JPWO2019088206A1 - H-section steel and manufacturing method thereof - Google Patents

Abstract

We propose a way to ensure high strength over YP355MPa and low temperature toughness at -40 ℃ at the flange part of H-section steel without increasing the manufacturing cost. C: 0.08 to 0.16%, Si: 0.05 to 0.60%, Mn: 0.10 to 1.80%, Nb: 0.005 to 0.060%, Ti: 0.001 to 0.020%, Al: 0.080% or less, N: 0.0010 to 0.0060%, P: Containing 0.030% or less and S: 0.030% or less in a range where Ceq is 0.44% or less, the balance is the component composition of Fe and inevitable impurities, and the microstructure is mainly composed of ferrite with a particle size of 15 μm or less The microstructure is such that the second phase is pearlite and / or bainite and the island martensite is 3% or less.

Description

  The present invention has a low temperature toughness at −40 ° C., which is used in marine structures, H-shape steels that are widely used as welded steel structures such as buildings, civil engineering, and bridges, especially in marine structures in cold regions. The present invention relates to an excellent high-strength H-section steel and a method for producing the same.

  Offshore structures that mine crude oil, natural gas, and the like are often operated in cold regions, and the H-shaped steel used requires excellent low-temperature toughness for both the base metal and the welded joint. In order to achieve both high strength and low temperature toughness, TMCP, which is a combination of controlled rolling and accelerated cooling, is widely used for thick steel plates, and is an effective technology for H-section steel. However, in manufacturing H-section steel, considering the formability, high-temperature heating of the material and rolling at a high temperature with low deformation resistance are necessary, and the structure tends to be coarse. Furthermore, controlled rolling in the low temperature range of austenite is important for refining the structure, but rolling at low temperature has problems in terms of increased rolling load and shape stability.

Until now, as H-shaped steel with excellent toughness, in Patent Document 1, in addition to adding no precipitation embrittlement element, by reducing the amount of solute N and applying accelerated cooling after rolling, A technique relating to a method for producing a rolled H-section steel that secures −40 ° C. toughness without performing controlled rolling is disclosed.
Further, as H-shaped steel having excellent low-temperature toughness used for offshore structures and the like, Patent Document 2 proposes a technique using an extremely low carbon component added with Nb or B. Furthermore, Patent Documents 3 and 4 disclose techniques for achieving excellent low temperature toughness at −40 ° C. with air cooling without adding Nb that inhibits productivity.

JP 2006-180584 A International Publication 2013/089156 JP 2016-84524 JP2016-156032

  Since the technique described in Patent Document 1 requires the application of accelerated cooling as a manufacturing method, there is a problem in achieving both material control and shape stabilization.

  Patent Document 2 discloses low-temperature toughness using a component in which N is combined with Nb and B in an amount of 0.040% or less in order to achieve Charpy absorbed energy at −40 ° C. and CTOD characteristics at −10 ° C. A technique related to an excellent H-section steel is disclosed. However, in order to reduce C substantially to about 0.020%, the refining time in the steelmaking stage becomes long, and in order to ensure strength, it is necessary to add a relatively large amount of alloy elements, which is expensive. It becomes.

  On the other hand, Patent Documents 3 and 4 increase the deformation resistance in hot rolling and inhibit productivity, and by appropriately controlling the amounts of V and N without adding Nb, This is a technology that has improved low-temperature toughness at 40 ° C and -60 ° C. However, in order to secure toughness more stably by controlling VN precipitates, it is necessary to secure an N content of 0.004% or more, so there is concern about cracks during continuous casting and toughness reduction due to residual free N. Is done.

  The present invention solves the above-mentioned problems, and in particular, to ensure high strength of YP355 MPa or more and low temperature toughness at −40 ° C. in the flange portion of the H-section steel without increasing the manufacturing cost. The purpose is to propose a way.

  Now, in order to produce a rolled H-section steel having high strength and excellent low-temperature toughness, it is important to apply controlled rolling to hot rolling. In particular, in order to effectively carry out controlled rolling in the austenite non-recrystallization temperature range, it is effective to increase the non-recrystallization temperature range by adding Nb. In order to exert the controlled rolling effect when Nb is not added, rolling in the low temperature range of austenite is necessary, the rolling load is increased, the rolling time is increased for temperature adjustment, and the dimensional accuracy of the H-section steel is deteriorated. Is a problem. Therefore, although Nb causes an increase in deformation resistance, it is an element that exhibits a controlled rolling effect in a high temperature range, and is therefore an extremely useful element from the viewpoint of material control. On the other hand, when Nb is added, the hardenability is improved in the cooling process after hot rolling, and a part of untransformed austenite becomes island martensite, which causes a problem of low temperature toughness deterioration.

  Therefore, the inventors have made utmost efforts to maximize the effect of controlled rolling with the addition of a small amount of Nb, and to ensure strength of YP355 MPa or more and low temperature toughness at −40 ° C. especially in the flange part of H-section steel. As a result of the study, Nb was added to make the most of the controlled rolling effect by increasing the temperature of the austenite non-recrystallization temperature, and the ferrite grain size was refined by controlled rolling at a relatively high temperature. By reducing the amount of island-like martensite produced by optimization, it was found that both high strength and low temperature toughness can be achieved, and the present invention has been completed. That is, the gist of the present invention is as follows.

[1] By mass%
C: 0.08 to 0.16%,
Si: 0.05-0.60%
Mn: 0.10 to 1.80%
Nb: 0.005-0.060%,
Ti: 0.0010-0.0200%,
Al: 0.080% or less,
N: 0.0010 to 0.0060%,
P: 0.030% or less and S: 0.030% or less in a range where Ceq according to the following formula (1) is 0.44% or less, the balance is the component composition of Fe and unavoidable impurities, and the average particle size is 15 μm or less H-shaped steel having a microstructure with ferrite as a main phase, wherein the second phase is pearlite and / or bainite, and island martensite is 3.0% or less.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
However, the element display in the formula indicates the content of the element, and the elements not included are zero.

[2] The component composition is further mass%,
V: 0.050% or less,
Cu: 1.0% or less,
Ni: 1.0% or less,
Cr: 1.0% or less and
Mo: H-section steel according to the above [1], containing one or more of 1.0% or less.

[3] TR in which the steel material having the composition described in [1] or [2] is heated at 1150 ° C. or higher and lower than 1300 ° C., and at least the surface temperature of the flange-corresponding portion is calculated by the following formula (2) A method for manufacturing H-section steel, which performs hot rolling at a cumulative reduction of 20% or less at a temperature of ℃ or less.
Record
TR = 174 log [Nb × (C + 12 / 14N)] + 1344 (2)

  According to the present invention, an appropriate amount of Nb can be added to increase the temperature range of austenite non-recrystallization, and the controlled rolling effect can be utilized to the maximum. As a result, without requiring accelerated cooling after hot rolling, in other words, the strength of the flange portion is YP355 MPa or more and the toughness of the flange portion is −40 ° C. even if air cooling is performed after hot rolling. It is possible to provide an H-section steel having excellent low-temperature toughness having a Charpy absorbed energy of 50 J or more.

  Hereinafter, the H-section steel of the present invention will be described in detail. First, the reasons for limiting the component composition of the H-section steel of the present invention will be described. In addition, unless otherwise indicated, the "%" display regarding a component shall mean "mass%".

C: 0.08-0.16%
C is an element necessary for improving the strength of steel, and in order to ensure strength without accelerated cooling after hot rolling, the lower limit of the C content is 0.08%. The C content is preferably 0.10% or more. On the other hand, when the C content is excessively large, the amount of the second phase such as pearlite and bainite increases, and the base metal toughness and weld zone toughness are lowered. Therefore, the upper limit of the C content is set to 0.16%. Preferably, it is 0.08 to 0.14%.

Si: 0.05-0.60%
Si is effective as a deoxidizing element or a solid solution strengthening element, and at least 0.05% is required to obtain the effect. On the other hand, if it exceeds 0.60%, the toughness of the base metal and the toughness of the welded portion are deteriorated. Preferably, it is 0.05 to 0.50%.

Mn: 0.10 to 1.80%
Mn needs to be 0.10% or more in order to ensure the strength of the base material. On the other hand, if added over 1.80%, the cold cracking susceptibility increases, so Mn was limited to the range of 0.10 to 1.80%. From the viewpoint of weld zone toughness, the upper limit is preferably 1.60%. More preferably, it is 0.30 to 1.60%.

P: 0.030% or less P is suppressed to 0.030% or less because the toughness of the welded portion decreases when the content exceeds 0.030%. Preferably, it is 0.020% or less. In order to suppress P to less than 0.005%, a large cost is required for the processing. Therefore, from the viewpoint of manufacturing cost, it is preferable to set 0.005% as the lower limit.

S: 0.030% or less S, like P, if contained in excess of 0.030%, the toughness of the base metal and the welded portion is reduced, so it is suppressed to 0.030% or less. Preferably, it is 0.005% or less. In order to suppress S to less than 0.001%, a large cost is required for the processing, and therefore 0.001% is preferably set as the lower limit from the viewpoint of manufacturing cost.

Nb: 0.005-0.060%
Nb is effective in refining the ferrite structure after rolling and cooling by forming Nb carbonitride and suppressing the coarsening of austenite grains during heating of the steel material, and at the austenite non-recrystallization temperature. It is a very important element for effective controlled rolling. It is also an effective element for increasing the strength by precipitation strengthening. In order to exhibit the effect and secure a strength of YP355 MPa or more, it is necessary to contain 0.005% or more. Furthermore, when high strength of YP420 MPa or more is required, it is preferable to contain it at 0.015% or more. On the other hand, if added over 0.060%, the toughness of the base metal and welded part due to the formation of island martensite becomes significant, so 0.060% was made the upper limit. In order to further suppress the formation of island martensite, the content is preferably 0.050% or less. More preferably, it is 0.040% or less, More preferably, it is 0.035% or less.

Ti: 0.0010-0.0200%
Ti is an element that forms TiN, suppresses austenite grain coarsening during heating of the steel material, and is effective for refining the ferrite structure after rolling and cooling. Therefore, it is contained at 0.0010% or more. On the other hand, it is also a precipitation strengthening element, and if added over 0.0200%, it causes precipitation embrittlement, so the upper limit is made 0.0200%. Preferably, it is 0.0050 to 0.0200%.

Al: 0.080% or less
Al is added to steel as a deoxidizer, and its effect is saturated when it exceeds 0.080%, so the upper limit of Al was made 0.080%. The lower limit is not particularly specified, but is preferably 0.003% or more in order to obtain a sufficient deoxidation effect. Preferably, it is 0.015 to 0.040%.

N: 0.0010-0.0060%
N is an element that forms nitrides such as Nb and Ti, and is useful for refining the structure, so 0.0010% or more is necessary. On the other hand, if excessively added N remains as solute N without forming a nitride, the toughness is reduced, so the upper limit is made 0.0060%. Preferably, it is 0.0020 to 0.0050%.

Each of the above components is contained, and the balance is Fe and inevitable impurities. In addition to this basic component, one or more of V: 0.050% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less are added as necessary. Can be contained.
That is, V is a precipitation strengthening element, and for that purpose, V is preferably contained in an amount of 0.005% or more. However, when 0.050% or more is contained, precipitation embrittlement is caused, so the upper limit is preferably made 0.050%. More preferably, it is 0.010 to 0.050%.

  Cu, Ni, Cr, and Mo are elements that contribute to strength improvement, and can be added as necessary within a range that does not exceed the upper limit of Ceq described later from the viewpoint of weldability. For that purpose, it is preferable to add each element at 0.01% or more. On the other hand, if each element exceeds 1.0%, it leads to a decrease in toughness and weldability and an increase in cost.

Ceq: 0.44% or less It is possible to increase the strength of the base metal by increasing the Ceq according to the following formula (1). However, if Ceq is too high, the base material toughness and weld toughness will be reduced. Is 0.44%. More preferably, it is 0.43% or less. In addition, the element display in Formula (1) shows content of this element, and the element which is not contained is set to zero.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)

  Here, heat corresponding to the manufacture of H-section steel with a flange thickness of 12 mm to 40 mm using a steel material whose chemical composition is changed from (0.10 to 0.13)% C-0.3% Si-1.5% Mn to the Nb content. Hot rolling was performed to evaluate various strengths and toughnesses and to analyze the microstructure. Based on the results, the microstructure and manufacturing conditions in the present invention were limited. The reasons for limitation concerning the microstructure and manufacturing conditions will be described below.

[Microstructure]
Ferrite average particle size: 15 μm or less The microstructure of the material having the above composition when air-cooled after hot rolling has ferrite as the main phase and the second phase is pearlite and / or bainite. In order to achieve the desired yield strength YP: 355 MPa or more and Charpy absorbed energy of −40 ° C .: 50 J or more, it is important to refine the ferrite grains. That is, if the ferrite average particle size exceeds 15 μm, the toughness at −40 ° C. is lowered, so the ferrite average particle size needs to be 15 μm or less.

Island-like martensite fraction: 3.0% or less The portion other than ferrite in the microstructure, that is, the second phase is pearlite and / or bainite. The bainite may contain some island-like martensite, but the island-like martensite is a hard phase and serves as a starting point for fracture. Therefore, when this island-like martensite is formed, the toughness at −40 ° C. decreases. Therefore, the area ratio needs to be 3.0% or less. Preferably, it is 2.5% or less.
In addition, the area ratio of island-shaped martensite here is an area ratio of island-shaped martensite with respect to the area of the whole structure. Further, the ferrite as the main phase has an area ratio of 70% or more, preferably 80% or more. On the other hand, the second phase, pearlite and / or bainite, is preferably 25% or less in terms of area ratio. This is because when the area ratio of hard pearlite and / or bainite exceeds 25%, the base material toughness decreases.

[Production conditions]
After heating the steel material having the above composition at 1150 ° C or higher and lower than 1300 ° C, at least the surface temperature of the portion corresponding to the flange is less than TR ° C calculated by the following formula (2). It is important to perform hot rolling.
Record
TR = 174 log [Nb × (C + 12 / 14N)] + 1344 (2)

Heating temperature: 1150 ° C or more and less than 1300 ° C In the production of H-section steel, it is important to control the shape by hot rolling, and it is necessary to heat to 1150 ° C or higher in order to work in a high temperature range with low deformation resistance . Furthermore, in order to sufficiently dissolve Nb (C, N), it is preferable to heat at 1200 ° C. or higher. On the other hand, if the heating temperature is too high, the TiN precipitate is dissolved, and the effect of suppressing the coarsening of the austenite grains is reduced. As a result, the structure becomes coarse and the toughness is reduced, so the heating temperature is less than 1300 ° C. And Preferably, it is 1290 ° C. or lower.

Hot rolling: At least the surface temperature of the portion corresponding to the flange is 20% or more when the cumulative temperature reduction is less than or equal to TR ° C calculated by the above formula (2). Here, the above formula (2) It is the result of having calculated | required experimentally the non-recrystallization temperature range of austenite at the time of adding. That is, it is possible to make maximum use of the controlled rolling effect by rolling at a cumulative reduction ratio of 20% or more at a temperature equal to or lower than the temperature calculated by the above formula (2) according to the amounts of C, N and Nb. Is possible. As a result, a strength of YP355 MPa or more and toughness at −40 ° C. can be secured stably. In addition, since the ferrite grain size becomes finer as the cumulative rolling reduction is higher and contributes to the improvement of strength and toughness, when a higher strength of YP420 MPa or more is required, the cumulative rolling reduction is preferably 30% or more. . On the other hand, if the cumulative reduction is applied excessively, it becomes difficult to increase the load during rolling and to ensure the shape. Therefore, the upper limit is preferably 50%. In addition, it is not necessary to prescribe | regulate especially the rolling reduction | decrease exceeding TR degreeC calculated by the said Formula (2), and desired intensity | strength and toughness can be ensured by prescription | regulation of the cumulative rolling reduction | decrease below TR degreeC.

  Here, the reason why the surface temperature of at least the portion corresponding to the flange is defined is that the surface temperature of the flange portion, which is the material evaluation position, is temperature-controlled with a radiation thermometer or the like and controlled rolling is performed.

  By following the above manufacturing conditions, after hot rolling, desired strength and toughness can be secured through air cooling (without accelerating cooling) and shape stabilization can be achieved. Moreover, by cooling at a cooling rate of about air cooling, it is possible to promote the decomposition of island martensite, which is a cause of toughness reduction, and improve low temperature toughness.

  Steel materials adjusted to various component compositions shown in Table 1 were hot-rolled according to the conditions shown in Table 2 to produce rolled H-section steels having different flange thicknesses. JIS No. 1A tensile test specimen was taken from the surface of the obtained H-shaped steel in parallel with the rolling direction from the 1/6 position of the flange width, and the tensile strength test was performed to determine the yield strength (YP) and tensile strength (TS). Asked. In addition, a Charpy impact test piece was taken in parallel to the rolling direction from a 1 / 4t (t: flange thickness) part below the surface at the flange width 1/6 position, and absorbed energy at 0 ° C. and absorbed at −40 ° C. The energy and the absorbed energy at −60 ° C. were evaluated. The evaluation results are also shown in Table 2.

  Further, a sample for microstructural observation is cut out from the flange width 1/6 position, the surface parallel to the rolling direction and the flange thickness direction is taken as the observation surface, this observation surface is polished, and after etching, microscopically at a magnification of 100 to 400 times using an optical microscope. Tissue observation was performed. Then, the microstructures of the main phase and the second phase were identified, and the ferrite fraction (area ratio) and the ferrite particle size (average particle size) were determined by image analysis. The microstructure observation sample was observed with a scanning electron microscope (SEM) at a magnification of 1000 times, and the area ratio (MA fraction) of island martensite was determined by image analysis. These results are also shown in Table 2.

  In the invention example, the yield strength is YP355MPa or more, the tensile strength is TS460 ~ 690MPa and Charpy absorbed energy at -40 ℃ is 50J or more. Not satisfied.

Claims (3)

% By mass
C: 0.08 to 0.16%,
Si: 0.05-0.60%
Mn: 0.10 to 1.80%
Nb: 0.005-0.060%,
Ti: 0.0010-0.0200%,
Al: 0.080% or less,
N: 0.0010 to 0.0060%,
P: 0.030% or less and S: 0.030% or less in a range where Ceq according to the following formula (1) is 0.44% or less, the balance is the component composition of Fe and unavoidable impurities, and the average particle size is 15 μm or less H-shaped steel having a microstructure with ferrite as a main phase, wherein the second phase is pearlite and / or bainite, and island martensite is 3.0% or less.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
However, the element display in the formula indicates the content of the element, and the elements not included are zero. The component composition is further mass%,
V: 0.050% or less,
Cu: 1.0% or less,
Ni: 1.0% or less,
Cr: 1.0% or less and
Mo: H-section steel according to claim 1, containing one or more of 1.0% or less. After heating the steel material having the component composition according to claim 1 at 1150 ° C or higher and lower than 1300 ° C, at least the surface temperature of the portion corresponding to the flange is cumulatively reduced below TR ° C calculated by the following formula (2) A method for manufacturing H-section steel that performs hot rolling at a rate of 20% or more.
Record
TR = 174 log [Nb × (C + 12 / 14N)] + 1344 (2)