A non-quenched and tempered steel with stable impact toughness and its preparation method and application

Abstract

The invention discloses a non-quenched and tempered steel with stable impact toughness, comprising the following elements by weight: C: 0.200% to 0.340%, Si: 0.320% to 0.410%, Mn: 1.520% to 1.760%, P: ≤0.02%, S: ≤0.01%, Cu: 0.065% to 0.082%, Ni: 0.180% to 0.231%, Cr: 0.063% to 0.076%, Mo: 0.020% to 0.024%, V: 0.04% to 0.09%, Nb: 0.030% to 0.041%, Ti: 0.010% to 0.035%, Al: 0.025% to 0.039%. The invention controls the temperature and cooling speed in the cooling process of the non-quenched and tempered steel, so that the strength of the non-quenched and tempered steel after cooling does not change significantly, and the steel has good and stable impact toughness and small fluctuation of impact toughness.

Description

Technical Field

[0001] The invention belongs to the technical field of controlled rolling and controlled cooling microalloyed steel pipes, and in particular relates to a non-quenched and tempered steel with stable impact toughness, and a preparation method and application thereof. Background Art

[0002] Non-quenched and tempered steel refers to the process of designing the composition of microalloyed steel so that the alloy elements are dissolved in austenite during heating, and then combined with controlled rolling and controlled cooling processes to precipitate the added microalloying elements in the form of fine carbides and nitrides in the parent phase, so that the steel is strengthened without quenching and tempering after hot rolling, and the mechanical properties reach or approach those of tempered steel. This type of steel can be used to prepare non-quenched and tempered steel pipes, and this type of steel pipe does not require quenching and tempering after hot rolling, thus greatly reducing the manufacturing cost, eliminating the need for quenching and tempering heat treatment and related straightening and transportation, saving heat treatment processes and heat treatment equipment, avoiding waste caused by deformation or quenching cracks during heat treatment, improving working conditions, and reducing pollution caused by heat treatment. The manufacturing cycle is shorter, capital occupancy is less, energy is saved, and the government's energy conservation and emission reduction requirements are responded to.

[0003] However, in the actual industrial production of non-quenched and tempered steel pipes, the original cooling method is usually used after final rolling, which exposes many problems. One of the more common problems is that the impact performance of non-quenched and tempered steel pipes fluctuates greatly. The impact toughness of samples obtained from different batches of non-quenched and tempered steel pipes, or even different positions of the same batch of non-quenched and tempered steel pipes, varies greatly, and there are large fluctuations (the impact energy sometimes differs by two to three times), and the impact performance fluctuates and cannot be stably controlled. Because the impact energy reflects the subtle defects and impact resistance of the steel pipe material, it is an important mechanical property index of the steel pipe material. Its practical significance lies in revealing the brittle tendency of the steel pipe material and reflecting the resistance of the steel pipe material to external impact loads. Good and stable impact toughness is a very important mechanical property parameter of the steel pipe. In actual service applications, the use scenarios of steel pipes are complex and diverse, covering a wide range, and the possibility of steel pipes being affected by impact loads is greatly increased. Fluctuations in the impact toughness of steel pipes may lead to potential fracture risks in field service. In some harsh service conditions, steel pipes with obvious fluctuation characteristics in impact toughness are not allowed to be used, which reduces the yield of steel pipe products and limits the application and development of such steel pipes.

[0004] In the prior art, in order to control the problem of unstable impact performance of non-quenched and tempered steel pipes, the composition of the micro-alloyed steel pipe is mainly adjusted, and then the controlled rolling and controlled cooling process parameters are adjusted. Although this method can effectively control the impact fluctuation problem of the finished non-quenched and tempered steel pipe, it will change other properties of the non-quenched and tempered steel pipe, and sometimes adjusting the production process requires adding other equipment, which makes the process complicated and the cost increased. Summary of the invention

[0005] The object of the present invention is to provide a non-quenched and tempered steel with stable impact toughness to solve at least one aspect of the problems and defects raised in the above-mentioned background technology.

[0006] The invention also provides a method for preparing the non-quenched and tempered steel with stable impact toughness.

[0007] The invention also provides application of the non-quenched and tempered steel with stable impact toughness.

[0008] The first aspect of the present invention provides a non-quenched and tempered steel with stable impact toughness, which is composed of the following elements in weight fractions:

[0009] C: 0.200% ~ 0.340%, Si: 0.320% ~ 0.410%, Mn: 1.520% ~ 1.760%, P: ≤0.02%, S: ≤0.01%, Cu: 0.065% ~ 0.082%, Ni: 0.180% ~ 0.231%, Cr: 0.063% ~ 0.076%, Mo: 0.020% ~ 0.024%, V: 0.04% ~ 0.09%, Nb: 0.030% ~ 0.041%, Ti: 0.010% ~ 0.035%, Al: 0.025% ~ 0.039%, the rest is Fe and unavoidable impurities.

[0010] The present invention controls the constituent elements of non-quenched and tempered steel, cooperates with a reasonable cooling process and cooling speed without changing the rolling parameter process, and can obtain non-quenched and tempered steel with stable impact toughness.

[0011] In some specific embodiments of the present invention, the content of the Mn element is 1.610% to 1.710%.

[0012] In the present invention, the content of Mn element has a great influence on the impact energy.

[0013] In some specific embodiments of the present invention, the content of the V element is 0.05% to 0.08%.

[0014] In some specific embodiments of the present invention, the content of the Nb element is 0.032% to 0.036%.

[0015] A second aspect of the present invention provides a method for preparing non-quenched and tempered steel with stable impact toughness, comprising the following steps:

[0016] S1. Cool the steel forging to 640℃~660℃;

[0017] S2, air cooling to a temperature of 440°C to 455°C;

[0018] S3. Finally, rapid cooling is performed.

[0019] The present invention controls the temperature and cooling speed of non-quenched and tempered steel forgings during cooling, so that the prepared non-quenched and tempered steel has good and stable impact toughness. By selecting two cooling temperatures and performing rapid cooling at the end, the precipitation and coarsening of the microstructure precipitation phase in the steel are controlled, thereby controlling the strength and impact toughness fluctuations of the steel.

[0020] The selection of temperature and the control of cooling rate of the present invention are based on the following: in view of the fact that the impact toughness of different batches of non-quenched and tempered steel or different positions of the same batch of non-quenched and tempered steel varies greatly, and there are large fluctuations (the impact energy sometimes differs by two to three times). After final rolling, the non-quenched and tempered steel enters the dual-phase region of ferrite + pearlite through cooling, obtains new ferrite recrystallized grains, and finally cools to room temperature. The cooling rate or cooling rate in this cooling process is a key factor affecting the microstructure of the non-quenched and tempered steel pipe. In the cooling process after final rolling, if forced rapid cooling is not adopted, the cooling rate or cooling rate will be different due to the difference in external factors such as cooling method, production environment temperature, stacking amount, etc. Even in the same batch, the cooling rate or cooling rate at different positions may be different, that is, the cooling rate fluctuates. This fluctuation in cooling rate will inevitably lead to different precipitation behaviors of microalloy carbonitrides in ferrite after the austenite phase transformation to ferrite.

[0021] An important feature of alloying elements in non-quenched and tempered steel is that they can form carbides, nitrides, and carbonitrides with carbon and nitrogen in non-quenched and tempered steel, and precipitate in the ferrite as a second phase during hot rolling or cooling after rolling. Especially at 640℃~660℃, fine precipitate phases can be precipitated from the supersaturated solid solution. For example, element V, the main carbide V4C3 in steel has the largest nucleation rate at 640℃~660℃, and the precipitation effect is most significant. The precipitation and growth of the second phase of alloying elements are closely related to the cooling rate during the cooling process. If the cooling rate is not fast enough, the precipitated second phase will coarsen and grow to a certain extent.

[0022] For non-quenched and tempered steel containing elements such as Cr, Mn, Ni, and P, if it stays in the range of 440°C to 660°C for a long time, carbides of elements such as Cr and Ni and impurity elements such as P, As, Sn, and Sb will precipitate at the grain boundaries. Ni and Cr not only promote the segregation of impurity elements, but also segregate themselves. If the temperature is below 440°C and then followed by slow cooling, these precipitated impurity phases will segregate and coarsen, sharply increasing the brittleness of the steel, that is, producing the so-called slow cooling brittleness. Therefore, in the cooling process after rolling, if the cooling rate at a temperature below 440°C is not fast enough, slow cooling brittleness will occur. If a rapid cooling method is adopted, the embrittlement phenomenon will disappear or be suppressed. Therefore, in the cooling process after the final rolling of the non-quenched and tempered steel of the present invention, forced rapid cooling is adopted to eliminate the fluctuation of the cooling rate, reduce the slow cooling brittleness, increase the impact work, and reduce the fluctuation of the impact toughness.

[0023] The main reason why the present invention first cools the steel forging to 640°C-660°C for air cooling is that non-quenched and tempered steel can precipitate fine precipitates from the supersaturated solid solution, and the precipitates will coarsen and grow, affecting the strength and toughness fluctuations of the steel. In the present invention, if the initial cooling temperature is greater than 660°C and the subsequent air cooling process is carried out, the carbide, nitride and carbonitride precipitates in the non-quenched and tempered steel will coarsen and grow too large, causing fluctuations in toughness. If the initial cooling temperature is less than 640°C and the subsequent air cooling process is carried out, the precipitates will not have time to precipitate or will not precipitate sufficiently, which will reduce the strength of the steel.

[0024] In the present invention, if conventional cooling is performed at a temperature below 440°C, the impurity phase precipitated in the second phase will be segregated and coarsened, which will sharply increase the brittleness of the steel, that is, the so-called slow cooling brittleness. Therefore, in the cooling process after rolling, if the cooling speed is not fast below 440°C, slow cooling brittleness will occur.

[0025] In some specific embodiments of the present invention, the rapid cooling method is at least one of water cooling, oil cooling and air cooling.

[0026] In some specific embodiments of the present invention, the cooling rate in step 1 is 0.5°C / s to 1°C / s.

[0027] The present invention firstly controls the cooling speed of the steel forging in the early stage, which can effectively control the formation of tissues in the steel and the content of each tissue, and can improve the accuracy of subsequent impact toughness fluctuation control.

[0028] In some specific embodiments of the present invention, the air cooling in step 2 is to a temperature of 440°C to 455°C.

[0029] The present invention controls the lower limit temperature of the slow cooling brittle temperature range, and can avoid the generation of a hard and brittle bainite phase structure during the cooling process after final rolling.

[0030] In some specific embodiments of the present invention, the air cooling rate in step 2 is 2.5°C to 3.5°C.

[0031] In some specific embodiments of the present invention, the rapid cooling rate is 135° C. / s to 165° C. / s.

[0032] A third aspect of the present invention provides use of non-quenched and tempered steel with stable impact toughness in the preparation of non-quenched and tempered steel pipes.

[0033] Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0034] The present invention firstly reasonably matches the constituent elements of non-quenched and tempered steel, and controls the temperature point in the cooling process and the cooling rate, thereby reducing the occurrence of the embrittlement phenomenon of non-quenched and tempered steel, thereby affecting the impact toughness of the non-quenched and tempered steel. By using the method for controlling the fluctuation of impact toughness in the cooling mode of the present invention, the strength of the non-quenched and tempered steel after cooling does not change significantly, and at the same time has good and stable impact toughness. DETAILED DESCRIPTION

[0035] The following will be combined with the embodiments to clearly and completely describe the concept of the present invention and the technical effects produced, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0036] In the description of the present invention, the description with reference to the terms "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0037] If the specific conditions are not specified in the examples, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. If the manufacturers of the reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0038] Specific embodiments of the present invention are described in detail below.

[0039] The thickness of the non-quenched and tempered steel used in the embodiments of the present invention and the comparative examples is 10 mm.

[0040] The impact energy in the embodiments of the present invention and the comparative examples is the impact absorption energy Ak value. Impact specimens are taken at intervals of 5 cm on the steel pipe to test the impact absorption energy values ​​at 7 different positions.

[0041] The fluctuation of impact toughness in the embodiments of the present invention and the comparative examples is represented by the standard deviation calculated from the impact energy at different positions. The smaller the standard deviation, the smaller the fluctuation of impact toughness.

[0042] Impact energy test reference: GB / T229-2020 Charpy pendulum impact test method for metal materials.

[0043] Yield strength test reference: GB / T228.1 Tensile test of metallic materials Part 1: Room temperature tensile test method.

[0044] Tensile strength test reference: GB / T228.1 Tensile test for metallic materials Part 1: Room temperature tensile test method.

[0045] Elongation test reference: GB / T228.1 Tensile test of metallic materials Part 1: Room temperature tensile test method.

[0046] Example 1

[0047] This embodiment is a non-quenched and tempered steel with stable impact toughness, and is composed of the following elements by weight:

[0048] C: 0.240%, Si: 0.380%, Mn: 1.660%, P: 0.015%, S: 0.003%, Cu: 0.079%, Ni: 0.207%, Cr: 0.073%, Mo: 0.022%, V: 0.07%, Nb: 0.035%, Ti: 0.03%, Al: 0.036%, and the rest are Fe and inevitable impurities.

[0049] The preparation method of non-quenched and tempered steel with stable impact toughness in this embodiment comprises the following steps: cooling the non-quenched and tempered steel after final rolling to a temperature of 650°C at a cooling rate of 1°C / s, immediately air cooling to a temperature of 450°C at a cooling rate of 2.5°C / s to 3.5°C / s, and finally rapidly cooling with water as a medium at a cooling rate of 60°C / s to a temperature of 25°C.

[0050] Example 2

[0051] This embodiment is a non-quenched and tempered steel with stable impact toughness, and is composed of the following elements by weight:

[0052] C: 0.230%, Si: 0.360%, Mn: 1.640%, P: 0.016%, S: 0.002%, Cu: 0.075%, Ni: 0.200%, Cr: 0.071%, Mo: 0.020%, V: 0.06%, Nb: 0.034%, Ti: 0.020%, Al: 0.035%, and the rest are Fe and inevitable impurities.

[0053] The preparation method of non-quenched and tempered steel with stable impact toughness in this embodiment comprises the following steps: cooling the non-quenched and tempered steel forging after final rolling to a temperature of 650°C at a cooling rate of 1°C / s, immediately air cooling to a temperature of 440°C at a cooling rate of 3.5°C / s, and finally rapidly cooling with water as a medium at a cooling rate of 60°C / s to a temperature of 25°C.

[0054] Example 3

[0055] This embodiment is a non-quenched and tempered steel with stable impact toughness, and is composed of the following elements by weight:

[0056] C: 0.200%, Si: 0.351%, Mn: 1.610%, P: 0.018%, S: 0.001%, Cu: 0.072%, Ni: 0.211%, Cr: 0.076%, Mo: 0.024%, V: 0.051%, Nb: 0.032%, Ti: 0.016%, Al: 0.026%, and the rest are Fe and inevitable impurities.

[0057] The preparation method of non-quenched and tempered steel with stable impact toughness in this embodiment comprises the following steps: cooling the non-quenched and tempered steel forging after final rolling to a temperature of 650°C at a cooling rate of 1°C / s, immediately air cooling to a temperature of 450°C at a cooling rate of 3.5°C / s, and finally rapidly cooling with water as the medium at a cooling rate of 60°C / s to a temperature of 25°C.

[0058] Example 4

[0059] This embodiment is a non-quenched and tempered steel with stable impact toughness, and is composed of the following elements by weight:

[0060] C: 0.310%, Si: 0.380%, Mn: 1.691%, P: 0.017%, S: 0.003%, Cu: 0.079%, Ni: 0.196%, Cr: 0.065%, Mo: 0.021%, V: 0.080%, Nb: 0.036%, Ti: 0.024%, Al: 0.034%, and the rest are Fe and inevitable impurities.

[0061] The preparation method of non-quenched and tempered steel with stable impact toughness in this embodiment comprises the following steps: cooling the non-quenched and tempered steel forging after final rolling to a temperature of 650°C at a cooling rate of 1°C / s, immediately air cooling to a temperature of 450°C at a cooling rate of 2.5°C / s, and finally rapidly cooling with water as the medium at a cooling rate of 75°C / s to a temperature of 25°C.

[0062] Comparative Example 1

[0063] This comparative example is a non-quenched and tempered steel with stable impact toughness, composed of the following elements by weight:

[0064] C: 0.240%, Si: 0.380%, Mn: 1.660%, P: 0.015%, S: 0.003%, Cu: 0.079%, Ni: 0.207%, Cr: 0.073%, Mo: 0.022%, V: 0.07%, Nb: 0.035%, Ti: 0.03%, Al: 0.036%, and the rest are Fe and inevitable impurities.

[0065] This comparative example adopts a conventional cooling method, specifically as follows: the non-quenched and tempered steel forging after final rolling is cooled to a temperature of 650°C at a cooling rate of 1°C / s, and finally air-cooled to 25°C.

[0066] Comparative Example 2

[0067] This comparative example is a non-quenched and tempered steel with stable impact toughness, composed of the following elements by weight:

[0068] C: 0.261%, Si: 0.242%, Mn: 1.796%, P: 0.021%, S: 0.012%, Cu: 0.067%, Ni: 0.261%, Cr: 0.065%, Mo: 0.024%, V: 0.050%, Nb: 0.035%, Ti: 0.016%, Al: 0.025%, and the rest are Fe and inevitable impurities.

[0069] The preparation method of the comparative example impact toughness stable non-quenched and tempered steel comprises the following steps: cooling the non-quenched and tempered steel forging after final rolling to a temperature of 580°C at a cooling rate of 1°C / s, immediately air cooling to a temperature of 450°C at a cooling rate of 2.5°C / s to 3.5°C / s, and finally rapidly cooling with water as a medium at a cooling rate of 60°C / s to a temperature of 25°C.

[0070] Comparative Example 3

[0071] This comparative example is a non-quenched and tempered steel with stable impact toughness, composed of the following elements by weight:

[0072] C: 0.224%, Si: 0.235%, Mn: 1.468%, P: 0.018%, S: 0.006%, Cu: 0.061%, Ni: 0.186%, Cr: 0.072%, Mo: 0.021%, V: 0.030%, Nb: 0.040%, Ti: 0.021%, Al: 0.031%, and the rest are Fe and inevitable impurities.

[0073] The preparation method of the impact toughness stable non-quenched and tempered steel of the comparative example comprises the following steps: cooling the non-quenched and tempered steel forging after final rolling to a temperature of 650°C at a cooling rate of 1°C / s, immediately air cooling to a temperature of 440°C at a cooling rate of 2.5°C / s to 3.5°C / s, and finally rapidly cooling with water as a medium at a cooling rate of 60°C / s to a temperature of 25°C.

[0074] The non-tempered chemical compositions obtained in Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 1.

[0075] The cooled non-quenched and tempered steels obtained in Examples 1 to 4 and Comparative Examples 1 to 3 were subjected to strength and impact energy tests. The results are shown in Table 2. The calculated impact toughness fluctuation results are shown in Table 3.

[0076] Table 1 Chemical component contents of non-quenched and tempered steels obtained in Examples 1 to 4 and Comparative Examples 1 to 3

[0077]

[0078]

[0079] Table 2 Test results of strength and impact energy of non-quenched and tempered steel obtained in Examples 1 to 4 and Comparative Examples 1 to 3

[0080]

[0081] Table 3 Fluctuation results of impact toughness of non-quenched and tempered steel obtained in Examples 1 to 4 and Comparative Examples 1 to 3

[0082] - Average impact energy Impact energy standard deviation Example 1 159.00 1.07 Example 2 158.86 1.36 Example 3 158.71 1.58 Example 4 158.14 1.81 Comparative Example 1 102.86 45.54 Comparative Example 2 140.14 19.40 Comparative Example 3 144.43 13.40

[0083] The difference between Comparative Example 1 and Example 1 is that the non-quenched and tempered steel is cooled by the existing cooling method. It can be seen from Tables 2 and 3 that without applying the rapid cooling process, the impact energy of Comparative Example 1 is not large, and the standard deviation of the impact energy is large, indicating that the impact toughness of the non-quenched and tempered steel obtained in Comparative Example 1 fluctuates greatly. At the same time, it can be seen from Tables 1 to 3 that when the chemical components of Comparative Example 1 and Example 1 are the same, the cooling process of the non-quenched and tempered steel after final rolling is controlled, and the strength of the obtained non-quenched and tempered steel does not change much, but the impact toughness fluctuates slightly.

[0084] In summary, the present invention firstly reasonably matches the constituent elements of non-quenched and tempered steel, and controls the temperature points in the cooling process and the cooling rate, thereby reducing the occurrence of embrittlement of non-quenched and tempered steel, thereby affecting the impact toughness of non-quenched and tempered steel. By using the method for controlling the fluctuation of impact toughness by the cooling method of the present invention, the strength of the non-quenched and tempered steel after cooling does not change significantly, and at the same time has good and stable impact toughness.

[0085] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A non-quenched and tempered steel with stable impact toughness, characterized in that: Composed of the following elements in weight fractions: C: 0.200%~0.340%, Si: 0.320%~0.410%, Mn: 1.520%~1.760%, P: ≤0.02%, S: ≤0.01%, Cu: 0.065%~0.082%, Ni: 0.180%~0.231%, Cr: 0.063%~0.076%, Mo: 0.020%~0.024%, V: 0.04%~0.09%, Nb: 0.030%~0.041%, Ti: 0.010%~0.035%, Al: 0.025%~0.039%, the rest are Fe and unavoidable impurities; The method for preparing the impact toughness-stabilized non-quenched and tempered steel comprises the following steps: S1. Cool the non-quenched and tempered steel after final rolling to a temperature of 640°C~660°C; S2, air cooling to a temperature of 440°C~455°C; S3, and finally cooling rapidly to a temperature of 25°C; The cooling rate in step S1 is 0.5°C / s~1°C / s; The air cooling speed in step S2 is 2.5°C / s to 3.5°C / s; The rapid cooling rate is 50°C / s to 75°C / s.

2. The impact toughness stabilized non-quenched and tempered steel according to claim 1, characterized in that: The content of the Mn element is 1.610% to 1.710%.

3. The impact toughness stabilized non-quenched and tempered steel according to claim 1, characterized in that: The content of the V element is 0.05% to 0.08%.

4. The impact toughness stabilized non-quenched and tempered steel according to claim 1, characterized in that: The content of the Nb element is 0.032% to 0.036%.

5. The impact toughness stabilized non-quenched and tempered steel according to claim 1, characterized in that: The rapid cooling method is at least one of water cooling, oil cooling and air cooling.

6. Use of the non-quenched and tempered steel with stable impact toughness as claimed in any one of claims 1 to 5 in the preparation of non-quenched and tempered steel pipes.

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