JPH06145786A - Production of wide flange shape minimal in difference in mechancal property in plate thickness direction - Google Patents

Abstract

PURPOSE:To produce a thick wide flange shape having uniform mechanical properties in a plate-thickness direction by properly controlling temp. hysteresis in the part of specific thickness from the surface part at the time of producing the thick wide flange shape. CONSTITUTION:A bloom of a steel which has a composition containing, by weight, 0.03-0.20% C, 0.05-0.50% Si, 0.5-2.0% Mn, 0.005-0.050% Al, and one or >=2 kinds among 0.005-0.5% Nb, 0.02-0.15% V, and 0.005-0.10% Ti or further containing one or >=2 kinds among <1.5% Ni, <1.5% Co, <1.0% 1.0% Cr, <0.5% Mo, <0.1% REM, and <0.01% Ca is heated up to 1200-1350 deg.C and hot-rolled at =10% cumulative draft at a finishing temp. between the Ar3 point and 950 deg.C, by which a thick wide flange shape of >=40mm thickness is formed. Subsequently, the part between thickness (1/4) (t) at an inner site 1/4 the thickness from the surface and the surface part is cooled from a temp. not lower than the Ar3 point at (1 to 25) deg.C/sec cooling rate, and cooling is stopped at the point of time when the temp. at the part of (1/4)(t) of plate thickness becomes a temp. not lower than the Ac1 point. By this method, the thick wide flange shape which is homegeneous in a thickness direction can be produced by a simplified process while obviating the necessity of hardening and tempering.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an H-section steel, and more particularly to a method for producing a thick H-section steel having a small difference in mechanical properties in the plate thickness direction.

[0002]

2. Description of the Related Art High-rise buildings, marine structures, etc. are becoming larger and larger, and steel materials used for large structures are becoming thicker. The steel materials used are required to have excellent strength, toughness, weldability, etc., but at the same time, thick steel materials are required to have small characteristic changes in the plate thickness direction.

When such a steel material is manufactured by quenching and tempering, even if the characteristic nonuniformity in the plate thickness direction occurs due to quenching, it will be uniformized by tempering, so there will be no problem. However, since quenching and tempering treatments are expensive, steel materials that can be used as they are rolled and cooled are required, and it is necessary to improve the quality of H-section steels.

As a means for omitting the quenching and tempering processes, a method of forcibly cooling immediately after the end of hot rolling is used (for example, JP-A-2-19422). However, this method has a problem in that even a steel sheet having a simple shape inevitably has nonuniform mechanical properties in the plate thickness direction.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing an H-section steel having a small difference in mechanical properties in the plate thickness direction, particularly an H-section steel having a wall thickness of 40 mm or more. .

[0006]

The present inventors have made various studies on cooling conditions after hot rolling in order to solve the above problems. As a result, when the hardness is non-uniform in the thickness direction, the vicinity of the wall thickness (1/4) t is due to the large increase in the hardness of the surface, that is, the wall thickness (1/4) t. We have found that controlling the increase in surface hardness is important for making the hardness distribution uniform in the thickness direction.

It is effective to control the temperature history of the (1/4) t portion, and further control of processing, that is, 950 ° C.
The present invention has been completed based on the finding that uniformity can be further improved by applying a cumulative reduction of 10% or more in a temperature range of Ar ₃ or higher. The present invention contains C: 0.03 to 0.20 wt% Si: 0.05 to 0.50 wt% Mn: 0.5 to 2.0 wt% Al: 0.005 to 0.050 wt% And Nb: 0.005 to 0.05% by weight V: 0.02 to 0.15% by weight Ti: 0.005 to 0.10% by weight, and the balance contains Fe and After heating a steel slab composed of unavoidable impurities, hot rolling is completed by applying a cumulative reduction of 10% or more in a temperature range of 950 ° C. or less and an Ar ₃ point or more, and thereafter, from the Ar ₃ point or more, a sheet thickness ( 1 /
4) Cool the t part at a cooling temperature of 1 to 25 ° C./s, and stop the cooling when the temperature of the plate thickness (1/4) t part is Ac ₁ point or more,
It is a method for producing an H-section steel having a small difference in mechanical properties in the plate thickness direction, which is characterized by allowing to cool. In this method, Ni: 1.5 wt% or less Cu: 1.5 wt% or less Cr: 1 wt% or less Mo: 0.5 wt% or less REM: 0.1 wt% or less Ca: 0.01 It is more preferable to contain one kind or two or more kinds by weight% or less.

[0008]

The reason for limiting the present invention will be described below. The limitation of the composition of steel will be described. C: 0.03 to 0.20 wt% C needs to be added in an amount of 0.03 wt% or more in order to secure the strength required as structural steel, but considering deterioration of toughness and weldability. Then, the upper limit is set to 0.20% by weight.

Si: 0.05 to 0.50 wt% Si is effective in promoting deoxidation and increasing strength, and is added in an amount of 0.05 wt% or more, but if it exceeds 0.50 wt%, HAZ is added.
The upper limit was made 0.50% by weight because it deteriorates the toughness of the part. Mn: 0.5 to 2.0 wt% Mn is effective for increasing strength and ductility, and 0.5 wt% or more is added. However, if it exceeds 2.0% by weight, the weld hardenability is significantly increased, so the upper limit is 2.0% by weight.
And

Al: 0.005 to 0.050 wt% Al is added in an amount of 0.005 wt% or more for deoxidizing steel. However, if the addition amount exceeds 0.050% by weight, the toughness of the welded portion is significantly deteriorated, so the upper limit is made 0.050% by weight. Nb: 0.005-0.05% by weight, V: 0.02-
0.15% by weight, Ti: 0.005 to 0.10% by weight Nb, V, and Ti are all precipitation hardening type elements and increase the strength without adding a large amount of alloying elements. Also,
In hot rolling, a non-recrystallized region is expanded to introduce a deformation zone into austenite to reduce ferrite grains after transformation and improve toughness. For that purpose, Nb, V, Ti
Are 0.005% by weight, 0.02% by weight, and 0.
005 wt% or more is required, but 0.05 wt%,
When added in excess of 0.15 wt% and 0.10 wt%,
Since the effect is saturated and the toughness of the welded portion is deteriorated, Nb, V, and Ti are each Nb: 0.005.
~ 0.05 wt%, V: 0.02-0.15 wt%, T
i: 0.005 to 0.10% by weight. Nb, V, T
The effect of i can be obtained by adding one or two or more.

For higher tensile strength or other purposes, one or more selected from Ni, Cu, Cr, Mo, REM and Ca can be added.
Even if these elements are added, the characteristics of the present invention are not lost, and the addition of the above elements achieves high tensile strength or the following effects. The purpose of adding the above components and the reason for limiting the addition amount will be described.

Ni: Ni is added in order to improve the strength and toughness of steel without adversely affecting the hardenability and toughness of the weld heat affected zone, but its upper limit is 1.5 from the viewpoint of cost. Weight% Cu: Cu improves the corrosion resistance in addition to having the same effect as Ni, but if added in an amount exceeding 1.5% by weight, hot brittleness is likely to occur, so the upper limit is made 1.5% by weight.

Cr, Mo: Cr and Mo can be expected to enhance the strength of the base material and improve the low temperature toughness of the base material by improving the hardenability and precipitation hardening. However, excessive addition exceeding the upper limit of each component is extremely harmful from the viewpoint of HAZ toughness and curability, so the upper limits of Cr and Mo were made 1.0 wt% and 0.5 wt%, respectively. Ca, REM: Ca and REM are MnS morphology control and LC
Is effective in improving the toughness in the direction, and REM oxysulfide of REM is effective in improving the toughness of the welded part. However, Ca exceeds 0.01% by weight and R exceeds 0.1% by weight.
The addition of EM deteriorates the cleanliness of the steel and causes internal defects, so the upper limits were made 0.01% by weight and 0.1% by weight, respectively.

Next, the limiting conditions of rolling-cooling conditions will be described. The steel material having the above composition range is heated for hot rolling, but the temperature is within the range for normal H-section steel production (1
200 to 1350 ° C.), and hot rolling is performed after heating. In the present invention, the rolling finishing temperature is set to 950 ° C. or lower and Ar ₃ point or higher, and a cumulative rolling reduction of 10% or higher is required. This is because when the finishing temperature exceeds 950 ° C., the crystal grains become coarse and the toughness deteriorates.

However, when the rolling finishing temperature is lower than the Ar ₃ point, the precipitated ferrite is subjected to work strain due to rolling, resulting in deterioration of toughness and ductility and an increase in yield ratio. Therefore, the lower limit of the finish rolling temperature was ₃ points Ar. Further, the cumulative rolling reduction in this temperature range needs to be 10% or more. This is because if it is less than 10%, a sufficient fine grain structure cannot be obtained.

After the above rolling, accelerated cooling is performed.₃ Point
Cool from above temperature. Cooling rate is (1/4) t part
At a cooling rate of 1 to 25 ° C / s, Ac₁ Above the point
Stop cooling. Ar₃ Accelerated cooling from below the point
Ar is not effective enough ₃ Open accelerated cooling from above the point
Need to start. The accelerated cooling rate is the plate thickness (1 /
4) When the cooling rate at the t-part is less than 1 ° C, the effect of accelerated cooling is not obtained.
In the case of thick and thick H-section steel, cooling exceeding 25 ° C / s
Higher speed requires higher equipment cost, so higher than 25 ℃ / s
It was limited.

The cooling stop temperature is an important regulation condition of the present invention, and the cooling stop temperature at the plate thickness (1/4) t portion is set to Ac ₁ point or higher. By doing so, the difference in strength between the surface portion and the central portion can be eliminated. In the vicinity of plate thickness (1/4) t
The strength increase in the portion of the surface is cooled stopped at by 400 ° C. or less subcooling addition to the cooling rate difference plate thickness (1/4) temperature of t section Ac ₁ point or more, heating condensate for more than _a point Ac, which As a result, it is possible to greatly improve the difference in strength between the wall thickness portions. Temperature distribution in the thickness direction in the two accelerated cooling, by the internal temperature and high temperature of at least ₁ point Ac, recuperation temperature of the cooling after the end of the surface is more than _one point Ac.

[0018]

EXAMPLE 13 beam blanks whose chemical compositions are shown in Table 1
It was heated to 00 ° C., and 500 × 592 × 50 × 70 mm H-section steel was obtained under various rolling conditions and cooling conditions shown in Table 2.
The JIS No. 4 tensile test piece and the JIS No. 4 impact test piece were sampled from the area 8 mm below the surface and the area (1/2) t of the flange width 1/2 of the H-section steel thus obtained, and the mechanical properties were investigated. did. The results are shown in Table 3.

The steels A to E in Table 1 are examples of the invention steels, and the F steels are comparative steels. As shown in Table 3, in the invention examples, the difference in strength and toughness depending on the plate thickness position is small. However, in Comparative Example No.
In No. 2, since the cooling rate is slow, both strength and toughness are low. No.
4 and No. In _No. 8, since the recuperation temperature after cooling is less than Ac ₁ point, the strength of the surface layer portion becomes too high. No. 6 is 950
The toughness is low due to insufficient cumulative reduction below ℃. No. In No. 10, since the rolling end temperature was at or below the Ar ₃ point, the strength increased and the toughness decreased. No. Since No. 11 has a chemical composition outside the scope of the present invention, both strength and toughness are low.

[0020]

EFFECTS OF THE INVENTION According to the present invention, the wall thickness H is suitable as a steel material for large-scale structures and has a small difference in mechanical properties in the plate thickness direction.
Shaped steel can be obtained.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

Claims (2)

[Claims] 1. C: 0.03 to 0.20% by weight Si: 0.05 to 0.50% by weight Mn: 0.5 to 2.0% by weight Al: 0.005 to 0.050% by weight In addition to containing one or more of Nb: 0.005 to 0.05 wt% V: 0.02 to 0.15 wt% Ti: 0.005 to 0.10 wt%, the balance is After heating a steel slab composed of Fe and unavoidable impurities, hot rolling is completed by applying a cumulative reduction of 10% or more in a temperature range of 950 ° C. or less and Ar ₃ or more, and then, from Ar ₃ or more, the plate thickness is increased. Of (1 /
4) Cool the t part at a cooling temperature of 1 to 25 ° C./s, and stop the cooling when the temperature of the plate thickness (1/4) t part is Ac ₁ point or more,
A method for producing an H-section steel having a small difference in mechanical properties in the plate thickness direction, characterized by allowing to cool. 2. Ni: 1.5 wt% or less Cu: 1.5 wt% or less Cr: 1 wt% or less Mo: 0.5 wt% or less REM: 0.1 wt% or less Ca: 0 The method for producing an H-section steel having a small difference in mechanical properties in the sheet thickness direction according to claim 1, wherein the H-section steel contains 0.1% by weight or less of one type or two or more types.