JP5543814B2 - Steel plate for heat treatment and method for producing steel member - Google Patents

Description

  The present invention relates to a steel plate for heat treatment and a method for producing a steel member. In detail, this invention relates to the manufacturing method of the steel plate for heat processing used in order to manufacture steel members, such as a motor vehicle part and a machine structural component, and the manufacturing method of the steel member produced using this steel plate for heat processing. .

  Some steel members such as automobile parts and machine structural parts are required to have excellent toughness and fatigue characteristics. Such a steel member is manufactured by performing a heat treatment (quenching and tempering treatment) after forming a heat-treated steel sheet into a desired part shape. From the viewpoint of quality stability, Hardenability and the like are required for the steel sheet for heat treatment. In addition, the steel member is desired to have high strength from the viewpoint of improving fatigue characteristics and weight reduction, but there is a concern that toughness and delayed fracture characteristics may be lowered by increasing the strength. That is, the strength of a steel member is generally determined mainly by the carbon content. Increasing the carbon concentration improves fatigue properties but decreases toughness and delayed fracture properties.

Various studies have been conducted to improve these various properties. For example, in Patent Document 1, by preparing a steel member using a heat-treating steel plate having a structure in which spherical carbide having a relatively large particle size is dispersed in equiaxed ferrite, the fracture starting point of the steel member is reduced. Improves toughness. Moreover, in patent document 2, the toughness of the steel member is improved by optimizing the carbon content at the time of solution treatment and controlling the size of undissolved carbide.
However, all of the techniques described in the above-mentioned patent documents improve the toughness of the steel member by optimizing the size and amount of undissolved carbide, and are produced by sufficiently performing austenitization (that is, The above technique cannot be applied to steel members that have no undissolved carbide. Moreover, the steel members described in these patent documents have insufficient fatigue characteristics and delayed fracture characteristics.

  On the other hand, the deterioration of delayed fracture characteristics of steel members is caused by hydrogen embrittlement, and many techniques for preventing this hydrogen embrittlement are known. As a typical thing, the thing regarding the steel member used for a volt | bolt is mentioned. This steel member has improved strength and delayed fracture characteristics by adding various special elements. However, due to the increase in material costs due to the addition of special elements, it is difficult to apply this technology to steel members such as automobile parts and machine structural parts.

  As another technique for preventing hydrogen embrittlement, Patent Document 3 proposes that the hydrogen embrittlement risk index (%) = 100 × (1−E1 / E0) be regulated to 30% or less. Here, E0 is the elongation at break of a steel member that does not substantially contain diffusible hydrogen, and E1 is the elongation at break of the steel member that contains diffusion hydrogen. However, Patent Document 3 only discloses that the hydrogen embrittlement risk index is specified, and the manufacturing conditions of the steel member are also unknown. Moreover, there is no description about the toughness and fatigue characteristics of the steel member, and the effect is unknown.

  In addition, Patent Document 4 has a structure containing 3.0% by volume or more of retained austenite, and tensile strength × elongation ≧ 18000 MPa ·%, prestrain: amount of limit diffusible hydrogen after application of 8% Has proposed a steel member with 0.5 ppm or more. However, Patent Document 4 does not describe the toughness and fatigue characteristics of the steel member, and the effect is unknown.

JP 2003-147485 A JP 2006-63384 A JP 2001-264240 A JP 2004-169180 A

As described above, fatigue characteristics (strength), toughness, and delayed fracture characteristics are contradictory characteristics, but it is desired to simultaneously improve these characteristics without causing an increase in cost.
That is, the present invention was made in order to solve the above-described problems, and a steel member having excellent workability and hardenability, and excellent fatigue characteristics, toughness, and delayed fracture characteristics. It aims at providing the manufacturing method of the steel plate for heat processing to give. Moreover, an object of this invention is to provide the manufacturing method of the steel member excellent in all the fatigue characteristics, toughness, and delayed fracture characteristics.

The inventors have found that the above problem can be solved by producing a steel sheet for heat treatment by a predetermined process using a slab having a predetermined composition, and have completed the present invention.
That is, the present invention is C: more than 0.1% by mass 0.4% by mass or less, Si: 0.5-1.5% by mass, Mn: 0.3-2% by mass, P: 0.02% by mass Hereinafter, S: 0.02 mass% or less, Cr: 0.1-2 mass%, Ti: 0.01-0.1 mass%, Nb: 0.01-0.1 mass%, Al: 0.1 mass% or less, B: 0.0022 to 0.01 mass%, N: see contains 0.01 mass% or less, after the balance was heated slab consisting of Fe and unavoidable impurities to a temperature above 1250 ° C., finishing Total rolling rate in rolling: 90% or more, Finishing temperature: Ar ₃ transformation point to Ar ₃ transformation point + 100 ° C., hot rolling at an average cooling rate: 40 ° C./sec or less, winding temperature: 450 to A coil is wound at 600 ° C. to form a hot rolled coil, and the hot rolled coil is pickled, spheroidized or cold rolled. It is the manufacturing method of the steel plate for heat processing used as the characteristic.
Moreover, this invention is a manufacturing method of the steel member characterized by performing the quenching and tempering process, after shaping | molding the steel plate for heat processing obtained by said manufacturing method.

  ADVANTAGE OF THE INVENTION According to this invention, while the workability and hardenability are favorable, the manufacturing method of the steel plate for heat processing which gives the steel member excellent in all the fatigue characteristics, toughness, and delayed fracture characteristics can be provided. Moreover, according to this invention, the manufacturing method of the steel member excellent in all the fatigue characteristics, toughness, and delayed fracture characteristics can be provided.

It is a graph which shows the relationship between the Vickers hardness of the heat-treated steel plate in Example 3, and a delayed fracture ratio. Sample No. in Example 3 46 (Example of the present invention) and Sample No. It is a microscope picture of the heat-treated steel plate of 51 (comparative example).

The method for manufacturing a steel sheet for heat treatment according to the present invention is characterized by using a slab having a predetermined composition and comprising a predetermined step. Here, in this specification, the “steel plate for heat treatment” means a steel plate before being subjected to heat treatment after being formed into a desired shape. Further, the “steel member” means a steel plate after being formed into a desired shape and heat-treated.
The composition of the slab used in the present invention is as follows.

<C: more than 0.1% by mass and less than 0.4% by mass>
C is an element necessary for ensuring strength and fatigue characteristics required for steel members such as automobile parts and machine structural parts. In the steel sheet for heat treatment that increases the strength of the steel member by heat treatment, it is desirable that the C content be greater than 0.1% by mass. On the other hand, if the C content exceeds 0.4% by mass, carbide precipitates on the prior austenite grain boundaries, and brittle fracture is likely to occur, and fatigue characteristics (particularly fatigue life) and delays due to a decrease in grain boundary strength. There is concern about the deterioration of fracture characteristics. In addition, workability and weldability are significantly deteriorated. For this reason, C content needs to be 0.1 mass% excess and 0.4 mass% or less.

<Si: 0.5 to 1.5% by mass>
Si is the most important element for improving toughness (particularly impact toughness), fatigue characteristics (particularly fatigue life), and delayed fracture characteristics. Here, Si suppresses the formation of film-like carbides during tempering, and precipitates fine carbides having an average particle size of 0.5 μm or less, thereby suppressing a decrease in grain boundary strength and improving fatigue life. Si is also an element effective in enhancing hardenability and temper softening resistance and ensuring strength after heat treatment. In order to obtain the above characteristics, the Si content needs to be 0.5 mass or more. However, if the Si content exceeds 1.5% by mass, coarse carbides are easily formed at the grain boundaries, and the fatigue life is reduced. Further, from the viewpoint of reducing the cold rollability, the Si content needs to be 0.5 to 1.5% by mass.

<Mn: 0.3-2 mass% mass%>
Mn is an element effective for ensuring hardenability and strength. In order to sufficiently obtain this effect, the Mn content needs to be 0.3% by mass or more. However, if the Mn content exceeds 2.0% by mass, the carbon equivalent also increases, which adversely affects workability and welded portion stability. Therefore, the Mn content needs to be 0.3-2% by mass. Further, from the viewpoint of reducing the prior austenite particle size after the heat treatment (quenching and tempering treatment), the Mn content is preferably 0.8% by mass or more.

<P, S: 0.02 mass% or less>
P is an element that lowers toughness and fatigue life by segregating to austenite grain boundaries during quenching and lowering the grain boundary strength. Therefore, the P content needs to be 0.02% by mass or less.
S is an element that forms MnS in steel, which becomes a starting point of cracks and causes a decrease in strength and toughness. Further, MnS segregates at the grain boundaries and reduces the fatigue life. Therefore, it is desirable to reduce the content as much as possible, and the S content needs to be 0.02% by mass or less.
By setting the upper limit of the content of P and S to 0.02% by mass, it is possible to suppress harmful effects caused by P and S.

<Cr: 0.1 to 2% by mass>
Cr is an element that is effective for improving the hardenability as well as Mn and increases the temper softening resistance. In order to sufficiently obtain this effect, the Cr content needs to be 0.1% by mass or more. However, if the Cr content exceeds 2% by mass, the structure after heat treatment (quenching and tempering treatment) contains a large amount of undissolved carbides, and this carbide serves as a starting point for promoting cracks, toughness and fatigue life. Cause a decline. Therefore, the Cr content needs to be 0.1 to 2% by mass.

<Ti: 0.01 to 0.1% by mass>
Ti is an element that contributes to securing solid solution B effective in improving hardenability by fixing N in steel as TiN. Ti is also an element that suppresses coarsening of the prior austenite grain size during quenching and improves the fatigue life. In order to obtain these effects sufficiently, the Ti content needs to be 0.01% by mass or more. However, even if adding Ti exceeding 0.1% by mass, the effect of suppressing the coarsening of the prior austenite grain size is saturated, and Ti-based inclusions that become the starting point of fatigue fracture increase. Therefore, the Ti content needs to be 0.01 to 0.1% by mass.

<Nb: 0.01 to 0.1% by mass>
Nb is an element that forms carbonitrides, suppresses coarsening of prior austenite crystal grains, improves toughness, and improves fatigue life. In order to sufficiently obtain these effects, the Nb content needs to be 0.01% by mass or more. However, even if Nb is added exceeding 0.1% by mass, the above effect is saturated and uneconomical. Therefore, the Nb content needs to be 0.01 to 0.1% by mass.

<Al: 0.1% by mass or less>
Al is an element effective for deoxidation and suppressing coarsening of austenite crystal grains during quenching. However, adding excess Al adversely affects toughness and fatigue life. Therefore, the Al content needs to be 0.1% by mass or less, preferably 0.05% by mass or less. Moreover, in order to acquire said effect effectively, it is preferable that the minimum of Al content shall be 0.01 mass%.

<B: 0.0005 to 0.01% by mass>
B is an element that enhances hardenability by adding a small amount. B is also an element that strengthens the prior austenite grain boundaries after heat treatment (quenching and tempering treatment), suppresses brittle fracture, and improves toughness. In order to sufficiently obtain these effects, the B content needs to be 0.0005% by mass or more. However, even if B is added over 0.01% by mass, the above effect is saturated and uneconomical. Therefore, the B content needs to be 0.0005 to 0.01% by mass, preferably 0.002 to 0.01% by mass.

<N: 0.01% by mass or less>
N is an element that inhibits the effect of B because B is consumed by the formation of BN. Therefore, the N content is desirably as low as possible. As a result of examining various compositions, the N content is allowed to be 0.01 mass, but is preferably 0.006 mass% or less.

<Ni: 0.5% by mass or less>
Ni is an optional component of the slab used in the present invention. Ni is an element effective for improving hardenability, toughness, and fatigue life. However, even if Ni is added in excess of 0.5% by mass, the above effect is saturated and uneconomical. Therefore, the Ni content needs to be 0.5% by mass or less. Moreover, in order to acquire said effect effectively, it is preferable that the minimum of Ni content shall be 0.1 mass%.

<Ca: 0.02 mass% or less>
Ca is an optional component of the slab used in the present invention. Ca is an element that reduces the anisotropy of the steel by spheroidizing the MnS inclusions. However, if Ca is added in excess of 0.02 mass%, Ca-based inclusions increase and fatigue characteristics are reduced. Therefore, the Ca content needs to be 0.02% by mass or less. Moreover, in order to acquire said effect effectively, it is preferable to make the minimum of Ca content into 0.001 mass%.

<Mo: 0.5% by mass or less>
Mo is an optional component of the slab used in the present invention. Mo is an element that has the effect of trapping hydrogen by forming carbides in addition to improving hardenability and temper softening resistance. Mo is also an element that suppresses toughness deterioration due to excessive addition of Mn and Cr. However, since Mo is an expensive element, it is uneconomical to add Mo in excess of 0.5 mass%. Therefore, the Mo content needs to be 0.5% by mass or less. Moreover, in order to acquire said effect effectively, it is preferable to make the minimum of Mo content into 0.1 mass%, Preferably it is 0.15 mass%.

<V: 0.5 mass% or less>
V is an optional component of the slab used in the present invention. V is an element that is effective for improving toughness by refining crystal grains during quenching and also has the effect of trapping hydrogen by forming carbides. However, since V is an expensive element, it is uneconomical to add V exceeding 0.5 mass%. Therefore, the V content needs to be 0.5% by mass or less. Moreover, in order to acquire said effect effectively, it is preferable to make the minimum of V content into 0.1 mass%.

<Remainder>
The balance of the slab consists of Fe and inevitable impurities. Here, in the present specification, “inevitable impurities” means impurities that are inevitably mixed although not intended.

Next, the manufacturing process of the steel plate for heat processing is demonstrated.
First, a slab having the above composition is heated to 1250 ° C. or higher. By setting the heating temperature within this range, solid solution in a matrix such as Ti or Nb is secured. When the heating temperature is less than 1250 ° C., the amount of Nb carbide precipitated in hot rolling is insufficient. As a result, prior austenite grains grow during quenching, and fatigue characteristics and toughness (especially impact toughness) are reduced.

Next, the heated slab is hot-rolled. In this hot rolling, finish rolling is performed under conditions of a total rolling ratio of 90% or more and a finishing temperature: Ar ₃ transformation point to Ar ₃ transformation point + 100 ° C. If the total rolling ratio is less than 90%, a fine structure effective for increasing the strength cannot be obtained sufficiently. If the finishing temperature is lower than the Ar ₃ transformation point, rolling is performed in a two-phase region, and the rolling conditions are likely to be unstable. On the other hand, if the finishing temperature exceeds Ar ₃ transformation point + 100 ° C., austenite grains become coarse after hot rolling, and a hot-rolled coil having a fine structure cannot be obtained.

  Next, the hot-rolled steel strip after hot rolling is cooled at an average cooling rate of 40 ° C./second or less, preferably 20-30 ° C./second. This average cooling rate is important for controlling the metallographic structure. When the average cooling rate exceeds 40 ° C./second, a martensitic structure may be generated in the hot-rolled steel strip. This martensite structure not only greatly changes the characteristics of the hot-rolled steel strip, but also causes plate cracking in the winding process.

  Next, the cooled hot-rolled steel strip is wound into a coil at a winding temperature of 450 to 600 ° C. to obtain a hot-rolled coil. By winding the hot-rolled steel strip at this winding temperature, a hot-rolled coil having fine ferrite + fine pearlite or bainite structure is obtained. When the winding temperature is less than 450 ° C., the coil is likely to be deformed, and the winding itself becomes practically difficult, and a plate crack from the coil edge may occur. When the coiling temperature exceeds 600 ° C., it becomes a hot rolled coil having a coarse ferrite + pearlite structure or a band-like structure, or surface decarburization and grain boundary oxidation progress, which becomes a starting point of fatigue failure.

Next, the hot-rolled coil is pickled, spheroidized, or cold-rolled. Since these treatments hardly affect the precipitation state of cementite generated by heat treatment (quenching and tempering treatment), any one can be selected as long as it gives strength and ductility that can be formed into the shape of the steel member. Good.
It does not specifically limit as a pickling method, It can carry out according to a well-known method. For example, a hot-rolled coil may be passed through a pickling tank containing hydrochloric acid or the like.
Conditions for performing spheroidizing annealing and cold rolling annealing are not particularly limited, and can be performed according to a known method.

When spheroidizing annealing is performed in a non-oxidizing atmosphere, it is preferable to perform it in a hydrogen gas atmosphere instead of a nitrogen atmosphere from the viewpoint of securing the hardenability of B. In addition, since the steel sheet for heat treatment is often cold formed into a desired product shape, annealing may be performed to a degree that gives a recrystallized structure, and complete spheroidizing annealing is required. There is no.
Moreover, when performing cold rolling annealing, it is preferable that an annealing temperature and annealing time determine optimal conditions according to the cold rolling rate before performing cold rolling annealing. For example, the higher the rolling rate of cold rolling, the more generally a fine recrystallized structure can be obtained at a low temperature and in a short time. Although there are some differences in the optimum conditions depending on the component composition of the steel, generally, when the total cold rolling reduction is 25%, the annealing time may be 670 to 750 ° C., and the annealing time may be 10 to 40 hours. .

The steel sheet for heat treatment manufactured as described above can be processed into a steel member by performing a quenching and tempering process after forming.
The forming method is not particularly limited, and a known method such as press forming can be used depending on the shape of the steel member.
The quenching and tempering treatment is a treatment necessary for enhancing the toughness and fatigue characteristics and suppressing the deterioration of delayed fracture characteristics while increasing the strength of the heat-treating steel plate. Quenching is preferably rapidly cooled after being held at a temperature of Ac ₃ transformation point + 50 ° C. or more for 60 seconds or more. The rapid cooling is not particularly limited as long as it is a cooling rate capable of causing martensitic transformation. Moreover, tempering should just hold | maintain for 10 to 60 minutes at the temperature of 180-500 degreeC.

  The hardness of the steel member obtained after tempering is preferably 500 HV or less, more preferably 470 HV or less. Here, “hardness” means Vickers hardness measured by a Vickers hardness tester. When the hardness exceeds 500 HV, the delayed fracture limit tends to rapidly decrease.

  In the steel member produced as described above, during heat treatment (quenching and tempering treatment), strengthening due to solid solution strengthening by Si and cementite formation suppressing effect, improving hardenability by Mn and B, It is considered that the desired effect can be obtained by synergistically acting toughness due to grain boundary strengthening by B, refinement of crystal grains by Nb carbide, hydrogen trapping effect, and the like. Although the detailed mechanism is unknown, in particular, due to the interaction between the effect of suppressing the formation of cementite by Si and the hydrogen trapping effect of Nb carbide, it suppresses the propagation of fatigue cracks and lowers the limit diffusible hydrogen, It is thought to improve all of toughness and delayed fracture characteristics. In addition, due to the effect of suppressing the formation of cementite by Si, the hardness can be maintained even when the tempering temperature is raised, and cementite is generated very finely at the grain boundaries and within the grains, so that the delayed fracture limit hardly decreases.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited by these.
The composition of the slab used in the examples is shown in Table 1. The balance of the slab is Fe and inevitable impurities.

Example 1
In Example 1, the effects of the slab composition, hot rolling conditions, quenching and tempering conditions, etc. on the workability of the heat-treated steel sheet and the fatigue properties and toughness of the heat-treated steel sheet were investigated.
A 200 mm-thick slab containing the components shown in Table 1 was heated at 1280 ° C. for 60 minutes, and then hot-rolled and cooled under the conditions shown in Tables 2-3 to produce a 4.0 mm-thick hot rolled sheet. And this hot-rolled sheet was wound up in the shape of a coil on the conditions shown in Tables 2-3, and it was set as the hot-rolled coil, and the steel plate for heat processing was obtained by pickling this hot-rolled coil. For some samples, spheroidizing annealing or cold rolling annealing was performed under the conditions shown in Tables 2 to 3 instead of pickling. The spheroidizing annealing was performed in a hydrogen atmosphere.
In order to evaluate workability about the obtained steel plate for heat treatment, a tensile test was performed in accordance with JIS Z2241, and tensile strength (TS) and tensile elongation (T.EL) were measured. The results are shown in Table 2.

Next, both surfaces of the steel plate for heat treatment obtained above were ground to 0.5 mm to a thickness of 3.0 mm, and cold-rolled to a thickness of 2.0 mm. Thereafter, heat treatment (quenching and tempering treatment) was performed to obtain a heat-treated steel sheet. Quenching was performed by holding at 900 ° C. for 15 minutes and then rapidly cooling with water. Tempering was performed by holding at 350 ° C. for 30 minutes.
About the obtained heat-treated steel plate, while measuring Vickers hardness, the impact test and the fatigue test were done.
In the impact test, a 2 mmU notch impact value at −45 ° C. was measured by a Charpy test in accordance with JIS Z2242.
In addition, for the fatigue test, a test piece based on JIS Z2275 was collected and tested at 22 Hz with a double swing using a plane bending fatigue tester. In this test, the fatigue strength value after 5 × 10 ⁶ cycles was defined as the fatigue limit.

The results of the above evaluations are shown in Tables 2-3. In the table, the fatigue limit ratio is a value obtained by dividing the fatigue strength by the tensile strength (fatigue limit ratio = fatigue strength / tensile strength).
As a standard for each evaluation, TS × T. If the value of EL is 16000 or more, it is considered that the workability of the steel sheet for heat treatment is good. If the Vickers hardness is 350 to 500 HV, the U-notch impact value is 80 J / cm ² or more, and the fatigue limit ratio is 0.4 or more, it is considered that the fatigue characteristics and toughness of the heat-treated steel sheet are good.

As shown in Tables 2 to 3, sample Nos. Prepared using slabs having a predetermined composition and under predetermined conditions. Steel plates for heat treatment of 2-3, 5-17, 19-20 and 22-23 are TS × T. The value of EL was 16000 or more, and the workability was good. Moreover, the heat-treated steel sheet produced from these heat-treated steel sheets had a U-notch impact value of 80 J / cm ² or more, a fatigue limit ratio of 0.4 or more, and good fatigue characteristics and toughness.

In contrast, sample no. Although the steel sheet for heat treatment No. 1 used a slab having a predetermined composition, martensite was generated because the average cooling rate was too high, and TS × T. The value of EL became low. That is, this heat-treatable steel sheet was not sufficiently workable.
Sample No. Although the steel plate for heat treatment No. 4 used a slab having a predetermined composition, the coiling temperature was too high, so that a coarse mixed structure of ferrite and pearlite was generated. As a result, pearlite was not sufficiently dissolved during quenching, and old austenite grains were mixed. That is, the heat-treated steel sheet obtained from this heat-treated steel sheet did not have sufficient fatigue strength.
Sample No. Although the steel plates for heat treatment of 18 and 21 used slabs having a predetermined composition, martensite was generated because the coiling temperature was too low, and TS × T. The value of EL became low. That is, these steel sheets for heat treatment were not sufficiently workable.

Sample No. The steel plate for heat treatment No. 24 had a low content of C and Si and used a slab containing no Cr, Ti, Nb and B. The value of EL became low. That is, this heat-treatable steel sheet was not sufficiently workable.
Sample No. Since the steel plate for heat treatment No. 25 used a slab with a low content of Si and Mn, TS × T. The value of EL became low. That is, this heat-treatable steel sheet was not sufficiently workable.
Sample No. The steel plate for heat treatment No. 26 uses a slab with a high Si content, so that it has high strength and TS × T. The value of EL became low. Moreover, in this steel sheet for heat treatment, since the coiling temperature was too low in addition to excessive Si, the transformation was delayed and the coil was deformed. That is, this heat-treatable steel sheet was not sufficiently workable. Furthermore, many scales were formed on the surface of the steel sheet for heat treatment, and the fatigue limit of the heat treated steel sheet was lowered. That is, this heat-treated steel sheet did not have sufficient fatigue properties.

Sample No. Since the steel plate for heat treatment No. 27 used a slab having a large Ti content, TiN was coarsened during the quenching and tempering treatment to become inclusions, and the U-notch impact property of the heat treatment steel plate was lowered. That is, this heat-treated steel sheet did not have sufficient toughness.
Sample No. Since the steel plate for heat treatment No. 28 used a slab having a low Si content, cementite was coarsened during the quenching and tempering treatment, and the U-notch impact property of the heat treated steel plate was lowered. That is, this heat-treated steel sheet did not have sufficient toughness.
Sample No. The steel plate for heat treatment No. 29 used was a slab with a high C content, so that it became high strength and TS × T. The value of EL became low. That is, this heat-treatable steel sheet was not sufficiently workable. Further, this steel sheet for heat treatment causes coarsening of cementite due to the low content of Si during the quenching and tempering treatment, and also due to insufficient strengthening of grain boundaries due to the absence of B, heat treatment. The U-notch impact property of the steel sheet was lowered. That is, this heat-treated steel sheet did not have sufficient toughness.
Sample No. Since the steel plate for heat treatment No. 30 used a slab having a high Nb content, NbN and NbC were coarsened into inclusions during the quenching and tempering treatment, and the U notch impact property of the heat treated steel plate was reduced. That is, this heat-treated steel sheet did not have sufficient toughness.

(Example 2)
In Example 2, the effect of the steel type and tempering temperature on the fatigue properties and toughness of the heat-treated steel sheet was investigated.
A 200 mm thick slab was heated at 1280 ° C. for 60 minutes, then hot-rolled at a finishing temperature of 890 ° C. and cooled at an average cooling rate of 30 ° C./second to produce a 4.0 mm thick hot rolled sheet. And this hot-rolled sheet was coiled at a coiling temperature of 550 ° C. to form a hot-rolled coil, and this hot-rolled coil was pickled to obtain a steel sheet for heat treatment.
Next, both surfaces of the steel plate for heat treatment obtained above were ground to 0.5 mm to a thickness of 3.0 mm, and cold-rolled to a thickness of 2.0 mm. Thereafter, heat treatment (quenching and tempering treatment) was performed to obtain a heat treated steel sheet. Quenching was performed by holding at 900 ° C. for 15 minutes and then rapidly cooling with water. Moreover, tempering was performed by hold | maintaining for 30 minutes in the range of 100-500 degreeC.

  The obtained heat-treated steel sheet was measured for yield strength (YS), tensile strength (TS), tensile elongation (T.EL), and Vickers hardness, and an impact test and a fatigue test were performed. Here, the yield strength (YS) was determined by performing a tensile test in accordance with JIS Z2241. Other measurements and tests were performed in the same manner as in the above examples. The results are shown in Table 4.

As shown in Table 4, while using the steel plate for heat treatment of the present invention, the sample No. 1 was prepared with a tempering temperature in the range of 180 to 500 ° C. The heat treated steel sheets of 32 to 35 had a U-notch impact value of 80 J / cm ² or more, a fatigue limit ratio of 0.4 or more, and good fatigue characteristics and toughness.
On the other hand, when the tempering temperature is less than 180 ° C., the heat-treated steel sheet becomes too hard even if the heat-treated steel sheet of the present invention is used, and fatigue characteristics and toughness are deteriorated (Sample No. 31). ). In addition, a heat-treating steel sheet that does not use a slab having a predetermined composition is a heat treatment that has insufficient fatigue characteristics and toughness not only when the tempering temperature is less than 180 ° C but also within the range of 180 to 500 ° C. Steel plates were provided (Sample Nos. 36-44).

(Example 3)
In Example 3, the effect of the steel type and the tempering temperature on the delayed fracture characteristics of the heat-treated steel sheet was investigated.
A 200 mm thick slab was heated at 1280 ° C. for 60 minutes, then hot-rolled at a finishing temperature of 890 ° C. and cooled at an average cooling rate of 30 ° C./second to produce a 4.0 mm thick hot rolled sheet. And this hot-rolled sheet was coiled at a coiling temperature of 550 ° C. to form a hot-rolled coil, and this hot-rolled coil was pickled to obtain a steel sheet for heat treatment.
Next, both surfaces of the steel plate for heat treatment obtained above were ground to 0.5 mm to a thickness of 3.0 mm, and cold-rolled to a thickness of 2.0 mm. Thereafter, heat treatment (quenching and tempering treatment) was performed to obtain a heat treated steel sheet. Quenching was performed by holding at 900 ° C. for 15 minutes and then rapidly cooling with water. Moreover, tempering was performed by hold | maintaining for 30 minutes in the range of 100-500 degreeC.

  The obtained heat-treated steel sheet was measured for Vickers hardness, delayed fracture limit, static bending fracture strength, total hydrogen content and diffusible hydrogen content, and the delayed fracture ratio was determined. Here, the delayed fracture limit, static bending fracture strength, total hydrogen and diffusible hydrogen were measured as follows. In addition, about the measurement of Vickers hardness, it carried out similarly to said Example.

  The delayed fracture test was carried out using a cantilever type bending delayed fracture test apparatus (CLT-20C manufactured by Toshin Kogyo Co., Ltd.). As a test piece, a V-notch having a plate thickness of 2.0 mm, a width of 10 mm, and a length of 100 mm was formed by cold rolling, and a vertical angle of 45 °, a depth of 2.0 mm, and a notch bottom of 0.3 R was provided in the longitudinal center. A thing was used. One end of this test piece is fixed, the V notch is immersed in a 10% hydrochloric acid solution, a weight is attached to the other end, various bending stresses are applied to the bottom of the V notch, and the relationship between fracture stress and fracture time The S—N diagram was created by obtaining And the bending stress which does not break even if 100 hours pass was calculated | required, and this was made into the delayed fracture limit.

  The static bending fracture test was performed as follows. The shape of the test machine and the test piece is the same as the delayed fracture test. The same is true in that one end of the test piece is fixed, the V notch is immersed in a 10% hydrochloric acid solution, a weight is attached to the other end, and various bending stresses are applied to the bottom of the V notch. However, instead of obtaining the relationship between the breaking stress and the breaking time, the load of the heavy cone was gradually increased to obtain the stress at which a crack from the V-notch portion was generated, and this was defined as the static bending fracture strength.

  The total hydrogen amount was measured after a test piece of 2 mm × 5 mm × 10 mm was immersed in a 10% hydrochloric acid solution for 100 hours, and then the corrosion product on the surface was ground. This test specimen for measurement was stored in a nitrogen atmosphere until the total amount of hydrogen was measured. Then, the total amount of hydrogen released when the test piece was heated from room temperature to 600 ° C. was measured by a gas chromatography method, and this was defined as the total amount of hydrogen.

The amount of diffusible hydrogen was measured after immersing a test piece of 2 mm × 5 mm × 10 mm in a 10% hydrochloric acid solution for 100 hours and then grinding the corrosion product on the surface. This test specimen for measurement was stored in a nitrogen atmosphere until the total amount of hydrogen was measured. And this test piece was heated from room temperature to 245 degreeC with the temperature increase rate of 12 degree-C / min, and the amount of hydrogen discharge | released was measured using the quadrupole mass spectrometer. And the integral value (area of the 1st peak) of the amount of hydrogen obtained in this temperature range was made into the amount of diffusible hydrogen.
The results are shown in Table 5. In Table 5, the delayed fracture ratio is a value obtained by dividing the delayed fracture limit by the static bending fracture strength (delayed fracture ratio = delayed fracture limit / static bending fracture strength). As an evaluation standard of this delayed fracture ratio, if the delayed fracture ratio is 0.5 or more, it is considered that the delayed fracture characteristics of the heat-treated steel sheet are good.

As shown in Table 5, the heat-treated steel sheet produced using the heat-treated steel sheet of the present invention and having a tempering temperature in the range of 180 to 500 ° C. had good delayed fracture characteristics (Sample No. 45). ~ 49).
On the other hand, the steel sheet for heat treatment not using a slab having a predetermined composition gave a heat treated steel sheet with insufficient delayed fracture characteristics even when the tempering temperature was in the range of 180 to 500 ° C. (Sample No. 50-53). This result is considered to be caused by an increase in the amount of diffusible hydrogen because the slab does not have a predetermined composition.

In order to consider the above results, a graph showing the relationship between the Vickers hardness of the heat-treated steel sheet and the delayed fracture ratio is shown in FIG. As can be seen from FIG. 1, even when the heat-treated steel sheet has the same degree of hardness, the heat-treated steel sheet of the present invention has better delayed fracture characteristics than the heat-treated steel sheet of the comparative example.
Next, as a representative example of each of the above samples, Sample No. 46 (Example of the present invention) and Sample No. A micrograph of the heat-treated steel sheet 51 (comparative example) is shown in FIG. As can be seen from FIG. In the heat-treated steel plate No. 46, sample No. Cementite that was finer than that of the heat-treated steel plate 51 was uniformly precipitated. It is considered that the above effect was brought about by the uniform precipitation of fine cementite.

  As can be seen from the above results, according to the present invention, there is provided a method for producing a steel sheet for heat treatment, which provides a steel member having excellent workability and hardenability and excellent fatigue properties, toughness and delayed fracture properties. Can be provided. Moreover, according to this invention, the manufacturing method of the steel member excellent in all the fatigue characteristics, toughness, and delayed fracture characteristics can be provided.

Claims (4)

C: 0.1 mass% excess 0.4 mass% or less, Si: 0.5-1.5 mass%, Mn: 0.3-2 mass%, P: 0.02 mass% or less, S: 0.0. 02 mass% or less, Cr: 0.1-2 mass%, Ti: 0.01-0.1 mass%, Nb: 0.01-0.1 mass%, Al: 0.1 mass% or less, B: from 0.0022 to 0.01 mass%, N: see contains 0.01 mass% or less, after the balance was heated slab consisting of Fe and unavoidable impurities to a temperature above 1250 ° C., the total rolling reduction at the finish rolling : 90% or more, Finishing temperature: Hot rolling at Ar ₃ transformation point to Ar ₃ transformation point + 100 ° C., average cooling rate: cooling at 40 ° C./sec or less, coiling temperature: 450-600 ° C. A steel sheet for heat treatment, which is wound into a hot-rolled coil, and the hot-rolled coil is pickled, spheroidized or cold-rolled annealed Manufacturing method.   The slab is one or more selected from the group consisting of Ni: 0.5% by mass or less, Ca: 0.02% by mass or less, Mo: 0.5% by mass or less, and V: 0.5% by mass or less. The manufacturing method of the steel plate for heat processing of Claim 1 characterized by the above-mentioned.   A method for producing a steel member, comprising performing a quenching and tempering treatment after forming the heat-treating steel plate obtained by the production method according to claim 1.   The method for producing a steel member according to claim 3, wherein a tempering temperature in the quenching and tempering treatment is 180 to 500 ° C.

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