JPS607686B2 - Manufacturing method for line pipe steel with excellent resistance to hydrogen-induced cracking - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing line pipe steel having excellent resistance to hydrogen-induced cracking and used as a line pipe material for natural gas containing 2S concentration.

Generally, it is well known that if the MnS-based inclusions contained in steel are spheroidized, good results can be obtained in the impact properties, hydrogen-induced cracking, reduction of area in the plate thickness direction, etc. of the finished product.

Methods for spheroidizing MnS-based inclusions include improving the rolling method or adding rare earth elements, Ca, etc. to the molten steel. However, the former method results in a decrease in rolling efficiency and
Perfect spheroidization is difficult. Therefore, the latter rare element, C
Additions such as a to the molten steel have been adopted for this purpose. However, when rare earth elements (mainly Ca) are added to molten steel, the specific gravity of the rare earth elements and their sulfides and oxides is higher than the specific gravity of molten steel, so they aggregate and accumulate at the bottom of the steel ingot or in the continuous casting slab. , which deteriorates the cleanliness of the steel and results in inclusion defects in the product. On the other hand, when Ca is added to molten steel, the specific gravity of Ca, calcium oxide, and calcium sulfide is lighter than the specific gravity of molten steel, so the phenomenon similar to rare element additives is not observed, and it is not ideal. It can be said to be an additive element.

However, in the case of the steel ingot method, since the specific gravity of Ca is lower than that of molten steel, the Ca content becomes low in the axial center of the steel ingot due to flotation. Due to the concentration segregation, it is difficult to completely spheroidize the sulfide inclusions in the S concentration segregation zone at the axial center by adding Ca alone. Therefore, the inventors of this invention added Ca to the molten steel as much as possible in the steel ingot method, and furthermore, for the above-mentioned reasons, the inventors of the present invention added Ca to the molten steel as much as possible, and furthermore, for the above-mentioned reasons, the inventors of the present invention By supplementing and adding a small amount of Ca to make up for the shortage, M
We have discovered a method for manufacturing line pipe steel that has excellent hydrogen-induced cracking resistance and does not impair cleanliness while making nS-based inclusions perfectly spherical. On the other hand, in the continuous casting method, the promotion of levitation of Ca is not as pronounced as in the ○ part of the steel ingot, so in the continuous casting method, the MnS-based inclusions can be fully spheroidized by adding Ca alone. An embodiment of this invention will be described below.

In the steel ingot method, in order to spheroidize MnS-based inclusions throughout the entire product, Ca is added to the molten steel as much as possible, and the inclusions at the S-concentrated bridge bend in the axial center of the steel ingot are spheroidized. This is a steel in which a small amount of Ce is added to compensate for the deficiency in MnS-based inclusions, and the MnS-based inclusions are completely spheroidized. In this case, the following (1)
Ca and Ce are added within the range of formula and formula (2). However, Ca, Ce, and S are concentrations (M%) in each finished product. 5.0mi (a+Ce in 3.)/S...mC
e Mi 0.025 ・・・・・・
{2l In other words, the diagram shown in Figure 1 shows Ca in steel ingots.
This is the actual value when combined addition of
Converted to Ca-Ce-○-S-A-Mn-based inclusions,
As shown in Figure 3, no inclusion enrichment phenomenon was observed at the bottom of the steel ingot.

On the other hand, in the composition range (B) in FIG. 1, an inclusion enrichment phenomenon occurs at the bottom of the steel ingot, resulting in UST defects or hydrogen-induced cracking starting from these giant inclusions. In addition, in the component range (C) in Figure 1, the complete spheroidization of MnS-based inclusions is insufficient, causing hydrogen-induced cracking.
There is no inclusion enrichment phenomenon at the bottom of the steel ingot. In addition, in the continuous casting method, in order to spheroidize the MnS-based inclusions throughout the resulting product, Ca is added to the molten steel to completely spheroidize the MnS-based inclusions.

In this case, Ca is added within the range of the Lake formula and [4- Formula] below. However, Ca and S are concentrations (M%) in each layer of the product. Ca x S Mi 4 x 10-5...
...[311.4miCa/S
...[4] In other words, the chart shown in Figure 2 shows actual values when Ca is added alone in continuous casting materials, but in the component range (A) shown by diagonal lines in Figure 2, No hydrogen-induced cracking occurs.

That is, this is a region where MmS-based inclusions are completely spheroidized, and in the component range (C) in FIG. 2, complete spheroidization is insufficient and hydrogen-induced cracking occurs. In addition, Ca has a limit to its solubility in steel, and the maximum dissolved Ca concentration is approximately 0.01.
%, therefore, there are no actual measured values in the component range (B) in FIG. Next, the reason why the combined addition of Ca and Ce in the steel ingot was limited to the above range is that when the MnS-based inclusion spheroidization index = (3. a + Ce) / S is 5 or less, NmS
System inclusions are the limit value for complete spheroidization, and when the Ce concentration exceeds 0.025%, inclusion enrichment phenomenon occurs at the bottom of the steel ingot, so this is the limit value.

In addition, the coefficient 3.5 in the MnS-based inclusion spheroidization index is C
The atomic weight ratio of e and Ca is shown. In addition, the reason why the individual addition of Ca in the continuous casting method was limited to the above range is that when the MnS-based inclusion spheroidization index = Ca/S is below 1.4, M
nS-based inclusions are at the limit value to become completely spherical, and Ca×S
When the concentration product exceeds 4 x 10-5 (%), the cleanliness deteriorates and this becomes a critical value that causes hydrogen-induced cracking.
In addition, since the maximum dissolved concentration of Ca in steel is 0.01%, there is no plot of Ca concentration higher than this.
The morphology of MnS-based inclusions in each region in FIG. 1 is as shown in FIG. 4. That is, FIG. 4A shows the morphology of MnS-based inclusions in the component range (A), FIG. 4 in the component range (B), and FIG. 4C in the component range. Next, we will discuss the hydrogen-induced cracking test results.

The basic composition of the material used in this test is shown in Table 1, to which Ca and Ce are added. The test method for hydrogen-induced cracking is Casel (artificial seawater Toka 2S gas)
In liquid PH5.1 Caseo (pure water ten ratio S gas)
Tests were conducted under three conditions: pH 4.5 in liquid Casem (0.5% acetic acid gas) pH 3.2 in liquid, and the individual test results are shown in Table 2.

The charts in FIGS. 1 and 2 are a compilation of Table 2. The copper of this invention, which is controlled within the component range (A) shown in Figures 1 and 2, does not generate hydrogen-induced cracking.

Therefore, when steel with excellent resistance to hydrogen-induced cracking is used for line pipe materials for natural gas containing 2S concentration, it is extremely effective.

[Brief explanation of drawings]

Figure 1 is a diagram showing the appropriate range of ingredients that does not cause hydrogen-induced cracking in steel ingots, Figure 2 is a diagram that shows the appropriate ingredient range that does not cause hydrogen-induced cracking in continuous casting materials, and Figure 3 is a diagram showing the appropriate ingredient range that does not cause hydrogen-induced cracking in continuous casting materials. FIG. 4 is a diagram showing the relationship between the height position of the steel ingot and the degree of inclusion accumulation. Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Scope of Claims] 1. A line with excellent hydrogen-induced cracking resistance, characterized by adjusting the addition of Ca alone or the combined addition of Ca and Ce to satisfy the following formula when melting steel for line pipes: Manufacturing method of steel for pipes. In the case of adding Ca alone ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ In the case of combined addition of Ca and Ce ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ However, for Ca, Ce, and S, the concentration (wt
%)