JPS6017061A - Free cutting steel and its production - Google Patents

Abstract

PURPOSE:To improve cold plastic workability by accelerating the flotation of alumina cruster in a molten steel by addition of Ca and converting a slight amt. of remaining S to spherical sulfide. CONSTITUTION:A carbon steel for machine structural purpose and an alloy steel for machine structural purpose are incorporated therein with, by weight %, >=0.0002 Ca, <=0.0015 O, <=0.25 Si, >=0.003atom% in total of C and N and >=0.003 atom% one or >=2 kinds among Nb, V, Ti and Ta. Said steels are further incorporated therein with <=0.01 1 or >=2 kinds among <=0.12 Pb, <=0.12 Bi and <=0.015 Te. A casting billet or steel ingot is produced by a continuous casting method or ingot making method from such molten steel and after the billet or ingot is heated to 1,150-1,350 deg.C, the billet or ingot is rolled to a slab. The slab is then rolled at 850-1,000 deg.C and a free cutting steel which has a pearlite strucand of which the crystal grain size number of ferrite is >=9 is produced.

Description

[Detailed description of the invention]

The present invention relates to a free-cutting steel that has excellent cold plastic workability and machinability and can be satisfactorily subjected to cold plastic working such as cold forging and cutting, and also relates to a method for producing the free-cutting steel. It is something. In recent years, industrial production of low-S steel has become possible with the development of outside-furnace refining technology, and low-S steel for cold plastic forming is being put into practical use.However, conventional low-SfM steels have poor machinability. There were drawbacks. Therefore, some of the present inventors first attempted to obtain a steel with excellent cold plastic workability and machinability by cold forging, etc. As a result of detailed investigation of the effects of element content and structure, we have developed a free-cutting steel with excellent cold plastic formability and machinability. This invention is a further improvement on this, and Ca
As a result of the discovery that the addition of alumina chloride in molten steel promotes flotation and separation, and that the small amount of S remaining in the steel is converted into spherical sulfide, cold plastic workability is further improved. This is what we have come to. That is, the free-cutting steel with excellent cold plastic workability and machinability according to the present invention has S≦0.009% by weight and Ca≧0.0 in carbon steel for machine structures and alloy steel for machine structures.
002% by weight,

[0]≦0.0015% by weight, St≦0
．． 25% by weight, a total of 0.003 atomic% or more of C and N,
and one or more of Nb, V, Ti, Ta in total of 0.003 atomic % or more, roughly pb≦0.1
It is characterized by containing at least 0.01% by weight in total of one or more of the following: double fluoride, Bi≦0.12% by weight, and Te≦0.015% by weight. In producing cut steel, it is more preferable to produce slabs or steel ingots from molten steel having the following components by a continuous casting method or an ingot making method, and to produce slabs or steel ingots with a
By standing or reheating the steel billet at a temperature of 850 to 1150 °C and then rolling it to a temperature of 750 to 1000 °C. The present invention is characterized by obtaining a steel having a ferrite + pearlite structure and having a ferrite grain size number of 9 or more. As the carbon steel for machine structures and the alloy steel for machine structures that are applied in this invention, S-C material is used. 5-GK material, SNC material, SN0M material, SCr material. 30M material, SMn material, SMnC material, SNB material. There are SACM materials, etc., and the range of basic component elements is, for example, C≦0908%, Mn≦1.65%, Ni
≦4.5%, Cr66%, MO≦0.65%, A text≦1
．． It is more preferable to set it to 2%, etc., and it can also be applied to this type of structural steel to which conventionally known alloying components are added. Next, in such carbon steel for machine structures and alloy steel for machine structures, the content of elements is determined as shown in the claims of this specification for the following reasons. S≦0.009% by weight S is an element that improves the machinability of steel, but 0.00%
If it exceeds 9% by weight, cold plastic workability such as cold forging will deteriorate significantly, so the content of S was set to 0.009% by weight or less, and it was decided that improvement in machinability due to S was not expected. Ca≧0.0002% by weight Ca is an element that promotes flotation of alumina clusters in steel and forms sulfides.
When added in an amount of 0.0002% by weight or more, it combines with a small amount of S to make the sulfide spheroidal and is effective in significantly improving cold plastic workability, so it is added in an amount of 0.0002% by weight or more. However, if it is contained in a large amount, it will impair the toughness and ductility of the steel, so the content is more preferably 0.1% by weight or less.

[0]≦0.0015% by weight 0 is an element that forms oxides in steel, O,0
If O exceeds 15'mJi%, the cold plastic workability deteriorates significantly, so the content of O is set to 0.0015% by weight or less. Si≦0.25% by weight St is an effective element for deoxidizing steel, but 0.25% by weight
If St
The content was set to 0.25% by weight or less. A total of 0.003 atomic % or more of C, N, and Nb, V
, Ti, Ta(7)') (7) Type 1 material is 2 g or more in total of 0.003 atomic % or more N b , V , Ti , Ta plays the role of grain refinement, and Combines with C and N to improve the toughness of steel. In order to effectively obtain such effects, it is necessary to contain a total of 0.003 atomic % or more in each element. Pb≦0.12% by weight, Bi≦0.12% by weight. Pb, Bi, and Te are all effective elements for improving the machinability of steel. In order to obtain such an effect, it is necessary to contain 0.01% by weight or more in total. However, if Pb exceeds 0.12% by weight, Bi exceeds 0.12% by weight, and Te exceeds 0.015% by weight, the cold plastic workability deteriorates significantly, so the respective ranges are set as above. In addition, carbonitrides necessary for the purpose of grain refinement are:
Sufficient production occurs within the above ranges of C, N and Nb, V, Ti, Ta, but for the purpose of deoxidation and crystal grain adjustment,
．． 0.06% by weight or less of AA. may be added. In addition to this purpose,
Soft nitrided steel with A content added up to 0.5% by weight, or 1% A content added.
．． The present invention can also be applied to nitrided steel (SACM) added at 2% by weight. The above-mentioned free-cutting steel with excellent cold plastic workability and machinability is produced by manufacturing a slab or steel ingot from molten steel by a continuous casting method or an ingot forming method according to a conventional method. The slab or steel ingot is manufactured by rolling the molten steel to a single piece and finishing rolling the steel slab, but more preferably, the slab or steel ingot is manufactured from the molten steel by a continuous casting method or an ingot making method, and the slab or steel ingot is is left or reheated at a temperature of 1150-1350°C and then rolled into a steel billet, and then the steel billet is heated to a temperature of 850-1350°C.
Steel having a ferrite + pearlite structure and a ferrite grain size number of 9 or more is obtained by standing or reheating at a temperature of 150 °C and then rolling at a finishing temperature of 750 to 1000 °C. . The reason why it is more desirable to employ such a manufacturing process is as follows. That is, 1150 to 135 when rolling slabs or steel ingots.
The temperature is set at 0°C by adding fine Nb, V, and
In order to precipitate carbonitrides of Ti and Ta, large amounts of N crystallized or precipitated during solidification and cooling of the slab or steel ingot are removed.
b, V, Ti. This is to once dissolve the Ta carbonitride in solid solution. In other words, if the temperature is lower than 1150°C, large carbonitrides will not form a solid solution, and if it is higher than 1350°C, austenite crystal grains will become large, and this effect will be carried over to the product, resulting in fine ferrite crystal grains. This is because it becomes impossible to obtain. Next, Nb obtained by the above rolling. 85 steel pieces with sufficient solid solution of carbonitrides of V, Ti, and Ta
When the temperature is 0 to 1150°C, carbonitrides, which are effective in refining austenite grains, precipitate. However, if the steel slab is heated to a temperature higher than 1150° C., the austenite crystal grains will become coarse, making it impossible to obtain fine ferrite crystal grains with a grain size number of 9 or more in the product, which is not preferable. Furthermore, if the steel slab is rolled at a temperature lower than 850° C., the deformation resistance of the rolled material will increase, making rolling difficult, which is not preferable. The finishing temperature in this rolling is 750 to 10
The reason for this is that the finishing temperature of rolling is 75°C.
This is because if it is lower than 0°C, austenite grains will not recrystallize and the ferrite grains obtained by transformation will not have a grain size number of 9 or higher.If it is higher than 1ooo°C, recrystallized austenite grains will rapidly I have grown up,
This is because the ferrite grains 1:i' obtained by transformation do not have a grain size number of 9 or more. The structure of the free-cutting steel obtained in this way is limited to a ferrite + pearlite structure, and when this rolled material is subjected to cold plastic processing such as cold forging as it is, the bainite structure has a ferrite + pearlite structure. This is because the hardness is greater than that of pearlite structure, and the life of the processing mold is shortened. In addition, the reason why the ferrite grain size number is limited to 9 or more is because when the rolled material is subjected to cold plastic processing such as cold forging as it is, if it is less than 9, the ductility is insufficient and cracks occur more frequently. It is. Examples will be described below. Example 1 Low alloy steel for mechanical structure (
After melting SCM), performing ladle refining and vacuum degassing, the obtained molten steel was continuously cast to produce slabs. Note that Pb and Bi are added to the molten steel. Te and Ca were added by adding a wire into the tundish immediately before casting. Then,
A round bar with a diameter of 34 mm was manufactured from the above slab by a normal rolling process. Next, test pieces were cut out from the surface layer of each of the round bars, and after polishing and corrosion, the ferrite crystal grain size was measured and the chemical composition was investigated. The results are shown in Table 1. Next, in order to evaluate cold forgeability, each round bar was
It was cut into lengths of mm and upset forged using a 600 ton press. In this forging, 20 test pieces were each processed at an upsetting rate of 75%, and the crack occurrence rate after processing was estimated at 1. The results are shown in Table 2. Subsequently, in order to evaluate the machinability by H/P, cutting speed: 50 to 200 mm/min, feed rate: 0, 05 to 0, 3 On+m/rev, depth of cut: 0. Forty combinations were made within the range of 5 to 2 mm, and chips were collected after processing for several seconds under each condition using a carbide tool. Then, the ratio (percentage) of the conditions for obtaining crushed chips with a length of 50+ nm or less to the total conditions was determined as the chip friability index. The results are also shown in Table 2. No. 1 No. 2 Table 14 As is clear from Tables 1 and 2,
, 1 to 5 are clearly superior in cold forgeability and machinability, but comparative steel No. 11-1.5
The results showed that although the workpiece 1!1 area was excellent, the cold forging property was not as good as the adjustable one. Firefly ffL? Machine structural carbon steel (S-C) was melted using a 70-ton electric arc furnace, and a slab was produced in the same manner as in Example 1. After that, it was rolled into a round bar, and the ferrite grain size was measured. At the same time, we investigated the chemical composition. The results are shown in Table 3. Next, cold forgeability and machinability were evaluated in the same manner as in Example 1. These results are shown in Table 4. As is clear from the results shown in Tables 3 and 4, the present invention #PINo, 6, and 7 have excellent cold forgeability, but the comparative steel It was confirmed that the cold forgeability of No. 16.17 was not so good. "Fire Engraving" Process: After melting low-alloy steel (SCr) for machine structures using a 701-year arc furnace, performing ladle refining and vacuum degassing, the resulting molten steel is continuously cast and cast. A piece was made. Note that addition of Pb, Bi, Te, and Ca (7) was performed in the same manner as in Example 1. Next, the slabs listed in - were rolled under the conditions shown in Table 5 to produce steel slabs, and then the slabs were rolled under the same conditions shown in Table 5 to obtain a steel slab with a diameter of 34 mra. A round bar was manufactured. Next, the ferrite crystal grain size of each specimen was measured in the same manner as in Example 1, and
The chemical composition was investigated. The results are shown in Table 6. moreover,
Cold forgeability (upset rate of 80% was added) and machinability were evaluated in the same manner as in Example 1. These results are shown in Table 7. Table 7 As shown in Tables 5 to 7, No. 10, which satisfies the first invention of the present invention, is No. 20j, which satisfies the first invention of the present invention, and No. 20j: Rimo, which has extremely excellent cold forgeability, but satisfies the second invention. No, which also satisfies
It was confirmed that Nos. 8 and 9 had even better cold forgeability. Note that Examples 1 to 3 in Section 1 show the case where slabs obtained by the continuous casting method were used, but the same results can be applied to steel ingots obtained by the ingot making method. I was able to get results. As described above, according to the present invention, in carbon steel for machine structures and alloy steels for machine structures, cold forming such as cold forging, 1. By examining the effects of content and structure in detail, we have regulated the chemical composition range of steel and, if desired, determined the rolling conditions for slabs or steel ingots, thereby improving cold plastic formability and machinability. This has the great effect of making it possible to obtain extremely superior free-cutting steel. Patent applicant: Daido Steel Co., Ltd. Representative patent attorney: Yutaka Oshio

Claims (1)

[Claims] (+) In carbon steel for m-plate structure and alloy steel for machine structure, S≦0.009% by weight, Ca≧0.0002 weight 1
%, [0]≦0.0015% by weight, Si≦0.25% by weight, C, N in total 0.003 atomic% or more, and Nb
,V,Tf. A total of 0.003 atomic % or more of one or more of Ta, roughly Pb≦0.12, Bi≦0.1
2% by weight, one or more of the following: Te≦0.015% by weight in total of 0.01% or more. Free-cutting steel. (2) S≦0.009% by weight, Ca≧o, oo. 2% by weight, [0]≦0.0015% by weight, St≦0.2
5% by weight, a total of 0.003 atomic % or more of C, N, and 0.003 atomic % or more of one or more of Nb, V, Ti, Ta, roughly Pb≦ 0.1
2fii1%, Bi≦0.12wt%, Te≦0.01
"One or more of 5% by weight": A slab or steel ingot is produced from the molten steel of carbon steel for machine structures and alloy steel for machine structures by a continuous casting method or an ingot-making method, and Alternatively, the steel ingot is heated to a temperature of 1150 to 1350°C and then rolled to a slab, and then the steel slab is heated to a temperature of 850 to 1150°C.
After rolling to a temperature of °C, the finishing temperature is 750 to 1000 °C.
A method for producing free-cutting steel with excellent cold plastic workability and machinability, characterized by obtaining steel having a ferrite + pearlite structure and a ferrite grain size number of 9 or more by rolling the steel. .