JP2011177775A - Manufacturing method of bar steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an effective method which can suppress surface decarburization and can manufacture a bar steel excellent in peelability of scale. <P>SOLUTION: The method is a method for manufacturing the bar steel by rolling a steel containing 0.10 mass% or more of Si and/or 0.1 mass% or more of Cr. Prior to hot rolling by a plurality of rolling mills, the steel is heated to the surface temperature of 900°C or lower in a heating furnace, and then rolled under a condition that cooling speed from the extraction from the heating furnace till the first pass of hot rolling is controlled at 30°C/second or less. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for producing a steel bar such as a wire rod and a bar steel, which is applied as a steel part in the field of automobiles and various industrial machines, and in particular, a steel bar having a reduced surface flaw without impairing productivity. The present invention relates to a useful method for manufacturing.

  “High carbon chromium bearing steel” such as SUJ2 defined in JIS standard, “alloy steel for mechanical structure” such as SCr420 and SCM435, or “carbon steel for mechanical structure” such as S45C, etc. Conventionally used as a material for steel parts used in various fields such as various industrial machines.

  It is necessary to ensure that the surface defects are in an acceptable range for steel products such as wire rods and steel bars manufactured by hot rolling. In recent years, the surface quality required for these steel strip products has become severe year by year, and it is required to minimize the occurrence of surface flaws that cause product cracking during secondary processing.

  The inventors of the present invention have previously proposed a technique for improving the surface flaw of the bar steel (for example, Patent Document 1). In this technique, the circumferential compressive strain generated during rolling is controlled. By appropriately controlling this compressive strain (for example, hole shape, roll gap, entry side shape, etc.), surface defects are controlled. It enables hot rolling without generating it.

  With the above technology, it was possible to suppress the occurrence of surface flaws in the bar steel, but there were problems when the heating temperature during hot rolling was high or when the time spent in the heating furnace was long due to rolling troubles, etc. It was often observed that surface flaws occurred.

  The above-mentioned steel bar parts are manufactured as steel wire rods and steel bars by hot rolling a steel material (billet). The scale is an iron oxide in a heating furnace before this hot rolling. Will be formed. In particular, in a steel material containing a relatively large amount of Si and Cr, an oxide (subscale) of Si, Cr, and Fe is generated at the boundary between the steel material and wustite (FeO).

  In order to remove the subscale as described above, the water flow rate and pressure of the apparatus for removing the scale with high-pressure water (hereinafter referred to as `` descaler '') is increased, and the steel heating temperature is a temperature at which the subscale is not formed. Various attempts have been made to reduce the production amount of the subscale itself by reducing the amount of the subscale. However, the fact is that these ideas do not lead to a fundamental solution to reducing surface defects.

  For example, Non-Patent Document 1 discloses that the subscale is poorly peelable, and that such a subscale is partially left by a descaler using high-pressure water. Further, even if the flow rate or pressure of water in the descaler is increased, the subscale peelability is poor, and as described later, it causes surface flaws and leads to a decrease in productivity.

JP 2007-90429 A

CAMP-ISIJ Vol. 20 (2007) -983

  The present invention has been made in view of such circumstances, and its purpose is to control the subscale to a form that does not become surface defects without impairing productivity, thereby reducing the surface defects. It is in providing the useful method for manufacturing.

  The manufacturing method of the steel bar of the present invention that has achieved the above object is a method of manufacturing a steel bar by rolling a steel material containing Si: 0.10% by mass or more and / or Cr: 0.1% by mass or more. Then, before performing hot rolling with a plurality of rolling mills, the surface temperature of the steel material is heated to 900 ° C. or lower in the heating furnace, and the first hot rolling from the heating furnace extraction is performed. It has a gist in that it operates at a cooling rate of up to 30 ° C./second.

  In the said method of this invention, it is preferable to operate by keeping the surface temperature of steel materials below 900 degreeC until hot rolling is complete | finished.

  According to the method of the present invention, the heating temperature before hot rolling is strictly controlled, and the cooling rate until hot rolling after being extracted from the heating furnace is appropriately controlled, so that the subscale is made to have surface defects. The steel strip obtained in this way is extremely useful as a material for various structural steels.

FIG. 1 is a diagram for explaining a situation in which a subscale remains on the surface of a steel material. FIG. 2 is a graph showing the relationship between the heating temperature in the heating furnace and the Cr concentration in the subscale. FIG. 3 is a graph showing changes in rolling line temperature (steel surface temperature) when the finish rolling speed is changed.

The present inventors have studied from various angles with the aim of realizing a strip with reduced surface flaws. As a result, the following findings (A) to (D) were obtained.
(A) In a steel material containing Si or Cr, a hard and brittle subscale is generated on the surface of the steel material in a heating process in a heating furnace.
(B) Even if the subscale as described above increases the pressure of the descaler, the subscale partially remains.
(C) The remaining hard subscale is pushed into the steel surface along with deformation during hot rolling.
(D) The remaining subscale is elongated in the longitudinal direction by rolling, and develops into an intermittent linear surface defect.

FIG. 1 is a diagram for explaining a situation in which a subscale remains on the surface of a steel material. For example, in a hot-rolled steel material containing a relatively large amount of Si, the surface of the steel material 1 / Fe ₂ SiO ₄ (subscale layer 2) / FeO layer 3 / (Fe ₃ O ₄ The layer 4) is formed in the order of Fe ₂ O ₃ [FIG. 1 (a)]. Of these, the subscale 2 is known to have poor peelability (Non-Patent Document 1), and even if a high-pressure descaler is used, the subscale 2 partially remains [FIG. (B)]. In addition, the amount of subscale generated is an oxidation reaction, so that the higher the heating temperature and the longer the heating time, the more the subscale is generated. (C)].

  That is, as shown in FIG. 1, when the thickness of the subscale is relatively thin, the subscale is partially left by the descaler [FIG. 1 (b)]. On the other hand, when the thickness of the subscale is relatively large, the subscale remains partially due to the descaler, but the adhesion increases as the generation of the subscale progresses further. A portion (uneven portion) that is destroyed by this is also seen [FIG. 1 (c)].

The hardness of each scale (hardness by Vickers: HV) when the heating temperature during hot rolling is 1000 ° C. (ordinary heating temperature) is, for example, about HV 13.6 with Fe ₂ O ₃ in the case of Cr-containing steel, Fe ₃ O ₄ is about HV9.4, FeO is about HV8.1, Fe ₂ Cr ₂ O ₄ (subscale) is about HV126, and the subscale has the highest hardness. Further, since the subscale is generated on the steel material side, the sub-scale is pushed into the steel material by the subsequent rolling process, and unevenness is generated. And this unevenness | corrugation becomes a linear wrinkle intermittent in a longitudinal direction by carrying out multiple pass rolling.

Therefore, as an effective improvement means for eliminating surface flaws, it is conceivable that the subscale is completely peeled off by the descaler or the subscale is not generated in the heating furnace. For example, it may be possible to descale at an ultrahigh pressure such that the discharge pressure exceeds 500 kgf / cm ² (49 MPa), but this is not realistic in terms of capital investment costs. Even if an ultra-high pressure descaler is installed, it is necessary to manage the wear of nozzles and peripheral devices due to high-pressure water splashing, so it is difficult to adopt it in consideration of actual operation.

On the other hand, in order not to generate subscale in the heating furnace, it is necessary to maintain the temperature in the heating furnace at about 550 ° C. At such a temperature, the steel material cracks due to an increase in rolling load or insufficient ductility during rolling. This causes problems such as For example, FIG. 2 is a graph showing the relationship between the heating temperature in the heating furnace and the Cr concentration in the subscale. At this time, a steel material containing 1.5% by mass of Cr is used (therefore, the broken line portion indicates the Cr concentration of the steel material). The atmosphere in the heating furnace is adjusted to N ₂ -18% H ₂ O-1% O ₂ . FIG. 2 shows that the Cr concentration in the scale increases as the subscale is formed, and that the subscale is not generated at a temperature of about 550 ° C. or lower.

The present inventors further examined the means that can avoid all the above problems from various angles. As a result, it has been found that the conditions may be set so as to satisfy the following requirements (i) and (ii).
(I) In the heating furnace, heating is performed under the condition that the subscale thickness is as thin as possible, so that unevenness due to cracking of the subscale itself does not occur before rolling.
(Ii) In the process from the heating furnace to rolling, partial cooling subscales are avoided by slowing the cooling rate (intentionally, a thin scale is left over the entire rolling surface).

  In the present invention, heating is performed under conditions such that the subscale thickness is as thin as possible in the heating furnace, but from this viewpoint, before performing hot rolling with a plurality of rolling mills, in the heating furnace, It is necessary to heat the steel material so that the surface temperature is 900 ° C. or lower. If the surface temperature of the steel material exceeds 900 ° C., the thickness of the subscale is increased by the descaler after that, and generation of surface flaws cannot be avoided. In addition, the heating temperature at this time should just be 900 degrees C or less, and although the minimum is not specifically limited, From a viewpoint that a crack does not generate | occur | produce at the time of rolling, it is preferable that it is 600 degrees C or more.

  The surface temperature of the steel material may increase due to heat generated during processing during hot rolling, and such a temperature increase may promote the generation of subscales. From such a viewpoint, it is preferable that the surface temperature of the steel material is kept at 900 ° C. or less until the hot rolling is finished.

  On the other hand, it is also important to set the cooling rate from extraction in the heating furnace to hot rolling in the first pass to 30 ° C./second or less. By slowing the cooling rate in this way, the subscale is formed uniformly thin. Thus, it is possible to avoid dispersion of subscales that cause surface flaws. The cooling rate at this time is preferably 20 ° C./second or less. However, from the viewpoint of not promoting the growth of scale between extraction and hot rolling in the first pass, the cooling rate is set to 0. The temperature is preferably 1 ° C./second or more.

  As described above, in order to reduce surface flaws, the steel material surface temperature in the heating furnace and the cooling rate from the heating furnace extraction to hot rolling (up to the first pass hot rolling) are appropriately controlled. It is necessary to. Although it is necessary to confirm the influence of these requirements on the scale form of the steel material, it is difficult to investigate the form of the scale in the hot state with an optical microscope or the like. In particular, since a secondary scale is generated even during a temperature drop from room temperature to room temperature, the state cannot be directly grasped.

  Therefore, the present inventors confirmed the effects of the heating temperature and the cooling rate on the surface defects by experiments using actual machines. The cooling rate in this experiment was adjusted by changing the discharge pressure in the descaler (that is, the cooling rate during descaling before hot rolling). The cooling rate of the steel material passing through the descaler can be calculated from the measured values of the temperatures on the entry side and the exit side of the descaler.

  In the present invention, a hard subscale containing Si or Cr as a steel material component is assumed. From this point of view, the content of Si and Cr in the steel material is 0.10% by mass or more and 0.0. The effect of the present invention is effectively exhibited when the steel material (Si: 0.10% by mass or more and / or Cr: 0.1% by mass or more) containing 1% by mass or more is targeted. In addition, although there is no particular limitation on the upper limit of the content of Si or Cr, it is preferable to set the content to 4% by mass or less in any case because excessive content impairs ductility.

  The strips used in the present invention are assumed to be used as bearing steels and mechanical structural steels, but the basic components (C, Mn, etc.) other than Si and Cr described above are usually contained in a normal amount. It may be. Further, the steel material may contain inevitable impurities (for example, S, P, Cu, Ni, Al, etc.) that are brought in depending on the status of raw materials, materials, manufacturing equipment, and the like. Further, from the viewpoint of increasing the strength of the steel material, a material (for example, SNCM815) containing a predetermined amount (0.90% by mass or less) of Mo can also be used.

  The steel bars targeted by the present invention are made into bearing parts and machine structural parts after being made into a predetermined part shape and then quenched and tempered. Both the linear shape and the rod shape are included, and the size can be appropriately determined according to the final product.

  Hereinafter, the present invention will be described in more detail by way of examples.However, the present invention is not limited by the following examples as a matter of course, and may be implemented with modifications within a range that can meet the gist of the preceding and following descriptions. Of course, they are all possible and are included in the technical scope of the present invention.

[Example 1]
Billets (cross-sectional size: 155 mm × 155 mm) having various chemical composition compositions (steel types A to D) shown in Table 1 below were melted.

These billets are heated to the steel surface temperature (1100 ° C, 1000 ° C, 900 ° C) shown in Tables 2 and 3 below, and then the finish rolling speed is changed while the water pressure from the descaler is changed (high pressure, medium pressure, low pressure). : Hot rolled at 10 m / sec to obtain a wire with a diameter of 10 mmφ. The water pressure (high pressure, medium pressure, low pressure) at this time is as follows, and is obtained by calculation from the relationship between the flow rate and the pressure. The atmosphere in the heating furnace was N ₂ 71%, H ₂ O 18%, CO ₂ 10%, O ₂ 1%, and the steel surface temperature was controlled with a radiation thermometer.
[Water pressure from the descaler]
High pressure: 120 kgf / cm ² (11.8 MPa) [Flow rate: 594 L / min]
Medium pressure: 47 kgf / cm ² (4.6 MPa) [Flow rate: 371 L / min]
Low pressure: 8 kgf / cm ² (0.8 MPa) [Flow rate: 148 L / min]

  The cooling rates shown in Tables 2 and 3 below are obtained by calculation based on the temperatures of “Descaler entry side” and “Descaler exit side”.

The maximum surface wrinkle depth observed at 10 or more cross sections perpendicular to the longitudinal direction of the rolled wire (the rolling direction of the steel material) was measured, and the average value (maximum wrinkle depth / measured number) was calculated. Then, from the average value of the average surface wrinkle depth, the surface wrinkles were evaluated by ranking as follows. When the surface defect rank is 1 or less (ranks 0 and 1), it means that there is no problem as a product. The results are shown in Tables 2 and 3 below together with the heating temperature, descaler conditions, descaler entry side temperature, exit side temperature, cooling rate, and surface defect rank.
[Evaluation criteria for surface defects]
Rank 0: average surface wrinkle depth of 0.00 mm or more and less than 0.01 mm Rank 1: average surface wrinkle depth of 0.01 mm or more and less than 0.02 mm Rank 2: average surface wrinkle depth of 0.02 mm or more, Less than 0.03 mm Rank 3: Average surface wrinkle depth is 0.03 mm or more, less than 0.04 mm Rank 4: Average surface wrinkle depth is 0.04 mm or more

  From these results, it can be considered as follows. In the case of medium temperature heating (1000 ° C.) and high temperature heating (1100 ° C.) (test Nos. 1 to 8, 13 to 20, 25 to 32, 37 to 44), there is a difference depending on the steel type, but the water pressure in the descaler is The higher the surface, the more the surface defects are reduced. However, in all cases, the surface defect rank is 2 or more (ranks 2, 3, 4).

  When the heating temperature is high, the reason why the surface flaw is reduced as the water pressure in the descaler is increased is considered to be because the remaining subscale is reduced. However, as described above, the rank remains only 2 because it remains partially. The reason why surface flaws are worse in high-temperature heating than in medium-temperature heating is considered to be because the amount of subscales originally generated is higher at higher temperatures.

  On the other hand, in the case of low-temperature heating (Test Nos. 9-12, 21-24, 33-36, 45-48), the surface flaw rank deteriorates under conditions where the water pressure in the descaler is high (Test No. 12). 24, 36, 48). Under the condition that the water pressure at the descaler is equal to or lower than the medium pressure by low-temperature heating (that is, the cooling rate is equal to or lower than 30 ° C./second), the surface defect rank satisfies 1 or lower. This is presumably because the subscale is uniformly left on the entire surface because the subscale generation amount is small (thin) and the cooling rate is low in low temperature heating.

  When the above results are examined for each steel type, (a) the surface defect rank of steel type A with a high content of Si and Cr is the worst, and (b) the surface defect rank of steel type D with no Cr content and a low Si content. Is the best. Moreover, from the comparison of steel types A and B having the same Si content and different Cr contents, it can be confirmed that the surface defect rank tends to deteriorate as the Cr content increases.

  From these test results, it can be seen that the cause of the surface flaws in the steel bar is partial remaining of the subscale containing Si and Cr, and the surface flaws can be improved by optimizing the heating temperature and cooling rate. .

[Example 2]
The generation of the subscale is mostly in the heating furnace. However, in the rolling of strip steel, the temperature rises due to processing heat generation during rolling, so a subscale (secondary scale) that grows during rolling is also conceivable. Therefore, with the aim of further improving surface defects, experiments were also conducted to control the temperature during rolling and investigate the influence of secondary scale.

  Using steel type A shown in Table 1 above, the surface temperature (heating temperature) in the heating furnace was set to 900 ° C., and the experiment was conducted with a finish rolling speed of 3 m / sec for the purpose of suppressing processing heat generation during rolling. The effect of changes in surface temperature on surface defects was investigated. At this time, the experiment was also performed when the finish rolling speed was 10 m / sec. At this time, the descaler was not used.

  The results are shown in Table 4 below. Moreover, the rolling line temperature (steel surface temperature transition during rolling) when the finish rolling speed is changed is shown in FIG. The temperature between the stands (measurement position) during rolling was measured with a radiation thermometer.

  From these results, it can be considered as follows. That is, on the condition that the finish rolling speed is 3 m / sec, the steel surface temperature is 900 ° C. or less over the entire rolling line. On the other hand, when the finish rolling speed is 3 m / sec, it takes a long time between the stands, and during this period, the temperature rise due to heat generation is suppressed, and the temperature rise is reduced, or the heat generation itself is reduced. When rolling is performed under such conditions, it can be seen that the surface defect rank is 0 for steel type A as well. This is presumably because the generation of subscales during rolling was reduced by suppressing the temperature rise due to processing heat generated during rolling.

  Incidentally, as a means for suppressing the growth of the secondary scale, it is conceivable to cool the surface of the steel material during the rolling, and the same result as above will be obtained. When such means are employed, the growth of the thin subscale is originally suppressed, so that it is considered that the cooling rate itself during water cooling does not affect the subscale form.

Claims (2)

A method of rolling a steel material containing Si: 0.10% by mass or more and / or Cr: 0.1% by mass or more to produce a bar steel,
Before performing hot rolling with a plurality of rolling mills, the steel material is heated to a temperature of 900 ° C. or less in the heating furnace, and the cooling rate from the heating furnace extraction to the first pass hot rolling Is manufactured at a temperature of 30 ° C./second or less.   The manufacturing method of the bar steel of Claim 1 which maintains and operates the surface temperature of steel materials at 900 degrees C or less until hot rolling is complete | finished.

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