HIGH TENSILE STRENGTH, REFRACTORY STEEL HAVING EXCELLENT WELDABILITY AND GAS CUTTABILITY AND METHOD FOR PRODUCING SAME

- Nippon Steel Corporation

Accordingly to an exemplary embodiment of the present invention, a high tensile strength, refractory steel can be provided which comprises, in mass %, approximately C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and the balance consisting of iron and unavoidable impurities. For example, a weld crack sensitive composition PCM can be defined by the following equation may be about 0.25% or less: PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B. An area fraction of polygonal ferrite or pseudo polygonal ferrite in a ¼ thick position in the plate thickness direction of the steel plate of the final rolling product is about 10% or less.

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Description
CROSS REFERENCE TO RELATED APPLICATION(S)

The present application is a national phase application of International Application PCT/JP2006/304127 filed on Mar. 3, 2006 and published as International Publication WO 2006/093282 on Sep. 8, 2006. This application claims priority from the International Application pursuant to 35 U.S.C. § 365. .The present application also claims priority from Japanese Patent Application No. 2005-060601 filed on Mar. 4, 2005 under 35 U.S.C. § 119. The disclosures of these applications are incorporated herein in their entireties.

FIELD OF THE INVENTION

The present invention relates to a high tensile strength, refractory steel having excellent weldability and gas cuttability, and a method of producing the same.

BACKGROUND INFORMATION

As a refractory steel for architectural construction, intended to provide a high temperature strength at a time of fire or the like, a refractory steel obtained by hot rolling a billet or slab has been described (see, for example, Japanese Unexamined Patent Application , First Publication No. H2-77523).

This refractory steel generally belongs to so called 400 MPa class steel or 490 MPa class steel, and can include several examples of so called 590 MPa class steel having an yield strength of 440 MPa (45 kgf/mm2) or more.

On the other hand, as a refractory steel corresponding to the 590 MPa class steel, steel containing Mo of 0.7% or more has been described (see, e.g., Japanese Unexamined Patent Application, First Publication No.2002-12939).

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

In “rolled steel for architectural construction” JIS G 3136 according to Japanese Industrial Standard and “high performance 590 N/mm2 steel for architectural construction (SA440B,C)” qualified by the Minister of Land, Infrastructure and Transport of Japan as examples of construction steel, the plate thickness can be regulated up to 100 mm. However, in the conventional types of refractory steel mainly constituting 400 MPa class steel and 490 MPa class steel, the plate thickness of the 590 MPa class steel can be at most, e.g., 40 mm, and thicker steel may not be supplied.

Recently, there has been an increasing demand for steel having a yield strength of 440 MPa or more and belonging to a class not lower than so-called 590 MPa class. Such a steel can be subjected to thermal refining. After the thermal refining, a hot rolled steel can have a metallic texture mainly composed of, e.g., polygonal ferrite or pseudo polygonal ferrite having low strength. Therefore, even when a thick steel plate having a thickness of about 100 mm is produced by hot rolling, it may be difficult to ensure the strength of the steel stably by a technical control.

On the other hand, a refractory steel corresponding to 590 MPa class steel may have a steel composition containing Mo of 0.7% or more, likely resulting in inferior cuttability by gas-cutting and high production cost. In addition, although weld crack sensitive composition (PCM) can be controlled in this refractory steel, Mo generally enhances the hardenability of the steel. From the view point of weldability, it may be preferable that the Mo content can be controlled to a low level.

Exemplary embodiments of the present invention may take the above issues into consideration. Thus, one of the objects of the present invention is to provide a high tensile strength, refractory steel and method of producing the same which has an excellent weldability and cuttability by gas cutting so as to allow mass production at low cost of a high tensile strength steel having a yield strength of 440 MPa or more and possibly having sufficient high temperature strength under a high temperature environment, such as, e.g., a fire.

Research for the above-described exemplary issue has been performed. Based on such research, it has been determined that by compound addition of Nb while suppressing the content of Mo, it is possible to stably provide and/or ensure the high temperature strength of a high tensile strength steel having a yield strength of 440 MPa or more. By suppressing the Mo content in the steel, it is possible to suppress the deterioration of weldability and gas cuttability of the steel to a minimum level. At the same time, by controlling PCM and the contents of respective alloy elements such as C, Si, Mn, and by further limiting the microstructure of the steel and manufacturing conditions for the microstructure, it is possible to obtain consistently complex properties including excellent high temperature strength, weldability, and gas cuttability.

According to one exemplary embodiment of the present invention, a high tensile strength, refractory steel having excellent weldability and gas cuttability can be provided. For example, this steel can include, in mass %, approximately, : 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and the balance consisting of iron and unavoidable impurities. A weld crack sensitive composition PCM of the steel is 0.25% or less, and can be defined by the following equation:


PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B,

and the area fraction of polygonal ferrite or pseudo polygonal ferrite in a ¼ thick position in the plate thickness direction of a steel plate of the final rolling product can be 10% or less.

This exemplary embodiment of the steel can further include, in mass %, approximately, Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, and one or two or more selected from Cr: 0.05 to 1.0%, V: 0.01 to 0.06%, B: 0.0002 to 0.0030%, Ti: 0.005to 0.025%, Mg: 0.0002 to 0.0050%, and the Ni content may be at least half of the Cu content. Such exemplary steel can further comprise, in mass %, approximately, one or two selected from Ca: 0.0005 to 0.0040% and REM: 0.0005 to 0.0100%. The yield strength of the steel can be 440 MPa or more.

In accordance with another exemplary embodiment of the present invention, a method for manufacturing a high tensile strength, refractory steel having excellent weldability and gas cuttability, can be provided. For example, according to this exemplary method, a steel member in a form of billet or slab can be heated at a temperature of about 1100 to 1300° C., with the billet and/or slab having the steel composition that includes, in mass %, approximately, : 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and the balance consisting of iron and unavoidable impurities. The steel member is rolled at a temperature of 800 to 950° C. The steel member is directly quenched from a temperature not lower than a higher one selected from about 750° C. or a temperature about 150° C. lower than a temperature at a time of completing the rolling. The steel member is tempered at a temperature not higher than Ac1(e.g., a temperature at which generation of austenite starts at a time of heating).

According to still another exemplary embodiment of the present invention, a method for manufacturing high tensile strength, refractory steel having excellent weldability and gas cuttability can be provided. For example, a steel member can be hot-rolled in a form of billet or slab, which has the steel composition that includes, in mass %, approximately, : 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and the balance consisting of iron and unavoidable impurities. The steel member is self-cooled, and quenched after reheating the steel member at a temperature of about 900 to 950° C. The steel member is tempered at a temperature not higher than Ac1.

According to a further exemplary embodiment of the present invention, a high tensile strength, refractory steel having excellent weldability and gas cuttability can be provided which may have a weld crack sensitive composition PCM of about 0.25% or less and the balance consisting of iron and unavoidable impurities. For example, PCM can be defined by the following equation: PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B. Further, an area fraction of polygonal ferrite or pseudo polygonal ferrite in a ¼ thick position in the plate thickness direction of a steel plate of the final rolling product can be about 10% or less. According to such a high tensile strength, refractory steel, it may be possible to perform mass production at low cost of high tensile strength steel having a yield strength of about 440 MPa or more, having excellent weldability and gas cuttability, and having sufficient high temperature strength under a high temperature environment such as a fire.

A construction steel for architectural construction according to yet another exemplary embodiment of the present invention can be provided, composed of, e.g., the high tensile strength, refractory steel which can be applied as general construction steel for various applications including civil engineering, marine structures, ships and vessels, various storage tanks, industrial facilities such as plate-mills or the like. Since the exemplary embodiment of the high tensile strength, refractory steel according to the present invention has sufficient high temperature strength even under a severe environment, for example, at a time of fire, in which the steel is exposed to high temperature conditions, it may be possible to further enhance the safety of weld constructions.

In accordance with yet a further exemplary embodiment of a method for manufacturing high tensile strength, refractory steel having excellent weldability and gas cuttability according to the present invention, a steel member in a form of billet or slab having the steel composition can be heated at a temperature of about 1100 to 1300° C., and rolled at a temperature of about 800 to 950° C. After that, the steel member may be directly quenched from a temperature of not lower than a higher one selected from about 750° C. or a temperature about 150° C. lower than a temperature at a time of completing the rolling, and tempered at a temperature not higher than Ac1. Therefore, it is possible to perform mass production at low cost of a high tensile strength steel having a yield strength of 440 MPa or more and having excellent weldability and gas cuttability and sufficient high temperature strength under a high temperature environment such as a fire, where the steel is exposed to high temperature conditions.

In accordance with still another aspect of a method for manufacturing high tensile strength, refractory steel having excellent weldability and gas cuttability according to the present invention, after hot-rolling a steel member in a form of billet or slab having the steel composition as described herein above, the exemplary steel member can be self-cooled, and quenched after being reheated at a temperature of about 900 to 950° C., and tempered at a temperature of not higher than Ac1. Therefore, it is possible to perform mass production at low cost of a high tensile strength steel having a yield strength of about 440 MPa or more and having excellent weldability and gas cuttability and sufficient high temperature strength under a high temperature environment such as a fire, where the steel is exposed to high temperature conditions.

These and other objects, features and advantages of the present invention will become apparent upon reading the following detailed description of embodiments of the invention, when taken in conjunction with the appended claims.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF INVENTION

An exemplary embodiment of high tensile strength, refractory steel having excellent weldability and gas cuttability according to the present invention and the method for manufacturing the same are described herein.

It should be understood that the exemplary embodiment of the present invention described herein are not limited to the description, and can be used and/or applicable in various other ways and for various applications.

For example, an exemplary embodiment of a high tensile strength steel according to the present invention can includes, in mass %, approximately, C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and the balance consisting of iron and unavoidable impurities. A weld crack sensitive composition PCM can be defined by the following exemplary equation can be 0.25% or less:


PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B,

and an area fraction of polygonal ferrite or pseudo polygonal ferrite in a ¼ thick position in the plate thickness direction of a steel plate of the final rolling product can be about 10% or less.

Further, it is possible to limit the composition of the high tensile strength, refractory steel. For example, C likely has an influence on the property of the steel. The preferably minimum value of approximately 0.04% can be a minimum content so as to ensure the strength, and may be used to reduce or inhibit the over softening than necessary of a heat-affected portion such as a weld portion. On the other hand, too high a C content may enhance the hardenability of the steel to an unnecessary level, and can have a negative influence on the strength, toughness balance, and weldability as intrinsic properties of the steel. Therefore, the upper limit of the C content may be set to be about 0.14%.

Si can have an influence on cleanability, weldability, and weld-portion toughness of the steel. Therefore, it may be used to control its upper limit value. Therefore, the Si content can be set to be about 0.50% or less. Si may also have an effect of deoxidizing of the steel. However, deoxidization of the steel can be performed by Ti or Al. Therefore, where weldability and weld-portion toughness are preferably used, it may not be necessary to add Si to the steel.

Mn can be an important element for ensuring strength and toughness of the steel, and its minimum content may be, e.g., about 0.50%. On the other hand, too high a Mn content can enhance the hardenability of the steel, deteriorate weldability and toughness of weld-heat affected portion of the steel, as well as enhance the segregation of the central portion of the slab during continuous casting process. Therefore, the upper limit of the Mn can be set to be about 2.00%.

P can constitute an impurity in the steel of an exemplary embodiment of the present invention. By reducing the P content, grain boundary deformation in the weld heat-affected portion may be reduced. Therefore, it may be preferable to control the P content to as low as possible. Therefore, so as not to deteriorate low temperature toughness of the base metal and weld heat-affected portion, the upper limit of the P content can be set to be about 0.020%.

As with P, S may constitute an impurity in an exemplary embodiment of the steel of the present invention. In order to ensure low temperature toughness of the steel, it may be preferable to control the S content to as low as possible. Therefore, so as not to deteriorate low temperature toughness of the base metal and weld heat-affected portion, the upper limit of the S content can be set to be about 0.010%.

Nb is an element which can play a role in an exemplary embodiment of the present invention where Mo content can be depressed as far as possible. Firstly, as a general effect, Nb can be an important element for elevating the recrystallization temperature of austenite, and exerting the effect of controlled rolling at a time of hot-rolling. In order to realize such effects, it may be preferable for the steel to contain Nb of at least about 0.01%.

Nb may also contribute to grain refining of heated austenite at a time of reheating preceding the rolling and has an effect of enhancing strength of the steel by precipitation hardening. In addition, by composite addition with Mo, Nb can contribute to the high temperature strength of the steel. However, too high a Nb contents may result in a deterioration of toughness of the weld portion. Therefore, so as not to generate the deterioration of toughness of the weld portion, the upper limit of the Nb content can be set to be about 0.005%.

Mo can be an important element for ensuring high temperature toughness of the steel.

In order for the exemplary embodiment of the steel to have sufficient high temperature strength under an environment, for example, at a time of fire, where the steel is exposed to high temperature conditions, it may be preferable for the steel to contain Mo of about 0.30% or more. On the other hand, too high of a Mo content can deteriorate weldability and gas cuttability of the steel. Therefore, the upper limit of the steel can be set to be less than 0.70%.

Al is a deoxidizing element. However, deoxidization of the steel can be sufficiently performed, e.g., solely by Si or by Ti. Therefore, the lower limit of Al content may not be set according to an exemplary embodiment of the present invention. On the other hand, too high an Al content may impair cleanability of the steel, deteriorate toughness of the base metal, and deteriorate toughness of the weld-heat affected portion. Therefore, the upper limit of Al content may be set to be about 0.060%.

N can be contained as an unavoidable impurity in the steel. By bonding with the above-described Nb, N can form carbonitride and enhance the strength of the steel. In addition, where the below-mentioned Ti is added, N can enhance strength of the steel by forming TiN. In order to obtain such effects, it may be preferable for the steel to contain N of at least about 0.0010%.

On the other hand, an increased N content has an adverse effect on toughness of the weld heat-affected portion and weldability. Therefore, the upper limit of N content can be set to be about 0.0060%.

In addition to the above-described exemplary composition, the high tensile strength steel according to the exemplary embodiment of the present invention can further include, in mass %, about Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, and one or two or more selected from Cr: 0.05 to 1.0%, V: 0.01 to 0.06%, B: 0.0002 to 0.0030%, Ti: 0.005to 0.025%, Mg: 0.0002 to 0.0050%, wherein the Ni content may be at least half of the Cu content.

One of the reason for adding these elements to the above-described exemplary basal composition can be to improve the properties such as strength and toughness of the steel without impairing excellent characteristics of the steel according to the exemplary embodiment of the present invention. Therefore, loadings of the exemplary elements may be restricted.

Where too much loading is avoided, Ni can improve strength and toughness of the steel without having a negative influence on weldability and toughness of the weld heat-affected portion of the steel. To realize such exemplary effects, it may be preferable for the steel to contain Ni of at least about 0.05% or more. On the other hand, too high a loading of Ni may elevate the price of the steel and also has an undesirable effect on weldability. Therefore, the upper limit of Ni content can be set to be about 1.0%.

Where Cu is added, in order to prevent the occurrence of Cu-cracks during the hot-rolling, it is necessary to control the Ni content to be not lower than ½ of the Cu content while controlling the Ni content in the above-described range.

Cu can show nearly similar functions and effects as those of Ni. However, in addition to deteriorating the weldability, too high a loading of Cu may cause Cu-cracks to occur at the time of hot-rolling and makes it difficult to produce the exemplary steel. Therefore, the upper limit of the Cu content may be set to be about 1.0%. On the other hand, in order to obtain a substantial effect, it may be preferable for the steel to contain a minimum amount of Cu. Therefore, the lower limit of the Cu content can be set to be about 0.05%.

Cr may improve strength and toughness of the base metal. However, too high a Cr content can deteriorate the toughness of the base metal and weld portion and weldability. Therefore, the upper limit of the Cr content may be set to be about 1.0%. On the other hand, in order to obtain a substantial effect, it may be preferable for the steel to contain a minimum amount of Cr. Therefore, the lower limit of the Cr content can be set to be about 0.05%.

The above-described Ni, Cu and Cr can be effective for improving weather resistance as well as for improving strength and toughness of the base metal. For this purpose, it can be preferable for the steel to contain these elements while controlling their amounts in a range not impairing weldability.

V has similar effects as Nb. However, its effect may be less than that of Nb. V may have an influence on hardenability and contributes to improvement of high temperature strength.

In order to realize the same effect as Nb, it is necessary for the steel to contain at least 0.01% V. On the other hand, where the steel contains excessive V, toughness of the weld portion may be deteriorated. Therefore, so as not to deteriorate the toughness of the weld portion, the upper limit of the V content can be set to be about 0.06%.

B can be segregated in the grain boundary of austenite, may depress occurrence of ferrite and thereby improves hardenability and strength of the steel. In order to realize such effects, it may be preferable for the steel to contain at least about 0.0002% of B. However, where too much B is contained, hardenability-improving effect is saturated, and there is a possibility of occurrence of B precipitates possibly having an adverse effect on the toughness of the exemplary steel. Therefore, the upper limit of the B content can be set to be about 0.003%.

When being used as steel for a tank or the like, there is a possibility of stress corrosion cracking of the exemplary steel. In such a case, reduction of hardness of the base metal and weld heat-affected portion are often important. For example, in order to prevent sulfide stress corrosion cracking (SSC), hardness of HRC≦22 (HV≦248) may be important. In such a case, it may be not as preferable to add B which enhances hardenability.

Where high toughness is preferred in the base metal and in the weld portion, it is preferable to load Ti in the exemplary steel. Where the Al content is low, for example, where the Al content is 0.03% or less, Ti can be bonded with O and may form precipitates mainly composed of Ti2O3 likely acting as nuclei of generation of ferrite by transgranular transformation and improving toughness of the weld portion. In addition, Ti can be effective by being bonded with N and forms TiN constituting fine precipitates in the slab, thereby depressing coarsening of austenite grain at the time of heating and reducing grain size of the rolled texture. In addition, fine TiN existing in a steel plate may reduce the grain size of the weld heat-affected portion at the time of welding.

In order to obtain such an effect, at least about 0.005% of Ti can be used. However, excessive Ti can form TiC and may deteriorate low-temperature toughness and weldability. Therefore, the upper limit of the Ti content may be set to be about 0.025%.

Mg can depress the grain growth of austenite grains in the weld heat-affected portion and reduce the grain size. As a result, the weld portion may be given a high toughness. In order to realize such an effect, it may be preferable for the exemplary steel to contain Mg of about 0.0002% or more. On the other hand, where the Mg content can be increased, it may not be cost effective, because there may be less enhancement of the effect of Mg compared with the increase of Mg content. Therefore, the upper limit of the Mg content can be set to be about 0.0050%.

In addition to the above-described exemplary composition, the high tensile strength, refractory steel of an exemplary embodiment of the present invention can further include, in mass %, one or two selected from Ca: about 0.0005 to 0.0040% and REM (Rare Earth Metal): about 0.0005 to 0.0100%.

As the REM, one or more selected from rare earth metals such as Ce, La, and Nd or the like may be used.

Ca and REM may be effective in controlling MnS morphologies and improving low-temperature toughness of the base metal. In addition, Ca and REM may be effective in reducing sensitivity for hydrogen-induced cracking, for example, hydrogen-induced cracking (HIC) under a humid hydrogen sulfide environment, SSC, and stress oriented HIC (SOHIC). In order to express such effects, at least a content of about 0.0005% can be used.

However, if too much Ca or REM is contained, cleanability of steel may be deteriorated, and toughness of the base metal and hydrogen induced cracking (HIC, SSC, SOHIC) sensitivity under a humid hydrogen sulfide environment may be enhanced. Therefore, the upper limit of the Ca content can be set to be about 0.0040% and the upper limit of the REM content may be set to be about 0.0100%. Since Ca and REM can express a nearly similar effect, it is possible to load one of Ca and REM in the above-described range, or it is possible to load a mixture of Ca and REM in the above-described exemplary range.

In the high tensile strength, refractory steel according to an exemplary embodiment of the present invention, in order to ensure the yield strength of about 440 MPa or more, and yield strength at about 600° C. of not less than about ⅔ of the yield strength at room temperature, that is, not less than about 294 MPa, while controlling Mo content to be lower than about 0.70%, it can be preferable to inhibit or reduce a control of the exemplary steel composition also to control its microstructure.

In the exemplary microstructure of the high tensile strength, refractory steel according an exemplary embodiment of the present invention, an area fraction of polygonal ferrite or pseudo polygonal ferrite in a ¼ thick position in the plate thickness direction of a steel plate of the final rolling product may be controlled to be about 10% or less.

In the steel having the steel composition according to the exemplary embodiment of the present invention in which the Mo content may be restricted to be lower than about 0.70%, especially in a thick steel plate thicker than about 40 mm, it may be difficult to ensure not only the room temperature strength, but also the high temperature strength of the steel, when the area fraction of polygonal ferrite or pseudo polygonal ferrite exceeds about 10%.

In the present invention, the microstructure is represented by the texture on the plane along the final rolling direction, where the plane is in a ¼ thick position with respect to the section of the plate thickness.

Although each component of the exemplary steel can be limited, it may be difficult to obtain preferable properties if the component system as a whole is not appropriately controlled. Therefore, a weld crack sensitive composition PCM can be limited to be about 0.25% or less, where the PCM may be defined by the following exemplary equation:


PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B.

The weld crack sensitive composition PCM can be a parameter indicating weldability, and weldability may be satisfactory as PCM shows a low value. In the steel according to the exemplary embodiment of the present invention, where the weld crack sensitive composition PCM may be 0.25% or less, it is possible to ensure excellent weldability while ensuring excellent high temperature strength.

Further, a method for manufacturing a high tensile strength, refractory steel according to an exemplary embodiment of the present invention is explained.

The high tensile strength, refractory steel according to the present invention can be manufacture by the first or by the second exemplary embodiment of a manufacturing method according to the present invention.

The first exemplary embodiment of the manufacturing method can include: heating a steel member in a form of billet or slab having the steel composition according to the exemplary embodiment of the present invention at a temperature of about 1100 to 1300° C.; rolling the steel member at a temperature of about 800 to 950° C.; directly quenching the steel member from a temperature of not lower than a higher one selected from about 750° C. or a temperature about 150° C. lower than a temperature at a time of completing the rolling; and tempering the steel member at a temperature not higher than Ac1.

The second embodiment of the manufacturing method can include: hot rolling a steel member in a form of billet or slab having the steel composition according to the exemplary embodiment of the present invention; self-cooling the steel member: quenching the steel member after reheating the steel member at a temperature of about 900 to 950° C.; and tempering the steel member at a temperature not higher than Ac1.

Firstly, the first exemplary embodiment of the manufacturing method as follows. A steel member in a form of billet or slab having the steel composition according to the exemplary embodiment of the present invention can be heated at a temperature of about 1100 to 1300° C. The exemplary reason for controlling the heating temperature preceding the rolling to be about 1100 to 1300° may be to inhibit the coarsening of austenite grains to an unnecessary size in the time of heating, and to refine the rolled texture. The temperature of approximately 1300° C. can be an upper limit of the temperature at which extreme coarsening of austenite is inhibited at the time of heating. Where the heating temperature exceeds the exemplary upper limit temperature, coarse grained austenite may be mixed in the texture, and rolled austenite grains also have a relatively large size. As a result, the metallographic structure after the phase transformation can be relatively coarse grained. In addition, in the transformation from the coarse austenite, the microstructure may tend to become a bainitic structure, possibly resulting in remarkable deterioration of toughness of the exemplary steel. On the other hand, the lower limit of the heating temperature can be set to be about 1100° C. based on the consideration of solution treatment of Nb so as to express the effect of controlled rolling at the time of hot-rolling and precipitation hardening.

The exemplary steel member thus heated can be rolled at a temperature of about 800 to 950° C. The rolling temperature may be limited to be in the range of about 800 to 950° C. for the following exemplary reason. Where the rolling is performed at a temperature exceeding about 950° C., even though Mo and Nb are compositely loaded, grain size refining of the rolled austenite may not be sufficient, and therefore low-temperature toughness may not be ensured stably even by performing subsequent direct quenching and tempering. On the other hand, at a temperature lower than about 800° C., depending on plate thickness, precipitation of ferrite can occur before the direct quenching and may cause difficulty in ensuring the microstructure, or Nb precipitates during rolling and does not contribute to the high temperature strength.

After completing the rolling, the steel member is directly quenched from a temperature of not lower than a higher one selected from 750° C. or a temperature 150° C. lower than a temperature at a time of completing the rolling, that is a rolling finish temperature minus 150° C.

For example, the direct quenching temperature can be limited in the above-described range. Firstly, in order to control the microstructure with a purpose of ensuring the microstructure, the temperature can be at least about 750° C. or more. On the other hand, even where the temperature is not lower than about 750° C., when a temperature drop from the rolling finish temperature exceeds about 150° C., there may be a high possibility of recovery and recrystallization after the rolling or precipitation of Nb. In such a case, there is a possibility of deterioration of toughness or reduction of strength including high temperature strength.

Therefore, the starting temperature of the direct quenching can be limited to be not lower than a higher one selected from about 750° C. or a temperature about 150° C. lower than a temperature at a time of completing the rolling.

After the direct quenching, a tempering treatment can be performed at a temperature not higher than Ac1.

In the exemplary embodiment of a steel member having a steel composition according to the present invention, in certain cases, a temperature of about 700° C. or less is not higher than Ac1. The practical treatment temperature can be set in accordance with target properties such as strength.

Considering productivity and controllability of the heat treatment furnace in the industrial production, the preferable temperature of the tempering treatment may be about 450 to 650° C.

In the above-description, rolling temperature or the like can denote a surface temperature of the steel plate which may be monitored.

By the above-described exemplary method, high tensile strength, refractory steel according to an exemplary embodiment of the present invention can be manufactured.

According to the second exemplary embodiment of the manufacturing method, after hot rolling a steel member in a form of billet or slab having a steel composition according to the exemplary embodiment of the present invention, the steel member may be self-cooled. In this exemplary case, the conditions of hot-rolling and self-cooling may not be limited because the metallographic structure and material quality of the steel member is determined depending on the subsequent treatments including reheating, quenching and tempering.

Further, the hot-rolled and self-cooled steel member can be reheated at a temperature of about 900 to 950° C. and subjected to quenching.

It may be preferable to control the reheating and quenching temperature to be higher than Ac3 ( a temperature at which transformation of ferrite to austenite is completed at the time of heating) in terms of metallurgical definition.

In the steel member having a steel composition according to an exemplary embodiment of the present invention, a temperature of about 900° C. or more may be sufficient as the temperature not lower than Ac3.

On the other hand, where reheating and quenching temperatures are too high, the metallic structure can be coarsened and low temperature toughness may be deteriorated. Therefore, the maximum temperature of the reheating and quenching can be set to be about 950° C.

In addition, the reheated and quenched steel member may be subjected to tempering treatment at a temperature of not higher than Ac1.

The conditions of the tempering treatment or the like may be similar or exactly the same as the above-described first exemplary embodiment of the manufacturing method.

By the above-described method, high tensile strength, refractory steel according to the exemplary embodiment of the present invention can be manufactured.

The high tensile strength, refractory steel according to the exemplary embodiment of the present invention can be applied to general weld construction steel not only for architectural construction but also for various applications including civil engineering, marine structures, ships and vessels, various storage tanks, or the like.

EXAMPLE

Further, high tensile strength, refractory steel having excellent weldability and gas cuttability according to an exemplary embodiment of the present invention is explained with reference to Examples 1 to 15 and Comparative Examples 16 to 22.

Firstly, using a steel converter, steel slabs having various compositions shown in Table 1 were produced as ingots. Next, the slabs were subjected to various steel manufacturing processes using conditions shown in Table 2, and steel plates each having a thickness (50 to 100 mm) shown in Table 2 were manufactured.

Next, as shown in Table 2, each steel plate of Examples 1 to 15 and Comparative Examples 16 to 22 was subjected to evaluation of the base metal structure, mechanical properties, toughness of weld heat-affected portion and roughness of gas-cut face.

In addition, as the mechanical properties, three parameters, that is, yield strength, tensile strength, and yield strength at 600° C. were measured, and yield ratio (yield strength/tensile strength (%)) was determined from the yield strength and tensile strength. Then the mechanical properties were evaluated.

With respect to the structure of the base metal, on a plane in a ¼ thick position with respect to the section of the plate thickness, 10 fields of view were observed using a microscope with a magnification ratio of 500. Thus, the area fraction (%) of polygonal ferrite (αp) and area fraction of pseudo polygonal ferrite (αq) were calculated.

With respect to yield strength and tensile strength, a test piece was sampled from a direction perpendicular to the rolling direction in the central portion of the plate thickness. Configuration of the test piece was in accordance with a No.4 round bar for a test piece for testing tensile strength standardized by Japanese Industrial Standard JIS Z 2201 “metallic material tensile strength test piece”. After that, yield strength and tensile strength were evaluated based on measurements in accordance with Japanese Industrial Standard JIS Z 2241 “Method for tensile test of metallic material”.

For the evaluation of toughness of the base metal, a test piece was sampled from a direction perpendicular to the rolling direction in the central portion of the plate thickness. Configuration of the test piece was in accordance with a 2 mmV notch impact test specimen standardized by Japanese Industrial Standard JIS Z 2202 “impact test specimen of metallic material”. After that, toughness was evaluated based on the measurement of fracture appearance transition temperature (vTrs (° C.)) of the impact test specimen in accordance with Japanese Industrial Standard JIS Z 2242 “Method for impacting test of metallic material”.

For the evaluation of toughness of the weld heat-affected portion, a test piece was sampled from a ¼ thick position of the plate thickness. Configuration of the test piece was in accordance with an impact test specimen standardized by Japanese Industrial Standard JIS Z 2202 “impact test specimen of metallic material”. Each test piece was subjected to a heat cycle corresponding to submerged arc welding (plate thickness 50 mm) of energy input of 60 kJ/mm. The toughness was evaluated based on the measurement of absorbed energy (vE0) of the test piece at 0° C.

For the evaluation of roughness of gas-cut face, the highest height (Ry) of the surface roughness of the surface of each steel plate was measured, where the definition of Ry was in accordance with Japanese Industrial Standard JIS B 0601 “Geometrical Property Standard (GPS) of a product-Surface Property: profile curve method-term, definition, and surface property parameter”. Where the maximum height (Ry) was 50 μm or less, roughness was evaluated as satisfactory (A), and where the maximum height (Ry) exceeded 50 μm, roughness was evaluated as unsatisfactory (B).

Target values of respective properties were 440 MPa for yield strength, −40° C. or less for fracture appearance transition temperature (vTrs), 294MPa or more for yield strength at 600° C., and 100 J or more for absorbed energy (vE0) at 0° C.

Compositions of steels are shown in Table 1 and manufacturing processes of steel plates and various properties are shown in Table 2.

TABLE 1 Table CLASS OF COMPOSITION (mass %) STEEL C Si Mn P S Nb Mo Al N Ni EXAMPLE 1 0.04 0.36 1.98 0.009 0.005 0.04 0.68 0.031 0.0042 2 0.04 0.45 1.80 0.012 0.004 0.05 0.64 0.035 0.0035 3 0.05 0.32 1.62 0.010 0.003 0.04 0.60 0.003 0.0038 0.50 4 0.05 0.30 1.58 0.011 0.005 0.03 0.55 0.026 0.0032 5 0.06 0.24 1.55 0.008 0.006 0.01 0.58 0.042 0.0029 6 0.06 0.30 1.40 0.012 0.003 0.02 0.52 0.020 0.0034 0.14 7 0.07 0.35 1.46 0.012 0.004 0.02 0.48 0.002 0.0048 0.35 8 0.07 0.30 1.35 0.011 0.006 0.02 0.37 0.033 0.0036 9 0.08 0.30 1.50 0.012 0.005 0.03 0.45 0.040 0.0035 10 0.08 0.56 1.52 0.010 0.005 0.01 0.40 0.055 0.0055 0.25 11 0.09 0.41 1.24 0.010 0.003 0.02 0.31 0.025 0.0038 0.20 12 0.09 0.35 0.80 0.011 0.001 0.03 0.36 0.020 0.0064 13 0.10 0.25 0.85 0.012 0.004 0.02 0.45 0.029 0.0040 14 0.12 0.33 0.55 0.010 0.003 0.02 0.50 0.019 0.0048 15 0.14 0.31 0.71 0.009 0.005 0.03 0.49 0.020 0.0078 COMPAR- 16 0.03 0.35 1.48 0.012 0.005 0.04 0.50 0.025 0.0039 ATIVE 17 0.04 0.36 1.39 0.012 0.005 0.00 0.51 0.024 0.0041 18 0.04 0.08 0.45 0.014 0.004 0.03 0.60 0.026 0.0041 0.15 19 0.05 0.35 1.41 0.011 0.003 0.07 0.50 0.026 0.0038 20 0.15 0.43 1.18 0.010 0.006 0.02 0.49 0.030 0.0014 21 0.06 0.35 1.30 0.013 0.008 0.02 0.25 0.029 0.0028 22 0.06 0.35 1.31 0.013 0.007 0.02 0.75 0.028 0.0028 CLASS OF COMPOSITION (mass %) STEEL Cu Cr V B Ti Mg Ca PCM * EXAMPLE 1 0.014 0.196 2 0.24 0.0016 0.200 3 0.028 0.012 0.193 4 0.176 5 0.009 0.184 6 0.14 0.38 0.203 7 0.34 0.010 0.0013 0.210 8 0.15 0.0008 0.184 9 0.195 10 0.19 0.015 0.215 11 0.25 0.010 0.202 12 0.12 0.039 0.018 0.0014 0.176 13 0.044 0.185 14 0.005 0.192 15 0.010 0.0018 0.219 COMPAR- 16 0.149 ATIVE 17 0.156 18 0.60 0.138 19 0.166 20 0.256 21 0.153 22 0.187 * PCM = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B ** Underline denotes that the data is outside of the range of the present invention.

TABLE 2 TEMPER- TEMPER- ATURE ATURE QUENCH- HEATING AT AT ING TEMPERING PLATE αp OR CLASS PRODUC- TEMPER- FINISHING STARTING TEMPER- TEMPER- THICK- αq OF TION ATURE ROLLING ROLLING ATURE ATURE NESS FRACTION STEEL PROCESS * (° C.) (° C.) (° C.) (° C.) (° C.) (mm) (%) ** EXAMPLE 1 DQT 1150 900 850 — 600 60 1 2 DQT 1100 810 760 — 580 50 2 3 QT 1200 — — 910 550 80 0 4 DQT 1150 940 870 — 620 75 0 5 DQT 1150 870 800 — 600 60 0 6 QT 1200 — — 930 580 50 0 7 QT 1250 — — 900 640 55 0 8 DQT 1200 920 880 — 600 75 1 9 QT 1200 — — 920 500 100 0 10 DQT 1100 900 860 — 580 100 0 11 DQT 1200 930 850 — 470 85 0 12 QT 1250 — — 910 520 80 0 13 QT 1200 — — 940 600 75 0 14 QT 1250 — — 900 550 100 0 15 DQT 1250 900 860 — 620 75 0 COMPAR- 16 DQT 1150 930 770 — 600 50 12 ATIVE 17 DQT 1100 850 730 — 600 50 16 18 QT 1100 — — 900 600 50 3 19 AR 1200 860 — — — 75 79 20 QT 1250 — — 900 600 75 0 21 QT 1200 — — 900 600 75 1 22 AR 1250 900 — — — 75 72 TOUGHNESS TOUGH- OF ROUGH- YIELD NESS WELD-HEAT NESS CLASS YIELD TENSILE YIELD STRENGTH OF BASE AFFECTED OF OF STRENGTH STRENGTH RATIO AT 600° C. METAL PORTION GAS CUT STEEL (MPa) (MPa) (%) (MPa) vTrs(° C.) vE0(J) SURFACE EXAMPLE 1 520 623 83 345 −68 165 A 2 515 610 84 365 −72 171 A 3 567 663 86 381 −64 154 A 4 482 581 83 344 −59 149 A 5 525 619 85 372 −62 151 A 6 562 654 86 386 −57 160 A 7 543 628 86 370 −61 204 A 8 538 613 88 368 −55 143 A 9 496 616 81 325 −51 153 A 10 509 602 85 336 −54 167 A 11 485 619 78 317 −59 132 A 12 525 622 84 322 −53 129 A 13 533 614 87 371 −50 152 A 14 468 593 79 316 −56 187 A 15 531 636 83 353 −47 137 A COMPAR- 16 433 552 78 261 −62 174 A ATIVE 17 404 535 76 303 −58 168 A 18 436 554 79 327 −51 155 A 19 313 468 67 235 −74  21 A 20 507 598 85 299 −12  17 A 21 472 572 83 274 −52 104 A 22 324 491 66 243 −32  24 B * DQT: DIRECT QUENCHING - TEMPERING, QT: REHEATING QUENCHING-TEMPERING, AR: AS ROLLED (AIR COOL) ** αp: polygonal ferrite, αq: pseudo polygonal ferrite *** Underline denotes that the data is outside of the range of the present invention.

In accordance with the results of evaluation, all of Examples 1 to 15 showed satisfactory properties.

On the other hand, compared with Examples 1 to 15, Comparative Examples 16 to 22 having compositions out of the composition range of the present invention showed inferior values in fundamental properties such as strength and toughness, and in high temperature strength, toughness of weld-heat affected portion, gas cuttability or the like.

In Comparative Example 18, since the Ni content is lower than the Cu content, cracks occurred during the hot-rolling, making it difficult to produce the steel.

In Comparative Example 20, because of not only high C content but also of high PCM, root cracks were generated by an oblique y shape weld crack test at room temperature.

According to the exemplary embodiment of the present invention, by composite loading of Nb while suppressing the content of Mo, a high temperature strength of a high tensile strength steel having a yield strength of 440 MPa or more can be stably ensured. By suppressing the content of Mo, a deterioration of weldability and gas cuttability can be limited to a minimum level. At the same time, by limiting each of alloy elements such as C, Si, and Mn, as well as by limiting PCM, and further by limiting the microstructure of the steel and manufacturing conditions for same, composite properties such as excellent high temperature strength, weldability, gas cuttability are compatibly ensured. Such high tensile strength, refractory steel having excellent weldability and gas cuttability can be widely applied as general weld construction steel for architectural constructions, civil engineering, marine structures, ships and vessels, various storage tanks or the like, and therefore has very large industrial applicability.

The foregoing merely illustrates the exemplary principles of the present invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous modification to the exemplary embodiments of the present invention which, although not explicitly shown or described herein, embody the principles of the invention and are thus within the spirit and scope of the invention. All publications, applications and patents cited above are incorporated herein by reference in their entireties.

Claims

1-16. (canceled)

7. A high tensile strength, refractory steel having a particular weldability and a particular gas cuttability, comprising:

at least one portion of a steel plate of a rolling product comprising approximately, in mass %, C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and a balance consisting of iron and unavoidable impurities,
wherein a weld crack sensitive composition PCM is about 0.25% or less, PCM being defined as: PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B, and
wherein an area fraction of polygonal ferrite or pseudo polygonal ferrite in a ¼ thick position in a plate thickness direction of the steel plate is about 10% or less.

8. The steel according to claim 7, wherein the at least one portion further comprises approximately, in mass %, Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, and one or two or more selected from Cr: 0.05 to 1.0%, V: 0.01 to 0.06%, B: 0.0002 to 0.0030%, Ti: 0.005 to 0.025%, and Mg: 0.0002 to 0.0050%, and wherein a content of Ni is equal to or greater than half of a content of Cu.

9. The steel according to claim 7, wherein the at least one portion further comprises approximately, in mass %, at least one of Ca: 0.0005 to 0.0040% or REM: 0.0005 to 0.0100%.

10. The steel according to claim 7, wherein a yield strength of the steel is about 440 MPa or more.

11. A method for manufacturing a high tensile strength, refractory steel having a particular weldability and a particular gas cuttability, comprising:

heating a steel member at a temperature of about 1100 to 1300° C. in a form of at least one of a billet or a slab comprising approximately, in mass %, C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and a balance consisting of iron and unavoidable impurities;
rolling the steel member at a temperature of about 800 to 950° C.;
directly quenching the steel member from a temperature greater or equal to a greater of about 750° C. or a temperature of about 150° C. lower than a temperature at a time of completing the rolling; and
tempering the steel member at a temperature lower than or equal to Ac1.

12. A method for manufacturing a high tensile strength steel having a particular weldability and a particular gas cuttability, comprising:

hot rolling a steel member in a form of at least one of a billet or a slab, the steel member comprising approximately, in mass %, C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and a balance consisting of iron and unavoidable impurities;
self-cooling the steel member:
quenching the steel member after reheating the steel member at a temperature of about 900 to 950° C.; and
tempering the steel member at a temperature not higher than Ac1.
Patent History
Publication number: 20090025839
Type: Application
Filed: Mar 3, 2006
Publication Date: Jan 29, 2009
Applicant: Nippon Steel Corporation (Tokyo)
Inventors: Yoshiyuki Watanabe (Kimitsu-shi), Ryuuji Uemori (Kisarazu-shi), Kiyoshi Ishibashi (Kimitsu-shi), Kenichi Yoshii (Kimitsu-shi)
Application Number: 11/816,015
Classifications
Current U.S. Class: With Additional Nonworking Heating Step (148/653); Three Percent Or More Manganese Containing Or Containing Other Transition Metal In Any Amount (148/337); Copper Containing (148/332); Beryllium Or Boron Containing (148/330); Rare Earth Meal Containing (148/331); Including Cooling (e.g., Quenching, Etc.) (148/654)
International Classification: C21D 8/00 (20060101); C22C 38/12 (20060101); C22C 38/20 (20060101); C22C 38/32 (20060101); C22C 38/08 (20060101); C22C 38/16 (20060101);