JPS5938283B2 - Method for manufacturing PC steel bars with excellent stress corrosion resistance properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a PC steel bar with excellent stress corrosion resistance.

Conventionally, pearlite-based PC steel bars of thin, medium, and large diameters have been widely used, but the problem remains as to how to improve their stress corrosion characteristics.

For example, 0605~0.4%C bolts and 0.1%C213%Mn materials for oil pipelines will fracture due to hydrogen embrittlement (in the case of bolts, it is called delayed fracture), so Cu is added. Kokichi is being carried out to prevent this. For line pipes, the addition of Cu alone has no effect, and fractures start from MuS, so ultra-low sulfur Cu steel is considered. It seems that adding 0.95 to 5% Cu for bolt materials and adding 0.03% Cu for pipe materials is effective. However, bolt materials containing 0.5% or more of Cu have problems in terms of other properties such as hot workability. The mechanism of the effect of Cu on bolt materials is unknown, but it is thought that Cu-added steel forms dense products on the surface of line pipe materials, which makes it difficult for H2 to penetrate. Furthermore, even with the addition of Cu, the stress corrosion properties of high carbon steel cannot be improved. In view of these points, the present invention is designed to improve stress corrosion resistance using a simple method. That is, this invention has C: 0.5 to 1
．． 0%, Si■ 0.15-0.5%, Mn■ 0.3
A steel bar consisting of ~0.89% Cu, 0.2~1.0% Cu, and the balance iron and impurities is subjected to a stretch fern treatment of 0.3~3%. C content is 0.5%
If it is less, the strength will be insufficient, and if it exceeds 190%, the toughness will deteriorate, so 0.5 to 150% is suitable.

The above ranges for Si and Mn are normal ranges specified by JIS and are based on melting requirements. Cu
The effect of the addition of is suggested by the corrosion test shown in Figure 1.

In other words, the figure shows a comparison between a steel with and without Cu added by immersing it in a 20% NH4SCN aqueous solution. Curve 3|4 shows that 0635% of Cu is added. This shows that the addition of Cu improves corrosion resistance. Further, in the case where Cu is not added, corrosion is accelerated by applying stretch, that is, tensile strain, but in the case where Cu is added, corrosion resistance is improved by applying stretch. i.e. 0075%
In the case of C, O, and 35% Cu with 1% stretch added (curve 4), corrosion progresses in the initial stage of immersion, but corrosion hardly progresses thereafter. FIG. 2 shows a 400 times cross-sectional structure of the sample shown by curve 4 in FIG.

This is thought to be because the strain caused by stretching accelerates corrosion, and this dense corrosion product covers the surface and prevents subsequent corrosion. Further, in an experiment conducted to investigate the effects of the amount of Cu added and the amount of stretching, the results were as shown in FIG.

In other words, curves 5, 6, 7, 8, and 9 show that the steel rods were blue-inked at 400°C after being stretched by 3%, 1%, 0.3%, 0.2%, and 0%, respectively. ,2
Immersed in 0% NH4SCN aqueous solution, 70% of tensile strength
FIG. 3 is a diagram showing the relationship between Cu content and rupture time when stress is applied. This shows that stress corrosion resistance improves when the Cu content is 0.2% or more. however,
Even if the Cu content is increased beyond that, the effect remains the same.
Moreover, if the content is too large, the cracking during hot processing becomes significant, so it is preferably within 1%. Also, the stress corrosion resistance improves as the amount of stretching increases, but 0.3%
If it is more than 3%, it is sufficient, and if it exceeds 3%, the effect will be saturated, so a range of 0.3 to 3% is appropriate. If blue ink was not applied to the above curves 5, 6, 7, 8 and 9, the curves would be 50, 60, 70 and 80, respectively.
90, indicating that blue ink is not essential for improving stress corrosion resistance.

In addition, as shown in Table 1, a steel bar G of an embodiment of the present invention containing 0.35% Cu and a normal steel bar D were heated at 60°C.
When a stress of 70% of the tensile strength was applied in a 20% NH4SCN aqueous solution, the time until breakage was investigated, and the results are shown in Table 2.

In addition, both G and D materials have a diameter of 17 wI.
1. After being rolled into a round bar and left to cool, it was subjected to a 10% stretching treatment and then treated with blue ink at 350°C. As is clear from Table 2, the rupture time due to stress corrosion in this invention is approximately 10 times longer than in the conventional method.
It can be seen that Koki has significantly improved stress corrosion resistance.

In addition, heat-treated steel and pearlite steel are used as steel rods for prestressed concrete, but when these are compared with Material G in Tables 1 and 2 above, their tensile strength, toughness, and relaxation properties are almost the same. It was confirmed that the above-mentioned material G has significantly superior stress corrosion resistance properties.

As explained above, this invention applies stretching to steel containing an appropriate amount of Cu, and greatly improves stress corrosion resistance while maintaining general mechanical properties. It is.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the effects of Cu content and stretching on corrosivity, Figure 2 is a photograph of the cross-sectional structure of the steel obtained by this invention, and Figure 3 is the relationship between Cu content and amount of stretching and stress corrosion. It is a relationship diagram with cracking. 4...Corrosion characteristics of the product of the present invention, 5,6,7...
...Stress corrosion cracking resistance of the product of the present invention.

Claims (1)

[Claims] 1 C: 0.5-1.0%, Si: 0.15-0.5%
, Mn: 0.3-0.9%, Cu: 0.2-1.0%,
A method for producing a PC steel bar with excellent stress corrosion resistance, characterized by subjecting a steel bar consisting of the balance iron and impurities to a stretching treatment of 0.3 to 3%.