RU2765963C1 - Acid- and corrosion-resistant steel for pipeline with large wall thickness and method of production thereof - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to acid and corrosion resistant steel for making thick-walled pipeline. Steel contains the following components in wt. %: C: 0.01–0.02; Si: 0.10–0.35; Mn: 0.9–1.40; P≤0.012; S≤0.0010; Nb: 0.020–0.070; Ti: 0.006–0.020; Ni≤0.30; Mo: 0.10–0.30; Cr: 0.10–0.30; Cu: 0.10–0.30; Al: 0.015–0.050; Ca: 0.0005–0.0040; wherein the residue is Fe and impurities. In the process of steel production, the cast billet is subjected to austenitization at temperature of 1110–1120 °C. Water temperature at the inlet during rolling is 790–800 °C, and ultra-fast cooling is used for cooling to 280–300 °C to obtain an ultra-fine grain structure with a core less than 30 mcm.

EFFECT: higher resistance to acids and corrosion, including in the presence of hydrogen atoms.

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Description

Technical field

The present invention relates to the field of metallurgy, in particular to an acid and corrosion resistant steel for a pipeline with a thick wall and a method for producing the same.

State of the art

The acid and corrosion resistant pipeline steel is mainly resistant to hydrogen induced cracking (HIC) and sulfide stress corrosion cracking (SSC). Hydrogen sulfide is a weakly acid electrolyte and is mainly found in molecular form in an aqueous solution with a pH of 1 to 5. Hydrogen sulfide reacts with metals: H ₂ S+Fe→FeS+2H to produce atomic hydrogen. H ₂ S is used as a poisoning agent in order for hydrogen to form a combination with hydrogen molecules, facilitating the entry of atomic hydrogen into the steel matrix. Hydrogen atoms entering the steel cause defects through diffusion and precipitation into hydrogen molecules, generating high pressure. In the presence of tensile stress (additional and/or residual), hydrogen is enriched in the tripartite area of tensile stress provided by metallurgical defects (inclusions, grain boundaries, phase boundaries, shears, cracks, etc.). When the concentration of segregated hydrogen reaches a critical value, high-strength steel and components with high internal stress, etc. will crack as a result of the combined action of hydrogen and stress field. Thus, in terms of product design, the main problem has become how to prevent atomic hydrogen from entering the steel matrix so that the steel matrix meets the requirements for acid resistance and corrosion.

DISCLOSURE OF THE INVENTION

Aiming at the above technical problems and overcoming the shortcomings of the prior art, the present invention provides an acid and corrosion resistant steel for a thick wall pipeline and a method for producing the same, which is not susceptible to corrosion by hydrogen atoms and improves the resistance of the matrix to acids and corrosion. .

In order to solve the above technical problems, the present invention provides an acid and corrosion resistant steel for a pipeline with a thick wall with the following components and their weight percentages: C: 0.01% - 0.02%, Si: 0.10% - 0.35%, Mn: 0.9%-1.40%, P≤0.012%, S≤0.0010%, Nb: 0.020%-0.070%, Ti: 0.006%-0.020%, Ni≤0.30 %, Mo: 0.10%-0.30%, Cr: 0.10%-0.30%, Cu: 0.10%-0.30%, Al: 0.015%-0.050%, with the remainder being Fe and impurities.

Technical Results: In the present invention, an ultra-low carbon composition of less than 0.020% is used, which effectively prevents the formation of a pearlite structure in a thick-wall pipeline rolling plate. Appropriate use of Mo can effectively improve the hardenability of thick-walled rolling plate, which promotes uniformity of the surface core structure and has a grain refining effect. The use of Ni contributes to the refinement of the grain size of the structure, forming a dense multifaceted ferrite structure and preventing corrosion of the grain boundary by hydrogen atoms. The ultra-low temperature austenitization process effectively reduces the grain size of the structure in the thickness direction of the rolling plate to obtain an ultra-fine grain structure with a core grain size of less than 30 µm. The dense structure prevents atomic hydrogen from entering the steel matrix.

Another object of the present invention is to provide a method for producing acid and corrosion resistant steel for a thick wall pipeline applied to the above solution, comprising the following steps:

S1: performing desulfurization with lime and magnesium powder in the desulfurization station, and cleaning the slag after the desulfurization, wherein the S content in the molten iron in the furnace is ≤0.002%;

S2: performing high-temperature dephosphorization in the converter in a two-slag method, and when reaching 85% of the purge, the temperature of the auxiliary lance is measured and sampled; and at this time, the filling of the slag is carried out by means of an oxygen lance; after completion of the slag filling, a second purge is performed according to the sub-lance sampling to ensure that the outlet temperature in the converter is higher than 1680° C. and that the final product P is ≦0.012%;

S3: Adopting a weak deoxidation method for tapping out of the converter, adding 500 kg of low-carbon ferromanganese when tapping the metal, and adding aluminum blocks according to the specific oxygen of the auxiliary lance to ensure that the oxygen content of the molten steel is between 50 ppm and 80 ppm million after release;

S4: After the molten steel enters the RH furnace, measure and sample the temperature, perform decarburization treatment after the temperature is over 1580°C and the vacuum degree is ≤3.0mbar, and then perform the fusion treatment after the vacuum is generated and C ≤ 0.0050%, and maintaining a vacuum after the completion of the fusion;

S5: after the steel reaches LF, adding limestone to produce slag, mixing with argon at the bottom, controlling the sulfur content within 0.0010%, and performing calcium treatment after desulfurization is completed;

S6: complete pouring with full protection during continuous casting, superheat control at 10-20°C, adopting double-roll electromagnetic stirring technology, and low rated power is within C1.0;

S7: heating the casting billet in the heating furnace by the step heating system, wherein the target austenitization temperature is 1110°C-1120°C, the heating time is 10 min-13 min/cm, and the holding time is more than 60 min; and

S8: Performing the rolling by the thermo-mechanical control process, where the final reduction percentage per pass through the rough surface rolls is more than 22%, and the initial temperature of the two-stage rolling is 800°C-830°C, the inlet water temperature is 780°C -800°C, as well as cooling by ultrafast cooling to 280°C-300°C to obtain an ultrafine grain structure with a core grain size of less than 30 μm and a multifaceted ferrite/bainite structure, namely (PF+B).

The technical solution in the present invention is further limited as follows:

Next, after step S3, the aluminum mass is increased to 20 kg to increase to 600 ppm oxygen, and then the aluminum content is increased to 10 kg to obtain an increase per 100 ppm oxygen.

With respect to the above acid and corrosion resistant steel for thick wall pipeline and its production method, 150 m-200 m seamless pure calcium flux-cored wire is used in step S5.

With regard to the above acid and corrosion resistant steel for thick wall pipeline and the method for producing the same, in step S5, static mixing is performed for 15 to 20 minutes after the calcium treatment.

The beneficial effects of the present invention are as follows:

(1) The ultra-low austenitization temperature in the present invention effectively refines the grain size of the structure, achieves an ultra-fine grain structure of 30 µm in the core of the rolled sheet with a large thickness, and improves the acid and corrosion resistance of the matrix;

(2) In the present invention, after the molten steel reaches the RH furnace, the temperature is maintained above 1580°C, which ensures the fluidity of the vacuum cycle after the molten steel is fused;

(3) In the present invention, decarburization is performed after the vacuum degree has become lower than 3.0 mbar, and carbon is removed by using oxygen in the molten steel;

(4) In the present invention, vacuum maintenance is performed after the completion of alloying, and during the vacuum processing process, the alloying operation is not performed, which effectively reduces the gas content of the molten steel;

(5) In the present invention, after the molten steel reaches LF, it is mixed with argon at the bottom to desulfurize;

(6) In the present invention, static mixing is performed after calcium treatment to improve the purity of molten steel;

(7) In the present invention, the double-roll electromagnetic stirring technology is applied during continuous casting in order to improve the internal structure and control center segregation;

(8) In the present invention, in the product design part, a matrix is used that prevents atomic hydrogen from entering the steel. The low carbon, low phosphorus, low sulfur and fine grain structure is compact, so that the steel matrix satisfies the idea of acid and corrosion resistance. Due to the composition composition, an ultra-fast cooling process with low austenitization temperature, ultra-fine multifaceted ferrite and bainite structure with a core of 30 µm was obtained, thereby obtaining a steel sheet excellent in acid and corrosion resistance.

Brief description of the attached drawings

On FIG. 1 shows a typical microstructure of the rolled steel sheet in Embodiment 1 under a metal microscope;

On FIG. 2 shows a typical microstructure of the rolled steel sheet in Embodiment 2 under a metal microscope.

Implementation Options

Implementation Option 1

The method for producing acid and corrosion resistant steel for thick wall pipeline in the present embodiment includes the following steps:

S1: performing desulfurization with lime and magnesium powder in the desulfurization station, and cleaning the slag after the desulfurization, wherein the S content in the molten iron in the furnace is ≤0.002%;

S2: performing high-temperature dephosphorization in the converter in a two-slag method, and when reaching 85% of the purge, the temperature of the auxiliary lance is measured and sampled; and at this time, the filling of the slag is carried out by means of an oxygen lance; after completion of the slag pouring, a second purge is performed in accordance with the selection of the auxiliary lance to ensure that the metal tapping temperature in the converter is higher than 1680° C. and that the final product P is ≦0.012%;

S3: Applying a weak deoxidation method for tapping from the converter, adding 500 kg of low-carbon ferromanganese when tapping the metal, and adding aluminum blocks according to the specific oxygen of the auxiliary lance, the mass of aluminum is increased to 20 kg to increase to 600 ppm oxygen, and then the content aluminum is increased to 10 kg to obtain an increase for every 100 ppm of oxygen, to ensure that the oxygen content of the molten steel is 50 ppm to 80 ppm after tapping;

S4: After the molten steel enters the RH furnace, measure and sample the temperature, perform decarburization treatment after the temperature is over 1580°C and the vacuum degree is ≤3.0mbar, and then perform the fusion treatment after the vacuum is generated and C ≤ 0.0050%, and maintaining a vacuum after the completion of the fusion;

S5: after the steel reaches LF, add limestone to make slag, mix with argon at the bottom, control the sulfur content within 0.0010%, and perform calcium treatment with 150m-200m pure calcium seamless cored wire after desulphurization is completed, and static mixing for 15 to 20 minutes after the calcium treatment;

S6: complete pouring with full protection during continuous casting, superheat control at 10-20°C, adopting double-roll electromagnetic stirring technology, and low rated power is within C1.0;

S7: heating the casting billet in the heating furnace by the step heating system, wherein the target austenitization temperature is 1110°C-1120°C, the heating time is 10 min-13 min/cm, and the holding time is more than 60 min; and

S8: Performing the rolling by the thermo-mechanical control process, where the final reduction percentage per pass through the rough surface rolls is more than 22%, and the initial temperature of the two-stage rolling is 800°C-830°C, the inlet water temperature is 780°C -800°C, as well as cooling by ultrafast cooling to 280°C-300°C to obtain an ultrafine grain structure with a core grain size of less than 30 μm and a multifaceted ferrite/bainite structure, namely (PF+B).

Implementation Option 2

The acid and corrosion resistant thick wall pipeline steel of the present embodiment was obtained by the production method described in Embodiment 1, and its weight percent components are shown in Table 1.

Implementation Option 3

The acid and corrosion resistant thick wall pipeline steel of the present embodiment was obtained by the production method described in Embodiment 1, and its weight percent components are shown in Table 1.

Table 1

C% Mn% P% S% Si% Ni% Cr% Cu% Alt% Nb% Mo% V% Ti% Ca% Implementation Option 1 0.012 1.32 0.008 0.0006 0.22 0.012 0.18 0.21 0.030 0.063 0.16 0.002 0.013 0.016 Implementation Option 2 0.011 1.31 0.006 0.0007 0.21 0.011 0.16 0.22 0.031 0.061 0.18 0.002 0.013 0.017

By observing the microstructure of the pipeline steel obtained in Embodiment 1 and Embodiment 2 as shown in FIG. 1 and FIG. 2, it was found that there is no obvious pearlite structure in the matrix, and the core structure in the thick wall rolled sheet is less than 30 µm, while the grains are fine and dense, and also have excellent acid and corrosion resistance.

Performance characteristics at HIC are shown in Table 2:

table 2

holding solution CLR% CTR% CSR% Implementation Option 1 A 0 0 0 Implementation Option 2 A 0 0 0

Performance characteristics at SSC are shown in Table 3:

Table 3

Load voltage Test time EC cracking Grade Implementation Option 1 0.8*Y.S (actual) 720 h Not Passed Implementation Option 2 0.8*Y.S (actual) 720 h Not Passed

From Tables 2 and 3, it can be seen that both Embodiments 1 and 2 have excellent resistance to hydrogen cracking without any cracks, and also fully comply with NACE0177-2005: CLR≤10%, CTR≤3% , CSR≤ 2%, the performance of the product under HIC and SSC is compliant.

The present invention uses an ultra-low carbon composition of not more than 0.02%, which effectively prevents the formation of a pearlite structure in thick wall pipelines. A corresponding increase in Cr and Mo alloys improves the hardenability of the thick wall steel sheet, which is an advantage for core structure refinement. The ultra-low austenitization temperature creates conditions for the refinement of the original austenite grains and ensures the grain size of the structure of the rolled sheet. The ultra-fast cooling process creates conditions for the transformation of bainite into a carbide structure, promotes grain refinement, and also strengthens the compactness of the grain boundary structure. The fine grain structure effectively prevents the diffusion of hydrogen atoms along the grain boundaries, thus providing an ultrafine grain structure with a core less than 30 μm. The structure is a multifaceted ferrite/bainitic structure (PF+B). Multi-faceted ferrite and heterogeneous bainite are distributed alternately, which ensures the strength and rigidity of the steel, as well as the acid and corrosion resistance of the thick wall steel plate.

In addition to the embodiments described above, the present invention may include other embodiments. Any technical solution formed by equivalent substitution or equivalent transformation falls within the protection scope of the present invention.

Claims (12)

1. A method for producing acid and corrosion resistant pipeline steel, wherein the acid and corrosion resistant pipeline steel contains the following components in the following weight percentages: C: 0.01-0.02%, Si: 0.10-0.35 %, Mn: 0.9-1.40%, P≤0.012%, S≤0.0010%, Nb: 0.020-0.070%, Ti: 0.006-0.020%, Ni≤0.30%, Mo: 0, 10-0.30%, Cr: 0.10-0.30%, Cu: 0.10-0.30%, Al: 0.015-0.050%, Ca: 0.0005-0.0040%, with the remainder is Fe and impurities, characterized in that it includes the following steps: S1: performing desulfurization with lime and magnesium powder in the desulfurization station and cleaning the slag after the desulfurization, wherein the content of S in the molten iron in the furnace is ≤0.002%; S2: performing dephosphorization in the converter in a two-slag method, wherein when 85% of the purge is reached, the temperature of the auxiliary lance is measured and sampled; and at this time, the filling of the slag is carried out by means of an oxygen lance; after completion of the slag pouring, a second purge is performed according to the selection of the auxiliary lance to ensure that the metal tapping temperature in the converter is higher than 1680°C and that in the final product P is ≤0.012%; S3: Applying the deoxidation method for tapping from the converter, adding 500 kg of low-carbon ferromanganese when tapping the metal, and adding aluminum blocks according to the specific oxygen of the auxiliary lance, to ensure that the oxygen content of the molten steel is 50 to 80 ppm after tapping; S4: After the molten steel enters the RH furnace, measure and sample the temperature, perform decarburization treatment after the temperature is over 1580°C and the vacuum degree is ≤3.0mbar, and then perform the fusion treatment after the vacuum is generated and C ≤ 0.0050%, and maintaining a vacuum after the completion of the fusion; S5: after the steel reaches the LF furnace, add limestone to form slag, mix with argon at the bottom, control the sulfur content within 0.0010%, and perform calcium treatment after desulfurization is completed; S6: complete pouring with full protection during continuous casting, superheat control at 10-20°C, adopting double-roll electromagnetic stirring technology; S7: heating the casting billet in the heating furnace by the step heating system, wherein the target austenitization temperature is 1110-1120°C, the heating time is 10-13 min/cm, and the holding time is more than 60 min; and S8: executing the rolling by the thermomechanical control process, wherein the final reduction percentage per pass through the rough surface rolls is more than 22%, and the initial temperature of the two-stage rolling is 800-830°C, the inlet water temperature is 780-800°C, and also cooling by cooling to 280-300°C to obtain a grain structure with a core grain size of less than 30 μm and a multifaceted ferrite/bainite structure, namely (PF+B). 2. Method according to claim 1, characterized in that after step S3, the aluminum mass is increased to 20 kg to increase to 600 ppm oxygen, and then the aluminum content is increased to 10 kg to obtain an increase for every 100 ppm oxygen. 3. The method according to p. 1, characterized in that at step S5 a seamless flux-cored wire of pure calcium with a length of 150-200 m is used. 4. Method according to claim 1, characterized in that in step S5 static mixing is performed for 15 to 20 minutes after the calcium treatment.

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