CN112893790B

CN112893790B - Cast-rolling short-process-based uniform and fine duplex stainless steel thin strip and preparation method thereof

Info

Publication number: CN112893790B
Application number: CN202110064789.7A
Authority: CN
Inventors: 陈雷; 郝硕; 彭程; 王永鑫; 李志国
Original assignee: Yanshan University
Current assignee: Yanshan University
Priority date: 2021-01-18
Filing date: 2021-01-18
Publication date: 2021-12-14
Anticipated expiration: 2041-01-18
Also published as: CN112893790A

Abstract

The invention provides a thin duplex stainless steel strip based on a short casting and rolling process and a preparation method thereof, and relates to the field of steel material manufacturing. The preparation method includes: a twin-roll casting and rolling step, that is, after the temperature of the tundish is preheated to 1320-1380°C, molten steel with a temperature of 1480-1530°C is cast into the tundish, and then the molten steel flows into the molten pool with the temperature of the crystallizing roll. Rotational lead-out to form a single-phase ferritic cast strip with a thickness of 3 to 4 mm as well as heavy cold rolling, short solution treatment, mild hot rolling and coiling steps. By this method, the anisotropy of the duplex stainless steel can be effectively eliminated, the grain size of the two phases can be reduced, and the duplex stainless steel with higher yield strength, tensile strength and elongation can be obtained.

Description

Cast-rolling short-process-based uniform and fine duplex stainless steel thin strip and preparation method thereof

Technical Field

The invention relates to the field of steel material manufacturing, in particular to a uniform and fine double-phase stainless steel thin strip based on a casting and rolling short process and a preparation method thereof.

Background

Duplex stainless steel has a ferrite-austenite duplex structure, and is widely used in various fields because of its advantages of high strength of ferrite steel, high plasticity of austenite steel, good corrosion resistance and the like. However, the mechanical properties of the duplex stainless steel are greatly dependent on the distribution characteristics of the two phases, and after the conventional rolling process, the microstructure of the duplex stainless steel mostly presents two phases which are severely elongated along the rolling direction, so that the mechanically significant anisotropy is caused, and the forming properties of the duplex stainless steel are greatly limited. On the other hand, the mechanical properties of the duplex stainless steel are also influenced by the grain size, and according to the Hall-Petch relation, the strength of the material is increased along with the reduction of the grain size, and meanwhile, the plasticity and the toughness of the fine-grained material are also improved to different degrees.

Nowadays, the traditional duplex stainless steel thin strip is mostly prepared by a continuous casting and rolling (continuous casting) method, namely: molten steel smelting → continuous casting → high-temperature heat treatment → rough rolling → finish rolling → controlled cooling → curling. Compared with the early ingot mold casting cogging method, the continuous casting method improves the casting blank quality to a certain extent, saves energy, still has larger energy consumption and larger investment cost, and the finished product has obvious anisotropy. In addition, the rolling deformation in the prior art is in a two-phase region, and due to poor deformation compatibility of the two phases, the steel plate is easy to crack in the rolling process, so that the yield is influenced, the total rolling reduction is greatly limited, and the size of a thinner two-phase grain is difficult to obtain.

Disclosure of Invention

The invention aims to provide a uniform and fine duplex stainless steel thin strip based on a short casting-rolling process and a preparation method thereof.

In order to achieve the above purpose of the present invention, the following technical solutions are adopted:

a method for preparing a uniform and fine duplex stainless steel thin strip based on a casting and rolling short process comprises the following steps:

preheating a tundish to 1320-1380 ℃, then casting liquid steel with the temperature of 1480-1530 ℃ into the tundish, then leading out the liquid steel after flowing into a molten pool along with the rotation of a crystallization roller to form a single-phase ferrite casting belt with the thickness of 3-4 mm, wherein the rotating speed of the crystallization roller is 150-250 m/min, the cooling speed of the crystallization roller is 100-200 ℃/s, and the temperature of the discharged roller casting belt is 1280-1330 ℃;

carrying out constant-tension severe cold rolling on the single-phase ferrite cast strip, wherein the reduction of each pass is 0.01-0.3 mm, the total reduction is 60-70%, and preparing the cold rolled strip with the thickness of 0.9-1.6 mm;

heating the cold-rolled strip to a two-phase region temperature, carrying out short-time solution treatment, immediately carrying out one-pass mild hot rolling, cooling the obtained hot-rolled thin strip, and then curling.

Further, in a preferred embodiment of the present invention, the mild hot rolling is performed at a start rolling temperature of 1050 to 1100 ℃, a finish rolling temperature of 1000 to 1050 ℃, and a hot rolling reduction of 25 to 35%.

Further, in a preferred embodiment of the present invention, the cooling the hot-rolled thin strip comprises: and air cooling the hot-rolled thin strip to 700-800 ℃, then water cooling to 350-500 ℃ and then curling.

Further, in a preferred embodiment of the present invention, the short time solution treatment includes: and (3) introducing the cold-rolled strip into a heating coil through constant tension to perform induction heating to 1050-1200 ℃, and keeping the temperature for 3-6 min.

Further, in a preferred embodiment of the present invention, the superheat degree of the molten steel is controlled to be 50 to 80 ℃ when the molten steel is poured into the tundish.

Further, in a preferred embodiment of the present invention, the tension in the constant-tension severe cold rolling step and the short-time solution treatment step is constant at 20 to 30 MPa.

Further, in a preferred embodiment of the present invention, the mass fraction of N in the molten steel is less than 0.2%.

Further, in a preferred embodiment of the present invention, the molten steel comprises the following components by mass: c: 0.03-0.06%, Si: 0.2-2.0%, Mn: 1.0-5.0%, Ni: 1.0-9.0%, Cr: 19.0 to 21.0%, N: 0.05 to 0.2%, Mo: 0-0.5%, W: 0-0.5%, Cu: 0-0.5% and the balance Fe.

The method for preparing the uniform and fine duplex stainless steel strip based on the casting-rolling short process is characterized in that the thickness of the uniform and fine duplex stainless steel strip is 0.6-1.2 mm, and the volume fraction of austenite is 40-60%.

The invention has the following effects:

1. the preparation method of the uniform and fine duplex stainless steel strip provided by the invention adopts a twin-roll strip casting technology, and ensures that the temperature of the cast strip discharged from a roll is 1280-1330 ℃ by controlling the cooling speed and the rotating speed of a crystallization roll, namely the cast strip is crystallized in a ferrite single-phase region to obtain a single-phase ferrite cast strip. Because the cast strip is single-phase ferrite, the cold rolling deformation can reach 60-70%, the rolling is more thorough, and the large grains can be synchronously reduced in the rolling process.

2. Compared with the traditional process of double-roller casting and rolling in a double-phase region (cold rolling or hot rolling), the method has the advantages that when rolling is carried out in a large deformation amount, the blank is in a ferrite single-phase region, rolling cracking caused by two-phase uncoordinated deformation is avoided, the rolling deformation amount is increased, and a refined structure is obtained. Particularly, in the case of the dual-phase stainless steel with the TRIP effect, a large amount of martensite is generated during cold rolling, which is more unfavorable for rolling with large deformation, and the single-phase region cold rolling is adopted in the invention, so that the defect is avoided, and the refined grain structure is more favorably obtained.

3. The preparation method provided by the invention is based on a casting and rolling short process, wherein the casting and rolling are realized by adopting a twin-roll thin strip continuous casting technology, and the short process is realized by omitting complicated steps in the traditional process in the process of obtaining a uniform and fine two-phase structure. For the traditional process, a thick and uneven two-phase tissue is generally obtained, and then the tissue is uniformly refined through a series of processes. The short process has the advantages of shortening working hours, saving energy and protecting environment.

4. In the short-time solution treatment process, the cold-rolled strip is heated to a two-phase region, and short-time heat preservation is carried out, so that an austenite phase is uniformly precipitated in the cold-rolled strip, the precipitation amount is ensured to be 40-60%, a uniform two-phase structure is favorably obtained, and the anisotropy of the structure is improved. And then carrying out mild hot rolling to recrystallize the two phases to generate a more uniform and refined microstructure, and simultaneously controlling the cooling speed to avoid separating out impurity phases. The short-time solution treatment adopts an induction heating mode, so that energy and time are saved; the mild hot rolling has the advantages of avoiding hot rolling cracking caused by the hot rolling with large deformation in a two-phase region, and simultaneously enabling the two phases to generate dynamic recrystallization to further homogenize the structure.

5. The invention controls the casting and rolling cooling intensity in a ferrite single-phase region, although the nitrogen content of the raw material needs to be controlled in a middle-low proportion level, the invention has no other requirements on the components except the nitrogen content, and only needs to ensure that the component proportion accords with the traditional duplex stainless steel. Namely, the invention is suitable for preparing all the low-nitrogen and medium-nitrogen duplex stainless steel thin strips.

In a word, the preparation method can obtain the uniformly refined medium-low nitrogen duplex stainless steel thin strip on the basis of saving energy, greatly improves the tensile strength and the elongation percentage, provides a new idea for the preparation of high-strength steel, and has important value for the healthy development of the traffic and transportation industry of a new generation of automobiles and the like, the advanced manufacturing industry of paper making and the like, and the ocean industry of seawater desalination and the like.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.

FIG. 1 is a schematic flow chart of the production process of the present invention;

FIG. 2 is a single-phase metallographic image at the end of step 2 in example 1 of the present invention;

FIG. 3 is a grain size distribution diagram of the duplex stainless steel in example 1 of the present invention;

FIG. 4 is a fine, uniform grain metallographic structure of a duplex stainless steel according to example 2 of the present invention;

FIG. 5 is a fine, uniform grain metallographic structure of a duplex stainless steel in example 3 of the present invention;

fig. 6 is a graph showing a distribution of grain sizes of 2205 duplex stainless steel produced according to a conventional twin-roll strip casting process; and

fig. 7 is a comparison of mechanical curves for the finished ribbons of the examples and comparative examples.

Reference numbers: 1-tundish, 2-crystallizing roller, 3-water cooling device, 4-cold rolling roller, 5-induction coil, 6-hot rolling roller, 7-water cooling device and 8-curling.

Detailed Description

Embodiments of the present invention will be described in detail below with reference to examples, but it will be understood by those skilled in the art that the following examples are only illustrative of the present invention and should not be construed as limiting the scope of the present invention. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.

In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.

Example 1

A preparation method of a uniform and fine duplex stainless steel thin strip based on a short casting-rolling process is shown in figure 1 and comprises the following specific steps:

step 1, smelting molten steel:

the components are mixed according to the composition ratio of 2205 duplex stainless steel, namely (wt%): c: 0.06%, Si: 0.42%, Mn: 1.65%, Ni: 5.0%, Cr: 22.2%, N: 0.12% and the balance Fe. Smelting molten steel in a smelting furnace filled with argon and under high pressure. Wherein, the argon is filled for preventing other gases from entering; high pressure is beneficial to improving the yield of manganese and nitrogen.

Step 2, double-roller casting and rolling:

preheating a tundish 1, then casting molten steel into the tundish 1, and controlling the superheat degree to be 50-80 ℃, so that partial solidification (crystallization) of the molten steel in the tundish and even freezing and water gap blockage can be avoided; meanwhile, the condition that the composition segregation of molten steel/casting strip occurs due to too large superheat degree can be avoided. Molten steel flows into a molten pool through a water gap, and forms a single-phase ferrite cast strip after being led out by the crystallization roller 2 along with the strict control of the cooling speed and the rotating speed, wherein the thickness of the cast strip is 3.5 mm. Wherein gas solidified from the molten steel beyond the saturation degree of N is discharged along with the crystallization rolls 2 to prevent the formation of pores in the cast strip. The casting strip discharged from the roll is rapidly cooled to room temperature through a water cooling device 3 so as to keep a ferrite single-phase structure in the casting strip, a single-phase gold phase diagram of the casting strip is shown in figure 2, and as can be seen from figure 2, the metallographic structure of the casting strip is coarse single-phase ferrite without obvious austenite phase;

wherein the preheating temperature of the tundish is 1340 ℃; the casting temperature of the molten steel is 1495 ℃; the cooling speed of the crystallization roller is controlled to be 100-200 ℃/s, the rotating speed is 230m/min, and the temperature of the cast strip finally discharged from the roller is 1285 ℃.

Step 3, heavy cold rolling:

the cast strip with single-phase ferrite is sent into a cold rolling roll 4 through a pinch roll to be subjected to constant-tension severe cold rolling, the tension is constant at 20Mpa, the reduction of each pass is 0.3mm, the total reduction is 68%, and the cold rolling strip with the thickness of 1.1mm is prepared.

And 4, short-time solution treatment:

the cold-rolled strip enters a heating coil 5 through constant tension to perform double-phase zone induction heating to 1050 ℃, and the temperature is kept for 4min, so that austenite phase is uniformly separated out, and an austenite and ferrite double-phase structure is obtained. Wherein the tension is constant at 20Mpa, the temperature can be controlled by adjusting the voltage of induction heating, and the austenite precipitation amount is ensured to be between 40 and 60 percent.

Step 5, mild hot rolling:

after short-time solution treatment, the cold-rolled strip is immediately led into a hot roll 6 for one-time hot rolling, so that two phases are recrystallized to generate a uniformly refined microstructure. Wherein the initial rolling temperature is 1060 ℃, the final rolling temperature is 1010 ℃, and the hot rolling reduction is 27%.

Step 6, curling:

the hot-rolled thin strip is firstly air-cooled to 700-800 ℃, then is immediately cooled to 350-500 ℃ by a water cooling device 7, the purpose of step-by-step cooling is to save water to a large extent, but the temperature of 500-800 ℃ is in a precipitated phase sensitive region, and when the temperature of the thin strip is lower than 800 ℃, the thin strip needs to be rapidly cooled so as to avoid that the thin strip stays for a long time in the temperature region to cause more precipitated phases. Then, the film is curled 8, and a finished thin belt with the thickness of 0.8mm is obtained.

Other detailed parameters are shown in table 1, and the EBSD and grain size distribution diagram of the finished thin strip is shown in fig. 3; the stress-strain curve under the quasi-static tensile test is shown in fig. 7.

TABLE 1 specific process parameters of the examples and comparative examples and part of the parameters of the finished strip

Example 2:

smelting molten steel, which comprises the following components in percentage by weight: c: 0.03%, Si: 1.0%, Mn: 5.0%, Ni: 1.5%, Cr: 21.0%, N: 0.2%, Mo: 0.5%, Cu: 0.5 percent and the balance of Fe; subsequently, the duplex stainless steel thin strip is prepared according to the process flow of the embodiment 1, wherein the related specific process parameters are shown in the table 1; the gold phase structure of the fine uniform crystal grains of the finished thin strip is shown in figure 4; the stress-strain curve under the quasi-static tensile test is shown in fig. 7.

Example 3:

smelting molten steel to prepare a duplex stainless steel ribbon with TRIP effect, wherein the composition ratio of the duplex stainless steel ribbon is (wt%): c: 0.04%, Si: 1.6%, Mn: 2.9%, Ni: 2.0%, Cr: 20%, N: 0.19%, Mo: 0.5 percent and the balance of Fe; the duplex stainless steel strip was subsequently prepared according to the process flow of example 1. Wherein the specific process parameters are shown in Table 1; the metallographic structure of fine uniform crystal grains of the finished thin strip is shown in figure 5; the stress-strain curve under the quasi-static tensile test is shown in fig. 7.

For ease of comparison, the following comparative examples are provided:

comparative example 1

Molten steel was melted in the same composition ratio (2205 duplex stainless steel) as in example 1. A duplex stainless steel ribbon was subsequently prepared essentially following the process flow of example 1, except that:

step 2, double-roller casting and rolling:

preheating the tundish 1, then casting molten steel into the tundish 1, and controlling the superheat degree to be 50-80 ℃. Molten steel flows into a molten pool through a water gap, and forms a dual-phase casting belt after being led out by the crystallization roller 2 along with the strict control of the cooling speed and the rotating speed, wherein the thickness of the casting belt is 3.2 mm. That is, the cooling speed of the crystallizing roller in the step 2 is increased so that the temperature of the final cast strip discharged from the roller is in a two-phase region to obtain a two-phase cast strip, and the subsequent preparation process is the same as that of the embodiment of the invention.

Wherein the preheating temperature of the tundish is 1320 ℃; the casting temperature of the molten steel is 1510 ℃; the cooling speed of the crystallization roller is controlled to be 100-200 ℃/s, the rotating speed is 160m/min, and the temperature of the cast strip which is finally discharged from the roller is 1100 ℃.

Step 3, heavy cold rolling:

and (3) conveying the dual-phase cast strip into a cold roll 4 through a pinch roll to perform constant-tension severe cold rolling, wherein the tension is constant at 20Mpa, the reduction of each pass is 0.3mm, the total reduction is 50%, and the cold rolled strip with the thickness of 1.6mm is prepared.

The specific process parameters are shown in table 1; the EBSD and grain size distribution maps are shown in FIG. 6; the stress-strain curve under the quasi-static tensile test is shown in fig. 7.

Comparative example 2

The same composition ratio as in comparative example 1 was used, and the molten steel smelting step and the twin-roll casting step were the same as in comparative example 1. The difference lies in that: and (4) increasing the cold rolling deformation in the heavy cold rolling in the step (3). The side edge of the cold-rolled sheet has a cracking sign when the total rolling reduction of the cold rolling reaches 62%, and the side edge of the cold-rolled sheet has a cracking sign when the total rolling reduction of the cold rolling is 70%, wherein cracks are clearly visible, the length of the cold-rolled sheet reaches about 1-2 cm, and raw materials are greatly wasted.

As can be seen from FIGS. 3 to 5, the method of the present invention can obtain a uniformly refined two-phase structure, has no obvious morphological anisotropy, and is suitable for the preparation of most of low-nitrogen and medium-nitrogen duplex stainless steel thin strips, including duplex stainless steel with TRIP effect. Wherein the average grain size of austenite is about 3 μm, and the average grain size of ferrite is 4-7 μm;

as can be seen from fig. 6, the 2205 duplex stainless steel produced according to the conventional twin roll strip casting process of comparative example 1 has a relatively small grain size (the average grain size of austenite is about 11 μm, and the average grain size of ferrite is about 20 μm), but exhibits a significant anisotropy in its structure, and the two phases are significantly elongated in the rolling direction, which is disadvantageous to the formation of steel;

as can be seen from fig. 7, the duplex stainless steel ribbon prepared according to the process of the present invention has higher yield strength, tensile strength (both 100 to 200MPa higher than that of the duplex stainless steel ribbon prepared by the general process) and elongation, and especially in example 3, due to the presence of TRIP effect, the tensile strength and elongation are greatly improved, respectively 905MPa and 84%, which are 10 to 20% higher than those of the general TRIP type duplex stainless steel.

As can be seen from comparative example 2, the total reduction of cold rolling in the conventional "twin roll casting + two-phase zone cold rolling" is greatly affected by the two-phase inconsistent deformation, and the large cold rolling deformation is greatly limited. Therefore, this conventional process is not suitable for heavy cold rolling and is not favorable for refining the structure.

The above-mentioned embodiments are merely illustrative of the preferred embodiments of the present invention, and do not limit the scope of the present invention, and various modifications and improvements made to the technical solution of the present invention by those skilled in the art without departing from the spirit of the present invention shall fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing a uniform and fine duplex stainless steel thin strip based on a casting-rolling short process is characterized by comprising the following steps:

2. The method for preparing the uniform and fine duplex stainless steel thin strip based on the cast-rolling short process according to claim 1, wherein the mild hot rolling is performed at a start rolling temperature of 1050-1100 ℃, a finish rolling temperature of 1000-1050 ℃ and a hot rolling reduction of 25-35%.

3. The method for producing thin strip of cast-rolled short run-based uniform duplex stainless steel according to claim 1, wherein the step of cooling the hot-rolled thin strip comprises: and air cooling the hot-rolled thin strip to 700-800 ℃, then water cooling to 350-500 ℃ and then curling.

4. The method for producing the cast-rolling short-run based thin strip of uniform fine duplex stainless steel according to claim 1, wherein the short-time solution treatment comprises: and (3) introducing the cold-rolled strip into a heating coil through constant tension to perform induction heating to 1050-1200 ℃, and keeping the temperature for 3-6 min.

5. The method for preparing the uniform and fine duplex stainless steel thin strip based on the cast-rolling short process according to claim 1, wherein the superheat degree of the molten steel is controlled to be 50-80 ℃ when the molten steel is cast into the tundish.

6. The method for producing the thin strip of uniform and fine duplex stainless steel based on the cast-rolling short process according to claim 4, wherein the tension in the constant-tension severe cold rolling step and the short-time solution treatment step is constant at 20 to 30 MPa.

7. The method for producing the thin strip of homogeneous duplex stainless steel based on the short cast-rolling pass according to claim 1, wherein the mass fraction of N in the molten steel is less than 0.2%.

8. The method for preparing the thin strip of uniform and fine duplex stainless steel based on the cast-rolling short process according to claim 1, wherein the composition of the molten steel comprises, in mass percent: c: 0.03-0.06%, Si: 0.2-2.0%, Mn: 1.0-5.0%, Ni: 1.0-9.0%, Cr: 19.0 to 21.0%, N: 0.05 to 0.2%, Mo: 0-0.5%, W: 0-0.5%, Cu: 0-0.5% and the balance Fe.

9. Thin strip of uniform fine duplex stainless steel produced by the production method according to any one of claims 1 to 8, having a thickness of 0.6 to 1.2mm and containing 40 to 60% volume fraction of austenite.