JP4770238B2 - Warm shot peening method for thick steel plate - Google Patents

Description

  The present invention relates to a method for warm shot peening of a thick steel plate, and more particularly to a technique for producing a large structural material effective as a material for a hull, a pressure vessel or the like.

  Shot peening is a kind of surface treatment method in which a large amount of granular pig iron or steel (hereinafter also referred to as a shot) is hit against the surface of a steel product at high speed, and the above iron particles are mechanically blown or compressed air is used. This is done by spraying from the nozzle using This shot peening was originally developed with the aim of hardening the surface of steel products, but in recent years, it has been used for scale removal and cleaning instead of pickling, omitting polishing finish, etc., and fatigue strength. It has come to be widely used to increase

  Shot peening for the purpose of improving the fatigue strength of this steel material was mainly applied to relatively small parts such as springs, gears, crankshafts, propeller shafts, turbine blades, etc., where repeated stress is applied at high cycles ( Patent Literatures 1 to 3). In order to enhance the effect of shot peening, so-called “warm shot peening”, which is performed after raising the temperature of an object to about 150 to 300 ° C. in addition to normal temperature, may be applied (patent document). 4).

  On the other hand, there has been an attempt to improve the fatigue strength of conventional large-sized steel plates, such as pressure vessels and hulls, which are subjected to repeated stress in a low cycle, by performing shot peening at room temperature. Actually, the fatigue strength is not so high as expected, and the “fatigue test” for evaluating the improvement of the fatigue strength takes too much time.

However, recently, from the viewpoint of expanding the use of steel products, there has been a need to further increase the fatigue strength of such large steel plates. However, the conventional shot peening at normal temperature cannot answer this need. That is, (1) increase the hardness of the iron particles that collide with the thick steel plate, (2) increase the size, (3) increase the collision density per unit area, (4) measures such as extending the projection time of the shot, There is a problem that the existing projecting means is remodeled and the equipment cost increases. In addition, even if such measures are taken, the surface indentation increases, the roughness increases, the appearance deteriorates, and cracks occur, and this does not simply lead to a significant increase in fatigue strength. It was.
JP-A-1-279786 JP-A-6-172847 JP 2001-82518 A JP 5-140643 A

  In view of such circumstances, an object of the present invention is to provide a warm shot peening method for a thick steel plate capable of significantly improving fatigue strength even when an existing projection apparatus is used.

  The inventor has intensively studied to achieve the above object, and the results have been embodied in the present invention.

That is, the present invention is, repeated stress is Ru loaded with low cycle, size thickness 16mm or more, the width 2500mm above the surface of the steel plate over a large length 6000 mm, a large number of iron particles, collide at high speed, the shot When performing peening, as a means for projecting the iron particles, a centrifugal projection device that accelerates the iron particles with a rotating impeller is adopted, and shot peening is performed so that the fatigue strength of the steel plate after shot peening is 360 to 550 MPa. The previous thick steel plate is heated using a heat treatment apparatus disposed in the hot rolling line, and the iron particles are added in an amount of 0.5 to 2 in a state where the temperature of the heated thick steel plate is maintained at 200 to 700 ° C. This is a warm shot peening method for a thick steel plate, characterized in that it is caused to collide only for a minute. In this case, before Kitetsu particles made is that cast steel, 2 mm outer diameter with a shape close to a substantially spherical shape, preferably projecting at the collision density of 1 m ² per 150~200kg thick steel plate.

  ADVANTAGE OF THE INVENTION According to this invention, it will become possible to manufacture cheaply the thick steel plate which fatigue strength improved conventionally conventionally using the projection apparatus and heat processing apparatus.

  Hereinafter, the best embodiment of the present invention will be described based on the background of the invention.

First, the inventor has carried out shot peening on relatively small parts such as springs and gears in order to improve the fatigue strength. We focused on the large improvement effect. And the projection apparatus utilized when peening at the normal temperature the problem at the time of applying this warm shot peening to a large-sized thick steel plate (steel type: SM490, size: thickness 20mm, width 3000mm x length 10000mm) I decided to consider using In other words, the mechanical projection device is used that causes iron particles (shots) made of cast steel and having a particle diameter of 1 to 5 mm to collide with a thick steel plate with a collision density of 200 kg / m ^2. The experiment was repeated. The fatigue strength of the steel sheet was measured using a “fatigue test method” defined in JIS Z 2275.

  As a result, it has been found that if the peening temperature is set in the range of 200 to 700 ° C., which is almost the same as that of small parts, it is more effective than room temperature. The reason for limiting the peening temperature to the above range is that if it is less than 200 ° C., there is not much difference in effect from that at room temperature. This is because the appearance is deteriorated or cracks are generated.

  However, the important point of the present invention is that the peening time must be shorter than that at room temperature. Since the steel plate surface tends to dent more than normal temperature, if the time is too long, as described above, the shot impression on the steel plate surface increases and the roughness increases, compared to the case of peening at normal temperature, the appearance is worsened, This is because cracks occur. Therefore, when the peening time was examined for thick steel plates of various steel types within the above temperature range, there was no problem for any steel type as long as it was allowed to collide only for 0.5 to 2 minutes. It was found that a large peening effect can be obtained. That is, almost no effect is exhibited when the time is less than 0.5 minutes, and the problem described above occurs when the time exceeds 2 minutes.

  Thus, if appropriate selection is made in the range of temperature: 200 to 700 ° C. and time: 0.5 to 2.0 minutes, compared to peening at room temperature, even if it is a steel plate of any steel type, Since it was found that the fatigue strength was improved, the present invention was completed based on these requirements.

In carrying out the present invention in an actual manufacturing process, an appropriate temperature and time are determined in advance by a test operation. Moreover, in order to heat a thick steel plate to the said temperature range, it is necessary to provide a heating furnace separately. The heating furnace is preferably provided in the vicinity of an existing shot projection apparatus. However, when an existing projection apparatus is used instead of a new installation, there is often no space for newly installing a heating furnace. Therefore, in the present invention, since a heat treatment furnace is arranged in the hot rolling line for thick steel plates, the use thereof is recommended (see FIG. 1). Even between the shot projection device away Ri kana, because the steel sheet can be sufficiently high temperature increase than the temperature range. However, it is necessary that the temperature of the steel plate can ensure the temperature range while the steel plate is conveyed from the heat treatment furnace to the shot projection apparatus. For that purpose, it is desirable that the steel plate conveying device between them can freely change the steel plate conveying speed.

Further, conventionally used iron particles used as shots include cast iron shots, steel wire shots (thinly cut steel wires), cast steel shots, and the like. However, since the cast iron shot is brittle and easily broken, it has a lot of wear, and many steel wire shots have a sharp shape. Therefore, in the present invention, it is preferable to use a cast steel shot having a nearly spherical shape. In addition, if the outer diameter of the cast steel shot is too large, the indentation becomes too large, so 2 mm is recommended. According to the inventor's investigation, the case of 2 mm was the most effective for the expression of the peening effect. Further, in the present invention, the collision density of the shot to the steel plate is not particularly limited, but it is preferable to project at a collision density of 150 to 200 kg per 1 m ^{2 of} the thick steel plate. This is because it is not necessary to greatly improve the existing projection apparatus, and the equipment cost is not increased.

The warm shot peening method according to the present invention was applied to thick steel plates with different sizes and steel types manufactured in the hot rolling process. The shot projection device used is an existing rotary impeller system, and the shot iron particles have an outer diameter of 2 mm and are almost spherical, and are applied to each steel plate at a collision density of 200 kg / m ² (steel plate area). Projected. As a heating device, as shown in FIG. 1, the steel plate temperature is heated to about 200 to 700 ° C. using a heat treatment device 1 installed in a hot rolling factory, and then the speed of the conveying device 3 is adjusted. At the time of peening, the steel sheet was conveyed to an existing shot projection device 2 (not shown) of an adjacent shot peening factory so that the temperature of the steel plate became a predetermined temperature. In addition, the peening temperature and peening time appropriate for each steel plate were determined in advance from data obtained in the test operation and the past operation. Moreover, in order to confirm the effect of this invention, the conventional normal temperature peening was separately performed about the same kind of steel plate. The performance was measured by the “fatigue test method” defined in JIS Z 2275 as an evaluation based on the fatigue strength of the steel sheet.

  The results are shown in Table 1 together with the type, size, peening temperature and peening time of the steel sheet.

  From Table 1, it is clear that according to the present invention, all the steel plates have higher fatigue strength than the conventional room temperature peening, and the present invention is an excellent peening method. Further, since it is not necessary to newly install a separate device other than the conveying device for the implementation of the present invention, the present invention is a very economically advantageous method.

It is a top view which shows each apparatus arrangement | positioning at the time of implementing warm peening of the thick steel plate which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Heat processing apparatus 2 Shot projection apparatus 3 Steel plate conveyance apparatus
4 Conveyor newly established

Claims (2)

Repeated stress Ru loaded with low cycle, size thickness 16mm or more, the width 2500mm above the surface of the steel plate over a large length 6000 mm, a large number of iron particles, collide at high speed, when performing shot peening,
As the iron particle projecting means, a centrifugal projection device that accelerates the iron particles with a rotating impeller is adopted, and the steel plate before shot peening is made to have a fatigue strength of 360 to 550 MPa after shot peening. was heated using a heat treatment apparatus that is disposed in the hot rolling line, the temperature of the heated said steel plate being maintained at 200 to 700 ° C., the impinging iron particles only 0.5 to 2 minutes A method of warm shot peening of thick steel plates. Wherein the iron particles, the material is cast steel is 2mm outer diameter shape nearly spherical, steel plate of claim 1, wherein that you projected collision density of 1 m ² per 150~200kg thick steel plate Warm shot peening method.

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