and the like.
Many methods have been proposed for adding a large amount of expensive alloy elements such as i, Mo, and Co.

[0003]

However, Ni, Mo,
The mass addition of expensive alloying elements such as Co or special heat treatment such as ausforming is not suitable for mass-produced steel used as a structural material. On the other hand, although the controlled rolling is suitable for mass production, the conventional elongation has a limit to the refinement of the structure and a limit to the effect of improving the strength and toughness. The present invention provides a steel having an ultrafine structure and consequently having properties such as high strength and high toughness, and a method for producing steel in which these are mass-produced.

[0004]

For example, as a method for achieving both high strength and high toughness, it is known that microstructuring is very effective. The present inventors have conducted intensive research on a method of manufacturing a steel material to obtain an ultrafine structure,
The inventors have invented a steel having an ultrafine structure and a method for producing the same.

That is, the gist of the present invention is a steel having an ultrafine structure described below and a method for producing the same. (1) A hypoeutectoid steel containing 0.02% by weight or more of C and having an ultrafine structure characterized by having a main structure of two phases and an average particle size of 2 μm or less. (2) The steel having an ultrafine structure according to the above (1), wherein the two-phase structure is bainite martensite and processed ferrite. (3) The steel having an ultrafine structure according to the above (1), wherein the two-phase structure is ferrite / pearlite and processed ferrite. (4) The steel according to any one of (1) to (3), further comprising 0.5% by weight or more of Ni, the steel having an ultrafine structure. (5) A steel having an ultrafine structure, wherein the steel according to any one of (1) to (4) is tempered at a temperature equal to or lower than the Ac ₁ transformation point.

(6) A hypoeutectoid steel containing 0.02% by weight or more of C and having an average grain size of 40 μm or less, a ferrite and austenite two phase having an austenite structure fraction of 20% or more and less than 80%. After heating to a temperature range and hot rolling at a cumulative reduction of 50% or more, it is cooled at a cooling rate faster than air cooling, the main structure is composed of two phases, and the average grain size is 2 μm.
A method for producing steel having an ultrafine structure, characterized in that the structure is not more than m. (7) A hypoeutectoid steel containing 0.02% by weight or more of C and having an average grain size of 40 μm or less is subjected to a two-phase temperature range of ferrite and austenite having an austenite structure fraction of 20% to less than 80%. After heating and hot rolling at a cumulative reduction of 50% or more, cooling at a cooling rate faster than air cooling is performed at least once, and the main structure is composed of two phases and the average grain size is 2 μm.
A method for producing steel having an ultrafine structure, characterized in that the structure is not more than m. (8) The method for producing a steel having an ultrafine structure according to the above (6) or (7), wherein the cooling method after hot rolling is water cooling. (9) The method according to any one of (6) to (8), wherein the steel further comprises 0.5% by weight or more of Ni as a steel component. . (10) The method according to any one of (6) to (9), wherein the steel is further tempered at a temperature equal to or lower than the Ac ₁ transformation point. .

Here, the definition of the average grain size is a length obtained by dividing the length of a straight line drawn in the thickness direction of the steel sheet by the number of intersections with the crystal grain boundaries. Here, the crystal grain boundaries refer to grain boundaries of old austenite, ferrite, and processed ferrite. When the structure before heating in the two-phase region is bainite-martensite, ferrite means high-temperature tempered bainite / high-temperature-tempered martensite tempered at the two-phase region heating temperature.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the contents of the present invention will be described in detail. A feature of the present invention is that the hypoeutectoid steel is cast into a fine grain structure of 40 μm or less by ordinary hot rolling or the like, and then heated between the Ac ₁ transformation point and the Ac ₃ transformation point to ferrite (α). A 10-austenite (γ) two-phase structure, which is hot-rolled to form a mixed structure of fine processed α and γ, and then water-cooled to form a mixed structure of a structure transformed from processed α and processed γ, substantially Ultrafine structure or steel. Here, when the cooling rate from the two-phase region is high or when the hardenability of steel is high, a low-temperature transformation structure such as bainite martensite is formed, and when the cooling rate is low or the hardenability of steel is low. Becomes ferrite perlite.

Generally, in order to make the structure fine, it is necessary to make the austenite structure fine before the martensitic transformation. As a method for refining the austenite structure, there is rapid heating and quenching, but it is not suitable as a method for producing large structural steel. Generally, hot rolling is performed from the finest austenite grains as much as possible, recrystallization is performed, and then microstructure refinement is performed by rolling in an unrecrystallized region. Even if this method is carried out under the most ideal conditions, the austenite grain size immediately before rolling in the non-recrystallized region is several tens of μm, and even after rolling in the non-recrystallized region, only an average particle size of at most 5 to 10 μm can be obtained. Was. Therefore, it is impossible to obtain ultrafine particles having an average particle size of 2 μm or less by the conventional controlled rolling method.

However, it has been found that the microstructure after rolling can be made extremely fine by changing the microstructure before rolling to a two-phase microstructure of γ and α dispersed very finely from a γ single-phase microstructure. further,
In this method, despite the low hot rolling temperature, the hot deformation resistance includes a ferrite phase with a low resistance, so the deformation resistance is equal to or lower than that of the low-temperature rolling in the γ single phase region, There are few restrictions.

Hereinafter, the reasons for limitation of the present invention will be described.
First, the reason why the average crystal grain size of steel is limited to 2 μm or less will be described. It has been found that when the crystal grains have a grain size of 2 μm or less, the proportion of plastic deformation in the grain boundary portion during brittle fracture progression increases, and the occurrence of so-called brittle fractures is almost suppressed. Therefore, unlike the conventional technical idea of improving the low-temperature toughness by reducing the unit of the brittle fracture surface to improve the low-temperature toughness, the generation of the brittle fracture surface can be suppressed when the thickness is 2 μm or less.

Here, the average grain size is a measured value in the sheet thickness direction. In the final structure, the grain structure is elongated in the rolling direction. On the optical microscope structure, the grain size in the rolling direction may be large. However, even in such a case, in martensite or bainite, the inside is divided into packet grains, and this size decreases as the grain boundary spacing in the sheet thickness direction decreases.

In the case of ferrite / pearlite, a large number of crystal grains are generated from the original γ, so that the microstructure of the optical microscope is fine. Further, when such fine grains are formed, the probability that the crack propagation path crosses the crystal grains increases. For this reason, ultrafine steel having an average crystal grain size in the thickness direction of 2 μm or less was used.

Next, the reasons for limiting the chemical composition of steel will be described. The method of the present invention is characterized in that hot rolling is started from a two-phase region of α and γ. Such a structure at a high temperature cannot be obtained unless it is a hypoeutectoid steel. Although the upper limit of the amount of C that becomes hypoeutectoid varies depending on the content of other alloying elements, it is usually 0.7 to 0.1.
8% by weight. However, when the C content is less than 0.02% by weight, the temperature at which the α and γ2 phases are formed becomes narrow, and the hardness of martensite, which is determined to some extent by the C content, becomes low, so that high strength cannot be obtained. Therefore, C needs to be 0.02% by weight or more. In particular, when high low-temperature toughness is required for the manufactured steel, the range of 0.05 to 0.2% by weight is desirable.

Other alloying elements are not particularly necessary for forming a microstructure, but are added to improve various properties such as high strength, high toughness, and high corrosion resistance. For example, as main elements, Si <1.0, Mn <3%, Cr <3
%, Mo <1.5%, Cu and 1.5% (all are% by weight), and other elements used in ordinary structural steels such as V, Nb, Al, and Ti. Mn is the most important element that enhances hardenability and increases strength.
o and Cn contribute to higher strength, higher toughness, and higher corrosion resistance.
V, Nb, Ti and Al are useful for increasing the strength and miniaturizing the initial structure. Si, Al and Ti are also used as deoxidizing elements.

In order to make the final structure fine, the structure when heated to the two-phase region must be fine. For this purpose, the structure before heating in the two-phase region must be fine. If the average grain size of the structure before heating in the two-phase region is 40 μm or more, fine 2
No phase structure is obtained. In particular, when it is desired to finally obtain a structure of 1 μm or less, the grain size before heating in the two-phase region should be 20 μm or less when the structure is martensite bainite, and 10 μm or less when the structure is ferrite / pearlite. desirable.

Such a crystal grain size is a level that can be easily achieved by ordinary hot rolling or the like. For example, a 250 mm thick steel ingot is heated to 1150 ° C.
Rolled to 120mm at a temperature of over ℃, then 900-8
What is necessary is just to control-roll to 25 mm in the temperature range of 00 degreeC.

The steel whose structure is refined to 40 μm or less is reheated to a temperature at which the γ fraction is 20% or more and less than 80%. γ
Is less than 20%, α is too large to achieve fineness.
On the other hand, when γ is 80% or more, the structure becomes mainly γ and is not a fine structure. Therefore, the γ fraction is 20% or more and 80% or more.
%. The relationship between the γ-fraction and the temperature can be easily known by measuring the expansion curve of the steel used up to the Ac ₃ point or more.

Although the two-phase structure is further refined by hot rolling, if the rolling reduction is less than 50%, the effect on the refinement is small, so the cumulative rolling reduction is set to 50% or more. The upper limit of the rolling amount is not particularly defined because the larger the rolling amount is, the finer the finer the structure is. However, the upper limit is naturally determined from the thickness ratio of the material and the product.

During hot rolling, both γ and α are processed to flatten and increase the dislocation density. From this state, cooling is further performed. When the cooling rate is high or the hardenability of steel is high, the portion of γ becomes martensite, bainite, or a mixed structure of these. On the other hand, when the cooling rate is slow or the hardenability of the steel is low, a ferritic structure is formed.
Since martensite and bainite are harder than ferrite, the strength can be increased by increasing the cooling rate, that is, by cooling with water. The structure of the processed ferrite is substantially inherited during continuous rolling regardless of the cooling method.

The structure obtained in this way becomes finer as the structure before hot rolling becomes finer. Therefore, a finer structure can be obtained by repeating heating, hot rolling and cooling to the two-phase region.

Further, 0.5% by weight of Ni as a steel component
When the above content is contained, γ is generated not only at the crystal grain boundaries but also within the crystal grains when heating to the two-phase region, further promoting the miniaturization. In this case, when the heating rate up to the Ac ₁ transformation point is reduced, γ is more minutely generated. Also,
Ni addition also contributes to the improvement of the strength and low-temperature toughness of the manufactured material.

Further, if necessary, the steel produced by the above method may be tempered at a temperature lower than the Ac ₁ transformation point. The temper recovers the ductility moderately by tempering. The tempering treatment does not change the microstructure fraction itself, and does not impair the features of the present invention.

[0024]

EXAMPLES Five kinds of steels whose chemical components are shown in Table 1 were used as raw materials. After rolling under various conditions shown in Table 2, tempering was optionally performed. Thereafter, microstructure observation, tensile test at room temperature, V-notch Charpy test of 7.5 mm thickness (-1)
00 ° C.). The results are shown in Table 2.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

As described above, according to the present invention, a steel obtained by manufacturing a steel having a specific chemical composition by a specific method becomes a high-strength steel having an ultrafine structure.

Claims (10)

[Claims] 1. A hypoeutectoid steel containing C in an amount of 0.02% by weight or more, having a main structure of two phases and an average particle size of 2 μm.
A steel having an ultrafine structure characterized by the following. 2. The steel having an ultrafine structure according to claim 1, wherein the two-phase structure is bainite martensite and processed ferrite. 3. The steel having an ultrafine structure according to claim 1, wherein the two-phase structure is ferrite / pearlite and processed ferrite. 4. The steel according to claim 1, further comprising 0.5% by weight or more of Ni, the steel having an ultrafine structure. 5. A steel having an ultrafine structure, wherein the steel according to any one of claims 1 to 4 is tempered at a temperature not higher than the Ac ₁ transformation point. 6. A hypoeutectoid steel containing 0.02% by weight or more of C and having a mean grain size of 40 μm or less, and a ferrite / austenite two-phase temperature having an austenite structure fraction of 20% or more and less than 80%. Heating area, cumulative reduction amount is 50%
After the hot rolling as described above, the steel is cooled at a cooling rate faster than air cooling to produce a steel having an ultrafine structure characterized in that the main structure is two phases and the average grain size is 2 μm or less. Law. 7. A hypoeutectoid steel containing 0.02% by weight or more of C and having an average grain size of 40 μm or less, a ferrite and austenite two-phase temperature having an austenite structure fraction of 20% or more and less than 80%. Heating area, cumulative reduction amount is 50%
After the hot rolling as described above, cooling at a cooling rate faster than air cooling is performed once or more, and an ultrafine structure characterized by having a main structure of two phases and a mean grain size of 2 μm or less is obtained. Method of producing steel. 8. The method for producing a steel having an ultrafine structure according to claim 6, wherein the cooling method after hot rolling is water cooling. 9. The method for producing a steel having an ultrafine structure according to claim 6, further comprising 0.5% by weight or more of Ni as a steel component. 10. The method of producing a steel having an ultrafine structure according to claim 6, wherein the steel is further tempered at a temperature not higher than the Ac ₁ transformation point.