TWI799335B - Hot rolled steel and manufacturing method the same - Google Patents

Abstract

A hot rolled steel and a manufacturing method the same are provided. The manufacturing method includes the following steps: providing a steel billet; heating the steel billet above 1200 ℃ and holding for more than 2 hours, then cooling the heated steel billet to between （A _C1+100）℃ and （A _C1+400）℃ at a cooling rate of more than -10 ℃/s, and performing a continuous rough rolling, wherein a single rolling reduction rate is more than 10% and the total reduction rate is more than 50%; cooling the roughly rolled steel strip down to between （A _C1+50）℃ and （A _C1+100）℃ at a cooling rate of more than -10 ℃/s, and performing a finish rolling, wherein the rolling elongation ratio of the first five passes is controlled between 1.0 and 2.0, and the rolling elongation ratio of the last two passes is controlled between 1.0 and 1.5, and the rolling is carried out at a speed of 80 to 100 m/s; performing a laminar flow cooling at 80 to 100 ℃/s in the first section and at 50 to 60 ℃/s in the second section, then coiling the finish rolled steel strip with a coil temperature controlled above （A _C1-50）℃, and slowly cooling the coiled steel strip to room temperature at a cooling rate （CR3） below 0.01 ℃/s.

Description

Hot-rolled steel and its manufacturing method

The invention relates to steel and its manufacturing method, in particular to a hot-rolled steel that can effectively reduce the strength (hardness) of the steel and improve its performance and its manufacturing method.

Among the existing steel products, Pearlite is produced by Austenite with a carbon content (in terms of weight percent) of about 0.8wt% in extremely slow cooling, when it is cooled to about 723°C , that is, a phase change begins to occur, and Ferrite and Cementite are precipitated at the same time, and then become a wave iron structure. The reason why Pileite forms a layered structure is that according to R.F.Mehl’s theory, when a eutectoid reaction occurs, nuclei of snowy carbon iron will be generated from the grain boundaries of Worthfield iron, and form a thin sheet toward the Worthfield iron crystal. Intragranular growth. Due to the high carbon content of Xueming carbon iron, it will absorb carbon from the surrounding waustian iron when it is generated, thus gradually forming fertilized iron. However, fertilized iron can only dissolve a small amount of carbon in solid solution, so it has the effect of pushing out carbon, so that the side waustian iron precipitates the core of snowy carbon iron, and forms sheet-shaped snowy carbon iron. After the above-mentioned steps are repeated, Xueming carbon iron and fat grain iron continue to grow vertically, and increase horizontally layer by layer, and finally completely replace the entire Worth field iron grains, thus forming layered Xueming carbon iron and fat grain iron alternately. Organization of arrangement.

However, the wavelet iron obtained by the general hot rolling process (hot rolling, hot rolling cooling, and coiling) is a layered structure with long flakes of snowy carbon iron and ferrite iron alternately arranged. Through three-dimensional image analysis, the aspect ratio of this layered snow-white carbon-iron carbide is more than 30 and the average surface area per unit volume of the ply iron is more than 1.8x10 ⁶ m ^-1 . Due to the high mechanical strength of steel with this structure but poor ductility and toughness, it is necessary to carry out spheroidizing annealing treatment on the above-mentioned steel in order to convert the long sheet-shaped Xueming carbon iron into a columnar or spherical shape, which is convenient for subsequent processing. or apply. Therefore, it is necessary to provide a hot-rolled steel and its manufacturing method, through the control of the hot-rolling process, to obtain twisted, discontinuous or even nano-scale snow-white carbon-iron carbides, so as to solve the problems existing in the conventional technology .

One object of the present invention is to provide a hot-rolled steel and its manufacturing method. Through the control of the hot-rolling process, twisted, discontinuous or even nano-scale snow-white carbon-iron carbides can be obtained to reduce the hardness of the steel and increase the hardness of the steel. Application and heat treatment properties.

Another object of the present invention is to provide a hot-rolled steel and its manufacturing method. The microstructure of the hot-rolled steel includes a ferrite phase, a wave-iron phase, and a nanoscale carbide, mainly through the alloy carbon steel in the The controlled rolling of the hot rolling process makes the proportion of ferrite iron in the two-phase ratio of ferrite ferrite and pole iron reach more than 50%, and the grain size is below microns, and can be obtained in the ferrite structure at the same time Long flake, twisted or discontinuous carbides with an aspect ratio of less than 30 and nanoscale carbides with a size of less than 200 nanometers are precipitated in the ferrite base, thereby effectively reducing the strength (hardness) of the steel and Improve performance.

In order to achieve the above-mentioned purpose, the present invention provides a method for manufacturing hot-rolled steel, which includes the following steps: providing a steel billet; heating the steel billet to above 1200°C, and keeping the temperature for more than 2 hours before leaving the furnace, and then heating the steel billet at -10°C A first cooling rate (CR ₁ ) of more than 1/sec lowers the temperature to between (A _C1 transformation point plus 100)°C and (A _C1 transformation point plus 400)°C, and conducts a first stage of multi-pass continuous rough rolling The single-pass reduction rate is above 10% and the total reduction rate is above 50%; after the multi-pass continuous rough rolling in the first stage, a second cooling rate of -10°C/s or above (CR ₂ ) cool down to (A _C1 transformation point plus 50) ℃ to (A _C1 transformation point plus 100) ℃, and carry out a second stage of multi-pass finish rolling, in which the second stage of multi-pass For the time delay of finishing rolling, the rolling ratio of the first five passes is controlled at 1.0 to 2.0 and the rolling ratio of the last two passes is controlled at 1.0 to 1.5, and the rolling is carried out at a speed of 80 to 100 m/s to obtain a finished rolled steel material; after laminar cooling of the finished rolled steel with a cooling rate of 80 to 100° C./s in the front section and a cooling rate of 50 to 60° C./s in the rear section, coiling the finished rolled steel after laminar flow cooling, The coil temperature of the coil is controlled above (A _C1 transformation point minus 50) °C, and slowly cooled down to room temperature at a cooling rate (CR ₃ ) below 0.01 °C/s to obtain a hot-rolled steel coil .

A hot-rolled steel produced by the method for producing hot-rolled steel as described above, wherein the billet contains a carbon content of 0.20 to 0.70 wt% of the total weight of the alloy steel, a silicon content of 0.15 wt% of the total weight of the alloy steel to 0.35wt%, a manganese content of 0.60 to 1.20wt% of the total weight of the alloy steel, a phosphorus content of less than 0.03wt% of the alloy steel, and a sulfur content of less than 0.035wt% of the alloy steel 1. The content of chromium is less than 1.50wt% of the total weight of the alloy steel; the content of molybdenum is less than 1.0wt% of the total weight of the alloy steel; the content of niobium is less than 0.30wt% of the total weight of the alloy steel; The total weight of the alloy steel is less than 0.01wt%, and the rest is iron and impurities.

In an embodiment of the present invention, the microstructure of the hot-rolled steel includes a ferrite, a wrought iron and a nano-carbide, wherein the average particle size of the nano-carbide in the base of the ferrite is The diameter is less than 200 nanometers, and the average curvature (H) and Gaussian curvature (K) of the surface energy are greater than 0 and less than 0.001.

In one embodiment of the present invention, the morphology of carbides in the wavelet structure of the hot-rolled steel material includes a long flaky carbide, a twisted carbide, a discontinuous carbide and combinations thereof, wherein the The sum of the proportion of twisted carbides and the proportion of discontinuous carbides accounts for more than 60% of the wavelet structure, and the proportion of elongated carbides accounts for less than 40% of the wavelet structure.

In an embodiment of the present invention, dislocations exist around the twisted carbides and the discontinuous carbides, while no dislocations exist around the nanoscale carbides of the ferrite.

In one embodiment of the present invention, the nanoscale carbides in the ferrite base are selected from the group consisting of Fe ₃ C, (Fe,Cr) ₃ C and (Fe,Mo) ₃ C .

In one embodiment of the present invention, the aspect ratio of the elongated carbides is less than 30, and the surface area of the elongated carbides in the unit wavelet iron is less than ^{1.5x106m -1} , and the discontinuous The aspect ratio of the discontinuous carbide is less than 15, and the surface area of the discontinuous carbide in the unit wave iron is less than 1.1x10 ⁶ m ^-1 .

In one embodiment of the present invention, the angle between the maximum twisted position of the twisted carbide and the direction parallel to the original carbide growth is more than 15 degrees and less than 60 degrees.

In an embodiment of the present invention, the average surface curvature (H) and the Gaussian curvature (K) of the surface energy of the carbides in the Pelletic structure are both greater than zero and less than 0.1.

In one embodiment of the present invention, the proportion of the ferrite phase in the hot-rolled steel is more than 50wt% and the grain size is less than 3 microns, and the wavelet structure has a long flaky carbide, a twisted carbide Or a discontinuous carbide, and nanoscale carbide precipitation with a size below 200 nanometers in the fertilizer iron base, wherein the tensile strength of the hot-rolled steel is below 700MPa, the reduction ratio is below 0.65, and the total The elongation rate is above 28% and the hardness value is below HRB95.

S11~S14: Steps

FIG. 1 : a schematic flow chart of a method for manufacturing hot-rolled steel according to an embodiment of the present invention; FIG. 2 : a schematic diagram of the relative relationship between temperatures corresponding to some stages of the process in FIG. 1 .

In order to make the above and other objects, features, and advantages of the present invention more comprehensible, preferred embodiments of the present invention will be exemplified below in detail together with the attached drawings. Furthermore, the directional terms mentioned in the present invention are, for example, up, down, top, bottom, front, back, left, right, inside, outside, side, surrounding, central, horizontal, transverse, vertical, longitudinal, axial, The radial direction, the uppermost layer or the lowermost layer, etc. are only directions referring to the attached drawings. Therefore, the directional terms used are used to illustrate and understand the present invention, but not to limit the present invention.

0且

2的其他任何實值。 As used herein, reference to a numerical range for a variable is intended to mean that the variable is equal to any value within that range. Thus, for a variable that is not inherently continuous, the variable is equal to any integer value within the numerical range, including the endpoints of the range. Similarly, for a variable that is inherently continuous, the variable is equal to any real value within the numerical range, including the endpoints of the range. By way of example, and not limitation, a variable described as having a value between 0 and 2 takes on values 0, 1, or 2 if the variable itself is discontinuous, and 0.0, 0.1, A value of 0.01, 0.001 or

0 and

Any other real value of 2.

The term "A _C1 transformation point" used in this article means that when the steel is heated to a high temperature (above the A _C3 transformation point), the steel is a carbon solid solution of gamma iron, called austenite, and its iron atoms are in the form of surface Centered cubic crystal arrangement, and non-magnetic; when cooled below the A _C3 point, iron atoms form a new structure composed of body-centered cubic lattice, called ferrite or α (alpha) iron (α iron contains solid solution carbon, and the dissolution rate of carbon is much lower than that of gamma iron); when the steel continues to cool to the A _C1 transformation point, additional fertilized iron is formed, and when the steel continues to cool below the A _C1 transformation point, the residual Voss Tian iron will become another new structure called pearlite.

The average curvature (H) and Gaussian curvature (K) of the surface energy of the nanoscale carbides in the base of ferrite as described herein are greater than 0 and less than 0.001, which means that the nanoscale carbides in the base of ferrite The average curvature (H) and Gaussian curvature (K) of the surface energy of carbides have the minimum curvature, that is, ferrite The shape of the nanoscale carbides in the base is mainly spherical. On the contrary, the average surface curvature (H) and Gaussian curvature (K) of the surface energy of the carbides in the wavelet iron structure are both greater than 0.01 and less than 0.1, which means that the surface energy of the carbides in the wave iron structure The average curvature (H) and Gaussian curvature (K) have relatively large curvature, that is, the carbides in the wavelet iron structure are mainly in the form of short flakes or columns.

The embodiment of the present invention proposes a method 100 for manufacturing hot-rolled steel, which includes the following steps: providing a steel billet (step S11); heating the steel billet to above 1200° C. The first cooling rate (CR ₁ ) above ℃/s is lowered to (A _C1 transformation point plus 100)°C to (A _C1 transformation point plus 400)°C, and a first-stage multi-pass continuous roughing Rolling, its single-pass reduction rate is more than 10% and the total reduction rate is more than 50% (step S12); A second cooling rate (CR ₂ ) lowers the temperature to between (A _C1 transformation point plus 50)°C and (A _C1 transformation point plus 100)°C, and performs a second stage of multi-pass finish rolling, in which In the second stage of multi-pass finish rolling, the rolling ratio of the first five passes is controlled at 1.0 to 2.0 and the rolling ratio of the last two passes is controlled at 1.0 to 1.5, and the rolling is carried out at a speed of 80 to 100 m/s. To obtain a finished rolled steel product (step S13); after the finished rolled steel product is subjected to laminar flow cooling with a cooling rate of 80 to 100° C./second in the front section and a cooling rate of 50 to 60° C./second in the rear section, after laminar flow cooling The finished rolled steel is coiled, wherein the coil temperature of the coil is controlled above (A _C1 transformation point minus 50) °C, and the temperature is slowly cooled at a cooling rate (CR ₃ ) below 0.01 °C/s to room temperature to obtain a hot-rolled steel coil (step S14).

Please refer to FIG. 1 and FIG. 2, proceed to step S11: provide a steel billet. The main alloy composition ranges of the steel blanks provided by the embodiments of the present invention and comparative examples are as follows: a carbon content (hereinafter all in weight percentage) accounts for 0.20 to 0.70wt% of the total weight of the alloy steel, and a silicon content accounts for the total weight of the alloy steel. 0.15 to 0.35wt%, a manganese content of 0.60 to 1.20wt% of the total weight of the alloy steel, a phosphorus content of the alloy Less than 0.03wt% of the total weight of the steel, a sulfur content of less than 0.035wt% of the total weight of the alloy steel, a chromium content of less than 1.50wt% of the total weight of the alloy steel, and a molybdenum content of 1.0% of the total weight of the alloy steel wt% or less, the content of niobium is less than 0.30wt% of the total weight of the alloy steel, the content of nitrogen is less than 0.01wt% of the total weight of the alloy steel, and the rest is iron and impurities.

It should be noted that the steel of the present invention can be produced according to conventional steelmaking or electric furnace methods.

It is worth mentioning that the steel billet with the above alloy composition and content can be vacuum smelted and cast into a flat steel billet or a large steel billet, and then hot-rolled.

Next, proceed to step S12: heat the steel billet to above 1200°C (point 0 to point A in Figure 2), hold the temperature for more than 2 hours and then release it from the furnace (point A to point B in Figure 2), and then - A first cooling rate (CR ₁ , such as points B to C in Figure 2) of 10°C/s or more is cooled to (A _C1 transformation point plus 100, that is, A _C1 +100)°C to (A _C1 transformation point plus 450 , that is, between A _C1 +450) ℃, and carry out a multi-pass continuous rough rolling in the first stage (points C to D in Figure 2), the single-pass reduction rate is above 10% and the total reduction The rate is above 50%. Optionally, the multi-pass continuous rough rolling in the first stage may be 5 passes of continuous rough rolling.

Optionally, after the steel billet is heated to above 1200° C. and kept at the temperature for more than 2 hours and released from the furnace, the steel billet can be sprayed with water to remove rust, and then the subsequent manufacturing process can be carried out.

After carrying out the multi-pass continuous rough rolling of the first stage, the temperature is lowered to (A _C1 transformation point) at a second cooling rate (CR ₂ , such as points D to E in Figure 2) above -10°C/s Add 50, that is, A _C1 +50) ℃ to (A _C1 transformation point plus 250, that is, A _C1 +250) ℃, and carry out a second stage of multi-pass finish rolling (as shown in Figure 2 E to Point F), wherein in the multi-pass finish rolling delay of the second stage, the rolling ratio of the first five passes is controlled at 1.0 to 2.0, and the rolling ratio of the latter two passes is controlled at 1.0 to 1.5, and the rolling ratio of the second pass is controlled at 80 to 100 meters / second speed to obtain a finished rolled steel.

The finished rolled steel is subjected to laminar flow cooling (points F to G in Figure 2), with a cooling rate of 80 to 100 °C/s in the front section and a cooling rate of 50 to 60 °C/s in the back section for laminar flow cooling and then pan cooling. Coil (point G in Figure 2), wherein the temperature of one coil of the coil is controlled above (A _C1 transformation point minus 50, that is, A _C1 -50) ° C, and a cooling rate of 0.01 ° C / sec or less (CR ₃ , point G to P in FIG. 2 ) slowly cool down to room temperature to obtain a hot-rolled steel coil.

Optionally, the hot-rolled coil may be subjected to a pickling step to remove surface scale, and a reprocessing treatment.

The following is further illustrated with actual experimental data. Using the above-mentioned manufacturing method of the present invention, through the control of the hot rolling process, to obtain twisted, intermittent or even nanometer-sized snow-white carbon-iron carbides to reduce steel Hardness and improved application and heat treatment properties.

Please refer to the following Tables 1 to 3, wherein Table 1 is used to represent the controlled rolling parameters of the steel of the embodiment of the present invention and the controlled rolling parameters of the conventional steel of the comparative example, and Table 2 is the different process using Table 1 Analysis of the microstructure and mechanical properties of the steel obtained under the conditions, Table 3 is the carbide form in the wave iron structure of the steel obtained under different process conditions in Table 1.

The above-mentioned microstructure and precipitate analysis method is as follows: Electropolish the steel structure, and then use scanning electron microscope, transmission electron microscope and electron backscatter diffractometer (EBSD) to analyze the microstructure and nano-scale carbonization Object observation and grain size measurement, and identification and analysis of precipitates.

Furthermore, using the focused ion beam electron beam microscope (FIB-SEM) as a tool, the observation of the steel microstructure and the collection of sequential tissue images (each image is etched at a distance of 100 nanometers) were performed. The sequence of images is combined, aligned and image stacked under the software to make it into an image file that can be displayed continuously. Finally, with the help of computer software such as AVS express, AVIZO and MAVI, these sequence images are stacked and combined to construct a 3D structure, and then the aspect ratio, surface curvature, surface area and quantitative characteristics of carbides are discussed. The evaluation method is explained as follows:

(1) Calculation of carbide aspect ratio

The calculation of the aspect ratio of Xueming carbon iron carbide in the present invention is to adopt the method proposed by T.Inoue et al. to calculate and obtain the average value, wherein the width is selected from the widest area of a single sheet of Xueming carbon iron calculate.

(2) Analysis of carbide surface average curvature and Gaussian curvature

The driving force for the spheroidization of the Plexiferrite structure comes from reducing the surface area of the Xueming carbon iron per unit volume. However, the reduction in surface area can be achieved by mass-transfer diffusion from regions of higher curvature to regions of lower curvature. Furthermore, the characteristics of the surface curvature of an object can be expressed by mean curvature (H) and Gaussian curvature (Gaussian curvature, K), as shown in the following formulas (1) and (2):

K= k ₁ × k ₂ ...... Formula (2)

Among them, k ₁ and k ₂ are the maximum and minimum curvatures on the surface patch area of the object and define k ₁ =1/R ₁ and k ₂ =1/R ₂ (R ₁ and R ₂ are respectively the The maximum and minimum radius of curvature).

(3) The calculation method of the surface area of three-dimensional snow-bright carbon-iron carbide, as shown in formula (3):

Among them, S _V is the surface area per unit volume, t is the temperature holding time, and K _S is the surface energy constant, which is related to the material diffusion coefficient, interface energy and material parameters.

(4) Analysis of mechanical properties: the steel is processed into JIS No. 5 test piece, and the tensile test is carried out to measure its yield strength (YS), tensile strength (TS) and elongation (E1), etc. Carry out steel hardness value measurement.

After carrying out the hot rolling and controlled rolling process of the comparative example 1 in Table 1 with the above-mentioned steel, it is found that after the first stage of rough rolling under the condition of slow cooling and high temperature, it is coiled after the finishing rolling and finishing rolling at a higher temperature. The obtained tissue is composed of ferrite (F) and pleated iron (P), wherein the ferrite phase accounts for less than 50% of the phase of the overall tissue, and its average particle size is between 4.7 and 5.0 microns (such as shown in Table 2). In addition, the snow-bright carbon-iron carbides in layered Pileite include three types of long flakes, twisted and discontinuous (as shown in Table 3), mainly long flakes with an aspect ratio greater than 45 Shaped carbides are the main ones, accounting for about 80-90%, followed by twisted carbides with a deflection angle within 10 degrees along the parallel growth direction, and the rest are discontinuous carbides.

The aforementioned carbide includes Fe ₃ C, (Fe,Cr) ₃ C, (Fe,Mo) ₃ C or a combination thereof.

According to the experimental results, the long flaky carbides of Comparative Example 1 in Table 3 have a large surface area ((2.2~2.0)x10 ⁶ m ^-1 ), and their average curvature (H>>0) and Gaussian curvature ( K>>0) are much greater than zero, indicating that the carbide morphology is mainly in the form of long flakes. In addition, when the carbide forms a discontinuous shape, in addition to the reduction of its aspect ratio and surface area, its surface curvature (H>0, K>0) also decreases, indicating that the original long sheet shape is transformed into a short sheet or columnar shape. shape of shape.

The observation results of the steel strip shape of Comparative Example 1 in Table 3 show that there is a situation where the flatness is not good, which is mainly due to the fact that during the second stage of rolling (finish rolling), the rolling ratio of the last 2 passes is higher than that of the first 5 passes. Due to delay ratio. Furthermore, Comparative Example 2 in Table 1 shows that after the steel billet comes out of the furnace, the temperature of rough rolling and finish rolling is lowered, and the reduction rate of rough rolling is increased, which helps to refine the structure, although the proportion of its phase composition and mechanical The properties are not much different from those of Comparative Example 1, but the detailed microstructure, such as the aspect ratio and surface area of the elongated and discontinuous carbides, are slightly reduced. In addition, through the adjustment of the rolling pass ratio in the second stage of finishing rolling, the shape and flatness of the steel strip are good.

In addition, as shown in Example 1 in Table 1, after the steel billet is released from the furnace, the rough rolling reduction rate is further increased, and the rough and finish rolling temperatures are reduced. Nanoscale carbides with an average particle size below 200 nanometers can also be found in the base of the ferrite phase, and the average curvature (H~0) and Gaussian curvature (K~0) are close to zero, indicating that the Na The morphology of rice carbide is mainly spherical. The nanocarbides include _Fe3C , (Fe,Cr) _3C , (Fe,Mo) _3C , or combinations thereof.

As shown in Table 2, the tissue of Example 1 of the present invention is composed of ferrite, polite and nano carbides (θ). Because the ratio of ferrite and discontinuous carbides in the composition is significantly increased, The strength and hardness of the steel are lower than those of the comparative example, and the elongation is effectively improved.

The morphology of the carbides in Example 1 of the present invention is shown in Table 3. The aspect ratio and surface area of the long flaky carbides are smaller than those of the above-mentioned comparative examples, and their proportion is also reduced to less than 55%, and vice versa. The proportion of carbide-like carbides is significantly increased to 30 to 40%, and the angle of the maximum twisted position of the twisted carbides and the deflection angle parallel to the growth direction of the original carbides is also increased to more than 15 degrees.

Example 2 of the present invention in Table 1 shows that after the steel billet comes out of the furnace, the cooling speed is accelerated and the rolling temperature of rough and finish rolling is reduced, but the overall reduction rate is increased, which helps to further refine the structure, so that the average particle size of ferrite The diameter is between 1.2 and 1.6 microns, and the proportion of fertilizer and iron phase is increased to 55 to 60%. In addition, the average particle size of the nano-carbides precipitated in the base of the ferrite phase is refined to less than 150 nanometers, which effectively reduces the hardness of the steel.

Furthermore, as shown in the rolling temperature effect of rough and finish rolling in Table 1, the size of ferrite, polite and nano-carbides in the structure will be more refined as the rolling temperature decreases. . In addition, the aspect ratio and surface area of the carbides in the wavelet iron structure, no matter the long sheet or discontinuous carbides, also decrease with the decrease of temperature, and the deflection angle of the twisted carbides also increases accordingly. The minimum angle is above 15 degrees, while the maximum angle is below 60 degrees.

In addition, through observation of the microstructure, as shown in Table 1, the carbides of the examples of the present invention have dislocations around the twisted and discontinuous carbides, while no dislocations exist around the nanocarbides.

Overall, the tensile strength of the hot-rolled steel developed by the present invention is below 700 MPa, the yield ratio is below 0.65, the total elongation is above 28%, and the hardness value is below HRB95.

According to the manufacturing method of the present invention, through the establishment of steel phase composition and carbide morphology control technology, the microstructure of the steel includes a ferrite phase, a wave iron phase and nano-scale snowy carbon iron carbide, mainly through The controlled rolling of advanced alloy carbon steel in the hot rolling process makes the proportion of ferrite iron phase in the two-phase ratio of ferrite iron and wavelaid iron reach more than 50% and the grain size is less than 3 microns, and can be used in Long lamellar, twisted or discontinuous carbides with an aspect ratio of less than 30 and nanoscale carbides with a size of less than 200 nanometers were obtained in the ferrite structure and precipitated in the ferrite base.

These carbides have the following characteristics:

(1) Composed of M ₃ C structure, where M can be expressed as Fe, Cr, Mo, and is composed of a group formed by any combination of one or more elements.

(2) For long-flaky, twisted or intermittent carbides, the average surface curvature (H) and Gaussian curvature (K) of the surface energy are greater than zero (H>O, K>0), that is, in the form of short flakes Or the columnar shape is the main form, while the average curvature and Gaussian curvature of nano carbides are close to zero (H~O, K~0), that is, the spherical form is the main form.

(3) The aspect ratio of elongated carbides is less than 30, and the surface area in unit wavelet iron is less than 1.5x10 ⁶ m ^-1 , while the aspect ratio of discontinuous carbides is less than 15, and the surface area Below 1.1x10 ⁶ m ^-1 .

(4) The angle between the twisted carbide and the parallel direction of the original carbide growth is more than 15 degrees, and the maximum angle is less than 60 degrees.

According to the hot-rolled steel produced by the above-mentioned manufacturing method of hot-rolled steel, the hot-rolled steel developed by the present invention comprises a carbon content of 0.20 to 0.70 wt% of the total weight of the alloy steel, and a silicon content of 0.15 wt% of the total weight of the alloy steel. to 0.35wt%, a manganese content of 0.60 to 1.20wt% of the total weight of the alloy steel, a phosphorus content of less than 0.03wt% of the alloy steel, and a sulfur content of less than 0.035wt% of the alloy steel 1. The content of chromium is less than 1.50wt% of the total weight of the alloy steel; the content of molybdenum is less than 1.0wt% of the total weight of the alloy steel; the content of niobium is less than 0.30wt% of the total weight of the alloy steel; The total weight of the alloy steel is less than 0.01wt%, and the rest is iron and impurities.

In an embodiment of the present invention, the microstructure of the hot-rolled steel includes a ferrite, a wrought iron and a nano-carbide, wherein the average particle size of the nano-carbide in the base of the ferrite is The diameter is less than 200 nanometers, and the average curvature (H) and Gaussian curvature (K) of the surface energy are greater than 0 and less than 0.001.

In one embodiment of the present invention, the morphology of carbides in the wavelet structure of the hot-rolled steel material includes a long flaky carbide, a twisted carbide, a discontinuous carbide and combinations thereof, wherein the The sum of the proportion of twisted carbides and the proportion of discontinuous carbides accounts for more than 60% of the wavelet structure, and the proportion of elongated carbides accounts for less than 40% of the wavelet structure.

In an embodiment of the present invention, dislocations exist around the twisted carbides and the discontinuous carbides, while no dislocations exist around the nanoscale carbides of the ferrite.

In one embodiment of the present invention, the nanoscale carbides in the ferrite base are selected from the group consisting of Fe ₃ C, (Fe,Cr) ₃ C and (Fe,Mo) ₃ C .

In one embodiment of the present invention, the aspect ratio of the elongated carbides is less than 30, and the surface area of the elongated carbides in the unit wavelet iron is less than ^{1.5x106m -1} , and the discontinuous The aspect ratio of the discontinuous carbide is less than 15, and the surface area of the discontinuous carbide in the unit wave iron is less than 1.1x10 ⁶ m ^-1 .

In one embodiment of the present invention, the included angle between the twisted carbide and the direction parallel to the growth of the original carbide is not less than 15 degrees and not more than 60 degrees.

In one embodiment of the present invention, the average surface curvature (H) and Gaussian curvature (K) of the surface energy of the carbides in the pleite structure are both greater than 0.01 and less than 0.1.

In one embodiment of the present invention, the proportion of the ferrite phase in the hot-rolled steel is more than 50% and the grain size is less than 3 microns. Or a discontinuous carbide, and nanoscale carbide precipitation with a size below 200 nanometers in the fertilizer iron base, wherein the tensile strength of the hot-rolled steel is below 700MPa, the reduction ratio is below 0.65, and the total The elongation rate is above 28% and the hardness value is below HRB95.

Although the present invention has been disclosed with preferred embodiments, it is not intended to limit the present invention. Anyone skilled in this art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the present invention The scope of protection shall be determined by the scope of the attached patent application.

S11~S14: Steps

Claims (10)

A method for manufacturing hot-rolled steel, comprising the following steps: (a) providing a steel billet; (b) heating the steel billet to above 1200°C, keeping the temperature for more than 2 hours, and then releasing it from the furnace; A first cooling rate (CR ₁ ) to cool down to (A _C1 transformation point plus 100) ℃ to (A _C1 transformation point plus 400) ℃, and carry out a multi-pass continuous rough rolling in the first stage, which The single-pass reduction rate is above 10% and the overall reduction rate is above 50%; (c) After the first stage of multi-pass continuous rough rolling, a second cooling rate of -10°C/s or more (CR ₂ ) Cool down to (A _C1 transformation point plus 50) ℃ to (A _C1 transformation point plus 100) ℃, and carry out a second stage of multi-pass finish rolling, in which the second stage of multi-pass For the time delay of finishing rolling, the rolling ratio of the first five passes is controlled at 1.0 to 2.0 and the rolling ratio of the last two passes is controlled at 1.0 to 1.5, and the rolling is carried out at a speed of 80 to 100 m/s to obtain a finished rolled steel (d) After laminar cooling of the finished rolled steel with a cooling rate of 80 to 100°C/s in the front stage and a cooling rate of 50 to 60°C/s in the rear stage, the finished rolled steel after laminar flow cooling Coil, wherein the temperature of one coil of the coil is controlled above (A _C1 transformation point minus 50) °C, and slowly cooled down to room temperature at a cooling rate (CR ₃ ) below 0.01 °C/s to obtain a Hot rolled steel coil. A hot-rolled steel produced by the manufacturing method of hot-rolled steel as described in claim 1, wherein the hot-rolled steel contains a carbon content of 0.20 to 0.70wt% of the total weight of the alloy steel, a silicon content of 0.20% by weight of the alloy steel 0.15 to 0.35wt% of the total weight, a manganese content of 0.60 to 1.20wt% of the total weight of the alloy steel, a phosphorus content of less than 0.03wt% of the total weight of the alloy steel, and a sulfur content of 0.6% of the total weight of the alloy steel 0.035wt% or less, a chromium content of less than 1.50wt% of the total weight of the alloy steel, a molybdenum content of less than 1.0wt% of the alloy steel, a niobium content of less than 0.30wt% of the alloy steel, Nitrogen content accounts for less than 0.01wt% of the total weight of the alloy steel, and the rest is iron and impurities. The hot-rolled steel product as described in claim 2, wherein the microstructure of the hot-rolled steel product comprises a ferrite, a wave-iron and a nano-carbide, wherein the nano-scale carbide in the base of the ferrite The average particle size of the material is less than 200 nanometers, and the average curvature (H) and Gaussian curvature (K) of the surface energy of the nanoscale carbides in the ferrite base are greater than 0 and less than 0.001. The hot-rolled steel product as described in claim 3, wherein the carbide form in the wave iron structure composed of the steel phase of the hot-rolled steel product includes a long flaky carbide, a twisted carbide, and a discontinuous carbide And combinations thereof, wherein the sum of the proportion of the twisted carbide and the proportion of the discontinuous carbide accounts for more than 60% of the wavelet structure, and the proportion of the elongated carbide accounts for the wavelet structure 40% or less. The hot-rolled steel product as claimed in item 4, wherein there are dislocations around the twisted carbide and the discontinuous carbide, and there is no dislocation around the nanoscale carbide of the ferrite . The hot-rolled steel product as claimed in claim 3, wherein the nanoscale carbide in the ferrite base is selected from the group consisting of Fe ₃ C, (Fe, Cr) 3 C and (Fe, Cr) ₃ C and (Fe, Mo) ₃ C. The hot-rolled steel product as claimed in item 4, wherein the aspect ratio of the long flaky carbides is 30 or less, and the surface area of the long flaky carbides in a unit of wave iron is 1.5×10 ⁶ m ^-1 or less, And the aspect ratio of the discontinuous carbide is less than 15, and the surface area of the discontinuous carbide in the unit wavelet is less than 1.1x10 ⁶ m ^-1 . The hot-rolled steel product according to claim 4, wherein the angle between the maximum twisted position of the twisted carbide and the direction parallel to the growth of the original carbide is more than 15 degrees and less than 60 degrees. The hot-rolled steel product as claimed in claim 3, wherein the average surface curvature (H) and Gaussian curvature (K) of the surface energy of the carbides in the wave iron structure are both greater than 0.01 and less than 0.1. The hot-rolled steel product as described in claim 2, wherein the proportion of the ferrite phase in the hot-rolled steel product is more than 50% and the grain size is less than 3 microns, and there is a long flaky carbide, a Twisted carbide or a discontinuous carbide, and nanoscale carbide precipitation with a size of less than 200 nm in the ferrite base, wherein the hot-rolled steel has a tensile strength of less than 700 MPa and a yield ratio of Below 0.65, total elongation above 28% and hardness below HRB95.

Images