CN113751679B - Manufacturing method of cobalt-free maraging steel cold-rolled thin strip - Google Patents

Abstract

The invention discloses a method for manufacturing a cold-rolled thin strip of cobalt-free maraging steel. The specific steps are as follows: (1) molten steel with qualified components is smelted; (2) molten steel flows into a twin-roll thin strip continuous casting machine to cast a thickness of 3.0-5.0mm as-cast thin strip; (3) secondary cooling immediately after the thin strip is taken out of the roll, and rapidly cooled to room temperature; (4) cold rolling at room temperature; (5) annealing; (6) thin strip coiling. The invention takes advantage of the advantages of thin strip continuous casting and small crystal grains, eliminating the process of multi-pass hot rolling in the traditional process; using a large number of dislocations generated by cold rolling to promote the precipitation of the second phase, and improving the strengthening effect; Taking advantage of the sub-rapid solidification of thin strip continuous casting to inhibit element segregation, the long-term homogenization annealing and high-temperature solution annealing of traditional processes are eliminated, which greatly shortens the process flow, and at the same time ensures the strength and plasticity of the product, and improves the product toughness.

Description

A method for manufacturing a cobalt-free maraging steel cold-rolled thin strip

technical field

The invention belongs to the technical field of iron and steel metallurgy, and in particular relates to a method for manufacturing a cobalt-free maraging steel cold-rolled strip.

Background technique

Maraging steel is a high-strength steel that is superimposed by two strengthening effects of martensitic transformation strengthening and aging strengthening. Due to its extremely high strength and toughness and excellent processing and welding performance, it has been widely used in important fields such as aviation, aerospace, and machinery manufacturing. However, the high cost of alloying elements limits the scope of application of maraging steel. In order to reduce costs, cobalt-free maraging steel came into being. At present, the mainstream production method of maraging steel is double vacuum smelting, that is, vacuum induction melting and vacuum arc remelting refining. However, this production method is still limited to the category of traditional die casting, and has disadvantages such as complicated process flow, high energy consumption, and long production cycle.

In the twin-roll thin strip continuous casting process, molten steel is directly cast on a water-cooled copper crystallization roll into a thin steel strip with a thickness of 1-5mm. Since the repeated heating and hot rolling process of the traditional process is omitted, the twin-roll thin strip continuous casting production line can be greatly shortened. Therefore, major iron and steel companies in the world, such as Nippon Steel of Japan, NUCOR of the United States, POSCO of South Korea, domestic Baosteel Group and Jingye Group, are competing to develop twin-roll thin strip continuous casting technology, and have carried out industrial trial production practice and achieved important progress. But currently it is only used in the production of carbon steel, stainless steel, and silicon steel thin strips.

The invention patent with application number 202110321095.7 discloses a metal material forming technology for the production of maraging steel through the additive manufacturing technology of laser selective melting. This technology can directly print spherical powder into metal parts. However, this method has problems such as complex process, low yield, and high cost, and is not suitable for large-scale production.

The invention patent with application number 201610066981.9 discloses an aluminum-reinforced maraging steel and its preparation method, which mainly includes smelting, casting, forging, solution, cold rolling, recrystallization, aging treatment and other processes to obtain high The maraging steel strengthened by B2-NiAl intermetallic compounds, trace carbides and nano-clusters is highly innovative. However, its core element Al content is high, it is easily oxidized during smelting and casting, and its formability is poor, so thin strip continuous casting cannot be realized.

Contents of the invention

In order to solve the problems existing in the prior art, the present invention attempts the manufacture of cobalt-free maraging steel cold-rolled thin strip for the first time, taking advantage of the advantages of small segregation and fine grains of thin strip continuous casting, with appropriate ingredients and rapid cooling , cold rolling and annealing to obtain maraging steel with fine-grained martensite matrix and high-density second phase strengthening, and at the same time improve the strength and plasticity of thin strip products. The method can directly obtain the final martensitic steel thin strip product from molten steel, and has the advantages of significantly shortened process, high production efficiency, low steelmaking cost, low energy consumption, and environmental friendliness.

In order to achieve the above object, the present invention discloses a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip, comprising the following steps:

(1) Molten steel smelting: molten steel is obtained by smelting, the mass percentage of its chemical composition is: Ni: 14-18%, Mo: 3-4.5%, Ti: 0.5-1.5%, C: ≤0.010%, S: ≤0.006% , P: ≤0.020%, N: ≤0.007%, the balance is Fe and unavoidable impurities;

(2) Thin strip casting: the molten steel is cast by a twin-roll thin strip continuous casting machine into an as-cast thin strip with a thickness of 3.0-5.0mm;

(3) Secondary cooling: the as-cast strip is rapidly cooled to room temperature, and when the temperature of the strip is above 700°C, the cooling rate is higher than 150°C/s, preferably 200-350°C/s, and cooled between 700°C and room temperature The rate is higher than 10°C/s and less than the cooling rate of the strip above 700°C, preferably 20-50°C/s; in the secondary cooling process, the previous cooling rate is greater than or equal to 150°C/s, the purpose of which is to Prevent the segregation of elements and the growth of grains; later adopt a cooling rate greater than or equal to 10°C/s, the purpose of which is to prevent the aging precipitation of the second phase in the low temperature section.

(4) Room temperature cold rolling: carry out room temperature cold rolling to the thin strip prepared in step (3), the cold rolling temperature is not higher than 100°C, the cold rolling reduction rate is 40%-65%, and the strain rate is 0.1-0.5s ^{- 1} ;

(5) Annealing treatment: annealing the thin strip prepared in step (4), the annealing temperature is 520-550°C, and the annealing time is 1-1.5h;

(6) Strip coiling: cooling the strip obtained in step (5) to room temperature and coiling it online to obtain a finished strip steel coil.

As a preferred solution, the present invention provides a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip, wherein the chemical composition of the molten steel is: Ni: 15%, Mo: 3%, Ti: 0.8%, C: ≤0.010%, S: ≤0.006%, P: ≤0.020%, N: ≤0.007%, and the balance is Fe and unavoidable impurities.

As a preferred solution, the present invention provides a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip. In the step 1), the smelting method is an electric furnace or a converter steelmaking method, and the molten steel is refined, including vacuum degassing and refining. and ladle refining.

As a preferred solution, the present invention provides a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip. In the step 2), the superheat of the molten steel is 80-120°C.

As a preferred solution, the present invention provides a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip. In the step 2), the thickness of the as-cast thin strip is 3.5-4.5 mm.

As a preferred solution, in the method for manufacturing a cobalt-free maraging steel cold-rolled strip according to the present invention, in the step 3), the as-cast strip is cooled by spraying water to 700° C. and cooling to room temperature by spraying air.

As a preferred solution, the present invention provides a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip. In the step 4), the cold-rolling temperature is not higher than 50°C.

As a preferred solution, the present invention provides a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip. In the step 6), the coiling temperature is 50-100°C.

The invention discloses a method for manufacturing a cobalt-free maraging steel cold-rolled thin strip. The thickness of the obtained thin strip can be 1-3 mm.

The yield strength of the finished strip obtained in the invention is 1840-1890MPa, the tensile strength is 1920-1980MPa; the elongation is 8-10%.

The present invention is the first attempt to prepare high-quality cobalt-free maraging steel cold-rolled thin strips by strip continuous casting in the absence of Co. The yield strength of the finished strip obtained in the invention is 1840-1890MPa, the tensile strength is 1920-1980MPa; the elongation is 8-10%.

The average grain size of the finished ribbon obtained in the present invention is 7-9 μm.

The present invention adopts an ultra-low carbon route, and limits the carbon content below 0.01%. C is a very strong solid solution strengthening element, and can also be combined with metal elements to form carbides to increase strength. But at the same time, it will seriously affect the plasticity and toughness of the material, and consume carbide-forming elements such as Ti, V, and Nb. Through experimental optimization, the content of C in the solution of the present invention is finally controlled below 0.010 wt%.

According to the characteristics of thin strip continuous casting with fast cooling speed, thin size and small segregation, the present invention appropriately reduces the Ni content, i.e. controls the Ni content at 14-18%. This can not only save costs, but also realize the comprehensive performance of the product by coordinating the preparation process get promoted.

The reason why the present invention uses 3-4.5% Mo is that Mo is beneficial to improving the strength, toughness and corrosion resistance of the maraging steel. Precipitation of Ni ₃ Mo and Fe ₂ Mo at the early stage of aging plays an important role in strengthening and maintaining the toughness of steel. The presence of Mo can also prevent the precipitation of the precipitated phase along the prior austenite grain boundary, thereby avoiding the intergranular fracture and improving the fracture toughness. However, excessive addition of molybdenum will also generate retained austenite.

Titanium is the most effective strengthening alloy element in maraging stainless steel, because Ti can form Ni ₃ Ti precipitated phase with Ni, which has the effect of precipitation strengthening. However, Ti is easy to oxidize, and its addition should be limited from the perspective of technology, so its content is determined as follows: Ti: 0.5-1.5%, preferably 1.0-1.2%.

S is a harmful element in steel. It is very unfavorable to the toughness and stamping performance of the steel strip, and it is easy to cause the anisotropy of the mechanical properties of the steel strip. The lower the sulfur content in the steel is controlled, the better. Comprehensively considering the existing steelmaking level and economic factors, the present invention will The S content is controlled below 0.006%.

P has the effect of improving the weather resistance of martensitic steel, but too high P content is not conducive to the stamping performance, welding performance, low temperature toughness, etc. of martensitic steel plate, the present invention pays more attention to the mechanical properties of martensitic steel plate, so the present invention The P content is not higher than 0.02%.

N in steel is easy to combine with alloying elements to precipitate carbonitrides. When N content is too high, it is easy to form coarse nitrides with alloying elements in steel, which will adversely affect the plasticity and fatigue properties of steel. A small amount of N is conducive to the precipitation of fine carbonitride second phase particles, which is conducive to the improvement of the strength of steel. Therefore, in the present invention, N is controlled at a relatively low level, and the content is not higher than 0.007%.

The invention breaks the limitation of the traditional double vacuum smelting process, adopts the advanced thin strip continuous casting technology, cooperates with cold rolling, and can achieve the following effects:

a) The final cobalt-free maraging steel cold-rolled thin strip is directly obtained from molten steel. Compared with the traditional production process, the method involved in the present invention has the advantages of short process, high production efficiency, low energy consumption and environmental friendliness.

b) The thickness of the cast strip obtained by thin strip continuous casting is only 3.0-5.0mm. Compared with the thickness of tens of millimeters in the traditional process, the cast strip does not need repeated heating and hot rolling, which greatly shortens the process flow. The hardenability requirements of the steel are low, the addition amount of the alloy element Ni can be reduced, and the cost of steelmaking is reduced. The invention avoids the addition of Co and greatly reduces the production cost.

c) The solidification speed of the strip continuous casting process is fast (1000-10000°C/s), which inhibits the segregation of alloy elements, so it is not necessary to perform homogenization annealing at a high temperature of 1200°C; at the same time, the grains are refined, and the grain size after heat treatment less than 10μm.

Cold rolling and annealing process selection of the present invention are based on following considerations:

The main functions of cold rolling after the as-cast strip is cooled are: a) to improve the surface smoothness; b) to refine the grain; c) to retain a large number of dislocations and provide sites for the precipitation of the second phase. Due to the high alloy element content of maraging steel, the as-cast strip obtained by twin-roll strip continuous casting has obvious dendrite structure. As shown in Figure 2, the primary dendrites extend perpendicular to the surface of the strip and extend inward. The fine secondary dendrites are perpendicular to the primary dendrites. Through cold rolling, a large amount of deformation and dislocations are accumulated inside the as-cast strip, and recovery and recrystallization occur in the subsequent annealing process, forming fine grains with a grain size of less than 10 μm, as shown in Figure 3. At the same time, a large amount of dislocations along the second phase are precipitated, which has the effect of aging strengthening.

Description of drawings

Fig. 1 is a schematic diagram of the technological process of the twin-roll strip continuous casting unit of the present invention;

Fig. 2 is the as-cast strip metallographic microstructure of the present invention;

Fig. 3 is the metallographic microstructure of the final product of the present invention.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

The maraging steels of Examples 1-4 of the present invention and Comparative Examples 1-5 are all produced by the aforementioned manufacturing method of a cobalt-free maraging steel cold-rolled thin strip, and Comparative Example 6 adopts traditional double-vacuum smelting to combine uniform Chemical treatment, three-pass cold rolling and aging treatment production, the molten steel composition and process parameter of embodiment 1-4 and comparative example 1-6 are listed in table 1, specifically describe as follows:

Example 1

Example 1 steel composition: 0% Co, 15% Ni, 3% Mo, 0.8% Ti, ≤0.010% C, ≤0.007% N, ≤0.020% P, ≤0.006% S, the balance is Fe and not Avoid impurities. Referring to Fig. 1, after the converter and vacuum degassing refining, the molten steel in the ladle 1 enters the tundish 3 through the long nozzle 2 (the superheat degree of the molten steel in the tundish is 100°C), and then enters the crystallization roller 7a through the distribution nozzle 4 , 7b and side sealing plate 6 in the molten pool 5, in contact with the water-cooled copper crystallization rollers 7a, 7b, casting a thin as-cast strip 9 with a thickness of 4.5mm, Fig. 2 shows its branches perpendicular to the direction of the roller surface crystal organization. The width of the crystallization roll is 1200mm, the diameter of the crystallization roll is 500mm, and the casting speed of the casting machine is 90m/min. The area between the distribution nozzle 4 and the pinch roller 11 is a closed space 12 filled with inert gas, which is nitrogen, which can prevent the strip from being oxidized at high temperature.

After the as-cast thin strip 9 leaves the crystallization roll, it enters the cooling system 13 after being passed through the pinch roll 11, and starts cold rolling after rapid cooling to room temperature (the rapid cooling is: when the temperature of the thin strip is above 700° C., the cooling rate is 200° C. °C/s, the cooling rate between 700 °C and room temperature is 25 °C/s), the cold rolling temperature is 25 °C, the cold rolling reduction rate is 50%, and the strain rate is 0.1s ^-1 . The cold-rolled thin strip enters the tunnel annealing furnace 15 and is annealed at 530° C. for 1 hour, and then cooled to 100° C. through the air-cooling nozzle 16 . After cooling, under the guidance of the pinch roller 17, the cast strip 9 is rolled into a finished steel coil 18 (thickness is 2.25mm) by a strong coiler, and the steel coil is naturally cooled to room temperature. FIG. 3 shows the metallographic structure of the finished coil 18 .

Example 2-4

Examples 2-4 and Example 1 adopt a similar process, and the difference is that the only difference between Example 2 and Example 1 is that Example 2 contains 18% Ni. The only difference between embodiment 3 and embodiment 1 is that the thickness of the cast strip of embodiment 3 is 3mm. The only difference between Example 4 and Example 1 is that the cold rolling reduction of Example 4 is 40%.

Comparative example 1-5

Comparative Examples 1-5 and Example 1 adopt a similar process, and the difference is that: the only difference between Comparative Example 1 and Example 1 is that Comparative Example 1 contains 10% Co. The only difference between Comparative Example 2 and Example 1 is that Comparative Example 2 contains 0.1% Ti. The only difference between Comparative Example 3 and Example 1 is that the secondary cooling method of Comparative Example 3 is slow cooling (the slow cooling is: the cooling rate is constant at 5° C./s). The only difference between Comparative Example 4 and Example 1 is that Comparative Example 4 does not undergo cold rolling treatment. The only difference between Comparative Example 5 and Example 1 is that Comparative Example 5 is not annealed after cold rolling.

Comparative example 6

Comparative example 6 is produced by the traditional process of double vacuum smelting. The steel grade composition is: 18% Ni, 3% Mo, 1.5% Ti, 0.010% C, 0.008% N, 0.023% P, 0.005% S, and the balance is Fe and Unavoidable impurities; the thickness of the cast strip after vacuum induction melting and vacuum arc remelting and refining is 40mm; the cast strip is first homogenized and annealed at 1200°C for 10h, and then slowly cooled to room temperature (the slow cooling is: the secondary cooling rate is constant at 5 ℃/s); followed by three passes of cold rolling at room temperature with a reduction rate of 80% and a strain rate of 0.1s ^-1 ; after annealing (annealing at 530°C for 1h), cooling to room temperature to obtain the final product.

Table 2 lists the product properties of Examples 1-4 and Comparative Examples 1-6. Examples 1-4 have achieved excellent combinations of mechanical properties, and their properties slightly change with changes in Ni content, cast strip thickness, and cold rolling reduction. Comparative Example 1 added 10% Co to obtain better performance, but its cost was greatly increased, which is not in line with economic benefits; Comparative Example 2 illustrates the necessity of adding Ti element, which is an important precipitation strengthening element; Comparative Example 3 shows that the slow cooling of the secondary cooling method will lead to grain growth, which will lead to a decrease in plasticity and strength; comparative example 4 shows that without cold rolling, the grain size will be too large and the strength will be greatly reduced; comparative example 5 shows that without annealing, it cannot Play the role of aging strengthening, although its strength is high, but its plasticity is extremely poor, so it must be annealed after cold rolling; Comparative Example 6 uses the traditional double vacuum melting technology, the product strength is equivalent to that of Examples 1-4, but the plasticity and toughness Poor, and the process is cumbersome. In summary, the technological process and technical parameters adopted in the present invention have high innovation and advantages.

Chemical composition and process parameter of the molten steel of table 1 embodiment 1-4 and comparative example 1-6

Note: * represents that this parameter is completely consistent with the numerical values listed in Example 1.

The product performance of table 2 embodiment 1-4 and comparative example 1-6

Claims (7)

1. A method for manufacturing a cobalt-free maraging steel cold-rolled strip, characterized in that, comprising the steps: (1) Molten steel smelting: Molten steel is obtained by smelting, the mass percentage of its chemical composition is: Ni: 15%, Mo: 3%, Ti: 0.8%, C: ≤0.010%, S: ≤0.006%, P: ≤0.020% , N: ≤0.007%, the balance is Fe and unavoidable impurities; (2) Thin strip continuous casting: the molten steel is cast through a twin-roll thin strip continuous casting machine to cast a thin strip with a thickness of 3.0-5.0mm; in step (2), the superheat of the molten steel is 80-120°C; (3) Secondary cooling: the as-cast strip is rapidly cooled to room temperature. When the temperature of the strip is above 700°C, the cooling rate is 200-350°C/s; the cooling rate between 700°C and room temperature is 20-50°C /s; (4) Cold rolling at room temperature: Carry out cold rolling at room temperature on the thin strip prepared in step (3), the cold rolling temperature is not higher than 100°C, the cold rolling reduction rate is 40%-65%, and the strain rate is 0.1-0.5s ^{- 1} ; (5) Annealing treatment: annealing the thin strip prepared in step (4), the annealing temperature is 520-550°C, and the annealing time is 1-1.5h; (6) Ribbon coiling: the thin strip obtained in step (5) is cooled to room temperature -100°C for online coiling to obtain the finished thin strip steel coil; the yield strength of the resulting finished thin strip is 1840-1890MPa, the tensile strength It is 1920-1980MPa; the elongation is 8-10%. 2. The method for manufacturing a cobalt-free maraging steel cold-rolled thin strip according to claim 1, characterized in that: in the step (1), the smelting method is electric furnace or converter steelmaking method, molten steel After refining, including vacuum degassing refining and ladle refining. 3 . The method for manufacturing a cobalt-free maraging steel cold-rolled thin strip according to claim 1 , wherein in the step (2), the thickness of the as-cast thin strip is 3.5-4.5 mm. 4 . 4. A method for manufacturing a cobalt-free maraging steel cold-rolled thin strip according to claim 1, characterized in that: in the step (3), the as-cast thin strip is cooled by spraying water to 700 °C, cooled to room temperature with jet air. 5. The method for manufacturing a cobalt-free maraging steel cold-rolled thin strip according to claim 1, characterized in that: in the step (4), the cold-rolling temperature is not higher than 50°C. 6. The method for manufacturing a cobalt-free maraging steel cold-rolled thin strip according to claim 1, characterized in that: in the step (6), the coiling temperature is 50-100°C. 7. A method for manufacturing a cobalt-free maraging steel cold-rolled thin strip according to claim 1, characterized in that: the thickness of the obtained finished thin strip is 1.2-3mm.

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