JP5907415B2 - Hot work tool steel with excellent toughness - Google Patents

Description

  The present invention relates to a hot work tool steel with improved toughness that is optimal for various hot tools such as a press die, a forging die, a die casting die, and an extrusion tool.

  Since a hot tool is used while being in contact with a high-temperature work material or a hard work material, it must have both strength and toughness that can withstand thermal fatigue and impact. Therefore, conventionally, for example, SKD61-based alloy tool steel, which is a JIS steel type, has been used as a steel type used in the field of hot tools (hereinafter referred to as hot tool steel). And the method of improving the toughness of the hot tool steel by reviewing the addition amount of the main elements constituting the hot tool steel and further regulating and controlling various impurities such as As, Bi, Sn, Zn, Sb Has been proposed (see Patent Document 1). However, adjusting each of the various impurity elements within the specified range can lead to an increase in manufacturing cost.

  On the other hand, the present inventor has conducted intensive investigations on elements that have not been actively added as alloys in the field of steel materials, regardless of the addition of expensive and special elements. It has been found that the toughness can be greatly improved by adding Zn in a predetermined content range (see Patent Document 2). That is, hot work tool steel containing, in mass%, C: 0.3 to less than 0.55%, Si: 1.5% or less, Mn: 1.5% or less, Cr: 3.00 to 5.65% And it is hot tool steel of Zn: 0.001-0.015%.

JP 2003-155540 A JP 2007-224418 A

  The Zn-added technique proposed in Patent Document 2 is effective as a new technique for improving the toughness of hot tool steel. And by using the technique of patent document 2, the scrap of Zn-plated steel can be utilized as a recycling raw material, and it is suitable also for reduction of an environmental load. The inventor examined the possibility of compensating for the deterioration of toughness due to other impurity elements, paying attention to the effect of improving toughness by this positive Zn addition. If the permissible amount of these impurity elements can be increased appropriately, the amount of energy used for removing impurities can be reduced while increasing the usage rate of low scrap with a large impurity content, which is expected to increase in the future. It is possible to further reduce the environmental load in the manufacturing process of hot tool steel.

  An object of the present invention is to provide a hot work tool steel excellent in toughness capable of reducing an environmental load.

  The inventor examined the influence of impurity elements contained in hot tool steel on toughness and the environment. As a result, in particular, P (phosphorus) is an element that greatly reduces the toughness of hot tool steel, and the energy required for removal is large, and for these reasons, the use of low-grade scrap is also stagnant. The knowledge that is a large element was obtained. Therefore, in order to reduce this environmental load, a method that can maintain sufficient toughness even if the allowable amount of P is increased was examined. As a result, it has been found that the deterioration of toughness due to the increase in the P content can be compensated for by adding an appropriate amount of Zn to the P content. The present invention has been achieved by clarifying the specific relationship between P and Zn that can sufficiently utilize this toughness complementary effect.

  That is, the present invention includes, in mass%, C: 0.3 to less than 0.6%, Si: 1.5% or less, Mn: 1.5% or less, and Cr: 3.0 to less than 6.0%. Hot tool steel with excellent toughness characterized by Zn: more than 0.0025 to 0.025%, P: 0.005% or more, and Zn / P: more than 0.5 Hot tool steel. Preferably, P: 0.01% or more. Further, if necessary, Mo and W may be contained alone or in combination (Mo + 1 / 2W): 3.5% or less, or V: 1.5% or less.

  Specifically, by mass%, C: 0.3 to less than 0.6%, Si: 1.5% or less, Mn: 1.5% or less, Ni: 1.5% or less (including 0%) Cr: 3.0 to less than 6.0%, Mo and W are used alone or in combination (Mo + 1 / 2W): 3.5% or less, V: 1.5% or less, Nb: 0.3% or less (0 Co: 5.0% or less (including 0%), Zn: more than 0.0025 to 0.025%, P: 0.005% or more, and Zn / P: more than 0.5 And it is the hot work tool steel excellent in toughness which consists of remainder Fe and an unavoidable impurity. Preferably, P: 0.01% or more.

  According to the present invention, since sufficient toughness can be maintained without managing P contained in hot tool steel as an impurity to an extremely low value, energy consumption for lowering P can be saved and the environment can be saved. Can reduce the load. And the toughness of hot tool steel can be drastically improved, and it becomes an effective technique for the practical application of hot tool steel that can be applied to various applications and environments.

In Example 1, it is the figure which plotted the 2 mmU notch Charpy impact value in room temperature of this invention steel and comparative steel tempered to various hardness with respect to the hardness of a test piece. In Example 1, it is the figure which plotted the 2 mmV notch Charpy impact value between room temperature and 400 degreeC of this invention steel and comparative steel which were tempered to the hardness of 45HRC with respect to test temperature. In Example 2, it is the figure which plotted the 2 mmU notch Charpy impact value in room temperature of this invention steel and comparative steel tempered to various hardness with respect to the hardness of a test piece. In Example 2, it is the figure which plotted the 2 mmV notch Charpy impact value between room temperature to 400 degreeC of this invention steel and comparative steel tempered to the hardness of 45HRC with respect to test temperature.

  The feature of the present invention resides in that Zn which has been conventionally treated as an impurity is positively added in order to improve the toughness of the hot work tool steel. And P which is an element which greatly reduces toughness is in a place where its inclusion is allowed in a specific range by clarifying the relationship with the amount of added Zn of the present invention. In other words, for hot tool steel, it has been found that if Zn is used as an alloy element, the effect of improving toughness can be exhibited even if the P content increases. In the present invention, a large amount of P, which has conventionally been required to be extremely reduced, can be tolerated. Therefore, in the selection of raw materials, the amount of high-grade low P scrap used can be reduced, which is suitable for scrap recycling. . Furthermore, the energy and time required for removing P in the refining process can be reduced. Hereinafter, the reason for limiting the components of the steel of the present invention will be described (mass% is simply expressed as%).

-C: Less than 0.3-0.6% C is a part of the solid solution in the base to give strength, and part of it forms carbides to improve wear resistance and seizure resistance, heat It is an essential element that is important for intertool steel. Further, when C, which is a solid interstitial atom, is co-added with a substitution atom having a high affinity with C, such as Cr, the I (interstitial atom) -S (substitution atom) effect; solute atom dragging It acts as a resistance and is also expected to increase the strength of steel. However, excessive addition causes a decrease in toughness and hot strength. Therefore, it is made 0.3 to less than 0.6%. Preferably it is less than 0.55%.

Si: 1.5% or less Si is a deoxidizing agent during steel making and an element that enhances the machinability of the material. In order to obtain these effects, addition of less than 0.2% may be performed, but addition of 0.2% or more is preferable. However, if it is too much, it will lead to the formation of ferrite, so 1.5% or less.

-Mn: 1.5% or less Mn has the effect of improving hardenability, suppressing the formation of ferrite, and obtaining appropriate quenching and tempering hardness. Moreover, since it exists as MnS of a nonmetallic inclusion, there is a great effect in improving machinability. In order to obtain these effects, addition of less than 0.1% may be performed, but addition of 0.1% or more is preferable. However, if the amount is too large, the viscosity of the base is increased and the machinability is lowered.

Cr: 3.0 to less than 6.0% Cr is an element that enhances hardenability and forms carbides and has an effect on strengthening the base and improving wear resistance. And it is an essential element for the hot work tool steel of the present invention which contributes to improvement of temper softening resistance and high temperature strength. However, excessive addition causes a decrease in hardenability and high temperature strength. Therefore, it is set as 3.0 to less than 6.0%. Preferably it is 5.65% or less.

Zn: more than 0.0025 to 0.025%
Zn is the most important additive element for the present invention, and the toughness is remarkably improved by adding Zn. And this effect can fully be acquired by adding exceeding 0.0025%. Preferably it is 0.003% or more. On the other hand, even if a large amount is added, the effect reaches a peak. Furthermore, if excessive segregation occurs at the grain boundary or the like due to excessive addition, this can be a factor that deteriorates toughness. Further, the addition technique becomes complicated, so the upper limit was made 0.025%. Preferably it is 0.020% or less, More preferably, it is 0.015% or less.

P: 0.005% or more P is an element that segregates at the prior austenite grain boundaries during the heat treatment such as tempering and embrittles the grain boundaries. Therefore, in order to improve the toughness of hot tool steel, it is an impurity element that has been usually managed as low as possible. However, in the present invention, the toughness degradation effect due to P can be compensated for by making maximum use of the above-described toughness improving effect by the addition of Zn. And the remarkable effect of Zn addition required for this can be acquired by adjustment of Zn addition amount with respect to P content mentioned later. Thereby, the hot tool steel of the present invention can tolerate a P content of 0.005% or more. Even if it is preferably 0.01% or more, more preferably 0.02% or more, sufficient toughness can be maintained.

-Zn / P: More than 0.5 Even when the hot tool steel of the present invention contains 0.005% or more of P, it is necessary to ensure a Zn addition amount sufficient to maintain sufficient toughness. Therefore, it is necessary to adjust the Zn addition amount with respect to the P content. Specifically, sufficient toughness can be ensured by setting the value of Zn / P to more than 0.5. Preferably it is more than Zn / P: 0.55. A Zn / P value exceeding 0.55 is a preferable condition even when 0.01% or more of P, further 0.02% or more of P is contained.

-Preferably, Mo and W are single or composite (Mo + 1 / 2W): 3.5% or less Mo and W are used for precipitating or agglomerating fine carbides by tempering to give strength and improving softening resistance. They can be added alone or in combination. The amount of addition at this time can be defined together with the Mo equivalent of (Mo + 1 / 2W) since W is about twice the atomic weight of Mo (of course, either one or both may be added) Can be added together). And in order to acquire an above-described effect, although addition of less than 1.0% may be sufficient as the value of (Mo + 1 / 2W), addition of 1.0% or more is preferable. However, since it will lead to a decrease in machinability and toughness if it is too much, the value of (Mo + 1 / 2W) is preferably 3.5% or less.

-Preferably, V: 1.5% or less V has the effect of forming carbides and improving the reinforcement of the base and wear resistance. In addition, the temper softening resistance is increased and the coarsening of crystal grains is suppressed, thereby contributing to the improvement of toughness. In order to obtain these effects, addition of less than 0.5% may be performed, but addition of 0.5% or more is preferable. However, since too much will lead to a decrease in machinability and toughness, it is preferably made 1.5% or less.

-Preferably, Ni: 1.5% or less Ni is an element which suppresses the production | generation of a ferrite. In addition, C, Cr, Mn, Mo, W, etc., together with imparting excellent hardenability to the steel of the present invention, forming a martensite-based structure even when the quenching cooling rate is slow, reducing toughness It is an effective element for preventing. Furthermore, since the essential toughness of the base is also improved, it is added as necessary in the present invention. However, if it is too much, the base becomes thicker and the machinability decreases. Therefore, even when it is added, the content is preferably 1.5% or less. In addition, when adding, 0.1% or more is preferable.

-Preferably, Nb: 0.3% or less Nb has the effect of forming a carbide | carbonized_material and improving the reinforcement | strengthening of a base and abrasion resistance. In addition, the temper softening resistance is increased and the coarsening of the crystal grains is suppressed to contribute to the improvement of toughness. Therefore, in the present invention, it is added as necessary. However, if too much, machinability and toughness are reduced. Therefore, even when it is added, the content is preferably 0.3% or less. When adding, 0.05% or more is preferable.

-Preferably, Co: 5.0% or less Co uses the steel of the present invention as a tool, and forms a very dense and protective oxide film on the surface at the time of temperature rise. This oxide film prevents metal contact with the counterpart material, suppresses temperature rise on the tool surface, and provides excellent wear resistance. Therefore, in this invention, it adds as needed. However, if it is too much, the toughness is lowered, so even if it is added, it is preferably 5.0% or less. When adding, 0.3% or more is preferable.

  The toughness improving effect of Zn addition according to the present invention is greatly inhibited and weakened when a large amount of carbide is distributed in the structure. That is, the above-described toughness improving effect is sufficiently exhibited by hot tool steel with less carbides than cold tool steel in which many carbides are distributed in the structure. Therefore, the object of the present invention is limited to hot tool steel. The component composition at this time is a standard steel type having a component composition defined in, for example, JIS-G-4404 or the like in the category of hot tool steel defined by the C and Cr amounts of the present invention. Hot tool steel that has been used can also be applied. Element types other than those defined in the above hot work tool steel can be added as necessary.

  The main elements that may remain in the steel as inevitable impurities are S, Cu, Al, Ca, Mg, O (oxygen), N (nitrogen), and the like. In order to effectively obtain the effects of the addition of Zn of the present invention, these elements are preferably as low as possible. However, on the other hand, in order to obtain additional functions and effects such as control of the shape of inclusions, other mechanical properties, and improvement in production efficiency, some inclusions and / or additions can be made. In this case, S ≦ 0.01%, Cu ≦ 0.25%, Al ≦ 0.025%, Ca ≦ 0.01%, Mg ≦ 0.01%, O ≦ 0.01%, N ≦ 0.03 % Range is sufficiently acceptable and is the preferred upper limit of regulation of the present invention.

  The hot tool steel according to the present invention may be subjected to a homogenization heat treatment while processing a steel ingot after casting into a steel material, for example, as a form for sufficiently exerting the toughness improving effect by addition of Zn. preferable. Alternatively, the quenching and tempering hardness is preferably 50 HRC or less. More preferably, it is 48 HRC or less.

  A 7 to 10 kg steel ingot having the composition shown in Table 1 was melted by a vacuum induction melting furnace. A Zn-plated steel plate was used as the Zn addition source. Zn content was measured by fluorescent X-ray analysis. The steel of the present invention is made by adding Zn so as to satisfy the Zn / P ratio of the present invention to the component composition of the commonly used hot work tool steel JIS-SKD61 (P standard: 0.030% or less), A large amount of P is allowed. The comparative steel is obtained by increasing only the P content of SKD61 without adding Zn (except comparative steel 6). In all the steel ingots, S, Cu, Al, Ca, Mg, O, and N are not added (including the case where Al is added as a deoxidizer in the melting step), and S ≦ 0. The results were 01%, Cu ≦ 0.25%, Al ≦ 0.025%, Ca ≦ 0.01%, Mg ≦ 0.01%, O ≦ 0.01%, and N ≦ 0.03%.

  These steel ingots were subjected to a homogenization heat treatment at 1250 ° C. for 5 hours, and then hot forged at 1150 ° C. to produce a steel material having a thickness of 20 mm × 60 mm width × about 500 to 800 mm. Then, after annealing at 860 ° C., it is processed into the size of Charpy impact test piece used for the following evaluation, oil quenching from 1030 ° C., and tempering at various temperatures. The toughness evaluation sample was used.

[Test 1]
FIG. 1 shows the results of a 2 mm U notch Charpy impact test at room temperature for each of the present invention steels 1, 3, 5 to 7 and comparative steels 1 and 3 to 6. The Charpy test piece was in the TL direction in accordance with ASTM E399-90. In the combination of the inventive steels 1, 3 and comparative steel 1 containing the same level of P, the inventive steel 5 and comparative steel 3, and the inventive steels 6, 7 and comparative steel 4, the Zn / P of the invention Inventive steels 1, 3, and 5-7 added with Zn so as to satisfy the ratio are superior in Charpy impact value compared to comparative steels 1, 3, and 4 that did not contain Zn. Moreover, although the steel 6 of the present invention contains Zn at a high concentration exceeding 0.02%, the addition of Zn results in the same level as the comparative steel 1 having a P of less than 0.01%. Maintains toughness. The comparative steel 6 is obtained by adding Zn to the comparative steel 5 containing the same level of P, but does not satisfy the Zn / P of the present invention, and no improvement in toughness is observed.

[Test 2]
FIG. 2 shows the results of a 2 mmV notch Charpy impact test between room temperature and 400 ° C. when the steels of the present invention 2, 4 to 7 and comparative steels 2 to 6 were tempered to a hardness of 45 HRC. The Charpy test piece was in the TL direction in accordance with ASTM E399-90. In the combination of the present invention steels 2, 4 and comparative steel 2 containing the same level of P, the present invention steel 5 and comparative steel 3, and the present invention steels 6, 7 and comparative steel 4, the Zn / P of the present invention Inventive steels 2, 4 to 7 to which Zn is added so as to satisfy the ratio are superior in Charpy impact value compared to comparative steels 2 to 4 containing P at the same level at any test temperature. And this invention steel 6 which satisfied Zn / P of this invention by adding Zn while containing P with a high density | concentration exceeding 0.02% is comparative steel 2 with P less than 0.01%. Maintains the same level of toughness.

  A steel ingot having the component composition shown in Table 2 was produced in the same manner as described in Example 1. Invention steel A is obtained by adding Zn to the component composition of hot work tool steel so as to satisfy the Zn / P ratio of the present invention. And the comparison steel B is set as the component composition equivalent to this invention steel A except not adding Zn. In both steel ingots, S, Cu, Al, Ca, Mg, O, and N were not added (however, Al was added as a deoxidizer in the melting step), S ≦ 0.01%, Cu ≦ 0.25%, Al ≦ 0.025%, Ca ≦ 0.01%, Mg ≦ 0.01%, O ≦ 0.01%, and N ≦ 0.03%.

  Next, these steel ingots were subjected to the same hot forging and various heat treatments as in Example 1 to prepare Charpy impact test pieces conditioned to the respective hardness. And the Charpy impact tests 1 and 2 implemented in Example 1 were implemented, and the toughness of each sample was evaluated.

[Test 1]
FIG. 3 shows the results of the 2 mm U-notch Charpy impact test at room temperature for the steel A of the present invention and the comparative steel B. The Charpy test piece was in the LS direction in accordance with ASTM E399-90. These steels have high toughness due to the addition of Ni. And this invention steel A which added Zn so that the Zn / P ratio of this invention may be satisfy | filled on this is excellent in the Charpy impact value compared with the comparative steel B which did not add Zn.

[Test 2]
FIG. 4 shows the result of a 2 mmV notch Charpy impact test between room temperature and 400 ° C. when the steel A of the present invention and the comparative steel B were tempered to a hardness of 45 HRC. The Charpy test piece was in the LS direction in accordance with ASTM E399-90. The steel A of the present invention in which Zn was added so as to satisfy the Zn / P ratio of the present invention in both steels having higher toughness as compared with the comparative steel B in which Zn was not added at any test temperature, Excellent Charpy impact value.

Claims (3)

C: 0.3 to less than 0.6%, Si: 1.5% or less, Mn: 1.5% or less, Ni: 1.5% or less (including 0%), Cr: 3. 0 to less than 6.0% , Mo and W alone or in combination (Mo + 1 / 2W): 3.5% or less, V: 1.5% or less, Nb: 0.3% or less (including 0%), Co: 5.0% or less (including 0%), Zn: 0.0025 super to 0.025%, P: is 0.005% or more, and Zn / P: 0.5 I super der, toughness excellent hot work tool steel according to claim Rukoto a balance of Fe and unavoidable impurities. % By mass, S ≦ 0.01%, Cu ≦ 0.25%, Al ≦ 0.025%, Ca ≦ 0.01%, Mg ≦ 0.01%, O ≦ 0.01%, N ≦ 0. The hot work tool steel excellent in toughness according to claim 1, characterized by being restricted to 03%. The hot work tool steel excellent in toughness according to claim 1 or 2 , wherein P is 0.01% or more by mass%.