TWI450975B - Process for making cementite grains in pearlite of steel cylindrical or spherical - Google Patents

Description

Process for spheroidized or spheroidized steel in ferritic carbon

This invention relates to a process relating to the manufacture of steel, and more particularly to a process for a cementite in a pillared or spheroidized steel.

In a steel material, the carbon content of Austenite is about 0.8% by weight in terms of weight percent (wt%). When the Vostian iron is cooled to 723 ° C at a very slow cooling rate, a phase change begins to occur, and at the same time, ferrite and ferritic carbon are precipitated to form a Borne iron structure.

According to the theory of R. F. Mehl, when an eutectoid reaction occurs, the nucleus of ferritic carbon iron is formed from the grain boundary of the Worthite iron and grows in the form of flakes in the grains of the above-mentioned Worthite iron. Because of the high carbon content of the ferritic carbon iron, when it is formed, carbon is absorbed from the surrounding Worth iron, and the ferrite is gradually formed. However, since the ferrite iron can only dissolve a small amount of carbon, the effect of squeezing the carbon outward is generated, and the Worthite iron on the side is further precipitated from the nucleus of the ferritic carbon iron to further form flaky stellite. After the above-mentioned production steps are repeated several times, the Xueming carbon iron and the fertilized iron continue to grow in the longitudinal direction, and increase in the lateral direction layer by layer, and finally completely replace the entire Worthite iron grains to form Xueming carbon iron and Layered waves alternately arranged by ferrite and iron.

Therefore, the obtained steel is produced by a general hot rolling process (hot rolling, hot rolling cooling, coiling), in which the pulverized iron is a layered structure composed of alternating flaky stellite and ferrite iron. Although the steel having the above-mentioned microstructure has the advantage of high mechanical strength, it has the disadvantage of poor ductility. According to the research, if the spheroidizing annealing treatment is carried out on the iron-plated iron structure of the steel, the long-shaped stellite carbon can be converted into a columnar shape or a spherical shape, thereby overcoming The above disadvantages of poor ductility are further beneficial for subsequent processing or application.

However, the above-described spheroidizing annealing treatment for the Wolla iron structure of the steel material will prolong the manufacturing time of the entire steel material. Therefore, there is a need for a new type of steel process that, in addition to obtaining a columnar or spheroidized stellite structure to enhance the performance of the steel, can also take into account the overall manufacturing time of the steel without excessively extending the steel. The overall manufacturing time, and achieve the purpose of energy saving and carbon reduction.

Therefore, the object of the present invention is to provide a process for the preparation of a ferritic carbon in a ferritic or spheroidal steel, and to control the degree of subcooling of the steel during the hot rolling process (that is, the temperature before and after cooling of the steel). Poor), rolling reduction rate and strain rate, thereby obtaining a Borne iron structure having a columnar or spheroidal shape of stellite. In addition, the above problem of prolonging the manufacturing time of the entire steel material can be avoided.

According to an embodiment of the present invention, there is provided a process for spheroidizing or spheroidizing a stellite carbon in a steel-to-iron structure. The process comprises: providing a steel embryo, wherein the steel embryo comprises the following components in wt%, the components are: less than or equal to 0.8 wt% of carbon, an auxiliary component, and a residue composed of iron, and the above The auxiliary component may be 0.20 wt% or less of chromium (Cr), 0.30 wt% or less of molybdenum (Mo) or a combination of the two; heating the steel embryo to the first temperature, and maintaining the steel embryo at the first temperature and continuing a first time period, wherein the first temperature is greater than or equal to 1200 ° C, and the first time period is greater than or equal to 4 hours; cooling the steel blank to a second temperature at a first cooling rate, wherein the first cooling rate is greater than or equal to each second 10 ℃ (℃ / s), and the second temperature is more than A _C1 transformation point below 20 ℃ A _C1 transformation point to 50 ℃; steel embryo multipass first and rolling process, thereby obtaining a first and rolling billets Wherein the thickness ratio of the first rolled steel blank to the steel blank is 15% to 25%; the first rolled steel blank is cooled to a third temperature at a second cooling rate, wherein the second cooling rate is 5 ° C / s to 30 ℃ / s, and the third temperature A _C1 transformation point below 150 ℃ A _C1 transformation point or less to 50 ℃; fourth temperature Performing a second rolling process on the first rolled steel preform to obtain a second rolled steel preform, wherein after performing any of the plurality of second rolling processes, the first rolling steel preform The ratio of thickness reduction is 10% to 50%, and the strain rate is less than or equal to 5.0 s ^-1 , and the thickness ratio of the second rolled steel blank to the first rolled steel blank that has not undergone any second rolling process is 10% to 20%, and the fourth temperature is greater than 600 ° C; and cooling the second rolled steel to room temperature to obtain the above steel, wherein the average particle size of the stellite in the steel is less than 2.0 microns (μm) and the average aspect ratio is less than 5.0.

Since the process of the present invention can eliminate the time of subsequent spheroidizing annealing treatment (also referred to as spheroidizing heat treatment), it has the advantage of increasing the yield. In addition, since no additional spheroidizing annealing process is required, the energy required to manufacture the steel material can be reduced and the carbon emissions can be reduced, thereby reducing the overall manufacturing cost of the steel and taking into account environmental protection demands.

Please refer to FIGS. 1 and 2, which are respectively a flow chart showing the process of the ferritic carbon in the ferritic structure of the columnarized or spheroidized steel according to an embodiment of the present invention, and FIG. 1 Schematic diagram of the relative relationship between the temperatures corresponding to the partial stages of the process. The process 100 of the ferritic carbon iron in the ferritic or spheroidized steel material (hereinafter referred to as "process 100" for convenience of explanation) begins in step 102 to provide a steel blank. In the above step 102, since the related process for manufacturing a steel preform (for example, manufacturing a steel blank by a conventional steelmaking or electric furnace method) is well known to those skilled in the art, it will not be described in detail. And the steel embryo of step 102 comprises the following components in wt%, such as: less than or equal to 0.8 wt% of carbon, auxiliary components, and other parts composed of iron (may be referred to as residuals) The auxiliary component may be 0.20 wt% or less of chromium, 0.30 wt% or less of molybdenum or a combination of the two.

After completing the foregoing step 102, the process 100 proceeds to step 104 to heat the steel blank in step 102 to a first temperature (corresponding to the 0→A rolling step in FIG. 2), and maintain the steel embryo at The first temperature continues for a first period of time (corresponding to the A→B rolling step in FIG. 2). In step 104, the first temperature system is greater than or equal to 1200 ° C, and the first time period is greater than or equal to 4 hours.

After step 104 is completed, the process 100 proceeds to step 106 to cool the steel preform to a second temperature at a first cooling rate (corresponding to the B→C rolling step in FIG. 2). Wherein the first cooling rate is greater than or equal to 10 ℃ / s, and said second temperature is the A _C1 transformation point to below 20 ℃ A _C1 transformation point above 50 deg.] C wherein any temperature.

The process 100 then proceeds to step 108 to perform a plurality of first rolling processes for the steel preforms that have completed steps 102 to 106 (corresponding to the C→D rolling step in FIG. 2), thereby obtaining the first rolled steel. Embryo. Wherein, the thickness ratio of the first rolled steel blank obtained by the plurality of first rolling processes to the steel blank which has not undergone any rolling process is 15% to 25%. In other words, the above-mentioned multi-pass first rolling process has a reduction rate of 75% to 85% for the steel blank, wherein the reduction rate is used for description, and the steel embryo is in thickness before and after the process of step 106. The proportion reduced.

Subsequently, the process 100 proceeds to step 110 to cool the first rolled steel blank obtained through the step 108 to a third temperature at a second cooling rate (corresponding to the D→E rolling step in FIG. 2). Wherein the second cooling rate of 5 ℃ / s to 30 ℃ / s, compared to the third temperature A _C1 transformation point to below 150 ℃ A _C1 transformation point than 50 ℃.

Next, the process 100 proceeds to step 112 to perform a plurality of second rolling processes on the first rolled steel preform at the fourth temperature (corresponding to the E→F rolling step in FIG. 2), thereby obtaining The second rolled steel embryo. In step 112, after performing any of the plurality of second rolling processes, the thickness of the first rolled steel blank is reduced by 10% to 50%, that is, every second rolling process is performed. The ratio of the thickness of the first rolled steel blank after rolling to the thickness before rolling is 50% to 90%, or the cutting rate of the second rolling process for each first rolling mill 10% to 50%. Further, when the second rolling process of any of the above is performed, the strain rate of the first rolled steel blank is less than or equal to 5.0 s ^-1 . Furthermore, the second rolled steel preform described above is thicker than the first rolled steel preform (which may be referred to as an initial first rolled steel or first steel blank) which has not undergone any second rolling process. The ratio is 10% to 20%, that is, the above-mentioned multi-pass second rolling process has a presidential reduction rate of 80% to 90% for the initial first rolling steel. In addition, the fourth temperature needs to be greater than 600 ° C, so as to provide sufficient energy, so that the carbon atoms, alloying elements and defects at the interface between the ferrite iron and the ferritic carbon iron in the Borne iron structure have sufficient time to diffuse, and thus Helps to further form the histogram of the columnar or spheroidized stellite.

Following the process 100, a final step 114 is performed to cool the second rolled steel preform obtained in step 112 to room temperature by air cooling (corresponding to the F→G rolling step in FIG. 2, Call it the slow cooling phase) to get the steel you need. Wherein, the obtained steel material is processed through the above steps 102 to 114, wherein the stellite carbon has an average particle diameter of less than 2.0 μm and an average aspect ratio of less than 5.0.

In a particular embodiment, the ferritic carbon iron in the steel treated via process 100 described above is a carbide. Further, the composition of the above carbide may be iron carbide (Fe ₃ C), chromium carbide [(Fe, Cr) ₃ C], molybdenum carbide [(Fe, Mo) ₃ C], or any combination of the above.

In the present embodiment, for the convenience of storage and transportation, in the above process 100, before the step of cooling the second rolled steel to room temperature (step 114), the process 100 further includes the step 113, The second rolled steel preform is subjected to a coiling process.

In the present invention, the process 100 mainly controls the degree of subcooling, reduction rate and strain rate of the steel during the hot rolling process, thereby achieving the purpose of improving the strain energy and chemical energy of the steel in the phase transformation state. Among them, the deformation strain energy can cause the sheet-like ferritic carbon in the Borne iron structure generated by the Wolster iron phase metamorphosis process to be subjected to stress strain to cause bending or fracture. In addition, a large number of defects introduced by the rolling process at a high degree of subcooling and a high reduction rate suppress the formation of flaky ferritic carbon and contribute to the formation of a columnar or spheroidized form.

Furthermore, in process 100, the chemical energy provided by the rolling and slow cooling (step 114) at a low strain rate (less than or equal to 5.0 s ^-1 ) allows the ferrite and snow in the Borne iron structure. Carbon atoms, alloying elements and defects at the carbon-iron interface have sufficient time to diffuse, which also contributes to the formation of a columnar or spheroidized ferritic carbon.

Further, the actual experimental data will be described below, and the process 100 of the present invention described above can indeed be used to columnize or spheroidize a ferritic carbon in a steel structure.

Please refer to Tables 1 to 4 below, wherein Table 1 is used to indicate the content of each alloy component contained in the steel of the embodiment of the present invention and the conventional steel of the comparative example, and Table 2 is used to indicate the steel applied to Table 1. For the different process conditions, Table 3 is used to indicate the microstructure of ferritic carbon iron (carbide) in steel, and Table 4 is used to indicate the average carbide content after applying the steel in Table 1 under some of the process conditions in Table 2. Particle size and aspect ratio.

For the above Tables 3 and 4, the results are mainly observed by scanning electron microscopy (SEM) and the results of the Xingming carbon-iron aspect ratio are calculated. In Table 4, the calculation of the aspect ratio of Xueming carbon-iron carbides was carried out by the method proposed by T. Inoue et al., and then the average value was obtained to obtain the results of Table 4. In addition, the width in the above aspect ratio calculation is calculated by selecting the widest area of a single Xueming carbon iron. Furthermore, three-dimensional (3D) image reconstruction software (AVIZO) and analysis software (MAVI) are used to establish a three-dimensional image of the Borneo structure and quantitative data of the stellite carbon-iron carbide.

Note: The lanthanide system indicates that the ferritic carbon iron is not columnarized or spheroidized, and is still in the form of a sheet; the Φ system indicates that only part of the ferritic carbon iron is columnarized or spheroidized, and the rest is still in the form of a sheet; Indicates that the ferritic carbon iron is completely columnarized or spheroidized.

Note: The above carbides include Fe ₃ C, (Fe, Cr) ₃ C, (Fe, Mo) ₃ C or any combination of the above.

According to the above Tables 1 to 4, when the steels in Table 1 are subjected to the process conditions of Condition 1 in Table 2, as shown in Table 3, among all the steels, except for the low carbon steel waves of Comparative Example 1. In addition to the partial columnar carbides in the iron structure, the rest of the steel is still in the form of flaky carbides. According to the above results, it is known that under the same B→C rolling step (step 106), a lower degree of subcooling is used [ (A _C1 -50) ° C] and lower cooling rate (> 5 ° C / s) to cool the steel (D → E rolling step or step 110), will lead to the phase change state of the dynamic and strain energy Therefore, it is not conducive to the formation of carbides which are columnar or spheroidized.

When the steel in Table 1 was subjected to the process conditions of Condition 2 in Table 2, it was found that, as shown in Table 3, in addition to the steel of Example 2, some of the columnar carbides were obtained in the other steels. Compared with Condition 1, the main reason for the above difference is that in Condition 2, the reduction rate of the single rolling process of the second rolling process as in the above step (E→F rolling step) is increased ( 40%). The increase of the above reduction rate will help increase the strain energy of the steel and introduce more defects, thereby suppressing the formation of some flaky ferritic carbon iron and promoting the formation of spheroidized or spheroidized ferritic carbon iron. . However, in the steel material of the second embodiment, although the reduction rate of the single rolling process is improved, the sheet-shaped stellite in the wave-forming iron structure generated during the phase transformation state is bent or broken by stress to form a small amount. Flaky form. However, since the steel of Example 2 has high strength and a large amount of alloy added, the alloying elements added therein participate in the process of columnarization or spheroidization of the structure, thereby affecting the formation of a columnar or spherical shape. The time of carbide formation, so the swarf carbon iron in the steel wave iron structure is still in the form of flakes.

When the steel in Table 1 is subjected to the process conditions of Condition 3 in Table 2, as shown in Table 3, as the cooling rate increases in the D→E rolling step (Step 110) ( 25 ° C / s) and E → F rolling step (step 112) in the second rolling process of the single rolling process of the reduction rate ( The increase of 40%) increases the kinetic and strain energy obtained by the steel during the phase transition. Therefore, the flaky stellite in the generated iron structure during the phase transformation state may be bent or broken by stress [as shown in Annex 1 (a)], and the carbide having a small length and width may be The form of columnarization or spheroidization is directly formed. In addition, in the case of a small piece or a relatively large length and width, a rolling process of a low strain rate (<1.0 s ^-1 ) in the E→F rolling step, and a subsequent slow cooling process as described in the above step 114 The chemical energy provided by the ferritic structure further diffuses the carbon atoms, defects and alloying elements (such as Cr and Mo) at the interface between the ferrite iron and the ferritic carbon-iron in the Borne iron structure. Therefore, all of the conventional comparative examples and the steels of the examples can be obtained by obtaining ferrite iron and columnar carbides [α and θ] distributions as shown in Annexes 1 (a) and (b), respectively. Come to the iron organization. In the above carbide, the composition may be Fe ₃ C, (Fe, Cr) ₃ C, (Fe, Mo) ₃ C or any combination of the above, and the average particle size and average aspect ratio of the carbide are As shown in Table 4, they are 2.0 mm or less and 5.0 or less.

When the steel in Table 1 is subjected to the process conditions of Condition 4 in Table 2, as shown in Table 3, the cooling temperature range of the D→E rolling step in Table 2 [ie, the degree of supercooling becomes larger; (A _C1 -150) ° C], cooling rate ( 25 ° C / s) and the reduction rate of a single rolling process ( The increase of 40%) is more conducive to the formation of pillared or spheroidized carbides in steel. In addition, due to the increase in the dynamic force (undercooling) and strain energy of the steel in the phase transformation process (D→E rolling step), it can be found from the microstructure of the steel that there is a large amount of columnarization or even a ball. The carbides have been formed during the phase transformation process. The carbide average particle diameter and the average length and width are respectively 2.0 mm or less and 5.0 or less. In addition, compared with the above conditions 1 to 3, the condition of the B→C rolling step (step 106) of the condition 4 is different from the condition of the B→C rolling step of the conditions 1 to 3, and the temperature range of the cooling is larger. (that is, rolling the steel at a lower temperature), which is helpful for the subsequent fertilization and fine crystallization of the Worthfield. Therefore, the conditions of the B→C rolling step can be adjusted as needed, thereby controlling the grain size before the next stage of the process (D→E rolling step or step 110).

When the steel in Table 1 is subjected to the process conditions of Condition 5 in Table 2, as shown in Table 4, as the strain rate of step 110 increases to 1 s ^-1 (as shown in Condition 5 of Table 2), The microstructure of the steel in the steel is still in a columnar or spheroidized form, and its average particle size is below 2.0 mm, but the aspect ratio has slightly increased compared to Condition 4 (as shown in Table 4). In addition, as the strain rate of the B→C rolling step continues to increase (as in Condition 6 of Table 2), the proportion of columnarized or spheroidized carbides in the Borne iron structure is gradually reduced, and sheet carbonization Things will gradually increase. The main reason is that the carbon atoms, defects and alloying elements (such as Cr and Mo) at the interface between ferrite and ferritic carbon-iron in the Borne iron structure do not have sufficient time to diffuse to form a columnar or spheroidized form. Caused.

Furthermore, if the reduction rate of the single second rolling process in the E→F rolling step (step 112) is further improved ( 50%) and strain rate ( When 5 s ^-1 ) (as shown in Condition 2 of Table 2), it can be found from the microstructure of the steel that an excessively large single rolling process reduction rate will result in uneven internal deformation of the steel, too fast strain rate. Not conducive to the formation of columnar or spheroidized carbides. Therefore, as shown in Table 3, each of the steel materials shown in Table 1 was treated under the conditions of Condition 7 of Table 2, and the carbide structure obtained in each of the steel materials was in a sheet form.

According to the results of Tables 1 to 4 above, the process conditions of each step of the process 100 as described above may be summarized, wherein the steel embryo contains less than or equal to 0.8 wt% of carbon, an auxiliary component, and a residue composed of iron. The auxiliary component may be 0.20 wt% or less of chromium, 0.30 wt% or less of molybdenum or a combination of the two.

Although the present invention has been disclosed in the above embodiments, it is not intended to limit the present invention, and the present invention can be modified and modified without departing from the spirit and scope of the present invention. The scope is subject to the definition of the scope of the patent application attached.

100. . . Process

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For a better understanding of the present invention, reference is made to the above detailed description and the accompanying drawings. It is emphasized that, in accordance with the standard of the industry, the various features in the drawings are not to scale. In fact, the dimensions of the various features may be arbitrarily enlarged or reduced in order to clearly illustrate the above embodiments. The relevant schema description is as follows.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow chart showing the process of a ferritic carbon iron in a corrugated iron structure of a columnar or spheroidized steel according to an embodiment of the present invention.

Figure 2 is a schematic diagram showing the relative relationship between the temperatures corresponding to the partial stages of the process of Figure 1.

Annexes 1(a) and 1(b) show the two-dimensional (2D) and three-dimensional forms of ferrite iron (α) and columnar or spheroidized carbides (θ) distributed in the Bornerite structure, respectively. (3D) reconstructed images, in which 3D reconstructed images were constructed using 3D image reconstruction software AVIZO.

100. . . Process

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Claims (4)

A process for spheroidizing or spheroidizing a steel ferritic carbon in a ferritic structure comprising: providing a steel slab, wherein the steel slab comprises the following plurality of components calculated in weight percent (wt%), The composition is: less than or equal to 0.8 wt% of carbon, an auxiliary component, and a residue composed of iron, the auxiliary component being selected from the group consisting of 0.20 wt% or less of chromium, 0.30 wt% or less of molybdenum, and the like. Combining the group; heating the steel embryo to a first temperature, and maintaining the steel embryo at the first temperature for a first time period, wherein the first temperature is greater than or equal to 1200 ° C, the first time period Greater than or equal to 4 hours; cooling the steel preform to a second temperature at a first cooling rate, wherein the first cooling rate is greater than or equal to 10 ° C (° C / s) per second, the second temperature is A _C1 20 ° C below the abnormal point to 50 ° C above the A _C1 metamorphic point; performing a first rolling process on the steel blank, thereby obtaining a first rolled steel blank, wherein the first rolled steel blank and the steel embryo a thickness ratio of 15% to 25%; cooling the first rolled steel blank to a second cooling rate Three temperature, wherein the second cooling rate of 5 ℃ / s to 30 ℃ / s, the third A _C1 transformation point temperature is below 150 ℃ A _C1 transformation point or less to 50 ℃; the first at a fourth temperature Rolling a steel preform to perform a second rolling process to obtain a second rolled steel preform, wherein the thickness of the first rolled steel blank after performing any of the second rolling processes The reduction ratio is 10% to 50%, and the strain rate is less than or equal to 5.0 s ^-1 , and the second rolled steel blank and the first rolled steel blank that has not undergone any of the second rolling processes are a thickness ratio of 10% to 20%, and the fourth temperature is greater than 600 ° C; and cooling the second rolled steel blank to room temperature to obtain the steel; wherein the average grain size of the stellite carbon of the steel is less than 2.0 microns and an average aspect ratio of less than 5.0. The process of spheroidizing or spheroidizing a steel in a ferritic carbon steel according to claim 1, wherein the stellite carbon of the steel is a carbide. The method of claim 2, wherein the composition of the carbide is selected from the group consisting of iron carbide (Fe ₃ C), chromium carbide, and a spheroidized or spheroidized steel material. ((Fe,Cr) ₃ C), molybdenum carbide ((Fe,Mo) ₃ C), and any combination of the above three. The process of spheroidizing or spheroidizing a steel-plated iron-structured ferritic carbon iron according to claim 1, wherein before the step of cooling the second rolled steel slab to room temperature, the method further comprises: The second rolled steel preform is subjected to a coiling process.