KR102419239B1 - H-beam and its manufacturing method - Google Patents

Abstract

A method for securing high strength of YP 355 MPa or more and low-temperature toughness at -40°C in the flange portion of H-beam steel without increasing manufacturing cost is proposed. 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: 0.030% or less and S: 0.030% or less are contained within the range where Ceq becomes 0.44% or less, and the balance has a composition of Fe and unavoidable impurities, and a microstructure containing ferrite having a particle size of 15 µm or less as a main phase, In the microstructure, the second phase is pearlite and/or bainite, and the island phase martensite is 3% or less.

Description

H-beam and its manufacturing method

The present invention relates to an H-beam widely used as a material for welded steel structures such as offshore structures, buildings, civil engineering and bridges, particularly high-strength H-beams having excellent low-temperature toughness at -40°C, used in offshore structures in cold regions, and their manufacture it's about how

In offshore structures that are mined for crude oil or natural gas, they are often operated in cold regions, and the H-beam used is required to have excellent low-temperature toughness in both the base metal and the weld seam. In order to achieve both high strength and low-temperature toughness, TMCP, which combines controlled rolling and accelerated cooling, is widely used in thick steel sheets, and is also an effective technique for H-section steels. However, in the production of H-section steel, in consideration of formability, high-temperature heating of the raw material and rolling at high temperature with low deformation resistance are required, and the structure tends to become coarse. In addition, although controlled rolling in an austenite low temperature region is important for refining the structure, rolling at a low temperature has problems in terms of an increase in rolling load and shape stability.

Until now, as an H-beam steel having excellent toughness, in Patent Document 1, in addition to not adding a precipitation embrittlement element, reducing the amount of solid solution N, and applying accelerated cooling after rolling, without performing controlled rolling, -40°C toughness A technology related to a method for manufacturing a rolled H-beam that secures is disclosed.

Moreover, as an H-beam steel excellent in low-temperature toughness used for offshore structures, etc., in patent document 2, the technique using the ultra-low carbon component which added Nb or B is proposed. Moreover, in patent documents 3 and 4, the technique of achieving the outstanding low-temperature toughness in -40 degreeC until air cooling without adding Nb which inhibits productivity is disclosed.

Japanese Patent Laid-Open No. 2006-180584 International Publication No. 2013/089156 Japanese Patent Laid-Open No. 2016-84524 Japanese Laid-Open Patent Publication No. 2016-156032

Since the technique described in patent document 1 requires application of accelerated cooling as a manufacturing method, there exists a subject in coexistence of material control and shape stabilization.

In addition, in Patent Document 2, in order to achieve Charpy absorbed energy at -40°C and CTOD characteristics at -10°C, an H-beam steel having excellent low-temperature toughness with a C content of 0.040% or less and Nb and B compound added is used. A related technology is disclosed. However, in order to substantially reduce C to about 0.020%, the refining time in the steelmaking step becomes long, and in order to ensure strength, it is also necessary to add a comparatively large amount of alloying elements, which becomes high cost.

On the other hand, Patent Documents 3 and 4, by appropriately controlling the amounts of V and N without adding Nb, which increases the deformation resistance in hot rolling and impairs productivity, -40°C or -60°C It is a technology that improves the low-temperature toughness in However, in order to control the VN precipitates and secure the toughness more stably, it is necessary to secure the N content of 0.004% or more.

The present invention is to solve the above problems, in particular, without increasing the manufacturing cost, YP 355 MPa or more high strength in the flange portion of the H-beam steel, and a method for securing low-temperature toughness at -40 ° C. aim to

By the way, in order to manufacture a rolled H-section steel with high strength and excellent low-temperature toughness, it is important to apply controlled rolling to hot rolling. In particular, in order to effectively perform controlled rolling in the austenite non-recrystallization temperature range, it is effective to increase the temperature in the non-recrystallization temperature range by adding Nb. In order to exhibit the controlled rolling effect when this Nb is not added, rolling in the austenite low temperature range is required, and there are problems with an increase in the rolling load, an increase in the rolling time for temperature adjustment, and deterioration of the dimensional accuracy of the H-beam steel. becomes Therefore, although Nb causes an increase in deformation resistance, since it is an element exhibiting the controlled rolling effect in a high temperature range, it is a very useful element from the viewpoint of material control. On the other hand, when Nb is added, hardening property is improved in the cooling process after hot rolling, and since a part of untransformed austenite becomes island martensite, deterioration of low-temperature toughness becomes a problem.

Therefore, the inventors carefully studied a method for maximizing the controlled rolling effect by adding a small amount of Nb, and ensuring the strength of YP 355 MPa or more, and the low-temperature toughness at -40°C, especially in the flange portion of the H-beam. As a result, Nb was added to maximize the controlled rolling effect due to the high temperature of the austenite non-recrystallization temperature range, and the ferrite grain size was refined by controlled rolling at a relatively high temperature, and the ballast martens were obtained by optimizing the rolling conditions. By reducing the amount of zite production, it was found that high strength and low-temperature toughness were compatible, and the present invention was completed. That is, the gist of the present invention is as follows.

[1] In mass %,

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: 0.0010 to 0.0200%,

Al: 0.080% or less;

N: 0.0010 to 0.0060%;

P: 0.030% or less and

S: 0.030% or less

contains in the range where Ceq according to the following formula (1) is 0.44% or less, and the balance has a microstructure mainly consisting of Fe and unavoidable impurities and ferrite having an average particle size of 15 µm or less, and the The microstructure is an H-beam steel in which the second phase is pearlite and/or bainite, and the island phase martensite is 3.0% or less.

Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 … (One)

However, the element display in the formula indicates the content of the element, and an element not included is set to zero.

[2] The component composition is further in mass%,

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

The H-beam steel according to the above [1], containing one or two or more of them.

[3] After heating the steel material having the component composition described in [1] or [2] at 1150 ° C. or higher and less than 1300 ° C., at least the surface temperature of the flange equivalent portion is at TR ° C. or lower calculated by the following formula (2) A method of manufacturing an H-section steel in which hot rolling is performed in which the cumulative reduction ratio is 20% or more.

TR = 174 log [Nb × (C + 12/14N)] + 1344 … (2)

According to the present invention, Nb is added in an appropriate amount to increase the temperature of the austenite non-recrystallization temperature range, and the controlled rolling effect can be maximized. As a result, accelerated cooling is not required after hot rolling, in other words, even after hot rolling, even in air cooling, the strength of the flange portion is YP 355 MPa or more, and the toughness of the flange portion is the Charpy absorbed energy at -40 ° C. An H-beam steel having excellent low-temperature toughness having 50 J or more can be provided.

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

C: 0.08 to 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 made 0.08%. The C content is preferably 0.10% or more. On the other hand, when there is too much C content, since the formation amount of secondary phases, such as pearlite and bainite, increases, and base metal toughness and weld part toughness fall, the upper limit of C content is made into 0.16 %. Preferably, it is 0.08 to 0.14 %.

Si: 0.05 to 0.60%

Si is effective as a deoxidation element or a solid solution strengthening element, and in order to acquire the effect, at least 0.05 % is required. On the other hand, when it exceeds 0.60 %, since the toughness of a base material and weld part toughness will deteriorate, Si is made into the range of 0.05 to 0.60 %. Preferably, it is 0.05 to 0.50 %.

Mn: 0.10 to 1.80%

Mn is required to be 0.10% or more in order to secure the strength of the base material. On the other hand, when added exceeding 1.80 %, since low-temperature cracking susceptibility will increase, Mn was limited to the range of 0.10 to 1.80 %. In addition, from the viewpoint of weld toughness, it is preferable that the upper limit be 1.60%. More preferably, it is 0.30 to 1.60 %.

P: 0.030% or less

Since the toughness of a weld part will fall when content exceeds 0.030 %, P is suppressed to 0.030 % or less. Preferably, it is 0.020% or less. Moreover, in order to suppress P to less than 0.005 %, since much cost is required for the process, it is preferable to make 0.005 % into a lower limit from a viewpoint of manufacturing cost.

S: 0.030% or less

Since the toughness of a base material and a weld part will fall when S is contained in excess of 0.030 % similarly to P, it is suppressed to 0.030 % or less. Preferably, it is 0.005 % or less. Moreover, in order to suppress S to less than 0.001 %, since much cost is required for the process, it is preferable to make 0.001 % into a lower limit from a viewpoint of manufacturing cost.

Nb: 0.005 to 0.060%

Nb forms Nb carbonitride and suppresses coarsening of austenite grains during heating of a steel material, which is effective for refining the ferrite structure after rolling-cooling, and controlled rolling at austenite non-recrystallization temperature. It is a very important element for effective implementation. Moreover, it is an element effective also in high strength by precipitation strengthening. In order to express the effect and ensure the intensity|strength of YP 355 MPa or more, 0.005% or more of containing is required. Moreover, when the high strength of YP 420 MPa or more is requested|required, it is preferable to make it contain in 0.015 % or more. On the other hand, when adding exceeding 0.060 %, since the toughness fall of the base material and welding part by island martensite formation became remarkable, 0.060 % was made into the upper limit. In order to further suppress the formation of island martensite, it is preferable to set it as 0.050% or less. More preferably, it is 0.040 % or less, More preferably, it is 0.035 % or less.

Ti: 0.0010 to 0.0200%

Ti is an element effective in forming TiN, suppressing coarsening of austenite grains during heating of a steel material, and refining the ferrite structure after rolling-cooling. Therefore, it is made to contain 0.0010% or more. On the other hand, it is also a precipitation strengthening element, and since precipitation embrittlement will be caused when added in excess of 0.0200%, 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 since the effect is saturated when it exceeds 0.080 %, the upper limit of Al was made into 0.080 %. Although the lower limit is not particularly specified, in order to sufficiently obtain the deoxidation effect, it is preferably set to 0.003% or more. Preferably, it is 0.015 to 0.040 %.

N: 0.0010 to 0.0060%

N is an element that forms a nitride such as Nb or Ti, and is useful for refining the structure, so 0.0010% or more is required. On the other hand, if the excessively added N remains as solid solution N without forming a nitride, a decrease in toughness is caused, so the upper limit is made 0.0060%. Preferably, it is 0.0020-0.0050 %.

Each of the above components is contained, and the remainder is Fe and unavoidable impurities. In addition to this basic component, one or two 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 contained as needed. can do.

That is, V is a precipitation strengthening element, and for that purpose, it is preferable to contain 0.005% or more. However, since precipitation embrittlement is caused when 0.050% or more is contained, it is preferable to set the upper limit to 0.050%. More preferably, it is 0.010 to 0.050 %.

Moreover, Cu, Ni, Cr, and Mo are elements contributing to a strength improvement, and can be added as needed in the range which does not exceed the upper limit of Ceq mentioned later from a weldability viewpoint. For that purpose, it is preferable to add all of the elements in an amount of 0.01% or more. On the other hand, when each element exceeds 1.0 %, since it leads to the fall of toughness and weldability, and a raise of cost, it is preferable to set it as 1.0 % or less, respectively.

Ceq: 0.44% or less

By increasing Ceq according to the following formula (1), it is possible to increase the strength of the base material, but if Ceq is too high, the base material toughness and the weld joint toughness decrease, so the upper limit is made 0.44%. More preferably, it is 0.43 % or less. In addition, element display in Formula (1) shows content of the element, and an element which is not contained is made into zero.

Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 … (One)

Here, using a steel material whose chemical composition is (0.10 to 0.13)% C-0.3% Si-1.5% Mn and the amount of Nb changed, hot rolling corresponding to production of H-section steel with a flange thickness of 12 mm to 40 mm is performed. It performed and evaluated various strengths and toughness, and analyzed the microstructure. Based on the results, the microstructure and manufacturing conditions in the present invention were limited. Below, the reason for limitation regarding a microstructure and manufacturing conditions is described.

[micro organization]

Ferrite average particle size: 15 μm or less

The microstructure when the raw material of the above composition is 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 the Charpy absorbed energy of -40°C: 50 J or more in the present invention, it is important to refine the ferrite grains. That is, since toughness at -40 degreeC will fall when a ferrite average particle diameter exceeds 15 micrometers, it is necessary to make the ferrite average particle diameter into 15 micrometers or less.

Fraction of island martensite: 3.0% or less

A portion other than ferrite in the microstructure, that is, the second phase is pearlite and/or bainite. The bainite may contain some island martensite, but the island martensite is a hard phase and serves as a fracture origin. It is necessary to set it as 3.0 % or less. Preferably, it is 2.5 % or less.

In addition, the area ratio of island martensite here is the area ratio of island martensite with respect to the area of the whole structure|tissue. Moreover, the ferrite used as a main phase is 70 % or more in area ratio, Preferably it is 80 % or more. On the other hand, it is preferable that the 2nd phase pearlite and/or bainite is 25 % or less in area ratio. This is because, when the area ratio of hard pearlite and/or bainite exceeds 25%, the base metal toughness decreases.

[Manufacturing conditions]

After heating a steel material having the above component composition at 1150 ° C. or higher and less than 1300 ° C., hot rolling with a cumulative reduction ratio of 20% or more at TR ° C. It is important to carry out

TR = 174 log [Nb × (C + 12/14N)] + 1344 … (2)

Heating temperature: 1150℃ or more and less than 1300℃

In the manufacture of H-beams, shape control by hot rolling is important, and in order to process in a high-temperature region with low deformation resistance, it is necessary to heat to 1150° C. or higher. In addition, in order to sufficiently dissolve Nb(C, N), heating at 1200°C or higher is preferable. On the other hand, if the heating temperature is too high, TiN precipitates are dissolved in solid solution, and as a result, the effect of suppressing coarsening of austenite grains becomes small. Preferably, it is 1290 degrees C or less.

Hot rolling: At least the surface temperature of the flange equivalent portion has a cumulative reduction ratio of 20% or more at TR°C or lower calculated by the above formula (2)

Here, the above formula (2) is the result of experimentally finding the non-recrystallization temperature range of austenite in the case of adding Nb in the above-described component system. That is, it is possible to utilize the controlled rolling effect to the maximum by performing rolling with a cumulative reduction ratio of 20% or more at a temperature below the temperature calculated by the above formula (2) according to the amounts of C, N and Nb. As a result, the strength of YP of 355 MPa or more and the toughness at -40°C can be stably secured. In addition, the higher the cumulative reduction ratio, the finer the ferrite grain size, contributing to the improvement of strength and toughness. On the other hand, if excessive cumulative rolling reduction is applied, it becomes difficult to increase the load at the time of rolling and to secure the shape. Therefore, it is preferable to set 50% as the upper limit. In addition, there is no need to prescribe in particular the rolling-reduction|draft ratio above TR degreeC calculated by said Formula (2), and desired strength and toughness can be ensured by regulation of the cumulative rolling-reduction|draft ratio at TR degreeC or less.

Here, at least the surface temperature of the flange equivalent portion is prescribed in order to temperature-manage the surface temperature of the flange portion, which is a material evaluation position, with a radiation thermometer or the like to perform controlled rolling.

By complying with the above manufacturing conditions, desired strength and toughness can be secured through air cooling (without accelerated cooling and simple) after hot rolling, and shape stabilization is also achieved. In addition, by cooling at a cooling rate equivalent to air cooling, decomposition of island martensite, which is a factor of a decrease in toughness, is promoted, and it becomes possible to improve the low-temperature toughness.

Example

The steel raw material adjusted to the various component compositions shown in Table 1 was hot-rolled according to the conditions shown in Table 2, and rolled H-beam steel from which flange thickness differs variously was manufactured. A JIS No. 1A tensile test piece was taken from the surface of the obtained H-beam in parallel in the rolling direction from a position of 1/6 of the flange width, and a tensile test was performed to determine the yield strength (YP) and tensile strength (TS). Further, a Charpy impact test piece is taken parallel to the rolling direction from the 1/4 t (t: flange thickness) part under the surface at the position of 1/6 of the flange width, and the absorbed energy at 0°C and the absorbed energy at -40°C and absorbed energy at -60°C, respectively. The evaluation result is written together in Table 2.

Further, a sample for microstructure observation is cut out from a position of 1/6 of the flange width, a surface parallel to the rolling direction and the flange thickness direction is an observation surface, and the observation surface is polished and etched by an optical microscope at a magnification of 100 to 400 Micro-tissue observation was carried out in a vessel. Then, the microstructure of the main phase and the second phase were identified, and the ferrite fraction (area ratio) and ferrite particle size (average particle size) were determined by image analysis. Moreover, the said sample for microstructure observation was observed with a magnification of 1000 times with a scanning electron microscope (SEM), and the area ratio (MA fraction) of island martensite was calculated|required by image analysis. These results are also written together in Table 2.

In the invention example, the yield strength YP 355 MPa or more, the tensile strength TS 460 to 690 MPa, and the Charpy absorbed energy 50 J or more at -40°C are satisfied. not doing

Claims (3)

in mass %,
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: 0.0010 to 0.0200%,
Al: 0.080% or less;
N: 0.0010 to 0.0060%;
P: 0.030% or less and
S: 0.030% or less
is contained in a range such that Ceq according to the following formula (1) is 0.44% or less, and the remainder is a steel material having a component composition of Fe and unavoidable impurities After heating at 1150°C or more and less than 1300°C, at least a portion corresponding to the flange By performing hot rolling with a cumulative reduction ratio of 20% or more at a surface temperature of TR °C or less calculated by the following formula (2), and not performing accelerated cooling after the hot rolling, ferrite having an average particle diameter of 15 µm or less A method for producing an H-section steel to obtain an H-section steel having a microstructure as a main phase, pearlite and/or bainite as the second phase, and an island martensite content of 3.0% or less.
Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 … (One)
However, the element display in the formula indicates the content of the element, and an element not included is set to zero.
TR = 174 log [Nb × (C + 12/14N)] + 1344 … (2) The method of claim 1,
The component composition is further in mass%,
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
A method for producing an H-section steel containing one or two or more of them. delete