A continuous rolling method for ultra-low-temperature-resistant high-strength 316LN bar

Abstract

The application discloses a kind of ultra-low temperature resistant high-strength 316LN bar continuous rolling method, comprising the following steps: step one, raw material ingot is heated using heating furnace, heating furnace is divided into preheating section, heating 1 section, heating 2 section, heating 3 section and soaking section, wherein the heating temperature of preheating section is ≤650, the heating temperature of heating 1 section is 1120-1150 DEG C, the heating temperature of heating 2 section, heating 3 section and soaking section is all 1240-1260, and A product is obtained;Step two, A product is roughed, and the rolling pass is 23-25 passes, and B product is obtained;Step three, B product is continuously rolled, and the finished round steel with specification φ65-160mm is obtained.The application has the characteristics of obtaining ultra-fine grain structure and low-temperature mechanical property stable bar.

Description

Technical Field

[0001] This invention relates to a continuous rolling method for 316LN bars, and more particularly to a continuous rolling method for 316LN bars with high strength and resistance to ultra-low temperatures. Background Technology

[0002] Low-temperature resistant austenitic stainless steel, primarily represented by 316LN, is an economical and practical steel grade characterized by its specific composition and excellent single-phase ultrafine austenitic microstructure. Currently, the mainstream products on the market are in the form of medium-thick plates, thick-walled tubes, and large-diameter bars, mainly used in nuclear power plant cooling equipment and gas liquefaction equipment. Under the specific composition of 316LN steel, the grain size determines the stability of its low-temperature strength performance and its resistance to different low-temperature strength conditions.

[0003] Currently, the bars of this steel grade are mainly prepared by forging. It is difficult to control the grain size, and the forging heating temperature and forging process temperature are not easy to control. This often leads to severe mixed grain phenomenon on the surface of the round bars, with large grain size differences. It is necessary to peel off the surface of the round bars, resulting in a very low final yield and unstable low-temperature mechanical properties, which cannot meet the application requirements of low temperature and high strength. Summary of the Invention

[0004] The purpose of this invention is to provide a continuous rolling method for high-strength 316LN bars resistant to ultra-low temperatures. This invention features bars with ultra-fine grain structure and stable low-temperature mechanical properties.

[0005] The technical solution of this invention: A method for continuous rolling of ultra-low temperature resistant, high-strength 316LN bars, comprising the following steps:

[0006] Step 1: Heating the raw material ingots in a heating furnace. The heating furnace is divided into a preheating section, heating section 1, heating section 2, heating section 3, and a soaking section. The heating temperature of the preheating section is ≤650℃, the heating temperature of heating section 1 is 1120-1150℃, and the heating temperatures of heating sections 2, 3, and the soaking section are all 1240-1260℃, yielding product A.

[0007] Step 2: Roughly roll product A into a billet, with 23-25 ​​rolling passes, to obtain product B;

[0008] Step 3: Roll product B continuously to obtain finished round steel bars with specifications of φ65-160mm.

[0009] In the aforementioned method for continuous rolling of ultra-low temperature resistant high-strength 316LN bars, in step one, heating section 1, heating section 2, heating section 3 and soaking section are all heated by upper and lower burners.

[0010] In the aforementioned method for continuous rolling of ultra-low temperature resistant high-strength 316LN bars, in step two, during the rough rolling process, the surface heat deformation temperature of the billet is 1080-1200℃.

[0011] In the aforementioned method for continuous rolling of ultra-low temperature resistant high-strength 316LN bars, in step two, after the rough rolling is completed, the surface temperature of the billet is not lower than 980℃.

[0012] In the aforementioned method for continuous rolling of ultra-low temperature resistant high-strength 316LN bars, in step two, the average reduction per rolling pass is 30-40 mm.

[0013] In the aforementioned method for continuous rolling of ultra-low temperature resistant high-strength 316LN bars, in step three, the surface hot deformation temperature of the billet during continuous rolling is 1000-1050℃.

[0014] In the aforementioned method for continuous rolling of ultra-low temperature resistant high-strength 316LN bars, step three involves continuous rolling using eight rolling mills, which include four 650 mills and four 550 mills arranged alternately in a horizontal and vertical configuration.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0016] This invention utilizes a unique one-pass rolling method. First, a rough rolling process is performed to create the billet, followed by continuous rolling. The billet undergoes specific heating temperatures in a preheating section, heating section 1, heating section 2, heating section 3, and a soaking section, with hot processing at a deformation temperature of 1080-1200℃. The reduction is rationally allocated to ensure that the billet undergoes multiple large single-pass deformations (average single-pass reduction controlled at 30-40mm) in continuous rolling of large-diameter 316LN bars (φ65-φ160). The resulting bars have a uniform, ultra-fine grain structure with no mixed crystals on the surface, a grain size ≥8, and a grain size difference ≤2. The product performance can stably achieve a yield strength ≥900MPa, tensile strength ≥1200MPa, elongation at fracture ≥32%, and reduction of area ≥40% at -269℃. This method achieves advantages such as low production cost, high yield, high production efficiency, and stable mechanical properties at -269℃.

[0017] Therefore, the present invention has the characteristics of obtaining ultrafine-grained structures and stable low-temperature mechanical properties in bar stock. Attached Figure Description

[0018] Figure 1 This is a metallographic diagram of the final product, round steel bar, from Example 1.

[0019] Figure 2 This is the metallographic structure diagram of Comparative Example 1, where the grain size is mixed.

[0020] Figure 3 This is the low-temperature physical property test table for 316LN bars in Example 1. Detailed Implementation

[0021] The present invention will be further described below with reference to embodiments, but these embodiments are not intended to limit the scope of the invention.

[0022] Example 1:

[0023] Material: 316LN; Raw material ingot dimensions: 460*460*1800mm.

[0024] A continuous rolling method for high-strength 316LN bars resistant to ultra-low temperatures includes the following steps:

[0025] Step 1: A 460*460*1800mm raw material ingot of 316LN material is heated in a pusher-type heating furnace. The furnace body is 35 meters long and 4.5 meters wide. The furnace is divided into five heating zones: a preheating section, heating section 1, heating section 2, heating section 3, and a soaking section. The heating temperature in the preheating section is ≤650℃, the heating temperature in heating section 1 is 1120-1150℃, and the temperatures in heating sections 2-3 and the soaking section are 1240-1260℃. Heating sections 1, 2, 3, and the soaking section all use upper and lower burners. After heating, product A is obtained.

[0026] Step 2: Grade A is rough-rolled on a 750 reversible rolling mill with 25 rolling passes. The mill uses a 750 reversible 2-roll mill. During rough rolling, the surface heat deformation temperature of the billet is 1080-1200℃, and the average reduction per rolling pass is 31mm. During rough rolling, the billet is gradually deformed and cooled through reciprocating rolling, ensuring that the surface temperature of the billet is not lower than 980℃ after rough rolling, preventing the formation of a mixed crystal layer on the surface before continuous rolling, thus obtaining Grade B.

[0027] Step 3: The B-grade steel is continuously rolled through an 8-stand rolling mill. During the rolling process, the surface heat deformation temperature of the billet is 1000-1050℃, which yields finished round steel bars with specifications of φ65-160mm and a compression ratio greater than 10. In this embodiment, φ100 316LN finished round steel bars are obtained.

[0028] The eight continuous rolling mills consist of four 650 mills and four 550 mills arranged alternately in horizontal and vertical configurations. The motor power of the eight continuous rolling mills is 800KW, 800KW, 1000KW, 1000KW, 1000KW, 1000KW, 1250KW, and 1250KW, respectively. The speed of the eight continuous rolling mills is 520 / 1200RPM, 520 / 1200RPM, 520 / 1200RPM, 420 / 1000RPM, 420 / 1000RPM, 420 / 1000RPM, and 420 / 1000RPM, respectively.

[0029] Testing revealed that the 316LN finished round steel obtained in this embodiment has an ultrafine grain structure, as shown in the metallographic image below. Figure 1 As shown, Figure 1 The grain size is grade 10, there are no mixed crystals, and the grain size grade difference is 2.

[0030] Two samples were taken from the same 316LN finished round steel bar and subjected to tensile property testing at -269℃. The testing standard was GB / T228.4-1010. The test report results are as follows: Figure 3 As shown, at -269℃, the sample had an average tensile strength of 1589 MPa, an average yield strength of 1091 MPa, an average elastic modulus of 204.5 GPa, an average elongation after fracture of 43%, and an average reduction of area of ​​47%.

[0031] Comparative Example 1:

[0032] This comparative example is basically the same as Example 1, except that the heating temperatures of heating stage 2, heating stage 3, and soaking stage are all 1200-1220°C; the metallographic structure image of the final finished round steel is shown below. Figure 2 As shown, the grain size is grade 7, with mixed crystals and a grade difference of 3.

[0033] Comparative Example 2:

[0034] This comparative example is basically the same as Example 1, except that the heating temperature of heating section 2, heating section 3 and soaking section are all 1280-1300°C; the grain size of the final finished round steel is grade 4, with mixed crystals and a grade difference of 3.

[0035] As can be seen from Example 1 and Comparative Examples 1-2, when the temperature is too high or too low, the product will exhibit large differences in temperature and mixed crystal phenomenon. The optimal heating temperature for heating stage 2, heating stage 3 and soaking stage is 1240-1260℃.

[0036] Comparative Example 3:

[0037] This comparative example is basically the same as Example 1, except that: the number of rolling passes for roughing billet is 31, the average reduction per rolling pass is 26mm, and the grain size of the final finished round steel is grade 5, with mixed grains and a grade difference of 2.

[0038] Comparative Example 4:

[0039] This comparative example is basically the same as Example 1, except that the roughing rolling process involves 19 passes, with an average reduction of 42 mm per pass. The resulting finished round steel has a grain size of grade 7, exhibiting mixed grains and large grade differences. Due to the large deformation, this comparative example results in a significant core temperature rise and grain growth.

[0040] As can be seen from Examples 1 and Comparative Examples 3-4, different reduction amounts of heat deformation have different effects on the uniformity of the finished product's microstructure. When the reduction amount is too high or too low, the product will exhibit large grade differences and mixed crystal phenomena. Therefore, the optimal number of roughing passes is 23-25.

[0041] It should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Those skilled in the art can modify the technical solutions described in the above embodiments, or make equivalent substitutions for some of the technical features; and all such modifications and substitutions should fall within the protection scope of the appended claims of the present invention.

Claims

1. A method for continuous rolling of ultra-low temperature resistant high strength 316LN bar, characterized in that: The method comprises the following steps: Step one, the raw material ingot is heated by a heating furnace, the heating furnace is divided into a preheating section, a heating 1 section, a heating 2 section, a heating 3 section and a soaking section, the heating temperature of the preheating section is ≤650℃, the heating temperature of the heating 1 section is 1120-1150℃, the heating temperature of the heating 2 section, the heating 3 section and the soaking section is 1240-1260℃, and A product is obtained; Step two, the A product is roughed and bloomed, in the roughing and blooming process, the surface thermal deformation temperature of the steel billet is 1080-1200℃, the rolling passes are 23-25, the single rolling pass reduction is 30-40mm, and B product is obtained; Step three, the B product is continuously rolled to obtain finished round steel with a specification of φ65-160mm, the grain size of the finished round steel is ≥8 levels, the grain size difference is ≤2 levels, the yield strength after breaking at-269℃ is ≥900MPa, the tensile strength is ≥1200MPa, the elongation after breaking is ≥32%, and the reduction of area is ≥40%.

2. The method of continuously rolling ultra-low temperature resistant high-strength 316LN bar according to claim 1, characterized in that: In step one, the heating 1 section, the heating 2 section, the heating 3 section and the soaking section are all heated by upper and lower burners.

3. The method of continuously rolling ultra-low temperature resistant high-strength 316LN bar according to claim 1, characterized in that: In step two, after the roughing and blooming, the surface temperature of the steel billet is not lower than 980℃.

4. The method of continuously rolling ultra-low temperature resistant high-strength 316LN bar according to claim 1, characterized in that: In step three, in the continuous rolling process, the surface thermal deformation temperature of the steel billet is 1000-1050℃.

5. The method of continuously rolling ultra-low temperature resistant high-strength 316LN bar according to claim 1, characterized in that: In step three, 8 continuous rolling mills are used for continuous rolling, the 8 continuous rolling mills include 4 650 rolling mills and 4 550 rolling mills which are distributed in a horizontal-vertical alternating mode.

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