JPS5944366B2 - Manufacturing method for steel with less grain boundary embrittlement - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is designed to prevent grain boundary carbon/nitrides or Si and Ti around grain boundaries even if impurities such as P, Sn, and Sb segregate at grain boundaries.
This invention relates to a method for manufacturing steel materials that reduces or prevents grain boundary embrittlement by controlling the concentration.

It has been known that temper embrittlement occurs in low alloy steels containing impurity elements such as P, Sn, and Sb, resulting in a significant decrease in toughness.

The cause of these embrittlements is that impurity elements such as P, Sn, and Sb segregate at grain boundaries, which lowers the grain boundary energy. It is necessary to. Furthermore, the amount of grain boundary segregation of these elements depends on alloying elements such as Mn and Ni, grain size,
It is a well-known fact that it is influenced by the structure, etc., and it has also been shown that it is related to the precipitation of grain boundary carbides. In research on the embrittlement of such steel, the present inventors investigated the transition temperature increase Δ'vTrs and the grain boundary impurity concentration CB in the 2vtmV notch Charpy test after embrittlement treatment.
It was found that α changes in the relationship ΔvTrs■αCB+β.

FIG. 1 shows, as an example, the results of analyzing the influence of Si on 2Otsu Cr-IMo steel (the chemical composition is shown in Table 1).

It is a well-known fact that reducing the amount of Si reduces temper embrittlement, but the mechanism is not well understood. According to the analysis by the present inventors, the amount of Si does not change the amount of grain boundary segregation of P, nor does Si alone increase embrittlement. It has become clear that increasing α in the above equation promotes grain boundary embrittlement due to P. Figure 2 shows an analysis of the influence of B on Si-Mn steel (chemical composition shown in Table 2) as an example. When BN precipitates at the grain boundaries, it becomes brittle for the same amount of grain boundary segregation of P. It is clear that the change will be very large. The cause of this is not grain boundary segregation B, but B precipitates (BN in this case) that are precipitated at the grain boundaries to a size of 0.2μ or less. Therefore, it was found that grain boundary precipitates increase α. Therefore, such precipitates must be reduced as much as possible. The present inventors have discovered that even if the same amount of P is segregated at grain boundaries, the embrittlement becomes larger or smaller (alpha changes).

This α corresponds to the ease of stress relaxation around the grain boundaries, and increases when there are fine carbides, nitrides, or segregation of Si or Ti at the grain boundaries. On the other hand, P, Sn, Sb, etc. are segregated only in the very vicinity of grain boundaries (<10 people), and it is unlikely that lowering grain boundary energy would increase embrittlement and hinder stress relaxation. Therefore, even if there is grain boundary segregation such as P, grain boundary embrittlement can be prevented or reduced by removing as much as possible what hinders stress relaxation around the grain boundaries. The present invention is based on such knowledge, and contains P in an amount of 0.005% by weight or more and 0.05% by weight or less, and contains CO. 20% or less, S
iO. 5% or less, Mn2. 0% or less (both weight%)
When producing low-alloy steel containing quenched, tempered, directly quenched, tempered or quenched after rolling, it is maintained at a temperature at which nitrides such as BN, AAN, and TiN precipitate in the grains in the austenite, Or carbonitrides with a diameter of 0.2 μm or less on the grain boundaries are maintained at a temperature of 650°C or more and A3 point or less, where carbides such as TiC, V4C3, and NbC precipitate in the grains, and left cooled or tempered. is the grain boundary C+N
The present invention relates to a method for manufacturing steel materials with less grain boundary embrittlement, characterized in that the concentration is 0.05% by weight or less.

The steel type to which the present invention can be applied may be any steel containing 0.005% by weight or more and 0.05% by weight or less of P, but particularly preferred components are CO. 20% or less, SiO. 5% or less, Mn2
．． 0% or less, and if necessary, CrlO% or less, Ni
1O% or less, MOl. 5% or less, CuO. 5% or less, Z
Carbon steel or alloy steel selectively containing 0.2% or less of each of r, Ti, Nb, V, and Al (all by weight) is preferable.

In addition, in the present invention, in addition to P, Sn, S
It is also applicable to steels containing at least one grain boundary embrittlement element such as b, and where grain boundary embrittlement is a problem. Further, the heat treatment may be any heat treatment such as quenching, tempering, direct quenching and tempering, or rapid cooling after rolling, but in particular, during manufacturing, the temperature range between 400 and 600 degrees Celsius is slowly cooled or the temperature range between this range is maintained. Of particular interest are steel materials and those used at temperatures between 400 and 600°C. Next, the reason why we limited the diameter of carbonitrides on the grain boundaries to 0.2 μm or less is that most of the BN shown in Figure 2 is 0.2 μm or less, and in general, larger BNs are less than 0.2 μm in diameter. This is because the number will be very small and will not affect stress relaxation, and other AI
This is because the same can be said about precipitates such as IN and TiN.

In addition, the amount of precipitates is 0.05% by weight when the C+N concentration at the grain boundary is
The following is limited to the following: When examining the relationship between α and grain boundary N concentration (this can be measured by Auger analysis, etc.), α increases when there is a nitride with a grain boundary N concentration of 0.05% or more. This is because it is remarkable. The same thing can be said about carbides; those that increase α are smaller than those of nitrides and have a diameter of 0.025 μm or less, and their amount becomes remarkable when converted to grain boundary C concentration of 0.05%. Become. The present invention relates to a method for manufacturing steel materials characterized by reducing grain boundary carbides and nitrides as described above. First, regarding nitrides such as BN, 1) grain boundary N concentration is 0.05
% or less. This can be determined because the grain boundary N concentration can actually be measured (by Augier analysis, etc.). In addition, in this case, when heating the steel to 1000℃ or less, N (3A-TiP
The amounts of N, Ti, Al, and B may be limited according to A/3.4-Al%/4-B% and 0.003%, and 1250
℃ or below 1000℃ or more, NPA-Ti (4/
3.4- According to A7%/4≦0.003%, N, Ti
, the amount of Al is limited, and the amount of B is set to 0.
0.002% or less.

In addition, when heating at 1250°C or lower below 1400°C, N (%
The content of N and Ti is limited according to -TiK/3.4≦0.003%, and the content of B is limited to 0.002% or less.

Further, at temperatures above 1400°C, it is necessary to limit the content of N to 0.003% or less, and it is necessary to limit the B content to 0.002% or less. 2) In the case of heat treatment and hot working, if the precipitate is BN, hold the temperature at 1000°C or lower at point A3 or higher for 1 minute or more, or increase this temperature range to 4°C.
Cool at a cooling rate of /S or less.

In addition, hot working or rolling of 20% or more is performed in this temperature range.
In the case of AlN and TiN, either hold the temperature below 1000℃ for 10 minutes or more at A3 point or above, or keep this temperature range at 0.4℃/S.
Cool at the following cooling temperature. In addition, hot working or rolling of 20% or more is performed in this temperature range. This can be achieved with etc.

Next, regarding carbides, 1) Even if all of the solid solute C concentration precipitates at the grain boundaries, the grain boundary C
It is precipitated within the grains so that the concentration is 0.05% or less.

In practice, hot working and rolling of 30% or more are carried out in the unrecrystallized austenite region of A3 point or higher at 950° C. or lower and immediately quenched to introduce defects within the grains and cause precipitation within the tempered grains. Tempering may be performed at any temperature up to 3001A3 point. @ Maintain the temperature within 5 minutes (Ars2O ℃) during quenching, or cool at a cooling rate of 0.05 ℃/S or more, and immediately water quench below this temperature range.

This causes the formation of ferrite and paynite at grain boundaries,
This is to suppress precipitation of grain boundary carbides. In this case, the precipitation temperature may be 400°C or lower. 2) As is well known, add Ti, Nb, V, etc. and heat for 30 seconds or more at a temperature of 650°C or higher and lower than A3 point to convert C into 'Ri' in advance.
TiC, NbC should be precipitated in the grains as c, v, c3, and NbC, and should not be contained in amounts exceeding the respective chemical equivalents to prevent excess Ti, Nb, and V from segregating at grain boundaries.
After heating to 1250 ° C. or higher to fully dissolve the solid solution, hot working of 30% or more in the non-recrystallized temperature range of 950 ° C. or lower and A3 point or higher to cause austenite intragranular precipitation, In addition, this can be achieved by not allowing excess Ti and Nb to be contained in amounts greater than the chemical equivalent of each so as not to segregate at grain boundaries.

Next, a persimmon example of the present invention will be explained.

Persimmon Example 1 A steel having component A in Table 3 was heated to 1250°C, then quenched, and then tempered at 650°C.
When embrittled, large embrittlement is shown as shown in FIG. 3 (HQT).

Simultaneously with the segregation of P at the grain boundaries of the embrittled material, precipitation of BN with a grain boundary N concentration of 0.06% is observed. Grain boundary carbides with a diameter of 02 μm or less have a grain boundary C concentration of 0.03%,
grain! '+N concentration was 0.09%, which did not satisfy the conditions of the present invention. Component B has a B content of 0.0002% or less to prevent such precipitation of grain boundary BN, and with such a component, the grain boundary N concentration is approximately 1% and almost O. Grain boundary C+N of carbonitride with a diameter of 0.2 μm or less
The concentration is 0.03%, and as shown in FIG. 3, grain boundary embrittlement is significantly reduced. In addition, 950℃ heating quenching and tempering material is 9
Since BN is precipitated in the grains at 50°C, embrittlement is not large even with component A, so even if the B content is reduced like component B, the degree of embrittlement does not change. Actual Winding Example 2 Figure 4 shows the delayed fracture characteristics of a material obtained by heating steel having the components shown in Table 3 to 1250°C, quenching it, and tempering it at 500°C for 1 hour.

Compared to steel B, steel A has BN precipitated at the grain boundaries, so the grain boundary C+N concentration of grain boundary carbonitrides with a diameter of 0.2 μm or less is 0.
10%, causing grain boundary embrittlement and deteriorating delayed fracture characteristics. However, if it is held in austenite and BN is precipitated within the grains to prevent BN from precipitating at the grain boundaries, the diameter is 0.2μ.
The grain boundary C+N concentration of the grain boundary carbonitride of m or less is 0.04%, and stress relaxation around the grain boundary is no longer inhibited, and the delayed fracture characteristics are improved as shown in the figure. Actual paddle example 3 Figure 5 shows 2'/4Cr-IMO steel (chemical composition is shown in Table 1).

) shows the relationship between grain boundary P content and embrittlement when the quenched and tempered material is subjected to embrittlement treatment. a is 0.0 at grain boundaries at 0.1 μm intervals
Carbides with a diameter of 25 μm or less were precipitated by 0.5% in terms of grain boundary C concentration (Steels 1, 2, 3, and 4), and the embrittlement was quite large. If the grain boundary carbide concentration is reduced to 0.05% or less in terms of grain boundary C concentration (Steel 8), embrittlement can be minimized even though the grain boundary P concentration is large, as shown in b. In this steel, the grain boundary N concentration of grain boundary nitrides of 0.2 μm or less is approximately O. Example 4 FIG. 6 shows C-Si-Mn steel (chemical compositions are shown in Table 5).

) shows the relationship between toughness and grain boundary P concentration when SR treatment (600°C for 2 hours→furnace cooling) is performed on the welding simulated thermal cycle material. When Nb and V are not included, carbonitrides of 0.2 μm or less are hardly observed on grain boundaries, and the grain boundary C+N concentration is 0.02% or less. In those containing Nb and V, carbides and nitrides of Nb and V are finely precipitated at grain boundaries, and the C+N concentration of carbonitrides with a diameter of 0.2 μm or less is 0.08% by weight. When performing SR processing, N
It can be seen that by reducing b and V to prevent the formation of fine precipitates on the grain boundaries, it is possible to prevent the toughness from deteriorating even when the P content is high.

[Brief explanation of drawings]

Figure 1 shows the grain boundary P concentration CP and Δ in 2ZCrIM0 steel.
VTrs (here, BΔVTrs indicates the increase in fracture surface transition temperature in the Charpy impact test due to embrittlement treatment), and Figure 2 shows the grain boundary P concentration CBc! in Si-MnP steel. : Graph showing the relationship between VTrs, Figure 3 is Δ
A graph showing the effects of ingredients and heat treatment on VTrs,
Figure 4 is a graph showing the effects of B addition and heat treatment on the delayed fracture properties of Si-Mn steel, and Figure 5 is a graph showing the effects of B addition and heat treatment on the delayed fracture properties of Si-Mn steel.
- A graph showing that the relationship between grain boundary P concentration CB and ΔVTrs in IMO steel changes depending on the state of grain boundary carbide precipitation. Figure 6 shows the toughness and grain boundaries after welding simulated thermal cycle + SR of Nb and V additives. It is a graph showing the relationship between P concentration.

Claims (1)

[Claims] 1 Contains 0.005% by weight or more and 0.05% by weight or less of P, C0.20% or less, Si0.5% or less, Mn2.
When manufacturing low-alloy steel containing 0% or less (both by weight) by quenching, tempering, direct quenching and tempering, or as rapidly cooled after rolling, BN, AlN, Ti in austenite is produced.
It is maintained at a temperature at which nitrides such as N precipitate in the grains, or T
When held at a temperature of 650°C or above and below A_3 point, where carbides such as iC, V_4C_3, and NbC precipitate in the grains, the diameter on the grain boundary is 0.2 μm when left cooled or tempered.
A method for producing a steel material with less grain boundary embrittlement, characterized in that the following carbonitrides are used to reduce the C+N concentration at grain boundaries to 0.05% by weight or less.