JP2607796B2 - Method for producing low alloy rolled section steel with excellent toughness - Google Patents

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 a low alloy rolled section steel having excellent toughness and used as a structural member of a building.

[0002]

2. Description of the Related Art Due to the increase in height of buildings and stricter safety standards, steel materials used for pillars, for example, H-shaped steel having a large plate thickness (hereinafter, referred to as extra-thick H-shaped steel) are being used. Further, higher strength, higher toughness and lower yield ratio are required. In order to satisfy such required characteristics, conventionally, a heat treatment such as a normalizing process has been performed after the completion of rolling. The addition of heat treatment causes a significant increase in cost, such as a decrease in heat treatment cost and production efficiency, and has a problem in economy. To solve this problem, it was necessary to develop a new alloy design and manufacturing method so that high-performance material properties could be obtained as-rolled.

Generally, a section steel having a flange, for example, H
When a section steel is manufactured by universal rolling, the web, flange,
Differences occur in the rolling finish temperature, rolling reduction, and cooling rate at each part of the fillet. As a result, variations in strength, ductility, and toughness occur between the portions, and for example, a rolled steel material for a welding structure (JI
There are sites that do not meet the criteria such as SG3106). In particular, when rolling an extremely thick H-section steel as a continuous cast slab as a raw material, the maximum thickness of the slab that can be manufactured by the continuous casting facility is limited, so that the reduction ratio is low. Further, the flange portion having a large thickness is subjected to high-temperature rolling due to the restriction of dimensional accuracy in the rolling molding, and the steel material after the rolling is gradually cooled, and the microstructure is coarsened.

[0004] A fine graining method using TMCP is also well known.
Due to the limitations of modeling, rolling of steel bars does not allow a large reduction in the manufacturing method of steel sheets. In the field of thick steel plates, high strength and high toughness steels are produced by utilizing the precipitation effect of VN.
No. 0548 and Japanese Patent Publication No. Sho 62-54862 have been proposed. However, in this conventional method, since the deoxidation of molten steel is performed by a general Al deoxidation treatment, fine composite oxides that are effective in refining the structure are not formed as intragranular ferrite generation nuclei, and Was not sufficiently refined. That is, the conventional Al deoxidation is performed at the initial stage of the melting process.
The deoxidation of molten steel and the generated Al ₂ O ₃ were floated and separated for high purification. That is, conventionally, emphasis has been placed on how to reduce the oxygen concentration of molten steel and the number of primary deoxidized oxides in steel.

[0005]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a novel manufacturing method which comprehensively covers steelmaking, rolling and cooling after rolling. To provide a low-alloy rolled section steel with excellent strength and toughness, that is, a means for increasing the number of ferrite nucleation sites in the manufacturing process, suppressing the growth of nucleated ferrite, and reducing the grain size. The purpose is to provide.

[0006]

The gist of the present invention is that C: 0.04 to 0.20% by weight and Si: 0.0% by weight.
5 to 0.50%, Mn: 0.5 to 1.8%, N: 0.0
The molten steel containing 0.3 to 0.012% and Al ≦ 0.005%, the balance being Fe and unavoidable impurities is dissolved in a pre-deoxidation treatment to reduce dissolved oxygen by weight to 0.003 to 0.015%.
%, The titanium content is further deoxidized, the titanium content is 0.005 to 0.025% by weight, and -0.006 ≦ [Ti%] − based on the dissolved oxygen [O%] of the molten steel. 2 [O
%] ≦ 0.008, and after reheating the slab to a temperature range of 1100 to 1300 ° C., rolling is started, and the temperature of the surface layer portion of the steel material is changed to Ar between passes in the intermediate rolling process.
₃ water cooling to -20 ° C. or less · Ar ₃ -100 ° C. or more, then rolling at least once in its recuperation process, toughness cooled to 600-400 ° C. at a cooling rate of 1 to 30 ° C. / S after the final finish rolling Method of producing low alloy rolled section steel with excellent C: 0.04 to 0.20% by weight, Si: 0.0% by weight
5 to 0.50%, Mn: 0.5 to 1.8%, N: 0.0
03-0.012%, Al ≦ 0.005%, V ≦ 0.20%, Cr ≦ 0.7%, Mo ≦ 0.3%,
Nb ≦ 0.05%, Ni ≦ 1.0%, Cu ≦ 1.0%,
The molten steel containing one or more of the following, the balance being Fe and unavoidable impurities, is adjusted to 0.003 to 0.015% by weight of dissolved oxygen by pre-deoxidation treatment.
Titanium is deoxidized, and the titanium content is 0.005% by weight.
0.025% and relative to the dissolved oxygen [O%] of the molten steel
Cast into slabs satisfying the relationship of 0.006 ≦ [Ti%] − 2 [O%] ≦ 0.008, and cast the slabs from 1100 to 1300
Rolling is started after reheating to a temperature range of ° C., and the temperature of the surface layer portion of the steel material is Ar ₃ -20 ° C.
₃ Low-rolled rolled alloy with excellent toughness that is water-cooled to -100 ° C or more, rolled at least once in the recuperation process, and cooled to 600 to 400 ° C at a cooling rate of 1 to 30 ° C / S after final finish rolling In the method of manufacturing steel.

According to the present invention, in order to refine the structure, an appropriate deoxidation treatment is performed in the steel making process, and the molten steel is highly purified.
By controlling the dissolved oxygen concentration and finally deoxidizing titanium, a large number of fine composite oxides that act as intragranular ferrite formation nuclei are dispersed in the steel, and further cooled with water between hot rolling passes to form the surface layer of the steel sheet. Gives a temperature difference between the inside and the inside, and even under low pressure conditions, enhances the penetration under pressure into the higher temperature inner layer, and introduces processing dislocations that become intragranular ferrite formation nuclei,
This is to increase the number of intragranular ferrite nuclei. In addition, by controlling the γ / α transformation temperature range after rolling, the growth of the nucleated ferrite can be suppressed, the microstructure can be refined, and high efficiency and low production cost can be achieved. It enables the production of steel.

[0008]

Hereinafter, the operation of the present invention will be described in detail based on embodiments. Higher strength of steel is achieved by finer ferrite crystals, solid solution strengthening by alloying elements, dispersion strengthening by hardened phases, and precipitation strengthening by fine precipitates. In addition, toughness can be improved by refining the crystal and dissolving N, C in the mother phase (ferrite).
This is achieved by reducing and miniaturizing high carbon martensite and coarse oxides / precipitates in the hardened phase, which is a starting point of fracture.

In general, toughness is reduced by increasing the strength of steel, and it is necessary to contradict high strength and toughness.
The only metallurgical factor that satisfies both at the same time is grain refinement.

The feature of the present invention is that in the steelmaking process, the control of the amount of dissolved oxygen in molten steel and the proper deoxidation treatment with Ti, a weak deoxidizing element, without performing deoxidation with the strongly deoxidizing elements Al, Ca, etc. In the hot rolling process, the process of dispersing a large number of fine Ti-based oxides in the steel and repeating the process of water cooling between hot rolling passes and rolling at the time of recuperation are repeated. In addition, the cooling after rolling is controlled, the growth of ferrite is suppressed, the microstructure is refined, and high strength and high toughness of the base material are achieved as rolled.

Next, the reasons for limiting the range of the basic components of the shaped steel according to the present invention will be described. First, C is added as an effective component for improving the strength of steel. If it is less than 0.04%, the strength required for structural steel cannot be obtained, and if it exceeds 0.20%, excessive addition of Since the base material toughness, weld cracking resistance, toughness of the weld heat affected zone, etc. are significantly reduced, the lower limit is set to 0.04.
% And the upper limit was set to 0.20%.

Next, Si is necessary for securing the strength of the base material, pre-deoxidizing the molten steel, and the like.
Generates high-carbon martensite having a hardened structure in the structure, and significantly reduces the toughness of the welded joint. If the content is less than 0.05%, the necessary preliminary deoxidation of molten steel cannot be performed, so the Si content is limited to the range of 0.05 to 0.50%.

Mn is 0.5 to secure the strength and toughness of the base material.
% Or more is necessary, but the upper limit is set to 1.8% within an allowable range such as toughness and cracking of the welded portion.

N is an extremely important element for the precipitation of TiN and VN. If the content is less than 0.003%, the amount of TiN and VN is insufficient, and the precipitation strengthening and the sufficient amount of intragranular ferrite structure can be obtained. Therefore, it was made 0.003% or more. When the content exceeds 0.012%, the toughness of the base material is reduced, and surface cracks of the steel slab during continuous casting are caused, so that the content is limited to 0.012% or less.

[0015] Ti is used as a deoxidizing agent to form a Ti-based oxide, which promotes the formation of intragranular ferrite during rolling and precipitates fine TiN to promote austenite grain refinement and the formation of intragranular ferrite. Improve the toughness of materials and welds. Therefore, if the content is 0.005% or less, Ti
The lower limit of the Ti content is set to 0.005% or more because the content is insufficient and the effect as the intragranular ferrite generation nucleus is reduced. However, if it exceeds 0.025%, excess Ti becomes Ti
C was formed, precipitation hardening occurred, and the toughness of the heat affected zone was significantly reduced. It should be noted that the Ti content is defined as −0.006 ≦ [T
i%]-2 [O%] ≤ 0.008% by weight
In this relation, when Ti is excessive with respect to the [O] concentration by weight%, the excess Ti generates TiN more than necessary, thereby impairing slab cracks and impairing base material toughness. When the content of Ti is too small relative to the [O] concentration in% by weight, the number of Ti-based oxides serving as intragranular ferrite nuclei does not exceed the required number of 40 / mm ² ,
This is limited. Al is a strong deoxidizing element,
If the content exceeds 0.005%, a Ti-based oxide or a Mn-Si-based oxide which promotes intragranular ferrite transformation is not formed, and the toughness is reduced. Therefore, the content is limited to 0.005% or less.

The amounts of P and S contained as inevitable impurities are not particularly limited, but welding cracks due to solidification segregation,
Since toughness is reduced, the content of P and S should be less than 0.02%.

The basic components of the steel of the present invention have been described above. For the purpose of increasing the strength of the base material and improving the toughness of the base material, V, C
One, two or more of r, Mo, Nb, Ni, and Cu can be contained. First, V is an important element as VN for grain refinement due to the formation of intragranular ferrite structure and high strength due to precipitation strengthening. However, if it exceeds 0.20%, the amount of precipitation becomes excessive and the base material toughness decreases. 0.20% to decrease
Limited to the following.

[0018] Cr is effective in strengthening the base material by improving the hardenability. However, excessive addition exceeding 0.7% is harmful from the viewpoint of toughness and curability, so the upper limit was made 0.7%.

Mo is an element effective for securing the base material strength, but is limited to 0.3% or less because it is expensive.

Nb is effective for strengthening the base material,
Excessive addition exceeding 5% is harmful from the viewpoint of toughness and hardenability, so it was made 0.05% or less.

Ni is a very effective element for increasing the toughness of the base material, but the addition of more than 1.0% increases the alloy cost and is not economical, so the upper limit was made 1.0%.

Although Cu is an element effective for strengthening the base material and weathering resistance, the upper limit is set to 1.0% in order to promote temper brittleness by stress relief annealing, weld cracking, and hot work cracking.

Next, the dissolved oxygen of the molten steel composed of the above components is controlled by a preliminary deoxidation treatment. The control of dissolved oxygen is extremely important for purifying molten steel at the same time as generating fine composite oxides in the slab. The reason for controlling the dissolved oxygen in the range of 0.003 to 0.015% by weight is that if the [O] concentration after preliminary deoxidation is less than 0.003%, intragranular ferrite nuclei that promote intragranular ferrite transformation are promoted. Of composite oxides, and cannot be refined to improve toughness. on the other hand,
If it exceeds 0.015%, even if other conditions are satisfied, the oxide becomes coarse and becomes the starting point of brittle fracture, and the [O] concentration after preliminary deoxidation for reducing toughness is reduced to 0% by weight. .
003 to 0.015%. The preliminary deoxidizing treatment was performed by vacuum degassing and deoxidizing Al, Si, Ca, and Mg. The reason is that vacuum degassing directly removes oxygen in molten steel as gas and CO gas, and removes Al, Si, Ca, M
Oxide-based inclusions generated by strong deoxidation, such as g, float and are easily removed, which is extremely effective for cleaning molten steel.

The slab that has undergone the above treatment is then subjected to 1100-1
Reheat to a temperature range of 300 ° C. The reason for limiting the reheating temperature to this temperature range is that the production of a shaped steel by hot working requires heating at 1100 ° C. or more to facilitate plastic deformation, and sufficiently removes elements such as V and Nb. Since it is necessary to form a solid solution, the lower limit of the reheating temperature was set to 1100 ° C. The upper limit was set to 1300 ° C. in view of the performance and economy of the heating furnace.

The heated steel material is rolled and formed by the steps of rough rolling, intermediate rolling and finish rolling. The feature of the rolling step of the present invention is that in the intermediate rolling step, the temperature of the surface layer of the slab is changed between the rolling passes by Ar. ₃ -20 ℃ or less · Ar ₃ -1
Cooling to a temperature of at least 00 ° C. and performing hot rolling in the process of recovering the surface of the steel material is to be performed at least once in the intermediate rolling step. This is because water cooling between rolling passes gives a temperature difference between the surface layer and the inside of the slab to allow the inside to work even under low pressure reduction conditions, and to perform low-temperature rolling efficiently in a short time. is there. The number of passes of recuperative rolling depends on the thickness of the material to be rolled, for example, the thickness of the flange in the case of H-section steel, and is performed plural times when the thickness is large. here,
The temperature of the steel strip surface layer portion Ar ₃ -20 ° C. or less · Ar ₃ -10
The reason for cooling at a temperature of 0 ° C. or higher is that cooling is carried out at an ordinary γ temperature range because of accelerated cooling subsequent to rolling, so that the surface layer is baked and a hardened phase is formed, thereby impairing workability. Once cooled within this temperature range, the γ / α transformation temperature is temporarily cut off, and the surface layer is reheated to the temperature before the next rolling, and it is processed in a two-phase coexisting temperature range to significantly reduce hardenability. The hardening of the surface layer due to accelerated cooling can be prevented. After the intermediate rolling step, the final product is obtained in the finish rolling step. In the present invention, after the final finish rolling is completed, the steel material is cooled to 600 to 400 ° C. at a cooling rate of 1 to 30 ° C./S.

The so-called accelerated cooling suppresses the ferrite grain growth and increases the pearlite and bainite structure ratios, so that the desired strength can be obtained with a low alloy. 600-40
The reason why the accelerated cooling is stopped at 0 ° C. is that the accelerated cooling at 600 ° C. or more causes the Ar point to be higher than or equal to 1, and a part of the γ phase remains, so that the grain growth of ferrite cannot be suppressed and the pearlite and bainite structure ratios cannot be increased. is there. 400 ° C
If the cooling is lower than this, C and N which are dissolved in the ferrite phase in a supersaturated state in the ferrite phase cannot be precipitated as carbides and nitrides, and the ductility of the ferrite phase decreases. did.

[0027]

EXAMPLE A prototype steel was melted in a converter, added with an alloy, preliminarily deoxidized, the oxygen concentration of the molten steel was measured, and a Ti amount corresponding to the amount was added.
After casting into a m-thick slab, it was rolled into an H-beam by the universal rolling mill row shown in FIG. Although illustration of the rough rolling step is omitted, a water cooling device 5 is provided before and after the intermediate universal rolling mill 4.
a, water cooling between rolling passes is performed by repeating spray cooling and reverse rolling of the inner and outer surfaces of the flange, and after finishing rolling in the finishing universal rolling mill 6, the product is cooled by a cooling device 5b provided on the rear face of the finishing universal rolling mill. The flange and the web were subjected to accelerated cooling by spraying.

The mechanical properties are shown in FIG. 2 at the center (1 / 2t ₂ ) of the thickness t ₂ of the flange 2 of the H-section steel 1 １ ／ of the full length (B) of the flange width (B). 4B, 1 / 2B), a test piece was collected from the center of the thickness of the web 3 at 1 / 2H of the web height. It should be noted that the characteristics of these locations were obtained by calculating the average mechanical properties of the flange 1 / 4F and the web 1 / 2w in the flange 1 / 4F, and the flange 1 / 2F 1 This is because the mechanical test characteristics of the H-section steel can be represented by these three locations because the strength decreases.

Table 1 shows the chemical component values of the prototype steel, and Table 2
Shows mechanical test characteristics for rolling and accelerated cooling conditions. It is well known that the rolling heating temperature is adjusted to 1280 ° C., in general, it is well known that the reduction of the heating temperature improves the mechanical properties, and it is estimated that the high temperature heating condition shows the lowest value of the mechanical properties. Was determined to be able to represent the characteristics at a lower heating temperature.

As shown in Table 2, steels 1 to 6 according to the present invention
Is the target base material strength (JISG3106) and -5 ° C
Satisfies a Charpy value of 47 (J) or more. On the other hand, the steel 7 of the comparative steel is subjected to ordinary Al deoxidation and is not subjected to the Ti deoxidation of the present invention and the accelerated cooling treatment after rolling, so that the base material strength satisfies the standard, but the flange thickness 1
In the case of / 2, the toughness of the width 1/2 part does not satisfy the target value. Steel 8 was subjected to only Ti deoxidation treatment but not to other treatments, so that the grain refinement of the structure was not sufficient, the base material strength was not satisfied, and the thickness of the flange was 1 / 2 width 1/2
The toughness of the part does not satisfy the target value. The steel 9 was subjected to only the accelerated cooling treatment in the present invention, and a low alloy was achieved. However, since other treatments were not performed, the toughness of the flange portion did not satisfy the target value. Since gamma refinement has not been achieved, the flange surface layer is quenched by accelerated cooling, and the surface hardness of the outer surface is much higher than Hv 240 or less at the target Vickers hardness, resulting in a reduction in workability.

That is, when all the requirements of the present invention are satisfied, the flange plate thickness 1/2 and width, which hardly satisfy the mechanical test characteristics of the rolled steel, as shown in Tables 1 to 6 shown in Table 2. Even in a half part, a rolled section steel having sufficient strength and excellent toughness can be manufactured. Note that the rolled section steels to which the present invention is applied are not limited to the H-section steels of the above-described embodiment, but include section steels having other flanges such as I-section steels, angle irons, channel steels, unequal-thickness angle irons. Of course, it can also be applied.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

According to the present invention, the rolled section steel has a sufficient strength even at a flange plate thickness of 1/2 and a width of 1/2 part where mechanical test characteristics are most difficult to be guaranteed, and is a low alloy rolled section having excellent toughness. Steel production becomes possible online, and there are extremely significant industrial effects such as improvement in reliability of large buildings, safety assurance, and economic efficiency.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an example of an apparatus arrangement for implementing the present invention.

FIG. 2 is a diagram showing a cross-sectional shape of an H-section steel and a sampling position of a mechanical test piece.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... H-shaped steel 2 ... Flange 3 ... Web 4 ... Intermediate rolling mill 5a ... Water cooling device of the front and back surface of an intermediate rolling mill 6 ... Finish rolling mill 5b ... Cooling device of the rear surface of a finishing rolling mill

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location C21D 8/00 9270-4K C21D 8/00 B 9/00 102 9352-4K 9/00 102A C22C 38 / 00 301 C22C 38/00 301A 38/06 38/06 (72) Inventor Naoki Oda 1 Chikako-Hachiman-cho, Sakai City Inside Nippon Steel Corporation Sakai Works (56) References JP-A-4-157117 ( JP, A) JP-A-4-279247 (JP, A) JP-A-5-271754 (JP, A)

Claims (2)

(57) [Claims] C. 0.04 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.5 to 1.8%, N: 0.003 to 0.012 by weight% %, Al ≦ 0.005%, the balance being molten iron consisting of Fe and unavoidable impurities.
After adjusting to 15%, the titanium was further deoxidized, and the titanium content was 0.005 to 0.025% by weight, and -0.006 ≦ [Ti%] based on the dissolved oxygen [O%] of the molten steel. -2
[O%] <= 0.008, cast into a slab that satisfies the relationship, re-heated the slab to a temperature range of 1100 to 1300 ° C, started rolling, and measured the temperature of the surface layer of the steel material between passes of the intermediate rolling process. Water cooled to Ar ₃ -20 ° C or lower and Ar ₃ -100 ° C or higher,
Rolled at least once in the recuperation process, and 600-400 ° C at a cooling rate of 1-30 ° C / S after the final finish rolling.
A method for producing a low alloy rolled section steel having excellent toughness, characterized by cooling to a low temperature. 2. In% by weight, C: 0.04 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.5 to 1.8%, N: 0.003 to 0.012 %, Al ≦ 0.005%, V ≦ 0.20%, Cr ≦ 0.7%, Mo ≦ 0.3
%, Nb ≦ 0.05%, Ni ≦ 1.0%, Cu ≦ 1.0
% Or more, and the balance of molten steel consisting of Fe and inevitable impurities is reduced to 0.003 to 0.0% by weight of dissolved oxygen by preliminary deoxidation treatment.
After adjusting to 15%, the titanium was further deoxidized, and the titanium content was 0.005 to 0.025% by weight, and -0.006 ≦ [Ti%]-based on the dissolved oxygen [O%] of the molten steel. 2 [O
%] ≦ 0.008, and after reheating the slab to a temperature range of 1100 to 1300 ° C., rolling is started, and the temperature of the surface layer portion of the steel material is changed to Ar between passes in the intermediate rolling process.
₃ water cooling to -20 ° C. or less · Ar ₃ -100 ° C. or more, that the rolled at least once in recuperation process is cooled after the final finish rolling to 600-400 ° C. at a cooling rate of 1 to 30 ° C. / S A method for producing a low alloy rolled section steel having excellent toughness characterized by the following.