KR100388041B1 - A Method for Manufacturing Hot-rolled Steels Having High Hardness - Google Patents

Abstract

The present invention relates to a method for producing a high hardness hot rolled steel sheet, the object of which is to produce a large amount of hot rolled coil in a short time by using a high speed hot rolling mill by applying a slab controlled in an appropriate component range, As such, it is intended to provide a method for obtaining a hot rolled coil at a low cost while having a suitable quality for a material requiring high hardness.

The present invention for achieving the above object by weight ratio, C: 0.5-0.8%, Mn: 0.2-2.0%, Si: 0.1-1.0%, Cr: 2.0% or less, Mo is represented by the following formula 1 and 2 High hardness, characterized in that it is contained in the range to be satisfied, hot-rolled steel made of the remaining Fe and unavoidable impurities, and wound by CT (winding temperature) of 550-620 ℃ so that the surface hardness is in the range of HRC23-30 The summary relates to a method for producing a hot rolled steel sheet.

[Equation 1]

Mo (wt%) ≥ {13-6C-4 (Si + Cr + Mn) +0.1 (CT-550)} / 35

[Equation 2]

Mo (wt%) ≤ {20-6C-4 (Si + Cr + Mn) +0.1 (CT-550)} / 35

Description

A Method for Manufacturing Hot-rolled Steels Having High Hardness

The present invention relates to a high hardness hot rolled steel sheet manufacturing method that can be applied for blades and the like used to cut stones and the like.

Stone blade is a consumable blade that is used to cut the raw stone collected from the quarry to the desired thickness, and it is required to have a low cost product with sufficient wear resistance and permanent deformation.

In order to secure the abrasion resistance of the blade, steel having high carbon is generally advantageous because the hardness is high and the amount of carbides is high, but if the hardness is too high, brittleness becomes large, and thus there is a risk of breakage during use. In addition, the yield strength of the material must be high in order to secure the permanent deformation, but if the yield strength is too high, it is susceptible to cracking. Therefore, a material having moderate ductility and high hardness and yield strength is required. Representative examples of blade manufacturing methods known to date are as follows.

The first method is a widely used method of hot-rolling a billet of a small width of high carbon steel having a blade size and then air-cooled to cut the length of the blade. In this method, a relatively good blade product can be made, but since the width of the billet is small, there is a problem in that the productivity of the product is low and thus the price is high.

Secondly, in order to solve the problems of the first method, a method of cutting a high carbon steel hot rolled coil into a blade size is appropriately cut, and subjected to hardening and tempering heat treatment to increase yield strength and hardness, and to have a proper ductility. It became. According to this method, it is possible to make an optimum blade with optimum hardness, yield strength, and ductility by optimizing heat treatment conditions, but since the length of the blade is about 4m, it is very long. It is hard to carry out and the deformation occurs a lot during heat treatment, so the error rate is small. Therefore, there is a problem in that the quality is excellent but the price of the product is high again.

The present invention is to solve the above problems, the purpose is to apply a slab controlled in the appropriate component range to produce a large amount of hot rolled coil in a short time by using a high speed hot rolling mill, such as a blade It is to provide a method to obtain a hot rolled coil at a low cost while having a suitable quality for materials requiring high hardness.

1 is a graph showing the change in surface hardness according to the coiling temperature

2 is a graph comparing the performance value and the calculated value of the surface hardness

In general, the quality characteristics of the product are greatly changed by the difference between the method of manufacturing the blade using the billet and the method of manufacturing the hot rolled coil. In the case of using the billet, the length of the billet is short. Since there is no coil winding process after hot rolling, a relatively high hardness can be obtained by air cooling. However, in the case of manufacturing the hot rolled coil, the cooling speed is very slow due to the large coil weight. Therefore, spheroidization of the pearlite occurs due to self-annealing during cooling, and in addition, transformation heat generated by phase transformation during cooling. There is a problem that the hardness of the furnace coil is rather increased and the hardness is even lower. In order to solve this problem, the present inventors have comprehensively studied the effects of alloying elements such as Mn, Si, Cr, Mo, V and hot rolling conditions on the hardness reduction due to self-annealing of hot rolled coils. By appropriately adjusting the production conditions, it has been found that hot rolled steel sheets capable of producing inexpensive blade steels having a suitable quality for stone blades can be obtained.

The present invention proposed on the basis of the above research results, by weight ratio, C: 0.5-0.8%, Mn: 0.2-2.0%, Si: 0.1-1.0%, Cr: 2.0% or less, Mo is the following formula It is contained in a range satisfying 1 and 2, hot rolled steel made of the remaining Fe and unavoidable impurities, and wound by CT (winding temperature) at 550-620 ° C. to have a surface hardness of HRC23-30. It relates to a manufacturing method of high hardness hot rolled steel sheet.

Mo (wt%) ≥ {13-6C-4 (Si + Cr + Mn) +0.1 (CT-550)} / 35

Mo (wt%) ≤ {20-6C-4 (Si + Cr + Mn) +0.1 (CT-550)} / 35

Next, the steel component used by this invention is demonstrated.

C is an element essential for securing the wear resistance by increasing the hardness and cementite amount of steel. In order to obtain the hardness (HRC 23-30) necessary for the blade through hot rolling, it is preferable to contain at least 0.5%. If the carbon content is too high, brittleness is strong and the risk of breakage during use is limited to 0.8% or less. desirable.

The Mn serves to increase hardness and yield strength by inhibiting transformation, solid solution, and spheroidization of pearlite. In addition, it forms a MnS in combination with sulfur, and also serves to prevent cracking during slab manufacture. To this end, it is preferable to add 0.2% or more, and if the amount of Mn is too large, the effect is saturated, and rather toughness may be reduced and the price is also increased, so it is preferable to limit it to 2.0% or less.

Si is a very important element as a deoxidizer and a solid solution strengthening element. If the amount of Si is too small, it contains 0.1% or more because it has little effect of deoxidation and solid solution strengthening. If the amount of Si is too high, the surface decarburization becomes severe during reheating and the red scale is generated, resulting in deterioration of the surface quality. It is preferable to limit to.

The Cr serves to increase hardness and yield strength by inhibiting transformation strengthening and spheroidization of pearlite. However, if the amount of Cr is too large, brittleness and the price also increase due to the formation of Cr carbide, so it is preferable to limit it to 2.0% or less.

The Mo is the most effective in increasing the hardness and yield strength by the addition of a small amount because the transformation strengthening and pearlite finer and suppress the spheroidization. The correlation between the surface hardness of hot rolled steel and the suitability for blades showed that the surface hardness of blade steel should be at least HRC 23 and the upper limit should be HRC 30 or less. The reason is that when the surface hardness is less than HRC 23, the stone is not cut to a uniform thickness due to the increase of the blade due to plastic deformation when cutting the stone, and the surface hardness is less than the toughness at HRC 30. However, even when manufacturing the blade by unrolling the hot rolled coil, there is a high risk of breakage, and it is difficult to manufacture the blade because it does not spread well. The amount of Mo added to obtain the range of hardness values suggested above was directly influenced by the addition amount of other alloying elements and the hot rolled coiling temperature.

That is, the following equation (3) was obtained by analyzing the correlation between the surface hardness of the hot-rolled steel, the alloy component and the winding temperature by using the regression analysis.

Surface Hardness (HRC) = 10 + 6C + 4 (Si + Cr + Mn) + 35Mo-0.1 (CT-550)

(Where 'CT' is the winding temperature)

The surface hardness range required for the blade using Equation 3 can be expressed as Equation 4 since the minimum value is HRC 23 and the maximum value is HRC 30.

23≤10 + 6C + 4 (Si + Cr + Mn) + 35Mo-0.1 (CT-550) ≤30

If Equation 4 is expressed as a function of Mo, it can be expressed as Equations 1 and 2 above.

Next, the manufacturing conditions of the present invention will be described in detail.

In general, since the hardness and yield strength of the hot rolled steel sheet mainly depend on the winding temperature, the finish rolling temperature may be performed at 800-950 ° C. which is a normal condition. Winding temperature is most important for microstructure control, and the most preferable microstructure as a blade is to refine the pearlite to increase strength and hardness, and to secure ductility. However, if the coiling temperature is too low, bainite is formed, the hardness is too high, the winding is difficult, and there is a fear of breakage during winding. In addition, there is a high risk of being broken when the hot rolled coil is completely cooled after being fully cooled. Therefore, the minimum of winding temperature was made into 550 degreeC. However, when the coiling temperature is too high, the pearlite spheroidizes due to magnetic annealing, and thus the hardness and yield strength are lowered, so the upper limit is set to 620 ° C.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

Steels having the composition shown in Table 1 below were prepared.

Chemical composition (wt%) Remarks C Si Mn Cr Mo Steel grade 1 0.6 0.41 1.2 0.3 0.2 Basic composition Grade 2 0.4 0.42 1.2 0.3 0.2 C decrease Grade 3 0.8 0.4 1.18 0.31 0.2 C increase Grade 4 0.6 0.41 1.17 0.28 0.4 Mo increase Grade 5 0.6 0.41 1.22 0.28 0.6 Mo increase Grade 6 0.61 0.42 0.8 0.3 0.2 Mn decrease Grade 7 0.60 0.4 1.21 0.8 0.2 Increase Cr Grade 8 0.60 0.8 1.20 0.3 0.2 Si increase

As can be seen in Table 1, when steel grade 1 is the basic composition, steel grades 2 and 3 are steels whose carbon content is changed in the basic composition, steel grades 4 and 5 are steels with increased Mo in the basic steel grades, and steel grade 6 , 7, and 8 are steels in which Mn, Cr, and Si are changed in the basic composition steel.

All the steels having such components were heated to 1200 ° C. in the same manner and maintained for 2 hours, and then a hot rolled steel sheet having a thickness of 4.2 mm was prepared at a rolling finish temperature of 900 ° C. and a winding temperature of 600 ° C. The surface hardness of the manufactured hot rolled steel sheet was measured using a Rockwell hardness tester, and the results are shown in Table 2 below.

Steel grade 1 Steel grade 2 Steel grade 3 Steel grade 4 Steel grade 5 Steel grade 6 Steel grade 7 Steel grade 8 Surface Hardness (HRC) 23.5 21.8 24.7 30.5 36.5 22.1 24.9 25

As shown in Table 2, it can be seen that under the same rolling conditions, the surface hardness increases as the amount of the alloying component increases, and in this case, Mo increases the hardness even with a small increase compared to other alloying elements. The reason is that when Mo is added, the perlite transformation temperature is greatly reduced and the transformation occurs at low temperature, so that the microstructure is fine and the spheroidization of the ferrite is suppressed after the transformation. As a result, Mo is the most effective element to increase the surface hardness to a small amount of addition compared to other alloying elements, and accordingly, if used properly, it is possible to manufacture hot-rolled steel with low cost and high quality.

Using the steel grade 1 in Table 1 to investigate the surface hardness change according to the coiling temperature. Hot-rolled steel sheet manufacturing, as described above was heated to 1200 ℃ for 2 hours to maintain a rolling finish temperature 900 ℃, winding temperature was changed to 540-680 ℃ to produce a hot rolled steel sheet of 4.2mm thickness. The results are shown in Table 3 and FIG. 1.

Winding temperature (℃) 540 560 580 600 620 640 660 680 Surface Hardness (HRC) 29.6 27.0 25.5 23.1 21.5 20.6 20.2 20

As can be seen in Table 3 and FIG. 1, the surface hardness increased as the coiling temperature was lowered, and the effect of the coiling temperature was particularly increased as the coiling temperature was lowered below 620 ° C. The reason for this is that the winding is performed before the transformation starts at 620 ℃ or higher, so the effect of the winding temperature is less, and the transformation is accompanied by the winding during the winding temperature below 620 ℃. . Therefore, in order to obtain the effect of a coiling temperature, it turned out that it is desirable to make a coiling temperature below 620 degreeC. However, if the coiling temperature is too low, the hardness is too high and accordingly the rolling workability is greatly reduced and there is a risk of breakage during rolling or when the rolled coil is unfolded, so the coiling temperature is preferably 550 ℃ or more. there was.

By this experiment, the following Equation 3 could be derived.

[Equation 3]

Surface Hardness (HRC) = 10 + 6C + 4 (Si + Cr + Mn) + 35Mo-0.1 (CT-550)

In order to confirm the accuracy of the above Equation 3, the performance value and the calculated value by the equation are simultaneously shown in Table 4 and FIG. 2.

Coiling temperature Surface Hardness (Performance, HRC) Surface hardness (calculated value, HRC) Steel grade 1 600 ℃ 23.5 23.2 Grade 2 600 ℃ 21.8 22.1 Grade 3 600 ℃ 24.7 24.3 Grade 4 600 ℃ 30.5 30.2 Grade 5 600 ℃ 36.5 37.2 Grade 6 600 ℃ 22.1 21.7 Grade 7 600 ℃ 24.9 25.2 Grade 8 600 ℃ 25 24.8 Steel grade 1 540 ℃ 29.2 29.6 Steel grade 1 560 ℃ 27.2 27 Steel grade 1 580 ℃ 25.2 25.5 Steel grade 1 620 ℃ 21.2 21.5

As can be seen from Table 4 and Figure 2, the performance value and the calculated value were almost identical. Therefore, by using Equation 3, the value of HRC23-30, which is the surface hardness range required for the blade, is applied, and this is expressed as a function of Mo to obtain Equations 1 and 2.

[Equation 1]

Mo (wt%) ≥ {13-6C-4 (Si + Cr + Mn) +0.1 (CT-550)} / 35

[Equation 2]

Mo (wt%) ≤ {20-6C-4 (Si + Cr + Mn) +0.1 (CT-550)} / 35

For example, C: 0.6%, Mn: 1.0%, Si: 0.2%, Cr: 0.3%, and the amount of Mo required to obtain the surface hardness HRC 23-30 required for the blade at CT of 580 ° C is 0.14. -0.35%.

As a result of cutting the stone by cutting the blade size using the hot rolled coil manufactured to satisfy the method of the present invention, the blade not only showed an excellent cutting effect, but also a low cost in manufacturing. It was confirmed that the quality is good.

Claims (1)

By weight ratio, C: 0.5-0.8%, Mn: 0.2-2.0%, Si: 0.1-1.0%, Cr: 2.0% or less, Mo contained in the range which satisfy | fills following formula 1 and 2, and remainder Fe And hot rolled steel made of unavoidable impurities, and wound by CT (winding temperature) of 550-620 ° C. such that the surface hardness is in the range of HRC23-30. [Equation 1] Mo (wt%) ≥ {13-6C-4 (Si + Cr + Mn) +0.1 (CT-550)} / 35 [Equation 2] Mo (wt%) ≤ {20-6C-4 (Si + Cr + Mn) +0.1 (CT-550)} / 35