HK1122072B

HK1122072B - Steel for hot tooling, and part produced from said steel, method for the production thereof, and uses of the same

Info

Publication number: HK1122072B
Application number: HK09102542.0A
Authority: HK
Inventors: 尼古拉斯‧比诺; 安德烈‧格雷利耶; 皮埃尔-埃马纽埃尔‧里奇
Original assignee: 奥贝特迪瓦尔公司
Priority date: 2005-11-29
Filing date: 2006-11-28
Publication date: 2012-11-16

Description

Steel for hot forming tools, component made of said steel, method for the production thereof and use thereof

Technical Field

The technical field to which the invention relates is steels for hot forming tools, which can be used for casting and moulding, forging, drawing or extrusion.

Disclosure of Invention

A preferred, but not limiting, field of application of the invention is the production of large-size molds for the casting under pressure of light alloys based on aluminium or magnesium or copper alloys (cupreus alloys).

During application, tools for thermoforming are subjected to cyclic stresses which can lead to their failure.

The sources of these stresses are:

mechanical stresses caused by forces directly applied by machines such as (presses);

-thermal stress: the sudden temperature changes due to the alternating contact with the hot substance to be converted and cooling by the sprayed lubricating oil or refractory coating, thus generating an expansion gradient, which is responsible for the local mechanical stresses.

In some cases, the failure is due to sudden fracture, which can cause momentary damage to the tool when the material is not sufficiently ductile. Typically this is caused by cracks that develop during the first few hundred cycles of application and then develop gradually until after tens or hundreds of thousands of cycles, catastrophic failure of the tool occurs. This process is commonly referred to as "thermal fatigue".

The failure from thermal fatigue requires sufficient toughness at the lowest point of temperature during thermal cycling. This property is generally measured by the impact bending energy of a standard test specimen, which is tested at a temperature between ambient temperature and 150 ℃. It is also desirable to have sufficient hardness and resistance to softening during use at the highest temperature point during cycling.

The production of large-sized molds or tools (e.g. having a thickness greater than 200 mm) requires higher performance of the steels from which they are made. Since the cooling rate is naturally moderated by the heat flux confined to the surface during the quenching process, and the producer is concerned that the part is not deformed or damaged, the steel does not produce a martensitic quenched structure that is the predominant one that yields the best performance properties. For each composition, the QCC diagram (continuous cooling quenching) can describe the properties of the phases formed according to the cooling rate, but it is well known that such a diagram does not allow the calculation of the amount of loss of toughness in the quenched/tempered state due to the decrease in the quenching rate.

Known steels for this purpose may be:

-AISI H11 steel containing approximately 0.40% C, 0.90% Si, 0.40% Mn, 5% Cr, 1.30% Mo and 0.5% V;

-AISI H13 steel, identical to the former, except that it contains V ═ 0.95%;

-W-1.2367 steel containing approximately 0.40% C, 0.30% Si, 0.40% Mn, 5% Cr, 2.9% Mo and 0.65% V;

-a steel comparable to AISI H11, but containing 0.3% Si and 0.2% Ni (see EP-B1-0663018); the nominal components are that C is 0.3-0.4%, Si is less than or equal to 0.8%, Mn is less than or equal to 0.8%, Cr is 4.5-5.8%, Mo is 0.75-1.75%, V is less than or equal to 1.3%, W is less than or equal to 1.5%, Ni is less than or equal to 0.5%, P is less than or equal to 0.008%, Sb is less than or equal to 0.002%, Sn is less than or equal to 0.003%, As is less than or equal to 0.005%, and 10P +5Sb +4Sn + As is less than or equal to 0.10%.

In order to improve the properties of these known steels, many studies have been carried out which focus on obtaining a better balance between hardness, toughness and stability of properties in use (in particular hardness). Therefore, the resistance to heat can be improved by increasing the contents of Mo and V in the steel as compared with H11 steel, such as the above-mentioned H13 steel and W-1.2367 steel, which, however, results in a decrease in toughness. On the other hand, if the content of Si is reduced or Ni is added, toughness can be improved and hardenability can also be improved. However, Ni reduces hardness and yield strength when heated.

The object of the present invention is to provide a new grade of steel for hot forming tools which achieves an excellent balance between the above mentioned properties.

To this end, the invention relates to a steel for hot forming tools having a composition, expressed in weight percent, of:

-0.30％≤C≤0.39％

-4.00％≤Cr≤6.00％

trace Si is less than or equal to 0.50 percent

Trace Mn is less than or equal to 0.80 percent

W is less than or equal to-trace and less than or equal to 1.45 percent

-trace amount is less than or equal to Co and less than or equal to 2.75 percent

-0.80％≤Ni≤2.80％

Mo is between 1.50% and 2.60%, Mo is between 1.50% and 0.65W is between 3.20%

-0.55％≤V≤0.80％

And-0.65. ltoreq. K.ltoreq.0.65

Wherein K is K2-K1

K2＝0.75×(Ni-0.60)

K1＝1.43×(V-0.40)+0.63×[(Mo+0.65W)-1.20]

Trace amount of Al is less than or equal to 0.080 percent

Trace amount of S is less than or equal to 0.0040 percent

Trace amount of P is less than or equal to 0.0200%

Trace amount of Ti is less than or equal to 0.05 percent

Trace Zr is less than or equal to 0.05 percent

Trace amount of Nb is less than or equal to 0.08 percent

Trace amount is less than or equal to N and less than or equal to 0.040 percent

-10P+As+5Sb+4Sn≤0.21％

30ppm or less of trace O

The balance of iron and inevitable impurities.

Preferably, 0.33% to 0.38% C.

Preferably, the trace amount is ≦ Si ≦ 040%.

Preferably, the trace amount of Mn is 0.60% or less.

Preferably, 4.6% to 6.0% of Cr.

Preferably, 1.60% to 2.00% of Mo and 1.60% to 0.65W to 2.20%.

Preferably, the trace amount is less than or equal to 0.030 percent of Al.

Preferably, the trace amount is ≦ S ≦ 0.0010%.

Preferably, the trace amount is ≦ P ≦ 0.0080%.

Preferably, the trace amount is less than or equal to 0.01% of Ti.

Preferably, the trace amount of Zr is less than or equal to 0.02 percent.

Preferably, the trace amount is less than or equal to Nb less than or equal to 0.01 percent.

Preferably, the trace amount is equal to or less than 0.01 percent.

Preferably, 10P + As +5Sb +4Sn is less than or equal to 0.10 percent.

Preferably, the trace amount is ≦ O ≦ 15 ppm.

Preferably, -0.35. ltoreq. K.ltoreq.0.35.

Preferably:

-0.335％≤C≤0.375％

-1.50％≤Ni≤2.10％

mo is between 1.60% and 0.65W and between 2.20% and Mo is between 1.60% and 2.00%

-0.62％≤V≤0.75％。

Preferably:

-0.335％≤C≤0.375％

-2.00％≤Ni≤2.40％

mo is more than or equal to 1.80 percent and less than or equal to 2.90 percent of 0.65W, Mo is more than or equal to 1.80 percent and less than or equal to 3.40 percent of W, and W is less than or equal to 0.90 percent of

-0.66％≤V≤0.76％。

Preferably:

-0.335％≤C≤0.375％

-0.90％≤Ni≤1.50％

mo is more than or equal to-1.50 percent and less than or equal to 0.6W and less than or equal to 1.90 percent, and W is less than or equal to 0.40 percent

-0.55％≤V≤0.63％。

Preferably:

c is more than or equal to 0.335% and less than or equal to 0.375%, Cr is more than or equal to 4.60% and less than or equal to 6.00%, Si is more than or equal to 0.40% in trace, Mn is more than or equal to 0.60% in trace, W is more than or equal to 1.45% in trace, Co is more than or equal to 2.75% in trace, Ni is more than or equal to 1.50% and less than or equal to 2.10% in trace, Mo + 0.60% and more than or equal to 2.20% in trace, Mo is more than or equal to 1.60% and less than or equal to 2.00% in trace, V is more than or equal to 0.62% and less than or equal to 0.75% in trace, K is more than or equal to 0.35 and less than or equal to 0.030% in trace, S is more than or equal to 0.0010% in trace, P is more than or equal to 0.0080.

The invention also relates to a method for producing a component made of steel, characterized in that: the steel is prepared from the steel, is subjected to austenite transformation at 1000-1050 ℃, and is then quenched.

Preferably, the austenite transformation temperature is 1015-1040 ℃.

Preferably, after quenching, the part is tempered at 550-650 ℃ for at least two times, so that the hardness of the part is 42-52 HRC.

The invention also relates to a part made of steel obtained by the above method, characterized in that: the part is a part of a tool for thermoforming.

The component has a thickness greater than or equal to 200 mm.

The component is a mold for casting a light alloy or a copper alloy under pressure.

The component is a forging tool.

The part is a forging die.

The tool is a drilling or rolling tool for steel pipes.

The component is a tool for glass forming.

The component is a tool for plastic molding.

The parts are made of the following steels: c is more than or equal to 0.335% and less than or equal to 0.375%, Ni is more than or equal to 2.40% and more than or equal to 2.00%, Mo and 0.65W are more than or equal to 1.80% and less than or equal to 2.90%, Mo is more than or equal to 1.80% and less than or equal to 3.40%, W is less than or equal to 0.90%, V is more than or equal to 0.66% and less than or equal to 0.76%, and the part is an extrusion die for aluminum alloy casting.

The invention also relates to the use of a part for a tool for thermoforming, characterized in that: the component is prepared from the following steel materials: c is more than or equal to 0335% and less than or equal to 0375%, Ni is more than or equal to 2.40% and more than or equal to 2.00%, Mo and 0.65W are more than or equal to 1.80% and less than or equal to 2.90%, Mo is more than or equal to 1.80% and less than or equal to 3.40%, W is less than or equal to 0.90%, V is more than or equal to 0.66% and less than or equal to 0.76%, and the working temperature of the surface of the alloy is kept to.

The invention also relates to the use of a part for a tool for thermoforming, characterized in that: the component is prepared from the following steel materials: c is more than or equal to 0.335% and less than or equal to 0.375%, Ni is more than or equal to 0.90% and less than or equal to 1.50%, Mo and 0.6W are more than or equal to 1.90%, W is less than or equal to 0.40%, V is more than or equal to 0.55% and less than or equal to 0.63%, and the surface temperature is kept lower than 770 ℃ when the alloy is used.

It will be appreciated that the invention is particularly suitable for simultaneously matching the elements Mo and V which serve for softening and stabilising with Ni which neutralizes their weakening effect, in comparison with the known steels mentioned above, in particular the steels of EP-B1-0663018. All this allows the quenchability to be improved, reproducing these properties on large parts, which previously could only be obtained on small parts.

Since the inventors first worked to effectively measure the instantaneous heat flow through the surface of the hot forming tool when it was in use, it was possible to optimize the composition of the steel of the present invention. The inventors then derived by calculating the instantaneous thermal stress due to thermal shock (which could lead to the creation of cracks). This allows a better understanding of the mechanical behaviour of the material when it is in operation. Due to experimental measurements carried out on samples reproducing the industrial quenching speed, and thermodynamic simulations, the inventors have made it possible to establish a link between the composition of the steel, the heat treatment parameters before application and the microstructure. In particular, the inventors have demonstrated that the interdependence between the components and the quenching temperature has a particularly important influence on obtaining a balance of various mechanical properties which in turn have a particularly important role in steels for hot forming tools.

The invention will be better understood from a reading of the following description and a reference to the accompanying drawings:

FIG. 1 shows the variation of the fraction of undissolved carbides for the reference compositions (FIG. 1a) to FIG. 1e)) and for the composition according to the invention (FIG. 1f)) as a function of the admissible temperature.

Fig. 2 is a QCC curve for a reference steel (fig. 2a)) and a steel according to the invention (fig. 2 b)).

FIG. 3 is a comparison of the fracture energy after quenching, performed under laboratory and industrial conditions, of different reference and inventive test specimens.

The tests were performed on samples having the compositions listed in table 1, and the specific test methods are described below. In the table, the coefficients K2, K1 and K correspond to the following quantities, where the contents are expressed in wt.%:

K2＝0.75×(Ni-0.60)

K1＝1.43×(V-0.40)+0.63×[(Mo+0.65W)-1.20]

K＝K2-K1

the invention is based essentially on the study of the effects and interactions of the elements carbon, chromium, molybdenum, vanadium and nickel and the effect of the austenite transformation temperature before quenching on the mechanical properties of the steels studied.

Influence of the austenite transformation temperature:

the austenite transition temperature determines the distribution of the alloying elements between the undissolved carbides and the matrix. The amount of undissolved carbides increases with increasing temperature.

The undissolved carbides must be maintained in a suitable amount in the final product to control grain size. Fine grain size is necessary to ensure toughness and fatigue resistance.

The alloying elements dissolved in the matrix control the quenchability, the resistance to annealing and generally all the mechanical properties.

Table 2 is the effect of the quenching temperature of one of the compositions studied (see melt 10) on microstructure and properties.

Table 2-experimental castings of reference example 10: austenite transformation temperature vs. microstructure (elements C and distribution of V) and the influence of mechanical properties

As the vanadium carbides become increasingly soluble, in this case, an increase in the austenite transformation temperature leads to an improvement in resistance to softening and a loss in toughness when heated.

This indicates that both the composition and austenite transformation conditions must be considered to obtain the optimum material for the intended application. Using calculation software commonly used by metallurgistsThe phase equilibrium is described by which thermodynamic simulations are performed, providing for each element, for each carbide (VC, M) not dissolved₂₃C₆And possibly M₆C，Fe₃C，M₂C … …). Fig. 1 is drawn by means of this simulation. Fig. 1 shows the variation of the fraction of undissolved carbides for five reference compositions (fig. 1a) to 1e)) and one composition according to the invention (fig. 1f)) as a function of the austenite transformation temperature.

The competitive relationship between the elements Mo and V for fixing carbon by their preferred carbide form is well established. The addition of nickel has only a minor effect on this mechanism (secondary effect).

Observation of the experimental microstructure in the as-quenched condition confirmed the simulated predictive trend. The austenite transformation temperature is optimized by the following principle:

at a suitable temperature, the carbide M₆C and M₂₃C₆The control of the grain size is not very effective and must be dissolved in order to release the metallic elements M and C, providing the potential for the best quenchability of the matrix.

According to thermodynamic simulations, it is necessary that the minimum value of the molar fraction of undissolved vanadium carbides is of the order of 0.20%, so as to ensure uniformity and fineness of the grains; the austenite transformation temperature must be kept below the corresponding threshold.

For this reference, deviations of 10 to 15 ℃ above and below a specific temperature must be taken into account, which corresponds to the temperature profile typical for industrial mass production.

The austenitic transformation temperatures of the different compositions are summarized in table 3:

table 3: ideal austenite transformation temperatures for the different experimental melts.

Definition of optimized composition and key performance measures:

as mentioned above, a basic object of the present invention consists in balancing the following aspects:

on the one hand, the elements molybdenum, vanadium and optionally tungsten, contribute to softening and softening resistance during handling, but have a weakening effect (welkening effect).

On the other hand, nickel contributes to the toughness, but is detrimental to the hardness when heated.

It is to be understood that the steels of the invention must have sufficient hardness to avoid pitting (recoiling) and to resist fatigue when heated, and that, to a first approximation, their hardness when heated has the same relationship to the hardness at 20 c, and that the quenched and tempered heat treated states are compared at the same hardness at 20 c. The preselected hardnesses were 47, 45 and 42 HRC.

According to the inventive method, laboratory samples which can be quenched at high speed and samples which reproduce the quenching speed of typical industrial parts on laboratory equipment (selected at 22 ℃ per minute and in the temperature range of 900/400 ℃) were measured systematically and simultaneously.

The measurement includes:

-describing the change in hardness as a function of tempering temperature for 2 hours of 2 tempering treatments, thus determining the tempering to be carried out to obtain the desired hardness;

-softening resistance is obtained by measuring the amount of hardness loss due to retention at 560 ℃ for 80 hours, the initial hardness value being 47 HRC;

and measuring the toughness through impact elastic bending energy when the V-shaped notch impact test sample is damaged at different temperatures of + 20-200 ℃.

Austenite new transformation point (Re-austenitation point Ac 1):

in operation, this point must not be exceeded, since the structure of the component material will change, which will result in a significant change in the mechanical properties.

Table 4 shows the most representative results obtained from the different samples, which confirms that the effect of the Mo and V elements is not significant; on the other hand, as the nickel content increases, the Ac1 point decreases. Therefore, in applications where the surface temperature is very high during operation (as in some forging tools), compositions with high nickel content must be avoided; they have many uses, however, such as light alloy molds, where their surface temperature is milder.

TABLE 4 Austenite reconversion Ac1 Point vs. composition

Casting piece	Nickel (%)	Molybdenum (%)	Vanadium (%)	Ac1 Point (. degree.C.)
					1	0.06	1.21	0.47	825
7	0.08	2.29	0.57	820
					8	0.15	1.62	0.64	805
12	1.42	1.21	0.46	770
					13	2.93	1.23	0.47	680

20	0.59	2.14	0.77	800
					22 (present invention)	1.63	1.82	0.71	755
23 (present invention)	1.05	1.78	0.70	785
					26 (present invention)	2.19	2.28	0.70	710

Resistance to tempering and softening during handling

Table 5 shows the effect of the alloying elements on the resistance to hardness degradation at high temperatures.

The hardness values obtained after the two tempers were 47 and 42HRC, the first tempering temperature being 550 ℃ and the second tempering being carried out at the characteristic temperatures appearing in the table.

The initial value for the hardness loss measurement was 47 HRC.

Table 5A shows the results obtained from reference sample 1 and two samples 12 and 13 having a higher nickel content than the reference sample. Table 5B shows the results obtained from sample 1, sample 3, sample 5, sample 6 and sample 8, in which the contents of Mo and possibly V are higher than in sample 1. Table 5C shows the results obtained from samples 8 and 22 and samples 6 and 26, which have higher Ni, Mo and V contents than sample 1.

TABLE 5 influence of the alloying elements on tempering temperature and softening behaviour when held for a long period of time

Table 5-A shows the detrimental effect of simply adding nickel-a very significant reduction in tempering temperature for a given hardness and an increase in hardness loss after a long period of holding in the heated state. The reduction in tempering temperature is destructive, since the steel must provide its highest possible working temperature, at least 600-630 ℃, in order to avoid excessive softening.

Since the surface temperature of the part at the time of aluminum injection (injection) is typically close to 520 ℃ and 560 ℃ and even higher at the time of forging, this criterion is an important factor to consider in order to determine whether a given composition will meet given application requirements.

Table 5-B shows that simply adding molybdenum and vanadium has a beneficial effect on improving tempering and softening resistance during handling. On the other hand, the decrease in quenching speed between laboratory and industrial conditions has an adverse effect on these parameters, which is caused by insufficient quenchability of the material.

Comparison of compositions in Table 5-C for (pairs) (8, 22) and (6, 26) shows that under laboratory conditions, the hardness loss resistance of nickel-containing castings is lower than that of corresponding low nickel-containing castings, but their performance is very similar under industrial quenching conditions.

To summarize: under industrial heat treatment conditions, the combined and balanced addition of nickel, molybdenum and vanadium imparts to the material a tempering resistance and a softening resistance after a prolonged period of retention, these properties being comparable to those without the addition of nickel.

These advantageous results can be explained by the significant improvement in quenchability shown in fig. 2, which fig. 2 compares the QCC continuous cooling profile of reference composition 1 (fig. 2a) with that of composition 22 of the invention (fig. 2b), reference composition 1 undergoing austenite transformation at 990 ℃ for 30 minutes and composition 22 undergoing austenitization at 1030 ℃ for 30 minutes.

The composition of the invention has a pearlite phase region and a bainite phase region, and the effect of a low cooling rate can be significantly counteracted with respect to the reference composition. Thus, for the tool to be treated, a typical industrial quenching (in the manner already shown in bold in fig. 2a and 2b) makes it possible to reach 400 ℃ in 1000 to 5000 seconds, depending on the dimensions of the part and the position in the part, the composition of the invention being capable of producing an exclusive martensitic transformation. In contrast, the reference composition necessitates the formation of a large amount of bainite, which is less favorable for obtaining the desired properties.

Toughness of：

The adverse effect of the decrease in cooling rate under laboratory and industrial conditions is more pronounced in the fracture energy of the V-notch impact specimen that bends due to impact.

Table 6 shows typical trends of the results obtained by screening, while the addition of Ni, Mo and V has a beneficial effect on the casting 21 of the present invention, since it achieves the highest elasticity value (resilience value) after processing under industrial conditions, with the least reduction caused by the reduction in the quenching speed.

TABLE 6-V-notch impact test specimens from several typical castings as a result of impact on their perimeter The energy of fracture generated, wherein:

r: rapid quenching (oil quenching of sample)

L: slow quench (industrial quench rate reproduced in the laboratory).

Figure 3 compares the values obtained after quenching at commercial speed and rapid quenching of metals of the same composition for all castings, with samples annealed to obtain hardnesses of 42, 45 and 47HRC and samples broken at 20 ℃ and 100 ℃. Each point represents the hardness value and failure temperature of the sample. The results show that the amount of hardness loss due to the decrease in quenching speed is much less for the compositions of the present invention.

The trend of the results of the laboratory tests was confirmed by the test results of the tool blocks processed under the following conditions:

tool block size 570X 450X 228mm

At the same position in the furnace

Quenching was carried out in the same industrial furnace at a gas pressure of 5 bar and at the same gas flow rate

The temperatures of the two tempers were adjusted separately to obtain a hardness of 46+/-0.5HRC

Selecting a V-notch impact specimen on the cross section: the selection is made on the large face center near the surface and core portions of the tool block.

The average values of the impact bending energy are given in table 7, which confirms the excellent properties of the steel 22 of the invention, in particular in the core region of the block, which can represent a component of larger dimensions.

TABLE 7 test results of impact bending energy of tool blocks processed under industrial conditions

Casting piece	Ni％	Mo％	V％	Breaking energy KV (Joule) of sample at periphery	Breaking energy KV (Joule) of core part sample
						2	0.11	1.27	0.49	32	16
5	0.17	2.74	0.48	18	14
						7	0.08	2.29	0.57	23	20
9	0.25	1.59	0.66	24	19
						22 (present invention)	1.63	1.82	0.71	28	26

All these mechanical test results show that reducing the quenching speed has an adverse effect, in particular:

when the hardness is the same, the impact bending energy is lowered

The hardness loss increases when the alloy is kept at 560 ℃ for a long time

However, the magnitude of these changes is not the same for all compositions, and it has been demonstrated that the simultaneous and balanced addition of alloying elements according to the rules described below can significantly reduce this difference.

The function of the alloy elements is as follows:

by comparative studies of the properties of experimental castings and those of thermodynamic simulations, it has been possible to evaluate the effects of various alloying elements and their interactions. By following the rules set out above with respect to the quenching conditions, the following trends were confirmed:

carbon (C)Is advantageous for quenchability, can raise the ideal austenite transformation temperature, and determines the maximum hardness obtained after annealing treatment at 550 ℃. But is detrimental to toughness. In combination with high contents of molybdenum or vanadium, the formation of eutectic carbides, which are detrimental to microstructure and toughness, results. Its content should be at least 0.30% to obtain sufficient hardness, and at most 0.39% to avoid irreparable fatigue. The most preferable range is 0.33 to 0.38%.

Chromium (III)Is advantageous for quenchability. Plays an important role in temper hardening and this property is advantageous for the preferred application expected by the present invention, i.e. large size parts requiring high hardness (42-52 HRC). However, the carbides produced therefrom rapidly develop into a more stable state, and it has been confirmed that the resistance to hardness reduction at high temperatures is not very highAnd (5) effect. Therefore, it is necessary to supplement Cr with other carbide-forming elements such as Mo and V. The content of chromium element must be kept at least 4.0% in order to obtain quenchability, at most 6.0%, if it exceeds 6.0%, the action of chromium will inhibit the action of V and Mo. Preferably, the content of chromium is 4.6% to 6%.

Molybdenum (Mo)The hardenability can be improved. Which coexists with chromium in the chromium-based carbide, can increase the amount of chromium-based carbide. When high in content, it forms a specific M₂C and M₆C. In terms of macroscopic properties, it can increase hardness and tempering resistance, and reduce toughness. Its content is 1.50-2.60%. Tungsten, if present, is also taken into account, as will be explained below. Preferably, the content of Mo is 1.60-2.00% and Mo +0.65W is 1.60-2.20%.

Vanadium oxideOn the face covered by the experimental casting, the carbides form a significant majority of the precipitates that are insoluble at the austenite transition temperature, thus ensuring that the grains cannot grow. During the tempering carried out after quenching, new micro-and nano-scale carbides are precipitated and they play a positive role in the resistance to softening when subjected to the action of temperature and cyclic stresses during secondary softening and handling, through interaction with martensitic crystal defects. On the other hand, excessive carbides generated during tempering cause a significant reduction in performance. Within the scope of the compositions studied here, and following the summarized selection principle regarding the austenite transformation temperature, the content of V must be between 0.55% and 0.75%.

Nickel (II)Has an adverse effect on the hardness in the treated state; it can lower the tempering temperature to obtain the desired hardness and softening resistance. Moreover, in the temperature range employed, amounts exceeding 3% can significantly reduce the austenite reconversion point, which should absolutely be avoided. On the other hand, the nickel can improve the quenchability, and particularly, when the content is 1-3%, the toughness can be obviously improved. The content of Ni is 0.80-2.80% of the total weight of the composition. The side effects of the large addition of nickel can be compensated by the amounts of Cr, Mo, V and W mentioned above.

TungstenOptional additional elements, with the condition that Mo +0.65W is 1.50-3.20% and the content of Mo is 1.50-2.60%, the content of W is at most 1.45%, and when the content of Mo is 1.60-2.00%, the content of W is preferably 1.60-2.20%. In fact, tungsten can compensate for the effect of molybdenum, 1% tungsten being equivalent to 0.65% molybdenum. The addition of tungsten has limited side effects on toughness and quenchability, and has a positive effect on softening resistance when heated, in particular at test temperatures above 560 ℃ (e.g. 600 ℃).

CobaltThe upper limit of the addition is 2.75%. For softening resistance, it is advantageous, in particular, when the temperature is of the order of 600 ℃; but is disadvantageous in terms of quenchability. In view of the high price of such added elements, the use thereof is not particularly recommended.

Furthermore, in order to achieve a desired balance between the properties in the application, it is also necessary to add molybdenum, vanadium, nickel and possibly tungsten in a balanced manner and to satisfy the following relationship:

k is between-0.65 and +0.65, preferably between-0.35 and +0.35, and most preferably as close to zero as possible, where:

K＝K2-K1

K2＝0.75×(％Ni-0.60)

K1＝1.43×(％V-0.40)+0.63×[％Mo+(0.65×％W)-1.20]

it can be seen that table 1 lists the K1, K2, K values for all castings.

The best results are obtained when the following conditions are simultaneously satisfied:

0.335％≤C≤0.375％；

1.50％≤Ni≤2.10％；

mo is more than or equal to 1.60 percent and less than or equal to 2.00 percent of 0.65W, and Mo is more than or equal to 1.60 percent;

0.62％≤V≤0.75％。

for more specific applications, it is also recommended that the following conditions are fulfilled:

0.335％≤C≤0.375％

2.00％≤Ni≤2.40％

mo is more than or equal to 1.80 percent and less than or equal to 2 and 0.65W is less than or equal to 2,90 percent, more than or equal to 1.80 percent and less than or equal to 2.40 percent, and W is less than or equal to 0.90 percent

0.66％≤V≤0.76％

The application occasions are as follows: when it is necessary to obtain excellent quenchability for the production of large-sized parts such as extrusion dies for casting Al alloys, the working temperature of the surface is kept below 680 c, considering the reduction of the phase transition point Al by Ni.

0.335％≤C≤0.375％

0.90％≤Ni≤1.50％

Mo is more than or equal to 1.50 percent and less than or equal to 1.90 percent of 0.65W and less than or equal to 0.40 percent of W

0.55％≤V≤0.63％

The application occasions are as follows: to meet the performance requirements of medium size parts and for applications where the surface temperature is below 770 c during operation.

In addition, other elements must be mentioned which must or can be present in a defined content limit.

SiliconDue to the detrimental toughness, the content is kept low, not exceeding a limit of 0.50%, preferably 0.40%, while the cost of industrial production can allow.

Manganese oxideAdvantageous for hardenability but detrimental for toughness, in amounts of not more than 0.80%, preferably not more than 0.60%.

Sulfur, phosphorus, arsenic, tin, antimony, titanium, zirconium, niobium, nitrogenIs unfavorable for toughness and can be produced during operationThe weakening must be limited to the minimum level achievable by industrial and economic means. The maximum allowable content is:

s: 0.0040%, preferably 0.0010%

P: 0.0200%, preferably 0.0080%

Ti: 0.05%, preferably 0.01%

Zr: 0.05%, preferably 0.02%

Nb: 0.08%, preferably 0.01%

N: 0.0400%, preferably 0.0100%

Moreover, the contents of P, As, Sb and Sn must satisfy:

10P + As +5Sb +4Sn is less than or equal to 0.21 percent, preferably less than or equal to 0.10 percent.

AluminiumThe content of (B) must be in the range of trace to 0.080%, preferably in the range of trace to 0.030%. Its function is to remove oxygen from the steel and thus to limit the amount of oxide inclusions in the steel, which oxides can lead in particular to a reduction in the fatigue resistance of the steel. For this reason the oxygen content must not exceed 30ppm, preferably 15 ppm. The high content of Al reduces the content of O dissolved in the liquid steel, but it also makes the liquid steel more sensitive to atmospheric reoxidation during casting, thus increasing the risk of the formation of harmful oxide inclusions.

In a general way, the steel of the present invention can be divided into two grades.

"Standard" grades, which are obtained when the content of each element in the composition is not absolutely within the optimum range as defined above, are obtained. The improvement is in quenchability compared to the prior art. This enables production of a large-sized product having high hardness and uniformity in various portions of the entire product.

A "premium" grade is obtained when the content of each element is within the optimum range defined above. In this case, in addition to the improved quenchability, high toughness is obtained, as well as good thermal fatigue resistance and sudden fracture resistance due to the high toughness and high hardness.

In order to achieve this result, it is necessary to resort to a method which comprises remelting the consumable electrode after primary refining in an electric furnace or ladle using a Vacuum Arc Remelting (VAR) process or an electrode conductive slag remelting (ESR) process, which in particular ensures the desired very low O content. Likewise, as is usual for the treatment of these types of steel, it is necessary to subject the cast steel to a thermodynamic process of rolling and annealing which makes the structure of the steel compact, coherent, fine and uniform; at the same time, a solidification process is also carried out, which can produce dendrites that are very small and not separated from each other.

Parts which can be produced in the steel of the invention produced according to the method described above, generally, include parts of tools for hot forming, in particular:

-the component is a casting die under extrusion of a light alloy or a copper alloy;

-a forging die;

drilling or rolling tools for steel pipes.

-glass and plastic material forming tools.

The invention is preferably used to produce parts having a thickness of 200mm or more.

Claims

1. Method for producing a steel component, characterized in that: the part is prepared by carrying out austenite transformation at 1000-1050 ℃ on steel for a hot forming tool, wherein the steel comprises the following components in percentage by weight, and then quenching the steel:

-0.30％≤C≤0.39％

-4.00％≤Cr≤6.00％

trace Si is less than or equal to 0.50 percent

Trace Mn is less than or equal to 0.80 percent

W is less than or equal to-trace and less than or equal to 1.45 percent

-0.80％≤Ni≤2.80％

Mo is between 1.50% and 2.60%, Mo is between 1.50% and 0.65W is between 3.20%

-0.55％≤V≤0.80％

--0.65≤K≤0.65

Wherein K is K2-K1

K2＝0.75×(Ni-0.60)

K1＝1.43×(V-0.40)+0.63×[(Mo+0.65W)-1.20]

Trace amount of Al is less than or equal to 0.080 percent

Trace amount of S is less than or equal to 0.0040 percent

Trace amount of P is less than or equal to 0.0200%

Trace amount of Ti is less than or equal to 0.05 percent

Trace Zr is less than or equal to 0.05 percent

Trace amount of Nb is less than or equal to 0.08 percent

Trace amount is less than or equal to N and less than or equal to 0.040 percent

-10P+As+5Sb+4Sn≤0.21％

Trace of O.ltoreq.30 ppm

The balance being iron and unavoidable impurities.

2. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: c is between 0.33 and 0.38 percent.

3. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace Si is less than or equal to 0.40 percent.

4. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount of Mn is less than or equal to 0.60 percent.

5. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: cr is between 4.6 and 6.0 percent.

6. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: mo is between 1.60% and 2.00%, and Mo +0.65W is between 1.60% and 2.20%.

7. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount of Al is less than or equal to 0.030 percent.

8. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount is less than or equal to S and less than or equal to 0.0010 percent.

9. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount of P is less than or equal to 0.0080%.

10. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount is less than or equal to Ti and less than or equal to 0.01 percent.

11. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace Zr is less than or equal to 0.02 percent.

12. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount is less than or equal to Nb and less than or equal to 0.01 percent.

13. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount is less than or equal to N and less than or equal to 0.01 percent.

14. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: 10P + As +5Sb +4Sn is less than or equal to 0.10 percent.

15. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount is less than or equal to O and less than or equal to 15 ppm.

16. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: k is more than or equal to-0.35 and less than or equal to 0.35.

17. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent:

-0.335％≤C≤0.375％

-1.50％≤Ni≤2.10％

-0.62％≤V≤0.75％。

18. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent:

-0.335％≤C≤0.375％

-2.00％≤Ni≤2.40％

mo is between 1.80% and 0.65W and between 2.90%, Mo is between 1.80% and 2.40%, W is between 0.90%

-0.66％≤V≤0.76％。

19. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent:

-0.335％≤C≤0.375％

-0.90％≤Ni≤1.50％

mo is more than or equal to-1.50 percent and less than or equal to 1.90 percent of 0.65W, and W is less than or equal to 0.40 percent

-0.55％≤V≤0.63％。

20. The method of claim 1, wherein: the steel for hot forming tools has the following composition, expressed in weight percent:

21. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace Si is less than or equal to 0.40 percent.

22. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount of Mn is less than or equal to 0.60 percent.

23. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: cr is between 4.6 and 6.0 percent.

24. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: mo is between 1.60% and 2.00%, and Mo +0.65W is between 1.60% and 2.20%.

25. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount of Al is less than or equal to 0.030 percent.

26. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount is less than or equal to S and less than or equal to 0.0010 percent.

27. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount of P is less than or equal to 0.0080%.

28. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount is less than or equal to Ti and less than or equal to 0.01 percent.

29. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace Zr is less than or equal to 0.02 percent.

30. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount is less than or equal to Nb and less than or equal to 0.01 percent.

31. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount is less than or equal to N and less than or equal to 0.01 percent.

32. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: 10P + As +5Sb +4Sn is less than or equal to 0.10 percent.

33. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: trace amount is less than or equal to O and less than or equal to 15 ppm.

34. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent: k is more than or equal to-0.35 and less than or equal to 0.35.

35. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent:

-0.335％≤C≤0.375％

-1.50％≤Ni≤2.10％

-0.62％≤V≤0.75％。

36. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent:

-0.335％≤C≤0.375％

-2.00％≤Ni≤2.40％

-0.66％≤V≤0.76％。

37. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent:

-0.335％≤C≤0.375％

-0.90％≤Ni≤1.50％

-0.55％≤V≤0.63％。

38. The method of claim 2, wherein: the steel for hot forming tools has the following composition, expressed in weight percent:

39. The method of any one of claims 1-38, wherein: the temperature for carrying out austenite transformation is 1015-1040 ℃.

40. The method of any one of claims 1-38, wherein: after quenching, tempering the part at 550-650 ℃ for at least two times to ensure that the hardness of the part is 42-52 HRC.

41. A steel component produced by the method of any one of the preceding claims, characterized in that: the part is a part of a tool for thermoforming.

42. The component of claim 41, wherein: the thickness of the part is greater than or equal to 200 mm.

43. The component of claim 41, wherein: the component is a mold for casting a light alloy or a copper alloy under pressure.

44. The component of claim 41, wherein: the component is a forging tool.

45. The component of claim 41, wherein: the part is a forging die.

46. The component of claim 41, wherein: the component is a drilling or rolling tool for steel pipes.

47. The component of claim 41, wherein: the component is a tool for glass forming.

48. The component of claim 41, wherein: the component is a tool for moulding of plastics material.

49. The component of claim 41, wherein: the part is prepared by the method of claim 18 or 36, and the part is an extrusion die for casting an aluminum alloy.

50. Use of the component of claim 41, wherein: the component is prepared by the method of claim 18 or 36, the working temperature of the surface of which is maintained below 680 ℃.

51. Use of the component of claim 41, wherein: the part produced by the method of claim 19 or 37, which in operation has a surface temperature maintained below 770 ℃.