JPH04259352A - Steel excellent in hydrogen-induced cracking resistance and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture a steel excellent in hydrogen-induced cracking by subjecting molten iron to preliminary dephosphorizing and desulfurizing, thereafter performing steel refining in a converter, furthermore subjecting it to desulfurizing and refining in a ladle and degassing treatment and thereafter blowing Ca therein. CONSTITUTION:Blast furnace molten iron is subjected to preliminary dephosphorizing and desulfurizing treatment to reduce the content of P and S in the molten iron to <0.015% and <0.004%. Next, this molten iron is subjected to desulfurizing and refining in a converter into molten steel, is thereafter charged to a ladle and is freed of mixed slag to prevent the return of phosphorus from the slag. Then, desulfurizing and refining are performed by arc heating and atmospheric regulation to regulate the content of S to <0.0008%, and is successively subjected to degassing and refining to <0.00015% H2 and <0.000% N2 by an RH type vacuum degassing apparatus or the like, in which Ca-Si is blown by Ar, etc., within 3min in the ratio of 0.05 to 0.10kg in terms of Ca per one ton molten steel. The steel contg., by weight, 0.02 to 0.05% C, 0.01 to 0.05% Si, 0.60 to 2.00% Mn, 0.01 to 0.10% Al, <0.001% S and 0.0005 to 0.002% Ca and excellent in hydrogen induced cracking resistance can be manufactured.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing steel with excellent hydrogen cracking resistance.

0002

[Prior Art and Problems] In recent years, many plans have been made to develop oil fields and lay pipelines due to the increase in oil consumption. When the hydrogen sulfide contained in these steels coexists with water, the steel surface is severely corroded. In other words, hydrogen generated by corrosion under the above conditions invades the steel and aggregates around the drawn sulfide inclusions such as MnS, increasing the internal pressure.
Hydrogen-induced cracks occur parallel to the plate thickness direction, and in some cases, they penetrate through the plate thickness direction and destroy the steel material. According to research to date, hydrogen-induced cracking occurs when hydrogen that has entered the steel from the environment due to corrosion collects at the boundary between nonmetallic inclusions and the steel base, and is caused by the gas pressure. There is a strong correlation with the amount and degree of spreading of MnS, which tends to cause stress concentration due to the shape effect (notch effect) at the tip of inclusions, and the less elongated MnS there is, the lower the susceptibility to cracking becomes.

Therefore, steel materials used in the harsh corrosive environment of oil fields are made by reducing the S content to reduce sulfide-based inclusions, which are the starting point for hydrogen-induced cracking, and by adding Ca to reduce non-metallic inclusions. Controlling the shape of objects has been attempted to reduce susceptibility to hydrogen-induced cracking.

[0004] As a method for producing steel materials with excellent hydrogen cracking resistance, as disclosed in Japanese Patent Publication No. 57-14747, less than 0.0020 wt% of S and less than 0.0020 wt% of Ca are used.
Ca is added to the molten steel using a Ca wire so that it contains 0.002 wt% or more and less than 0.0050 wt%.

However, in this method, the yield of Ca addition is at most 18%, and excess Ca forms oxides, some of which remain in the steel material as inclusions. For this reason, as the specifications for steel materials with high strength and excellent hydrogen-induced cracking resistance have become stricter in recent years, it has been difficult to manufacture high-quality steel materials that stably satisfy the specifications.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a steel material having high strength and excellent resistance to hydrogen-induced cracking, and a method for manufacturing the same.

[0007]

[Means and effects for solving the problem] The steel material with excellent hydrogen-induced cracking resistance according to the first invention has a C; 0.02.
~0.05t%, Si:0.01~0.05wt%,
Mn: 0.60-2.00wt% and Al: 0.0
In steel materials containing 1 to 0.10 wt% and used in environments containing hydrogen sulfide, it contains S reduced to less than 0.001 wt% and Ca of 0.002 to 0.005 wt%, with the balance is characterized in that it consists essentially of Fe including impurities.

[0008] A method for manufacturing a steel material with excellent hydrogen-induced cracking resistance according to the second invention is to reduce the P content to 0.015 wt% or less and the S content to 0.015 wt% or less in the hot metal by pre-treating the hot metal.
004 wt% or less, S is reduced to 0.0008 wt% or less by ladle refining of molten steel decarburized by converter refining, H is reduced to 0.0001 wt% or less by degassing treatment, and then Ca is added to the molten steel. Features.

In this case, the pure Ca content is 0.05 to 0.10
It is desirable to add Ca to the molten steel by passing kg/ton of CaSi alloy powder into the molten steel in a ladle through a top blowing lance together with Ar gas within 3 minutes.

0010

[Example] The manufacturing process of this example is as follows. First, the hot metal tapped from the blast furnace is desiliconized in the cast bed to remove Si.
After setting the concentration to ≦0.10wt%, S
≦0.004wt%, then P by hot metal dephosphorization equipment
≦0.015wt%. The hot metal that has been subjected to this hot metal pretreatment is charged into a top-bottom blowing converter in which inert gas is blown through a bottom-blowing nozzle made of a collection of thin tubes, and decarburization and dephosphorization are performed. P ≦0.010 wt%, C ≦ 0.
After charging the sample into a ladle at 0.40 wt%, the sludge is removed using a vacuum sludge remover to prevent rephosphorization. The molten steel after the slag removal is desulfurized and deoxidized in a ladle refining device with arc heating and atmosphere adjustment functions, and S ≦0.0008wt%.
, T O ≦0.0030wt%. Next, dehydrogenation and denitrification are performed in an RH degassing device, and H
≦0.00010wt%, N≦0.004
It is set to 0wt%. After the degassing treatment, CaSi is injected into the molten steel using a powder injection device that injects powder together with an inert gas into the molten steel. Addition of Ca is 0.04 to 0.10 kg/ pure Ca.
ton of Ar gas is blown into the molten steel as a carrier gas at a flow rate of 2000 to 3000 Nl/min. If the amount of Ca added is less than 0.04 kg/ton, the morphology control of MnS will be insufficient, and if it exceeds 0.10 kg/ton, the amount of Ca added will be insufficient.
A-based sulfates increase.

[0011] Next, this molten steel is cast into a vertical bending type continuous casting machine to form a slab, which is rolled in a plate mill and then pipe-formed in a UOE mill.

[0012] In recent years, customer specifications for hydrogen-induced cracking (HIC) resistance have become stricter. Table 1 shows, as an example, HIC specifications for X65 and X60 of the API standard.

[0013]

[Table 1]

CLR (Crack Len) shown in Table 1
gth Ratio), CSR (Crack Sens.
CTR (Crack
Thickness Ratio indicates a measurement method for testing the hydrogen cracking resistance of steel materials, and is specified by ASTM. To briefly explain this measurement method, a sample is cut out from the line pipe made as described above, and the sample is mixed with a so-called NACE solution specified by ASTM 01-77, that is, (5% NaC
l + 0.5% CH3COOH + H2S saturation,
After immersion in a solution with a pH of 3.5 to 3.8 for 96 hours, the cracks that have occurred are observed under a microscope over an area of 3 x 20 mm. Regarding the observed cracks, CLR
is the crack length, CTR is the thickness when multiple cracks are layered in the plate thickness direction, and CSR is the product of the crack length and thickness divided by the observed cross-sectional area as %. This is shown in . Table 1 also shows the yield strength (Ys, kg
/mm2) and tensile strength (Ts, kg/mm2) are also shown, but although the strength is higher than that of conventional low-grade API standard products, SLR,
The specifications for cracks such as SCR and STR have not been relaxed, and the specifications shown in Table 1 are actually strict. In the conventional manufacturing method, the X60 fully satisfies the specifications, but the X65 does not meet the specifications in Table 1, resulting in approximately 13.6% of products being defective, and this defective rate is It was strongly desired to lower the

[0015] Various studies have been conducted for the purpose of reducing the defective rate, and one of them is to use EPMA (Electron Micro Prob
e Analyzer). An example is shown in Figure 1. According to Figure 1, the observed inclusions are Ca-Al
It turned out to be a type sulfate. It is well known that controlling the morphology of MnS by adding Ca is important for hydrogen-induced cracking resistant steel, but it is also necessary to thoroughly study the behavior of excessive Ca. Addition of more than necessary for morphology control produces the above-mentioned Ca-based sulfides, which increases the occurrence of cracks.

[0016] Focusing on this point, the yield of added Ca was studied. The results are shown in FIG. Figure 2 shows the amount of CaSi alloy added (kg/
ch), the concentration of Ca, and the yield of Ca. The CaSi alloy used here contains Ca; 30wt%
, Si; 60 wt%, balance Fe powder. Therefore, in this example, the addition amount (kg/ch) of the CaSi alloy is 235 ton/ch for the molten steel, and
Considering the yield of a, the added amount of pure Ca (kg/ch) can be easily determined. As shown in Figure 2, CaS
The addition yield of Ca is improved by reducing the amount of i added, and the amount of Ca in the product is reduced to 40 kg/ch.
Even if it is reduced to 16 to 20 ppm. Furthermore, since the Ca yield is improved to 30%, excess oxides and sulfides are significantly reduced. In addition, S in the product
By setting Ca/S to 8 ppm or less, it is possible to ensure Ca/S of 2 or more. Therefore, as shown in Fukumi et al., Tetsu-to-Hagane, 69 (1983) S214, it is possible to control the morphology of MnS if Ca/S is 2 or more. As shown above, it is understood that in the production of HIC-resistant steel, it is necessary to improve the Ca yield and reduce Ca sulfide (Oxysulfide) generated as Ca loss. Table 2 shows that the effect of Ca addition is clear even when S is 10 ppm or less.

[0017]

[Table 2]

[0018] Table 2 shows the results for four levels A to D. A and B are for this example in which S is 10 ppm or less, and C and D are comparative examples for this example except for the addition of CaSi. Similar to certain A and B, the samples were manufactured according to the above-mentioned process. In A and B, the amount of CaSi alloy added is 0.05 kg/ton. As shown in this table, the maximum CLR for A and B is 7%, which is sufficient.
Satisfies HIC resistance specifications. On the other hand, for C and D without Ca addition, CL
R reaches 26% at the minimum and 33% at the maximum, which does not satisfy the specifications in Table 1 above. Therefore, ultra-low sulfidation treatment (
S ≦8 ppm) alone indicates that MnS as an inclusion becomes the starting point of cracking.

Table 3 shows the component analysis values of X65 grade steel produced in actual operation for 10 different samples, all expressed as weight ratios. All of these satisfy the specifications in Table 1. Arbitrary elements Cu, Ni, V,
Nb and Ti are added as necessary to further improve the properties of the manufactured steel. One or more of these additive elements can be added within an appropriate range without impairing the hydrogen-induced cracking resistance.

[0020]

[Table 3]

Next, the reasons for limiting the components of the steel material having excellent hydrogen-induced cracking resistance according to the present invention will be explained. C: If it is less than 0.01wt%, the required strength cannot be obtained, and 0.
．． If it exceeds 0.05 wt%, unfavorable effects will appear on the toughness and weldability of the steel, so the content was set at 0.02 to 0.05 wt%. Si: An element necessary for deoxidation, but if it is less than 0.01 wt%, there is no deoxidizing effect, and if it exceeds 0.50 wt%, there is a risk of deterioration of toughness, so Si: 0.01 to 0.50 wt%.
And so. Mn: If it is less than 0.60 wt%, it will be difficult to meet the strength requirements, while if it exceeds 2.00 wt%, it will have a negative effect on toughness and weldability.
It was set as wt%. Al: is an element necessary for deoxidation, but if it is less than 0.01wt%, the deoxidation effect will not actually occur, but at 0.10w
If it exceeds t%, it promotes coarsening of crystal grains and deteriorates the material quality, so it is set at 0.01 to 0.10wt%. S: If it exceeds 0.001 wt%, it is not possible to effectively control the morphology of MnS, which becomes the starting point of cracks, so the content was set to 0.001 wt% or less. Ca: If it is less than 0.0005 wt%, the morphology of MnS cannot be controlled sufficiently, and if it is more than 0.002 wt%, the production of Ca-based sulfates cannot be effectively reduced. It was set as .002 wt%.

[0022]

According to the present invention, desulfurization and dephosphorization are performed through hot metal pretreatment, and Ca addition is performed through converter refining, ladle refining, and degassing treatment, so the amount of Ca added can be reduced by extremely low sulfidation. It is possible to reduce the amount of Ca and improve the yield, and it is possible to reduce sulfates caused by excess Ca and improve resistance to hydrogen-induced fragility.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between X-ray energy and intensity according to EPMA analysis.

[Figure 2] Addition amount (kg/ch) of CaSi alloy and
FIG. 2 is a graph diagram showing the relationship between the concentration of Ca and the yield of Ca.

Claims (3)

[Claims] Claim 1: C; 0.02 to 0.05t%, Si
:0.01~0.05wt%, Mn:0.60~2.
00 wt% and Al: 0.01 to 0.10 wt% and used in an environment containing hydrogen sulfide, S and 0.00 wt% are reduced to less than 0.001 wt%.
A steel material having excellent resistance to hydrogen-induced cracking, characterized in that it contains 0.02 to 0.005 wt% of Ca, and the remainder, including impurities, consists essentially of Fe. [Claim 2] P in hot metal is reduced by hot metal pretreatment.
0.015wt% or less, S 0.004wt
% or less, S is reduced to 0.0008 wt% or less by ladle refining of molten steel decarburized by converter refining, H is reduced to 0.00015 wt% or less by degassing treatment, N is reduced to 0.00015 wt% or less, and N
A method for producing a steel material with excellent hydrogen-induced cracking resistance, which comprises adding Ca to molten steel after reducing the content of the steel to 0.0050 wt% or less. [Claim 3] Ca pure content: 0.05 to 0.10 kg/
ton by CaSi alloy powder within 3 minutes.
Pass the gas through the top blowing lance through the molten steel in the ladle,
3. The method for producing a steel material with excellent hydrogen-induced cracking resistance according to claim 2, characterized in that Ca is added to the molten steel.