JP2008231544A - Non-tempered steel product and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an S-containing middle carbon non-tempered steel product having high upper shelf energy and excellent toughness. <P>SOLUTION: The non-tempered steel product contains 0.30 to 60% C, 0.05 to 0.5% Si, 0.2 to 1.50% Mn, ≤0.035% P, 0.03 to 0.10% S, ≤0.02% Ti, ≤0.05% Al, and ≤0.03% N, and consists of the balance Fe and impurities, in which the form and distribution of MnS at a plane parallel to a hot working direction satisfy the conditions that (1) the area of MnS in ≤10% of an aspect ratio is ≥50% with respect to the total MnS area, and that (2) the area of MnS of ≤2 μm in the width perpendicular to the maximum longitudinal direction is ≥75% with respect to the total MnS area, and in which the texture of the matrix is a ferrite-pearlite texture of 40 to 60% in the ratio of ferrite. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a non-heat treated steel material and a method for producing the same, and more particularly, to an S-containing medium carbon non-heat treated steel material used in hot working and a method for producing the same, and more particularly to automobiles, industrial machinery and construction machinery. The present invention relates to an S-containing medium carbon non-heat-treated structural steel material suitable as a material for non-heat treated machine parts and the like and a method for producing the same.

  Conventionally, machine parts such as automobiles, industrial machines, and construction machines have been manufactured by repeatedly using hot working such as rolling and forging using medium carbon structural steel as a raw material, but this hot working is required. Since the main objective is to shape to product dimensions, the resulting structure is generally a coarse, two-phase structure of a mixed structure of ferrite and pearlite. For this reason, refinement | miniaturization of a structure | tissue is achieved by giving a tempering process (quenching-tempering process), and the said mechanical parts are made to have a desired mechanical property.

  However, from the viewpoint of cost reduction and energy saving, it is desired to omit the tempering process, and in recent years, the demand has become particularly large.

  On the other hand, there is a problem that non-heat treated steel materials that are used without being subjected to tempering treatment while being hot worked are inferior in mechanical properties, in particular, toughness, as compared with tempered steel materials.

  For this reason, for example, Patent Literature 1 and Patent Literature 2 propose a technique for obtaining a medium carbon non-tempered steel material having high toughness even if the tempering treatment is omitted.

Specifically, in Patent Document 1, in mass%, C: 030 to 0.80%, Si: 0.1 to 2.5%, Mn: 0.30 to 2.0%, Al: 0.001 -0.06%, N: 0.005-0.10%, P: 0.30% or less (including 0%), S: 0.12% or less (including 0%), Cr: 1.0 % Or less (including 0%), Cu: 0.3% or less (including 0%) and Ni: 0.3% or less (including 0%), and if necessary, Pb: 0.3 % Or less (excluding 0%), Zr: 0.2% or less (not including 0%), Ca: 0.010% or less (not including 0%), Te: 0.10% or less (0% And Bi: not less than 0.1% (not including 0%), the balance being Fe and inevitable impurities, and [element] in the formula being mass% [Si] + 3.4 × [Mn] + 19.5 × [P] −13.4 × [S] + 2.7 × [Cr] ≧ 3.5 and [C] + 1.1 × [ Mn] -1.9 × [S] + 1.5 × [Cu] + 1.8 × [Ni] + 0.6 × [Cr] ≦ 2.6 Furthermore, Ti: 0.05% or less It is contained (not including 0%), and the Ti in any cross section carbides, nitrides, there sulfides or average particle size 10nm or more precipitates consisting of complex compounds, or 3 per 1 [mu] m ² “Non-tempered steel for hot forging” having a tensile strength of 600 to 900 N / mm ² is disclosed.

  In Patent Document 2, in terms of% by weight, C: 0.2-0.6%, Si: 0.05-1.0%, Mn: 0.1-3.0%, Al: 0.01-0. 05%, N: 0.005 to 0.050%, B: 0.0005 to 0.01%, V: 0.01 to 0.4%, Nb: 0.001 to 0.05%, Ti : One or more of 0.001 to 0.05% is included, and if necessary, (a) S: ≦ 0.15%, Pb: ≦ 0.3%, Ca: ≦ 0 0.005%, Bi: ≦ 0.3%, Te: ≦ 0.3%, or 2 or more types, (b) Cr: 0.01-2.0%, Ni: 0.01-2 0.0%, Mo: 0.01 to 1.0% of one or more elements selected from the group of one or more elements, the balance being substantially Fe and inevitable impurities "High toughness non-heat treated steel" It has been disclosed.

JP-A-9-310152 JP-A-6-256892

  The technique disclosed in Patent Document 1 utilizes the pinning effect caused by precipitates made of Ti carbide, nitride, sulfide, or a composite compound thereof to refine austenite crystal grains and improve toughness. It is intended.

  Further, the technique disclosed in Patent Document 2 aims to improve toughness by finely dispersing BN and utilizing its pinning effect.

  When the steel in which pinning particles are precipitated, as proposed in Patent Document 1 and Patent Document 2, is hot-worked, the grain size becomes finer and the ductile brittle transition temperature called “fracture surface transition temperature” decreases. Thus, as schematically shown in FIG. 1, the absorbed energy at room temperature is certainly increased and the toughness is improved. In addition, what was shown with the dotted line in FIG. 1 is a "Charpy absorbed energy-temperature" curve when there is little S content and the particle size is not refined | miniaturized.

  However, in recent years, in order to improve the machinability of Pb-free (no Pb added) medium carbon structural steel due to environmental problems, a large amount of S, which is well known as a so-called “free-cutting element”, may be contained. . And in steel containing a large amount of S, since MnS generally precipitates, stress concentration occurs in the MnS when subjected to external stress, and it becomes a starting point of fracture. As shown schematically in FIG. The upper shelf energy will be lower.

  For this reason, the refinement of austenite crystal grains contributes to the improvement of toughness from the viewpoint of obtaining a low “fracture surface transition temperature” because the ductile brittle transition temperature is low, but in steels containing a large amount of S, room temperature Since the absorbed energy at the time becomes low, it may not be useful for improving toughness from the viewpoint of obtaining high “absorbed energy”.

  Accordingly, an object of the present invention is to provide an S-containing medium-carbon non-heat treated steel material having a high upper shelf energy and excellent toughness, and a manufacturing method thereof, in particular, a non-containing steel containing 0.03% or more by mass%. Even in the case of tempered steel, it has a high upper shelf energy, and it has excellent toughness as a material for non-tempered machine parts such as automobiles, industrial machinery and construction machinery. It is providing the manufacturing method of quality steel materials.

  In order to solve the above-mentioned problems, the present inventors conducted a test simulating hot working using medium carbon steel containing 0.03% or more by mass%, and the morphology and distribution of MnS are The influence on the upper shelf energy and the influence of the conditions of thermomechanical treatment on the morphology and distribution of MnS were examined in detail.

  As a result, the following findings (a) to (e) were obtained.

  (A) The decrease in the upper shelf energy of the medium carbon steel material containing 0.03% or more of S by mass% is the aspect ratio of MnS by hot working, that is, the maximum length of MnS (hereinafter, “maximum length”). This is also because the ratio between the width perpendicular to the maximum length direction and the width perpendicular to the maximum length direction increases, and the probability that MnS exists on the fracture surface increases.

  FIG. 2 is a conceptual diagram regarding the “aspect ratio” and “width” of the MnS.

  (B) Even if it is a medium carbon steel material containing 0.03% or more of S by mass%, the area of MnS having an aspect ratio of 10 or less as the form and distribution of MnS in the plane parallel to the hot working direction If the area of MnS that is 50% or more with respect to the total MnS area and the width perpendicular to the maximum length direction is 2 μm or less is 75% or more with respect to the total MnS area, the upper shelf energy is reduced. Absent.

  (C) If the MnS in the plane parallel to the hot working direction of the steel material containing 0.03% or more of S by mass% is the form and distribution of (b) above, the upper shelf energy does not decrease. In addition, by promoting the formation of intragranular ferrite with MnS as a nucleus, the proportion of ferrite in the structure (hereinafter also referred to as “ferrite ratio”) increases, and the upper shelf energy further increases.

  (D) In a steel material containing 0.03% or more of S by mass%, in order to have the form and distribution of MnS in a plane parallel to the hot working direction as described in (b) above, For example, if a workpiece is heated to 1000 to 1250 ° C. and subjected to hot working with a cumulative area reduction of 90% or more in a temperature range of 900 ° C. or higher, the workpiece is held for 3 minutes or more in a temperature range of 900 ° C. or higher. Good.

  This is because MnS is deformed with hot working, but is heated to 1000 to 1250 ° C., and undergoes processing at a high surface reduction rate of 90% or more in a temperature range of 900 ° C. or higher, and the width is small. In the case of the formed MnS, if the temperature is kept at 900 ° C. or higher immediately after processing for 3 min or longer, the maximum length is shortened by dividing from the narrowed portion due to the surface tension, and the width of the MnS is reduced. This is because the aspect ratio is small because the maximum length is short even if it is small.

  (E) Moreover, in order to increase the ferrite ratio of (c) and obtain a higher upper shelf energy, for example, a steel material that has been hot-worked under the condition (d) above is further 700 ° C. or higher. What is necessary is just to perform hot working with a cumulative surface reduction rate of less than 90% in a temperature range of less than 900 ° C.

  That is, by further performing hot working with a cumulative area reduction of less than 90% in a temperature range of 700 ° C. or more and less than 900 ° C., not only ferrite transformation from the processed austenite occurs, but also the intragranular ferrite with MnS as the core. Since the generation also occurs, the ferrite ratio can be increased to 40% or more.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in the non-heat treated steel material shown in (1) below and the production method shown in (2).

(1) By mass%, C: 0.30 to 0.60%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.5%, P: 0.035% or less, S: Contains 0.03-0.10%, Ti: 0.02% or less, Al: 0.05% or less, and N: 0.03% or less, with the balance being Fe and impurities, parallel to the hot working direction The shape and distribution of MnS on a rough surface satisfy the conditions <1> and <2> below, and the matrix structure is a ferrite pearlite structure with a ferrite ratio of 40 to 60%. Quality steel.
<1> The area of MnS having an aspect ratio of 10 or less is 50% or more based on the total MnS area;
<2> The area of MnS whose width perpendicular to the maximum length direction is 2 μm or less is 75% or more with respect to the total MnS area.

  (2) By mass%, C: 0.30 to 0.60%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.5%, P: 0.035% or less, S: 0.03 to 0.10%, Ti: 0.02% or less, Al: 0.05% or less, and N: 0.03% or less, with the balance being made of steel with Fe and impurities, 1000 to 1250 After heating to 90 ° C. and performing hot working with a cumulative area reduction of 90% or more in a temperature range of 900 ° C. or higher, hold for 3 to 10 minutes in a temperature range of 900 ° C. or higher, and then further 750 ° C. or higher and lower than 900 ° C. The method for producing a non-tempered steel material according to claim 1, wherein hot working is performed in a temperature range of 5% or more and less than 90%.

  Note that the “surface parallel to the hot working direction” is, for example, “the surface in the rolling direction” if the hot working is “hot rolling”, and the hot working is “hot forging”. For example, "plane parallel to the training axis".

“Total MnS area” means that the observation surface is 64 fields of view at a magnification of 400 times and a total of 1.44 mm ² according to “Microscopic test method for non-metallic inclusions in steel” described in JIS G 0555 (2003). The total of MnS areas when a considerable area is observed.

  It is assumed that the ferrite constituting the pearlite is not included in the ratio of ferrite in the ferrite-pearlite structure as the matrix.

  “Raw steel” refers to, for example, a steel ingot, a continuous cast material, or a billet obtained by subjecting them to a hot work, such as a billet obtained by split rolling.

  The above “temperature” refers to the “surface temperature” of the target material steel or hot-worked steel.

  Hereinafter, the invention related to the non-heat treated steel material of (1) and the invention related to the method of manufacturing the non-heat treated steel material shown in (2) are referred to as “the present invention (1)” and “the present invention (2)”, respectively. . Also, it may be collectively referred to as “the present invention”.

  Although the non-tempered steel material of the present invention contains 0.03% or more of S by mass%, high upper shelf energy can be obtained even if the tempering treatment after hot working such as rolling or forging is omitted. Therefore, it is suitable as a material for non-tempered machine parts such as automobiles, industrial machines and construction machines, and greatly contributes to a reduction in manufacturing costs. In addition, this non-tempered steel material can be manufactured by the method of the present invention.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”.

(A) Chemical composition:
C: 0.30 to 0.60%
C is effective for ensuring the strength of steel, and is also very effective for ensuring wear resistance in non-heat treated machine parts such as automobiles, industrial machines and construction machines. In order to acquire these effects, it is necessary to make C content 0.30% or more. However, if the C content increases, especially when it exceeds 0.60%, the upper shelf energy is reduced. Therefore, the content of C is set to 0.30 to 0.60%. In addition, the desirable range of C content is 0.38 to 0.55%.

Si: 0.05-0.5%
Si is an element having a deoxidizing effect, and is also an element effective for strengthening the solid solution of ferrite, so 0.05% or more is contained. However, since the hot workability is impaired when the Si content is excessive, the upper limit of the content is 0.5% necessary and sufficient for solid solution strengthening of ferrite. Therefore, the Si content is set to 0.05 to 0.5%. In addition, the desirable range of Si content is 0.15-0.4%.

Mn: 0.2 to 1.5%
Mn is an element necessary for improving the machinability by forming MnS in steel, and is contained in an amount of 0.2% or more. However, if the Mn content is excessive, the amount of MnS is increased unnecessarily, leading to a decrease in the upper shelf energy. In particular, when it exceeds 1.5%, the upper shelf energy is significantly decreased. Therefore, the Mn content is set to 0.2 to 1.5%. In addition, the desirable range of Mn content is 0.4 to 1.0%.

P: 0.035% or less P is an element contained as an impurity in steel and segregates at the grain boundary to promote grain boundary embrittlement cracking. In particular, when the content exceeds 0.035%. , Grain boundary embrittlement cracking is likely to occur. Therefore, the content of P is set to 0.035% or less. In addition, since there exists an intensity | strength improvement effect | action in P, when acquiring this effect, it is desirable that content of P shall be 0.005-0.035%.

S: 0.03-0.10%
S is an element necessary for forming MnS to improve machinability. In addition, the MnS has the effect of promoting the formation of intragranular ferrite using this as a nucleus, increasing the ferrite rate, and increasing the upper shelf energy, that is, increasing the toughness. In order to acquire such an effect, S needs to be 0.03% or more. However, if the content of S is too large, not only the effect is saturated but also the hot workability is lowered, and particularly when it exceeds 0.10%, the hot workability is significantly lowered. Therefore, the content of S is set to 0.03 to 0.10%. In addition, the desirable range of S content is 0.03 to 0.07%.

Ti: 0.02% or less Ti is an element that may be contained as an impurity in steel, and if contained excessively, lowering of the upper shelf energy is caused. In particular, its content exceeds 0.02%. And the fall of the upper shelf energy becomes remarkable. Therefore, the Ti content is set to 0.02% or less. In addition, since Ti has the effect | action which forms a nitride with N and makes a particle size fine, when it wants to acquire this effect, content of Ti may be 0.005-0.02%. desirable.

Al: 0.05% or less Al is also an element that may be contained as an impurity in steel. If excessively contained, the upper shelf energy will be reduced, and in particular, its content exceeds 0.05%. And the fall of the upper shelf energy becomes remarkable. Therefore, the Al content is set to 0.05% or less. Al has a deoxidizing action, and also has an action of forming a nitride with N to make the particle size finer. Therefore, when these effects are desired, the Al content is 0.005. It is desirable to set it to -0.05%.

N: 0.03% or less N is an element contained as an impurity in steel, and if excessively contained, bubbles are generated. Particularly, when the content exceeds 0.03%, bubbles are generated. It becomes remarkable and damages the material, and it is often industrially difficult to contain N in an amount of 0.03% or more, resulting in an increase in cost. Therefore, the N content is set to 0.03% or less. N has an effect of increasing strength by solid solution strengthening and an effect of forming a nitride together with Ti and Al to make the particle size fine. Is preferably 0.003 to 0.03%.

  For the above reasons, the chemical composition of the non-tempered steel material according to the present invention (1) contains C, Si, Mn, P, S, Ti, Al and N in the above-mentioned range, and the balance is made of Fe and impurities. It was decided to become.

  In the non-heat treated steel material according to the present invention (1), elements other than the elements C to N described above are essentially impurities, and are not intentionally added.

  Here, from the standpoint of avoiding unnecessarily high costs in the steelmaking process for removing impurities and preventing hot cracking due to excessive inclusion, the contents of Cr, Cu and Ni in the impurities are all In the range of 0.3% or less, it is acceptable.

(B) MnS morphology and distribution in a plane parallel to the hot working direction:
Even when the non-tempered steel material according to the present invention (1) has the chemical composition described in the item (A), the form and distribution of MnS in the plane parallel to the hot working direction is “aspect ratio”. That the area of MnS of 10 or less is 50% or more with respect to the total MnS area, and “the area of MnS whose width perpendicular to the maximum length direction is 2 μm or less is the total MnS” When it deviates from at least one of the above-mentioned <2> conditions of “75% or more with respect to the area”, the upper shelf energy is reduced due to MnS.

  Therefore, in the non-tempered steel material according to the present invention (1), in order to prevent the lower shelf energy from being reduced due to MnS, the generation of intragranular ferrite with MnS as a nucleus is further promoted. In order to have a higher upper shelf energy by increasing the ferrite ratio, the MnS morphology and distribution in the plane parallel to the hot working direction is the total MnS area with an aspect ratio of MnS of 10 or less The area of MnS whose width perpendicular to the maximum length direction is 2 μm or less is 75% or more with respect to the total MnS area.

  As already described, the “surface parallel to the hot working direction” means, for example, “the surface in the rolling direction” if the hot working is “hot rolling”, and the hot working is “ “Hot forging” refers to “a plane parallel to the forging axis”.

In addition, “total MnS area” means that the observation surface is 64 fields of view at a magnification of 400 times and a total of 1. in accordance with “Microscopic test method of non-metallic inclusions in steel” described in JIS G 0555 (2003). The total area of MnS when an area corresponding to 44 mm ² is observed.

(C) Matrix organization:
Even when the non-heat treated steel material according to the present invention (1) has the chemical composition described in the item (A), the ferrite ratio in the ferrite pearlite structure as the matrix structure is less than 40%. Does not provide higher upper shelf energy.

  On the other hand, when the ferrite ratio in the ferrite-pearlite structure as the matrix structure exceeds 60%, the desired strength as the medium carbon structural steel cannot be maintained.

  Therefore, in the non-heat treated steel material according to the present invention (1), in order to have a higher upper shelf energy, the matrix structure is a ferrite pearlite structure having a ferrite ratio of 40 to 60%. Stipulated.

  As described above, the ferrite constituting the pearlite is not included in the ratio of ferrite in the ferrite-pearlite structure as the matrix.

(D) Production method of non-heat treated steel:
The non-heat treated steel according to the present invention (1) is, for example, a material steel having the chemical composition described in the above item (A), “cumulative surface reduction in a temperature range of 900 ° C. or higher by heating to 1000 to 1250 ° C. After performing hot working at a rate of 90% or higher, hold at a temperature range of 900 ° C. or higher for 3 to 10 minutes, and then, at a temperature range of 750 ° C. or higher and lower than 900 ° C., a cumulative area reduction rate of 5% or higher and lower than 90% According to the present invention (2), which is characterized by performing “hot working”.

That is, in the plane parallel to the hot working direction, the area of MnS having an aspect ratio of 10 or less described in the above (B) is 50% or more with respect to the total MnS area, and perpendicular to the maximum length direction. MnS morphology and distribution in which the area of MnS having a width of 2 μm or less is 75% or more with respect to the total MnS area, and a matrix of ferrite and pearlite structure in which the proportion of ferrite described in the section (C) is 40 to 60% To get
[1] The material steel is heated to 1000 to 1250 ° C. and hot-worked with a cumulative area reduction of 90% or more in a temperature range of 900 ° C. or higher, and then held for 3 to 10 minutes in a temperature range of 900 ° C. or higher. ,
[2] After the treatment of [1], hot working is further performed with a cumulative area reduction of 5% or more and less than 90% in a temperature range of 750 ° C. or more and less than 900 ° C.
It is effective to perform the process in two stages.

  In addition, as already stated, the “material steel” referred to in the above-mentioned process [1] is, for example, a steel ingot, a continuous cast material, or a billet obtained by subjecting them to hot rolling, etc. Point to.

  In addition, as described above, the “temperature” refers to the “surface temperature” of the target material steel or hot-worked steel.

(D-1) Regarding the processing of [1]:
By performing the treatment [1], the width of MnS is reduced, and the aspect length is also reduced because the maximum length is shortened by dividing from a particularly narrow portion by surface tension.

  When the heating temperature is lower than 1000 ° C., the deformation resistance of MnS itself is increased and MnS is difficult to deform, so that it is difficult to effectively reduce the width of MnS. In addition, even when the heating temperature is higher than 1250 ° C., the difference in deformation resistance between MnS and austenite, which is a matrix, is large, and MnS is difficult to deform compared to the matrix, so it is difficult to reduce the width of MnS. .

  Similarly, when the temperature of hot working is lower than 900 ° C., it is difficult to effectively reduce the width of MnS. In addition, it is difficult to effectively reduce the width of MnS even when the cumulative area reduction in a temperature range of 900 ° C. or higher is less than 90%.

  In addition, in the hot working of [1] above, in the case where hot working cannot be performed such that the cumulative area reduction in a temperature range of 900 ° C. or higher is 90% or more by only one heating, 1000 to What is necessary is just to reheat to the temperature range of 1250 degreeC, and to make it the cumulative area reduction rate in the temperature range of 900 degreeC or more becoming 90% or more.

  If the cumulative area reduction rate of 900 ° C. or higher is excessively increased, the processing time and the processing cost are unnecessarily increased. Therefore, the cumulative area reduction rate in the temperature range of 900 ° C. or higher is desirably 98% or less. .

  Furthermore, after performing hot working with a cumulative area reduction ratio of 90% or more in a temperature range of 900 ° C. or higher, and holding for 3 minutes or more in a temperature range of 900 ° C. or higher, MnS is removed from a portion that is particularly narrow due to surface tension. The maximum length is shortened by dividing, and even if the width of MnS is small, the maximum length is short, so that the MnS has a small aspect ratio as shown in FIG.

  On the other hand, when the time kept in the temperature range of 900 ° C. or higher after the hot working is less than 3 min, there is not enough time for MnS to divide. The aspect ratio becomes large as shown in b).

  3A and 3B show the form and distribution of MnS in test number 3 and test number 12 in the examples described later, respectively.

  The holding time of 3 min or longer in the temperature range of 900 ° C. or higher may be secured by continuously cooling after the hot working, or may be secured by supplementary heating or heating. Further, supplementary heating, heating and cooling may be ensured repeatedly as long as “the time maintained in the temperature range of 900 ° C. or higher is 3 min or longer”.

  However, if the structure is held for a long time in a temperature range of 900 ° C. or higher, the structure is unnecessarily coarsened and the absorbed energy is reduced. Therefore, the upper limit of the holding time in the temperature range of 900 ° C. or higher is set to 10 min.

  Even when hot working with a cumulative area reduction ratio of 90% or more is performed in a temperature range of 900 ° C. or higher, the time that is maintained in the temperature range of 900 ° C. or higher is less than 3 minutes, and is temporarily 900 ° C. When the temperature is lowered to less than 90 ° C. and then reheated to 900 ° C. or higher and held at 900 ° C. or higher for 3 min or longer, no significant MnS fragmentation occurs. From this, after performing hot working with a cumulative area reduction of 90% or more in a temperature range of 900 ° C. or higher, the MnS fragmentation by holding for 3 min or longer in the temperature range of 900 ° C. or higher is the heat of [1]. It is inferred that the machining strain existing immediately after the inter-machining occurs as a driving force.

(D-2) Regarding process [2]:
After performing the process [1], the process [2] is further performed, that is, hot working with a cumulative area reduction of 5% or more and less than 90% in a temperature range of 750 ° C. or more and less than 900 ° C. This not only causes ferrite transformation from the processed austenite, but also produces intragranular ferrite with MnS as the nucleus, so that the ferrite ratio increases.

  Even if MnS is divided by the treatment [1], if the cumulative area reduction in the temperature range of 750 ° C. to less than 900 ° C. in the treatment [2] is 90% or more, the divided MnS Will be stretched and the aspect ratio will increase, making it difficult to obtain the desired form and distribution of MnS.

  Further, even if the cumulative area reduction ratio in the treatment of [2] is less than 90%, when the processing temperature is 900 ° C. or higher, MnS is more easily stretched, so the aspect ratio is increased, It becomes difficult to obtain the desired form and distribution of MnS. Moreover, since it is not possible to promote the formation of intragranular ferrite with MnS as a nucleus, it is difficult to improve the ferrite rate.

  When the processing temperature is less than 750 ° C., it is not processing in the austenite single-phase region, and ferrite transformation from the processed austenite cannot be caused. The lower limit temperature of the temperature range below 900 ° C. is 750 ° C.

  Further, in order to cause ferrite transformation from the processed austenite, the lower limit of the cumulative area reduction in the hot working performed in a temperature range of 750 ° C. or higher and lower than 900 ° C. is set to 5%.

  In addition, the hot working in the process [2] may be performed continuously from the process [1], or the temperature once lowered after the process [1] You may apply after heating again at 800 degreeC.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steels 1 to 12 having the chemical composition shown in Table 1 were melted in a vacuum melting furnace to produce a cylindrical ingot having a diameter of about 230 mm.

  In addition, the steel 1-3 in Table 1 and the steel 9-12 are steel which has a chemical composition in the range prescribed | regulated by this invention. On the other hand, Steels 4 to 8 are steels in which the S content deviates from the conditions specified in the present invention.

  After heating said ingot to 1200-1250 degreeC and hold | maintaining for 2 hours, hot forging was performed and the square material of thickness 100mm x width 100mm, thickness 60mm x width 60mm, and thickness 30mm x width 60mm was manufactured. .

  During hot forging, the forging process was repeated by reheating to the same temperature as the initial heating temperature so that the temperature during forging did not fall below 900 ° C. Therefore, the cumulative area reduction ratios in the temperature range of 900 ° C. or higher of the above-mentioned square members having a thickness of 100 mm × width of 100 mm, thickness of 60 mm × width of 60 mm, and thickness of 30 mm × width of 60 mm are 75%, 90% and 95%, respectively. It is.

  In addition, although the completion temperature of said hot forging changes with heating temperature and the dimension (thus total area reduction rate) of the manufactured square bar, it was made not to fall below 1050 degreeC.

  After completion of hot forging, the product was cooled to room temperature by cooling in the air or water cooling.

  Table 2 shows details of the heating temperature for hot forging, the cumulative area reduction in a temperature range of 900 ° C. or higher, and the holding time in a temperature range of 900 ° C. or higher.

  In Table 2, each test number having a holding time of 3 to 4 minutes in a temperature range of 900 ° C. or higher is left to cool in the air after hot forging. In addition, each test number having a holding time of 3 sec in a temperature range of 900 ° C. or higher is water-cooled after completion of hot forging.

  In addition, as already stated, each said temperature points out the "surface temperature" of what became the target ingot and it hot forged.

  About the square material of thickness 100 mm × width 100 mm, thickness 60 mm × width 60 mm and thickness 30 mm × width 60 mm obtained as described above, a test piece is cut out from the central portion of the thickness and width, embedded in resin, and heated. The surface parallel to the forging direction, that is, the surface parallel to the forging axis is mirror-polished so that it becomes the observation surface, and then corroded with nital, and described in JIS G 0551 (2005). The ferrite ratio was calculated by image analysis of the field of view corresponding to a circle with a diameter of 0.8 mm.

  In addition, the remaining portions of the square material having a thickness of 100 mm × width of 100 mm, thickness of 60 mm × width of 60 mm, and thickness of 30 mm × width of 60 mm were heated to 1050 ° C. and held for 30 minutes, and then at 800 ° C. Hot rolling with a reduction in area of 40% was performed to finish steel sheets each having a thickness of 52 mm × width of 115 mm, thickness of 31 mm × width of 70 mm, and thickness of 16 mm × width of 68 mm.

  The 40% area reduction rate at 800 ° C. corresponds to the cumulative area reduction rate in the temperature range below 900 ° C. in the present invention (2). In Table 2, the hot rolling is also shown.

Next, a test piece is cut out from the center of each rolled steel plate obtained as described above, embedded in resin, and mirror-polished so that the surface parallel to the hot rolling direction, that is, the surface in the rolling direction becomes the observation surface. According to JIS G 0555 (2003), “Microscopic test method for non-metallic inclusions in steel”, the observation surface is 64 fields at a magnification of 400 times, and a total area equivalent to 1.44 mm ² is confocal. Observation was made using a microscope, and the morphology and distribution of MnS were determined by image processing. In addition, the mirror-polished surface is corroded with nital, the matrix structure is observed, and a circle with a diameter of 0.8 mm is used in accordance with “steel-crystal grain size microscopic test method” described in JIS G 0551 (2005). The ferrite ratio was calculated by image analysis of the visual field of the area corresponding to.

  Further, a “U-notch test piece” having a width of 10 mm described in JIS Z 2202 (1998) was cut out from a direction parallel to the rolling direction of the central portion in the thickness direction of each rolled steel sheet, and a Charpy impact test was performed at 100 ° C. Carried out.

  Table 3 summarizes the above test results.

The matrix structure of the hot-rolled steel sheet was all “ferrite / pearlite structure”. For this reason, in Table 3, description of the matrix structure of the hot-rolled steel sheet was omitted. In the Charpy test, since ductile fracture surfaces were obtained for all the test pieces, in Table 3, the absorbed energy at 100 ° C. was expressed as upper shelf energy ( _U E _USE ).

  FIG. 3 shows the influence of the retention time in the temperature range of 900 ° C. or higher on the shape and distribution of MnS after the hot working with a cumulative area reduction of 90% or higher in the temperature range of 900 ° C. or higher. As an example, the case of test number 3 and test number 12 is shown. FIG. 4A shows the case of test number 3 and the holding time is 3 min. FIG. 5B shows the case of test number 12, and the holding time is 3 sec.

  From Table 3, in the case of Test Nos. 1 to 3 using steels 1 to 3 whose chemical composition is within the range specified in the present invention, the form of MnS, despite containing 0.03% or more of S, Since the distribution is within the range specified in the present invention (1) and the ferrite ratio after hot rolling is increased with respect to after hot forging, the upper shelf energy is 108.8 to 119.2 J, Test numbers 4 to 8 using steels 4 to 8 which are not only higher than those of test numbers 9 to 12 using steels 9 to 12 having the same S content, but also have a low S content of 0.008 to 0.018%. It is higher than that of 8 and it is clear that the toughness is excellent.

  On the other hand, in the case of test numbers 4 to 8 using steels 4 to 8 whose S content deviates from the conditions specified in the present invention, the upper shelf energy is 95 at most regardless of the form and distribution of MnS. Only 2J.

  Moreover, in the case of test numbers 9-12, although the chemical composition of the steels 9-12 used was in the range prescribed | regulated by this invention, the form and distribution of MnS are outside the range prescribed | regulated by this invention (1). Therefore, the upper shelf energy is low.

  That is, in the case of test numbers 9 and 10 using steels 9 and 10, since the area of MnS whose width perpendicular to the maximum length direction is 2 μm or less is less than 75% with respect to the total MnS area, Shelf energy is 79.2 J and 80 J, respectively, and the S content is lower than that of test numbers 4 to 8 using steels 4 to 8 with a low S content of 0.008 to 0.018%, and the toughness is extremely inferior It is.

  In the case of the test numbers 9 and 10, the area of MnS whose width perpendicular to the maximum length direction with respect to the total MnS area is 2 μm or less is small as shown in Table 2 in hot forging. This is based on the fact that the width of MnS could not be effectively reduced because the cumulative area reduction in the temperature range of 900 ° C. or higher is as low as 75%.

  Similarly, in the case of test numbers 11 and 12 using steels 11 and 12, since the area of MnS having an aspect ratio of 10 or less is less than 50% with respect to the total MnS area, the upper shelf energy is 84 respectively. 8J and 74.4J, and the S content is lower than that of test numbers 4 to 8 using steels 4 to 8 having a low content of 0.008 to 0.018%, and the toughness is extremely inferior.

  In the case of the test numbers 11 and 12, the area of MnS having an aspect ratio of 10 or less with respect to the total MnS area is small, as shown in Table 2, at a temperature of 900 ° C. or higher after completion of hot forging. Since the holding time in the region is as extremely short as 3 sec, even when MnS has a short width, it does not have time to divide it, so that it remains based on a large aspect ratio.

  Although the non-heat treated steel material of the present invention contains 0.03% or more of S by mass%, high upper shelf energy can be obtained even if heat treatment after hot working such as rolling or forging is omitted. Therefore, it is suitable as a material for non-tempered machine parts such as automobiles, industrial machines and construction machines, and greatly contributes to a reduction in manufacturing costs. In addition, this non-tempered steel material can be manufactured by the method of the present invention.

It is a figure which shows typically the improvement of toughness based on the fall of the ductile brittle transition temperature by refinement | miniaturization of a particle size, and the fall of toughness based on the upper shelf energy by MnS becoming low. It is a figure explaining the aspect-ratio and width | variety of MnS. It is a figure which shows an example of the influence which the holding time in the temperature range of 900 degreeC or more has received on the form and distribution of MnS after receiving the hot working of the cumulative surface reduction rate of 90% or more in the temperature range of 900 degreeC or more. . (A) is the case of test number 3 of an Example, The said holding time is 3 minutes. (B) is the case of test number 12 of the Example, and the holding time is 3 sec.

Claims (2)

In mass%, C: 0.30 to 0.60%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.5%, P: 0.035% or less, S: 0.03 -0.10%, Ti: 0.02% or less, Al: 0.05% or less, and N: 0.03% or less, with the balance being composed of Fe and impurities in a plane parallel to the hot working direction A non-tempered steel material in which the form and distribution of MnS satisfy the following conditions <1> and <2>, and the matrix structure is a ferrite pearlite structure with a ferrite ratio of 40 to 60%.
<1> The area of MnS having an aspect ratio of 10 or less is 50% or more based on the total MnS area;
<2> The area of MnS whose width perpendicular to the maximum length direction is 2 μm or less is 75% or more with respect to the total MnS area.   In mass%, C: 0.30 to 0.60%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.5%, P: 0.035% or less, S: 0.03 ~ 0.10%, Ti: 0.02% or less, Al: 0.05% or less, and N: 0.03% or less, with the balance being heated to 1000 to 1250 ° C with steel made of Fe and impurities After performing hot working with a cumulative area reduction ratio of 90% or more in a temperature range of 900 ° C. or higher, hold for 3 to 10 minutes in a temperature range of 900 ° C. or higher, and then further in a temperature range of 750 ° C. or higher and lower than 900 ° C. The method for producing a non-tempered steel material according to claim 1, wherein hot working is performed at a cumulative area reduction of 5% or more and less than 90%.

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