A refining method for reducing the content of cluster-shaped CaO-CaS inclusions in steel

Abstract

The application discloses a refining method for reducing the content of cluster CaO-CaS inclusions in steel, which comprises LF refining, RH vacuum treatment, molten steel calcium treatment and molten steel net blowing processes. min =max(1.3x[O],2S), the maximum value of calcium content Ca max =min(2x[O],4S); wherein [O] and S refer to the total oxygen and sulfur content of the molten steel after calcium treatment; in the molten steel net blowing process, the gas flow is controlled by a rectangular pulse model, and the Ca, S and [O] content of the treated molten steel meets the following conditions: 1.3<=Ca / [O]<=2 and 2<=Ca / S<=4. The number of CaO-CaS inclusions in the molten steel treated by the method is less than or equal to 1 per mm 2 , the size of a single inclusion is less than or equal to 3 um, the inclusion inspection of the steel is rated below 1.0 in the B inclusion category, and the qualified rate of HIC inspection is greater than or equal to 99.5%.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of steel metallurgy, and particularly relates to a refining method for reducing the content of cluster-shaped CaO-CaS inclusions in steel. BACKGROUND

[0002] With the rapid growth of global energy transportation demand, in order to reduce the construction cost of long-distance pipelines and improve the transportation efficiency, oil and gas long-distance pipelines are developing towards high strength. In recent years, the acid-resistant pipeline steel has been improved from X42 / X52MS level to X65 / X70MS level, and the conventional high-grade pipeline steel X80 / X100 has also entered the stage of engineering application. However, with the increase of the strength level, the hydrogen-induced cracking (HIC) sensitivity of the pipeline steel in the sulfur-containing medium environment is significantly intensified, which becomes the main bottleneck restricting the safe service of high-strength pipelines.

[0003] At present, the main means for reducing the hydrogen-induced cracking sensitivity of micro-alloyed high-strength steel in the steelmaking process is as follows: firstly, adding elements such as Ni and Cu to the steel to form effective hydrogen traps to hinder the diffusion of hydrogen; secondly, reducing the contents of easily-segregated components C, Mn and P in the steel, and adopting electromagnetic stirring / light pressing and other continuous casting processes to reduce the center segregation of the casting blank and prevent hydrogen from gathering along the segregation band; and thirdly, promoting the transformation of long strip-shaped MnS in the steel into spherical CaS inclusions through calcium treatment of the molten steel to avoid the gathering of hydrogen in the steel-inclusion interface gap.

[0004] However, it is found in actual industrial production that although the calcium treatment process for controlling the Ca / S of the molten steel above a certain critical value can completely transform the MnS, more than 15% of the test batches of the steel HIC inspection still fail to meet the standard requirements, and cluster-shaped CaO-CaS inclusions are found at the HIC crack fracture. It is of great significance to study how to reduce the cluster-shaped CaO-CaS inclusions in the pipeline steel blank for further improving the hydrogen-induced cracking sensitivity of the material, and it also has certain reference value for the development and production of other steel grades. SUMMARY

[0005] The application aims to provide a refining method for reducing the content of cluster-shaped CaO-CaS inclusions in steel, which can significantly reduce the number of large-size cluster-shaped CaO-CaS inclusions in the molten steel to 1 / mm2 or less, make the B-type inclusion rating of the steel <1.0, and effectively improve the cleanliness of the molten steel and reduce the hydrogen-induced cracking sensitivity of the material.

[0006] To achieve the above technical purposes, the specific technical solutions of the application are as follows:

[0007] A refining method for reducing the content of cluster-shaped CaO-CaS inclusions in steel, which comprises LF refining, RH vacuum treatment, calcium treatment of the molten steel and molten steel net blowing, and specifically as follows:

[0008] (1) LF refining: the Al2O3 content in the slag at the inlet of the furnace is controlled at 15% to 25%, the CaO / SiO2 ratio of the slag in the process is controlled at 3.0 to 8.0, and the S content in the molten steel at the end of the treatment is 8 to 15 ppm.

[0009] (2) RH vacuum treatment: before the treatment, a one-time conical steel slag baffle is installed at the immersion tube to prevent the slag from entering the vacuum tank; and the total oxygen content [O] in the molten steel at the end of the treatment is 16 to 24 ppm.

[0010] (3) Calcium treatment of the molten steel: the molten steel is subjected to quantitative calcium treatment, the minimum calcium content Ca min =max(1.3×[O], 2S), and the maximum calcium content Ca max =min(2×[O], 4S); wherein [O] and S represent the total oxygen and sulfur contents in the molten steel after the calcium treatment, max(1.3×[O], 2S) represents the larger one of 1.3×[O] and 2S, and min(2×[O], 4S) represents the smaller one of 2×[O] and 4S.

[0011] (4) Purification blowing of the molten steel: argon or other inert gas is blown into the bottom of the ladle, the gas flow is controlled by using a rectangular pulse model, the pulse period T is 30s≤T≤120s, the peak flow F max and the valley flow F min are executed as 0.05F max ≤F min ≤0.2F max , the duty cycle Dr is executed as 0.25≤Dr≤0.5, the duty cycle Dr refers to the ratio of the peak flow duration in a single pulse period, the bottom blowing gas amount is controlled to avoid the exposure and turbulence of the molten steel, and the Ca, S and [O] contents in the molten steel at the end of the treatment meet the requirements of 1.3≤Ca / [O]≤2 and 2≤Ca / S≤4.

[0012] Preferably, in the RH vacuum treatment process, the molten steel is circulated for 3 to 6 cycles at a vacuum degree of ≤1.2mbar.

[0013] Preferably, in the RH vacuum treatment process, the steel slag baffle should be made of a suitable material to ensure that it will not cause the composition of the molten steel to be inconsistent after being melted into the molten steel.

[0014] Preferably, in the calcium treatment process of the molten steel, a splash-proof calcium wire is used for calcium treatment.

[0015] Preferably, in the purification blowing process of the molten steel, the peak flow of the bottom blowing gas is set to the largest possible flow value under the premise of ensuring that the molten steel is not exposed and turbulent, and the bottom blowing time is 4 to 8 minutes.

[0016] The invention principle of the technical scheme of the present application is as follows:

[0017] The present application firstly pre-regulates the slag component and the content of S and [O] in molten steel in a refining LF-RH process, then quantitatively treats the molten steel, controls the Ca / S and Ca / [O] content of the molten steel in a specific range, greatly reduces the opportunity of generating cluster CaO-CaS under the premise of fully transforming MnS and Al2O3 inclusions in the steel, and finally removes the CaO-CaS inclusions generated in the steel by rectangular pulse airflow.

[0018] Compared with the prior art, the improvement of the present application mainly lies in:

[0019] 1) The molten steel composition adjusting method based on the multiple constraint conditions of CaO-CaS type inclusion regulation and quantitative calcium treatment. In order to fully transform MnS and Al2O3 inclusions in the steel, the conventional calcium treatment process generally determines the feeding amount of calcium wire according to the S content or Al2O3 inclusion content in the molten steel. When the feeding amount of calcium wire is too much, cluster CaO-CaS inclusions are easily generated, which affects the cleanliness of the molten steel. In order to solve this problem, based on the inclusion formation mechanism research and production test data analysis, the influence of Ca / [O] and Ca / S on the inclusion morphology and composition is fully considered, the molten steel Ca-[O]-S composition control range which can effectively reduce the generation of CaO-CaS inclusions is determined, and the quantitative calcium treatment method in which [O] and S of the molten steel are calculated simultaneously is formulated. Compared with the traditional process, the present method not only ensures the full transformation of MnS and Al2O3 inclusions, but also effectively reduces the generation of CaO-CaS inclusions, further improving the cleanliness of the molten steel.

[0020] 2) The rectangular pulse airflow control of the ladle for promoting the full floating of inclusions. In order to avoid the secondary oxidation of the molten steel caused by the exposure and turbulence of the molten steel, the traditional molten steel inclusion removal process usually adopts constant small flow rate ladle bottom blowing. Although it can promote the aggregation and floating of inclusions to a large extent, it has limited floating force for large size and cluster type inclusions, which easily makes the inclusions remain in the steel. In order to solve this problem, the rectangular pulse airflow control is adopted for the ladle bottom blowing gas flow, which is greater than the flow peak value set in the conventional process, has a high shear damage effect on the cluster type inclusions, and is less than the flow valley value set in the conventional process, so that the inclusions can better aggregate and float in the low turbulence state. The high-frequency variable flow ladle bottom blowing method improves the inclusion removal efficiency by more than 20%.

[0021] 3) RH production process to prevent slag from re-entraining into molten steel. In the traditional RH process, slag inevitably enters the vacuum chamber from the riser pipe when treating molten steel, thereby re-entraining into molten steel, affecting the control of inclusions in molten steel and the cleanliness of molten steel. At present, a small number of steel plants test slagging before vacuum treatment, but it is easy to cause partial exposure of molten steel to secondary oxidation. The present application installs a one-time conical steel slag baffle on the immersion tube before RH treatment, which can 100% isolate the slag outside the vacuum chamber, prevent the slag from re-entraining into molten steel and affecting the cleanliness of molten steel, and has a beneficial effect on the precise control of [O] and S components in molten steel and the control of inclusions. The RH slag baffle process has low additional cost, the device is easy to install, and the slag baffle effect is good.

[0022] The beneficial effects produced by the above technical scheme are that: through the comprehensive application of molten steel composition and calcium treatment control process, rectangular pulse air flow bottom blowing and RH slag baffle process, the generation and residual amount of cluster CaO-CaS inclusions in steel are greatly reduced while MnS and Al2O3 inclusions in steel are fully transformed, the calcium wire usage is reduced, it is more low-carbon and environmentally friendly, the number of CaO-CaS inclusions in the treated molten steel is ≤1 / mm 2 , the size of a single inclusion is ≤3um, the B-type inclusion rating of steel inclusions is 1.0 or less, the qualified rate of HIC inspection is ≥99.5%, the hydrogen-induced cracking sensitivity of pipeline steel and other clean steels is effectively reduced, and the service safety of the material in a sulfur-containing medium environment is improved. In addition, the rectangular pulse air flow control technology of ladle bottom blowing and the RH slag baffle process also have a promoting effect on the reduction of other types of inclusions in steel. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The CaO-CaS-Al2O3 ternary phase diagram of the molten steel treated in Example 1.

[0024] Figure 2 The inclusion metallographic photo of the finished plate produced by the molten steel of Example 1. DETAILED DESCRIPTION

[0025] In order to better illustrate the present application, the following examples are further illustrated.

[0026] Examples 1-7

[0027] A refining method for reducing the content of cluster CaO-CaS inclusions in steel, comprising LF refining, RH vacuum treatment, molten steel calcium treatment and molten steel net blowing, specifically as follows:

[0028] (1) LF refining, adding slagging agent and aluminum deoxidizer at the entrance to control the Al203 content in the slag at 15% to 25%, to ensure that the slag has good fluidity and adsorption inclusion capacity; sampling and monitoring the slag basicity during desulfurization, and adding appropriate amount of lime if necessary, to stabilize the slag basicity CaO / SiO2 at 3.0 to 8.0, to ensure the desulfurization and inclusion removal efficiency; when the S content of the molten steel is removed to 8 to 15 ppm, and other metallurgical tasks are completed, the treatment is ended. The main parameters of the LF refining process of each example are shown in Table 1 below.

[0029]

[0030] (2) RH vacuum treatment process: before treatment, a one-time conical steel slag baffle is installed on the vacuum tank immersion tube to wrap the riser and downcomer, the slag baffle is made of appropriate material to ensure that it will not cause the composition of the molten steel to be inconsistent when it is melted into the molten steel; when the immersion tube starts to contact with the molten steel, the upper slag of the ladle is completely pushed away by the slag baffle and isolated outside the vacuum chamber, and at the same time, the immersion tube insertion speed into the molten steel is controlled, so that the slag baffle is completely melted by the molten steel before the immersion tube reaches the insertion depth of the molten steel vacuum treatment; the vacuum degree is maintained at 1.2 mbar or below during the molten steel circulation process, to carry out degassing and inclusion removal. The circulation time is controlled at 3 to 6 molten steel circulation periods, to avoid excessive deoxidation of the molten steel by long-time circulation; when the [O] content of the molten steel is 16 to 24 ppm and other metallurgical tasks are completed, the treatment is ended. The main parameters of the RH vacuum treatment process of each example are shown in Table 2 below.

[0031]

[0032] (3) Molten steel calcium treatment: using anti-splashing calcium wire for calcium treatment, controlling the argon or other inert gas blowing agitation gas amount and wire feeding speed when feeding the calcium wire, to ensure that the molten steel does not overturn; combining the molten steel weight, calcium content of the calcium wire, S and [O] content of the molten steel before calcium treatment, calcium alloying yield, etc. to determine the appropriate calcium wire feeding amount, to ensure that the calcium content of the molten steel after calcium treatment is between Ca min and Ca max ; wherein the minimum calcium content Ca min = max (1.3 x [O], 2S), and the maximum calcium content Ca max = min (2 x [O], 4S), max (1.3 x [O], 2S) refers to the larger value of 1.3 x [O] and 2S, and min (2 x [O], 4S) refers to the smaller value of 2 x [O] and 4S, and in the formula, [O] and S refer to the total oxygen and sulfur content of the molten steel after calcium treatment. The main process parameters of the molten steel calcium treatment are shown in Table 3.

[0033]

[0034] (4) Molten steel purging: after calcium treatment, switch the ladle bottom blowing stirring to purging mode, blow argon or other inert gas into the bottom of the ladle, the gas flow is controlled by using a rectangular pulse model, the model parameters include pulse period T, peak flow F max , valley flow F min and duty cycle Dr. Specifically: 30s≤T≤120s, 0.05F max ≤F min ≤0.2F max , 0.25≤Dr≤0.5. Under the premise of ensuring that the molten steel is not exposed and boiling, the peak flow F max of the bottom blowing gas is set to the largest possible flow value. After purging the molten steel for 4-8 minutes, the bottom blowing is turned off, the treatment is ended, and the Ca, S, [O] contents of the molten steel after treatment meet the conditions: 1.3≤Ca / [O]≤2 and 2≤Ca / S≤4. The main process parameters of the molten steel purging of each example are shown in Table 4.

[0035]

[0036] The molten steel and finished steel of each example were sampled for inclusion inspection and analysis, and the test results are shown in Table 5.

[0037]

[0038] As can be seen from Table 5, the main inclusions in the molten steel samples of each example are a small amount of spherical, fine CaO-Al2O3-Ca composite inclusions, and no large-size cluster CaO-CaS inclusions are found; the number of CaO-CaS inclusions is ≤1 per mm 2 , and the diameter of a single inclusion is ≤3um; the inclusion test of the steel sample mainly shows a small amount of Class B and Class Ds inclusions, and the inclusion rating is below 1.0.

[0039] The molten steel sampled at the end of Example 1 was analyzed for inclusions, and the types and components of the molten steel inclusions were mainly concentrated in the shadow area of Figure 1 , as shown in Figure 1 . As can be seen from Figure 1 , the molten steel produced by the method of this example greatly reduces the generation of CaO-CaS binary inclusions, effectively preventing the occurrence of large-size cluster CaO-CaS inclusions in the steel.

[0040] The inclusion metallographic photos of the finished plate produced by the molten steel of Example 1 are shown in Figure 2 . By observing Figure 2 , no long strip-shaped, large-size Class B non-metallic inclusions caused by the cluster CaO-CaS inclusions in the molten steel were found.

[0041] The rest of the example processing end molten steel inclusions CaO-CaS-Al203 ternary phase diagram, and the production of finished plate inclusions metallographic photographs with example 1, no longer one by one to provide.

[0042] The finished steel of each example was sampled, and HIC test was carried out on the sample under normal temperature and pressure according to the latest version of GB / T 8650 standard by using A solution. After 96 hours of immersion in H2S saturated solution, the results showed that the surface of the base metal sample was free of hydrogen bubbles; metallographic observation of the cross section showed that all the samples were free of HIC cracks, the crack length rate (CLR) of the steel sample of each example was 0%, the crack width rate (CTR) was 0%, the crack sensitivity rate (CSR) was 0%, and the HIC inspection qualified rate was 100%.

Claims

1. A refining method for reducing the content of clustered CaO-CaS inclusions in steel, characterized in that, It includes LF refining, RH vacuum treatment, steel calcium treatment, and steel cleaning and blowing processes; The steel calcium treatment process involves quantitative calcium treatment of the molten steel, with a minimum calcium content of Ca. min =max(1.3×[O],2S), the maximum calcium content Ca max =min(2×[O],4S); where [O] and S refer to the total oxygen and sulfur content of molten steel after calcium treatment, max(1.3×[O],2S) refers to the larger of 1.3×[O] and 2S, and min(2×[O],4S) refers to the smaller of 2×[O] and 4S. In the steel cleaning and blowing process, inert gas is blown into the bottom of the ladle. The gas flow rate is controlled using a rectangular pulse model, with the pulse period T ranging from 30s to 120s, and the peak flow rate F. max Valley flow F min Execution: 0.05F max ≤F min ≤0.2F max The duty cycle Dr is set to 0.25≤Dr≤0.5, where Dr refers to the percentage of peak flow duration within a single pulse cycle. The bottom blowing volume is controlled to prevent molten steel from being exposed and churning. The Ca, S, and [O] contents of the molten steel at the end of the treatment meet the following requirements: 1.3≤Ca / [O]≤2 and 2≤Ca / S≤4.

2. The refining method for reducing the content of clustered CaO-CaS inclusions in steel according to claim 1, characterized in that, In the LF refining process, the Al2O3 content in the incoming slag is controlled at 15% to 25%, the slag basicity CaO / SiO2 ratio is controlled at 3.0 to 8.0, and the S content in the molten steel at the end of the process is 8 to 15 ppm.

3. The refining method for reducing the content of clustered CaO-CaS inclusions in steel according to claim 1, characterized in that, Before the RH vacuum treatment process, a disposable conical steel slag stopper is installed on the immersion tube to prevent slag from entering the vacuum tank; after the treatment, the total oxygen content [O] of the molten steel is 16-24 ppm.

4. The refining method for reducing the content of clustered CaO-CaS inclusions in steel according to claim 3, characterized in that, The material of the steel slag baffle should be such that it will not cause inconsistencies in the composition of the molten steel after it is melted into the molten steel.

5. The refining method for reducing the content of clustered CaO-CaS inclusions in steel according to claim 3, characterized in that, In the RH vacuum treatment process, molten steel is circulated for 3 to 6 cycles at a vacuum level ≤ 1.2 mbar.

6. The refining method for reducing the content of clustered CaO-CaS inclusions in steel according to claim 1, characterized in that, The calcium treatment process for molten steel uses a calcium-splash-proof line.

7. The refining method for reducing the content of clustered CaO-CaS inclusions in steel according to claim 1, characterized in that, The bottom blowing process of the molten steel has a blowing time of 4 to 8 minutes.

8. The refining method for reducing the content of clustered CaO-CaS inclusions in steel according to claim 1, characterized in that, The number of CaO-CaS inclusions in the molten steel after the refining method is ≤1 per mm. 2 Individual inclusion size ≤3μm, steel inclusion inspection Class B inclusion rating below 1.0, HIC inspection pass rate ≥99.5%.

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