JP2013082992A - Method for producing steel material excellent in toughness for die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing steel material for a die, composed of martensitic stainless steel and excellent in toughness.SOLUTION: The method for producing steel material for a die is provided in which a martensitic stainless steel raw material having component composition containing by mass%, 0.3-0.5% C and 12.0 to 16.0% Cr, is hot-worked and successively, the hot-worked stainless steel raw material is annealed, wherein the hot working is carried out in such a manner that: the working ratio calculated using an expression of [(the thickness of the stainless steel raw material before hot working - the thickness of the stainless steel raw material after hot-working)/the thickness of the stainless steel raw material before subjected to the hot working]×100, is ≥55%; and the hot-worked stainless steel raw material is cooled to the temperature of the Ms point or lower and then successively, the stainless steel raw material is heated to the annealing temperature and subjected to the above annealing. In another embodiment, a method for producing steel material for a die is provided in which after the stainless steel raw material is annealed, the stainless steel raw material is hardened and tempered, and thus pre-hardened steel material is obtained.

Description

  The present invention relates to a method for producing a steel material for a mold having excellent toughness that can be used for various molds.

  In general, steel for molds is manufactured by hot working a material obtained by casting to adjust the shape, followed by annealing. Then, the annealed material is machined into a mold shape and quenched and tempered to produce a mold adjusted to a predetermined hardness. Conventionally, high C-high Cr martensitic stainless steel represented by JIS-SUS420J2 and its improved steel materials have been used for mold steel materials that require corrosion resistance and polishability such as plastic molding (patents) Literatures 1-3). Since the steel for a mold having these component compositions has a high hardness of 50 HRC by quenching and tempering, it is excellent in wear resistance during use. However, in the high hardness state after quenching and tempering, machining into a mold shape is not easy, and thus, generally, it is provided in the above-described annealed material state having a low hardness of less than 30 HRC. Is machined into a mold shape and then tempered to the above-mentioned hardness.

  Some of these mold steel materials are provided in a pre-hardened state that has been quenched and tempered in advance. The pre-hardened steel for molds is machined into a mold shape in the quenched and tempered state, so that the problem of heat treatment deformation caused by quenching and tempering can be solved. For example, what is provided by quenching and tempering the above-mentioned annealed material to about 30 HRC has lower machinability (tool life) during machining than that of the annealed material, but machining itself is not difficult. The provided steel for the mold is machined, and is tempered and tempered again to a working hardness of about 50 HRC. Alternatively, an annealed material that is quenched and tempered to about 40 HRC and provided in a pre-hardened state can be machined even at this hardness, so it can be machined and used as it is It is.

Japanese Patent Laid-Open No. 04-002745 Japanese Patent Laid-Open No. 03-097829 JP 2007-27739 A

  The component composition of martensitic stainless steel proposed in Patent Documents 1 to 3 is suitable for obtaining a high-hardness mold. However, since the component composition of these steel materials for molds contains a large amount of C, coarse carbides are likely to be distributed in the structure by the annealing after the hot working described above. And this coarse carbide | carbonized_material is hard to erase | eliminate by subsequent quenching and tempering. As a result, if there is a lot of this coarse carbide in the structure of the mold completed through machining, the toughness of the mold deteriorates, which may lead to early damage of the mold such as cracking during use. there were.

  An object of the present invention is to provide a manufacturing method capable of improving the toughness in providing a steel material for a mold made of martensitic stainless steel having a component composition proposed in SUS420J2, Patent Documents 1 to 3, and the like. is there.

  The inventor investigated the cause of the phenomenon that coarse carbides are distributed in the steel material after annealing. As a result, it has been found that this phenomenon occurs due to the temperature history until the transition to the next annealing on the structure of the material after hot working. Therefore, when this temperature history was mutually reviewed together with the conditions of the hot working as the previous process, if the condition management of the hot working and the temperature history of the material after the hot working were appropriately performed, It has been found that generation of coarse carbides can be suppressed, and the present invention has been achieved.

  That is, the present invention hot-processes a martensitic stainless steel material having a composition containing C: 0.3 to 0.5% and Cr: 12.0 to 16.0% by mass%, A method of manufacturing a steel for a mold for annealing the hot-worked stainless steel material, wherein the hot-working is [(thickness of stainless steel material before hot-working-stainless steel after hot-working. (Thickness of material) / thickness of stainless steel material before hot working] × 100 The processing rate calculated by the formula of 55 is equal to or higher than 55%, and the stainless steel material after hot working is at a temperature below Ms point. It is the manufacturing method of the steel material for metal mold | die excellent in the toughness characterized by heating to the annealing temperature after cooling to this, and annealing. Preferably, the annealing temperature is 600 ° C. or higher and / or 1000 ° C. or lower. Alternatively, the stainless steel material used for hot working is preferably produced by a remelting method.

  Moreover, this invention is the manufacturing method of the steel material for metal mold | die characterized by making the steel material for metal mold | die of a hardened state by quenching and tempering after the said annealing.

  According to the manufacturing method of the present invention, it is possible to improve the toughness of the steel for molds having the component composition proposed in SUS420J2, Patent Documents 1 to 3, and the like, and particularly a manufacturing method useful for providing a large mold. It is.

It is a drawing substitute photograph showing the metal microstructure after annealing which shows an example of the steel material for metal molds manufactured by the method of the present invention and the conventional method. It is a figure which shows an example of the influence of the processing rate at the time of hot working on the toughness of the steel material for metal mold | die manufactured by this invention method and the comparison method.

  The feature of the present invention is that the conventional temperature history, which has been passed until the material after hot working is annealed, is reviewed together with the conditions during the hot working so that optimum hot working and temperature management can be performed. A method has been proposed. Below, each component of this invention is demonstrated.

(1) Hot working a martensitic stainless steel material having a component composition containing C: 0.3 to 0.5% and Cr: 12.0 to 16.0% by mass%, followed by the heat It is the manufacturing method of the steel material for metal mold | die which anneals the said stainless steel raw material processed between.
As described above, an object of the present invention is to improve the toughness of the steel for molds having the component composition proposed in SUS420J2, Patent Documents 1 to 3, and the like. And as above-mentioned, this steel material for metal mold | dies is manufactured by carrying out the hot processing of the martensitic stainless steel raw material of this component composition, and subsequently annealing according to a normal method. Hereinafter, the component composition of the material (that is, the component composition of the steel material for molds) will be described (hereinafter, “mass%” is simply referred to as “%”).

・ C: 0.3-0.5%
C is an element necessary for improving the hardenability and further obtaining a sufficient quenching and tempering hardness of 50 HRC or more. However, if it is too much, it becomes easy for cracking to occur after quenching. Therefore, in the present invention, the content is set to 0.3 to 0.5%. Preferably, it is 0.33% or more and / or 0.45% or less.

・ Cr: 12.0 to 16.0%
Cr is an element necessary for improving hardenability and obtaining high quenching and tempering hardness. Furthermore, Cr is an important element that enhances the corrosion resistance of the steel for molds. However, if the amount is too large, the thermal conductivity is significantly reduced. When the thermal conductivity is high, the time required for the heat cycle of heating and cooling during use as a mold can be shortened, which is preferable. Therefore, in the present invention, the content is 12.0 to 16.0%. Preferably, it is 12.5% or more and / or 15.0% or less.

A preferable composition range including the other elements as a component composition of the steel for molds according to the production method of the present invention is as follows.
C: 0.3 to 0.5% (preferably 0.33% or more and / or 0.45% or less)
Si: 1.0% or less (preferably 0.2% or more and / or 0.7% or less)
Mn: 1.0% or less (preferably 0.2% or more and / or 0.7% or less)
P: 0.05% or less (preferably 0.03% or less)
S: 0.01% or less (preferably 0.005% or less)
Ni: No addition to 1.0% (preferably 0.5% or less)
Cr: 12.0 to 16.0% (preferably 12.5% or more and / or 15.0% or less)
One or two of Mo and W: 0 to 1.5% in the formula of (Mo + 1 / 2W) (preferably 0.1% or more and / or 1.0% or less)
Fe: Substantially the balance (for example, including the balance Fe and inevitable impurities)

Si has an effect of improving machinability, but if it is too much, it is an element that extremely lowers the thermal conductivity.
Mn has an effect of enhancing hardenability and suppressing the formation of ferrite, but if it is too much, it is an element that lowers machinability because it extremely increases the base viscosity.
P is an element that reduces hot workability and toughness when too large.
When S is too much, it is an element that decreases hot workability and corrosion resistance, and further promotes toughness anisotropy.
Ni has an effect of improving hardenability and improving corrosion resistance, but if it is too much, Ni is an element that decreases thermal conductivity.
Mo and W have an effect of increasing the hardness at the time of tempering, but if too much, they cannot be completely dissolved in the base at the time of quenching, and conversely, the tempering hardness is lowered.

(2) The hot working is expressed by the formula [(thickness of stainless steel material before hot working−thickness of stainless steel material after hot working) / thickness of stainless steel material before hot working] × 100. The calculated machining rate is 55% or more.
When hot working the stainless steel material according to the present invention, the material is usually heated to a temperature in the austenite single-phase region in order to improve the workability and homogenize the components by solid solution of carbide. From hot working. And for the present invention aiming at the refinement of carbides in the structure, it is effective that the crystal grains in the structure when the hot working is completed are fine. That is, after the cooling process described later, the austenite grain boundary in the structure becomes a carbide precipitation site, so by making the crystal grains before cooling fine, this precipitation site increases, and after annealing The carbide can be made fine.

Therefore, in the present invention, it is important that the processing rate when the above stainless steel material is hot-worked is 55% or more as calculated by the following formula. That is, by setting the processing rate to 55% or more, a dynamic recrystallization phenomenon in which accumulation of processing strain and recrystallization is repeated progresses in the structure during hot processing, and the crystal grains are sufficient. Therefore, the number of carbide precipitation sites can be effectively increased. The processing rate is preferably 60% or more, and more preferably 65% or more. Further, keeping the crystal grains fine also has an effect of improving mechanical properties such as toughness of the steel material for the mold by itself acting to reduce the fracture structural unit.
Processing rate (%) = [(thickness of stainless steel material before hot working−thickness of stainless steel material after hot working) / thickness of stainless steel material before hot working] × 100

(3) After the hot-worked stainless steel material is cooled to a temperature below the Ms point, it is subsequently heated to the annealing temperature and annealed.
If the material after the above hot working is left as it is, the temperature is lowered from the working end temperature. At this time, since the material passes through a temperature range in which carbides precipitate in the cooling process after processing, fine carbides are likely to precipitate particularly on the austenite grain boundaries. In actual operation, in order to improve efficiency by omitting a new heating step, the material during the cooling has been inserted into the annealing furnace. Therefore, conventionally, the material after hot working in which the fine carbides are deposited is heated to the annealing temperature again in a state where it is not sufficiently cooled, specifically around 300 to 500 ° C. for a long time. It was exposed to the annealing environment. And this inventor discovered that said fine carbide | carbonized_material grew to the coarse carbide | carbonized_material so that the toughness of steel for metal mold | dies might be reduced by this temperature history.

  Therefore, the present inventor has studied a method capable of suppressing the growth of the carbide in the next annealing step even if the fine carbide is deposited on the material after hot working. As a result, the material with increased carbide precipitation sites after the above-mentioned hot working is cooled to a temperature below the Ms point without transitioning to annealing during cooling, and the material is martensitic textured before annealing. It has been found that the coarsening of the carbide can be suppressed by reheating to a temperature. In other words, the martensitic transformation further increases the number of sites in the structure where carbides can precipitate in the next annealing step. As a result, the amount of carbon and chromium that can diffuse into the grain boundaries during annealing is reduced, and growth is suppressed even if carbides are present at the grain boundaries. In addition, the temperature which cools the raw material after hot processing is so preferable that it is lower than Ms point. The temperature is preferably (Ms point −30 ° C.) or lower, more preferably (Ms point −50 ° C.) or lower. Specifically, the lower the temperature in the order of 150 ° C. or lower, 100 ° C. or lower, and 50 ° C. or lower, the more preferable. However, if it is too low, there is a possibility of causing fire cracks. Therefore, the temperature is preferably 0 ° C. or higher. The cooling rate is not particularly required as long as the stainless steel material according to the present invention can be martensitic. And about this, it is enough if the quick cooling rate more than air cooling is securable, for example.

(4) Preferably, the said annealing temperature is 600 degreeC or more and / or 1000 degrees C or less.
When machining in an annealed state, annealing after hot working has the effect of reducing hardness and improving workability. Moreover, it has the effect of suppressing cracks and bends that occur in subsequent processes. In addition, in stainless steels with the above component composition, the “grain boundary pinning effect”, which is manifested by the uniform precipitation of Cr carbide in the structure, suppresses grain coarsening during subsequent quenching. And has an effect of suppressing a decrease in toughness. In order to sufficiently obtain these effects, particularly the pinning effect, the annealing temperature is preferably 600 ° C. or higher. However, if the annealing temperature is too high, precipitation of the above Cr carbide becomes difficult, so 1000 ° C. or less is preferable. More preferably, it is 650 degreeC or more and / or 950 degreeC or less. The annealing may be repeated twice or more, for example, with cooling to room temperature or lower.

  And it is more preferable that said annealing temperature shall be more than A3 point. For example, it is 750 ° C. or higher. When annealing is performed at the point A3 or higher, new austenite grains are formed in the structure, and the sites where carbides can precipitate increase accordingly. As a result, carbide growth can be further suppressed by the same action as described above. Moreover, the increase in the precipitation site | part of a carbide | carbonized_material will also increase the number density of the carbide | carbonized_material in a structure | tissue. Many of these carbides exhibit the above-mentioned “pinning effect” during subsequent quenching and act on the refinement of crystal grains. When repeating annealing more than once, it is preferable to carry out at least once at a temperature of A3 point or higher.

(5) Preferably, the stainless steel material subjected to hot working is obtained by a remelting method.
In the case of the stainless steel produced by the present invention, it is preferable to uniformly precipitate Cr carbide during the annealing. Therefore, it is preferable that the steel ingot as the starting material has component segregation reduced as much as possible. Further, when the stainless steel is used particularly for a mold for plastic molding, it is preferable to reduce as much as possible non-metallic inclusions such as Al ₂ O ₃ which adversely affect the polishability of the steel material. From the above, it is preferable that the steel material used for hot working is obtained by a consumable electrode type remelting method such as an electroslag remelting method or a vacuum arc remelting method.

(6) Preferably, after annealing, quenching and tempering to obtain a pre-hardened steel for molds.
As described above, the steel for molds after annealing can be provided in a pre-hardened state that is appropriately quenched and tempered to a required hardness, for example, a hardness of 25 to 45 HRC. Then, if importance is attached to the machinability (tool life) during machining, it is preferable to quench and temper to about 25 to 35 HRC. In addition, after the machining, if re-quenching and tempering is not performed, it is preferable to quench and temper to about 35 to 45 HRC in consideration of use hardness.

  An ingot of martensitic stainless steel having chemical components shown in Table 1 was prepared by vacuum arc remelting. These steel ingots have an Ms point of about 200 ° C. and an A3 point of about 800 ° C. Next, these steel ingots are heated to 1100 to 1200 ° C. and subjected to hot working with a working rate of about 58% according to the above formula, then cooled according to the conditions in Table 2, and then once or twice. Annealing was performed. At this time, the cooling after hot working was air cooling, and the cooling temperature was controlled by the surface temperature of the material. Moreover, annealing was performed by furnace-cooling the raw material hold | maintained for the predetermined time in the heating furnace. These annealed materials were subjected to normal quenching from 1030 ° C. and 350 ° C. applied to quenching and tempering of 50HRC prehardened, and the hardness and toughness after quenching and tempering were evaluated. For evaluation of toughness, 10R Charpy impact test specimens were collected and the impact value at room temperature was measured.

  Sample No. manufactured by the method of the present invention was used. Since the steel materials for molds 1 to 5 were cooled to room temperature after hot working, the microstructure after the subsequent annealing had fine carbides on the austenite grain boundaries (in FIG. (The metal microstructure (* 400 times) after annealing of a sample is shown). And sample No. manufactured by the method of the present invention. The subsequent quenching and tempering characteristics of the steel materials for molds 1 to 5 were as follows: Sample No. The impact value more than double that of No. 6 steel for molds was shown.

  An ingot of martensitic stainless steel having chemical components shown in Table 3 was prepared by a vacuum arc remelting method. These steel ingots have an Ms point of about 200 ° C. and an A3 point of about 800 ° C. Next, after heating these steel ingots to 1100 to 1200 ° C. and performing hot working with the processing rate according to the above formula as shown in Table 4, until the surface temperature of the material reaches room temperature of 25 ° C. Cooling was followed by two annealings, 780 ° C. for the first time and 860 ° C. for the second time. At this time, the procedure of cooling and annealing after hot working was in accordance with that of Example 1. Then, these annealed materials were quenched from 1030 ° C. and tempered at 350 ° C. to adjust the hardness to about 50 HRC, and toughness was evaluated. For evaluation of toughness, 10R Charpy impact test specimens were collected and the impact value at room temperature was measured.

Sample No. The steel materials for molds 11 to 17 have almost the same component composition, but their toughness at a hardness of about 50 HRC improved as the processing rate during hot working increased (the processing rate is shown in FIG. 2). And the impact value). And in addition to having implemented cooling after hot working to room temperature, Sample No. 5 with a working rate during hot working increased to 55% or more. The steel materials for molds 11 to 16 exhibited impact values exceeding 60 J / cm ² .

Claims (5)

A martensitic stainless steel material having a component composition containing C: 0.3 to 0.5% and Cr: 12.0 to 16.0% by mass% was hot-worked, followed by the hot-working. A method for manufacturing a steel material for a mold for annealing the stainless steel material,
The hot working is calculated by the formula [(thickness of stainless steel material before hot working−thickness of stainless steel material after hot working) / thickness of stainless steel material before hot working] × 100. The processing rate is 55% or more, and
A method for producing a steel material for a mold having excellent toughness, wherein the stainless steel material after hot working is cooled to a temperature below the Ms point, and then is annealed by heating to an annealing temperature.   The said annealing temperature is 600 degreeC or more, The manufacturing method of the steel material for metal mold | die excellent in toughness of Claim 1 characterized by the above-mentioned.   The said annealing temperature is 1000 degrees C or less, The manufacturing method of the steel material for molds excellent in toughness of Claim 1 or 2 characterized by the above-mentioned.   The method for producing a steel material for a mold having excellent toughness according to any one of claims 1 to 3, wherein the stainless steel material is obtained by a remelting method.   The method for producing a steel material for a mold having excellent toughness according to any one of claims 1 to 4, wherein the steel material for a mold in a hardened state is quenched and tempered after the annealing.