JP2004277818A - Free cutting steel for metal mold for molding plastic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for a metal mold for molding plastics having excellent machinability even in the case where machining is applied in a prehardened condition and also to provide steel for a metal mold for molding plastics having superior mirror finish properties in addition to excellent machinability with respect to an article requiring mirror finish properties. <P>SOLUTION: The steel for the metal mold for molding plastics has a composition consisting, by weight, of 0.03 to 0.3% C, >1 to 3% Si, 0.05 to 3% Mn, >0.01 to 0.3% S, 0.1 to 5% Cu, 1.0 to 5% Ni, 0.02 to <3% Cr, >0.5 to 2.5% Al and the balance essentially Fe. Moreover, with respect to the article requiring mirror finish properties in addition to machinability, practically no S is added. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００５３】
【表２】

【００５４】
（イ）溶解
真空誘導炉，大気誘導炉によりインゴット２００ｋｇを製造した。
（比較鋼Ｎｏ．１，６，発明鋼Ｎｏ．１，１３については大気誘導炉で製造）
【００５５】
（ロ）鍛造
１１５０℃加熱保持後、断面９３×３６に鍛造した。
【００５６】
（ハ）試験片粗加工
シャルピー試験片：Ｌ方向（ＪＩＳ Ｚ ２２０２：３号試験片）
被削性評価試験片（７０×３０×２００ｍｍ）
鏡面性評価試験片（４５×５０×１０ｍｍ）
溶接性評価試験片（１５０×２００×１５ｍｍ）
【００５７】
（ニ）熱処理
▲１▼焼入れ又は溶体化処理（ＳＴ）：８５０〜１０３０℃×３０〜９０ｍｉｎ保持後、衝風冷却した。
▲２▼焼戻し又は時効処理（ＡＧ）：２００〜６８０℃×３０ｍｉｎ〜５ｈｒ保持後、空冷した。
【００５８】
［ＩＩ．］試験及び測定条件
（ホ）シャルピー試験（靭性評価）
常温で試験を実施した。評価は試験片数３本の平均値とした。
試験片硬さは表３，表４に記載した硬さに調整した。
【００５９】
（ヘ）被削性評価試験
以下の条件で試験を行った。
・超硬エンドミル：ＵＴｉ２０Ｔを使用
・工具径φ２０ｍｍ，スローアウェー，ダウンカット，１枚刃
・深さ５ｍｍ，切込幅１．５ｍｍ
・切削速度３００ｍ／ｍｉｎ，送り速度０．０２５ｍｍ／刃，乾式
評価は比較鋼１で工具摩耗量が３００μｍに達したときの切削距離を２００、比較鋼４で工具摩耗量が３００μｍに達したときの切削距離を１００とし、発明鋼その他比較鋼の切削距離を相対比較評価した。
【００６０】
（ト）鏡面性評価試験
機械研磨により＃１５０→＃４００→＃８００→＃１５００→＃３０００→＃８０００で鏡面研磨加工した。
そして任意に選んだ部位５箇所について１５ｍｍ測長で最大面粗さＲｙ（μｍ）を測定し、それぞれの最大面粗さの平均値で評価した。
【００６１】
（チ）溶接性評価試験
ｙ形溶接割れ試験方法（ＪＩＳ Ｚ ３１５８）により実施した。
試験片温度は２５℃とし、
表面割れ率Ｃｆ（＝Σｌｆ／Ｌ×１００：％）により評価した。
但し Σｌｆ：表面割れの合計長さ（ｍｍ）
Ｌ：試験ビードの長さ（ｍｍ）
【００６２】
（リ）介在物長さ
鏡面性評価試験片の鏡面加工面を光学顕微鏡で観察した（倍率２００倍，視野数２０）。
そして全視野の中で観察された介在物の最大長（μｍ）とする。
【００６３】
これらの試験及び測定の結果が表３，表４に示してある。
表１，表２及び表３，表４において、発明鋼Ｎｏ．１〜９はＳを０．０１超〜０．３％の範囲内で添加し且つ硬さ（ＨＲＣ）を約４０に調整した例を比較鋼Ｎｏ．１〜３とともに示したものであり、また発明鋼Ｎｏ．１０〜１６は実質的にＳを添加せず（Ｓ：≦０．０１％）且つ硬さ（ＨＲＣ）を発明鋼Ｎｏ．１〜９と同じく約４０に調整した例を比較鋼Ｎｏ．４，５とともに示したものである。
【００６４】
更に発明鋼Ｎｏ．１７〜２３はＳを０．０１超〜０．３％の範囲内で添加し且つ硬さ（ＨＲＣ）を約３０に調整した例を比較鋼Ｎｏ．６，７とともに示したもので、発明鋼Ｎｏ．２４〜３１は実質的にＳを添加せず（Ｓ：≦０．０１％）且つ硬さ（ＨＲＣ）を約３０に調整した例を比較鋼Ｎｏ．８とともに示したものである。
【００６５】
【表３】

【００６６】
【表４】

【００６７】
表３，表４の結果に見られるように、本発明に従ってＳｉを１超〜３％の範囲内で添加し、更にＣｕを０．１〜５％の範囲内で添加し、加えてＳを０．０１超〜０．３％の範囲内で添加した発明鋼Ｎｏ．１〜９，１７〜２３は特に優れた被削性を示しており、併せて靭性，溶接性も良好な値を示している。
【００６８】
一方実質的にＳを含有させていない発明鋼Ｎｏ．１０〜１６及び２４〜３１は優れた鏡面性の値を示している。
またこれら発明鋼Ｎｏ．１０〜１６及び２４〜３１は実質的にＳを含有させていないにも拘わらずＳｉ，Ｃｕの含有によって被削性においても優れた値を示している。
【００６９】
＜実施例２＞
［Ｉ．］試験片の作製
表５に示す化学組成の各種のプラスチック成形用の金型用鋼を以下の条件で溶解（一部については再溶解），圧延，熱処理，試験片精加工を行い、被削性評価試験片及び鏡面性評価試験片を作製した。
【００７０】
【表５】

【００７１】
（イ）溶解
大気誘導炉によりインゴット３３００ｋｇを製造した。
（ロ）再溶解
真空アーク溶解を実施し、インゴットを製造した（３０００ｋｇ）。
（ハ）圧延
１２００℃加熱保持後、断面５０×１２０に鍛造した。
【００７２】
（ニ）熱処理
▲１▼焼入れ又は溶体化処理（ＳＴ）：８５０〜１０３０℃×２０〜９０ｍｉｎ保持後、衝風冷却した。
▲２▼焼戻し又は時効処理（ＡＧ）：２００〜６８０℃×３０ｍｉｎ〜５ｈｒ保持後、空冷した。
【００７３】
（ホ）試験片精加工
被削性評価試験片（５０×５０×２００ｍｍ）
鏡面性評価試験片（４５×５０×１０ｍｍ）
にそれぞれ精加工した。
【００７４】
［ＩＩ．］試験及び測定条件
（ヘ）被削性評価試験
以下の条件で試験を行った。
・ハイスドリル：ＳＫＨ５１を使用
・工具径φ１０ｍｍ，コーティングなし（ホモ処理）
・深さ２０ｍｍ（非貫通）の穴開け加工
・切削速度４０〜１００ｍ／ｍｉｎ，送り速度０．１５ｍｍ／刃，湿式
評価は工具溶損，折損に到ったときの切削距離を工具寿命と定義して評価した。
また比較鋼９において工具寿命が３０００ｍｍとなる切削速度を２００、比較鋼１０において工具寿命が３０００ｍｍとなる切削速度を１００とし、発明鋼その他比較鋼の工具寿命３０００ｍｍとなる切削速度を相対比較評価した。
【００７５】
（ト）鏡面性評価試験
機械研磨により＃１５０→＃４００→＃８００→＃１５００→＃３０００→＃８０００で鏡面研磨加工した。
そして任意に選んだ部位５箇所について１５ｍｍ測長で最大面粗さＲｙ（μｍ）を測定し、それぞれの最大面粗さの平均値で評価した。
【００７６】
表５に見られるように、本発明に従う発明鋼は鏡面性，被削性等の特性において優れているとともに、特に再溶解を実施したものについては、再溶解を実施していないものに対して鏡面性を示す値が更に高くなっていることが分る。
【００７７】
以上本発明の実施例を詳述したがこれはあくまで一例示であり、本発明はその趣旨を逸脱しない範囲において種々変更を加えた態様で実施可能である。
【００７８】
【発明の効果】
以上のように本発明によれば、一般的に難加工となるプレハードン状態で金型加工を行う場合においても容易に切削加工を行うことができ、金型製造のための期間の短縮化，加工費用の低減を達成することができ、またＳを添加しないものについては併せて高い鏡面研磨性を確保することができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a mold steel for plastic molding, and more particularly to a free-cutting mold steel.
[0002]
Problems to be solved by the prior art and the invention
Conventionally, molds for plastic molding have been widely used in various fields.
For example, molds for molding resin products such as daily goods, home appliance exterior / interior / parts, OA equipment exterior / interior / parts, mobile phone parts, automobile interior parts and light covers, optical lenses, food containers , Medical equipment, cosmetic containers, molds for molding precision products, IC sealing molds, molds for molding optical discs (CDs and DVDs), molds for forming light guide plates, molds for forming reflectors, and the like. is there.
[0003]
By the way, in recent years, higher quality molds have been required for plastic molding dies in order to meet demands for higher precision, higher precision, designability, shorter delivery time and the like of products.
Mainly for high accuracy, a material (pre-hardened steel: HRC 28 to 45) in which a mold is heat-treated in advance and adjusted to a predetermined hardness is often used.
[0004]
This prehardened steel is generally difficult to machine.
Among the pre-hardened steels, those having a relatively high hardness of HRC 35 to 45 are particularly difficult to cut, so that S is added as a free-cutting element and MnS inclusions are generated in the steel to enhance machinability. This is often done.
[0005]
However, in the method of improving machinability by generating MnS inclusions by adding S, the mirror polishing property and toughness at the time of manufacturing a mold are reduced.
Among plastic products, those which are directly visible to the human eye are required to have good appearance such as smoothness and glossiness of the product surface.
Therefore, a mold for molding such a plastic product is required to have high specularity, but the one in which S is added and MnS inclusions are formed in steel as described above has poor mirror polishability, Also, the toughness is reduced.
[0006]
Therefore, for a mold in which mirror polishing and toughness are important, cutting is performed without intentionally adding a free-cutting element such as S. In this case, processing is naturally difficult.
If processing is difficult, the productivity of the mold decreases, and the manufacturing cost of the mold increases accordingly.
[0007]
On the other hand, among the pre-hardened steels, those having a relatively low hardness of HRC 28 to 35 can be said to be relatively easy to cut because the hardness is lower than that of the HRC 35 to 45 described above.
As a material for the hardness region, an SC or SCM material is often used.
Since these materials are usually heat-treated by quenching and tempering, when a large-sized mold is manufactured, the hardness of the central portion cannot be sufficiently secured.
[0008]
Also, in this hardness region, although hardness is low and cutting is relatively easy, there are many large dies, and in this case, excellent cutting workability is required. In such a case, a free-cutting element such as S may be added.
Thus, if a free-cutting element such as S is added, it is inevitable that the mirror polishing property and toughness are reduced.
[0009]
In addition, as a well-known technique close to the present invention, there is the following.
Patent Literature 1 and Patent Literature 2 below disclose a plastic molded prehardened steel having excellent machinability by using a Zr compound.
In Patent Documents 1 and 2, the machinability is obtained by a Zr compound, and is not intended to improve the machinability by positively adding Si as in the present invention.
The role of Si in Patent Documents 1 and 2 is added to improve oxidation resistance and hardenability, and it is described that if the content exceeds 1.5%, the machinability decreases. There is no description of improvement in machinability by addition).
In Patent Documents 1 and 2, it is necessary that the machinability by the Zr compound is sufficiently exhibited, and the viewpoint is different from the improvement of machinability by Si in the present invention.
[0010]
Patent Literature 3, Patent Literature 4, Patent Literature 5, and Patent Literature 6 below disclose steel for plastic molding dies excellent in toughness, mirror finish, machinability and the like.
In any of these, Si utilizes the effect as a deoxidizing agent and is limited to the addition of 1% or less, which is different from the present invention.
[0011]
[Patent Document 1]
JP-A-60-174854
[Patent Document 2]
JP-A-62-167863
[Patent Document 3]
JP-A-11-335775
[Patent Document 4]
JP-A-2002-309341
[Patent Document 5]
JP 2001-152246 A
[Patent Document 6]
JP-A-5-70888
[0012]
[Means for Solving the Problems]
The free-cutting mold steel for plastic molding of the present invention has been devised to solve such problems.
According to the first aspect, C: 0.03 to 0.3%, Si: more than 3%, Mn: 0.05 to 3%, S: more than 0.01 to 0% by weight. 0.3%, Cu: 0.1 to 5%, Ni: 1.0 to 5%, Cr: 0.02 to less than 3%, Al: more than 0.5 to 2.5%, the balance being substantially Fe It is characterized by being.
[0013]
In claim 2, C: 0.03 to 0.3%, Si: more than 3%, Mn: 0.05 to 3%, S: ≦ 0.01%, Cu: 0 by weight%. 0.1 to 5%, Ni: 1.0 to 5%, Cr: 0.02 to less than 3%, Al: more than 0.5 to 2.5%, the balance being substantially Fe and present in steel The maximum length of the inclusion represented by MnS is 50 μm or less.
[0014]
According to a third aspect, in any one of the first and second aspects, Mo: ≦ 3%, W: ≦ 0.5%, V: ≦ 0.5%, Co: ≦ 0.5% by weight%. , Nb: ≤ 0.3%, Ta: ≤ 0.3%, B: ≤ 0.01%, Ca: ≤ 0.1%, Mg: ≤ 0.1%, Pb: ≤ 0.3%, P : Characterized by further containing one or more of ≦ 0.2%.
[0015]
According to a fourth aspect of the present invention, in any one of the first to third aspects, the unavoidable content of N and O is% by weight, and N: ≦ 0.0150% and O: ≦ 0.0150%. Features.
[0016]
A fifth aspect of the present invention is characterized in that in any one of the first to fourth aspects, the pre-hardened state has a hardness of HRC 28 to 45.
[0017]
[Action]
According to the first aspect of the present invention, Si and Cu are added together with Cr and Al to achieve free cutting without impairing the mirror polishing property.
The addition of S as a free-cutting element causes MnS inclusions formed in the steel to be scraped off or fall off from the machined surface during mirror polishing, causing irregularities to occur there, and the specularity is significantly deteriorated.
However, Si is a solid solution element and usually does not become inclusions or carbides, a uniform matrix can be obtained even in a microscopic view, and the addition of Si does not particularly deteriorate the mirror polishing property.
[0018]
On the other hand, the added Si reacts with oxygen during cutting to form an oxide film, and reduces the friction between the chips and the cutting tool to protect the tool. By this effect, it functions to effectively enhance machinability in manufacturing a mold.
In the present invention, in order to sufficiently bring out the function of Si, a large amount of more than 1% is contained in steel.
[0019]
On the other hand, Cu is an element that contributes to age hardening, and by including a large amount of Cu, the machinability in manufacturing a mold can be improved.
Cu is finely dispersed and precipitated from the matrix by aging heat treatment, and its size is very fine (1 μm or less in diameter), so that it does not adversely affect the mirror polishing property.
On the other hand, Cu slightly lowers the toughness of the matrix due to precipitation, and serves to enhance the machinability by making it slightly brittle.
[0020]
However, in claim 1, S is contained in an amount of more than 0.01% and 0.3% or less together with these Si and Cu.
Some plastic products are not particularly required to clean the surface skin, and a mold for molding such a plastic product is not particularly required to have specularity.
Therefore, in such a case, it is a good measure to actively add S to enhance the machinability. In this way, the manufacture of the mold can be made easier, and the cost for the manufacture of the mold can be reduced.
[0021]
Therefore, in the first aspect of the present invention, S is contained in a predetermined range particularly for those which are not required to have high specularity.
However, even in that case, in the first aspect of the present invention, since the machinability is enhanced by the addition of Si and Cu, the required amount of S can be reduced.
[0022]
On the other hand, a second aspect of the present invention does not substantially add S (S: ≦ 0.01%) and regulates the maximum length of the inclusion represented by MnS to 50 μm or less. According to No. 2, both excellent machinability by Si and Cu and excellent mirror polishing can be realized.
[0023]
Common free-cutting elements (S, Se, Te, etc.) combine with Mn to form inclusions such as sulfides.
These inclusions are dispersed in the steel and have a size (maximum length of 10 to 100 μm) that can be sufficiently observed with an optical microscope. You may not be able to obtain the expected mirror surface due to being dug into the ground.
[0024]
Therefore, it is indispensable to reduce the free-cutting elements in order to improve the mirror polishing property.
Therefore, when mirror polishing is important, it is desirable to reduce S and the like as free-cutting elements as much as possible.
[0025]
In the case of these mold steels according to claims 1 and 2, it is possible to secure hardness by age hardening as compared with ordinary SC-based and SCM-based steels. Even if it is, it is possible to secure uniform hardness up to the center.
[0026]
At that time, the desired hardness can be adjusted by adjusting the amounts of Ni, Cu, Al, C, Mn, and Cr.
Further, the age hardened steel is superior in machinability to ordinary SC-based and SCM-based steels, and more excellent machinability can be obtained by using the free-cutting method of the present invention.
[0027]
In Claims 1 and 2, any one or more of Mo, W, V, Co, Nb, Ta, B, Ca, Mg, Pb, and P are contained in the above-described amounts as necessary. (Claim 3).
[0028]
Next, claim 4 regulates each of N and O unavoidably contained to 0.0150% or less.
O and N are easily associated with the additional elements in the steel, and generate unexpected oxides and nitrides. These inclusions fall off during mirror polishing and cause pinholes. Since the occurrence of pinholes is the one that most deteriorates the mirror polishing property, it is necessary to reduce these inclusions as much as cost permits.
[0029]
Since the steel of the present invention contains a large amount of Si and Al, it is necessary to reduce the amounts of O and N as much as possible.
Specifically, the amount of O and N is reduced as much as possible in the first melting (by sufficient refining in the atmosphere or changing to vacuum melting, etc.), and if the manufacturing cost permits, the first melting material is used as a raw material. The second melting (vacuum arc melting or electroslag melting) is performed to reduce the amounts of O and N.
The application of the secondary dissolution is also effective in reducing segregation and the like, and also has the effect of improving the mirror polishing property.
[0030]
According to a fifth aspect of the present invention, the steel for the mold for plastic molding has a hardness of HRC 28 to 45 in a pre-hardened state.
In a state where the pre-hardened steel is adjusted to HRC 28 to 45, high mirror polishing can be obtained (the harder the steel, the higher the mirror finish). This hardness is a level that can be sufficiently cut.
If it is too hard, free cutting properties will be lost, and if it is too soft, the mirror polishing properties will be impaired.
[0031]
The mold steel for plastic molding of the present invention is finally subjected to a surface treatment to further improve the surface roughness, hardness, corrosion resistance and the like.
In the case where such a surface treatment is performed, the mold steel for plastic molding of the present invention, particularly, the steel not containing S (S: ≦ 0.01%), has uneven color due to the surface treatment. Etc. can be suppressed.
[0032]
Here, as the surface treatment, for example, formation of a hard film by nitriding, Cr plating, Ni plating, CVD, PVD, TRD (thermal reaction deposition / diffusion), PCVD (plasma CVD), or graining with an acid or base solution ( Light corrosion of the surface).
The surface treatment mainly for the formation of the hard film is carried out to improve the wear resistance and the mirror polishing of the mold surface, while the surface treatment for the light corrosion of the surface is carried out to obtain the intended design. Is done.
[0033]
In this case, in the case of the mold steel for plastic molding of the present invention, Si or Cu is dissolved in the steel or is finely dispersed in the steel. Unevenness, thickness unevenness, etc.) do not occur.
When a free-cutting element (S, etc.) is added, the surroundings of inclusions such as sulfides present in the matrix are selectively corroded, and the thickness of the surface treatment varies, resulting in unevenness in the micro region. turn into. Therefore, from the viewpoint of surface treatment, it is desirable that free-cutting elements are not added.
[0034]
Next, the reasons for limiting each chemical component in the present invention will be described in detail below.
C: 0.03 to 0.3%
C is necessary to ensure the hardness used as tool steel, and it is necessary to add 0.03% or more.
However, excessive addition hardens the hardness more than necessary and also reduces the machinability, so the upper limit is 0.3%.
[0035]
Si: More than 1 to 3%
Si is an element used as a deoxidizing agent. The effect of improving machinability can be obtained by adding more than 1% of Si.
On the other hand, if the content is too large, the weldability is reduced, so the upper limit is 3%.
[0036]
Mn: 0.05-3%
Mn is an austenitizing element, and must be added in an amount of 0.05% or more to obtain the required hardness.
On the other hand, if it is too large, the hardness becomes more than necessary, so the upper limit is 3%.
[0037]
S: more than 0.01 to 0.3% or ≦ 0.01%
When S is added as a free-cutting element, it combines with Mn to form inclusions, and the presence of the inclusions improves machinability.
However, there arises a problem that the mirror polishing property (for example, surface roughness) is lowered and the toughness is lowered.
The addition of S is determined by the balance between the machinability and the mirror polishing property, and when the mirror polishing property is particularly required, the S content is set to 0.01% or less.
On the other hand, when high mirror polishing is not required but rather machinability is required, it is contained in the range of more than 0.01 to 0.3%.
[0038]
Cu: 0.1-5%
Addition of Cu has an effect of improving machinability and also has an effect of improving hardness by aging heat treatment, and is added by 0.1% or more. From the viewpoint of improving machinability, it is desirable to add 0.5% or more.
On the other hand, excessive addition impairs hot workability, so the upper limit is 5%.
[0039]
Ni: 1.0 to 5%
Ni has the effect of improving hardness by aging heat treatment by being added simultaneously with Al. To obtain this effect, 1.0% or more must be added.
However, excessive addition reduces the machinability, so the upper limit is 5%.
[0040]
Cr: less than 0.02 to 3%
Cr is an element that contributes to improvement in hardness and forms carbide. When Mn is not added in an amount of 0.5% or more, it is necessary to add a predetermined amount instead of Mn in order to secure hardness. However, the addition of 0.02% or more is necessary in order to contribute to the improvement of the hardness. On the other hand, excessive addition increases the hardness more than necessary and lowers the machinability, so that the addition is set to less than 3%.
[0041]
Al: more than 0.5 to 2.5%
Al is an element having a deoxidizing effect. Also, by adding simultaneously with Ni, improvement in hardness is obtained, and in order to obtain this effect, it is necessary to add more than 0.5%.
On the other hand, an excessive addition causes a decrease in toughness and a decrease in mirror polishing, so the upper limit is 2.5%.
[0042]
Mo: ≤ 3%, W: ≤ 0.5%, V: ≤ 0.5%, Co: ≤ 0.5%, Nb: ≤ 0.3%, Ta: ≤ 0.3%, B: ≤ 0 .01%
This has the effect of preventing crystal grains from being coarsened by the formation of carbides and improving the strength of crystal grain boundaries, and can be positively added for the purpose of improving toughness.
On the other hand, if the amount is too large, the machinability decreases.
Further, even when added in combination, each element exerts a unique effect, and therefore a combination thereof can be selected according to the purpose.
[0043]
The same effect as these elements can be obtained even when Ti: ≤ 0.03%, Zr: ≤ 0.03%, Be: ≤ 0.5%, and H: ≤ 0.01%.
[0044]
It is desirable to reduce Mo as much as possible from the viewpoint of improving the weldability.
Similarly, since other elements also affect the weldability, it is desirable to reduce any of the elements from the viewpoint of improving the weldability.
[0045]
Ca: ≦ 0.1%, Mg: ≦ 0.1%, Pb: ≦ 0.3%
Any of these elements can be added for the purpose of improving machinability.
Since Ca and Mg form sulfides and oxides and form a protective film for the cutting tool during cutting, the wear amount of the cutting tool can be reduced.
Pb is dispersed in the steel and improves the machinability by a lubricating effect that reduces the coefficient of friction between the cutting tool and the chip during cutting.
However, excessive addition of any of the elements has a greater effect of deteriorating the toughness of the steel than an effect of improving machinability, and thus the upper limits are specified as described above.
[0046]
The same machinability-improving effect as the elements listed here can also be obtained by adding Bi: ≦ 0.3%, Sn: ≦ 0.05%, and As: ≦ 0.3%.
[0047]
P: ≦ 0.2%
P is mixed in from the scrap raw material and inevitably exists in the steel.
Generally, P exists at the crystal grain boundary and has an effect of reducing the toughness of steel. Since the steel becomes brittle due to the decrease in toughness, the friability of chips and the like are improved and the machinability is improved. However, if the amount is too large, the toughness is greatly reduced.
Usually, since the machinability is sufficiently ensured by other elements, it is desirable to reduce P, and it is desirable to reduce the P to 0.05% or less in consideration of the manufacturing cost.
[0048]
[Other impurity elements]
Steel may contain Se, Te, Zn, Y, or a rare earth element (REM) as an impurity element entering from scrap or the like.
Se and Te play a role as a substitute element for S, and are effective in improving machinability. Zn, Y, and rare earth elements fix impurities such as O and P, increase the cleanliness of the matrix, and improve toughness. Has the effect of improving.
Therefore, in the present invention, these elements are contained in a range of Se: ≦ 0.3%, Te: ≦ 0.3%, Zn: ≦ 0.1%, Y: ≦ 0.05%, REM: ≦ 0.05%. Tolerate.
[0049]
N: ≦ 0.0150%, O ≦ 0.0150%
N and O, which are inevitably contained in steel, combine with Si, Ti, Zr, V, Al, etc. contained in steel to form nitrides and oxides.
These nitrides and oxides are hard, and tend to increase in number and size as the amount of N or O increases. Since these reduce the mirror polishing property, as well as the machinability and toughness, it is desirable to suppress them as low as possible.
However, it is preferable to set N: ≦ 0.01% and O: ≦ 0.005 in consideration of the manufacturing cost.
[0050]
[The maximum length of inclusions represented by MnS in steel is 50 μm or less]
It is general that inclusions formed by free-cutting elements such as S are as small as possible and smaller in size from the viewpoint of mirror polishing.
In particular, in mold steel requiring high mirror polishing, it is desirable to limit the maximum length of inclusions represented by MnS to 50 μm or less (a size invisible for plastic products).
For this purpose, secondary melting (vacuum arc melting, electroslag melting) is performed in the manufacturing process to reduce the number of inclusions, and the size of the inclusions is increased by minimizing the size of the mold and increasing the solidification rate. In some cases, it is necessary to make it into the manufacturing process, for example, to reduce the size.
[0051]
【Example】
Next, examples of the present invention will be described in detail below.
<Example 1>
[I. ] Preparation of test pieces
Under the following conditions, various types of plastic mold steels having the chemical compositions shown in Tables 1 and 2 were melted, and then forged, roughed, heat-treated, and refined. Each test piece was prepared for a machinability evaluation test, a mirror surface (polishing) evaluation test, and a weldability evaluation test.
[0052]
[Table 1]

[0053]
[Table 2]

[0054]
(B) Dissolution
200 kg of ingot was manufactured by a vacuum induction furnace and an air induction furnace.
(Comparative steel Nos. 1, 6 and invention steels Nos. 1 and 13 are manufactured in an air induction furnace.)
[0055]
(B) Forging
After heating and holding at 1150 ° C., it was forged to a cross section of 93 × 36.
[0056]
(C) Rough processing of test pieces
Charpy test piece: L direction (JIS Z 2202: No. 3 test piece)
Machinability evaluation test piece (70 × 30 × 200 mm)
Specularity evaluation test piece (45 × 50 × 10 mm)
Test piece for evaluating weldability (150 x 200 x 15 mm)
[0057]
(D) Heat treatment
{Circle around (1)} Quenching or solution treatment (ST): After holding at 850 to 1030 ° C. for 30 to 90 minutes, it was cooled by blast.
{Circle around (2)} Tempering or aging treatment (AG): After holding at 200 to 680 ° C. × 30 min to 5 hr, air-cooled.
[0058]
[II. ] Test and measurement conditions
(E) Charpy test (evaluation of toughness)
The test was performed at room temperature. The evaluation was an average value of three test pieces.
The test piece hardness was adjusted to the hardness described in Tables 3 and 4.
[0059]
(F) Machinability evaluation test
The test was performed under the following conditions.
・ Carbide end mill: UTi20T is used
・ Tool diameter φ20mm, throw away, down cut, single blade
・ Depth 5mm, cut width 1.5mm
・ Cutting speed 300m / min, feed speed 0.025mm / tooth, dry type
In the evaluation, the cutting distance when the tool wear amount reached 300 μm in Comparative Steel 1 was 200, and the cutting distance when the tool wear amount in Comparative Steel 4 reached 300 μm was 100. Relative comparative evaluation was performed.
[0060]
(G) Specularity evaluation test
Mirror polishing was performed in the order of # 150 → # 400 → # 800 → # 1500 → # 3000 → # 8000 by mechanical polishing.
Then, maximum surface roughness Ry (μm) was measured at 15 mm length measurement at five arbitrarily selected portions, and the average value of each maximum surface roughness was evaluated.
[0061]
(H) Weldability evaluation test
The test was performed according to a y-type weld crack test method (JIS Z 3158).
The specimen temperature is 25 ° C,
It was evaluated by the surface cracking rate Cf (= Δlf / L × 100:%).
Where Σlf: total length of surface cracks (mm)
L: Length of test bead (mm)
[0062]
(I) Inclusion length
The mirror-finished surface of the specularity evaluation test specimen was observed with an optical microscope (magnification: 200, field of view: 20).
Then, the maximum length (μm) of the inclusions observed in the entire visual field is set.
[0063]
Tables 3 and 4 show the results of these tests and measurements.
In Tables 1 and 2 and Tables 3 and 4, invention steel No. Comparative Steel Nos. 1 to 9 are examples in which S was added in a range of more than 0.01 to 0.3% and the hardness (HRC) was adjusted to about 40. No. 1 to No. 1 and the invention steel No. Nos. 10 to 16 do not substantially contain S (S: ≦ 0.01%) and have a hardness (HRC) of Invention Steel No. Comparative steel Nos. 1 to 9 were adjusted to about 40. It is shown together with 4,5.
[0064]
In addition, invention steel No. Comparative steel Nos. 17 to 23 are examples in which S was added in a range of more than 0.01 to 0.3% and hardness (HRC) was adjusted to about 30. 6 and 7 are shown together with Invention Steel No. Comparative steel Nos. 24 to 31 are examples in which S was not substantially added (S: ≦ 0.01%) and hardness (HRC) was adjusted to about 30. 8 are shown together.
[0065]
[Table 3]

[0066]
[Table 4]

[0067]
As can be seen from the results in Tables 3 and 4, according to the present invention, Si was added in the range of more than 1 to 3%, Cu was added in the range of 0.1 to 5%, and S was added. Invention steel No. added in the range of more than 0.01 to 0.3%. Nos. 1 to 9, and 17 to 23 show particularly excellent machinability, and also show good values for toughness and weldability.
[0068]
On the other hand, inventive steel No. 10 to 16 and 24 to 31 show excellent specularity values.
In addition, these invention steel Nos. Nos. 10 to 16 and 24 to 31 show excellent values in machinability due to the inclusion of Si and Cu even though S is not substantially contained.
[0069]
<Example 2>
[I. ] Preparation of test pieces
Various types of plastic mold steels having the chemical compositions shown in Table 5 were melted (partially re-melted), rolled, heat-treated, and refined under the following conditions. A specularity evaluation test piece was prepared.
[0070]
[Table 5]

[0071]
(B) Dissolution
3300 kg of ingots were produced by an air induction furnace.
(B) Remelting
Vacuum arc melting was performed to produce an ingot (3000 kg).
(C) Rolling
After heating at 1200 ° C., forging was performed to a cross section of 50 × 120.
[0072]
(D) Heat treatment
{Circle around (1)} Quenching or solution treatment (ST): After holding at 850 to 1030 ° C. for 20 to 90 minutes, the product was cooled by blast.
{Circle around (2)} Tempering or aging treatment (AG): After holding at 200 to 680 ° C. × 30 min to 5 hr, air-cooled.
[0073]
(E) Fine processing of test pieces
Machinability evaluation test piece (50 × 50 × 200 mm)
Specularity evaluation test piece (45 × 50 × 10 mm)
Each was finely processed.
[0074]
[II. ] Test and measurement conditions
(F) Machinability evaluation test
The test was performed under the following conditions.
・ High speed drill: Use SKH51
・ Tool diameter φ10mm, no coating (homo treatment)
・ Drilling 20mm (non-penetrating) depth
・ Cutting speed 40 ~ 100m / min, feed rate 0.15mm / blade, wet type
The evaluation was performed by defining the cutting distance at the time of tool melting and breakage as tool life.
The cutting speed at which the tool life of the comparative steel 9 was 3000 mm was 200, the cutting speed at which the tool life of the comparative steel 10 was 3000 mm was 100, and the cutting speed at which the tool life of the invention steel and the comparative steel was 3000 mm was comparatively evaluated. .
[0075]
(G) Specularity evaluation test
Mirror polishing was performed in the order of # 150 → # 400 → # 800 → # 1500 → # 3000 → # 8000 by mechanical polishing.
Then, maximum surface roughness Ry (μm) was measured at 15 mm length measurement at five arbitrarily selected portions, and the average value of each maximum surface roughness was evaluated.
[0076]
As can be seen from Table 5, the inventive steel according to the present invention is excellent in properties such as specularity, machinability, etc. It can be seen that the value indicating the specularity is even higher.
[0077]
Although the embodiment of the present invention has been described in detail, this is merely an example, and the present invention can be implemented in variously modified forms without departing from the spirit thereof.
[0078]
【The invention's effect】
As described above, according to the present invention, it is possible to easily perform cutting even when performing die processing in a pre-hardened state, which is generally difficult, and to shorten the period for manufacturing a die and to perform processing. Cost reduction can be achieved, and high mirror polishing properties can be ensured for those without addition of S.

Claims (5)

In weight percent,
C: 0.03 to 0.3%
Si: More than 1 to 3%
Mn: 0.05-3%
S: more than 0.01 to 0.3%
Cu: 0.1-5%
Ni: 1.0 to 5%
Cr: less than 0.02 to 3% Al: more than 0.5 to 2.5%
A free-cutting mold steel for plastic molding, the balance being substantially Fe. In weight percent,
C: 0.03 to 0.3%
Si: More than 1 to 3%
Mn: 0.05-3%
S: ≦ 0.01%
Cu: 0.1-5%
Ni: 1.0 to 5%
Cr: less than 0.02 to 3% Al: more than 0.5 to 2.5%
The balance is substantially Fe, and the maximum length of inclusions typified by MnS present in the steel is 50 μm or less, for a free-cutting mold for plastics molding excellent in mirror polishing properties. steel. In any one of Claims 1 and 2,
Mo: ≦ 3%
W: ≦ 0.5%
V: ≦ 0.5%
Co: ≦ 0.5%
Nb: ≦ 0.3%
Ta: ≦ 0.3%
B: ≦ 0.01%
Ca: ≦ 0.1%
Mg: ≤0.1%
Pb: ≦ 0.3%
P: ≦ 0.2%
A free-cutting mold steel for plastic molding, characterized by further containing one or more of the following. In any one of claims 1 to 3, N and O unavoidably contained are in% by weight,
N: ≦ 0.0150%
O: ≦ 0.0150%
A free-cutting mold steel for plastic molding, characterized in that: The free-cutting mold steel for plastic molding according to any one of claims 1 to 4, having a hardness of HRC 28 to 45 in a pre-hardened state.