JPH0611902B2 - Stainless steel section and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a shaped steel of austenitic stainless shaped steel used for building structural members, that is, H-shaped steel, angle steel, channel steel, steel sheet pile, and the like. It relates to their manufacturing method.

(Prior Art) The steel materials used for structural members for construction are stipulated by the Building Standards Law, but the regulations do not include the shape of stainless steel. It can be said that this is because stainless steel does not have characteristics as a structural member, but rather because it has a problem in price and is too expensive to be used as a structural member. However, the rapid rise in land prices in recent years has reduced the relative cost of buildings built on it, and relatively expensive materials have been used as building construction materials. For example, a building in the city center
Unlike the conventional cost-based design, the design tends to be based on the idea of landscape or function.

There are activities such as the "Landscape Materials Study Group" as an advisory body of the Ministry of International Trade and Industry Bureau of the Ministry of International Trade and Industry and the "Comprehensive Technology Development Project" of the Ministry of Construction as movements in line with the above trends. Especially in the latter case, the "Structural Material Design and Construction Standards Preparation Subcommittee" established in the Stainless Steel Association in April 1986 conducted a survey and experimental research for 2 years and 6 months to produce a beautiful austenitic stainless steel. An attempt was made to apply it as a structural member, and the mechanical properties necessary for using austenitic stainless steel (JIS SUS304) as a structural member for construction were clarified. The contents are 0.1% proof stress of 24 Kgf / mm ² or more, tensile strength of 53 kgf / mm ² or more, yield ratio of 0.60 or less, and elongation of 35% or more.

However, currently produced austenitic stainless steel materials do not always satisfy this mechanical property. Among them, the rolled steels represented by the H-shaped steels are particularly problematic. Generally, austenitic stainless steel is subjected to solution treatment for the purpose of improving corrosion resistance, but rolled steel has a low yield point because it does not undergo a cold straightening process such as other plate materials, bars and wires. In order to guarantee the above-mentioned 0.1% proof stress, it is necessary to newly install cold working equipment and perform cold working after solution treatment to increase the strength. However, since the sectional shape of the shaped steel is complicated, it is difficult to perform cold working, and the processing equipment is required to be sophisticated, and the equipment introduction cost is enormous and the feasibility is poor. In order to apply rolled steel to building structural members, it is necessary to improve mechanical strength such as proof stress by a low-cost manufacturing method that does not require new cold working equipment.

(Problems to be Solved by the Invention) As a method of improving the yield strength of austenitic stainless steel, introduction of processing strain by rolling (hot rolling) is considered. However, in ordinary austenitic stainless steel, the formation of carbides at grain boundaries cannot be suppressed by rolling as it is, and deterioration of corrosion resistance due to the Cr-deficient layer at grain boundaries cannot be avoided. In order to guarantee the original corrosion resistance of austenitic stainless steel, solution treatment must be carried out, and the yield strength will drop significantly. If cold working is performed after the solution heat treatment, the yield strength will be recovered, but as described above, there is a major drawback in cold working of shaped steel.

The object of the present invention is to provide a shaped steel which has the inherent corrosion resistance of austenitic stainless steel and which also satisfies the above-mentioned strength criteria, and to which such shaped steel is not subjected to solution treatment and is therefore subject to subsequent cooling. An object of the present invention is to provide a method for manufacturing without requiring interworking.

(Means for Solving the Problem) In order to produce an austenitic stainless steel section steel satisfying the above-mentioned strength criteria, especially 24% Kgf / mm ² or more at 0.1% proof stress at low cost, it has sufficient corrosion resistance even as rolled. Therefore, it is necessary to devise the material and processing method. That is, as long as it has excellent corrosion resistance as rolled, it is not necessary to perform solution treatment, and therefore the high strength obtained by rolling is not lost, so cold working for high strength can be performed. It becomes unnecessary. In this case, the rolling conditions must be selected such that the shaped steel can be provided with strength satisfying the above criteria and corrosion resistance comparable to that of the solution heat-treated material.

In order to improve the yield strength of austenitic stainless steel, it is necessary to make austenite grains (γ grains) finer. Further, in order to improve the corrosion resistance, it is necessary to suppress the formation of carbides at grain boundaries and suppress the formation of a Cr-deficient layer. The present inventors sought a processing method satisfying these conditions and obtained the following findings. In other words, for the refinement of γ grains, high-temperature finish rolling at 850 ° C or higher is used to utilize recrystallization, or the processing strain introduced through rolling is performed by performing rapid heating and short-time heating after rolling. It is effective to accelerate the recrystallization by utilizing.

It is difficult to suppress the formation of carbides at the grain boundaries if the rolling is continued. However, the formation of carbides at grain boundaries depends on the carbon content of the steel, and by suppressing the carbon content to 0.03% or less, the formation of carbides at grain boundaries can be suppressed even in the as-rolled state.

If recrystallization is preceded before the formation of carbide, the formation of carbide at the grain boundary is delayed.

The present invention made based on the above findings, the following (1) ~
(4) is the summary.

(1)% by weight, C: 0.03% or less, Si: 1.00% or less, Mn:
0.30-2.00%, P: 0.040% or less, S: 0.03% or less, C
r: 17 to 20%, Ni: 7.0 to 10.5%, N: 0.06 to 0.14%, balance Fe and unavoidable impurities, the amount of residual ferrite or martensite is 10% or less, and the 0.1% proof stress is 24 Kgf. / mm ² or more, an austenitic stainless shaped steel for building structural members.

(2) In addition to the above component (1), Mo: 0.05 to 0.70%,
Nb: 0.005-0.080%, V: 0.005-0.15%, Cu: 0.10-
0.50% and Ti: 0.005 to 0.60%, containing at least one of Fe and inevitable impurities, and the total amount of residual ferrite or martensite is 10% or less,
Austenitic stainless shaped steel for building structural members, which has a 0.1% proof stress of 24 Kgf / mm ² or more.

(3) 1000 to 1250 steel having the composition described in (1) or (2) above
After heating in the temperature range of ℃ and rough rolling, 1000 ~ 12 again.
Heated to a temperature of 50 ℃, subjected to hot working, formed into a shaped steel of a predetermined shape in a temperature range of 850 ~ 1000 ℃ or more, and then air-cooled, (1) or (2) A method for producing an austenitic stainless shaped steel for building structural members.

(4) 1000 to 1250 steel having the composition described in (1) or (2) above
After heating in the temperature range of ℃, hot working is performed under the condition that the finishing temperature is 900 ℃ or less, and it is formed into a shaped steel of the specified shape, and immediately
The austenitic stainless steel type for building structural members according to claim (1) or (2), which is isothermally maintained at a temperature of 900 to 1100 ° C. or higher for 1 to 60 minutes and then cooled at a cooling rate of air cooling or higher. Steel manufacturing method.

Shaped steel of the present invention, the strength criteria described above, that is, 0.1% proof stress is 24 Kgf / mm ² or more, tensile strength is 53 Kgf / mm ² or more, yield ratio is 0.60 or less, and elongation is 35% or more, In addition, it is an austenitic stainless steel having a corrosion resistance equal to or higher than that of the solution heat-treated material. And the use is a building structure member.

The manufacturing method of the present invention is a method for manufacturing the shaped steel as described above without a solution treatment or a cold working step. Hereinafter, each will be described in detail.

(Operation) First, the chemical composition of the shaped steel of the present invention will be described. In addition,% about a component content means weight%.

C: 0.03% or less C is an element that stabilizes the austenite phase, but if it is contained in the steel in an amount of more than 0.03%, preferential precipitation of carbides at grain boundaries cannot be suppressed, and the thermal history causes Causes a Cr-deficient layer near the grain boundaries, resulting in reduced corrosion resistance. Therefore, the C content is 0.03% or less.

Si: 1.00% or less Si is required as a deoxidizing agent at the time of steel making, but if the content exceeds 1.00%, ductility deteriorates significantly. Therefore Si
Content is 1.00% or less.

Mn: 0.30-2.00% Mn is an austenite-forming element, and S in steel is Mn.
Fixed as S to improve hot workability. For that purpose, the content of 0.30% or more is necessary, but if the content exceeds 2%, the effect is not increased and only the material price is increased. Therefore, the Mn content is set to 0.30 to 2.00%.

P: 0.040% or less P is an element contained in steel as an unavoidable impurity,
Although it is preferable that the amount is small, the allowable upper limit of P is 0.04% in consideration of the manufacturing cost.

S: 0.03% or less S, like P, is an element contained in steel as an unavoidable impurity and precipitates as a low melting point compound at the grain boundary to significantly deteriorate hot workability. The lower the S content is, the more preferable it is. However, if it is 0.03% or less, there is no particular problem in hot workability, so the allowable upper limit value is 0.03%. 0 is more desirable.
It is 010% or less.

Cr: 17 to 20% Cr is an essential element for guaranteeing corrosion resistance. 17
If it is less than%, sufficient corrosion resistance cannot be obtained. On the other hand, if the content exceeds 20%, the formation of ferrite will be reduced to 10 for the reasons described below.
%, It is necessary to increase the amount of Ni added, which significantly increases the manufacturing cost. Therefore, Cr
The proper content of is 17 to 20%.

Ni: 7.0 to 10.5% Ni is a basic element that constitutes austenitic stainless steel and is an element that improves corrosion resistance. For that purpose, the content of 7.0% or more is required. However, even if the content exceeds 10.5%, only by raising the material price, the effect of improving corrosion resistance tends to saturate, so the content is set to 7.00 to 10.5%.

N (nitrogen) is a stabilizing element of austenite, and
The addition of 06 to 0.14% serves to replace expensive Ni and maintain the austenite balance of the steel, and has the effect of suppressing the amount of residual ferrite and martensite to 10% or less. However, if the content exceeds 0.14%, surface cracking of the continuously cast steel slabs increases, leading to an increase in manufacturing cost due to a decrease in yield. If N is less than 0.06%, the amount of Ni required to keep the austenite balance properly increases, which also increases the cost.

In addition to the above components, the balance is Fe and inevitable impurities in the shaped steel of (1) above. On the other hand, when higher toughness or higher strength is required, one or more of Mo, Nb, V, Cu, and Ti can be contained within the following range of content.

Mo: 0.05 to 0.70% Mo is an element effective for increasing the strength, and if this effect is expected, it is necessary to contain 0.05% or more. However, if the content exceeds 0.70%, an excessive amount of ferrite may occur, so the upper limit is preferably set to 0.70%.

Nb 0.005% to 0.080% Nb is an element that increases strength. 0.005 for that
%, But if it exceeds 0.080%, the effect of suppressing recrystallization of austenite grains becomes apparent, and it is attempted to increase the strength by promoting recrystallization and refining the austenite grains. Against the purpose of.

V: 0.005 to 0.15% V is an element that increases strength like Nb. For that purpose, the content of 0.005% or more is necessary, but even if it exceeds 0.15%, the effect does not increase and only the increase of the material price is caused, so 0.005 to 0.15% is appropriate.

Cu 0.10 to 0.50% Cu is an element effective in improving the high temperature strength and the corrosion resistance, but for that purpose, it is necessary to contain 0.10% or more. However, if it exceeds 0.5%, surface cracking during rolling tends to increase, and weld cracking tends to be promoted.

Ti: 0.002 to 0.60% Ti is an element that increases strength like V and Nb. For that purpose, the content of 0.002% or more is required, but if it exceeds 0.60%, the toughness of the base material is impaired, so 0.002 to 0.60% is an appropriate content.

Next, the metallurgical structure of the shaped steel of the present invention will be described. The shape steel of the present invention is made of austenitic stainless steel. Therefore, in principle, its structure is completely austenite, but 10% or less by volume of ferrite,
Or it may contain martensite.

The amount of residual ferrite and martensite produced depends on the chemical composition of the steel and the production conditions (in the case of the method of the present invention, cooling conditions during and after rolling, and conditions when reheating is performed).
Depends on. The presence of residual ferrite and martensite increases the strength of the product steel, but on the other hand reduces the ductility. When the amount of these exceeds 10% by volume, the ductility is remarkably deteriorated, and cracks are likely to occur particularly in the stress concentration portion around the welded portion, which is unsuitable as a structural member.

Due to its chemical composition, it contains 10 residual ferrite or martensite.
The desirable condition to keep the percentage below is both Ni equivalent (Nieq) ≥ 0.9 x Cr equivalent (Creq) -7.2 ... (a) Ni equivalent (Nieq) ≥ -0.8 x Cr equivalent (Creq) + 25 ... (b) It is to satisfy the formula. The expression (a) is a condition for reducing the residual ferrite content to 10% or less, and the expression (b) is a condition for reducing the production of martensite to 10% or less.

However, Nieq = Ni% + 30 × C% + 30 × N% + 0.5 × Mn% Creq = Cr% + Mo% + 1.5 × Si% + 0.5 × Nb%.

Hereinafter, a method for manufacturing the shaped steel of the present invention will be described.

1 and 2 are explanatory views of a heat pattern in the method for manufacturing a shaped steel of the present invention.

First, a description will be given with reference to FIG.

As mentioned earlier, when high-temperature finish rolling at 850 ° C or higher and recrystallization by rolling are used, γ
Grains are made finer and the proof stress is improved. However, it is difficult to secure a high finishing temperature of 850 ° C or higher because rolling of shaped steel generally has a complicated pass schedule. Therefore, the method of FIG. 1 employs a method of reheating after rough rolling.

In Fig. 1, heating (a) before rough rolling is 1000 to 1250 ° C.
Done in. This temperature must be 1000 ℃ or more, Nb, V
Since it is impossible to form a solid solution of carbonitrides such as, it becomes impossible to utilize their precipitation strengthening. On the other hand, even in a component system that does not contain alloy components such as Nb and V, austenitic stainless steel has a high resistance to hot deformation, so if it is not heated to a temperature of 1000 ° C or higher, it can be formed into a predetermined shape. Can not. However, heating at a temperature above 1250 ° C. leads to coarsening of γ grains in the initial stage of rolling, and the effect of grain refining utilizing recrystallization by rolling becomes small.

Since the main purpose of the rough rolling (b) is to eliminate the casting structure, it is desirable to set the processing rate to 50% or more. There is no particular restriction on the lower limit temperature of this rough rolling, but it is 60
It is preferable to set the temperature to about 0 ° C.

After rough rolling, reheat to a temperature range of 1000 to 1250 ℃
(c). The reason for selecting this temperature range is the same as the reason for selecting the heating temperature before the rough rolling described above.

After reheating, finish rolling is performed to obtain a shaped steel product having a predetermined product shape (d). At this time, the finishing temperature (end rolling temperature) is set to 8
It is important that the temperature is 50 ° C or higher. The method of the present invention aims to improve the strength by making fine γ grains by utilizing recrystallization by rolling. The recrystallization temperature range of such austenitic stainless steel exists on the high temperature side as compared with carbon steel, but it changes depending on the amount of processing and the strain rate in processing. However, in the processing conditions of the rolling process of shaped steel, the temperature range in which recrystallization appears is 75
It is 0 ° C or higher. On the other hand, even if the grain size is reduced by recrystallization and the strength is increased, it is meaningless that carbides are generated at the grain boundaries and the corrosion resistance is reduced due to the Cr-deficient layer. Precipitation of carbides is generally promoted in the cooling process after rolling, but in the austenitic stainless steel targeted by the present invention, the precipitation nose is around 850 ° C., so if finish rolling is performed at a temperature lower than this temperature, carbides Cannot be avoided. For the above reasons, it is necessary to carry out finish rolling in a temperature range of 850 ° C. or higher so that recrystallization by rolling can be expected and at least carbide is not generated during rolling. However, if the temperature of finish rolling exceeds 1000 ° C, the growth of crystal grains will be significant, so the upper limit is made 1000 ° C.

After finish rolling, air cool to prevent precipitation of carbides
(e). When cooled under conditions faster than air cooling, the processed γ
Since the recrystallization of grains is delayed, the processed structure may remain, and the isotropy of mechanical properties is impaired. On the other hand, in slow cooling such as furnace cooling, recrystallization proceeds well, but formation of a Cr-deficient layer due to precipitation of carbides at grain boundaries is significant, which causes a problem in corrosion resistance.

FIG. 2 is another heat pattern of the present invention in which the material is reheated after rolling and held isothermally.

The temperature of heating (a) before rolling is 1000-1 for the same reason as above.
250 ℃

In this case, rolling (f) is performed by one heating (without reheating) from rough rolling to finish rolling. Rolling finishing temperature is
Keep below 900 ℃.

The method shown in FIG. 2 is characterized in that the processing strain introduced by rolling is utilized to heat for a short time in a temperature range where recrystallization occurs to promote recrystallization. For that purpose, it is necessary to perform hot working in the unrecrystallized region of austenite below 900 ° C.
The lower the finishing temperature, the larger the stored energy, so the driving force for recrystallization increases and it is more effective.
Since the load on the rolling mill also increases, it is necessary to determine the finishing temperature in consideration of these factors. In actual operation, the proper finishing temperature is
600 ~ 900 ℃, the most effective is to finish around 800 ℃.

Immediately after rolling, heat to a temperature of 900 to 1100 ° C for 1 to 6
Hold isothermally for 0 minutes (g). The purpose of maintaining the isothermal temperature after rolling is to cause static recrystallization by maintaining the isothermal temperature to reduce the grain size of γ grains. In order to cause this static recrystallization, it is necessary to hold at a temperature of 900 ° C. or higher, and at that temperature for at least 1 minute or longer. The longer the holding time is, the more the recrystallization proceeds.However, the holding time up to 60 minutes gives the required recrystallization, and the holding time longer than this leads to a decrease in productivity and unnecessary grain growth. 1-60 minutes is suitable.
Further, holding at a temperature higher than 1100 ° C. is not preferable because it causes unnecessary grain growth. Note that isothermal holding does not necessarily mean strictly holding at a constant temperature. There is no problem even if the temperature fluctuates to some extent within the above temperature range.

After the above isothermal holding, cooling is performed at a cooling rate higher than air cooling. As described above, deterioration of corrosion resistance due to carbide formation occurs at a rate slower than air cooling. However,
In the case of the method of FIG. 2, recrystallization is almost completed during the isothermal holding, so there is no concern that the isotropy of mechanical properties will be impaired even if the subsequent cooling rate is faster than air cooling.

(Example) An H-section steel was manufactured using austenitic stainless steel having the composition shown in Table 1. Steel types A to H in Table 1 are steels having composition ranges defined in the present invention, and comparative steels I to L have nitrogen (N) content, M has carbon (C) content, and N has carbon and nitrogen. The content of each is outside the scope of the present invention.

Table 2 shows the manufacturing conditions. Test numbers 1 to 8 are examples of the method of the present invention, in which 2 to 4 are 1 of the heat pattern in FIG.
Finish rolling is performed by heating once. Comparative example 9
Nos. 18 to 18 are examples in which the composition of the raw material or the manufacturing conditions are out of the scope of the present invention.

The shapes of rough rolling and finish rolling in Table 2 are shown in FIGS. 3 and 4. The symbols in these figures (a, b, c,
D) corresponds to the same symbol in Table 2.

Table 3 shows the mechanical properties and the corrosion resistance test results of the H-section steels manufactured under the conditions of Table 2. The mechanical properties are
Base material 0.1% proof stress, tensile strength (TS), yield ratio [YR = (0.1
% Yield strength / tensile strength) × 100], elongation (EL), and bending test. The bending test is for TIG-welded joints 1
It was evaluated by bending at 80 degrees. A circle indicates that no crack was generated even when bent 180 degrees, and a cross indicates that a crack was generated in the heat-affected zone by bending at 180 degrees.

The corrosion resistance was evaluated by the following tests.

Intergranular corrosion evaluation test (sulfuric acid-copper sulfate corrosion test, JIS G 05
75, 75 hours) Crevice corrosion test (3t x 30w x 50 (mm) polishing test piece having a 4mm hole for bolting in the center was fastened with a 3mm Teflon bolt, and at 60 ℃, 800ppmCl-containing water environment. 10-day test) Here, the corrosion resistance was evaluated for both the base metal portion and the joint portion subjected to the covered arc welding.

According to the results of Table 3, in the test numbers 1 to 8 satisfying the conditions of the present invention, the mechanical properties all satisfy the target performance, and the base metal and the welded joint have sufficient corrosion resistance. is there.

On the other hand, although the composition standard of SUS 304 was satisfied, but the test numbers 9 to 14 which were out of the composition range of the present invention and out of the production conditions of the present invention resulted in not being able to satisfy the target performance. There is. In particular, since it is a low-temperature finish of 750 ° C, the structure obtained by rolling has inherent work strain, making it difficult to suppress YR to 60% or less. Further, since it is a low-temperature finish, it is not possible to suppress the formation of carbides after rolling, crevice corrosion occurs, and the original corrosion resistance of austenitic stainless steel cannot be secured.

Test Nos. 15 to 18 are examples in which the steel having the composition range of the present invention was used and manufactured under the conditions outside the manufacturing method of the present invention. Examples where the finishing temperature is too low (Test No. 16) or the holding temperature is too low even if it is reheated and held isothermally (Test No. 18), the holding temperature is a temperature corresponding to the method of the present invention. If the holding time is too short (test number 17),
The target value of mechanical properties is not always satisfied.

In the bending test, if the amount of ferrite or martensite is
Only test number 9, which was 12% (volume%), exhibited cracks. This is because the content of nitrogen (N) is low and Ni equivalent (Nieq)
This is because a small steel (I steel in Table 1) was used.

(Effects of the Invention) As described in detail above, the austenitic stainless steel section of the present invention has sufficient mechanical properties and corrosion resistance for practical use as a building structural member. According to the manufacturing method of the present invention, the shaped steel can be manufactured at low cost only by the hot rolling process.

The present invention is an invention that greatly contributes to the popularization of rolled shaped austenitic stainless steel.

[Brief description of drawings]

1 and 2 are heat pattern diagrams of the shaped steel manufacturing method of the present invention. FIG. 3 and FIG. 4 are views showing a rough rolling shape and a finish rolling shape in manufacturing the H-section steel in the example.

Claims (4)

[Claims] 1. By weight%, C: 0.03% or less, Si: 1.00% or less, Mn: 0.30 to 2.00%, P: 0.040% or less, S: 0.03%
Below, Cr: 17 to 20%, Ni: 7.0 to 10.5%, N: 0.06 to 0.1
Containing 4% and the balance Fe and unavoidable impurities, the amount of residual ferrite or martensite is 10% or less,
Austenitic stainless shaped steel for building structural members, which has a 0.1% proof stress of 24 Kgf / mm ² or more. 2. By weight%, C: 0.03% or less, Si: 1.00% or less, Mn: 0.30 to 2.00%, P: 0.040% or less, S: 0.03%
Below, Cr: 17 to 20%, Ni: 7.0 to 10.5%, N: 0.06 to 0.1
4%, Mo: 0.05 to 0.70%, Nb: 0.005 to 0.080
%, V: 0.005-0.15%, Cu: 0.10-0.50%, and T
i: 0.005 to 0.60%, containing at least one of Fe and unavoidable impurities, the amount of residual ferrite or martensite is 10% or less, and the 0.1% proof stress is 24 kgf / mm ²
The above is the austenitic stainless shaped steel for building structural members. 3. A steel having the composition according to claim 1 or 2.
After heating in the temperature range of 1000 to 1250 ℃ and rough rolling, it is heated again to the temperature of 1000 to 1250 ℃ and hot-worked.
The method for producing an austenitic stainless shaped steel for building structural members according to claim (1) or (2), wherein the shaped steel is formed into a predetermined shape in a temperature range of up to 1000 ° C and then air-cooled. 4. A steel having the composition according to claim 1 or 2.
It is heated in the temperature range of 1000 to 1250 ° C, hot working is performed under the condition that the finishing temperature is 900 ° C or less to form a shaped steel of a predetermined shape, and immediately at a temperature of 900 to 1100 ° C for 1 to 60 minutes. The method for producing an austenitic stainless shaped steel for building structural members according to claim 1 or 2, characterized in that the material is kept isothermal and cooled at a cooling rate higher than air cooling.