JP2000073138A - Steel for bolt excellent in sulfuric acid corrosion- resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for a bolt excellent in cold forgeability and sulfuric acid corrosion resistance. SOLUTION: For improving cold forgeability, the content of N is controlled to <=0.01%, one or >= two kinds among Al, Nb, V and Ti are added by suitable amounts and moreover, Ceq(=C+Si/24+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5) is controlled to <=0.500%. For improving sulfuric acid corrosion resistance, the content of C is controlled to >0.15 to <0.30% and Cr, Cu and Si are added respectively by suitable amounts.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to, for example, excellent sulfuric acid corrosion resistance used in a flue of a thermal power plant and the like.
The present invention relates to steel for bolts having excellent cold forgeability during bolt processing.

[0002]

BACKGROUND ART Conventionally, steel heavy oil-fired boiler economizer of the air preheater, various exchangers, flue, chimney, etc. etc., when used in an atmosphere in which SO ₂ is produced in the combustion gases, SO ₂
The phenomenon that sulfuric acid corrosion is caused by sulfuric acid generated from sulfur is well known. In sulfuric acid corrosion, a part of SO ₂ is oxidized,
The SO ₃ becomes SO ₃ , and the SO ₃ combines with the moisture in the gas to generate sulfuric acid gas and condenses on the surface of the steel material. As a result, the steam in the heavy oil combustion gas is about 8% and the dew point is 175 ° C. When the surface temperature increases, the concentration of sulfuric acid in the liquid condensing on the surface increases, reaching 80% at 130 ° C. JP-B-47-1876, JP-B-53-46776,
JP-A-61-3867, JP-A-9-25536, JP-A-10-110237 and the like relate to steel materials which improve the corrosion resistance in such a sulfuric acid corrosive environment.

[0003]

However, these steel materials are intended for thick plates, seamless steel pipes, electric resistance welded steel pipes, etc., and do not include bolts. The required cold forgeability is not mentioned. The sulfuric acid corrosion resistance after cold forging is also unknown. The present invention has been made in view of the above points, and its purpose is to
An object of the present invention is to provide a bolt steel having both sulfuric acid corrosion resistance and cold forgeability.

[0004]

Means for Solving the Problems In order to achieve the above object, the present inventors have studied the steel for bolts with respect to the sulfuric acid corrosion resistance after cold forging and the cold forgeability in terms of microstructure and The effects of chemical components were studied and the following findings were obtained. (1) The sulfuric acid corrosion resistance after cold forging indicates that the structure of pearlite is relatively large (C ≧ 0.15%), the pearlite structure is strengthened (lamellar spacing is reduced by adding Cr,
Suitable amount of C added), Enhancement of corrosion resistance of ferrite (Cu added)
And the suppression of exfoliation of the interface between ferrite and cementite caused by cold forging (addition of Si) enhances the effect. (2) The effect of (1) is further improved by adding at least one of Sn and Sb. (3) The cold forgeability reduces the amount of N, and the Al, V,
It is improved when N is fixed by adding one or more of Nb and Ti. The present invention has been completed on the premise of satisfying the hot workability and strength characteristics of a steel for bolts by obtaining these findings. That is, the present invention provides: C: more than 0.15% and 0.30% or less, Si:
0.1% or more and 1.5% or less, Mn: 0.1% or more and 1.0% or less
%, P: 0.05% or less, S: 0.05% or less, C
u: 0.1% to 1.0%, Ni: 0.1% to 0.6%, Cr: 0.2% to 1.0%, N:
0.01% or less, and Al: 0.01% or more and 0.10
%, Nb: 0.01% to 0.20%, V:
0.01% or more and 0.20% or less, one or more kinds of Ti: 0.01% or more and 0.10% or less, and
Ceq (= C + Si / 24 + Mn / 6 + Cu / 15 + N
i / 15 + Cr / 5 + Mo / 5 + V / 5) ≦ 0.500
%, The balance being substantially Fe, the bolt steel having excellent sulfuric acid corrosion resistance. Sn: 0.02% or more and 0.50% or less, Sb: 0.
2. Bolt steel excellent in sulfuric acid corrosion resistance according to 1 above, which contains one or more kinds of not less than 02% and not more than 0.50%. Mo: 0.05% or more and 0.2% or less, B: 0.00
3. Bolt steel excellent in sulfuric acid corrosion resistance according to 1 or 2 containing one or more of 30% or more and 0.010% or less.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical components of the present invention will be described in detail. C is added in excess of 0.15% in order to secure the amount of pearlite necessary for the sulfuric acid corrosion resistance of the steel material after cold working, but if it exceeds 0.30%, the sulfuric acid corrosion resistance deteriorates. More than 15% and not more than 0.30%. S
i is necessary for deoxidation and is also effective in improving sulfuric acid corrosion resistance. To obtain those effects, 0.1% or more is added. Deteriorates,
0.1% or more and 1.5% or less. Mn is necessary for deoxidation and ensuring strength, and is added in an amount of 0.1% or more. However, if it exceeds 1.0%, cold forgeability deteriorates.
The following is assumed. P hardens ferrite and deteriorates cold forgeability, so is made 0.05% or less. Since S becomes MnS and degrades the cold forgeability, S is set to 0.05% or less. Cu is 0.1% to improve the sulfuric acid corrosion resistance.
% Is added, but if it exceeds 1.0%, the hot workability deteriorates, so the content is made 0.1% or more and 1.0% or less. In the case of the steel of the present invention to which Cu is added, Ni is necessary for ensuring the hot workability, and is added in an amount of 0.1% or more. 0.1% or more
6% or less. Cr is added in an amount of 0.2% or more to improve the sulfuric acid corrosion resistance. However, if it exceeds 1.0%, the cold forgeability deteriorates. N is contained in steel as an impurity, but at the time of cold forging, it deteriorates cold forgeability due to strain aging.
0.01% or less. Also, one or more of Al, Nb, V, and Ti are added to fix N and improve cold forgeability. Al is added in an amount of 0.01% or more, but if it exceeds 0.10%, the effect is saturated and hot workability is deteriorated.
Nb is added in an amount of 0.01% or more, but if it exceeds 0.20%, the effect is saturated and hot workability deteriorates.
% To 0.2%. V is added in an amount of 0.01% or more, but if it exceeds 0.20%, the effect is saturated and hot workability is deteriorated. Ti is 0.0
If more than 0.1% is added, the effect is saturated and hot workability deteriorates.
0% or less. The above is the basic component composition. When the sulfuric acid corrosion resistance is further improved, one or more of Sn and Sb are added. Both Sn and Sb are added in an amount of 0.02% or more.
0.02% or more and 0.50% or less. When high strength is required, one or more of B and Mo can be added. B is added in an amount of 0.0030% or more.
If it exceeds 10%, the effect is saturated, so 0.0030%
At least 0.010%. Mo is added at 0.05% or more, but if it exceeds 0.2%, the effect is saturated.
0.05% or more and 0.2% or less. Further, in the present invention, in addition to the above-mentioned component limitation, Ceq
(= C + Si / 24 + Mn / 6 + Cu / 15 + Ni / 1
5 + Cr / 5 + Mo / 5 + V / 5) is set to 0.500% or less so that the cold forgeability does not deteriorate. In addition, the present invention can be applied to a high-strength bolt that is subjected to heat treatment (quenching and tempering), a nut that is cold-worked, and a washer, in addition to the bolt that is used in a cold forged state.

[0006]

EXAMPLE Steel of the chemical composition shown in Table 1 was melted in a 150 kg vacuum melting furnace to form a steel ingot, and then hot-rolled to form a steel ingot.
mm round bar. After that, it undergoes pickling, bonding, wire drawing (16 mm to 15.6 mm), cold head forging, and thread rolling, and then undergoes JISB 1180 hexagon bolt M16.
* 50 was manufactured. J from the center of the bolt to check the strength
An IS4 test piece (parallel portion 7 mm diameter) was cut out and subjected to a tensile test. The cold forgeability was determined when forming the bolt head (10
0 cracks) (%). Sulfuric acid corrosion resistance is measured by tightening five bolts of each steel type to the plate under normal conditions.
Conditions (30 ° C / 20% sulfuric acid, 50 ° C / 40% sulfuric acid, 13
After soaking for 5 hours at 0 ° C. · 80% sulfuric acid), corrosion weight loss due to corrosion was measured to determine the corrosion rate ^{(mg / cm 2 / 5h)} . Table 2 shows the tensile speed (average value of n = 3), cold forgeability, and corrosion rate (average value of n = 5). It can be seen that the steels of the present invention (Nos. 1 to 9) are excellent in both sulfuric acid corrosion resistance and cold forgeability. On the other hand, the comparative steel (No. 10)
17) are inferior in sulfuric acid corrosion resistance or cold forgeability.

[0007]

[Table 1]

[0008]

[Table 2]

[0009]

As described above, according to the present invention, a steel for sulfuric acid corrosion resistant bolts having not only excellent sulfuric acid corrosion resistance but also excellent cold forgeability can be obtained.
It can be used for air preheaters, various exchangers, stacks, chimneys, etc., and is extremely useful in industry.

Claims (3)

[Claims] C: more than 0.15% and not more than 0.30%,
Si: 0.1% to 1.5%, Mn: 0.1% to 1.0%, P: 0.05% or less, S: 0.05% or less, Cu: 0.1% to 1 0.0% or less, Ni: 0.1%
And 0.6% or less, Cr: 0.2% or more and 1.0% or less,
N: 0.01% or less, and Al: 0.01% or more.
10% or less, Nb: 0.01% or more and 0.20% or less,
V: 0.01% or more and 0.20% or less, Ti: 0.01%
0.10% or less, and Ceq (= C + Si / 24 + Mn / 6 + Cu / 15
+ Ni / 15 + Cr / 5 + Mo / 5 + V / 5) ≦ 0.5
Bolt steel excellent in sulfuric acid corrosion resistance, with the balance satisfying 00% being substantially Fe 2. Sn: 0.02% or more and 0.50% or less;
The steel for bolts having excellent sulfuric acid corrosion resistance according to claim 1, wherein b: one or more of 0.02% or more and 0.50% or less. 3. Mo: 0.05% or more and 0.2% or less, B:
The steel for bolts excellent in sulfuric acid corrosion resistance according to claim 1 or 2, which contains one or more of 0.0030% or more and 0.010% or less.