A cold-rolled magnetic pole steel with a yield strength of not less than 350 MPa and its production method

Abstract

The present invention provides a cold-rolled magnetic pole steel with a yield strength of not less than 350 MPa and a production method thereof. The composition is as follows: C: 0.03 - 0.05%, Si: 0.20 - 0.40%, Mn = 1.5×(Si + P) + 2×S, P: 0.08 - 0.12%, S: 0.0040 - 0.0060%, Als: 0.0030 - 0.0050%, N: 0.0030 - 0.0050%, Ti: 0.0030 - 0.0050%, and the balance is Fe and inevitable impurities. Compared with the prior art, the finished product structure under the matching of the composition and process of the present invention is a pearlite structure. The precipitation particles of cementite Fe3C, MnS, AlN, and Ti(C, N) increase the strength and ensure the preferred orientation of the structure, so that the finished product has high mechanical property strength and magnetic induction intensity.

Description

Technical Field

[0001] The present invention belongs to the field of alloys, and particularly relates to a cold-rolled pole steel with a yield strength of not less than 350 MPa and a production method thereof, which is used for the production of large hydraulic generators. Background Art

[0002] Cold-rolled pole steel is an indispensable material in large hydraulic generating units, accounting for about 60-65% of the steel used in the entire generating unit. With the rapid growth of power installed capacity and the transformation of the energy structure towards clean energy, the proportion of hydropower is expected to exceed 20%, and the market demand is huge.

[0003] Due to the long operating life and high efficiency grade of large hydraulic generators, and the pole being an important magnetic conduction channel, high requirements are imposed on the yield strength, tensile strength, maximum working magnetic induction intensity, and stacking factor of the pole steel.

[0004] In order to obtain high magnetic induction intensity and strength indexes, British Patent GB1351884A increased the carbon, manganese, and silicon contents through alloying. Although high strength was obtained, the magnetic induction intensity was significantly lower. Chinese Patents CN200610019771.0, CN200610019772.5, CN201310226025.9, CN201310226038.6, CN201310226044.1, and CN201310227270.1 obtained high mechanical strength and relatively high magnetic induction intensity by adding different amounts of micro-alloying elements such as manganese, niobium, and titanium and controlling appropriate hot rolling and cold rolling annealing processes. However, the large addition of alloying elements increased the production cost. Summary of the Invention

[0005] The purpose of the present invention is to provide a cold-rolled pole steel with a yield strength of not less than 350 MPa and a production method thereof, without adding precious alloying elements such as Nb, V, and Cr. With a relatively low alloy cost and appropriate process adjustment, high mechanical strength, magnetic induction intensity, and stacking factor are achieved, and the product competitiveness is enhanced.

[0006] The specific technical solution of the present invention is as follows:

[0007] A cold-rolled pole steel with a yield strength of not less than 350 MPa, comprising the following components by mass percentage: C: 0.03-0.05%, Si: 0.20-0.40%, Mn = 1.5×(Si + P)+2×S, P: 0.08-0.12%, S: 0.0040-0.0060%, Als: 0.0030-0.0050%, N: 0.0030-0.0050%, Ti: 0.0030-0.0050%, and the balance is Fe and inevitable impurities.

[0008] According to Mn = 1.5×(Si + P)+2×S, the calculation result is reserved to two decimal places; Mn is an austenite-forming element, and Si and P are ferrite-forming elements. This kind of matching is considered for the smooth progress of hot rolling and cold rolling in the production process on the one hand; on the other hand, it comprehensively matches the mechanical properties (yield strength and tensile strength) and magnetic properties (magnetic induction intensity). The existing technologies or steel products mainly consider the component matching from the perspective of carbon equivalent to achieve the required mechanical properties, without considering both mechanical properties and magnetic properties.

[0009] The composition of the cold-rolled pole steel with a yield strength not less than 350 MPa also satisfies: Mn / S > 30;

[0010] The structure of the cold-rolled pole steel with a yield strength not less than 350 MPa is pearlite structure, and the pearlite grain size rating is 10 or above. The higher the pearlite grain size rating, the finer the structure, and the higher the corresponding mechanical strength.

[0011] The yield strength of the cold-rolled pole steel with a yield strength not less than 350 MPa is ≥350 MPa, and the yield ratio is 0.80 - 0.90.

[0012] The magnetic induction intensity B of the cold-rolled pole steel with a yield strength not less than 350 MPa 100 ≥1.83 T, B 150 ≥1.91 T, and the stacking factor ≥0.98.

[0013] A production method of a cold-rolled pole steel with a yield strength not less than 350 MPa provided by the present invention includes the following technological processes:

[0014] Smelting molten steel → Continuous casting → Heating of casting blank → Hot rolling → Pickling and cold rolling → Annealing → Skin pass → Finishing and packaging.

[0015] For the smelting of molten steel, the smelting is to meet the above composition requirements.

[0016] For the continuous casting, the mold electromagnetic stirring is put into use, and the thickness of the continuous casting billet is 200 - 260 mm, so that the equiaxed crystal ratio of the casting billet ≥60%. Too low equiaxed crystal ratio will cause corrugated surface defects in the finished product.

[0017] For the hot rolling, the heating temperature of the casting blank is 1100 - 1200 °C, and the hot rolling thickness is 2.8 - 3.2 mm. The reduction ratio between this hot rolling thickness and the casting blank thickness can, on the one hand, ensure that the columnar crystals in the casting blank are fully broken to avoid corrugated marks on the surface of the finished product, and on the other hand, ensure a {100} component texture with sufficient strength;

[0018] For the hot rolling, the finish rolling temperature is 830 - 870 °C, and the finish rolling temperature can ensure that the hot rolling is carried out in the austenite phase region to ensure the stable and smooth progress of the hot rolling process;

[0019] For the hot rolling, the coiling temperature is controlled at 650 - 700 °C; this coiling temperature can ensure that the deformed grains after hot rolling can complete sufficient recrystallization, making microscopic tissue preparations for subsequent cold rolling; the fully recrystallized equiaxed grain structure is beneficial for subsequent cold rolling. If the recrystallization is incomplete and deformed grains still remain, it is easy to break the strip and cause edge cracking during cold rolling.

[0020] The pickling and cold rolling can be carried out according to the requirements of conventional cold-rolled products.

[0021] For the annealing, the annealing is carried out at a temperature of 650 - 750 °C and maintained for 0.5 - 1 h in a reducing atmosphere. This temperature can ensure that the deformed structure after cold rolling undergoes partial recovery and recrystallization, ensuring the yield ratio (the ratio of yield strength to tensile strength), and providing sufficient safety margin for subsequent use in generators.

[0022] For the skin pass rolling, a shape control mode is adopted, and the elongation is 0.3 - 0.5%, to eliminate waviness, improve the shape of the finished product, and ensure the stacking coefficient.

[0023] The design concept of the present invention is as follows:

[0024] The matching of C, Si, and Mn can, on the one hand, ensure sufficient austenite content during hot rolling, facilitating the smooth progress of hot rolling. On the other hand, the carbon equivalent of this matching (the carbon equivalent is to convert the contribution of various components to strength into the influence of C. The component design in the present invention is optimized and matched on the basis of the carbon equivalent, taking into account both mechanical properties and magnetic properties) can ensure a sufficient number of cementite Fe3C in the finished product to ensure strength. Excessive alloy content will significantly deteriorate magnetic induction and increase alloy cost; the addition of P can, on the one hand, appropriately increase strength, and on the other hand, P segregation along the grain boundaries can increase the {100} component and reduce the {111} component, which is beneficial to the improvement of magnetic induction intensity. However, too high P content will make it brittle and deteriorate cold workability; in the present invention, Mn / S > 30 ensures good hot workability and coarsens MnS, promoting the strengthening of the {100} and {110} components and the weakening of the {111} component, improving magnetism. In addition, MnS particles will inhibit the grain growth during annealing and increase strength; the content matching of Als, N, and Ti (within the content range of the present invention) can form AlN and Ti(N, C) particles, which will inhibit the grain growth during annealing and increase strength. In addition, Al can be used as a fine-tuning deoxidizer in the later stage of the steelmaking process and added in trace amounts. Ti is an impurity element that is difficult to remove from molten iron, and the production control cost is relatively low at this content.

[0025] Compared with the prior art, the present invention can obtain cold-rolled pole steel products with excellent properties on the basis of ordinary carbon steel. The finished product structure under the matching of the composition and process of the present invention is a pearlite structure. The precipitation particles of cementite Fe3C, MnS, AlN, and Ti(C, N) increase the strength and ensure the preferred orientation of the structure, so that the finished product has high mechanical property strength and magnetic induction intensity. The product does not add relatively expensive alloy elements such as Nb, V, and Cr, and has a low cost; the yield strength of the cold-rolled pole steel product produced according to the technical solution of the present invention is ≥350 MPa, and the yield ratio is stably between 0.80 and 0.90, providing a sufficient strength safety margin for subsequent use on generators; the magnetic induction intensity B 100 ≥1.83 T, B 150 ≥1.91 T, and the stacking factor ≥0.98, which guarantees the efficiency of subsequent generators. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is the structure diagram of Example 1, and the grain size rating is 10;

[0027] Figure 2 is the structure diagram of Comparative Example 1, and the grain size rating is 9. DETAILED DESCRIPTION OF THE INVENTION

[0028] Example 1

[0029] A production method of cold-rolled pole steel with a yield strength not less than 350 MPa, including the following technological processes:

[0030] Smelting molten steel → Continuous casting → Slab heating → Hot rolling → Pickling and cold rolling → Annealing → Skin pass → Finishing and packaging.

[0031] Specifically: The composition of the molten steel is smelted according to mass percentage: C: 0.03%, Si: 0.20%, Mn: 0.43%, P: 0.08%, S: 0.0060%, Als: 0.0050%, N: 0.0050%, Ti: 0.0050%, and the rest is Fe and inevitable impurities.

[0032] Then the smelted molten steel is continuously cast into a continuous casting slab with a thickness of 200 mm, and the proportion of equiaxed crystals in the slab is 60%. Then the slab is heated in a heating furnace at 1100°C for 3 h, and then hot-rolled to 2.8 mm, controlling the finish rolling temperature at 830°C and the coiling temperature at 650°C; then the hot-rolled coil is pickled and cold-rolled to a thickness of 1.0 mm, and then the cold-rolled coil is annealed. The annealing process is to maintain for 0.5 h in a reducing atmosphere at 650°C. Finally, the annealed coil is skin passed according to the plate shape mode with an elongation of 0.5%.

[0033] The yield strength of the cold-rolled pole steel product produced according to the technical solution of Example 1 is 353 MPa, the yield ratio is 0.89, the magnetic induction intensity B 100 = 1.85 T, B 150 = 1.93 T, and the stacking factor is 0.99.

[0034] Example 2

[0035] A production method of cold-rolled pole steel with a yield strength not less than 350 MPa, including the following technological processes:

[0036] Smelting molten steel → Continuous casting → Billet heating → Hot rolling → Pickling cold rolling → Annealing → Skin pass → Finishing and packaging.

[0037] Specifically:

[0038] The molten steel is smelted according to the mass percentage of components: C: 0.05%, Si: 0.40%, Mn: 0.79%, P: 0.12%, S: 0.0040%, Als: 0.0050%, N: 0.0050%, Ti: 0.0050%, and the rest is Fe and inevitable impurities.

[0039] Then, the smelted molten steel is continuously cast into a continuous casting billet with a thickness of 260 mm. The proportion of equiaxed crystals in the billet is 62%. Then, the billet is heated in a heating furnace at 1200 °C for 3 h, and then hot rolled to 3.2 mm, controlling the finish rolling temperature at 870 °C and the coiling temperature at 700 °C; then, the hot rolled coil is pickled and cold rolled to a thickness of 1.0 mm, and then the cold rolled coil is annealed. The annealing process is to maintain it in a reducing atmosphere at 750 °C for 0.5 h. Finally, the annealed coil is skin passed with an elongation of 0.5% according to the plate shape pattern.

[0040] The yield strength of the cold-rolled pole steel product produced according to Example 2 is 392 MPa, the yield ratio is 0.80, the magnetic induction intensity B 100 = 1.84 T, B 150 = 1.92 T, and the stacking factor is 0.99.

[0041] Example 3

[0042] A production method of cold-rolled pole steel with a yield strength not less than 350 MPa, including the following technological processes:

[0043] Smelting molten steel → Continuous casting → Billet heating → Hot rolling → Pickling cold rolling → Annealing → Skin pass → Finishing and packaging.

[0044] Specifically:

[0045] The composition of the molten steel is smelted by mass percentage: C: 0.03%, Si: 0.20%, Mn: 0.43%, P: 0.08%, S: 0.0041%, Als: 0.0035%, N: 0.0033%, Ti: 0.0030%, and the rest is Fe and inevitable impurities.

[0046] Then, the smelted molten steel is continuously cast into a continuous casting billet with a thickness of 200 mm. The proportion of equiaxed crystals in the billet is 60%. Then, the billet is heated in a heating furnace at 1100 °C for 3 h, and then hot-rolled to 2.8 mm, controlling the finishing rolling temperature at 870 °C and the coiling temperature at 700 °C. After that, the hot-rolled coil is pickled and cold-rolled to a thickness of 0.5 mm, and then the cold-rolled coil is annealed. The annealing process is to hold for 1 h in a reducing atmosphere at 750 °C. Finally, the annealed coil is leveled and lightly rolled according to the plate shape mode (elongation rate of 0.3%).

[0047] The yield strength of the cold-rolled pole steel product produced according to the technical solution of Example 3 is 350 MPa, the yield ratio is 0.80, and the magnetic induction intensity B 100 = 1.83 T, B 150 = 1.91 T, and the stacking factor is 0.98.

[0048] Example 4

[0049] A production method of cold-rolled pole steel with a yield strength not less than 350 MPa includes the following technological processes:

[0050] Smelting molten steel → Continuous casting → Billet heating → Hot rolling → Pickling and cold rolling → Annealing → Leveling → Finishing and packaging.

[0051] Specifically:

[0052] The composition of the molten steel is smelted by mass percentage: C: 0.05%, Si: 0.40%, Mn: 0.79%, P: 0.12%, S: 0.0057%, Als: 0.0030%, N: 0.0035%, Ti: 0.0030%, and the rest is Fe and inevitable impurities.

[0053] Then, the smelted molten steel is continuously cast into a continuous casting billet with a thickness of 260 mm. The proportion of equiaxed crystals in the billet is 61%. Then, the billet is heated in a heating furnace at 1200 °C for 3 h, and then hot-rolled to 3.2 mm, controlling the finishing rolling temperature at 830 °C and the coiling temperature at 650 °C. After that, the hot-rolled coil is pickled and cold-rolled to the order thickness of 0.5 mm, and then the cold-rolled coil is annealed. The annealing process is to hold for 0.8 h in a reducing atmosphere at 700 °C. Finally, the annealed coil is leveled and lightly rolled according to the plate shape mode (elongation rate of 0.3%).

[0054] The yield strength of the cold-rolled pole steel product produced according to the technical solution of Example 4 is 377 MPa, the yield ratio is 0.86, and the magnetic induction intensity B 100 = 1.84 T, B 150 = 1.93 T, and the stacking factor is 0.99.

[0055] Comparative Example 1

[0056] A production method of cold-rolled pole steel includes the following technological processes:

[0057] Smelting molten steel → Continuous casting → Slab heating → Hot rolling → Pickling cold rolling → Annealing → Skin pass → Finishing and packaging.

[0058] Specifically:

[0059] The molten steel is smelted according to the mass percentage of the composition: C: 0.03%, Si: 0.20%, Mn: 0.21% , P: 0.08%, S: 0.0060%, Als: 0.0050%, N: 0.0050%, Ti: 0.0050%, and the rest is Fe and inevitable impurities.

[0060] Then the smelted molten steel is continuously cast into a continuous casting slab with a thickness of 200 mm. The proportion of equiaxed crystals in the slab is 60%. Then the slab is heated in a heating furnace at 1100 °C for 3 h, and then hot rolled to 2.8 mm, controlling the finish rolling temperature at 830 °C and the coiling temperature at 650 °C; then the hot rolled coil is pickled and cold rolled to the order thickness of 1.0 mm, and then the cold rolled coil is annealed. The annealing process is to maintain it for 0.5 h in a reducing atmosphere at 650 °C. Finally, the annealed coil is skin passed with a flatness pattern (elongation of 0.5%).

[0061] The yield strength of the cold-rolled pole steel product produced according to the technical solution of Comparative Example 1 is 342 MPa , the yield ratio is 0.85, Magnetic induction intensity B 100 = 1.82 T, B 150 = 1.90 T, and the stacking factor is 0.99. Compared with Example 1, the Mn content in Comparative Example 1 is lower, resulting in a decrease in strength and magnetic induction.

[0062] Comparative Example 2:

[0063] A production method of cold-rolled pole steel includes the following technological processes:

[0064] Smelting molten steel → Continuous casting → Slab heating → Hot rolling → Pickling cold rolling → Annealing → Skin pass → Finishing and packaging.

[0065] Specifically:

[0066] The molten steel is smelted according to the mass percentage of the composition: C: 0.05%, Si: 0.40%, Mn: 0.21%, P: 0.12%, S: 0.0040%, Als: 0.0050%, N: 0.0050%, Ti: 0.0050%, the balance being Fe and inevitable impurities. Then the molten steel after smelting is continuously cast into a continuous casting slab with a thickness of 260 mm, and the proportion of equiaxed crystals in the slab is 62%. Then the slab is heated in a heating furnace at 1200 °C for 3 h, and then hot-rolled to 3.2 mm, controlling the finish rolling temperature at 870 °C and the coiling temperature at 700 °C; then the hot-rolled coil is pickled and cold-rolled to the order thickness of 1.0 mm, and then the cold-rolled coil is annealed. The annealing process is to hold for 0.5 h in a reducing atmosphere at 750 °C. Finally, the annealed coil is leveled and lightly rolled according to the plate shape pattern (elongation of 0.5%).

[0067] The yield strength of the cold-rolled pole steel product produced according to the technical solution of Comparative Example 2 is 377 MPa, the yield ratio is 0.80, Magnetic induction intensity B 100 = 1.81 T, B 150 = 1.90 T , and the stacking factor is 0.99. The lower Mn content in Comparative Example 2 results in a decrease in strength and magnetic induction.

[0068] Comparative Example 3:

[0069] A production method of cold-rolled pole steel includes the following technological processes:

[0070] Smelting molten steel → Continuous casting → Slab heating → Hot rolling → Pickling and cold rolling → Annealing → Leveling → Finishing and packaging.

[0071] Specifically:

[0072] The molten steel is smelted according to the mass percentage of the components: C: 0.03%, Si: 0.20%, Mn: 0.50% , P： 0.13 %, S：0.0065％ , Als: 0.0035%, N: 0.0033%, Ti: 0.0030%, the balance being Fe and inevitable impurities. Then the molten steel after smelting is continuously cast into a continuous casting slab with a thickness of 200 mm, and the proportion of equiaxed crystals in the slab is 60%. Then the slab is heated in a heating furnace at 1100 °C for 3 h, and then hot-rolled to 2.8 mm, controlling the finish rolling temperature at 870 °C and the coiling temperature at 700 °C; cracks occur during hot rolling and cold rolling breakage is frequent. P is a grain boundary segregation element, and too high S will form a low-melting-point FeS phase, both of which will greatly deteriorate the rolling.

[0073] Comparative Example 4:

[0074] A production method of cold-rolled pole steel includes the following technological processes:

[0075] Smelting molten steel → Continuous casting → Slab heating → Hot rolling → Pickling and cold rolling → Annealing → Leveling → Finishing and packaging.

[0076] Specifically:

[0077] The molten steel is smelted according to the mass percentage of components: C: 0.05%, Si: 0.40%, Mn: 0.70%, P: 0.12%, S: 0.0057%, Als: 0.0030%, N: 0.0035%, Ti: 0.0030%, and the rest is Fe and inevitable impurities. Then the smelted molten steel is continuously cast into a continuous casting slab with a thickness of 260 mm, and the proportion of equiaxed crystals in the slab is 61%. Then the slab is heated in a heating furnace at 1200°C for 3 h, and then hot-rolled to 3.2 mm, controlling the finish rolling temperature at 830°C and the coiling temperature at 650°C; then the hot-rolled coil is pickled and cold-rolled to the order thickness of 0.5 mm, and then the cold-rolled coil is annealed. The annealing process is to maintain it for 0.8 h in a reducing atmosphere at 700°C. Finally, the annealed coil is (Elongation 0.1%) leveled and lightly rolled.

[0078] For the cold-rolled pole steel product produced according to the technical solution of Comparative Example 4 Yield strength is 345 MPa, yield ratio is 0.92 , the magnetic induction intensity B 100 = 1.84 T, B 150 = 1.93 T, Poor plate shape, stacking coefficient is 0.95 The elongation rate is equivalent to a slight cold rolling reduction, and the strength is improved by the principle of work hardening; in addition, applying a certain elongation rate plays a role in leveling and improves the sheet shape, so the natural stacking coefficient is large; while the elongation rate of the leveling and light rolling reduction in Comparative Example 4 is low, resulting in a low strength of the product, poor sheet shape, and a reduced stacking coefficient.

[0079] In this specification, specific examples are used to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A cold-rolled magnetic pole steel with a yield strength of not less than 350 MPa, characterized in that, The cold-rolled magnetic pole steel with a yield strength of not less than 350 MPa comprises the following components by mass percentage: C: 0.03 - 0.05%, Si: 0.20 - 0.40%, Mn = 1.5×(Si + P) + 2×S, P: 0.08 - 0.12%, S: 0.0040 - 0.0060%, Als: 0.0030 - 0.0050%, N: 0.0030 - 0.0050%, Ti: 0.0030 - 0.0050%, and the balance is Fe and inevitable impurities; The magnetic induction intensity B of the cold-rolled pole steel with a yield strength of not less than 350 MPa 100 ≥1.83 T, B 150 ≥1.91 T, and the stacking factor ≥ 0.98; The structure of the cold-rolled magnetic pole steel with a yield strength of not less than 350 MPa is pearlite structure, and the pearlite grain size rating is 10 or above.

2. The cold-rolled pole steel with a yield strength of not less than 350 MPa according to claim 1, wherein The yield strength of the cold-rolled magnetic pole steel with a yield strength of not less than 350 MPa is ≥350 MPa, and the yield ratio is 0.80 - 0.

90.

3. A production method of the cold-rolled magnetic pole steel with a yield strength of not less than 350 MPa according to any one of claims 1 - 2, comprising the following technological processes: smelting molten steel → continuous casting → heating the cast slab → hot rolling → pickling and cold rolling → annealing → skin pass rolling → finishing and packaging.

4. The production method according to claim 3, characterized in that, For the continuous casting, electromagnetic stirring of the mold is used, the thickness of the continuous casting slab is 200 - 260 mm, so that the equiaxed crystal ratio of the cast slab is ≥60%.

5. The production method according to claim 3, characterized in that, For the hot rolling, the heating temperature of the cast slab is 1100 - 1200 °C, and the hot rolling thickness is 2.8 - 3.2 mm.

6. The production method according to claim 3, characterized in that For the hot rolling, the finish rolling temperature is 830 - 870 °C, and the coiling temperature is 650 - 700 °C.

7. The production method according to claim 3, characterized in that, For the annealing, the annealing temperature is 650 - 750 °C, and it is maintained for 0.5 - 1 h in a reducing atmosphere.

8. The production method according to claim 3, characterized in that For the skin pass rolling, the skin pass elongation is 0.3 - 0.5%.

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