JPS6112849A - Reinforced steel bar having excellent low-temperature toughness and sea water resistance - Google Patents

Abstract

PURPOSE:To produce a reinforced steel bar for reinforced concrete having excellent sea water resistance without decrease in toughness at a cryogenic temp. by using a low-Ni steel as a starting stock and subjecting the material to hot rolling, hardening and tempering under specific conditions thereby producing the reinforced steel bar. CONSTITUTION:The billet of the low-Ni alloy steel which contains <0.17% C, <0.03% Si, <0.70% Mn, <0.015% P, <0.005% S and 2-4% Ni and in which the content of P and S is decreased as far as possible is hot rolled to a steel bar at 750-850 deg.C finishing temp. The bar is forcibly cooled to harden the surface part and is then placed on a cooling bed to recuperate the heat in the steel bar and to temper automatically the steel bar. The reinforced steel bar which has the tempered martensite or bainite layer of >=3mm. depth in the outside circumferential part of the steel bar, has internally the structue consisting of ferrite and pearlite and has >=40kg/mm.<2> yield strength and >=4kg-cm Carpy impact value at -150 deg.C as well as the excellent low-temp. toughness and sea water resistance is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a reinforced steel bar with excellent low-temperature toughness and seawater resistance.

Demand for liquefied natural gas has increased due to the oil situation in recent years, and the demand for reinforced steel bars for use in constructing storage bases has increased.

That is, safety is emphasized in the natural gas storage containers installed recently, and the outer shell is being covered with concrete, and the demand for reinforcing steel bars is increasing. The characteristics required for concrete reinforced steel bars for natural gas storage are excellent seawater resistance because sea sand is used as a raw material for concrete due to the depletion of river sand, and because the boiling point of natural gas is -165°C. It does not exhibit brittleness even at low temperatures of -165°C, and it has high yield strength, which is a general characteristic required of reinforced steel bars.

As is well known, high-Ni steel has excellent low-temperature toughness and is used in low-temperature pressure vessels, etc., but it is very expensive, so if high-temperature toughness is not required, steel containing a few percent of Ni is used at low temperatures. It is used in pressure steel plates and low-temperature steel pipes.

However, this is manufactured through expensive heat treatments such as normalizing and tempering by reheating, and quenching and tempering, and due to the heavy use of sea sand in concrete, there has been a demand for durability in recent years. Seawater properties were not always satisfactory.

On the other hand, reinforcing steel bars have a unique elongated shape with a diameter of several tens of millimeters and a length of several meters, so bending due to heat treatment is unavoidable in the manufacturing method using reheating quenching and tempering, and the straightening process and hardening due to straightening are unavoidable. It requires an additional annealing process to remove it, which requires complicated labor and high manufacturing costs, so it has not been carefully considered in the past. As one of the simple heat treatment methods for such steel bars, a method has been proposed in which the retained heat immediately after hot rolling is used to rapidly cool red-hot steel materials in the temperature range of the austenitic structure and quench harden only the surface layer. This method is a manufacturing method that causes very little bending due to heat treatment and is relatively inexpensive.

Conventionally, surface hardening was applied to low carbon steel reinforcing bars to reduce alloying elements and improve weldability. It was difficult to obtain toughness comparable to that of recycled material. As a result of research on surface quenching after rolling of low-Ni steel bars, the present inventors found that by controlling the finish rolling temperature and making the grain size finer, the low-temperature toughness of the non-hardened parts inside the steel bars can be improved by combining the effect of Ni with the effect of Ni. It has been found that the toughness can be improved by further reducing the Si, P, and S contents in the steel components without decreasing the toughness.

Next, we tested the seawater resistance of Ni steel, and found that the seawater resistance was slightly better than that of general carbon steel due to the corrosion resistance of Ni, but martensitic and bainitic steels due to quenching are chemically stable even after tempering. Due to its stability, corrosion resistance was poor, and the target seawater resistance could not be achieved. However, even in these tempered martensitic or bainitic steels, si.

Seawater resistance is significantly improved by reducing P and S.
I found out that it does. In particular, the influence of the amount of S is extremely significant.

The present invention is based on the above findings, and provides a reinforcing steel bar with outstanding seawater resistance and low-temperature toughness.

That is, the gist of the present invention is as follows: C: 0.17% or less, Si: Q, Q 3% or less, Mn:
0.70% or less, P: 0.015% or less, S: O.

005% or less, Ni: 2 to 4%, the balance is iron and unavoidable impurities, the outer periphery has a tempered martensite or bainite layer with a depth of 3 mm or more, and the interior is made of ferrite and pearlite. Yield strength: 40 kg/mrr
? Above, Charpy impact value 4kg at -150℃
It is a reinforcing steel bar with excellent low temperature toughness and seawater resistance, and has a toughness of at least m.

Next, the reason for determining the above-described range of components in the present invention and the manufacturing method suitable for the steel of the present invention will be described.

The reason why C is set to 0.17% or less is that C is an element with high hardenability, and if it exceeds 0.17%, toughness deteriorates. When the amount of C increases in combination with other elements, the toughness of not only the unhardened portion inside the steel bar but also the quenched and tempered portion of the surface layer decreases. Although low-temperature toughness cannot be obtained. When Si is buried in concrete, it becomes enriched in the film formed on the surface of the reinforcing bars, forms silicates, and has a greater tendency to be eroded by the alkaline liquid that constitutes the concrete. Therefore, in order to reduce this tendency, the amount of Si is 0.03
% or less. Mn is used as a reinforcing element that causes less deterioration in toughness, but if it exceeds 0.70%, the hardenability becomes excessive and like C, the toughness deteriorates, so it is set to 0.07% or less. P is an element that cannot be avoided as an impurity element and is 0
．． If it exceeds 0.015%, seawater resistance and low-temperature toughness deteriorate, so the content was set at 0.015% or less. It has been found that seawater resistance and low temperature toughness are significantly improved by reducing S to 0.005% or less in combination with P.
The following was made. Ni is a solid solution in the base iron to improve strength and toughness, and if it is less than 2%, the effect will be small, and in consideration of alloy cost, the range is set to 2 to 4%. The depth of the tempered martensite or bainite layer in the surface layer is set to 3 mm or more because if it is less than 3 mm, the required yield strength and Charpy impact value cannot be obtained.

Next, a manufacturing method suitable for the steel of the present invention will be described.

The Ar transformation point of the steel of the present invention is 750 to 800°C, and the Ar transformation point is 670 to 680°C, and the crystal grains are fine in both rolling in the austenite region and rolling in the austenite-ferrite dual phase region. The rolling temperature in the area is 750°C to 900°C to obtain good properties.
It is desirable to set it to ℃. However, within this range, 750
Low-temperature finish rolling at 0C to 850C is even more preferred.

If the lower limit temperature is 750°C, the deformation resistance during rolling becomes large and exceeds the capacity of a normal rolling mill. Furthermore, although the strength of the material improves, the toughness deteriorates. Note that the material properties obtained by rolling in the two-phase region at 790° C. to 750° C. are such that the ferrite/pearlite structure in the non-quenched portion becomes finer grained than in rolling in the austenite region, resulting in excellent low-temperature toughness and yield strength. However, the deformation resistance is greater than in austenite region rolling.

Therefore, it is more advantageous in terms of rolling load to perform rolling in the austenite region if it is acceptable to sacrifice some low-temperature toughness and yield strength. The upper limit temperature is 900° C. If the temperature exceeds 900° C., the austenite crystal grains become coarse and the fuel cost increases to 1 due to the increase in heating temperature.

Further, it is effective to set the cooling rate after rolling (from the γ range to 400°C) to 20'C/see or higher; if the cooling rate is lower than this, the surface layer will not become a hardened structure.

Next, examples of the present invention will be described. Table 1 shows the types and chemical compositions of the test materials.

In addition to the steel of the present invention, 3,5Ni steel (JIS G3127゜5L3N), which is normally produced as a comparative material, was used as a test material.
45), deformed reinforced steel bar type 3 (JIS G31'12
, 5D35) and class 1 steel bars for reinforced concrete (JI
'SG31'12. 120 billet having a chemical composition compatible with '5R24) was used.

Each sample billet was heated to 1100°C to 1150°C and rolled into a deformed steel bar (D25). The finishing rolling temperature at this time was controlled to be 800°C to 850°C. Among the test materials immediately after finish rolling, the present invention steel and 3.5% steel are strong - after forced cooling in a ring trough and quenching of the surface of the steel material, the steel is reheated automatically using the heat inside the steel material on a cooling bed. Tempered to °. Sample material 5D
35°5R24 was simply cooled on a cooling bed as usual without performing forced cooling.

FIG. 1 shows the cooling curve of the sample material subjected to surface hardening treatment. The average cooling rate at the outermost surface is 120°C/see
In this case, the depth of the hardened structure at the outer periphery is about 5 m.
It was m. Next, using the rolled material, tensile and low-temperature impact tests were conducted to investigate the mechanical properties. JIS No. 2 (L=8D) was used as the tensile test piece, and JIS No. 4 (V-notch) was used as the impact test piece. Table 2 shows the mechanical test results.

The tensile strength of the steel of the present invention and the 3,5 q i steel is higher than that of 5D35 and 5R24 produced by simple cooling, because the steel was cooled during use using a cooling trough. On the other hand, the low-temperature impact value of the steel of the present invention with reduced Si, P, and S content is 10 kg-m at -165°C.
These results indicate that the material exhibits excellent low-temperature toughness despite its high strength compared to other test materials.

Next, using the steel of the present invention having the components shown in Table 1, quenching and tempering were performed (holding at 800°C for 30 minutes → water cooling, 60
0°C x 6Q minutes → water cooling). Table 3 also lists the characteristic values when quenching and tempering were performed and when surface quenching was performed.

The surface-hardened steel produced by forced cooling using a cooling trough had almost the same tensile properties and low-temperature toughness as the conventional hardening and tempering method.

FIG. 2 shows the microstructure of the steel of the present invention having the components shown in Table 1 after being quenched and tempered and subjected to the surface hardening treatment according to the present invention. The structure of the quenched and tempered material is a co-quenched and tempered martensite and bainite structure in the surface layer and center.
The surface hardened material has a tempered martensite and bainite structure in the surface layer, and extremely fine ferrite in the center.
It has a perlite structure.

Next, in order to investigate the seawater resistance, using the test materials shown in Table 1 above, the test method was to anodic polarize the test materials using 0.2% saline solution at a liquid temperature of 25°C by constant potential method. Curve measurements were taken. FIG. 3 shows potential-current density curves for each sample material. As shown in the figure, in the steel of the present invention, the current density does not increase up to the highest potential.

In other words, the seawater resistance is 3.5% of that of conventional reinforcing steel.
It can be seen that it is extremely superior when compared to Ni1ll.

As described above, the steel of the present invention is a high-strength reinforced steel bar with unprecedented low-temperature toughness and seawater resistance, and can be manufactured at an extremely low cost without the conventional reheating heat treatment. This is an extremely useful invention.

[Brief explanation of drawings]

FIG. 1 shows the cooling curve of the steel bar according to the present invention. Figure 2 shows the microstructure. Figure 3 shows the seawater resistance test results.

Claims (1)

[Claims] C: 0.17% or less, Si: 0.03% or less, Mn: 0
．． 70% or less, P: 0.015% or less, S: 0.005
% or less, Ni: 2-4%, the balance consists of iron and unavoidable impurities, the outer periphery has a tempered martensite or bainite layer with a depth of 3 mm or more, and the interior consists of ferrite and pearlite. Yield strength 40 kg/mm^ A reinforced steel bar with excellent low-temperature toughness and seawater resistance, having a Charpy impact value of 4 kg-m or more at -150°C.