JPH0830241B2 - Steel sheet having excellent workability and toughness and good hardenability, and a method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) By making the microstructure of high-carbon steel mainly composed of a ferrite phase and a graphite phase, workability and toughness in cold forming, and further quenching With regard to a steel sheet having improved properties and a method of manufacturing the steel sheet, a new material particularly suitable as a material for a tiller nail is newly pioneered.

(Prior Art) As is clear from its application, the tiller nail is required to have abrasion resistance against earth and sand, mud, clay and the like. Therefore, at present, most of the materials are spring steels with excellent wear resistance (JIS standard SUP6, C: 0.55 to 0.65%, Si: 1.50 to 1.80%, Mn:
0.70-1.0%) is used.

These spring steels used as materials for cultivator claws are first manufactured by melting or refining the steel in the above-mentioned composition range in an electric furnace or a converter into blooms or billets by casting or slabbing, and then heating furnaces. After heating at about 6 ~ 12mm thickness and about 50 ~ 10 width by hot rolling mill for flat steel
Finish a flat bar with a length of 0 mm and a length of 6-10 m.

Next, the cultivator claws should be fitted with the above flat bar with a length of about 200 to 2
After cutting into small pieces of 50 mm, generally, it is heated and processed into a predetermined tiller claw shape by a hot forming process step, and subsequently reheated to the austenite region to impart wear resistance and then quenched. After that, it was customary to carry out a low temperature tempering treatment so as to obtain a predetermined hardness (nail base: HRC45 to 50, claw blade: HRC55 to 60) and a predetermined impact characteristic.

(Problems to be solved by the invention) By the way, in recent years, in order to further reduce the manufacturing cost, the manufacturing industry of cultivator claws has been increasingly motivated to conserve energy and labor through continuous production facilities. .

As an example of such a continuous production facility, it is considered that one of the means is to replace some of the conventional manufacturing processes by hot forming with cold punching.

When adopting such a cold stamping forming process, the following two points are important as the requirements required for the nail material.

First, it is a material that is very easy to process in the nail forming process, while it has high hardenability in the quenching process and has a high hardness after quenching.

Secondly, in order to maintain high productivity in the manufacturing equipment, it is necessary to keep the supply of material on the inlet side of the manufacturing line uninterrupted. In this case, the material form can be easily loaded into the material supply device without stopping the manufacturing equipment. However, looking at the current situation from the first point, the SUP 6 flat bar currently used is finished in the state of being air-cooled after hot rolling, so that the tensile strength is about 90 to 110 kgf / mm ² It is unsuitable for cold punching with this strength as it is, and if this is forcibly performed, the life of the tool will be significantly shortened and it will be difficult to maintain the dimensional accuracy after punching. .

Regarding the second point, the maximum length of the current flat bar is 10 m at the maximum, it is too short for continuous material supply, and even if a long flat bar is used, It is disadvantageous in terms of equipment space and manufacturing cost to make a feeder suitable for such a long material.

As described above, it is the current situation that the manufacturing process of the tiller nail cannot be made more efficient and continuous due to the material and morphological restrictions of the material that is the material.

Based on the recognition that the cause of hindering the efficiency of the cultivating claw manufacturing process is due to the material, it is possible to provide a material that solves the above-mentioned problems, and to provide a manufacturing method thereof. Its purpose is based on the following perspectives.

First, in order to satisfy the above first requirement, (1) the material properties of the material are soft during forming, easy to cold form, good quenchability during heat treatment, and To exhibit excellent wear resistance and impact characteristics of the tiller nail products.

(2) In order to achieve such material properties, the microstructure is adjusted to a structure in which fine graphite particles are uniformly dispersed in the ferrite phase (hereinafter referred to as ferrite / fine graphite structure).

(3) As manufacturing conditions necessary to obtain such a microstructure, it is important to adjust the chemical composition of steel, the rolling conditions during hot rolling, and the annealing conditions during annealing. Is.

By the way, if the structure is mainly composed of a ferrite phase and a graphite phase and that the cold workability is improved, it is disclosed and proposed in JP-A-60-128245, for example. This proposal is not directed to materials in the field of quenching as in the present invention, but rather to structural materials that utilize the damping properties of the graphite phase.

From the viewpoint of damping property, it is necessary that the graphite phase existing in the structure has a large particle size, and therefore a structure having coarse graphite particles is desirable. On the contrary, in the field of heat treatment targeted by the present invention, the structure having such coarse graphite particles is not suitable. The reason for this is as follows.

In general, when the steel is heated to the austenitizing temperature, the solubility of C in the steel in the austenite phase is higher in the case where C is in the graphite state than in the case where C is in the ordinary cementite state. It is known to be inferior. This tendency becomes stronger as the graphite particles are coarser. Therefore, in the case of such a coarse graphitized structure, it is not possible to use it as a heat-treating steel because the specified quenching hardness cannot be obtained unless the graphite is melted and placed for a sufficient time during austenitization. Of.

Therefore, a material having a coarse ferrite-graphite structure as disclosed in JP-A-60-128245 is not suitable for heat treatment. In addition to the above, no prior art is found for a material having a ferrite-graphite structure and satisfying both cold formability and hardenability at the same time, or a proposal for a method for producing such a material.

When the ferrite / graphitized structure is used, the inventors have made it possible to obtain a ferrite / fine graphite structure in which the graphite particles in the structure are extremely finely and uniformly dispersed without causing deterioration of hardenability. We found that a material with excellent cold formability was obtained,
The present invention is constructed based on this knowledge.

Next, in order to satisfy the second requirement, a hot rolled steel strip whose material length can be made much longer than the ferrite bar currently used is prepared. It is possible to make the material into a compact form and to easily transport and supply the material to the manufacturing equipment for the cultivator claws by dividing the blade into the width direction and forming a slit coil with the same width as the current flat bar. is necessary.

(Means for Solving the Problems) The above-mentioned object of the present invention is achieved by a configuration having the following points as its gist.

(1) As a basic component, C: 0.40 to 0.80 wt% (hereinafter simply expressed as%) Si: 0.2 to 2.00% Mn: over 0.50 to 1.50% Al: 0.001 to 0.150% P: 0.018% or less S: 0.010% or less N: 0.0050% or less, consisting of balance Fe and unavoidable impurities, and having a graphitization ratio mainly composed of a ferrite phase and a graphite phase.
It has a structure with 40% or more and an average particle size of graphite of 10 μm or less, and a soft material with TS ≤ 60 kgf / mm ² , which is excellent in workability and toughness, and has good hardenability. Steel plate (first invention).

(2) In addition to the above-mentioned basic components, Ti: 0.10% or less, Nb: 0.05% or less, Zr: 0.050% or less, B: 0.01% or less, and one or more kinds of them are additionally contained. A steel plate similar to (1) above (second invention).

C: 0.40 to 0.80%, Si: 0.2 to 2.00%, Mn: over 0.50 to 1.50%, Al: 0.001 to 0.150%, P: 0.018% or less, S: 0.010% or less, N: 0.0050% or less The steel contained is hot-rolled at a finishing rolling temperature of 800 ℃ or less and 500 ℃ or more, and wound into a coil to form a hot-rolled steel strip. It consists of annealing for 1 to 200 hours. TS ≦ 60kg with a structure mainly composed of ferrite phase and graphite phase with a graphitization ratio of 50% or more and a graphite average particle size of 10 μm or less.
A method for producing a steel sheet having excellent workability, toughness, and hardenability (third invention), characterized by exhibiting a soft material of f / mm ² .

(Function) In the present invention, in order to uniformly disperse the graphite fine particles in the ferrite by using the steel in which the high C and Si are added as described above and the P, S and N are reduced, The inter-finishing rolling temperature is set to a temperature of 800 ° C or lower and 500 ° C or higher to increase the dislocation density in the material after rolling and to increase the micro-transformation structure, thereby producing destabilized cementite in ferrite. The key point is to efficiently cause nucleation and growth of graphite from the destabilized cementite by annealing at a low temperature of 500 to 750 ° C.

The reason for the numerical limitation in this invention is as follows.

Regarding chemical composition: C is an essential element for ensuring quenching forming, and 0.40% or more is necessary from the viewpoint of wear resistance of the tiller nail. On the other hand, the reason why the upper limit of the amount of C is 0.80% was determined from the viewpoint of impact resistance. That is, the cultivator claw needs to have a shock resistant property because a large impact force is applied to the base of the claw by rotation during the cultivating work.

This is because if the C content exceeds 0.80%, the effect of improving the hardenability and the hardness after quenching is almost saturated and only the impact resistance property deteriorates, which is not preferable.

 Si is an essential element for the following two reasons.

First of all, the solid solution strengthens the steel substrate,
This is because it is easy to obtain high strength in a range that cannot be achieved only by quenching and strengthening by, and thereby improving wear resistance.

Secondly, in order to obtain good moldability,
As described above, medium- and high-carbon steels have a microstructure that is essentially as hot-rolled as ferrite and pearlite, or a structure that contains bainite in these and is usually very high in strength, so that the formability is extremely poor. In order to improve this, in the present invention, the microstructure is changed to a structure mainly composed of ferrite and graphite by annealing,
Si has the effect of promoting spheroidization of cementite that precedes graphitization and subsequent graphitization during this annealing, and therefore contributes to the improvement of moldability.

In order to combine the above two effects, Si needs to be 0.2% or more, while the upper limit is set to 2.0% from the viewpoint of manufacturing cost. That is, even if added over 2.0%, the improvement effect in terms of wear resistance and molding processability is saturated,
This is because the manufacturing cost only increases.

Mn is an element that improves the hardenability, and since it has a large effect of lowering the critical cooling rate in the quenching treatment process in particular, it is possible to slow the cooling rate during quenching from the viewpoint of preventing quenching distortion. , At least 0 to get this effect.
While it is necessary to make it over 50%, if it exceeds 1.5%, the susceptibility to cooling cracks after casting becomes large and surface defects are likely to occur, so it is limited to over 0.50 to 1.5%. .

Since Al acts on N in steel and fixes it as AlN, it is an element that has the effect of eliminating the adverse effects of N in steel. To use this effect, 0.001% or more is necessary, and 0.005% or more is particularly contained. desirable. However, if it exceeds 0.150%, the hardenability is adversely affected, so 0.150% was made the upper limit.

P is generally known as an element that has an adverse effect on both the hardenability and the formability in terms of the effect on the transformation characteristics of steel and segregation. In the present invention as well, first of all, It is an unfavorable element for the same reason as above. Secondly, P has a function of dissolving a trace amount in cementite and stabilizing it, and it is also limited because it becomes difficult to obtain a structure mainly composed of ferrite and graphite during annealing, and moldability is impaired. I have to. In order to avoid such an adverse effect of P, it is necessary to set it to 0.018% or less.

S has the disadvantage that it easily forms non-metallic inclusions, which deteriorates the formability and stabilizes cementite during annealing, which hinders spheroidization and graphitization. The reason why the upper limit of the amount of S is set to 0.010% or less is that if it exceeds this, the above-mentioned adverse effect becomes significantly large.

N is an unfavorable element because it has a strong effect of dissolving in cementite and stabilizing it, and it becomes difficult to obtain a structure mainly composed of ferrite and graphite during annealing. If the N content exceeds 0.0050%, the above-mentioned adverse effects become remarkably large, so the upper limit was made.

In addition to the above, Ti, Nb, and Zr are all elements that have a strong tendency to form nitrides, so that harmful solid solution N is fixed as a nitride to reduce its adverse effect, and these nitrides are annealed. At that time, it acts as a core of spheroidization and graphitization of cementite and promotes softening,
Furthermore, it exhibits a beneficial effect such as improving the impact characteristics when finished into a final product by the structure refining effect,
When used in an appropriate amount as a selection component, a further effect of improving quality can be obtained.

In order to utilize this effect, in the case of Ti 0.02% or more,
In the case of Nb and Zr, addition of 0.01% or more is necessary.
However, in the case of any element, even if added over 0.10%, the effect is saturated and it is not economical, so the upper limit is 0.1.
%.

B also has the effect of fixing the solid solution N as in the case of Ti, Nb and Zr, and has the effect of improving the hardenability when it is contained in an appropriate amount. In order to obtain such an effect, 0.0005% or more is required to obtain such an effect, but if 0.0100% or more, the effect is saturated and it is not economical, so the upper limit is 0.010%.
And Microstructure In order to satisfy both cold formability and hardenability at the same time, the present invention is limited to a structure mainly composed of a ferrite phase and a graphite phase. The reason for this will be explained based on the results of research by the inventors.

FIG. 1 has a composition within the range of components according to the present invention
By changing the graphitization ratio in the structure and the size of the graphite particles by various methods using a small sample taken from a hot rolled steel strip with a thickness of 8 mm, the remaining C was spheroidized cementite. It is the result of investigating the tensile property, the Charpy impact property and the hardenability by adjusting the structure. Further, FIG. 2 shows the difference in the hardenability when the graphite particles are different. The evaluation of hardenability is made when materials with different graphitization ratios of 80% or more and different average graphite particle sizes are used, and after various heating and holding times at 860 ° C and after quenching at a cooling rate of 50 ° C / s. The cross-section average hardness is shown.

According to the results shown in FIGS. 1 and 2, (1) the tensile properties and impact properties depend on the graphitization ratio, and when it is 40% or more, the tensile strength is low and the elongation properties and impact properties are good.

(2) On the other hand, the hardenability depends on the average particle size of graphite, and in the case of large graphite particles exceeding 10 μm, the heating time required for austenitizing becomes remarkably long.

(3) By having a fine graphite structure in which the graphitization ratio is increased and the average grain size is adjusted to 10 μm or less, cold moldability and hardenability are simultaneously satisfied.

The knowledge was obtained.

Hot rolling conditions In this invention, the hot rolling finishing temperature is set to 80 as the hot rolling condition.
Although it is limited to the range of 0 ° C or less and 500 ° C or less, the reason is that spheroidization and graphitization of cementite can be easily progressed in the annealing process without adopting special annealing conditions, and the uniformity is excellent. This is done for the purpose of obtaining an organization.

The effect of the hot rolling finishing temperature will be described below based on the results of the investigation by the inventors.

Figure 3 shows the chemical composition of C0.6%, Si1.64%, Mn0.86%, P0.01.
5%, S0.007%, Al0.004%, N0.048% The balance of the steel consisting essentially of Fe is hot rolled steel strip with the hot rolling finish temperature changed from 900 ℃ to 600 ℃ at 720 ℃. It shows tensile properties and impact properties after annealing for 40 hours.

As is clear from this, when the hot rolling finish temperature exceeds 800 ° C, the strength is high and the elongation and impact properties are poor. It can be seen that the ductility and toughness are remarkably improved.

By lowering the hot rolling finish temperature in this way,
The reason why the subsequent annealing brings about softening and improvement of ductility and toughness is due to the following mechanism.

As the first mechanism, the recrystallization of γ grains is extremely delayed in the rolling temperature range of 800 ° C or lower, so that the transformation structure after rolling becomes fine. When the structure thus refined is annealed next, nucleation sites for spheroidization and graphitization of cementite are increased, softening is promoted, and the spheroidized cementite particles and graphite grains formed are uniform and fine. Since a dispersed structure is obtained, ductility and toughness are improved.

Further, such finely and uniformly dispersed spheroidized cementite particles and graphite particles are easily dissolved in the austenite phase at the time of heating during the quenching treatment, so that they also have the effect of improving the quenchability.

As the second mechanism, rolling strain is introduced into the transformation structure due to retardation of recrystallization of γ grains due to low temperature rolling, which causes an increase in dislocation density of the base ferrite phase and an increase in internal stress for cementite. This effect is further enhanced when the hot rolling finishing temperature is further lowered and rolling is performed in a part of the γ + α region. As a result, cementite in the transformed structure after hot rolling becomes very unstable and dislocations become nucleation sites for cementite spheroidization and graphitization.
Annealing easily causes spheroidization and graphitization, and softening is significantly promoted. Further, the decrease in hot rolling finish temperature makes the black scale scale thin, as described later, so that surface decarburization is less likely to occur even if the post-process annealing is performed with the black scale still attached. It also has advantages. In particular, the thickness of the black leather scale has a hot rolling finish temperature of 720.
When the rolling is completed in the region of less than ° C, the thickness is remarkably reduced, and thus the adoption of such rolling conditions is effective in preventing surface decarburization.

In the present invention, the upper limit of the hot rolling finishing temperature is specified to 800 ° C. based on the above findings. By the way, the effect of such hot rolling finishing temperature is not limited to saturation at less than 500 ° C., the rolling load becomes large, and the wear of the rolling rolls becomes large.

Regarding the annealing conditions, the range of the annealing conditions was selected from the two viewpoints of obtaining an annealing structure that is most advantageous for softening and forming processability and that the annealing cost is low. This is because the lower limit of the annealing temperature range defined by the present invention is not economical because annealing at less than 500 ° C. significantly slows the progress of softening. Also, the upper limit is 75
This is because if the temperature exceeds 0 ° C., the proportion of the austenite phase increases during annealing, and this portion remains as a pearlite phase after annealing, which is a cause of hindering softening and homogeneity of the structure, which is not preferable.

A suitable annealing time is about 1 to 200 hours, but a lower annealing temperature requires a longer time. In terms of material, the optimum annealing temperature range is 650 ° C. to the A ₁ transformation point range. Particularly, if the temperature just below the A ₁ transformation point is selected, a good material can be obtained by annealing in a short time. In addition, as an annealing cycle, for example, once heating to a temperature in the α + γ2 phase temperature range, then slowly cooling to a temperature range below the A ₁ transformation point at a very slow cooling rate, or holding in the temperature range below the A ₁ transformation point Even if this method is adopted, the annealing time can be shortened and the material can be improved.

Hereinafter, when manufacturing the steel of the present invention, when annealing is performed with the black scale still attached, C of the steel surface layer portion during the annealing and FeO, Fe ₃ O ₄ , Fe ₂ O _{3 in the} scale composition are produced. And the like may react with each other to form a decarburized layer on the surface layer of the steel sheet. When the depth of such a decarburized layer is considerably large, the surface hardness after quenching decreases and the wear resistance deteriorates. In order to avoid this, if the black scale is removed using a pickling device or a mechanical descaling device and then annealed, no problem will occur, but it is disadvantageous in terms of manufacturing cost. In order to eliminate this point, it is necessary to take measures so that surface decarburization does not occur even when the black scale is annealed, and the following three points can be mentioned as means.

First of, the surface decarburization reaction becomes active enough annealing temperature becomes high, particularly alpha + gamma becomes a region with 500 ° C. or less ₁ point A when the reaction rate of annealing remains black bark scale since significantly larger It is desirable to adopt an annealing temperature in the range of.

Second, if the annealing atmosphere gas contains H2 gas, or if the dew point is high, the decarburization reaction will be accelerated.
It is preferable to use an inert gas such as N ₂ gas or Ar gas.

Third, since the depth of the surface decarburized layer increases in proportion to the thickness of the black scale, in order to reduce the surface decarburization, as mentioned above, the reduction of the rolling finish temperature during hot rolling or the coiling temperature is required. It is effective to take measures such as reduction of the thickness in advance and sufficiently reduce the thickness of the black scale.

As described above, by taking measures in view of the annealing condition and the hot rolling condition, it is possible to avoid the problem of the surface decarburization even if the black leather scale descaling step is omitted.

The hot-rolled steel strip after annealing can be provided as a raw material for a cultivator claw after being divided into a predetermined width by using a width dividing device such as a slitter, and then wound as a slit coil. Since the plate made of is soft and has excellent workability, it can be sheared very easily, and it is also excellent in cold punching workability for the processing of cultivator claws. There is no need for labor-intensive processing such as hot roll forming A, B, and C after heating the small piece 2 obtained by shearing the flat bar 1 shown in the figure.

(Examples) Table 1 shows the chemical compositions of the test steels, and Table 2 summarizes the hot rolling and subsequent annealing conditions and the results depending on them.

(Effect of the Invention) According to the present invention, as a typical example of a cultivator claw, of a wear-resistant part that requires forging such as hot roll forming after heating a shearing piece from a conventional flat bar. A material that not only makes it possible to perform simpler cold working instead of hot working, but also can comply with the demand for hardenability by heat treatment after forming, and is also advantageous for the operation of the above cold working equipment. It is also useful for smooth and stable supply. Further, the present invention can be widely used not only in the field of cultivating claws, but also in the fields of blades, springs, and various other machine parts for processing purposes, and has a great effect.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of the graphite ratio on the mechanical properties and hardenability, and FIG. 2 is a graph showing the effect of the size of graphite particles on the hardenability. FIG. 4 is a graph showing the relationship between hot finishing temperature and mechanical properties, and FIG. 4 is an explanatory diagram of a conventional procedure for processing a tiller claw.

Claims (3)

[Claims] 1. C: 0.40 to 0.80 wt%, Si: 0.20 to 2.00 wt%, Mn: over 0.50 to 1.50 wt%, Al: 0.001 to 0.150 wt%, P: 0.018 wt% or less, S: 0.010 wt% Hereafter, N: 0.0050 wt% or less, consisting of balance Fe and unavoidable impurities, and having a structure mainly composed of ferrite phase and graphite phase and having a graphitization ratio of 40% or more and a graphite average particle size of 10 μm or less. , TS ≦ 60 kgf / mm ² of a soft material, a steel sheet with excellent workability and toughness and good hardenability. 2. C: 0.40 to 0.80 wt%, Si: 0.2 to 2.00 wt%, Mn: more than 0.50 to 1.50 wt%, Al: 0.001 to 0.150 wt%, P: 0.018 wt% or less, S: 0.010 wt% In the following, N: 0.0050 wt% or less, and Ti: 0.10 wt% or less, Nb: 0.05 wt% or less, Zr: 0.050 wt% or less, and B: 0.01 wt% or less, including one or more types. , Balance Fe and unavoidable impurities, mainly composed of ferrite phase and graphite phase, with a graphitization ratio of 40% or more and a graphite average particle size of 10 μm
A steel sheet having excellent workability and toughness, and having good hardenability, which is characterized by having a soft material having the following structure and TS ≦ 60 kgf / mm ² . 3. C: 0.40 to 0.80 wt%, Si: 0.2 to 2.00 wt%, Mn: over 0.50 to 1.50 wt%, Al: 0.001 to 0.150 wt%, P: 0.018 wt% or less, S: 0.010 wt% Hereinafter, steel containing N: 0.0050 wt% or less as a basic component is hot-rolled at a finish rolling temperature of 800 ° C or more and 500 ° C or more, and wound into a coil to form a hot-rolled steel strip. It consists of annealing the steel strip in the temperature range of 500 ℃ ~ 750 ℃ for 1 ~ 200 hours, mainly composed of ferrite phase and graphite phase, the graphitization ratio is 40% or more and the average graphite particle size is 10%.
A method for producing a steel sheet having excellent workability and toughness and good hardenability, which is characterized by expressing a soft material with TS ≤ 60 kgf / mm ² in a structure of μm or less.