JPH02197548A - Preharden steel having excellent specular finishing properties and machinability - Google Patents

Abstract

PURPOSE:To obtain the preharden steel having specified low hardness and excellent specular finishing properties and machinability by specifying the compsn. constituted of C, Si, Mn, P, S, Cr, Mo, V, Al and Fe. CONSTITUTION:The preharden steel contains 0.15 to 0.25% C, 0.05 to 0.80% Si, 0.50 to 2.00% Mn, <=0.020% P, <=0.003% S, 0.50 to 2.50% Cr, 0.05 to 1.00% Mo, 0.01 to 0.10% V, <=0.004% Al and <=0.0040% O, furthermore contains, at need, one or both of 0.0005 to 0.0030% B and 0.005 to 0.02% Ti and the balance Fe with inevitable impurities and furthermore has 25 to 32 HRC Rockwell hardness, which has excellent specular finishing properties and machinability. The steel can be obtd. by limiting S, Al, O, etc., to suppress the formation of nonmetallic inclusions and by limiting C to properly regulate the Rockwell hardness, thereby simultaneously improving the specular finishing properties and machinability.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a pre-hardened steel that is used for molds and the like and has excellent mirror workability and machinability.

(Prior technology) For transparent plastic products such as optical lenses and front covers of televisions, it is essential that the surface be scratchable and smooth. Requires polished skin.

However, pre-hardened steel used in general molds has
It contains a large amount of non-metallic inclusions such as AlzO*, which is a deoxidation product during steelmaking, and HnS, which imparts free machinability, and these may fall off or be dug up during polishing, resulting in pinholes. occurs.

In addition, if the hardness of the base material is lower than Rockwell hardness HRC35, the base material around the non-metallic inclusions will be scraped off during polishing, making it easier for the non-metallic inclusions to fall off, promoting the formation of pinholes. be done.

In order to solve this problem, we developed a steel for plastic molds (Japanese Patent Application Laid-open No. 63-16283) which aims to reduce non-metallic inclusions.
No. 7) etc.

Furthermore, a steel for pre-hardened molds for plastic molding (Japanese Patent Laid-open No. 183158/1983) has been proposed which has excellent machinability and mirror workability in which the hardness of the base material is increased to Rockwell hardness HRC40 or higher.

(Problems to be Solved by the Invention) However, the steel for plastic molds proposed in the above-mentioned Japanese Patent Application Laid-Open No. 63-162837 requires electroslag melting, electrobeam melting, etc. after vacuum melting the raw materials. Since it is manufactured using a special melting method that involves re-melting it using a melting method or the like, its manufacturing cost is high.

In addition, a steel for plastic molding pre-hardened molds with excellent machinability and good mirror finish properties, which has a hardness of the base material increased to Rockwell hardness HRC40 or higher, proposed in JP-A No. 63-183158, is available in large quantities. Because it is a precipitation hardening type containing Ni, Al, and Cu, it is expensive to manufacture, and its Rockwell hardness is extremely high at HRC43 or higher, so expensive cutting tools are required when processing it into molds. .

(Means for Solving the Problems) - The present invention is based on the inventors' intensive research in order to simultaneously improve mirror finishing and machinability in view of the above-mentioned problems with conventional pre-hardened steel for mirror finishing. As a result, S is 0.0
03% or less, ^l 0.004% or less, O 0.004
By adjusting C to 0% or less, the formation of nonmetallic inclusions can be suppressed without adopting any special melting method, and furthermore, C can be reduced to 0%.
．． Make the hardness of the base material 25% or less to Rockwell hardness HR
This invention was completed based on the knowledge that mirror workability and machinability could be improved at the same time by lowering C0.15 to 0.25% and Si to about 30%.
0.05-0.80%, Mn 0.50-2. OO%
, P 0.020% or less, S 0.003% or less, Cr
0.50-2.50%, Mo 0.05-1.00%
, VO0O1-0.10%, Al 0.004% or less,
00.0040% or less, with the remainder consisting of Fe and unavoidable impurities.

In addition to the conditions of the first invention, the second invention further includes B 0.0005 to 0.0030% and Ti 0.0%.
It contains one or two selected from 0.05 to 0.020%.

(Function) Hereinafter, the function of the present invention will be explained in detail, focusing on the knowledge obtained based on the inventors' experimental results.

As mentioned above, with regard to mirror workability, which is one of the characteristics that steel for molds should have, it is important to reduce non-metallic inclusions in the steel as much as possible, and in this regard, the present inventors have This will be explained by an experiment conducted.

First, 1.7%Cr-0,45%Mo-0,06%V-
Several types of steel slabs with varying amounts of Al and 0 added to a composition system of 0,002% S are hot rolled into steel plates with a thickness of 50 mm, and then quenched and tempered to achieve Rockwell hardness. Adjusted to HRC 25-32. Test pieces were taken from these steel plates, and the surface corresponding to the center of the plate thickness in a cross section perpendicular to the steel plate surface was polished to 11200 with vapor, and further,
After polishing to 112,000 using a diamond compound, the polished surface was observed with a microscope at a magnification of 400 times, and mirror workability was evaluated based on the area ratio of AhOz within the field of view. The results are shown in FIG.

FIG. 1 shows the effect of Al in steel on mirror workability of steel.

0, indicating the effect of quantity.

The rating for the amount of Altos in the steel in the figure is evaluated on a 5-level scale, ^lx0. The one with less I is O5 and the one with more I is 4. In addition, the mirror finish rating is 1 = excellent,
2 is good, 3 is acceptable (with pinhole), and 4 is unusable.

As is clear from FIG. 1, it has been found that a high degree of mirror workability can be obtained by suppressing the formation of Altos in the steel by reducing 81 and O in the steel.

Next, the present inventors investigated the influence of the amount of C on the end mill workability of pre-hardened steel. The test method will be explained below.

The test steel is 1.7%Cr-0.45%Mo-0.06%
v-0. o.

Several types of steel slabs with a composition system of 2% S and varying amounts of C were hot rolled into steel plates with a thickness of 50 IIff, and then quenched and tempered to achieve a Rockwell hardness of HRC 25 to 3.
It has been adjusted to 2.

Test pieces were taken from these steel plates and subjected to an end mill wear test. The end mill wear test was conducted using a two-blade end mill (material 5KH56, diameter 10++ @) as a tool, without using cutting oil, under the conditions of a depth of cut of 15 mm x 1 am, a rotation speed of 66 Orpm +, and a feed rate of 100 am/sin. End mill workability was determined by the cutting length until the flank wear width of the end mill reached 0.21.

The results are shown in FIG.

As is clear from FIG. 2, it was found that the end milling property decreased as Cl increased, and decreased rapidly when the C content exceeded 0.25%.

Furthermore, the present inventors investigated the influence of steel hardness on the drillability of pre-hardened steel. The test method is described below.

The test steel is 1.7%Cr-0.45%Mo-0.06%
v-0. o.

Several types of steel pieces with a composition of 2% S were hot rolled to a plate thickness of 50 mm.
The steel plate is finished into a steel plate with a thickness of 1.5 mm and then quenched and tempered to have a Rockwell hardness of HRC 25 to 40. Test pieces were taken from these steel plates and subjected to a drilling test.

The drilling test was conducted using a standard straight drill (material 5KH51, diameter 5 mm) as a tool, without using cutting oil, with a cutting length of 15 a+m penetration and a cutting speed of 15 m+*/
sin, rotation speed 955rp+w, feed 0.09va/r
It was carried out under ev conditions. Drillability was determined by the number of holes drilled through. The results are shown in FIG.

As is clear from Figure 3, the number of drilled holes decreases rapidly as the hardness of steel increases. It can be said that it is necessary to suppress the upper limit of hardness to below Rockwell hardness HRC32.

The reasons for limiting the individual chemical components will be explained below.

C is an element that has a large effect of improving hardenability and is inexpensive, and in the present invention, the Rockwell hardness HR
In order to ensure a hardness of C25 or higher, at least 0.
Requires addition of 15% or more.

However, as is clear from FIG. 2, if too much is added, machinability decreases, and furthermore, quench cracks are more likely to occur during hardening and weldability deteriorates, so the upper limit of the amount added is set at 0. It shall be 25%.

Si is an essential element during steel manufacturing as a deoxidizing element,
At least 0.05% is added. However, when adding too much, the toughness deteriorates, so the upper limit should be set at 0.
It shall be 80%.

Mn is 0.50% for deoxidation and improvement of hardenability.
It is necessary to add the above. However, when added in excess of 2.00%, the toughness deteriorates, so the upper limit is set at 2.00%.

P promotes segregation and causes a local increase in hardness inside the steel, deteriorating machinability, so the upper limit is set to 0.020.
%.

S is contained as an impurity in steel, but it impairs mirror workability and toughness, so it is desirable to reduce it as much as possible.
Therefore, in the present invention, the upper limit of S is set to 0.003%.

C" is an essential element for hardening to the inside of thick-walled materials, and it is necessary to add 0.50% or more. However, when added in excess, it may cause embrittlement due to the precipitation of Cr carbides. However, the upper limit of the amount added is set at 2.50% because it deteriorates the toughness and thus tends to cause damage to minute parts during precision processing.

Mo needs to be added in an amount of 0.05% or more in order to improve hardenability and temper softening resistance. However, since it is a very expensive element, from a practical standpoint, the upper limit is set at 1.00%.

V improves resistance to temper softening and also has the effect of improving toughness through grain refinement. In order to effectively exhibit such an effect, it is necessary to add at least 0.01%.

However, even if added in excess, the effect will be saturated and manufacturing costs will increase, so the upper limit of the amount added is set at 0.10%.

Al is usually added as a deoxidizing element, but as is clear from FIG. 1, in the present invention, AlA O1 remaining in the steel deteriorates mirror workability, so the upper limit is set to 0. 004%.

As is clear from FIG. 1, 0 forms oxide-based inclusions, leading to the formation of underground formations and deterioration of machinability, mirror workability, etc., so it is desirable to reduce it as much as possible, but on the other hand, Since an extreme reduction will result in a significant increase in cost in steel manufacturing, in the present invention, the allowable upper limit is set to 0.00.
It shall be 40%.

B improves hardenability when added in a small amount, but when the amount added is 0. When it is less than ooos%, the effect is poor, and on the other hand, when it is added in excess of 0.0030%, the hardenability deteriorates, so the upper limit is set at 0.0%.
030%.

Ti has the effect of improving toughness by making crystal grains finer, and in order to effectively obtain this effect, it is necessary to add at least 0.005%. However, when adding too much, TiN, which is a hard inclusion, is generated and the mirror workability is deteriorated, so the upper limit is set at 0.020.
%.

Next, the reason for limiting the hardness of steel will be explained.

Steel hardness is one of the important quality characteristics required for mold steel, and if the Rockwell hardness is less than HRC25,
It has low strength and poor wear resistance and cannot be used as a pre-hardened steel. On the other hand, as is clear from Fig. 3, when the hardness of the steel exceeds the Rockwell hardness HRC32, the machinability is improved as in the present invention. If the steel does not contain elements that improve the hardness, the machinability will be extremely low, so in order to ensure machinability that does not pose a practical problem in mold machining, the upper limit of the hardness of steel is set by Rockwell. Hardness: HRC32 (Example) Although the structure of the present invention is as described above, an example will be described below.

The test steel was produced by manufacturing steel slabs having the chemical composition shown in Table 1 in accordance with a conventional method and heat rolling them to a thickness of 50 mm.
It was finished to m+s, and then quenched and tempered.

Test pieces were taken from these steel plates, and mirror workability and machinability were investigated according to the test method described in the operational section. The results are shown in Table 1.

Table 1 summarizes the chemical composition, Rockwell hardness, mirror workability, and machinability of the invention steel and comparative steel.

All of the steels of the present invention have a Rockwell hardness of HRC25 to 32.
It is within the range of , and shows good properties in terms of both mirror finish and machinability.

On the other hand, none of the comparative steels could satisfy the three requirements of hardness, mirror workability, and machinability at the same time.

Comparative Steel 11 has a high content of S and 0, and therefore has poor mirror workability.

Comparative Steel 12 has a low C content and a low Rockwell hardness of HRC23, which causes non-metallic inclusions to fall off during polishing, promoting the formation of pinholes and deteriorating mirror workability. There is.

Comparative Steel 13 has high hardness and poor machinability because of its high C content, and also has poor mirror workability because of its high A1 content.

Comparative w414 has a high C content, so it has high hardness and poor machinability.

Compare! 1i115 has a high S content and Zr is added, so it has excellent machinability, but on the other hand, M
Mirror workability is poor due to nonmetallic inclusions of nS and ZrN.

Comparative Steel 16 has a high content of A1 and 0, and therefore has poor mirror workability, and also has poor machinability due to the hard Alz(h).

Comparative Steel 17 has a high content of ^l and 0, and also has high hardness, so it is unsatisfactory in both mirror workability and machinability.

As is clear from the results of the above examples, the pre-hardened steel according to the present invention has excellent mirror finish properties and machinability.

In addition, although the above example is about a thick steel plate,
It goes without saying that the present invention can also be applied to other steel products, such as forged steel products.

(Effects of the Invention) As explained above, since the pre-hardened steel according to the present invention has the above structure, it has excellent mirror workability and machinability without going through the special melting process.
Since the C content is kept low, it also has excellent effects on toughness and weldability.

[Brief explanation of the drawing]

Figure 1 shows the effect of A+g0 in steel on mirror workability of steel. FIG. 2 is a graph showing the effect of C amount on end mill workability, and FIG. 3 is a graph showing the effect of hardness on drill workability. Figure 1 Patent Applicant: Kobe Steel Binka Co., Ltd. Patent Attorney Akira Yomitsu - ← - Less Much - Rating of the amount of Al2O3 in steel Figure 2 Amount of C (%) Figure 3 Rockwell hardness (HRC)

Claims (2)

[Claims] (1) C0.15-0.25%, Si0.05-0.8
0%, Mn 0.50-2.00%, P 0.020% or less, S 0.003% or less, Cr 0.50-2.50%, M
o0.05~1.00%, V0.01~0.10%, A
Pre-hardened steel with excellent mirror workability and machinability, characterized by containing 0.004% or less of l, 0.0040% or less of 0, the remainder consisting of Fe and unavoidable impurities, and having a Rockwell hardness of HRC 25 to 32. steel. (2) C0.15-0.25%, Si0.05-0.8
0%, Mn 0.50-2.00%, P 0.020% or less, S 0.003% or less, Cr 0.50-2.50%, M
o0.05~1.00%, V0.01~0.10%, A
Contains 0.004% or less of l, 0.0040% or less of 0, and furthermore, 0.0005 to 0.0030% of B, and 0.00% of Ti.
Contains one or two selected from 5 to 0.020%,
A pre-hardened steel having excellent mirror workability and machinability, the balance being Fe and unavoidable impurities, and having a Rockwell hardness of HRC 25 to 32.