JP2003129175A - High strength hot-rolled steel sheet for architectural metallic-fitting superior in workability and galvanizing property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive high-strength hot-rolled steel sheet having high strength and workability required for architectural metallic-fittings. SOLUTION: The high strength hot-rolled steel sheet for the architectural metallic-fittings superior in workability and galvanizing properties has a chemical composition comprising, by mass%, 0.15-0.30% C, 0.5% or less Si, 1.0-2.5% Mn, 0.025% or less P, 0.01% or less S, 0.5-1.5% Cr, 0.1-0.5% Mo, 0.0005-0.0050% B, 0.01-0.05% Ti, 0.01% or less N+O, 0.02-0.08% Al, and the balance Fe with unavoidable impurities, and an X value of 6.5 or more, which is defined by the expression (1): X=10×C+2.0×Mn+2.5×Cr+3.5×Mo+50×B, and has 10 μm or less of a controlled grain boundary oxidation depth from the steel sheet surface. The metal structure can have a bainite content of 70 vol.% or more, preferably 95 vol.% or more.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high strength for construction metal fittings having good workability and good galvanizability, which are used as materials for ceiling hangings, connecting fittings for beams, columns and floors of wooden or reinforced structure houses. The present invention relates to a hot rolled steel sheet.

[0002]

^2. Description of the Related Art Conventionally, building metal fittings have been generally used by processing 440N / mm ² grade ordinary steel and applying zinc plating.

Recently, high-strength metal fittings have been desired as a countermeasure against earthquakes, and particularly, heat-treated special steel products may be used for ceiling hangings and for connecting fittings for beams, columns and floors of wooden or reinforced structure houses. is there. For example, many ceiling hangings have claws having an interval that matches the pitch width of the corrugation of the deck, but it is particularly desired to increase the load bearing capacity in order to improve safety. Therefore, medium and low carbon special steels such as S45C and S55C are used to form metal fittings, which are then hardened and tempered to a hardness of about Hv400. Similar special steel heat-treated products may be used to obtain high-strength products such as beams, columns, and floor fittings.

[0004]

It is relatively easy to increase the load bearing capacity of a building metal fitting by using the high strength special steel as described above. However, special steel is expensive and
The cost increases because the strength is increased by heat treatment such as quenching and tempering.

On the other hand, a method of adopting an inexpensive high-strength hot-rolled steel sheet which does not require a special heat treatment can be considered. But,
There is a problem in workability and it is difficult to obtain a building fitting having a desired shape by simply using an existing high-strength hot-rolled steel sheet.
That is, in the manufacture of building metal fittings, since bending is usually performed after punching, it is required to have ductility capable of withstanding severe bending R. It is also desired to have excellent hole expandability. In the case of ordinary steel with a tensile strength of 440 N / mm ² grade, this is not a serious problem, but in the case of high-strength steel, it is inherently inferior in "local" ductility, so cracking may occur during bending or hole expansion. Easy to break.

Further, when the existing high-strength hot-rolled steel sheet is used, there is a concern that the galvanized layer may swell or peel off. Furthermore, it is feared that sufficient high strength cannot be maintained due to softening in the weld heat affected zone and softening due to heating during galvanization.

Therefore, the present invention is a high-strength hot-rolled steel sheet that is compatible with high strength and workability suitable for building metal fittings in the state of an unheat-treated steel sheet as hot-rolled, and has good galvanizability. It is an object of the present invention to provide an inexpensive element in which the content of an expensive element is suppressed as much as possible.

[0008]

Means for Solving the Problems The inventors have studied various methods for obtaining an inexpensive hot-rolled steel sheet having strength, workability, and galvanizability suitable for construction metal fittings. As a result, they have found that it is very effective to strictly control the chemical composition of the steel sheet and to suppress the grain boundary oxidation depth on the surface of the hot rolled steel sheet to a certain level or less. It was also found that the metallographic structure of the hot-rolled steel sheet has a great influence on the properties, and particularly when the bainite-based metallographic structure is used, the load bearing capacity can be significantly increased while maintaining the workability. The present invention has been completed based on these findings.

That is, the above-mentioned object is C: 0.
15 to 0.30%, Si: 0.5% or less, Mn: 1.0 to 2.5%, P:
0.025% or less, S: 0.01% or less, Cr: 0.5 to 1.5%, M
o: 0.1-0.5%, B: 0.0005-0.0050%, Ti: 0.01-0.
05%, N + O: 0.01% or less, Al: 0.02 to 0.08%, the balance being Fe and inevitable impurities, and having a chemical composition with an X value defined by the following formula (1) of 6.5 or more, This is achieved by a high-strength hot-rolled steel sheet for building metal fittings, in which the grain boundary oxidation depth from the steel sheet surface is suppressed to 10 μm or less and which has good workability and galvanizability. X = 10 × C + 2.0 × Mn + 2.5 × Cr + 3.5 × Mo + 50 × B ・ ・ (1)

Here, the content of each element expressed in mass% is substituted for the element symbol on the right side of the equation (1).

The above hot-rolled steel sheet having a metal structure containing 70% by volume or more of bainite, and
Provided is one having a notch tensile elongation Elv5 mm of 15% or more and a hardness of Hv250 or more and not subjected to heat treatment after hot rolling. Furthermore, it has a metallographic structure containing bainite of 95% by volume or more, and the notch tensile elongation Elv5mm is 15% or more and the tensile strength is
Provided is a high-strength hot-rolled steel sheet for building metal fittings having a hardness of 1200 N / mm ² or more and a hardness of Hv350 or more, which has not been subjected to heat treatment after hot rolling.

[0012] Here, "not subjected to heat treatment after hot rolling" means that after hot rolling is completed (in the case of winding,
It means that it has not undergone a history of "heating" or "sub-zero treatment" such that the metal structure changes after the coiling). Generally, a so-called "hot-rolled" steel sheet or a steel sheet subjected to only pickling after hot rolling corresponds to this. The normal cooling process after hot rolling is not included in the “heat treatment” here, even if the change in the metal structure is accompanied.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Items specifying the present invention will be described below. C is an important element for ensuring strength. The lower the C content, the better the ductility, but the strength of the hot-rolled steel sheet decreases, so the lower limit was made 0.15 mass%. On the other hand, if the C content exceeds 0.30 mass%, the bending workability deteriorates. Therefore, the C content is set to 0.15 to 0.30 mass%.

Si is a solid solution strengthening type element, and is an element effective in securing strength. However, if the Si content exceeds 0.5% by mass, firelight is likely to be generated during heating of the slab, and the descaling property during hot rolling tends to deteriorate. Scale remains on the surface of hot-rolled steel sheet, S
The presence of the i-enriched layer causes non-plating and swelling when galvanizing. Therefore, the upper limit of the Si content is 0.5% by mass. A more preferable Si content is 0.
It is 3% by mass.

Mn is indispensable for ensuring the strength, and it is necessary to contain at least 1.0 mass% or more. However, if the amount of Mn exceeds 2.5% by mass, the toughness deteriorates, the hardenability increases, and the workability deteriorates. Therefore, the Mn content is set to 1.0 to 2.5 mass%.

Since P deteriorates the workability, the content of P is limited to 0.025 mass% or less. A more preferable upper limit of the P content is 0.015%.

S produces MnS and becomes a non-metallic inclusion extending in the rolling direction in the hot-rolled steel sheet, deteriorating the workability. Particularly in the case of high-strength steel sheets, if the S content exceeds 0.01% by mass, the workability deteriorates significantly. Therefore, S
The content is regulated to 0.01% by mass or less. A more preferable upper limit of the S content is 0.005% by mass.

N forms TiN and has an effect of suppressing coarsening of crystal grains during welding. However, when a large amount of N is present, it is age-hardened by solid solution N and the workability is deteriorated.
O forms an oxide and serves as a starting point of destruction. Therefore, in the present invention, the total of N content and O content is 0.01% by mass.
Defined below.

Ti is an element which forms TiN and is effective for refining crystal grains and improving strength. In order to exert this effect, it is necessary to contain at least 0.01 mass% of Ti.
Ti is also effective in preventing softening of the heat-affected zone of welding, and is effective when it is necessary to weld the building fitting and other parts. However, when the amount of Ti exceeds 0.05 mass%, TiN becomes coarse, and this easily becomes a starting point of fracture, resulting in deterioration of workability. Therefore, the Ti content is set to 0.01 to 0.05 mass%.

When B is added in a very small amount, it lowers the strain energy at the crystal grain boundaries and improves workability and toughness. This effect is exhibited from 0.0005 mass%. Even if B is contained in an amount of more than 0.0050% by mass, the effect is saturated and the cost is increased. Therefore, the B content is set to 0.0005 to 0.0050 mass%.

Cr is an element effective for ensuring strength, and suppresses ferrite transformation during hot rolling to improve the stability of the material. Further, since the temper softening resistance is increased, softening of the welding heat affected zone is suppressed during welding with other parts. Further, softening due to heating during galvanization is also suppressed. If the Cr content is less than 0.5% by mass, these effects are not sufficiently exhibited. On the other hand, if it exceeds 1.5% by mass, the hardenability becomes excessive, and when welded to other parts, the weld becomes too hard and the workability deteriorates. Therefore, the Cr content is set to 0.5 to 1.5 mass%.

Mo, like Cr, is an element effective in securing strength, suppresses ferrite transformation during hot rolling to improve the stability of the material, and softens the welding heat affected zone, and Increases softening resistance during galvanization. In order to fully exert these effects, it is necessary to contain 0.1% by mass or more of Mo. However, if it exceeds 0.5%, the strength may excessively increase and the workability may deteriorate. Also, excessive M
Addition of o causes cost increase. Therefore, the Mo content is
It was set to 0.1 to 0.5 mass%.

Al is an element used as a deoxidizing agent for molten steel, and also acts to fix N. Such an effect becomes remarkable when the Al content is 0.02 mass% or more. But,
If the Al content in the steel exceeds 0.08 mass% and increases, the cleanliness of non-metallic inclusions is impaired, and flaws are likely to occur on the surface of the steel sheet. Therefore, the Al content is set to 0.02 to 0.08 mass%.

The X value is the content of C, Mn, Cr, Mo and B, the hardness of the hot rolled steel sheet, the susceptibility of ferrite transformation during hot rolling, the softening resistance of the weld heat affected zone, and the zinc plating. It is an index that defines the relationship with the softening resistance at time and is defined by equation (1). X = 10 × C + 2.0 × Mn + 2.5 × Cr + 3.5 × Mo + 50 × B ··· (1) As a result of the research conducted by the inventors, even if the content of each element is within the specified range, the X value A value of less than 6.5 has been found to lead to the following situations. The hardness of the hot-rolled steel sheet is easily lowered to less than Hv250. The effect of suppressing the ferrite transformation during hot rolling becomes small, and the material stability of the hot rolled steel sheet deteriorates. When welding with other parts, the heat affected zone may soften. The material may soften due to heating during galvanization. Therefore, in the present invention, it is important to control the content of each element so that the X value becomes 6.5 or more.

The grain boundary oxidation depth on the surface of the hot-rolled steel sheet is one of the surface properties that has a significant effect on the workability, galvanizability, and the load bearing capacity of the building metal fittings. In the conventional metal fittings of general strength level that have been widely used, that is, the metal fittings processed from ordinary steel having a tensile strength of 440 N / mm ² , the grain boundary oxidation depth on the surface of hot-rolled steel sheet is Does not affect load bearing capacity. However, since the steel targeted by the present invention is a high-strength steel, microscopic defects in the grain boundary oxidized portion cause a local decrease in ductility, which becomes a starting point of cracks during bending and hole expanding.

As a result of various investigations, when the grain boundary oxidation depth on the surface of the hot-rolled steel sheet is suppressed to 10 μm or less, adverse effects on bending workability, hole expandability, load resistance, etc. can be significantly suppressed. all right. It was also confirmed that in this case, the problem of swelling or peeling of the zinc plating layer due to the scale remaining in the grain boundary oxidized portion was solved. Therefore, in the present invention, the grain boundary oxidation depth on the surface of the hot rolled steel sheet is specified to be 10 μm or less.
The grain boundary oxidation depth can be examined by observing the cross section of the steel sheet with a microscope.

In order to reduce the grain boundary oxidation depth of the hot rolled steel sheet, the Si content of the steel is lowered, and the descaling is strengthened by increasing the number of times of descaling or increasing the water spraying pressure during hot rolling. In a hot rolling line, measures such as shortening the run-out table exposure time, lowering the coiling temperature, and increasing the coil cooling rate after coiling are effective.

The general metal structure of ordinary steel is ferrite +
It is perlite. Since ordinary steel has low strength and high elongation, there is no problem in workability even if it has a ferrite + pearlite structure. In the case of high strength steel, the metal structure is ferrite +
With pearlite, cracking may occur at the boundary between ferrite and pearlite due to local processing strain when subjected to strong processing. In the high-strength hot-rolled steel sheet of the present invention, workability is basically improved by strictly observing the chemical composition and the grain boundary oxidation depth described above. Therefore, the metal structure may be a ferrite + pearlite structure.

However, if it is desired to further improve the strength, it is desirable that the metal structure be a structure mainly composed of bainite. Specifically, it is desirable to have a structure state in which bainite occupies 70% by volume or more. In particular, when the bainite content is 95% by volume or more, the tensile strength is 1200 N / mm ² or more and the hardness is Hv 350 or more, which is a remarkable improvement in strength, which is achieved in the unheat-treated steel sheet as hot-rolled. Moreover, the notch tensile elongation Elv5mm shows 15% or more, and it has excellent workability, so it can be sufficiently molded into building metal fittings. Here, in the structure mainly composed of bainite, it is desirable that the balance of the bainite is substantially pearlite.

The metal structure mainly composed of bainite can be obtained by adjusting the chemical composition and controlling the hot rolling conditions. That is, to add a large amount of Cr or Mo among the component elements, to adjust the X value to a high value, to increase the cooling rate in hot rolling, and to lower the hot rolling coiling temperature are It is effective in suppressing ferrite transformation and causing bainite transformation.

The notch tensile elongation is an index for evaluating the local ductility of the material. According to this index, the decrease in ductility due to the presence of the grain boundary oxide layer can also be evaluated. As a result of detailed investigation of the correlation between the degree of cracking and the test value of the notch tensile elongation when forming a high strength hot rolled steel sheet into a building metal fitting, when the "notch tensile elongation Elv5mm" is less than 15%, the bending occurs. Cracks due to the grain boundary oxide layer were sometimes generated during processing. Therefore, the notch tensile elongation Elv5mm is preferably 15% or more.

Here, "notch tensile elongation Elv 5 mm" means
The local elongation at a gauge length of 5 mm measured by a notch tensile test. Specifically, a JIS No. 5 tensile test piece with V notches with a depth of 2 mm on both sides in the width direction near the center of the parallel part, and a test piece with a 5 mm gap to include the V notch positions between the gauge points. Is prepared and a tensile test is performed using this, the elongation rate measured by the reference point.

Further, the hardness of the hot rolled steel sheet is preferably Hv250 or more. If the Hv is less than 250, the improvement of load resistance is small and the weight saving effect is small as compared with a metal fitting made of ordinary steel.

[0034]

[Examples] Table 1 shows the composition and X value of the test steel.
Steels A to D are steels of the present invention, and steels E to I are comparative steels. I
Steel is ordinary steel with tensile strength of 440 N / mm ² grade, and J steel is special steel S45C.
Is.

[0035]

[Table 1]

The slabs of A to I steels were hot-rolled under the following conditions to obtain a coil having a plate thickness of 2.0 mm.・ Slab heating temperature: 1250 ° C or higher ・ Total rolling ratio: 90% or higher ・ Finishing hot rolling outlet temperature: 800 to 850 ° C ・ Coiling temperature: about 500 ° C, B and C steels are 500 to 650 ° C, followed by continuous pickling And provided for the test. No heat treatment was applied to the hot rolled steel sheet. J steel (S45C) is made high by heating a hot-rolled steel sheet with a thickness of 2.0 mm hot-rolled by a normal method after heating at 900 ° C for 10 min, water quenching, and then tempering at 370 ° C for 30 min. It was strengthened and used for the test.

For each steel sheet, tensile strength TS, notch tensile elongation Elv 5 mm, hardness, bainite area ratio, grain boundary oxidation depth,
The limit bending radius, load resistance, and galvanizability were investigated. The tensile strength was determined by performing a tensile test in a direction perpendicular to the rolling direction using a JIS No. 5 test piece. Goods of 780 N / mm ² or more were considered good. The notch tensile elongation Elv5 mm was obtained by performing a tensile test in the direction perpendicular to the rolling direction by the method described above. Those with 15% or more were considered good. The hardness is 20kg for the L section of the hot rolled steel sheet using a Vickers hardness tester.
It was measured at. Those with Hv250 or more were considered good. The bainite area ratio and the grain boundary oxidation depth were determined from an image of a steel plate cross section observed with an optical microscope. Grain boundary oxidation depths of 10 μm or less were considered good.

The limit bending radius is 90 ° V for a test piece having a width of 35 mm and a length of 100 mm (longitudinal direction is perpendicular to the rolling direction).
Obtained by performing block bending. The limit bending radius at which cracking did not occur was determined, and this was taken as the limiting bending radius. Those with a thickness of 4.0 mm or more were considered good. For load bearing, a test piece with a width of 10 mm and a length of 40 mm (longitudinal direction is perpendicular to the rolling direction) was cut using a galvanized steel sheet, and the distance between fulcrums was 20
mm, punch radius R6 mm. The load when the test piece cracked was measured, and this was taken as the withstand load. Those of 2000N or more were considered good. The galvanizability was investigated by immersing the steel sheet in a galvanizing bath at 500 ° C. for 90 seconds, pulling it up, lowering it to room temperature, and visually observing the surface. A product having no gloss and no swelling and having excellent gloss was regarded as good. The results are shown in Table 2.

[0039]

[Table 2]

The invention examples (No. 1, 2, 3, 7, 8) in which the chemical composition of the hot rolled steel sheet is within the range specified by the present invention and the grain boundary oxidation depth of the steel sheet surface is suppressed to 10 μm or less, Load capacity of 20
High strength of over 00N is obtained and workability is limited to a bending radius of 4.0.
It was as good as mm or less. In particular, No. 1 (A steel) has bainite with a metal structure of 95% by volume or more, a tensile strength of 1200 N / mm ² or more, a high strength of hardness Hv 350 or more, and a notch tensile elongation Elv 5 mm. High workability of 15% or more.

On the other hand, the comparative examples are as follows. No. 4 (Steel B) has a chemical composition within the range specified by the present invention, but the hot rolling coiling temperature was raised to 650 ° C. to intentionally increase the grain boundary oxidation depth on the surface of the hot rolled steel sheet. . This was inferior in bending workability, and non-plating occurred even in galvanizing. No.5
(B steel) is a material in which some martensite is intentionally generated, and the bending workability is also poor. No. 6 (C steel) has a chemical composition within the specified range of the present invention, but has a hot rolling coiling temperature of 620.
The hardness is lowered by intentionally increasing the grain boundary oxidation depth by increasing the temperature to ℃ to generate ferrite and lowering the hardness. Further, since the grain boundary oxidation depth was large, swelling occurred in galvanization.

No. 9 (E steel) has a low Cr and Mo content, so it softens after plating and has a low withstand load. No.10
In the (F steel), the content of each element was within the specified range, but the X value was out of the specified range and was low. this is,
Low hardness and withstand load. No. 11 (G steel) has a high S content and thus is inferior in bending workability. Further, since Si was high, non-plating occurred in zinc plating.

No. 12 (H steel) has a low C content, so that ferrite and pearlite are separated from each other, and since Mn is high, it has a layered non-uniform structure. It becomes low and bending workability is poor. No.13 (I
Steel) is a normal steel with a tensile strength of 440 N / mm ² grade, so it has good workability but low withstand load. No.14 (J steel) is a hardened / tempered material of S45C, which has a high strength level but poor workability. Therefore, it is necessary to perform heat treatment such as quenching and tempering after processing the parts.

[0044]

According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having high strength and workability required for building metal fittings in a state of not being heat-treated as hot-rolled. The steel sheet of the present invention has the following merits. Compared with the conventional 440 N / mm ² class ordinary steel, the load capacity is significantly higher. Since the wall thickness can be reduced by increasing the strength, the weight of the metal fitting can be reduced. Since the amount of alloying elements is smaller than that of conventional special steel, raw material cost is reduced. Since the quenching and tempering treatments required for conventional special steels are not required, the productivity of metal fittings is improved and the manufacturing cost is reduced. High resistance to softening during heating during welding and galvanizing. Good galvanizability. The present invention can be easily implemented using a general production line for hot rolled steel sheets.

Claims (4)

[Claims] 1. C: 0.15 to 0.30% and Si: 0.5 by mass%
% Or less, Mn: 1.0 to 2.5%, P: 0.025% or less, S: 0.01
% Or less, Cr: 0.5 to 1.5%, Mo: 0.1 to 0.5%, B: 0.00
05 to 0.0050%, Ti: 0.01 to 0.05%, N + O: 0.01% or less, Al: 0.02 to 0.08%, the balance being Fe and inevitable impurities, and the X value defined by the following formula (1) is 6.5. With the above chemical composition, the grain boundary oxidation depth from the steel sheet surface is
High-strength hot-rolled steel sheet for construction metal fittings with good workability and galvanizability that is suppressed to 10 μm or less. X = 10 × C + 2.0 × Mn + 2.5 × Cr + 3.5 × Mo + 50 × B ・ ・ (1) 2. The high-strength hot-rolled steel sheet for building metal fittings according to claim 1, which has a metallographic structure containing 70% by volume or more of bainite. 3. A construction having good workability and galvanizability according to claim 1 or 2, which has a notch tensile elongation Elv5 mm of 15% or more and a hardness of Hv250 or more and has not been subjected to heat treatment after hot rolling. High strength hot rolled steel plate for metal fittings. 4. A hot rolled sheet having a metal structure containing 95% by volume or more of bainite, a notch tensile elongation Elv 5 mm of 15% or more, a tensile strength of 1200 N / mm ² or more and a hardness of Hv 350 or more. The high-strength hot-rolled steel sheet for building metal fittings according to claim 1, which has not been subjected to a heat treatment afterwards.