JP4619635B2 - Welding method for high carbon steel - Google Patents

Description

  The present invention relates to a method for welding high carbon steel materials such as high carbon steel, alloy tool steel and bearing steel which are hardened and fractured immediately after welding, and in particular, prevents fracture of welded parts in continuous processing lines such as rolling and pickling. About. The “steel material” as used in the present invention includes a steel strip, a steel plate, a bar steel, and a shaped steel.

  In general, steel materials are cast into ingots of molten steel whose components have been adjusted, and hot rolled into steel strips (also called steel strips), steel plates (often referred to as thick steel plates), steel bars, and shaped steels. If the steel is made of high carbon steel with component C: 0.3 mass% or higher, the welded joint is martensite in the process of cooling immediately after completion of welding (natural cooling in the atmosphere, that is, air cooling). It is easy to transform and harden. For this reason, when mechanical bending is applied to the welded portion, the crack may propagate from the microcracks on the surface of the welded portion and may break at the welded portion. When a fracture occurs at the weld, it becomes a big problem.

  As a means to prevent such fracture of the welded part, after welding, it is heated to a temperature below the Ac1 transformation point to convert hard hardened martensite to tempered martensite rich in toughness, and post-heat treatment is performed to generate cracks. There are welding methods to prevent this.

  For example, in Patent Document 1, each end of a steel sheet to be abutted is heated continuously or intermittently from before welding to immediately after the end of welding, and the temperature of the welded part and its vicinity immediately after welding is 80 to 400 ° C. There has been proposed a laser welding method in which the tempering heat treatment is performed and then tempering heat treatment is performed. In Patent Document 2, when joining the rear end of the preceding material of the special steel having a carbon content of 0.30% or more and the front end of the succeeding material by flash butt welding and passing through the continuous line, the welded portion is immediately after welding. There is an in-line joining method for special steel strips that, after forcibly cooling to a temperature that is 100 ° C lower than the Ms point, is then subjected to post-heat treatment at a temperature of about 700 ° C with a gas burner or the like to obtain a tempered martensite structure. Proposed.

In Patent Document 3, laser welding is applied when butt-welding a high carbon steel strip, and post heat treatment is performed within a temperature range of 400 ° C. or more and Ac ₁ point or less within 1 minute after the completion of welding. High-carbon steel strip welding methods have been proposed.
JP-A-61-79729 Japanese Examined Patent Publication No. 63-64501 JP-A-5-132719

  However, in the technique described in Patent Document 1, since heating (preheating) is performed before welding, thermal expansion occurs in the steel sheet, and as a result, irregularities in the butt gap in the steel sheet width direction often occur. In the technique described in Patent Document 1 using a thin energy beam such as laser welding, there is a problem in that there is a location where insufficient penetration occurs, and the steel strip may break from that portion. In addition, the unevenness of the butt gap is reduced by lowering the preheating temperature, but the effect of suppressing the cooling rate of the welded portion after welding, which is the original purpose of preheating, is reduced, and as a result, quenched martensite is generated. There is a problem that it is difficult to reduce the amount, and it is impossible to completely prevent the steel strip from breaking. Further, in the technique described in Patent Document 1, since heating is also performed after welding (post-heating), in the vicinity of both-side heaters sandwiching the welded portion, it is locally heated better than the welded portion, and the Ac1 transformation point is much higher. In some cases, the temperature is higher than that, and some of the martensite is reversely transformed into austenite, and quenching martensite is generated again at the time of cooling, which may easily break in the vicinity of the weld.

  In the technique described in Patent Document 2, martensitic transformation occurs during forced cooling after welding, and cracks may occur before post-heat treatment. Moreover, in the technique described in Patent Document 2, since the post-heat treatment is performed with a gas burner, the post-heat treatment temperature is not uniform in the steel strip width direction, and there are places where the temperature is much higher than the Ac1 transformation point. For this reason, there is a problem that part of martensite is reversely transformed into austenite (hereinafter also referred to as γ), and when quenched, martensite is generated again and is likely to break.

  Moreover, in the technique described in Patent Document 3, the hardness of the welded portion and its vicinity due to post-heat treatment within 1 minute after completion of welding is still insufficient and may be broken by bending with a bridle roll or looper roll. In addition to the above problem, there has been a problem that, in the subsequent cold rolling and temper rolling, there is a case where the welded portion is broken due to the difference in hardness between the base metal and the welded portion. Furthermore, if the roll of the rolling mill is released when the weld passes in order to prevent breakage, there is a problem in that a portion where the plate thickness exceeds a predetermined range occurs, resulting in a decrease in yield.

  An object of the present invention is to propose a welding method for high carbon steel materials containing 0.3 mass% or more of carbon, which can advantageously solve such problems of the prior art and suppress the occurrence of breakage in weld joints. .

In order to achieve the above-described problems, the present inventor has conducted a thorough examination focusing on post-weld heat treatment for various factors affecting the occurrence of cracks due to bending of a C: 0.3 mass% or higher high carbon steel weld. As a result, it was found that the heating (temperature increase) rate during post-weld heat treatment has a great influence on the precipitation carbide morphology in the tempering process and the phase transformation near the Ac1 transformation point. That is, after completion of welding, the post-heat treatment is performed by rapidly heating (heating) the welded joint at a rate of 1 ° C./s or more, so that the proper temperature range of the post-heat treatment is higher than previously considered. It has been found that the post-heat treatment (tempering) effect of the welded portion can be remarkably increased, and the crack resistance (breaking resistance) of the welded portion can be significantly improved.

The present invention is based on the above findings. That is, the gist of the present invention is as follows.
(1) C: In a welding method for high carbon steel materials in which high carbon steel materials having a composition containing 0.3 to 1.5 mass% are butted or overlapped with each other, and the high carbon steel materials are welded to each other. the weld formed by, heated to a temperature within the temperature range of 900 ° C. than Ac1 transformation point at 1 ° C. / s or higher heating rate, at said temperature, cool release Chi was held between 5~30s and facilities for heat treatment after that, the welding method of high carbon steel, wherein the hardness of the weld heat affected zone is HV400 or less.
(2) Oite to (1), in mass% in addition to the composition, Cr: 0.2~13.0%, Mo: 0.2~10.0%, W: 0.2~19.0%, V: 0.2~5.2%, Nb: 0.2 A method for welding high-carbon steel, comprising one or more selected from ˜5.0%.

  According to the present invention, since the occurrence of breakage from the welded joint can be prevented, for example, a long-time stop in a continuous processing line is prevented, productivity is remarkably improved, and a remarkable industrial effect is achieved. Further, conventionally, in order to prevent breakage in the steel strip welded portion, the rolling roll was controlled to be in an open state when passing the welded portion in continuous rolling. Control is not required. Therefore, there is no need to cut off the front and back of the welded portion, and the production yield of the steel strip is improved.

  The steel material targeted by the present invention is a high carbon steel material having a composition containing C: 0.3 to 1.5 mass%. In addition, in the present invention, in addition to the above-described composition, Cr: 0.2-13.0%, Mo: 0.2-10.0%, W: 0.2-19.0%, in addition to the above-described composition, from the viewpoint of suppressing the precipitation growth of composite carbide. You may contain 1 type (s) or 2 or more types chosen from V: 0.2-5.2% and Nb: 0.2-5.0%.

  For example, as a high carbon steel material, it contains C: 0.3-1.5 mass%, and also contains Si: 3.5 mass% or less, Mn: 1.0 mass% or less, Ni: 1.0 mass% or less as a component other than C, and the balance There is a carbon steel with a composition consisting of Fe and inevitable impurities. In addition, as an inevitable impurity, Cu: 0.3 mass% or less is acceptable.

  Moreover, as a high carbon steel material, C: 0.95-1.1 mass% is contained, Si: 0.15-0.70 mass%, Mn: 1.15 mass% or less, Cr: 0.9-1.6 mass% are contained, The remainder Fe and unavoidable There are bearing steels composed of mechanical impurities.

As the high carbon steel, C: 0.35 to 1.5 containing mass%, further Si: 0.3 5m ass% or less, Mn: 0.8 mass% or less, wherein the further, Cr: 0.2~1.5mass%, V: An alloy tool steel having a composition including one or two or more selected from 0.1 to 0.3 mass% and W: 1.0 to 3.5 mass%, the balance being Fe and unavoidable impurities.

  In the present invention, high carbon steel materials are butted or overlapped and welded together. For example, it is suitably applied to the case where the tail end of the preceding high carbon steel strip and the tip of the following high carbon steel strip are butted or overlapped and welded in the width direction of the high carbon steel strip. be able to. The welding method is not particularly limited, but laser welding, flash butt welding, lap current welding (seam welding), arc welding, or the like is preferable. The welded portion formed by the welding joint is then subjected to post heat treatment.

  The post heat treatment is a process of heating to a temperature within a temperature range of 600 ° C. to 900 ° C. at a temperature rising rate of 1 ° C./s or more, and then allowing to cool or gradually cool. Rapid heating at a rate of 1 ° C / s or higher suppresses the diffusion of solute C, promotes the generation of lattice defects, suppresses the start of transformation to the γ phase, and promotes softening of the weld. Is done. Furthermore, by setting the rate of temperature rise to 1 ° C./s or more, even if transformation into the γ phase occurs, diffusion of solid solution C in the γ phase is suppressed, so that C is uniform in the γ phase. In the subsequent cooling, only a part of the γ phase becomes a hard martensite phase. For this reason, even if a welding part is post-heated to the high temperature range which the transformation to (gamma) phase produces, it will not show remarkable hardening. In addition, it is preferable that a temperature increase rate shall be 1.0-200 degrees C / s from a viewpoint of precipitation growth and transformation suppression. More preferably, it is 10-200 degreeC / s.

In the post heat treatment of the present invention, A _c1 Heat to the temperature in the temperature range exceeding the transformation point and below 900 ° C. at the above-mentioned rate of temperature increase. " A _c1 " As the “transformation point”, a value obtained from a thermal expansion curve obtained by a thermal expansion measurement performed at a heating rate (temperature increase rate): 0.05 ° C./s is used.

  If the heating temperature is less than 600 ° C., sufficient weld softening due to less solid solution C precipitation of quenched martensite does not occur, and therefore cracking from the weld cannot be prevented. On the other hand, when the heating temperature is higher than 900 ° C., transformation to the γ phase occurs remarkably and a large amount of γ phase is present, and solid solution of C becomes active in the γ phase. C becomes uniform and most of the γ phase is transformed into a hard martensite phase by the subsequent cooling, and the weld is hardened. For this reason, the occurrence of cracks from the welded portion cannot be completely prevented.

  In the present invention, it is preferable that the holding time at the heating temperature by the above-described post-heat treatment is 5 to 30 seconds. If the holding time is less than 5 s, sufficient softening of the weld cannot be expected. On the other hand, even if the holding time is longer than 30 s, the effect is saturated and the diffusion and solid solution of C due to heat conduction starts, the growth of precipitates and α transformation occur, and the productivity takes longer. It becomes uneconomical and disadvantageous.

  In the present invention, the mixture is heated to the above temperature, preferably held at that temperature, and then allowed to cool or gradually cool without being rapidly cooled. After heating, by allowing to cool or gradually cool, some martensite generated from the γ phase during cooling also becomes soft, does not harden after cooling, can avoid the occurrence of fracture from the weld, and is resistant to fracture Improves. The cooling is air cooling in the atmosphere, and the cooling rate is approximately 20 ° C./s or less. Further, the slow cooling referred to here means cooling in a furnace at a cooling rate of 0.008 ° C./s or less.

  The present invention was applied to welding by a welding machine 10 installed on the inlet side of a cold rolling line for a steel strip as shown in FIG. Here, a high carbon steel strip (SK5: 3.5 mm thickness, SUJ2: 4.5 mm thickness) having the composition shown in Table 1 was used as the steel strip. From these steel strips, specimens for welding joining (size: 1000 w × 500 lmm) were collected, and the specimens were butted together and welded together. The welding machine used was a carbon dioxide laser welding machine with an output of 10 kW. Immediately after welding, post-heat treatment was performed on the weld. The post-heat treatment was performed under the post-heat treatment conditions (heating rate, heating temperature, holding time, cooling) shown in Table 2 using a high-frequency induction heating apparatus built in the welding machine.

  For the obtained welded portion, the maximum hardness HV of the heat affected zone was determined in accordance with the provisions of JIS Z 3101. Further, a bending test piece (flat plate t × 20 w × 150 lmm) was collected from the obtained welded portion, and a repeated bending test was performed. In the repeated bending test, 90 ° bending with a bending radius of 25 mm and bending back were mechanically performed 30 times on the weld line, and the number of repetitions at which cracking occurred was determined.

  The obtained results are shown in Table 2.

  In all of the examples of the present invention, the hardness of the weld heat affected zone is as low as HV400 or less, and the number of repeated bendings is 20 times or more, so that the weld zone is excellent in crack resistance and fracture resistance. In the comparative example, the welded heat affected zone has a hardness of Hv452 or higher and the welded portion is hardened, and the number of repeated bendings is as low as 10 or less, so that the welded portion is easily cracked.

  In this embodiment, an example using butt laser welding has been described. However, the present invention is not limited to this, and lap welding may be used, and the type of welding may be flash butt welding or seam welding. Needless to say, it may be used.

It is a schematic diagram which shows an example of the cold rolling line of a steel strip.

Explanation of symbols

1 Steel strip 2 Finish rolling mill row 5 Shear
10 Welding machine
20 payoff reel
30 tension reel
50 Bridle roll
60 Looper roll
100 cold rolling line

Claims (2)

C: In a high carbon steel material welding method in which high carbon steel materials having a composition containing 0.3 to 1.5 mass% are butted or overlapped with each other and welded to each other, the high carbon steel materials are formed by the welding joint after the welding joint. and the weld is heated to a temperature within a temperature range of 900 ° C. than Ac1 transformation point at 1 ° C. / s or higher heating rate, at said temperature, the heat treatment after that cool release Chi was held between 5~30s and facilities, and a method of welding high carbon steel material, wherein the hardness of the weld heat affected zone is HV400 or less. In addition to the above composition, it is 1% selected from Cr: 0.2-13.0%, Mo: 0.2-10.0%, W: 0.2-19.0%, V: 0.2-5.2%, Nb: 0.2-5.0% The high carbon steel material welding method according to claim 1, comprising seeds or two or more kinds.

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