JP2543282B2 - Method for producing controlled rolled 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 controlled rolled steel having excellent toughness which is used as a structural member of a building.

[0002]

2. Description of the Related Art Due to the construction of super-high-rise buildings and stricter safety standards, steel materials used for columns, such as H-section steel with a particularly large plate thickness (hereinafter referred to as extra-thick H-section steel) Are required to have higher strength, higher toughness, and lower yield ratio. In order to satisfy such required characteristics, conventionally, heat treatment such as normalizing treatment is performed after the rolling is completed. The addition of heat treatment causes a significant cost increase such as reduction of heat treatment cost and production efficiency, and there is a problem in economic efficiency. 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, shaped steel with a flange, such as 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 respective parts, and for example, rolled steel material for welded structure (J
Some parts do not meet the criteria such as IS G3106).
In particular, when an extremely thick H-section steel is rolled using a continuous casting slab as a raw material, there is a limit to the maximum thickness of the slab that can be produced by the continuous casting equipment, and therefore the lower pressure lower ratio is obtained. 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.

A fine graining method using TMCP is well known,
Due to the restrictions on shaping, large reduction of the steel sheet manufacturing method is not possible with shaped steel rolling. Further, in the field of thick steel plates, high strength and high toughness steel is manufactured by utilizing the precipitation effect of VN.
The techniques disclosed in Japanese Patent No. 50548 and Japanese Patent Publication No. 62-54862 are proposed. However, in this conventional method, since deoxidation of molten steel is performed by general Al deoxidation treatment, fine complex oxides that are effective in grain refinement of the structure are not generated as intragranular ferrite formation nuclei, The grain refinement of the tissue was not sufficient. That is, in the conventional Al deoxidation, Al was added in the initial stage of the melting process, and the deoxidation of the molten steel and the produced Al ₂ O ₃ were floated and separated for the purpose of high cleaning. 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 uses a novel manufacturing means comprehensively for the steps of steelmaking, rolling and cooling after rolling, and the structure as rolled To provide a method for producing a low alloy rolled steel having excellent strength and toughness, that is, to increase the nucleation sites of ferrite in the production process and suppress the growth of nucleated ferrite to reduce the grain size. The purpose is to provide a means to do so.

[0006]

The gist of the present invention is as follows: C: 0.04 to 0.20% by weight, Si: 0.05
~ 0.50%, Mn: 0.5-1.8%, N: 0.00
Molten steel containing 3 to 0.012%, Ti: 0.005 to 0.025%, and the balance Fe and unavoidable impurities,
By the pre-deoxidation treatment, the dissolved oxygen is reduced to 0.003% by weight.
After being adjusted to 0.015%, it is further deoxidized by adding metallic aluminum or ferroaluminum.
Is 0.005 to 0.015%, and -0.004≤ [Al%]-1.1 [O] with respect to dissolved oxygen [O%] of molten steel.
%] ≦ 0.006, cast into a slab, reheat the slab to a temperature range of 1100 to 1300 ° C., and then start rolling, and set the temperature of the surface layer of the steel material to Ar between passes in the intermediate rolling step.
₃ water cooling to -20 ° C. or less · Ar ₃ -100 ° C. or more, that the rolling at least once with recuperation process is cooled after the final finish rolling to six hundred to four hundred ° C. at a cooling rate of 1 to 30 ° C. / S A method for producing a controlled rolled steel with excellent toughness, characterized by: C: 0.04 to 0.2% by weight, Si: 0.05 to
0.50%, Mn: 0.5 to 1.8%, N: 0.003
.About.0.012%, Ti: 0.005 to 0.025, and in addition V.ltoreq.0.02%, Cr.ltoreq.0.7%, Mo.ltoreq.
0.3%, Nb ≦ 0.05%, Ni ≦ 1.0%, Cu ≦
After adjusting the dissolved oxygen to 0.003 to 0.015% by weight, the molten steel containing 1.0% of one kind or two or more kinds and the balance consisting of Fe and unavoidable impurities is further adjusted to 0.003 to 0.015% by weight. It is deoxidized by adding metallic aluminum or ferroaluminum, and the Al content is 0.005 to 0.
015% and -0 .. with respect to the dissolved oxygen [0%] of the molten steel.
It casts to the slab which satisfy | fills the relationship of 004 <= [Al%]-1.1 [O%] <= 0.006, The said slab is 1100-1300.
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 water cooling to -100 ° C. or higher, and rolled at least once in the recuperation process, after the final finish rolling at a cooling rate of 1 to 30 ° C. / S excellent toughness, characterized by cooling to six hundred to four hundred ° C. There is a method of manufacturing controlled rolled steel.

According to the present invention, in order to reduce the grain size of the structure, appropriate deoxidation treatment is performed in the steelmaking process to make the molten steel highly clean, control the dissolved oxygen concentration, and add Al in the final addition order. By controlling the amount of Al added and dispersing a large number of fine composite oxides that act as intragranular ferrite formation nuclei in the steel and water-cooling between hot rolling passes, A temperature difference is applied to the steel, and even under a low pressure condition, the infiltration into the inside at a higher temperature is enhanced, and a work dislocation that becomes an intragranular ferrite formation nucleus is introduced to increase the intragranular ferrite formation nucleus. In addition, by controlling the cooling of the γ / α transformation temperature range after rolling, the growth of the nucleated ferrite is suppressed and the microstructure can be made finer, and the production cost can be controlled at high efficiency. It enables the production of rolled steel.

[0008]

The operation of the present invention will be described in detail below with reference to examples. 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, toughening reduces the grain size of crystals, reduces the solid solution N and C of the parent phase (ferrite), and reduces the high carbon martensite in the hardening phase and the coarse oxides / precipitates that are the starting points of fracture. It is achieved by miniaturization and the like. In general, toughness is reduced by increasing the strength of steel, and contradictory measures must be taken to increase strength and toughness. The only metallurgical factor that satisfies both at the same time is grain refinement.

A feature of the present invention is that deoxidation is controlled in the steelmaking process, and a large number of fine composite oxides that act as intragranular ferrite formation nuclei are dispersed in the steel and between rolling passes in the hot rolling process. By repeating water cooling and rolling at the time of recuperation, both treatments increase intragranular ferrite formation nuclei, and in addition, accelerated cooling is carried out after rolling to suppress the growth of the ferrite and reduce the microstructure grain. The base material is strengthened and toughened by an in-line rolling process. Next, the reasons for limiting the basic composition range of the shaped steel targeted by 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%, it is excessive. Addition significantly lowers the base metal toughness, weld crack resistance, weld heat affected zone toughness, etc., so the lower limit is 0.04%,
The upper limit was 0.20%.

Next, Si is necessary for securing the strength of the base metal and pre-deoxidizing molten steel, but if it exceeds 0.50%, HAZ
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 for securing 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 precipitation of TiN and VN. If it is less than 0.003%, the amount of precipitation of TiN and VN is insufficient, and precipitation strengthening and a sufficient amount of intragranular ferrite structure are obtained. Since it does not exist, it is set to 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.

As a deoxidizing agent, Ti forms a Ti-based oxide and promotes the generation of intragranular ferrite during rolling, and the effect of precipitating fine TiN to refine austenite and promote the formation of intragranular ferrite. Improves the toughness of the base material and the welded part. Therefore, if it is 0.005% or less, the Ti content in the oxide is insufficient, and the action as intragranular ferrite formation nuclei is reduced, so the lower limit of the Ti content is 0.00
It was set to 5% or more. However, if it exceeds 0.025%, excessive Ti forms TiC, precipitation hardening occurs, and the toughness of the weld heat affected zone is remarkably reduced, so the Ti content is limited to this range.

Al is a strong deoxidizing element, and 0.015
%, The complex oxide that promotes the intragranular ferrite transformation is not generated, resulting in a decrease in toughness. Excess solute Al combines with N to form AlN, which reduces the amount of VN precipitation. Therefore, the content is limited to 0.015% or less. Also,
If it is less than 0.005%, the target composite oxide containing Al cannot be formed, so the content was made 0.005% or more. And the amount of Al is -0.% By weight% with respect to the dissolved oxygen [0%] of the molten steel.
004% ≦ [Al%] − 1.1 [0%] ≦ 0.006%
In the relational expression, when Al is excessive with respect to the [O] concentration in the above relational expression, the number of complex oxides formed decreases, and Al that is ineffective as intragranular ferrite formation nuclei is reduced. _{Since a} large amount of ₂ O ₃ is generated and the structure cannot be refined, the toughness is reduced, and when Al is too small with respect to the [O] concentration in weight%, the amount of compound oxide that becomes intragranular ferrite nuclei is significantly reduced. , Limited like this. The reason for adding Al in the latter stage of the steelmaking process is that Al forms stable Al ₂ O ₃ with a strong deoxidizing power, and it is difficult to form the target low melting point composite oxide.

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 above are the basic components of the steel of the present invention. V, C are used for the purpose of increasing the strength of the base metal and improving the toughness of the base metal.
One 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:

[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. N
b is effective for toughening the base material, but excessive addition exceeding 0.05% is harmful from the viewpoint of toughness and hardenability, so b was made 0.05% or less. Ni is an extremely effective element that enhances 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%. Cu is an element effective for strengthening the base material and weathering resistance. However, the upper limit is set to 1.0% in order to promote temper embrittlement by stress relief annealing, weld cracking, and hot work cracking.

Next, the dissolved oxygen containing the above components is controlled by pre-deoxidation treatment to control the dissolved oxygen. The control of dissolved oxygen is extremely important for highly cleaning molten steel and at the same time producing fine complex oxides in the slab. Dissolved oxygen is 0.
The reason for controlling to 003 to 0.015% is that when the [O] concentration after preliminary deoxidation is less than 0.003%, the complex oxide of intragranular ferrite-forming nuclei that promotes intragranular ferrite transformation decreases, and Toughness cannot be improved. On the other hand, 0.01
If it exceeds 5%, the oxide becomes coarse and becomes a starting point of brittle fracture even if other conditions are satisfied, and the [O] concentration after preliminary deoxidation for reducing toughness is 0.003% by weight. ~
It was limited to 0.015%. The preliminary deoxidizing treatment was performed by vacuum degassing and deoxidizing Al, Si, Ca, and Mg. The reason is that the vacuum degassing process directly removes oxygen in molten steel as gas and CO gas, and oxide inclusions generated by strong deoxidation of Al, Si, Mg, etc. float and are easily removed, so that the molten steel is cleaned. It is extremely effective for

The slab that has undergone the above treatment is then 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 economical efficiency of the heating furnace.

The heated steel material is roll-molded by each process of rough rolling, intermediate rolling and finish rolling. The characteristic feature of the rolling process of the method of the present invention is that the temperature of the surface layer of the slab is Ar between rolling passes in the intermediate rolling process. ₃ -20 ° C. or less, Ar ₃ -1
It is to perform hot rolling at least in the intermediate rolling step at least once in the process of cooling to 00 ° C. or higher and reheating the surface of the steel material. This is because water cooling between rolling passes creates a temperature difference between the surface layer and the inside of the steel slab so that the internal processing can permeate even under low pressure conditions, and low-temperature rolling can be performed efficiently in a short time. Is. 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 surface layer of the cast slab is set to Ar ₃ −20 ° C. or lower, Ar ₃ −
The reason why the cooling is limited to 100 ° C. or higher is that accelerated rolling is followed by accelerated cooling, so that cooling from the normal γ temperature range causes quenching in the surface layer portion to form a hardened phase and impairs workability. If cooled within the range limited to this temperature, once γ
/ Α transformation temperature is cut, the surface layer temperature rises by recuperative heating until the next rolling, and processing is performed in the two-phase coexisting temperature range, hardenability can be significantly reduced, and hardening of the surface layer due to accelerated cooling can be prevented. .

After the intermediate rolling process, the final product is obtained in the finish rolling process. In the present invention, the steel product is cooled to 600 to 400 ° C. at a cooling rate of 1 to 30 ° C./S after the final finishing rolling is completed. This so-called accelerated cooling suppresses the grain growth of ferrite and increases the pearlite and bainite structure ratios, so that the target strength can be obtained with a low alloy. 600-4
The reason why the accelerated cooling is stopped at 00 ° C. is that the accelerated cooling above 600 ° C. results in Ar ₁ point or more, and a part of the γ phase remains to suppress the grain growth of ferrite and to increase the pearlite and bainite structure ratio. Is. Also, 400
If the temperature is lower than 0 ° C, C and N that are solidly dissolved in the ferrite phase in a supersaturated state cannot be precipitated as carbides or nitrides by subsequent cooling, and the ductility of the ferrite phase is reduced. Limited

[0023]

[Examples] Prototype shaped steel was melted in a converter, pre-deoxidized after adding an alloy, oxygen concentration of the molten steel was measured, and an Al amount corresponding to the amount was added and continuous casting was carried out to 250 to 300 mm.
After being cast into a thick slab, it was rolled into an H-section steel by the universal rolling apparatus train shown in FIG. Although illustration of the rough rolling step is omitted, water cooling devices 5a are provided before and after the intermediate universal rolling machine 4, water cooling between rolling passes is performed by repeating spray cooling of the inner and outer surfaces of the flange and reverse rolling, and a finishing universal rolling machine 6 After the rolling was completed in step 1, accelerated cooling was performed by spray cooling of the product flange and web by a cooling device 5b provided on the rear surface of this finishing universal rolling machine.

The mechanical characteristics are the thickness t of the flange 2 shown in FIG.
1 of the total flange width (B) at the center of ₂ (1 / 2t ₂ ).
/ 4, 1/2 width (1 / 4B, 1 / 2B) to web 3
A test piece was collected from the center of the plate thickness of 1 / 2H of the web height. In addition, the characteristics of these portions were obtained because the flange 1 / 4F portion and the web 1 / 2w portion show the average mechanical characteristics of the flange portion and the web portion, respectively.
This is because the characteristics of the F portion are the most deteriorated, so that the mechanical test characteristics of the H-section steel can be represented by these three points.

Table 1 shows the chemical composition values of the trial steels, and Table 2
Shows mechanical test characteristics for rolling and accelerated cooling conditions. The rolling heating temperature is set to 1280 ° C. It is well known that reducing the heating temperature generally improves the mechanical properties, and it is estimated that the high temperature heating condition shows the lowest value of the mechanical properties. This is because it was determined that the characteristics at a heating temperature lower than that can be represented.

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, in the comparative steel, steel 7, in the Al deoxidation process, the amount of Al added exceeded the upper limit of the present invention with respect to the dissolved oxygen concentration, and the accelerated cooling treatment after rolling was not performed. Satisfies the standard, but the toughness of the flange thickness of 1/2 and the width of 1/2 portion does not satisfy the target value. Steel 8 was only subjected to the trace Al deoxidation treatment of the present invention, but was not subjected to any other treatment, so the grain refinement of the structure was not sufficient and the standard of the base metal strength was not satisfied. 1/2 thickness and 1 width
The toughness of the / 2 part also does not satisfy the target value. Further, although the steel 9 is subjected to only accelerated cooling treatment in the present invention, and the strength achieves the target value, the toughness of the flange portion does not satisfy the target value because no other treatment is carried out. Since gamma grain refinement on the outer surface of the flange has not been achieved, the surface layer of the flange is hardened by accelerated cooling, and the surface hardness of the outer surface far exceeds the target Vickers hardness of Hv240 or less, and the workability deteriorates. Cause

That is, when all the requirements of the present invention are satisfied, like the shaped steels 1 to 6 shown in Table 2, the flange plate thickness 1/2 and the width which are the most difficult to satisfy the mechanical test characteristics of the rolled shaped steels. It is possible to manufacture a rolled steel having a sufficient strength even in ½ part and excellent toughness. The rolled shaped steel to which the present invention is applied is not limited to the H-shaped steel of the above-described embodiment, but is also applicable to shaped steel having a flange such as I-shaped steel, chevron steel, channel steel, and unequal-thickness chevron steel. Of course you can.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

EFFECTS OF THE INVENTION According to the present invention, the rolled rolled steel is a controlled rolled steel having excellent toughness with sufficient strength even in the flange plate thickness 1/2 and width 1/2 part where it is most difficult to guarantee mechanical test characteristics. Can be manufactured online, and industrial effects such as reliability improvement, safety assurance, and economic efficiency of large-scale buildings are extremely remarkable.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an example of device arrangement for carrying out the method of the present invention.

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

[Explanation of symbols]

 1 H-shaped steel 2 Flange 3 Web 4 Intermediate rolling mill 5a Intermediate rolling mill Water-cooling device for front and rear surfaces 5b Finishing rolling mill Rear cooling device 6 Finishing rolling mill

Claims (2)

(57) [Claims] 1. C: 0.04 to 0.20% by weight%, Si: 0.05 to 0.50%, Mn: 0.5 to 1.8%, N: 0.003 to 0.012 %, Ti: 0.005 to 0.025%, and the balance of Fe and unavoidable impurities in a molten steel by preliminary deoxidation treatment to obtain a dissolved oxygen content of 0.0% by weight.
After adjusting to 03 to 0.015%, it is further deoxidized by addition of metal aluminum or ferroaluminum, the Al content is 0.005 to 0.015% by weight, and dissolved oxygen [O%] of molten steel. On the other hand, -0.004≤ [Al%]-1.1
[O%] ≦ 0.006 is cast into a slab, the slab is reheated to a temperature range of 1100 to 1300 ° C., and then rolling is started, and the temperature of the steel material surface layer portion between passes of the intermediate rolling step. Water-cooled to Ar ₃ -20 ° C or lower and Ar ₃ -100 ° C or higher,
Roll at least once in the recuperating process, and 600-400 ° C at a cooling rate of 1-30 ° C / S after final finishing rolling.
A method for producing a controlled rolled steel with excellent toughness, which is characterized by cooling up to 2. C .: 0.04 to 0.2%, Si: 0.05 to 0.50%, Mn: 0.5 to 1.8%, N: 0.003 to 0.012% by weight. Ti: 0.005 to 0.025% inclusive, in addition V ≦ 0.02%, Cr ≦ 0.7%, Mo ≦ 0.3
%, Nb ≦ 0.05%, Ni ≦ 1.0%, Cu ≦ 1.0
% Of the molten steel containing one or two or more of Fe and the balance of Fe and inevitable impurities, and adjusting the dissolved oxygen to 0.003 to 0.015% by weight by preliminary deoxidation. Deoxidized by addition of ferroaluminium, and the Al content is 0.005 to 0.015% by weight.
And -0.004 ≦ for dissolved oxygen [0%] of molten steel
[Al%]-1.1 [0%] ≤ 0.006 is cast into a slab, the slab is reheated to a temperature range of 1100 to 1300 ° C, and then rolling is started. The temperature of the surface layer of the steel material between the passes is Ar ₃ -20 ° C or less · Ar ₃ -10
Excellent toughness control characterized by cooling to 0 ° C or higher with water, rolling at least once in the recuperating process, and cooling to 600 to 400 ° C at a cooling rate of 1 to 30 ° C / S after final finishing rolling. Rolled shaped steel manufacturing method.