Low-delay cracking sensitivity heavy rail steel and preparation method thereof

Abstract

The invention discloses low-delay cracking sensitivity heavy rail steel and a preparation method. The steel comprises, by weight, 0.6%-0.75% of C, 0.33%-0.65% of Si, 1.0%-2.5% of Mn, smaller than or equal to 0.010% of P, smaller than or equal to 0.008% of S, smaller than or equal to 0.003% of Als, 2.8%-3.9% of Cr, 0.55%-0.65% of Cu, the mass ratio of Cr to Cu is 4.3%-6.5, and the balance Fe and impurity elements. By adding a certain amount of Cr and Cu and controlling the mass ratio of Cr to Cu, the repulsive action of Cu on hydrogen is guaranteed, hot brittleness is avoided, the hydrogen-induced delayed cracking resistance of the steel is effectively improved, and the steel has high tensile strength and excellent application prospects and economic benefits.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of high-strength steel production in the metallurgical industry, and particularly relates to a heavy rail steel with low delayed cracking sensitivity and a preparation method. BACKGROUND

[0002] High-speed rail and heavy haul railway are important infrastructure in China, and the service life and safety of steel rails are directly related to the maintenance cost and the safety of human life and property. The current tensile strength of steel rails is between 880 MPa and 1280 MPa, and the tensile strength of the mainstream product U75V is greater than 1000 MPa. Delayed cracking is prone to occur at this strength. Research shows that the delayed cracking of steel is caused by hydrogen in the material and the service environment of the material, and is unpredictable and sudden, often leading to serious safety problems. Therefore, delayed cracking has always been a problem that must be overcome for heavy rail products.

[0003] The main reason for hydrogen-induced delayed cracking of steel rails is that hydrogen atoms in the steel tend to gather at the grain boundaries to produce hydrogen gas, causing the steel to become brittle. Therefore, how to improve the toughness of the steel and reduce the aggregation of the steel at the grain boundaries is crucial to overcoming delayed cracking. SUMMARY

[0004] The present application provides a heavy rail steel with low delayed cracking sensitivity and a preparation method to overcome the deficiencies of the prior art.

[0005] To achieve the above-mentioned purpose, the present application provides a heavy rail steel with low delayed cracking sensitivity, and the steel composition includes, by weight percentage: C: 0.6-0.75%, Si: 0.33-0.65%, Mn: 1.0-2.5%, P≤0.010%, S≤0.008%, Als≤0.003%, Cr: 2.8-3.9%, Cu: 0.55-0.65%, and the mass ratio of Cr / Cu is 4.3-6.5, and the rest is Fe and impurity elements.

[0006] Cu and Cr are two elements commonly used in the development of steel grades. These two alloys have certain effects on suppressing delayed cracking, but the effect is limited when used alone. For example, Cu elements tend to gather at the grain boundaries and have good hydrogen repelling ability. When the grain boundaries are occupied by Cu, it is difficult for hydrogen atoms to gather there. As the Cu content increases, the effect of the steel on suppressing hydrogen will also increase significantly. However, Cu also has hot brittleness, and when the content is too high, it mainly shows hot brittleness. Therefore, the addition of other alloy elements is needed to play a better role. Cr elements can improve the hardenability of steel to improve the strength and toughness of steel, and have good synergistic effect with Cu to effectively suppress the hot brittleness caused by Cu. Reasonable use of Cu and Cr elements can improve the delayed cracking resistance of heavy rail steel.

[0007] The synergistic effect of Cu and Cr is as follows: Chromium is a typical passivation element, which can form a dense and stable Cr2O3passivation film on the surface of the steel, and this film has strong corrosion resistance to acid and can block the direct contact of acid with the substrate; copper can enrich in the defects of the passivation layer (such as microcracks and pores) to form a secondary protective film rich in Cu. On the one hand, Cu itself is prone to passivation in an acidic environment, which can fill the weak areas of the passivation layer; on the other hand, Cr and Cu can adjust the electrochemical potential and inhibit the corrosion process. The presence of Cr can increase the electrode potential of the steel and reduce the anodic dissolution rate. As a noble metal element, the enrichment area of Cu can act as a cathode to inhibit the dissolution reaction of the anode (iron matrix) through cathodic polarization, thereby reducing the corrosion of the environment to the matrix. The synergistic effect of the two can improve the overall electrochemical stability of the steel and slow down the redox reaction rate in an acidic environment.

[0008] However, the best comprehensive effect is obtained when the mass ratio of Cr / Cu is 4.3-6.5. This is mainly because the main passivation layer formed by Cr needs to be combined with the “repairing” effect of Cu in the defects. Cr maintains the chemical stability of the passivation layer, and if the passivation layer is damaged, Cu will quickly enrich in the damaged area to form a local corrosion-resistant microzone, thereby preventing the expansion of the corrosion pit and inhibiting the accumulation of acid in the pit. This synergistic mechanism significantly improves the corrosion resistance of the steel in an acidic environment and thus improves the hydrogen resistance of the steel. From the economic and applicability aspects, the content of Cr should not be too high. If the content of Cr is too high, the strength and surface hardness of the steel will increase, which will reduce the toughness of the steel and increase the cost. Therefore, the content of Cr is required to be in the range of 2.8-3.9%. Meanwhile, the content of Cu should not be too high, as too high content of Cu can cause thermal embrittlement of the steel. Therefore, the mass ratio of Cr / Cu is required to be in the range of 4.3-6.5, which can reduce the risk of thermal embrittlement caused by Cu and improve the overall toughness and hydrogen resistance of the steel.

[0009] Further, the mass ratio of Cr / Cu is 6.0-6.2.

[0010] Further, the mass percentage of Cr is 3.4-3.6%.

[0011] Further, the mass percentage of Cu is 0.56-0.58%.

[0012] Further, the mass ratio of Cr / Cu is 4.6-5.3.

[0013] Further, the mass percentage of Cr is 2.8-3.2%.

[0014] Further, the mass percentage of Cu is 0.60-0.65%.

[0015] Also provided is a method for preparing the low-delay cracking sensitive heavy rail steel as described above, comprising the following steps: Deep desulfurization of molten iron, converter smelting, scrap steel smelting, Al deoxidation; Soft blowing is performed in the LF refining process, lime and calcium carbide are added to the molten steel to form slag and perform desulfurization operation, composition fine tuning and alloying ensure the composition of the molten steel; then RH vacuum treatment is performed, and soft blowing is performed after the RH vacuum treatment is completed; Molten steel continuous casting, large pot long nozzle and crystallizer immersion nozzle protection casting are adopted, a cover heat preservation cover is added to the cast blank for slow cooling to promote hydrogen escape in the steel blank and reduce hydrogen content in the steel blank, and the slow cooling time is 24-48 hours; The steel blank is heated, rough rolling and finish rolling are performed to form a steel rail, and slow cooling to room temperature is performed in a slow cooling pit, and the slow cooling time is 48-72 hours.

[0016] Compared with the prior art, the heavy rail steel has the beneficial effects that: by adding a certain content of Cr and Cu and controlling the mass ratio of Cr / Cu, the heavy rail steel of the application not only ensures the repulsion of hydrogen by Cu, but also avoids the generation of hot brittleness, effectively improves the performance of the steel against hydrogen-induced delayed cracking, has high tensile strength, and has excellent application prospect and economic benefit. DETAILED DESCRIPTION

[0017] The application will be further described below in combination with specific examples.

[0018] Example 1 The components of the low-delay cracking sensitive heavy rail steel include C: 0.65%, Si: 0.45%, Mn: 1.5%, P: 0.010%, S: 0.008%, Als: 0.003%, Cr: 2.8%, Cu: 0.55%, and the mass ratio of Cr / Cu is 5.1, and the rest is Fe and impurity elements.

[0019] 1) Deep desulfurization of molten iron, converter smelting, scrap steel smelting, Al deoxidation; 2) Soft blowing is performed in the LF refining process, the soft blowing time is 5 min, lime and calcium carbide are added to the molten steel to form slag and perform desulfurization operation, composition fine tuning and alloying ensure the composition of the molten steel; RH vacuum treatment is performed.

[0020] 3) Molten steel continuous casting, large pot long nozzle and crystallizer immersion nozzle protection casting are adopted, a cover heat preservation cover is added to the cast blank for slow cooling to promote hydrogen escape in the steel blank and reduce hydrogen content in the steel blank, and the slow cooling time is 38 hours; 4) The steel blank is heated, rough rolling and finish rolling are performed to form a steel rail, and slow cooling to room temperature is performed in a slow cooling pit, and the slow cooling time is controlled to be between 48-72 hours.

[0021] Example 2 The low-delay cracking sensitive heavy rail steel composition includes, by weight percentage, C: 0.67%, Si: 0.53%, Mn: 1.5%, P: 0.008%, S: 0.005%, Als: 0.002%, Cr: 3.2%, Cu: 0.60%, the mass ratio of Cr / Cu is 5.3, and the rest is Fe and impurity elements.

[0022] 1) Deep desulfurization of molten iron, converter smelting, scrap steel smelting, Al deoxidation; 2) Soft blowing in the LF refining process, soft blowing time is 5 min, adding lime and calcium carbide in the molten steel to form slag and perform desulfurization operation, component fine tuning, alloying to ensure molten steel composition; RH vacuum treatment.

[0023] 3) Molten steel continuous casting, using large ladle long nozzle and crystallizer immersion nozzle protection pouring, covering the billet with a heat preservation cover for slow cooling to promote hydrogen escape in the billet and reduce hydrogen content in the billet, slow cooling time is 48 hours; 4) Heating the billet, rough rolling, after finish rolling, forming steel rail, slowly cooling to room temperature in the slow cooling pit, slow cooling time is controlled between 48-72 hours.

[0024] Example 3 The low-delay cracking sensitive heavy rail steel composition includes, by weight percentage, C: 0.68%, Si: 0.38%, Mn: 2.0%, P: 0.009%, S: 0.006%, Als: 0.0025%, Cr: 3.0%, Cu: 0.65%, the mass ratio of Cr / Cu is 4.6, and the rest is Fe and impurity elements.

[0025] 1) Deep desulfurization of molten iron, converter smelting, scrap steel smelting, Al deoxidation; 2) Soft blowing in the LF refining process, soft blowing time is 5 min, adding lime and calcium carbide in the molten steel to form slag and perform desulfurization operation, component fine tuning, alloying to ensure molten steel composition; RH vacuum treatment.

[0026] 3) Molten steel continuous casting, using large ladle long nozzle and crystallizer immersion nozzle protection pouring, covering the billet with a heat preservation cover for slow cooling to promote hydrogen escape in the billet and reduce hydrogen content in the billet, slow cooling time is 48 hours; 4) Heating the billet, rough rolling, after finish rolling, forming steel rail, slowly cooling to room temperature in the slow cooling pit, slow cooling time is controlled between 48-72 hours.

[0027] Example 4 The low-delay cracking sensitive heavy rail steel component includes, by weight percentage, C: 0.69%, Si: 0.45%, Mn: 2.5%, P: 0.007%, S: 0.007%, Als: 0.0015%, Cr: 3.6%, Cu: 0.58%, the mass ratio of Cr / Cu is 6.2, and the rest is Fe and impurity elements.

[0028] 1) Deep desulfurization of molten iron, converter smelting, scrap steel smelting, Al deoxidation; 2) Soft blowing in the LF refining process, soft blowing time is 5 min, adding lime and calcium carbide in the molten steel to form slag and perform desulfurization operation, component fine tuning, alloying to ensure molten steel composition; RH vacuum treatment.

[0029] 3) Molten steel continuous casting, adopting large ladle long nozzle and crystallizer immersion nozzle protection pouring, covering the billet with a heat preservation cover for slow cooling to promote hydrogen escape in the billet and reduce hydrogen content in the billet, slow cooling time is 48 hours; 4) Heating the billet, rough rolling, after finish rolling, forming steel rail, slowly cooling to room temperature in the slow cooling pit, slow cooling time is controlled between 48-72 hours.

[0030] Example 5 The low-delay cracking sensitive heavy rail steel component includes, by weight percentage, C: 0.70%, Si: 0.55%, Mn: 1.2%, P: 0.006%, S: 0.004%, Als: 0.001%, Cr: 2.9%, Cu: 0.62%, the mass ratio of Cr / Cu is 4.7, and the rest is Fe and impurity elements.

[0031] 1) Deep desulfurization of molten iron, converter smelting, scrap steel smelting, Al deoxidation; 2) Soft blowing in the LF refining process, soft blowing time is 5 min, adding lime and calcium carbide in the molten steel to form slag and perform desulfurization operation, component fine tuning, alloying to ensure molten steel composition; RH vacuum treatment.

[0032] 3) Molten steel continuous casting, adopting large ladle long nozzle and crystallizer immersion nozzle protection pouring, covering the billet with a heat preservation cover for slow cooling to promote hydrogen escape in the billet and reduce hydrogen content in the billet, slow cooling time is 48 hours; 4) Heating the billet, rough rolling, after finish rolling, forming steel rail, slowly cooling to room temperature in the slow cooling pit, slow cooling time is controlled between 48-72 hours.

[0033] Example 6 The low-delay cracking sensitive heavy rail steel component includes, by weight percentage, C: 0.72%, Si: 0.46%, Mn: 1.8%, P: 0.005%, S: 0.003%, Als: 0.002%, Cr: 3.4%, Cu: 0.56%, the mass ratio of Cr / Cu is 6.1, and the rest is Fe and impurity elements.

[0034] 1) Deep desulfurization of molten iron, converter smelting, scrap steel smelting, Al deoxidation; 2) Soft blowing in the LF refining process, soft blowing time is 5 min, adding lime and calcium carbide in the molten steel to form slag and perform desulfurization operation, component fine tuning, alloying to ensure molten steel composition; RH vacuum treatment.

[0035] 3) Molten steel continuous casting, adopting large pot long nozzle and crystallizer immersion nozzle protection pouring, covering the billet with a heat preservation cover for slow cooling to promote hydrogen escape in the billet and reduce hydrogen content in the billet, slow cooling time is 48 hours; 4) Heating the billet, rough rolling, after finish rolling, forming steel rail, slowly cooling to room temperature in the slow cooling pit, slow cooling time is controlled between 48-72 hours.

[0036] Example 7 The low-delay cracking sensitive heavy rail steel component includes, by weight percentage, C: 0.73%, Si: 0.55%, Mn: 2.2%, P: 0.004%, S: 0.002%, Als: 0.001%, Cr: 3.1%, Cu: 0.64%, the mass ratio of Cr / Cu is 4.8, and the rest is Fe and impurity elements.

[0037] 1) Deep desulfurization of molten iron, converter smelting, scrap steel smelting, Al deoxidation; 2) Soft blowing in the LF refining process, soft blowing time is 5 min, adding lime and calcium carbide in the molten steel to form slag and perform desulfurization operation, component fine tuning, alloying to ensure molten steel composition; RH vacuum treatment.

[0038] 3) Molten steel continuous casting, adopting large pot long nozzle and crystallizer immersion nozzle protection pouring, covering the billet with a heat preservation cover for slow cooling to promote hydrogen escape in the billet and reduce hydrogen content in the billet, slow cooling time is 48 hours; 4) Heating the billet, rough rolling, after finish rolling, forming steel rail, slowly cooling to room temperature in the slow cooling pit, slow cooling time is controlled between 48-72 hours.

[0039] Example 8 The low-delay cracking sensitive heavy rail steel component includes, by weight percentage, C: 0.75%, Si: 0.52%, Mn: 2.3%, P: 0.003%, S: 0.001%, Als: 0.0005%, Cr: 3.5%, Cu: 0.57%, the mass ratio of Cr / Cu is 6.1, and the rest is Fe and impurity elements.

[0040] 1) Deep desulfurization of molten iron, converter smelting, scrap steel smelting, Al deoxidation; 2) Soft blowing in the LF refining process, soft blowing time is 5 min, adding lime and calcium carbide in the molten steel to form slag and perform desulfurization operation, component fine adjustment, alloying to ensure the composition of the molten steel; RH vacuum treatment.

[0041] 3) Molten steel continuous casting, adopting large ladle long nozzle and crystallizer immersion nozzle protection pouring, covering the billet with a heat preservation cover for slow cooling to promote hydrogen escape in the billet and reduce hydrogen content in the billet, slow cooling time is 48 hours; 4) Heating the billet, rough rolling, after finishing rolling, forming a steel rail, slowly cooling to room temperature in the slow cooling pit, slow cooling time is controlled between 48-72 hours.

[0042] Comparative Example 1 The component includes, by weight percentage, C: 0.75%, Si: 0.45%, Mn: 2.1%, P: 0.003%, S: 0.001%, Als: 0.0005%, and the rest is Fe and impurity elements.

[0043] 1) Deep desulfurization of molten iron, converter smelting, scrap steel smelting, Al deoxidation; 2) Soft blowing in the LF refining process, soft blowing time is 5 min, adding lime and calcium carbide in the molten steel to form slag and perform desulfurization operation, component fine adjustment, alloying to ensure the composition of the molten steel; RH vacuum treatment.

[0044] 3) Molten steel continuous casting, adopting large ladle long nozzle and crystallizer immersion nozzle protection pouring, covering the billet with a heat preservation cover for slow cooling to promote hydrogen escape in the billet and reduce hydrogen content in the billet, slow cooling time is 48 hours; 4) Heating the billet, rough rolling, after finishing rolling, forming a steel rail, slowly cooling to room temperature in the slow cooling pit, slow cooling time is controlled between 48-72 hours.

[0045] Comparative Example 2 The component includes, by weight percentage, C: 0.75%, Si: 0.48%, Mn: 2.3%, P: 0.003%, S: 0.001%, Als: 0.0005%, Cr: 1.5%, and the rest is Fe and impurity elements.

[0046] 1) Deep desulphurization of the hot metal, smelting in a converter, addition of scrap, Al deoxidation; 2) Soft blowing during the LF refining process, soft blowing time 5 min, addition of lime and carbide to the bath to create the slag and perform the desulphurization operation, composition trimming, alloying to guarantee the bath composition; RH vacuum treatment.

[0047] 3) Continuous casting of the bath, protected casting with a long nozzle and a mould immersion nozzle, slow cooling of the strand with a cover to promote the hydrogen escape from the strand and reduce the hydrogen content in the strand, slow cooling time 48 hours; 4) Heating of the strand, roughing, finishing, and then rail, slow cooling to room temperature in a slow cooling pit, slow cooling time controlled between 48 and 72 hours.

[0048] Comparative Example 3 The components include, in percentage by weight, C: 0.75%, Si: 0.46%, Mn: 2.3%, P: 0.003%, S: 0.001%, Als: 0.0005%, Cr: 1.2%, Cu: 0.57%, mass ratio Cr / Cu 2.1, the rest being Fe and impurities.

[0049] 1) Deep desulphurization of the hot metal, smelting in a converter, addition of scrap, Al deoxidation; 2) Soft blowing during the LF refining process, soft blowing time 5 min, addition of lime and carbide to the bath to create the slag and perform the desulphurization operation, composition trimming, alloying to guarantee the bath composition; RH vacuum treatment.

[0050] 3) Continuous casting of the bath, protected casting with a long nozzle and a mould immersion nozzle, slow cooling of the strand with a cover to promote the hydrogen escape from the strand and reduce the hydrogen content in the strand, slow cooling time 48 hours; 4) Heating of the strand, roughing, finishing, and then rail, slow cooling to room temperature in a slow cooling pit, slow cooling time controlled between 48 and 72 hours.

[0051] The conventional mechanical properties of Examples 1-8 and Comparative Examples 1-3 were compared, and the results are shown in Table 1; the hydrogen-induced delayed cracking resistance of the test steel and the comparative steel was compared, and the test was carried out in 0.5 mol / L H2SO4, a dynamic hydrogen charging current was applied, the hydrogen charging current density was 0.5 mA / cm 2 , the tensile strain rate was 1.0 10 -5 / s, the hydrogen-induced delayed cracking resistance was evaluated by calculating the elongation loss after fracture (hydrogen embrittlement index I HE ) , I εThe smaller the value, the better the hydrogen-induced delayed cracking resistance. Table 1 shows the comparison of hydrogen-induced delayed cracking resistance between the comparative steel and the hot-formed hydrogen embrittlement resistant steel produced by the method of the present application.

[0052] Table 1 shows the performance test results of the embodiments and the comparative examples of the present application. As can be seen from the test results in Table 1, the embodiments 1-8 all have good performance, and in particular, the embodiments 4, 6 and 8 have significantly better elongation after hydrogen charging than the steel of the comparative example, and the hydrogen-induced delayed cracking sensitivity is greatly improved.

[0053] The specific embodiments are only the best examples, and are not a restrictive implementation of the technical solutions of the present application.

Claims

1. A low-delayed cracking susceptibility heavy rail steel, characterized by: The steel composition includes, by weight percentage: C: 0.6-0.75%, Si: 0.33-0.65%, Mn: 1.0-2.5%, P≤0.010%, S≤0.008%, Als≤0.003%, Cr: 2.8-3.9%, Cu: 0.55-0.65%, and the mass ratio of Cr / Cu is 4.3-6.5, and the rest is Fe and impurities.

2. The low-delayed cracking sensitive heavy rail steel according to claim 1, characterized in that: The mass ratio of Cr / Cu is 6.0-6.

2.

3. The low-delayed cracking sensitive heavy rail steel according to claim 1, characterized in that: The mass percentage of Cr is 3.4-3.6%.

4. The low-delayed cracking sensitive heavy rail steel of claim 1, wherein: The mass percentage of Cu is 0.56-0.58%.

5. The low-delayed cracking sensitive heavy rail steel of claim 1, wherein: The mass ratio of Cr / Cu is 4.6-5.

3.

6. The low-delayed cracking sensitive heavy rail steel of claim 1, wherein: The mass percentage of Cr is 2.8-3.2%.

7. The low-delayed cracking sensitive heavy rail steel of claim 1, wherein: The mass percentage of Cu is 0.60-0.65%.

8. A method of producing the low-delay cracking sensitive heavy rail steel according to claim 1, characterized by: The method comprises the following steps: Deep desulfurization of molten iron, converter smelting, scrap steel smelting, and Al deoxidization; Soft blowing is performed in the LF refining process, lime and calcium carbide are added to the molten steel to form slag and perform desulfurization operation, then RH vacuum treatment is performed, and soft blowing is performed after the RH vacuum treatment is completed; Continuous casting of molten steel, large ladle long nozzle and crystallizer immersion nozzle protection casting are adopted, a cover heat preservation cover is added to the cast blank for slow cooling, and the slow cooling time is 24-48 hours; The steel blank is heated, rough rolling and finish rolling are performed to form a steel rail, and slow cooling to room temperature is performed in a slow cooling pit, and the slow cooling time is 48-72 hours. The steel blank is heated, rough rolling and finish rolling are performed to form a steel rail, and slow cooling to room temperature is performed in a slow cooling pit, and the slow cooling time is 48-72 hours.

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