JP3535754B2 - B-containing steel excellent in cold workability and delayed fracture resistance, its manufacturing method and bolt - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a B-containing steel which is frequently used in bolts and the like used for vehicles such as automobiles and industrial machines, and particularly to cold workability and delayed fracture resistance. And a useful method for producing such a B-containing steel, a bolt produced by such a B-containing steel, and the like.

[0002]

2. Description of the Related Art When a steel material is processed into parts such as bolts, hot-rolled material (steel material as hot-rolled) or steel material after softening heat treatment is cold-drawn and then cold-forged. Often shaped into a shape. Further, after that, from the viewpoint of satisfying the final mechanical properties of the steel material, it is common to perform a heat treatment for obtaining a tempered martensite structure. In order to obtain good mechanical properties after such heat treatment, it is necessary that the hardenability is good, but the hardenability was improved by increasing the amount of C added or adding an alloying element. In this case, the strength of the as-hot-rolled material becomes too high and the cold workability is significantly reduced. as a result,
Softening heat treatment is required before cold forging, which causes a large cost increase.

By the way, since boron (B) is an element that greatly improves the hardenability by adding a small amount, C in steel is
Even if the amount is reduced, the same hardenability can be secured, and it is possible to achieve both cold workability and
It has been used in many fields in recent years. In order to exert the effect of improving the hardenability by B, it is necessary to suppress the precipitation of BN and to exist in the steel as free B. For that,
It is known that fixing N by Ti is the most effective. However, even in the B-containing steel whose composition has been adjusted in this way, it is required to reduce the strength of the as-hot-rolled material in order to further improve the cold workability. Many techniques aiming at softening by controlling the structure by controlled rolling have been proposed (for example, Japanese Patent Publication Nos. 61-59379 and 61-37).
333, Japanese Patent Publication No. 7-5960, etc.).

On the other hand, it has been noted that B-containing steel has good cold workability and hardenability, and even for high-strength bolts in which low alloy steel has been used in the past, B-containing steel as described above is used. Is being considered. However, in the case of B-containing steel, the former austenite grain size is likely to coarsen during tempering and the delayed fracture resistance is insufficient compared to low alloy steel, so it is applied as a steel for high strength bolts. Challenges remain to be addressed.

The inventors of the present invention are also conducting research from various angles to apply B-containing steel to steel for high-strength bolts, and improve delayed fracture resistance from the viewpoint of controlling precipitates in B-containing steel. We have developed a technology aimed at achieving this, and since its technical significance was recognized, we applied for a patent first (Japanese Patent Application No. 8-19113).
No. 4). Although this technique provided excellent characteristics for delayed fracture resistance, cold workability was not fully considered, and cold workability and delayed fracture resistance were not achieved at the same time. . As an attempt to achieve both cold workability and delayed fracture resistance, for example, the technique of Japanese Patent Laid-Open No. 8-60245 has been proposed, but since both of these characteristics are intended to be achieved only by adjusting the component balance. There was a limit to the improvement, and there was still room for improvement.

[0006]

The present invention has been made under such circumstances, and an object thereof is a B-containing steel having both cold workability and delayed fracture resistance, and such a B-containing steel. The present invention provides a useful method for manufacturing a steel, as well as a bolt made of the B-containing steel described above.

[0007]

The B-containing steel of the present invention capable of achieving the above object is C: 0.10 to 0.45%, B:
0.0003-0.0050%, Ti: 0.01-0.
1%, N: 0.0025~0.010% only contains, as yet another component, Si: 0.03 to 0.5%, M
n: 0.3 to 1.5% and Al: 0.01 to 0.10.
%, With the balance being Fe and unavoidable impurities, a hot-rolled material or a steel material cold-drawn after hot-rolling, which satisfies at least one of the following requirements (1) and (2): The point is to do. (1) The amount of Ti contained in the precipitate extracted by the extraction residue method and having a particle size of more than 0.1 μm is the total Ti contained in the steel material.
It is 60% or more of the amount. (2) Particle size: 0.01 to 0.2 μm observed by electron microscope observation by the extraction replica method In the observation field of view, the average number of Ti-based precipitates is 25 μm ^2.
In 10 to 500.

In the B-containing steel having the above-mentioned chemical components, it is effective to include the following components (i) to (iv), if necessary, and the characteristics thereof are further improved depending on the kind of the components contained. Be improved. (i) Cu: 2% or less (not including 0%), Ni: 2% or less (not including 0%) and Cr: 2% or less (0%)
One or more selected from the group consisting of (ii) Mo: 1% or less (not including 0%) (iii) Nb: 0.005 to 0.1% and / or V:
0.005 to 0.3% (iv) Ca: 0.0003 to 0.001% Further, by using a B content that satisfies the above requirements, cold workability and delayed fracture resistance are compatible. High strength bolts can be obtained.

On the other hand, the method for producing a B-containing steel of the present invention which has achieved the above-mentioned object is the cold workability which has the gist of operating while satisfying the following conditions (a) to (c). It is a method for producing a B-containing steel excellent in delayed fracture resistance. (A) Billet reheating temperature: 950 ° C. or higher (b) Finish rolling temperature (that is, before the final rolling pass or
Mean surface temperature measurable with a radiation thermometer in front of rolling rolls
Degree) : 700 to 900 ° C (c) After finish rolling, average cooling rate from 750 ° C to 650 ° C: 1 ° C / sec

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In the B-containing steel as described above, it is necessary to suppress the precipitation of BN in order to ensure the stable hardenability, and Ti is added to fix N. In order to improve the cold workability, it is necessary to reduce the strength of the as-hot-rolled material, but Ti may become TiN, TiC or fine precipitates similar to these in the steel, and precipitation strengthen the matrix. . From the viewpoint of improving the cold workability, it can be said that it is desirable to suppress such precipitation strengthening as much as possible. However, since these fine precipitates effectively act to prevent the coarsening of the austenite grain size after refining and to improve the delayed fracture resistance, they cannot be completely eliminated.

Therefore, the present inventors need to perform morphology control for forming precipitates optimal for strength reduction during hot rolling, while preventing coarsening of crystal grains and improving resistance to delayed fracture. I thought there was, and conducted research and development. Conventionally, as described above, a technology aiming at softening by controlling the metallographic structure of the as-hot-rolled steel by controlled rolling in B-containing steel (the above-mentioned Japanese Patent Publication Nos. 61-59379 and 61-3).
No. 7333, Japanese Examined Patent Publication No. 7-5960, etc.), or a patent in which cold workability and delayed fracture resistance are compatible with each other mainly in consideration of adjustment of component balance (the above-mentioned JP-A-8-60245).
No.) was proposed, but there was no case where attempts were made to achieve both softening and other properties by controlling the morphology of precipitates.

By the way, the fine precipitates affect the strength reduction of the as-hot-rolled material, the prevention of crystal grain coarsening during heat treatment, and the improvement of delayed fracture resistance. Depends on First, the strength of the as-hot-rolled material is most affected by the finely matched TiC precipitated in the ferrite, and the strength increases as the amount of finely matched TiC increases. Therefore, in order to reduce the strength of the as-hot-rolled material, it is necessary to suppress the TiC-based fine matching precipitates that precipitate in the ferrite region. By reheating the billet at a temperature that does not re-dissolve the coarse precipitates that had precipitated before reheating the billet, it is possible to suppress precipitation in the ferrite region, but with this method, as described below, coarse crystal grains And not effective in improving delayed fracture resistance. Since non-coherent precipitates precipitated in austenite do not contribute to strengthening so much, it is desirable to effectively utilize such precipitates.

Further, in order to prevent the crystal grain coarsening during the heat treatment for tempering, the effect cannot be exerted with a coarse precipitate which does not melt during reheating of the billet. Therefore,
It is necessary to reprecipitate fine precipitates after re-dissolving. However, even if a fine precipitate that precipitates in the ferrite region is precipitated in the as-hot-rolled material, it will re-dissolve in a short time during the holding of the austenite region in the subsequent tempering treatment, or it will grow in Ostwald, so austenite will grow. It cannot be said that it is effective in preventing the coarsening of crystal grains as compared with the precipitates deposited in the low temperature region.
Here, the low temperature range of austenite is generally 900 ° C.
The following austenite states are defined, and austenite in the untransformed state with Ae ₃ point or less is also included.

Further, regarding the improvement of delayed fracture resistance, T
It is known that it is effective to use the iC-based precipitate as a hydrogen trap site to reduce the amount of harmful diffusible hydrogen. Since hydrogen trapping due to TiC-based precipitates is considered to occur at the interface between the precipitates and the matrix, decreasing the precipitate size that increases the specific surface area improves the trapping ability and improves delayed fracture resistance. Considered to be effective. However, delayed fracture resistance becomes a problem after refining, so the precipitate morphology at that time is important. Therefore, no matter how many fine precipitates are deposited after rolling, they are not effectively utilized because they are re-dissolved during holding the austenite region or coarsen by Ostwald ripening. That is, the fine precipitates precipitated in the ferrite region cannot be expected to improve hydrogen trapping as compared with the precipitates precipitated in the low temperature austenite region. In addition, since the coarse precipitates that have been precipitated before the billet is reheated remain coarse during tempering, the effect of improving delayed fracture resistance is small.

Based on the above findings, in order to satisfy all the requirements of strength reduction of the as-hot-rolled material, prevention of crystal grain coarsening, and improvement of delayed fracture resistance, precipitation in the low temperature region of the austenite region is sufficient. It has been found that it is effective to deposit a large amount of precipitates. Specifically, in a hot-rolled material whose chemical composition has been appropriately adjusted as described below or a cold-drawn steel material after hot-rolling, at least one of the following requirements (1) and (2) It has been found that all the required characteristics described above can be satisfied by satisfying the above. (1) The amount of Ti contained in the precipitate extracted by the extraction residue method and having a particle size of more than 0.1 μm is the total Ti contained in the steel material.
It is 60% or more of the amount. (2) Particle size observed by electron microscope observation by the extraction replica method: 0.01 to 0.2 μm The average number of Ti-based precipitates is 10 to 5 in the observation field of 25 μm ^2.
It is 00.

Next, the reasons for limiting the requirements (1) and (2) will be described. Ultrafine precipitates that precipitate in the solid solution Ti or ferrite region cannot be measured by the extraction residue method because they pass through a filter having a pore size of 0.1 μm. Therefore,
The amount of Ti in the precipitate extracted by the extraction residue method is the sum of the amount of Ti fixed in the coarse precipitate that has not been re-dissolved during billet reheating and the precipitate that is newly precipitated in the austenite phase. If the amount of Ti is less than 60% of the total amount of Ti contained in the steel material, it indicates that a large number of fine Ti-based precipitates are precipitated in the ferrite, and the as-rolled strength is increased. More preferably, it is 80% or more.
In addition, the target particle size: a precipitate having a particle size of more than 0.1 μm is
Not only Ti-based precipitates but also non-Ti-based precipitates (Ti
It is also meant to include precipitates not containing). This is because it is difficult to separate only Ti-free precipitates (for example, cementite) by the extraction residue method.

The size of the Ti-based precipitate that precipitates in the austenite region is 0.01 to 0.2 μm. A Ti-based precipitate having a grain size of less than 0.01 μm is a coherent precipitate that mainly precipitates in the ferrite region, and raises the strength of the as-hot-rolled material, which poses a problem. Further, a precipitate having a grain size of more than 0.2 μm does not contribute to prevention of crystal grain coarsening or improvement of delayed fracture resistance. Therefore, the particle size is 0.01 μm
As described above, depositing a large number of Ti-based precipitates having a size of 0.2 μm or less prevents the strength of the as-hot-rolled material from decreasing and prevents the crystal grains from coarsening.
In addition, it is effective in satisfying all of the improvement of delayed fracture resistance. In order to exert such an effect, it is necessary to contain 10 or more Ti-based precipitates having a grain size within the above range on average in the observation visual field of 25 μm ² . More preferably, the average number of Ti-based precipitates is 20 or more.

On the other hand, the upper limit of the average number is set to 500, because a large amount of Ti must be added to obtain a larger number of precipitates, which may adversely affect toughness and the like. Since the efficiency of Ti addition becomes poor, this was made the upper limit. The preferable upper limit of the number of Ti-based precipitates is 300 on average.

Examples of the Ti-based precipitates that are the subject of the present invention include
For example TiC, TiN, TiS, Ti _Four C₂ S₂ Toko
These composite precipitates, as well as Nb, Mo, V, etc.
It is a generic term for some of the precipitates included. Further, in the present invention, "Ti
The reasons for defining "system precipitates" are shown in (I) to (III) below.
On the street. (I) Ti-based precipitates differ in both C and N due to individual precipitates.
Ti (C, N) contained in the amount
Many, but it is difficult to separate C and N, (II) Nb, M
The precipitate compounded with o, V, etc. also has Ti as the main component.
As long as it has no influence on the characteristics, it does not contain Nb or V
There is no big difference with the product, and it may be called Ti system, (I
II) Some of the above-mentioned precipitates are deposited in that type.
Temperature regions are different, but size regions overlap
The same size may affect the characteristics.
There is no need to think separately.

From the above viewpoints, the reasons for limiting the definition of the chemical composition of the steel of the present invention necessary for satisfying all the required properties while precipitating the precipitates are shown below.

C: 0.10 to 0.45% C is an element effective in imparting strength, and in order to exert such effects, it is necessary to contain C by 0.10% or more.
However, if it is contained excessively, the toughness after quenching decreases, so 0.45% or less is necessary. The preferable lower limit of the C content is 0.15%, and the preferable upper limit is 0.40%.

B: 0.0003 to 0.0050% B has the effect of improving the hardenability by adding a trace amount. In order to exert such effects, it is necessary to contain 0.0003% or more. However, if it is contained excessively, not only the effect is saturated but also the toughness and workability are deteriorated, so it is necessary to make it 0.0050% or less. The preferable lower limit of the B content is 0.0005%, and the preferable upper limit thereof is 0.002%.

Ti: 0.01 to 0.1% Ti is an element necessary for ensuring the hardenability effect of B. Further, it is an element effective for refining crystal grains and improving delayed fracture resistance. In order to exert such effects, it is necessary to contain at least 0.01% or more. However, if it is contained excessively, Ti-based precipitates are coarsened, which may adversely affect toughness and fatigue strength, and the strength of the as-hot-rolled material may increase. It should be 0.1% or less. The preferable lower limit of the Ti content is 0.025%, and the preferable upper limit thereof is 0.07%.

N: 0.0025 to 0.010% N is preferably as small as possible in order to secure the hardenability improving effect of B. On the other hand, if the amount is too large, TiN having a large grain size is likely to be formed, which may adversely affect the toughness and fatigue strength of the steel material. Therefore, the upper limit needs to be 0.010%. However, the N content is 0.0025
If it is less than%, a large number of fine TiC-based precipitates are generated in the ferrite region, and the strength of the as-hot-rolled material increases, so this was made the lower limit. The preferable lower limit of the N content is 0.004.
It is 0%, and the preferable upper limit is 0.0070%.

The basic chemical composition of the B-containing steel of the present invention is as described above, the balance being Fe and unavoidable impurities, but if necessary Si, Mn, Al, Cu, Ni, C
It is also effective to add alloying elements such as r, Mo, Nb, V and Ca. The reasons for limiting the components when adding these elements are as follows. The B-containing steel of the present invention includes
In addition to these, trace amounts of components that do not impair the characteristics of B-containing steel may be contained, and such B-containing steel is also included in the technical scope of the present invention.

Si: 0.03 to 0.5% Si is effective as a deoxidizer, and it is necessary to contain Si in an amount of 0.03% or more in order to exert its effect. However, if it is contained excessively, ductility and cold workability deteriorate, so it is necessary to set it to 0.5% or less. The preferable lower limit of the Si content is 0.05%, and the preferable upper limit is 0.1.
40%.

Mn: 0.3 to 1.5% Mn is effective as a deoxidizing agent, a desulfurizing agent and an element for improving hardenability, and 0.3% or more is required to exert its effect. However, if it is contained excessively, the structure becomes nonuniform due to segregation, and the toughness after quenching and the cold workability deteriorate. Therefore, it is necessary to set it to 1.5% or less. The preferable lower limit of the Mn content is 0.5%.
And the preferable upper limit is 1.2%.

Al: 0.01 to 0.10% Al is effective as a desulfurizing agent, and N is used as AlN.
Also exerts the effect of fixing. In order to exert these effects, Al needs to be contained by 0.01% or more.
However, if it is contained excessively, the toughness is adversely affected, so it is necessary to set it to 0.10% or less. The preferable lower limit of the Al content is 0.015%, and the preferable upper limit is 0.06%.

Cu: 2% or less (not including 0%), N
i: 1% or more selected from the group consisting of 2% or less (not including 0%) and Cr: 2% or less (not including 0%) These elements are delayed fracture resistant due to improvement in hardenability and corrosion resistance. It is an element effective in improving the properties. However, if added in a large amount, the strength of the as-hot-rolled material becomes too high, so it is necessary to make it 2% or less in all cases.

Mo: 1% or less (not including 0%) Mo is an element effective for improving hardenability, but if added in a large amount, the strength of the as-hot-rolled material becomes too high.
It should be 1% or less.

Nb: 0.005 to 0.1% and / or V: 0.005 to 0.3% Nb and V are elements effective for refining crystal grains and improving delayed fracture resistance. In order to exert such an effect, it is necessary to contain 0.005% or more in each case. However, if added excessively, the strength of the as-hot-rolled material becomes unnecessarily high, so Nb is 0.1% or less and V is 0.3% or less.
Must be less than or equal to%.

Ca: 0.0003 to 0.001% Ca controls the morphology of sulfide-based inclusions (see, for example, Japanese Patent Laid-Open No.
8-84960) and Ca _x P _y generation (for example, JP-A-6-212345), but in addition to these effects, electrons are emitted to cause the surrounding iron It has the effect of increasing the bond strength, and thereby exerts the effect of increasing the bond strength at the grain boundaries and suppressing hydrogen embrittlement. Further, by controlling the morphology of precipitates which become hydrogen top sites, it contributes to improve delayed fracture resistance. In order to exert these effects, the Ca content is 0.0
The content must be 003% or more, but if it becomes excessive, coarse inclusions will be generated, which will rather deteriorate the delayed fracture property, so it must be 0.001% or less.

Next, the manufacturing method of the present invention will be described. In producing the B-containing steel of the present invention, it is necessary to operate while satisfying the following conditions (a) to (c), and the reason for setting these conditions is as follows. (A) Billet reheating temperature: 950 ° C or higher (b) Finish rolling temperature: 700 to 900 ° C (c) After finish rolling, average cooling rate from 750 ° C to 650 ° C: 1 ° C / sec or less

In order to precipitate Ti-based precipitates in an amount necessary for preventing crystal grain coarsening and improving delayed fracture resistance in a low temperature range of austenite, it is first necessary to form a solid solution of Ti in the austenite range. To do this, set the billet reheating temperature to 9
It is necessary to set the temperature to 50 ° C or higher. If this temperature is lower than 950 ° C., the amount of solid solution is insufficient, and only the large-scale precipitates that have existed up to that point are formed, and Ti-based precipitates of the desired size are not formed. This temperature is preferably 100
It is better to set it to 0 ° C or higher.

Next, it is necessary to precipitate Ti-based precipitates in the austenite region. Precipitation of Ti-based precipitates in the austenite region generally takes a considerably long time, but the area of the austenite crystal grain boundaries, which is the nucleus of precipitation, increases, that is, the austenite crystal grains are refined and residual strain remains. By setting it to, it is possible to deposit in a practically sufficient time. In order to refine the austenite grains and to leave residual strain, it is necessary to reduce the finish rolling temperature. From this viewpoint, the upper limit was set to 900 ° C. At temperatures above 900 ° C., austenite crystal grains become coarse and residual strain does not remain, so that Ti-based precipitates in the austenite region do not deposit. However, if the temperature is too low, the rolling load increases and the surface flaws increase, which becomes unrealistic. Therefore, the lower limit is set to 700 ° C. Here, the finish rolling temperature is the average temperature of the surface that can be measured by the radiation thermometer before the final rolling pass or before the group of rolling rolls.

As a method for producing B-containing steel, Japanese Patent Laid-Open No. 8-
Although the method of No. 60245 has also been proposed, this method has a finish rolling temperature of 900 ° C. in order to improve cold workability.
It is defined as above. This requirement explains that when the finish rolling temperature is lower than 900 ° C., the structure becomes a fine structure, the strength is increased, and the cold workability is deteriorated. On the other hand, the present invention aims to prevent not only softening but also crystal grain coarsening and improvement of delayed fracture resistance by positively utilizing Ti precipitates, and the heating temperature is regulated from such a viewpoint. ing. Also in this temperature range,
There is certainly an effect of increasing the strength by lowering the finish rolling temperature, but the effect of softening by controlling the precipitate described in the present invention is more significant, and the total finishing rolling temperature is 700
It was found that the effect is greater when the temperature is in the range of up to 900 ° C.

In the present invention, after finish rolling, the average cooling rate from 750 ° C. to 650 ° C. is required to be 1 ° C./sec or less. This is to accelerate the precipitation of Ti precipitates in the austenite region. This is to secure sufficient time.
The precipitation in the size region, which is important for improving the characteristics, is 750 to 6
Since it occurs at 50 ° C, the cooling rate in this region becomes particularly important. Cooling from the finishing temperature to 750 ° C. should be rapidly cooled to prevent coarsening of fine austenite crystal grains, but this region may also be gradually cooled to promote precipitation of Ti precipitates. The finish rolling temperature is the specified minimum 7
When rolled at 00 ° C, the temperature of the material rises up to about 750 ° C due to heat generated during processing. Also, if the temperature drops below 650 ° C,
Since more than half of the austenite finishes transformation, slow cooling may be discontinued.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any change in the design of the present invention can be made without departing from the spirit of the preceding and the following. It is included in the technical scope.

[0039]

Example 1 A sample material having the chemical composition shown in Table 1 below was prepared, and the diameter:
It was rolled into a 13 mm wire. Billet reheat temperature 10
It was rolled at 50 ° C. and the finish rolling temperature was adjusted to 800 ± 30 ° C., then water-cooled to a temperature of 750 ° C., and mounted on a conveyor. The range up to 650 ° C. was cooled on the conveyor so that the average cooling rate in the coil sparse part where the cooling rate was relatively high was 0.8 ° C./s. A tensile test was performed after this rolling. The results (tensile strength TS) are also shown in Table 1.

[0040]

[Table 1]

After machining the sample into a delayed fracture test piece,
It was heated at 870 ° C. for 30 minutes and oil-quenched. afterwards,
Although tempering treatment was performed for 120 minutes, the tempering temperature was adjusted so that the tensile strength TS was 135 kgf / mm ² . JIS the austenite grain size after quenching and tempering
It was measured by a method (JIS G 0551) specified by For the delayed fracture resistance, a notched acceleration constant load tensile test with a stress concentration factor α of 10 was performed in water. Then, the strength that does not fracture even after 100 hours has passed is defined as the delayed fracture strength.

The results are shown in Table 2 below together with the measurement results of the precipitates. The size and amount of the precipitates were measured on the as-hot-rolled material. Further, the measurement of the amount of Ti contained in the Ti-based precipitate, the measurement of the number of precipitates, and the observation of the precipitates were carried out by the following methods.

<Measurement of Ti content in precipitate> 0.
The extraction residue method was used to measure the amount of Ti contained in the precipitates having a size of more than 1 μm. Precipitate was extracted from 1.0 g of a steel material sample by a commonly used electrolytic extraction method to obtain 0.1 μm
It was collected by the filter of. The Ti mass {Ti} (g) in the recovered precipitate was analyzed by the ICP method. Further, the ratio A to the total Ti amount [Ti] (mass%) contained in the steel material
(%) Was evaluated by the following formula (1). A (%) = [(100 / {Ti}) / [Ti]] × 100 (1)

<Measurement of the number of precipitates> Ti-based precipitates: 0.01 observed in a visual field of 25 μm ^2.
The number of Ti-based precipitates having a size of ˜0.2 μm was measured.

<Observation of Precipitates> Observation of the precipitates was performed by an extraction replica / electron microscope. This method is described, for example, in "Steel Manual IV: Steel Materials, Testing and Analysis" (3rd edition, Showa 5).
6th year, published by Maruzen, pp. 395-399). As for the sample, there is a possibility that the surface layer of the sample may change the morphology of the precipitate due to decarburization, so in order to avoid it, the sample was taken from a position 0.3 mm from the surface layer.
The electron microscope observation was performed at an accelerating voltage of 200 kV, and the tissue was identified by EDX (energy dispersive semiconductor detector). The photograph was taken at a magnification of 1500 times, and the size was measured on the photograph. At this time, in addition to Ti, in precipitates containing Nb, Mo, V, etc.,
If the strength of Ti was found to be the highest as measured by EDX, it was determined to be a Ti-based precipitate.

[0046]

[Table 2]

From these results, it can be judged as follows.
The tensile strength TS of the as-rolled material is shown in Table 1 above, and the grain size number and delayed fracture strength after heat treatment are shown in Table 2. However, in the comparative steel lacking any of the requirements specified in the present invention, all the required properties are shown. I can't be satisfied. For example, No. 23,
Those of 24, 27, 28, 31 to 34 have high tensile strength of the as-rolled material (poor cold workability). In addition, No. Two
Nos. 2, 25, 26, 29, and 30 to 34 have insufficient delayed fracture strength. In particular, No. No. 25 does not have B added and has insufficient hardenability,
The strength after heat treatment was also insufficient. In addition, No. The hardness of 30 is less than 135 kgf / mm ² .

On the other hand, in the steels of the present invention (Nos. 1 to 21) satisfying all the requirements specified in the present invention, it is understood that all the properties are good.

Example 2 An experiment was conducted to investigate the influence of rolling conditions. The experimental conditions at this time are shown in Table 3 below. As the working material, No. 1 in Table 1 was used. The same charge as 1 was performed, and only the rolling conditions were changed. The experimental conditions such as delayed fracture property and the measurement conditions of precipitates were the same as in Example 1.

The results are also shown in Table 3 below, and those satisfying the manufacturing conditions specified by the method of the present invention have a large number of Ti-based precipitates and have finer crystals after heat treatment (the particle size number increases). It can be seen that the delayed fracture strength is high.

[0051]

[Table 3]

[0052]

EFFECTS OF THE INVENTION The present invention is constituted as described above, and it is possible to realize a B-containing steel which can satisfy both the characteristics of cold workability and delayed fracture resistance. A high-strength bolt with excellent properties can be realized.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Yuichi Namimura, 2 Nadahama Higashi-cho, Nada-ku, Kobe City Kamido Steel Works, Ltd. Inside the Kobe Steel Works (56) Reference JP-A-4-28519 (JP, A) 58-84960 (JP, A) JP-A-11-92868 (JP, A) JP-A-11-43737 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 1 / 00-49/14 C21D 8/06 F16B 35/00

Claims (7)

(57) [Claims] 1. C: 0.10 to 0.45% (meaning mass%; the same applies hereinafter), B: 0.0003 to 0.0050%,
Ti: 0.01 to 0.1%, N: 0.0025 to 0.0
10% saw including, as yet another component, Si: 0.03 to 0.5%, M
n: 0.3 to 1.5% and Al: 0.01 to 0.10.
%, With the balance being Fe and unavoidable impurities, a hot-rolled material or a steel material cold-drawn after hot-rolling, which satisfies at least one of the following requirements (1) and (2): A B-containing steel excellent in cold workability and delayed fracture resistance, which is characterized by being (1) The amount of Ti contained in the precipitates extracted by the extraction residue method and having a particle size of more than 0.1 μm is 60% or more of the total amount of Ti contained in the steel material. (2)
The average number of Ti-based precipitates having a grain size of 0.01 to 0.2 μm observed by an electron microscope observation by the extraction replica method is 10 to 500 in the observation visual field of 25 μm ² . 2. As other components, Cu: 2% or less (0% is not included), Ni: 2% or less (0% is not included) and Cr: 2% or less (0% is not included) claim 1 is intended to contain one or more selected from the group consisting of
B-containing steel according to. As claimed in claim 2, further other components, Mo: 1% or less (not including 0%) according to claim 1 or those containing
B-containing steel according to 2. 4. Further, as another component, Nb: 0.005
To 0.1% and / or V: 0.005 to 0.3%, B according to any one of claims 1 to 5.
Containing steel. 5. As another component, Ca is 0.000.
B-containing steel according to any one of claims 1 to 4, those containing from 3 to 0.001%. 6. those produced by B-containing steel according to any one of claims 1 to 5 volts. 7. A method for producing a B-containing steel according to any one of claims 1 to 5 cold, characterized in that to operate while satisfying the following conditions (a) ~ (c) A method for producing a B-containing steel excellent in workability and delayed fracture resistance. (A) Billet reheating temperature: 950 ° C. or higher (b) Finish rolling temperature (that is, before the final rolling pass or
Mean surface temperature measurable with a radiation thermometer in front of rolling rolls
Degree) : 700 to 900 ° C (c) Average cooling rate from 750 ° C to 650 ° C after finish rolling: 1 ° C / sec or less