JP6421900B2 - Rolled H-section steel and its manufacturing method - Google Patents

Description

(Cross-reference of related applications)
This application claims priority based on Japanese Patent Application No. 2006-166535 for which it applied to Japan on August 29, 2016, and uses the content here.

  The present invention relates to a rolled H-section steel manufactured by hot rolling a steel slab and a method for manufacturing the same.

  H-shaped steel has been widely used as a material for buildings, civil engineering, marine structures, etc., and various cross-sections are used. In particular, H-section steel produced by hot rolling using a rectangular cross-section slab obtained by continuous casting with high productivity is low in production cost and is used in many fields. Conventionally, H-section steel manufactured from a slab has been manufactured by the edging method shown in FIG. In the edging method, first, a groove for guiding the steel material to the center of the hole mold of the roll is formed at the end of the slab, rolled in the width direction of the slab, and the flange is formed by extending the end of the slab in the thickness direction of the slab. It is a rolling method. Alloy elements such as Mn are concentrated in the central segregation portion formed when the slab is cast. By rolling by the edging method, the central segregation portion may further aggregate at a portion where the web and the flange intersect, that is, a so-called “fillet portion”, which may adversely affect toughness.

  In view of such problems, in order to eliminate macro segregation (aggregation of the central segregation part), it is effective to diffuse Mn by heating at a high temperature for a certain period of time. A method of performing heat treatment has been proposed (see, for example, Patent Document 1). Moreover, in order to promote diffusion, it is effective to maintain strain at high temperature after applying strain by rolling, and a method of reheating before intermediate rolling after rough rolling the steel slab has been proposed ( For example, see Patent Documents 2 and 3.)

  In addition to heat treatment, methods for eliminating macro segregation have been proposed (see, for example, Patent Documents 4 and 5). Patent Document 4 discloses a method of applying a reduction before completely solidifying by continuous casting. On the other hand, Patent Document 5 discloses a method of forming an edging hole mold having a slab width of a rough rolling mill into a box hole mold having a flat hole mold bottom, and is referred to as a wedge method.

JP 2012-180584 A Japanese Patent Laid-Open No. 6-122921 JP-A-6-122922 JP-A-5-305395 JP 7-88502 A

  As described above, conventionally, various measures have been proposed in order to suppress a decrease in toughness of the fillet portion of the rolled H-section steel due to macro segregation. However, each measure has a problem in that productivity is impaired as compared with the conventional edging method.

  Therefore, in view of such circumstances, an object of the present invention is to provide a rolled H-section steel in which macrosegregation of the fillet portion is reduced and a method for producing the same without impairing productivity with respect to the conventional edging method. There is.

The present invention is characterized in that there is a step of forming an interruption by a shaping hole mold in which a projecting portion for making an interruption vertically is formed with respect to the width direction of the material to be rolled, and sequentially bending it from the starting point. According to such a process, when forming a flange from a slab, a center segregation part is disperse | distributed to the whole flange, and aggregation of the center segregation part in a fillet part can be suppressed, without impairing productivity.
The gist of the present invention is as follows.

[1] By mass%
C: 0.01 to 0.25%
Si: 0.05% to 0.50%
Mn: 0.40 to 2.50%,
P: 0.050% or less,
S: 0.050% or less,
N: 0.020% or less,
Cu: 0.70% or less,
Ni: 0.70% or less,
Cr: 0.50% or less,
V: 0.12% or less,
Mo: 0.30% or less,
Nb: 0.08% or less,
Ti: 0.05% or less,
Al: 0.07% or less,
REM: 0.010% or less,
Ca: 0.0050% or less,
Balance: Fe and inevitable impurities,
A rolled H-section steel having a chemical composition of
The upper 5% average value of the Mn concentration at the most brittle part in the flange is 1/6 in the flange width direction from the end face in the flange width direction, and the flange thickness direction from the flange surface located on the opposite side of the web Less than 1.6 times the Mn concentration at the 1/4 position,
The upper 5% average value of the Mn concentration in the central segregation part is 15 mm or more from the center of the flange width toward one end face or both end faces in the flange width direction and within the flange surface layer within 2 mm in the thickness direction. 1.1 or more times the Mn concentration at a position 1/6 in the flange width direction from the end face in the flange width direction and at a position 1/4 in the flange thickness direction from the surface of the flange on the opposite side of the web. 6 times Ri der below,
The most brittle part in the flange is a part where a straight line indicating a position 3/4 in the flange thickness direction from the surface of the flange located on the opposite side of the web and a part where the central segregation part is aggregated,
The upper 5% average value of the Mn concentration is an average value of 12,500 points that identify a visual field of 10 mm × 10 mm, and among the 500 points × 500 points in the visual field, the value is higher than the upper 5%. Rolled H-section steel.
[2] A method for producing a rolled H-section steel according to [1], in which a steel piece having a rectangular cross section is heated to 1100 to 1350 ° C., and a rough rolling step, an intermediate rolling step, and a finish rolling step are sequentially performed. The rolling mill that performs the rough rolling process is provided with a plurality of three or more hole molds for shaping the material to be rolled, and at least one of the plurality of hole molds interrupts perpendicularly to the width direction of the material to be rolled. The interrupt forming hole type provided on the pair of upper and lower rolls formed with the projections for inserting the interrupting part, and the divided portions formed by the interrupt forming hole type are sequentially bent at the subsequent stage of the interrupt forming hole type. A method for producing a rolled H-section steel, wherein a shaping hole mold is provided.
[3] The method for producing rolled H-section steel according to [2], wherein a tip angle of the protrusion formed in the interrupt forming hole mold is 40 ° or less.
[4] The interruption length H formed by the protrusions, the thickness T of the steel piece having the rectangular cross section, and the width F of the flange of the rolled H-section steel formed by the finish rolling process are as follows: The method for producing a rolled H-section steel according to [2] or [3], wherein the formula (1) is satisfied.
H ≧ 0.5F−0.5T (1)

  According to the present invention, it is possible to obtain an H-section steel having excellent fillet portion toughness by a simple process without performing special heat treatment such as preheating, reheating after rolling, or temperature maintenance. Therefore, the industrial contribution of the present invention is extremely remarkable, such as the reliability of the steel structure including the rolled H-section steel as a member can be further improved without impairing the economy.

It is a schematic explanatory drawing about the comparison with "edging method" and "split method". It is a figure which shows the correlation of segregation degree and Charpy transition temperature difference (DELTA) vTrs. It is a schematic explanatory drawing which shows the position which performed the mechanical test and observation of the metal structure. It is a schematic explanatory drawing which shows the manufacturing process of the H-section steel which concerns on embodiment of this invention. It is a schematic explanatory drawing which shows the shape of the roll used for rough rolling, and a to-be-rolled material.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the present specification and drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant description is omitted.

The inventors have obtained the knowledge that, when forming the flange portion, interrupting and bending and manufacturing the flange portion, segregation is dispersed throughout the flange and segregation of segregation in the fillet portion is improved. . First, this knowledge will be briefly described.
In addition, the manufacturing method of H-section steel which performs the rolling shaping | molding by bending the flange part which concerns on this Embodiment is called a "split method" in this specification.

  First, the outline of the “split method” will be briefly described with reference to FIG. FIG. 1 shows a so-called “edging method” which is one of rough rolling methods in a conventional method for manufacturing H-section steel, and a so-called “split method” which is a rough rolling method in the method for manufacturing H-shaped steel according to the present embodiment. It is a schematic explanatory drawing about a comparison with "."

  As shown in FIG. 1 (a), the edging method provides a groove for guiding the slab to the center of the hole mold at the end of the slab at the time of rough rolling when manufacturing the H-section steel from the slab. In this method, hot rolling is performed by a perforated roll attached to a machine. The slab heated in the heating furnace is rolled in the width direction, and the flange portion is formed by extending the end of the slab in the thickness direction of the slab. In order to further precisely adjust the shape and dimensions of the product with the flange portion formed in this way, intermediate rolling by an intermediate rolling mill or finish rolling by a finishing mill is performed, and the final H Shaped steel products are manufactured.

  On the other hand, as shown in FIG. 1 (b), in the split method, a deep groove (interrupt) is formed on the end surface of the slab at the time of rough rolling when manufacturing H-section steel from the slab. It is given by the hole type. Then, rolling modeling is performed on the applied groove so as to split the slab end portion which is a divided portion by using a hole roll of a modeling hole type in which a protrusion for expanding the groove is formed. Is called. The split method is a method of forming the flange portion by performing such split rolling and shaping, for example, by changing the angle a plurality of times. Thus, intermediate rolling, finish rolling, etc. are further performed with respect to the to-be-rolled material in which the flange part was formed, and a final H-section steel product is manufactured.

When comparing the edging method shown in FIG. 1 and the split method, the present inventors pay attention to the central segregation part, which is a portion having a high Mn concentration, present in the slab, and the rough rolling by the edging method and the split method. In rough rolling, it was found that there is a great difference in the state of aggregation or dispersion of the central segregation part of the slab.
That is, as shown in FIG. 1 (a), it is known that the center segregation part aggregates into the fillet part when the slab is rolled in the width direction by the perforated roll in the edging method. On the other hand, as shown in FIG. 1B, in the split method, the slab is hardly rolled in the width direction and the flange portion is split and the center segregation portion is dispersed throughout the flange portion, and the fillet portion is dispersed. Rough rolling is performed without agglomeration. In particular, it has been found that the aggregation of the central segregation portion can be suppressed by setting the tip end angle of the interrupting hole-type protrusion to an acute angle of 40 ° or less.

  Then, the inventors of the present invention, by split method, have VTrs (Charpy transition temperature) of 0 ° C. or less in F / 6, which shows the average mechanical properties of H-section steel, and as shown in FIG. It has been found that the difference in vTrs with the most brittle part where the toughness is most deteriorated can be suppressed within 40 ° C. This is presumably because embrittlement due to MnS present in the central segregation part having a high Mn concentration, island-like martensite (MA) which is a hard phase, and upper bainite was suppressed.

  Hereinafter, the rolled H-section steel according to the present embodiment and the manufacturing method thereof according to the above-described knowledge will be described in detail. In addition, in the following description, the description of “%” regarding the component means “% by mass” unless otherwise specified.

  First, the component composition (chemical composition) of H-section steel will be described.

(C: 0.01-0.25%)
C promotes the formation of MA at the fillet portion and reduces toughness. However, it is possible to improve the strength of C at a low cost, and removing C completely in the steelmaking process leads to an increase in cost, so the C content is set to 0.01% or more. On the other hand, if the amount of C exceeds 0.25%, MA increases at the position where the central segregation portion of the fillet portion is aggregated and the toughness is lowered, so the amount of C is limited to 0.25% or less. Preferably, the C content is 0.20% or less, more preferably less than 0.17%.

(Si: 0.05 to 0.50% or less)
Si is a deoxidizing element and contributes to improvement in strength, but like C, it is an element that generates MA. When the amount of Si exceeds 0.50%, the toughness of the base material and the weld heat affected zone decreases due to the generation of the hard phase, so the Si amount is limited to 0.50% or less. The amount of Si is preferably 0.30% or less, more preferably 0.20% or less, and still more preferably 0.10% or less. However, if Si is not contained, the cost increases in the deoxidation process, so that Si is contained in an amount of 0.05% or more.

(Mn: 0.40 to 2.50%)
In the H-shaped steel manufactured by the edging method, the center segregation part of the slab is aggregated in the fillet part. Mn tends to agglomerate particularly in the central segregation part, and the concentration of Mn locally increases to form MA as an embrittled phase, increase in coarse bainite as a coarse structure, increase in MnS, and increase in hardenability. Increase in hardness is promoted. As a result, the toughness is significantly reduced. In particular, when Mn exceeding 2.50% is contained, the toughness of the base material and the weld heat affected zone is impaired due to an increase in inclusions in the fillet portion. For this reason, the amount of Mn is limited to 2.50% or less. The amount of Mn is preferably 2.00% or less, more preferably 1.80% or less. On the other hand, since Mn is an element effective for reducing the crystal grain size, 0.40% or more is contained.

(P: 0.050% or less)
Since P causes weld cracking due to solidification segregation and a decrease in toughness, it should be reduced as much as possible. The amount of P is preferably limited to 0.050% or less, more preferably 0.010% or less. In addition, about a lower limit, since it will raise steel-making cost large if it removes to less than 0.001%, 0.001% or more may be sufficient.

(S: 0.050% or less)
S forms MnS in the central segregation part formed by solidification segregation, and causes not only weld cracking and toughness degradation but also hydrogen cracking, so it should be reduced as much as possible. The amount of S is preferably limited to 0.050% or less, and more preferably 0.010% or less. In addition, about a lower limit, since it will raise steel-making cost large if it removes to less than 0.001%, 0.001% or more may be sufficient.

  Furthermore, for the purpose of improving strength and toughness, one or more of Cu, Ni, Cr, V, Mo, Nb, Ti, Al, and N may be included as an optional additive element. In addition, since arbitrary addition elements do not necessarily need to be added, the lower limit of the content of each optional addition element is 0%.

(Cu: 0.70% or less)
Cu is an element that contributes to improvement in strength. However, if the Cu content exceeds 0.70%, the strength increases excessively and the toughness decreases, so the Cu content is limited to 0.70% or less. The Cu amount is preferably 0.50% or less, more preferably 0.30% or less, and still more preferably 0.10% or less. The lower limit of the amount of Cu is preferably 0.01%.

(Ni: 0.70% or less)
Ni is an extremely effective element for increasing strength and toughness. However, Ni is an expensive element, and in order to suppress an increase in alloy cost, the amount of Ni is limited to 0.70% or less, preferably 0.50% or less, more preferably 0.30% or less, and still more preferably. Is 0.10% or less. The Ni content is preferably 0.01% or more, more preferably 0.02% or more.

(Cr: 0.50% or less)
Cr is also an element contributing to the improvement of strength. However, if Cr is added in excess of 0.50%, carbides may be generated and the toughness may be impaired. Therefore, the Cr content is limited to 0.50% or less, preferably 0.30% or less. The lower limit of the Cr amount is preferably 0.01%.

(V: 0.12% or less)
V is an element forming nitride (VN), and may be contained in an amount of 0.01% or more in order to increase the strength of the base material. Preferably, the V amount is 0.02% or more, more preferably 0.03% or more. On the other hand, since V is an expensive element, the upper limit of the amount of V is limited to 0.12%, preferably 0.08%.

(Mo: 0.30% or less)
Mo is an element that enhances hardenability and contributes to improvement in strength. However, if Mo is added exceeding 0.30%, precipitation of Mo carbide (Mo ₂ C) and generation of MA in the fillet portion are promoted, and in particular, the toughness of the weld heat affected zone may be deteriorated. The amount is limited to 0.30% or less, preferably 0.15% or less. The lower limit of the Mo amount is preferably 0.01%.

(Nb: 0.08% or less)
Nb is an element that refines ferrite and improves toughness. However, if added over 0.08%, ferrite transformation is excessively suppressed and the formation of MA is promoted, so the amount of Nb is limited to 0.08% or less, preferably 0.05% or less, more preferably 0.03% or less.

(Ti: 0.05% or less)
Ti is an element that forms TiN. When Ti content exceeds 0.05%, TiN becomes coarse and becomes the starting point of brittle fracture, so the Ti content is limited to 0.05% or less. Preferably, the Ti content is 0.03% or less, more preferably 0.02% or less. The lower limit of the amount of Ti may be 0%, but fine TiN contributes to the refinement of the structure, so 0.005% or more may be contained.

(Al: 0.07% or less)
Al is a deoxidizing element, but if the Al content exceeds 0.07%, the toughness of the base metal and the weld heat affected zone is lowered by inclusions, so the Al content is limited to 0.07% or less. The Al content is preferably 0.05% or less, more preferably 0.04% or less, and still more preferably 0.03% or less. The lower limit of the amount of Al is not specified and may be 0%, but Al is a useful deoxidizing element and may contain 0.01% or more.

(N: 0.020% or less)
N is an element that lowers the toughness of the base material and the weld heat affected zone. If the N content exceeds 0.020%, the low temperature toughness is impaired by the formation of solid solution N or coarse precipitates, so the N content is limited to 0.020% or less. The N amount is preferably 0.010% or less, more preferably 0.007% or less. On the other hand, if it is attempted to reduce the N content to less than 0.002%, the steelmaking cost increases, so the N content may be 0.002% or more. From the viewpoint of cost, the N amount may be 0.003% or more.

  Furthermore, for the purpose of controlling the form of inclusions, one or two of REM and Ca may be added as optional additional elements.

(REM: 0.010% or less, Ca: 0.0050% or less)
Since REM and Ca are deoxidizing elements and contribute to control of the form of sulfide, they may be added. However, since REM and Ca oxide easily float in molten steel, the amount of REM contained in the steel is limited to 0.010% or less and the amount of Ca is limited to 0.0050% or less. Preferably, the lower limits of the REM amount and the Ca amount are each 0.0005%.

  Next, the metal structure and characteristics of the rolled H-section steel according to the present invention will be described. FIG. 3 is a schematic explanatory view showing a position where the mechanical test and the observation of the metal structure are performed. Below, the result of having verified about a metal structure | tissue and a characteristic mainly in the position shown in FIG. 3 is demonstrated.

As shown in FIG. 3, the flange width direction end face of the flange is 1/6 in the flange width direction, and the flange face located on the opposite side of the web (ie, the outer face) is 1/4 in the flange thickness direction. The position of is in the middle between the flange end portion where the temperature tends to decrease during hot rolling and the flange center portion where the temperature does not easily decrease. Further, the central segregation part is not observed at this site. Therefore, this position is considered to show the average chemical composition and mechanical properties of the H-section steel from the temperature distribution.
In this specification, the position is expressed as “F / 6-t / 4” using the flange width F and the flange thickness t.

  The H-section steel according to the present embodiment suppresses material variations in the flange. For this reason, the observation of the metal structure of the H-section steel and the measurement of mechanical properties (strength and Charpy absorbed energy) were carried out using the most brittle part in the vicinity of F / 2-3t / 4 of the H-section steel shown in FIG. Sample pieces are collected from each position of / 6-t / 4.

  The position of the most brittle portion is not constant in the left-right direction of the figure, that is, the flange width direction, depending on the situation during rough flange rolling. Therefore, after the portion where the central segregation portion is agglomerated is revealed by the nital corrosion liquid, the position of 3/4 (3t / 4) is indicated in the flange thickness direction from the surface of the flange opposite to the web. The portion where the straight line and the portion where the central segregation portion agglomerates intersects was determined as the position of the most brittle portion. A sample piece was taken from the most embrittled portion whose position was specified, and the metal structure was observed and the mechanical properties were measured.

The metal structure of the rolled H-section steel of the present invention is evaluated by an optical microscope, a scanning electron microscope (SEM), and an electron beam microanalyzer (EPMA). A 10 mm × 10 mm visual field centered on the most brittle portion shown in FIG. 3 is identified by an optical microscope. In the identified field of view, the Mn concentration at the position of the most embrittled portion determined was measured under the conditions of an acceleration voltage of 20 kV after electropolishing, a beam shape of a belt shape having a length of 20 μm, and a step of 20 μm. Of 500 points × 500 points in the field of view, an average value of 12500 points, which is the value of the top 5% or more (referred to as “the top 5% average value”), is obtained, and the Mn concentration ( CMn-max)
It was.
On the other hand, a sample is taken from the position of F / 6-t / 4, and according to JIS G0404 (2014 edition), the chemical component of the sample is analyzed to obtain the Mn concentration value at the position of F / 6-t / 4. Mn concentration (CMn) in Furthermore, the value (CMn-max) / (CMn) obtained by dividing (CMn-max) by (CMn) was evaluated as the degree of segregation.

  The target value of the strength of the rolled H-section steel according to the present invention was set based on the steel material standard EN10225 adopted in Europe. Yield point (YP) or 0.2% proof stress measured at room temperature is 325 MPa or more and tensile strength (TS) is 450 MPa or more using a sample piece taken from the position of F / 6-t / 4. Is desirable. The target value of toughness is set to ΔvTrs ≦ 40 ° C.

FIG. 2 is a diagram showing a correlation between the segregation degree and the Charpy transition temperature difference ΔvTrs in the H-section steel. The segregation degree in FIG. 2 is the concentration ratio of Mn at the most brittle portion and the position of F / 6-t / 4 described above with reference to FIG.
As shown in FIG. 2, in the case of the rolled H-section steel manufactured by the conventional edging method, the segregation degree exceeds 1.6, the most brittle part, and the position of F / 6-t / 4 And the Charpy transition temperature difference ΔvTrs with respect to. In this state, a large amount of Mn segregates in the most embrittled portion to form MnS, island martensite (MA) which is a hard phase, upper bainite, and the like, and embrittlement cannot be suppressed.
On the other hand, the rolled H-section steel manufactured by the split method has a Charpy transition temperature difference ΔvTrs of 40 ° C. or less between the most brittle portion and the position of F / 6-t / 4. That is, in a state where the segregation degree is 1.6 or less, aggregation of the center segregation portion is suppressed, and a rolled H-section steel having excellent uniformity in the cross section of the flange as compared with the conventional product can be obtained.
In addition, when a steel structure building used under general temperature conditions is subjected to seismic force or the like, in order to satisfy a predetermined mechanical characteristic without causing brittle fracture of the H-shaped steel of the member, F / 6-t It is desirable that vTrs at the position of / 4 is 0 ° C. or lower.

  As described above, in the rolled H-section steel according to the present invention, the segregation degree shown in FIG. 2 is preferably 1.6 or less. Furthermore, the lower the degree of segregation, the more the aggregation of the central segregation part is suppressed and the embrittlement characteristics are improved. Further, the degree of segregation does not fall below 1.0 in terms of numerical characteristics, and is preferably 1.0 or more or 1.1 or more, for example.

  Next, the manufacturing method of the H-section steel which concerns on this Embodiment is demonstrated. In this embodiment, in the process shown in FIG. 4, a rectangular steel slab having excellent productivity is heated, hot rolling comprising a rough rolling process, an intermediate rolling process, and a finish rolling process is performed, and accelerated cooling is performed by a water cooling device. To produce an H-section steel. Of the hot rolling, rough rolling is performed by the split method shown in FIG.

  In the steel making process (upstream side of the heating furnace in FIG. 4), the chemical components of the molten steel are adjusted and then cast to obtain a rectangular steel piece (also called “slab”). The casting is preferably continuous casting from the viewpoint of productivity. The thickness of the steel slab is preferably 200 mm or more from the viewpoint of productivity, and is preferably 350 mm or less in consideration of reduction of segregation, uniformity of heating temperature in hot rolling, and the like.

  Next, a steel slab is heated using a heating furnace and hot rolling is performed. Subsequently, rough rolling is performed by the split method shown in FIG. Thereafter, intermediate rolling is performed using an intermediate universal rolling mill (intermediate rolling mill) and a water cooling device. Subsequently, finish rolling is performed using a finish rolling mill and hot rolling is finished. At this time, the H-section steel may be water-cooled at a timing as required. Hereinafter, conditions and the like in each step will be described.

(Heating temperature of steel slab: 1100-1350 ° C.)
The heating temperature of a steel piece shall be 1100-1350 degreeC. When the heating temperature is low, the deformation resistance becomes high. On the other hand, when the heating temperature of the steel slab exceeds 1350 ° C., the oxide on the surface of the steel slab, which is the raw material, may melt and the inside of the heating furnace may be damaged. In order to sufficiently dissolve elements that form precipitates such as Nb, the lower limit of the heating temperature of the steel slab is preferably set to 1150 ° C. or higher. In particular, when the plate thickness of the product is thin, the cumulative rolling reduction increases, so it is preferable that the heating temperature of the steel slab is 1200 ° C or higher. In order to make the structure fine, it is preferable that the upper limit of the heating temperature of the steel slab is 1300 ° C. or less.

(Regulation of interrupt length H in rough rolling process)
In the rough rolling by the split method, the thickness T of the steel piece having a rectangular cross section and the width F of the flange of the rolled H-section steel formed by the finish rolling process are the predetermined hole tip angle (inside the hole mold) in FIG. The interrupt length H may be set so as to satisfy the interrupt length H based on the hole type of the peripheral protrusion tip angle) and the following equation (1).
H ≧ 0.5F−0.5T (1)

  As the above formula (1), the lower limit of the interrupt length H is 0.5F-0 with respect to the thickness T of the steel piece having a rectangular cross section and the width F of the flange of the rolled H-section steel formed by the finish rolling process. .5T or more. This is to suppress the amount of reduction in the obtuse hole type in which the central segregation part easily aggregates by performing rolling shaping by the split method until the flange width after rough rolling becomes equal to the flange width of the product. . The upper limit of the interrupt length H is not particularly set, but if it exceeds 0.8F-0.5T, excessive edging rolling is required at the time of intermediate rolling, and productivity is lowered, so 0.8F-0.5T or less is desirable.

(Projection tip angle in the hole type at the time of interruption)
The hole tip angle shown in FIGS. 1B and 5 (the protrusion tip angle of the inner periphery of the hole mold) may be an angle that is sufficiently acute to form an interrupt. For example, the upper limit is 40 °. It may be set to. This is because when the die tip angle exceeds 40 °, the center segregation portion of the slab is not dispersed by the flange and aggregates in the fillet portion as in the edging rolling shown in FIG. When the hole tip angle is set to 40 ° or less, as shown by the split method of FIG. 1 (b), the center segregation part is dispersed without being aggregated in the flange during rolling with the interrupt forming hole mold, It becomes possible to suppress a decrease in toughness.
There is no particular lower limit for the hole tip angle, but if it is less than 25 °, the roll may be broken during rolling, so 25 ° or more is preferable.
At this time, the center segregation portion of the slab is not divided into the left and right flanges in the I posture as shown in FIG.

For example, when a rolled H-section steel having a flange width of 150 mm or more is manufactured by the split method shown in FIG. 1B, the center segregation portion is dispersed in the flange portion, and from the vicinity of the center of the flange width in the flange to the flange width direction. It remains in an area of 15 mm or more toward one end face or both end faces and within 2 mm in the thickness direction in the flange surface layer (from the flange face located on the side opposite to the web in the flange thickness direction). When the rolled H-section steel is manufactured by the split method shown in FIG. 1B, the central segregation portion dispersed in the flange portion remains over a predetermined length in the region. The central segregation portion dispersed in the vicinity of the surface layer can be revealed by identification with the aforementioned nital corrosion solution.
The upper 5% average concentration of Mn at the central segregation part dispersed in the vicinity of the surface layer is defined as (CMn-surface), and the segregation degree (CMn-surface) / (CMn) at this position is 1.1 or more and 1.6 or less. It is desirable that In the split method, the segregation degree of the flange surface layer tends to be higher than in the edging method. When the degree of segregation is 1.1 or more, there is an advantage that surface cracks can be visually confirmed and inspection becomes easy, and it is also possible to trace a plurality of manufactured products as individual bodies based on surface cracks. Is possible. On the other hand, when the degree of segregation exceeds 1.6, a large number of cracks are easily formed on the flange surface. Therefore, the degree of segregation is desirably 1.1 or more and 1.6 or less. The method for obtaining the upper 5% average concentration in (CMn-surface) conforms to the method for obtaining the upper 5% average concentration in (CMn-max). That is, the method for obtaining the numerical value is basically the same except that the sampling position of the sample is different.

(Intermediate rolling process)
In the intermediate rolling process of hot rolling, controlled rolling by an intermediate universal rolling mill may be performed. Control rolling is a manufacturing method for controlling the rolling temperature and the rolling reduction. In the intermediate rolling of hot rolling, it is preferable to perform one pass or more of water-cooling rolling between passes. In the water-cooled rolling process between passes, water cooling is performed between rolling passes to give a temperature difference between the surface layer portion and the inside of the flange and roll. The inter-pass water-cooled rolling process is, for example, a manufacturing method in which the flange surface temperature is water-cooled to 700 ° C. or lower by water cooling between rolling passes and then rolled in the reheating process.

  When performing water-cooled rolling between passes, it is preferable to perform water cooling between rolling passes using water cooling devices provided before and after the intermediate universal rolling mill, and repeats spray cooling and reverse rolling of the flange outer surface by the water cooling device. Preferably it is done. In the inter-pass water-cooled rolling process, even when the rolling reduction is small, it is possible to introduce a processing strain to the inside of the plate thickness. Further, productivity is improved by lowering the rolling temperature in a short time by water cooling.

  In addition, after completion | finish of the hot rolling as an intermediate rolling process and a finishing rolling process, you may perform accelerated cooling to the inner surface and outer surface of a flange as it is with the water-cooling apparatus provided in the exit side of the finishing mill. The cooling rate of the inner and outer surfaces of the flange becomes uniform, and the material and shape accuracy can be improved. The upper surface of the upper surface of the web after the rough rolling process is cooled by cooling water sprayed on the inner surface of the flange. In order to suppress the warping of the web, cooling may be performed from the lower surface of the web.

  In the rolled H-section steel manufactured by the method for manufacturing the H-section steel according to the present embodiment described above, the central segregation portion existing in the slab before rolling shaping is dispersed without being aggregated in the fillet portion and rolled. Modeling can be completed. Specifically, a rolled H-section steel having a ΔvTrs of 40 ° C. or less is manufactured in the flange after the rolling shaping, and the segregation degree thereof is 1.6 or less (see FIG. 2).

  In such a rolled H-section steel, it is avoided that the central segregation portion aggregates in the fillet portion of the flange and adversely affects toughness and embrittlement characteristics. That is, the manufacture of H-shaped steel products having excellent toughness and embrittlement characteristics is realized. Moreover, the center segregation part dispersed in the flange is 15 mm or more from the center of the flange width toward one end surface or both end surfaces in the flange width direction, and from the surface located on the side opposite to the web in the flange thickness direction. However, it is presumed that there is almost no influence on toughness and embrittlement characteristics because it does not agglomerate. Furthermore, various inspections and experiments have been conventionally required to investigate the internal state of the flange, but in the H-section steel product according to the present embodiment, the flange surface located on the opposite side of the web is visually examined. be able to.

  As mentioned above, although an example of embodiment of this invention was demonstrated, this invention is not limited to the form of illustration. Various changes or modifications can be adopted within the scope of the idea described in the claims, and it is understood that these also belong to the technical scope of the present invention.

  As an example of the present invention, a sample was collected from a rolled H-section steel manufactured to satisfy the component composition and manufacturing conditions described in the above embodiment, and the sample was subjected to chemical analysis. On the other hand, as a comparative example, a sample was taken from a rolled H-section steel that did not satisfy any of the component composition and manufacturing conditions described in the above embodiment, and the same chemical analysis was performed. Hereinafter, comparison of detailed examples and comparative examples will be described.

(Example)
First, in Example No. Steel pieces having the composition (unit: mass%) shown in Table 1 were melted as 1 to 13 and 28, and steel pieces having a thickness of 250 to 300 mm were produced by continuous casting. The steel was melted in a converter, subjected to primary deoxidation, an alloy was added to adjust the components, and vacuum degassing was performed as necessary. And the obtained steel slab was hot-rolled on the manufacturing conditions shown in Table 2. In hot rolling, following rough rolling, using an intermediate universal rolling mill and a water cooling device provided before and after that, spray cooling of the flange outer surface, reverse rolling, and water cooling after rolling were performed as necessary. .

And from each position (refer FIG. 3) of the most brittle part and F / 6-t / 4, the test piece which makes a rolling direction a length direction was extract | collected, and the mechanical characteristic was measured. As mechanical properties, yield point (YP), tensile strength (TS), and vTrs were measured. The tensile test was conducted according to JIS Z 2241 (2011 edition), and the Charpy impact test was conducted according to JIS Z 2242 (2005 edition). In addition, a sample is taken from each position of the most brittle part and F / 6-t / 4, and a region within a 10 mm (longitudinal direction) × 10 mm (flange thickness direction) square in which the central segregation part is aggregated, EPMA (CMn-max) and (CMn) were measured and calculated by the methods described in JIS G0404 (2014 edition), respectively.
Further, center segregation remains within 2 mm of the surface layer over 15 mm or more from the center of the flange width toward at least one end face in the flange width direction, and the center segregation parallel to the flange thickness direction is performed as the Mn concentration of the surface layer portion. (CMn-surface) was measured and calculated by EPMA for a region 10 mm below the flange surface layer in the thickness direction (see FIG. 3).
The measurement and calculation results are shown in Table 3 below.

  In addition, the target value of each characteristic of the H-section steel to be manufactured is a normal temperature yield point (YP) or 0.2% proof stress of 335 MPa or more, a tensile strength (TS) of 450 MPa or more, and ΔvTrs of 40 ° C. or less.

  As shown in Table 3, the example No. In Nos. 1 to 13 and 28, the intensity at normal temperature is in the target range, and ΔvTrs satisfies the target value of 40 ° C. or less. Further, the segregation degree of Mn was 1.6 or less. The segregation degree of Mn is desirably 1.5 or less, and more desirably 1.4 or less.

(Comparative example)
Comparative Example No. Steel having the component composition shown in Table 4 was melted as 14 to 27, and steel pieces having a thickness of 250 to 300 mm were produced in the same manner as in the above examples. And the obtained steel slab was hot-rolled on the manufacturing conditions shown in Table 5.
In addition, the location which underlined in the following Table 4 and Table 5 is a location which does not satisfy | fill the component composition and manufacturing condition which concern on this invention demonstrated in the said embodiment.

And from the most brittle part and F / 6-t / 4 position (refer FIG. 3), the test piece which makes a rolling direction the length direction was extract | collected, and the mechanical characteristic was measured similarly to the said Example. As mechanical properties, yield point (YP), tensile strength (TS), and vTrs were measured. Further, samples were taken from the most brittle part, surface layer part, and F / 6-t / 4 positions, and (CMn-max) and (CMn-surface), JIS G0404 (EPM), as in the above example. (CMn) was measured and calculated by the method described in (2014 edition).
The measurement and calculation results are shown in Table 6 below. In Table 6 below, underlined portions are values that deviate from the target values of the characteristics of the H-section steel to be manufactured.

  As shown in Table 6, no. 14, 16 and 18 are insufficient in strength due to the small amounts of C, Mn and Si. No. 15 has a large amount of C. No. 17 has a large amount of Si, and vTrs at F / 6-t / 4 is 0 ° C. or higher due to the increase and coarsening of the hard phase, and the toughness is also lowered in the most brittle part. No. No. 19 has a large amount of Mn, vTrs at F / 6-t / 4 is 0 ° C. or more, the degree of central segregation deteriorates in the most brittle part, and the toughness deteriorates due to MnS and MA. No. No. 20 has a large amount of P. No. 21 has a large amount of S and has a low toughness. No. No. 22 has a rough rolling hole tip angle of more than 40 °, and the slab center segregation portion is aggregated without being dispersed, so that the toughness of the most brittle portion is lowered. No. In Nos. 23 and 24, the length of the interruption is insufficient, and the slab center segregation part is aggregated without being dispersed, so that the toughness of the most brittle part is lowered. No. No. 25 has a large amount of Nb. No. 26 has a large amount of Mo. No. 27 has a large amount of REM, and the toughness of the most brittle part is lowered.

  The present invention can be applied to a rolled H-section steel manufactured by hot rolling a steel slab and a manufacturing method thereof.

Claims (4)

% By mass
C: 0.01 to 0.25%
Si: 0.05% to 0.50%
Mn: 0.40 to 2.50%,
P: 0.050% or less,
S: 0.050% or less,
N: 0.020% or less,
Cu: 0.70% or less,
Ni: 0.70% or less,
Cr: 0.50% or less,
V: 0.12% or less,
Mo: 0.30% or less,
Nb: 0.08% or less,
Ti: 0.05% or less,
Al: 0.07% or less,
REM: 0.010% or less,
Ca: 0.0050% or less,
Balance: Fe and inevitable impurities,
A rolled H-section steel having a chemical composition of
The upper 5% average value of the Mn concentration at the most brittle part in the flange is 1/6 in the flange width direction from the end face in the flange width direction, and the flange thickness direction from the flange surface located on the opposite side of the web Less than 1.6 times the Mn concentration at the 1/4 position,
The upper 5% average value of the Mn concentration in the central segregation part is 15 mm or more from the center of the flange width toward one end face or both end faces in the flange width direction and within the flange surface layer within 2 mm in the thickness direction. 1.1 or more times the Mn concentration at a position 1/6 in the flange width direction from the end face in the flange width direction and at a position 1/4 in the flange thickness direction from the surface of the flange on the opposite side of the web. 6 times Ri der below,
The most brittle part in the flange is a part where a straight line indicating a position 3/4 in the flange thickness direction from the surface of the flange located on the opposite side of the web and a part where the central segregation part is aggregated,
The upper 5% average value of the Mn concentration is an average value of 12,500 points that identify a visual field of 10 mm × 10 mm, and among the 500 points × 500 points in the visual field, the value is higher than the upper 5%. Rolled H-section steel. A method for producing a rolled H-section steel according to claim 1, wherein the steel pieces having a rectangular cross section are heated to 1100 to 1350 ° C, followed by a rough rolling step, an intermediate rolling step, and a finish rolling step.
The rolling mill that performs the rough rolling step is provided with a plurality of three or more hole molds for shaping the material to be rolled,
At least one of the plurality of hole molds is an interrupt forming hole mold provided on a pair of upper and lower rolls on which protrusions for interrupting vertically with respect to the width direction of the material to be rolled are formed,
A method for producing a rolled H-section steel, characterized in that a shaping hole mold for sequentially bending the divided portions formed by the interruption formation hole mold is provided in a subsequent stage of the interruption formation hole mold. The method for manufacturing a rolled H-section steel according to claim 2, wherein a tip angle of the protrusion formed in the interrupt forming hole die is 40 ° or less. The length H of the interruption formed by the protrusion, the thickness T of the steel piece having the rectangular cross section, and the width F of the flange of the rolled H-section steel formed by the finish rolling process are expressed by the following formula (1). The method for producing a rolled H-section steel according to claim 2 or 3, wherein:
H ≧ 0.5F−0.5T (1)