JP5000367B2 - High strength galvanized bolt with excellent hydrogen embrittlement resistance - Google Patents

Description

The present invention is suitable for use in corrosive environments, it relates to a galvanized bolt having excellent hydrogen embrittlement resistance.

  In order to reduce the weight and performance of automobiles and various industrial machines, or to reduce the construction costs of civil engineering and building structures, the strength of bolts has been increased. Conventional high-strength bolts are manufactured, for example, by using a low alloy steel such as SCM435 or SCM440 defined in JIS G 4105, cold forming into a predetermined shape, and then quenching and tempering. However, there has been a problem that delayed fracture is likely to occur due to an increase in tensile strength.

  For such a problem, a method has been proposed in which cold-forged pearlite steel is cold-forged into a bolt shape and subjected to heat treatment (see, for example, Patent Documents 1 and 2). In addition, some of the present inventors have proposed a high-strength bolt with improved delayed fracture resistance by controlling the shape of pearlite (see, for example, Patent Document 3).

However, since corrosion resistance is required in a corrosive environment, galvanized bolts with galvanized surfaces are prone to hydrogen embrittlement, especially when the strength is high, because hydrogen easily penetrates into the bolts during plating. It was. Moreover, the technique which suppresses the amount of diffusible hydrogen and improves the delayed fracture resistance of a galvanized bolt is proposed (for example, refer patent document 4). However, the galvanized bolt described in Patent Document 4 is not intended to improve the delayed fracture resistance of the steel material.
JP 2001-348618 A JP 2005-281860 A JP 2004-307929 A Japanese Patent Laid-Open No. 64-68452

The present invention has been made in view of the fact that a galvanized bolt having a metal structure made of pearlite having excellent delayed fracture resistance and having a strength of 1200 MPa or more has not been put into practical use, has corrosion resistance, and has hydrogen resistance. An object of the present invention is to provide a high-strength galvanized bolt having a strength of 1200 MPa or more that is excellent in embrittlement characteristics.

  The present inventors paid attention to the excellent delayed fracture resistance of the pearlite structure, and studied a bolt that does not cause hydrogen embrittlement even when galvanized, and a method for manufacturing the same. As a result, it is possible to prevent the occurrence of vertical cracks due to hydrogen that has penetrated into the bolt during plating by limiting the hardness difference between the inside of the bolt and the surface layer. Furthermore, by limiting the amount of hydrogen in the bolt, delayed fracture characteristics Found to improve.

This invention is made | formed based on such knowledge, The place made into the summary is as follows.
(1) By mass%, C: 0.70 to 1.10%, Si: 0.05 to 2.00%, Mn: 0.20 to 2.00%, P: 0.020% or less S: 0.020% or less, N: 0.0150% or less, Al: 0.005-0.100%, Ti: 0.002-0.100%, Nb: 0.002-0. It contains any one or two or more of 100%, the balance is made of Fe and inevitable impurities, the metal structure is made of pearlite with an area ratio of 90% or more, the Vickers hardness of the surface layer of the shaft part and the Vickers hardness of the center High strength galvanized bolt excellent in hydrogen embrittlement resistance, characterized in that the difference from the above is 30 or less, the tensile strength is 1200 MPa or more, the delayed fracture limit diffusible hydrogen content is 0.2 ppm or more .
(2) The hydrogen embrittlement resistance according to (1) above, characterized by containing one or both of V: 0.05 to 2.00% and Cr: 0.05 to 2.00% by mass%. High-strength galvanized bolt with excellent heat treatment characteristics.
(3) By mass%, C: 0.70 to 1.10%, Si: 0.05 to 2.00%, Mn: 0.20 to 2.00%, P: 0.020% or less , S: 0.020% or less, N: 0.0150% or less, V: 0.05 to 2.00%, Cr: 0.05 to 2.00% is contained, the balance Is made of Fe and inevitable impurities, the metal structure is made of pearlite with an area ratio of 90% or more, the difference between the surface layer Vickers hardness and the center Vickers hardness is 30 or less, and the tensile strength is 1200 MPa or more. A high-strength galvanized bolt excellent in hydrogen embrittlement resistance, characterized by having a delayed fracture limit diffusible hydrogen content of 0.2 ppm or more.
(4) The high-strength galvanizing excellent in hydrogen embrittlement resistance according to any one of (1) to (3) above, wherein the amount of diffusible hydrogen in the steel is 0.10 ppm or less bolt.
The galvanized bolt according to any one of the above (1) to (3) is, for example, after hot rolling a steel material comprising the component according to any one of the above (1) to (3). , Cooled to a temperature range of 550 to 700 ° C. at a cooling rate of 30 ° C./s or more, held for 30 to 300 s in the temperature range, and then cooled to room temperature, and then stretched to a coefficient of friction of 0.1 or less. After wire processing, it is manufactured by forming into a bolt shape and applying electrogalvanization or hot dip galvanization .
The galvanized bolt according to any one of the above (1) to (3) is, for example, after hot rolling a steel material comprising the component according to any one of the above (1) to (3). It is manufactured by cooling as it is, after wire drawing, forming into a bolt shape, and applying electrogalvanization or hot dip galvanization .
The galvanized bolt according to any one of the above (1) to (3) is, for example, after hot rolling a steel material comprising the component according to any one of the above (1) to (3). It is manufactured by reheating to 900 ° C., cooling, forming into a bolt shape after wire drawing, and applying electrogalvanizing or hot dip galvanizing .
Moreover, the galvanized bolt as described in said (4) is manufactured by heating the galvanized bolt obtained by the method in any one of the above in the temperature range of 200-400 degreeC , for example. Further, the galvanized bolt may be heated while applying a tension of 20 to 60% of the tensile strength .

According to the present invention, it is possible to provide a high-strength galvanized bolt having excellent hydrogen embrittlement resistance, in particular, delayed fracture resistance and longitudinal crack resistance, and good corrosion resistance. Extremely prominent.

Hereinafter, the high-strength galvanized bolt excellent in hydrogen embrittlement resistance and the method for producing the same according to the present invention will be described in detail.
When zinc plating is applied to a bolt obtained by drawing a high-strength steel having a pearlite structure with excellent hydrogen embrittlement resistance, vertical cracks after plating, which may be caused by intrusion hydrogen during plating, may occur. was there. In order to prevent this, the present inventors examined the occurrence of vertical cracks after plating using a high-strength bolt made of drawn pearlite.

  The present inventors pay attention to the structure of bolts, particularly the relationship between hardness homogeneity and longitudinal cracks after plating, change the friction coefficient, and perform wire drawing on a steel wire having a pearlite structure to perform plating. Later vertical cracks were evaluated. As a result, when the coefficient of friction at the time of wire drawing was lowered, the bolt structure became homogeneous, the difference in hardness between the surface layer and the center became small, and it was found that longitudinal cracks can be prevented.

  This is considered to be an effect of reducing the difference in hardness between the surface layer and the central portion when the wire drawing processing at the surface layer and the central portion is made uniform, and reducing the internal stress load and stress concentration. Specifically, the vertical crack after plating can be prevented by setting the difference between the Vickers hardness of the surface layer of the bolt shaft portion and the Vickers hardness of the center to 30 or less.

  In addition, the measurement of Vickers hardness was performed based on JISZ2244. The sample was taken with the cross section perpendicular to the longitudinal direction of the shaft portion of the bolt as the measurement surface. The measurement surface of the sample was mechanically polished and measured at three locations within a range of 200 μm within the central portion in the direction perpendicular to the longitudinal direction and at three locations at 200 μm from the surface layer, and the average value was determined. The load was 98.07 N (10 kgf).

  In order to make the difference between the Vickers hardness of the surface layer of the bolt shaft portion and the Vickers hardness of the center 30 or less, it is necessary to limit the friction coefficient during wire drawing to 0.1 or less. If the friction coefficient during wire drawing exceeds 0.1, the difference in hardness between the surface layer and the center of the wire drawing material becomes large, and vertical cracks due to intruding hydrogen tend to occur. Moreover, when the friction coefficient is 0.08 or less, the effect of making the hardness uniform becomes remarkable, which is preferable.

  Examples of a method for reducing the friction coefficient during wire drawing include a method using a diamond die, a method using a lubricant film containing zinc calcium phosphate, and a method using a soap lubricant during wire drawing. The coefficient of friction at the time of wire drawing can be obtained by measuring the drawing force during wire drawing and using the Siebel equation.

  Furthermore, in order to evaluate the relationship between the metal structure and the delayed fracture resistance, the present inventors processed the steel wire after wire drawing into a bolt and measured the amount of critical diffusible hydrogen. The critical amount of diffusible hydrogen is the maximum amount of hydrogen that does not cause delayed fracture, and after containing various levels of diffusible hydrogen by electric field hydrogen charging, plating is performed so that hydrogen is not released from the sample into the atmosphere. After that, a load of 90% of the tensile strength was applied, and the maximum value of the diffusible hydrogen amount at which delayed fracture did not occur was evaluated.

  As a result, it was found that when the area ratio of the pearlite structure was 90% or more, the limit diffusible hydrogen amount was 0.2 ppm or more. Therefore, in order to have sufficient strength and to have excellent hydrogen embrittlement resistance, it is necessary to have a metal structure containing 90% or more of pearlite.

  If the pearlite is less than 90%, other tissues will increase. For example, if bainite increases, the strength decreases and sufficient strength cannot be obtained. Moreover, if proeutectoid cementite and martensite increase, the hydrogen embrittlement resistance will deteriorate. The balance of pearlite is ferrite, bainite, martensite, and proeutectoid cementite.

  The area ratio of pearlite is obtained by taking a sample from the wire before drawing, mirror-polishing the cross section perpendicular to the longitudinal direction, etching with Picral solution, and observing any 10 fields of view at 2000 times with a scanning microscope Then, the photograph can be taken, the pearlite portion can be determined by visual observation, and it can be obtained by image analysis.

  The area ratio of pearlite is controlled by cooling conditions or reheating conditions after hot rolling. Specifically, at the time of cooling after hot rolling or after reheating, it is cooled to a temperature range of 500 to 700 ° C. at a cooling rate of 30 ° C./s or more, and is maintained for 30 to 300 s in this temperature range. And further cooled to room temperature.

  If the cooling rate is less than 30 ° C./s, pearlite transformation occurs during cooling and the tensile strength decreases, so the lower limit is made 30 ° C. or more. Although the upper limit of the cooling rate is not specified, it is technically difficult to exceed 500 ° C./s.

  The reason why the cooling stop temperature and holding temperature are limited to 550 to 700 ° C. is to promote pearlite transformation and improve strength and hydrogen embrittlement resistance. When the cooling stop temperature and the holding temperature are less than 550 ° C., bainite is likely to occur, the area ratio of pearlite decreases, and the strength reduction or hydrogen embrittlement resistance deteriorates. On the other hand, when the cooling stop temperature and the holding temperature exceed 700 ° C., the tensile strength after pearlite transformation decreases.

  Furthermore, in order to make the area ratio of pearlite 90% or more, the holding time in the temperature range of 550 to 700 ° C. is important. If the holding time at 550 to 700 ° C. is short and the area ratio of pearlite decreases, bainite or martensite that adversely affects the hydrogen embrittlement resistance is generated, so the holding time is 30 s or more. If the holding time is long, productivity is hindered. In the component range of the present invention, if the holding time is 300 s or less, the pearlite area ratio can be 90% or more. The holding at 550 to 700 ° C. may be carried out by immersing in a salt bath or a lead bath after hot rolling or reheating.

  If the final finishing temperature of the hot rolling is less than 850 ° C., the deformation resistance becomes large and it is difficult to control the shape of the rolled material, and if it exceeds 1150 ° C., the austenite grains become coarse and the ductility after wire drawing may decrease. Therefore, the final finishing temperature of rolling is preferably in the temperature range of 850 ° C to 1150 ° C.

  In the present invention, after hot rolling, it may be cooled as it is, or may be once cooled to room temperature and then reheated. When the steel material after hot rolling is reheated, if the reheating temperature is less than 900 ° C, the solution effect is insufficient and undissolved carbide tends to remain, and the effect of the alloy may decrease. It is preferable that Further, if the upper limit of the reheating temperature is too high, the austenite grains are coarsened, and ductility is lowered after the wire drawing.

Next, the reasons for limiting the components of the steel that is the subject of the present invention will be described.
C is an essential element for securing the strength of the bolt, but if it is less than 0.70%, sufficient strength after wire drawing cannot be obtained, while if it exceeds 1.10%, coarse precipitation of pro-eutectoid cementite. Since the delayed fracture resistance is deteriorated by pro-eutectoid cementite, it is limited to the range of 0.70 to 1.10%.

  Si is an element that increases the strength by solid solution strengthening, but in order to obtain this effect, addition of 0.05% or more is necessary, and if it exceeds 2.00%, an effect commensurate with the addition amount is obtained. Therefore, the content is limited to 0.05 to 2.00%.

  Mn is not only necessary for deoxidation and desulfurization, but is an element that improves the hardenability of the steel and increases the strength, but in order to obtain this effect, addition of 0.20% or more is necessary. is there. Further, when Mn is added excessively, martensite is generated in the central segregation part and the wire drawing workability is lowered, so the upper limit is limited to 2.00% or less.

  P and S are unavoidable impurities, and if they are contained in excess, the hydrogen embrittlement resistance deteriorates, so they are limited to 0.020% or less.

  N is an impurity, and if it exceeds 0.0150%, the ductility is lowered, so the range is 0.0150%. In addition, when Al, Ti, and Nb are contained in order to form a nitride effective for refining the austenite (γ) grain size, it is preferable to contain N in an amount of 0.0020% or more.

  Further, V and Cr may be selectively added to increase the strength, and Al, Ti and Nb may be selectively added to refine the crystal grains.

  V is an element effective for increasing the strength of the pearlite transformation structure, and is preferably added in an amount of 0.05% or more. Moreover, in order to improve the delayed fracture resistance of a high-strength bolt made of a pearlite structure, it is preferable to add 0.20% or more. On the other hand, even if V is added in excess of 2.00%, it is difficult to obtain an effect commensurate with the cost.

  Cr has an effect of refining the lamella spacing of the pearlite structure, and is preferably added in an amount of 0.05% or more. Moreover, in order to raise intensity | strength, addition of 0.10% or more is preferable, On the other hand, even if it adds exceeding 2.00%, an effect will be saturated.

Al is added in order to refine the γ grain size at the time of heat treatment by deoxidation and the pinning effect of fine Al ₂ O ₃ or AlN precipitation. In order to obtain this effect, addition of 0.005% or more is preferable. On the other hand, even if added over 0.100%, the effect is saturated.

Ti is an element that refines the γ grain size during heat treatment by deoxidation and a pinning effect caused by precipitation of oxides such as TiO ₂ and TiN. In order to obtain this effect, it is preferable to add 0.002% or more of Ti. On the other hand, the effect is saturated even if Ti is added in excess of 0.100%.

  Nb is added to refine the γ grain size during heat treatment due to the pinning effect of the Nb carbonitride. In order to obtain this effect, Nb is preferably added in an amount of 0.002% or more. On the other hand, when the amount of Nb exceeds 0.100%, the effect is saturated, so the content is limited to the range of 0.002 to 0.100%.

  The plating of the present invention may be either electrogalvanization or hot dip galvanization, but if hot dip galvanization, the treatment is performed in the order of general methods such as degreasing, pickling, flux, hot dipping, and cooling. For electrogalvanizing, it is preferable to use a zinc chloride bath that has good throwing power and excellent cathode current efficiency. The process is performed in the order of degreasing, pickling, electric field cleaning, and plating, for example.

  If the amount of diffusible hydrogen in the steel exceeds 0.10 ppm, the bolt of the present invention is more likely to cause vertical cracking or delayed fracture, so it is preferable that the bolt be 0.10 ppm or less. The amount of diffusible hydrogen is determined by performing a temperature rising hydrogen analysis using a gas chromatograph, and the amount of hydrogen released in the range from room temperature to 200 ° C. at a temperature rising rate of 100 ° C./min.

  In order to reduce the amount of diffusible hydrogen in the bolt, it is preferable to perform a heat treatment after the bolt is cold-formed after the wire drawing and is subjected to Zn plating. Further, it is more preferable to apply tension during heating after plating.

  The heat treatment after galvanization is preferably performed at 200 ° C. or higher in order to sufficiently release diffusible hydrogen in the bolt. On the other hand, when the temperature of the heat treatment exceeds 400 ° C., the galvanizing is melted, so the range of the heating temperature is limited to 200 to 400 ° C. The heating time is not particularly limited because it varies depending on the heating furnace, but is preferably in the range of 10 s to 15 ks in order to sufficiently exhibit the above effects.

  In addition, the hydrogen embrittlement resistance is further improved by performing heat treatment while applying tension to the bolt after plating. If the lower limit of the tension is less than 20% of the tensile strength, the effect of improving hydrogen embrittlement resistance is small, and even if a tension exceeding 60% is applied, the effect is saturated. . As a method of performing heat treatment while applying a tension, for example, a method of performing heat treatment in a state where a bolt is fastened to a jig may be mentioned.

  Hereinafter, the effects of the present invention will be made clearer by examples. In addition, this invention is not limited to a following example, In the range which does not change the summary, it can change suitably and can implement.

  Steel having the chemical composition shown in Table 1 is melted and hot-rolled under the conditions shown in Table 2 and immediately immersed in a salt bath, or reheated after hot-rolling, immersed in a lead bath, and transformed into pearlite. It was. Thereafter, wire drawing with a true strain of 0.4 to 2 was performed with the friction coefficient shown in Table 2, and after forming into a bolt by cold working, the bolt was subjected to electrogalvanization or hot dip galvanization. Some of the plated bolts were subjected to heat treatment under the conditions shown in Table 2 while applying a tension while a bolt was fastened to a jig. In addition, the blank in Table 1 means that intentional addition is not performed, and “-” in Table 2 means that the corresponding processing is not performed.

  And No. 2 shown in Table 2. The bolts 1 to 22 were evaluated for hydrogen embrittlement resistance. The evaluation results are shown in Table 3.

The tensile strength of the bolts shown in Table 3 was obtained by a tensile test based on JIS B 1051.
The hardness difference shown in Table 3 is the difference between the Vickers hardness of the surface layer of the shaft portion and the Vickers hardness of the center portion, and the cross section perpendicular to the longitudinal direction of the shaft portion of the bolt is the measurement surface, and the range within 200 μm of the center portion. And the difference of the average value of three Vickers hardness each was calculated | required in the position of 200 micrometers from the surface layer. The Vickers hardness was measured based on JIS Z 2244 with a load of 98.07N. The unit HV10 means Vickers hardness measured with a load of 98.07 N.
The limit diffusible hydrogen amount shown in Table 3 is that after adding the diffusible hydrogen amount by electric field hydrogen charge, plating is performed so that hydrogen is not released from the sample into the atmosphere, and a load of 90% of the tensile strength is applied. The maximum amount of diffusible hydrogen that was loaded and did not cause delayed fracture was evaluated.
The amount of diffusible hydrogen shown in Table 3 was evaluated as the amount of hydrogen released at a temperature rising rate of 100 ° C./min in the range from room temperature to 200 ° C., and measured by performing a temperature rising hydrogen analysis using a gas chromatograph. Furthermore, electrogalvanization or hot dip galvanization was performed, and the presence or absence of occurrence of vertical cracks was confirmed.
In Table 3, “-” means that the corresponding measurement is not performed.

As shown in Table 3, no. Each of the bolts 1 to 13 (examples of the present invention) is made of pearlite having a metal structure of 90% or more in area ratio, the difference between the surface layer of the shaft portion and the Vickers hardness at the center is 30 or less, and the tensile strength is 1200 MPa. High strength as described above, excellent hydrogen embrittlement resistance, and no vertical cracks after plating.
In addition, No. In the bolts 1 to 12, the amount of diffusible hydrogen in the steel is 0.1 ppm or less. In the bolt No. 13, the amount of diffusible hydrogen in the bolt exceeds 0.1 ppm, and the probability of occurrence of delayed fracture is slightly high.
On the other hand, no. The bolts 14, 16 and 17 (comparative examples) have a C amount, an Si amount, and an Mn amount that are less than the lower limit of the present invention, respectively, so that the tensile strength is below 1200 MPa.
No. The 15 bolt (comparative example) has a C amount higher than the upper limit of the present invention, and the delayed fracture resistance is reduced.
No. In 18 bolts (comparative examples), the amount of Mn exceeds the upper limit of the present invention, so that the wire drawing workability deteriorates and breaks during wire drawing.
No. No. 19 bolt (comparative example) has a low cooling rate after rolling and has a tensile strength of less than 1200 MPa.
No. No. 21 bolt (comparative example) has a tensile strength of less than 1200 MPa because the transformation temperature after rolling exceeds 700 ° C.
No. In the case of 20 bolts (comparative example), the transformation temperature after rolling is lower than 550 ° C., so that bainite is generated and the critical diffusible hydrogen content is lower than 0.2.
No. The bolt 22 (comparative example) has a friction coefficient of more than 0.1 at the time of wire drawing, so the difference between the surface layer of the bolt shaft and the Vickers hardness of the center exceeds 30, and vertical cracking after plating. Has produced.

Claims (4)

% By mass
C: 0.70 to 1.10%,
Si: 0.05 to 2.00%,
Mn: 0.20 to 2.00%
Containing
P: 0.020% or less,
S: 0.020% or less,
N: limited to 0.0150% or less,
Al: 0.005 to 0.100%,
Ti: 0.002 to 0.100%,
Nb: 0.002 to 0.100%
Contains one or more of them,
The balance consists of Fe and inevitable impurities, the metal structure consists of pearlite with an area ratio of 90% or more, the difference between the Vickers hardness of the surface layer of the shaft part and the Vickers hardness of the center is 30 or less, and the tensile strength is 1200 MPa or more A high-strength galvanized bolt excellent in hydrogen embrittlement resistance, characterized by having a delayed fracture limit diffusible hydrogen content of 0.2 ppm or more. % By mass
V: 0.05-2.00%
Cr: 0.05-2.00%
The high-strength galvanized bolt excellent in hydrogen embrittlement resistance according to claim 1, comprising one or both of the following: % By mass
C: 0.70 to 1.10%,
Si: 0.05 to 2.00%,
Mn: 0.20 to 2.00%
Containing
P: 0.020% or less,
S: 0.020% or less,
N: limited to 0.0150% or less,
V: 0.05-2.00%
Cr: 0.05-2.00%
One or both of
The balance consists of Fe and inevitable impurities, the metal structure consists of pearlite with an area ratio of 90% or more, the difference between the Vickers hardness of the surface layer of the shaft part and the Vickers hardness of the center is 30 or less, and the tensile strength is 1200 MPa or more A high-strength galvanized bolt excellent in hydrogen embrittlement resistance, characterized by having a delayed fracture limit diffusible hydrogen content of 0.2 ppm or more.   The high-strength galvanized bolt excellent in hydrogen embrittlement resistance according to any one of claims 1 to 3, wherein the amount of diffusible hydrogen in the steel is 0.10 ppm or less.