JPH03211251A - High strength for welding structure having excellent fracture toughness in heat affected zone - Google Patents

Abstract

PURPOSE:To manufacture a high strength steel for welding structures having excellent fracture toughness in the heat affected zone by preparing a steel in which the heat affected zone after welding is formed of a specified compsn. and having a structure constituted of crystalline grains surrounded by lumpy pro-eutectoid ferrite. CONSTITUTION:This steel is prepd. in such a manner that in which the heat affected zone after welding (HAZ) contains, by weight, 0.07 to 0.16% C, <=0.020% P, 0.15 to 0.30% Si, <=0.020% S, 1.20 to 1.50% Mn, 0.005 to 0.020% Ti and 0.005 to 0.10% Al, contains, at need, one or >=2 kinds among <=1.0% Cu, <=0.02% Nb, <=1.0% Ni, <=0.1% Ca, <=0.1% V and <=0.0015% B as well as satisfies Mn/C<=15 and C%+Si%/24+Mn%/6+Ni%/40+V%/14=0.32 to 0.44% and the balance Fe with inevitable components and the structure is constituted of crystalline grains surrounded by lumpy pro-eutictoid ferrite. In this way, the high strength steel for wilding structures in which the HAZ has excellent value in a deep notch test can be obtd.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a high-strength welded structural steel material whose weld heat affected zone (hereinafter referred to as HAZ) has an excellent deep notch test value (fracture toughness value = Kc). It is something.

<Conventional technology> YP24kg steel was initially used as welded structural steel materials used in fields such as shipbuilding and bridges, and later YP3
2 kg steel has been used.

In the meantime, the guarantee and confirmation of the material quality of the steel between the manufacturer and the user is disclosed in, for example, Japanese Patent Application Laid-open No. 155418/1983.
JP-A-51-39523, JP-A-51-4162
As described in Publication No. 0, Charpy test values were used.

However, as is well known, the Nyalpy test value indicates the average characteristics of the notch, which is composed of various tissues, so although it can be used for safety discussions based on experience, it is still used for high strength If steel is to be put into practical use, fracture mechanics studies are required.

In other words, with the current demand for higher toughness in the HAZ of welded structural steel materials, in order to accurately guarantee the soundness of structures, 1 (
The Charpy test value, which does not show the characteristics of the most bent tissue at the tip of the AZ notch, cannot be used alone because it is guaranteed by its average value.

For this reason, the present inventors conducted a fracture mechanical study using a deep notch test, etc., which shows the characteristics of the most bent structure at the notch tip of IAZ, and used material quality assurance methods that have been used and trusted for many years. We proceeded with the study of adding the fracture toughness value Kc obtained from the deep notch test to the above Charpy test value to ensure complete guarantee.

This deep notch test, as is well known, is a test method established in Japan, and is used in this field over 20 years.
This is a test method that has been used for more than a year when strict guarantees are required.

Based on the above background, the present inventors have developed HAZ&
When examining the influence of changes in the weave on Charpy test values and deep notch test values, it was found that the deep notch test values deteriorate significantly as the strength increases.

For example, when we conducted this deep notch test on a YP40kg steel joint using IAZ (a structure made from welded structural steel that has already obtained a passing value of 4.0kgf-■ or higher), Among them, there are welded structural steel materials that do not reach the required fracture toughness value of c500kgf/am'' (0°C) or higher in the applications of the currently used YP40 kg steel, and the above-mentioned HAZ toughness This confirmed the need for a guarantee method.

On the other hand, regarding the improvement of HAZ toughness of steel materials under such circumstances,
For example, as described in Japanese Unexamined Patent Publication No. 63-103051, there is a proposal to disperse fine TiN of 0.02 μm or less to refine the austenite grain size of the HAZ and improve the HAZU property.

However, in the IAZ (which is subjected to heat cycles of 1400°C or more), the finely dispersed TiN in the base metal is melted during welding, and a large amount of TiN cannot be dispersed and precipitated again in the HAZ after welding, so the structure becomes finer. ) No improvement in IAZ toughness can be expected, and it is necessary to reduce N in order to prevent embrittlement due to N freed during melting at this time.
Normally, N is set to 0.004Z or less.

As an alternative to this, in recent years, intragranular transformed ferrite with fine ferrite dispersed within the grain, that is, rFP, has been
There are proposals to improve AZ toughness.

However, as a result of the detailed analysis of the HAZ fracture by the present inventors, it was found that the IFP does not directly improve the toughness of the HAZ, but that when the IFP space factor increases, the ferrite side, which directly controls the HAZ toughness, Plate (hereinafter referred to as FSP and Sakakisu)
It has been found that there are some cases where the size of the steel is reduced, and this improves toughness.

Based on this fact, the present inventors found that it is not possible to directly and completely control the shape of FSP, which is a HAZO latex structure, simply by increasing the IFP space factor, and that the fracture characteristics of It was discovered that the deep notch test value, which indicates the

In addition, in the rFP formation structure, coarse FSP is formed along the plate-shaped pro-eutectoid ferrite in proportion to the length of the plate-shaped ferrite.
It has been found that there is a part where Kc is generated, and this part becomes one of the starting points of brittle fracture, making it impossible to obtain a sufficient Kc value.

<Problems to be Solved by the Invention> The present invention does not indirectly improve HAZ toughness by increasing the space factor of IFP, but by directly controlling the shape of FSP, which directly controls HAZ toughness. We accurately guarantee the strength of YP40 kg steel and even YP43 kg steel,
The Charpy test value exceeds 4.0 kgf 1, and the fracture toughness value Kc exceeds 500 kgf /am '・' (0°C
) To provide economical high-strength welded structural steel materials of YP40kg steel grade and YP43kg steel grade that do not require TiO treatment, REM treatment, etc. that increase costs in conventional methods. The challenge is to

<Means for Solving the Problems> In order to achieve the above-mentioned problems, the present invention has the following objectives: (1) The heat affected zone, or HAZ, after welding has a C: 0.07 to 0.07% by weight.
162 P: ≦0.020! St: 0.
15~0.30Z S: ≦0.008%Mn
: 1.20~1.50Z Ti: 0.0
05 to 0.020
i:≦1.0Z Ca:≦0.1χV
: ≦0.1%B: ≦O, 1 or 2 of 0O15Z
Contains more than one species, and Mn/C≦15 and Cχ+SiZ/2
4+MnZ/6 +Niz/40+VZ/14 Ka0.
32 to 0.442, other unavoidable components and Fe
The first means is to use a high-strength welded structural steel material with excellent fracture toughness in the weld heat-affected zone, which is characterized by a structure consisting of crystal grains surrounded by massive pro-eutectoid ferrite; (2) The heat affected zone or HAZ after welding is in weight percent, c:
0.07-0.16%P:≦0.008%Si:
0.15~0.30zS: ≦0.020! Mn
: 1.20~1.50Z Ti: 0.005
~0.020%Al: 0.005~0.10χB
:≦0.0O15ZN: 0.003 to 0.008z, as necessary, Cu:≦1.0%Nb:≦0.02%Ni:≦
1.0zCa: ≦0.1χ ■: ≦0.1χ Contains one or more types, and Ti/N = 2.0 to 3
．． 2 and CZ + SiZ/24 + Mn%/6 +
Welding heat effect characterized by NiZ/40+V%/14 satisfying 0.32 to 0.44χ, consisting of other unavoidable components and Fe, and having a structure composed of crystal grains surrounded by massive pro-eutectoid ferrite. The second method is to use high-strength welded structural steel materials with excellent fracture toughness.

The reason for limiting each component of the above-described first and second means will be explained below.

C is determined to ensure the required structure consisting of grains surrounded by massive pro-eutectoid ferrite found by the present inventors in a high Kc HAZ, and to satisfy the required strength for the application, and St is: It is added to maintain the strength of the base metal and to preliminarily deoxidize the molten steel, and the upper limit is limited to prevent the formation of island-like martensite in the segregated area.Mn, like C, is added to the HAZ to form massive pro-eutectoid ferrite. P is determined to obtain a structure consisting mainly of surrounded grains, to ensure a high Kc of IAZ, and to obtain base metal strength, and P is determined to prevent deterioration of HAZ toughness due to micro-segregation. The upper limit is set for S to prevent the formation of coarse A-based inclusions and deterioration of the toughness and anisotropy of the base metal, and for AI, deoxidation, It is determined to refine the base material structure, fix solid solution N, etc., and to prevent deterioration of the cleanliness of steel. In order to act as a formation nucleus for the steel, and also to prevent a decrease in the toughness of the matrix and the promotion of the formation of high carbon martensite in the HAZ, a lower limit is set to prevent a decrease in the cleanliness of the steel and embrittlement due to the precipitation of Tic. An upper limit is set.

The above is the amount of each element added as a basic component of the welded structural steel material of the present invention and the reason for its addition.

Furthermore, (1) one or more of Cu, Ni, V, and Nb for the purpose of increasing the strength of the base material and improving the toughness of the base metal and HAZ, and (2) preventing coarsening of the crystal grains of Z and the base material. One or more of REM, Ca, and Mg are used for the purpose of reducing the anisotropy of the material, and in reality, either (1) or (2) or both (1) and (2) are added. are doing.

However, although Cu in group (1) increases the strength of the base material, the increase in hardness of HAZ is small, but stress relief annealing and HA
Since the hardenability of Z increases, in order to prevent this increase, 1.
The upper limit is 0χ.

Ni is added to simultaneously increase the strength and toughness of the base metal and the HAZ toughness, but the increase in hardenability may inhibit the formation of IFP in the HAZ, so in order to prevent this, 1. The upper limit of the amount of addition is set at Ol.

Nb is used in YP40 to increase the strength of the base metal and welded joint by improving hardenability and precipitation effect without increasing Ceq, which is one of the indicators of weldability, and to ensure the low-temperature toughness of the base metal. It is an essential additive element for high-strength welded structural steel materials of kg or more.

However, the addition of Nb delays ferrite transformation and promotes the degree of supercooling during ferrite transformation, which is necessary for FSP generation that directly controls HAZ toughness.
The upper limit is 02z.

In addition, the elements of group (2) mentioned above are oxides, sulfides, or t
It is added for the purpose of forming llPi compounds, coarsening the crystal grains of HAZ, and reducing the anisotropy of the base material, but JI! In order to secure MnS composite precipitates that can become nuclei, both when one or more of these elements are added as a mixture and when each is added individually, each is 0. The upper limit is lZ.

The above-mentioned components can be added in the same manner as necessary in both the first means and the second means of the present invention to obtain the same effects.

The present invention aims to obtain a HAZ characterized by being composed of crystal grains surrounded by massive pro-eutectoid ferrite that suppresses FSP, which directly controls fracture toughness, in order to achieve both toughness, particularly Kc value and Charpy toughness. That is.

Specifically, the first method is to reduce the degree of supercooling required for FSP generation by limiting Nb and Mn, which retard the intragranular ferrite transformation after grain boundary ferrite formation, and to reduce the amount of FSP from the grain boundaries. The second method is to secure a large amount of TiN, 8N, which precipitates in the temperature range just above the grain boundary pro-eutectoid ferrite transformation, as a production site for pro-eutectoid ferrite, and to suppress the formation of pro-eutectoid ferrite. By metamorphosing from the generation site, the shape becomes lumpy,
This suppresses plate-like ferrite having a linear shape necessary as an FSP generation site, thereby suppressing the generation of FSP.

Mn/C, which is a limiting condition for the first means, prevents appropriate cooling from the transformation temperature after pro-eutectoid ferrite precipitation, considering the influence of Mn and C added to ensure strength on ferrite transformation behavior. An upper limit is set to suppress the generation of FSP.

Further, N and Ti/N, which are limiting conditions specific to the second means, are determined for the following reasons.

N is added to precipitate TiN and BN, which are the nuclei for producing massive pro-eutectoid ferrite that reduces the shape and amount of FSP, and ensures a large amount of TiH and
A lower limit is set to ensure BN that precipitates on N at a temperature just above ferrite transformation and locally promotes ferrite transformation, and an upper and lower limit of Ti/N is set to prevent excessive free N and free Ti.

Regarding B, in the first means it is a selective component, and in the second means it is a limited component, but in both cases, -
BN is precipitated by combining with dissolved TiN or free N, and becomes a metamorphic nucleus of pro-eutectoid ferrite agglomeration that suppresses the generation of FSP, suppressing grain boundary FSP that is harmful to HAZ toughness and suppressing HAZ solid solution N. Although it has the effect of improving IAZ toughness (fixing, etc.), adding a large amount leads to a decrease in toughness due to the precipitation of Fe23(CB)6 and an increase in the hardenability of I(AX) due to free B. The upper limit is .0015χ.

Also, for boat fishing, Ceq is 1 (
Since it reduces the AZ M property, it is set to 0.45X or less, but in the present invention, the above-mentioned required structure is generated to achieve high K.
In order to obtain c, basic means and concrete means are 0.
．． It is limited to 38r or less, and for applied means that additionally uses a selective component, it is limited to 0.44Z or less.

Calculation of this Ceq is based on the following equation.

Ceq, = (J+Si!/24+Mnz/6 +Ni!
/40+Cr! 15 +Hoz/4+VZ/14 The above are the amounts of each component added in the first and second means of the present invention and the reasons for their limitations.

Effects> The effects of the above-mentioned means, which the present inventors believe will achieve the above-mentioned objects of the present invention, will be explained below.

Based on the above knowledge, the present inventors classified products with good Charpy test value only and products with good Charpy test value and fracture toughness value based on usage conditions and component conditions, and based on this, the products shown in Table 1 were prepared. FAB welding (a method of single-sided high heat input welding) was performed using sample steels A, B, and C, and the conditions for welded structural steel materials that would yield the required HAZ Charpy test (iivETe and fracture toughness value Kc were examined. We investigated the results.
As shown in the figure.

As is clear from the figure, high Charpy test values and deep notch test values were obtained for sample steels A (marked with O) and B (Δ), which are typical welded structural steel materials.

In contrast, other sample steel C (
It was confirmed that although the Charpy test value of the product (marked with an X) exceeds 4.0 kgf., the deep notch test value, which indicates the fracture toughness value at each operating temperature required by the user, is not necessarily satisfied.

Therefore, the present inventors investigated in detail the structure of the fracture initiation point in the deep notch test based on the structure photographs of a large number of test materials, and found that samples 11A (○ mark), B (Δ mark), and other similar HA of each welded structural steel material that obtained high Kc with the composition of
Z is formed by a structure consisting of crystal grains surrounded by massive pro-eutectoid ferrite shown as 1 in Fig. 2, but sample steel C
In the HAZ of each welded structural steel material for which a high Kc could not be obtained with the same composition, including ( They discovered that it was a lath-like structure, and that this FSP formed the point where fracture occurred.

It is known that this FSP is generated after the precipitation of pro-eutectoid ferrite, when the degree of cooling below the transformation temperature is large in the ferrite transformation behavior within the grains.

Therefore, the present inventors utilized this established theory to lower Mn, which is an element that delays ferrite transformation, and suppressed the appropriate cooling state by using C, which is an element that has a small effect of delaying ferrite transformation, to ensure strength. By promoting grain boundary ferrite transformation to generate massive pro-eutectoid ferrite and suppressing plate-like pro-eutectoid ferrite, which is the FSP generation site, we have created a structure that can control the shape of FSP to a small and small amount and obtain a high Kc. I tried to form it.

As a result, it was found that, in addition to the necessary components, the nuclear structure was stably formed with Ceq within a limited range and Mn/C of 15 or less.

Based on the above knowledge, the present inventors directly controlled the size of FSP that controls the fracture characteristics of HAZ, made pro-eutectoid ferrite into lumps, and created a structure consisting of crystal grains surrounding the lumped pro-eutectoid ferrite. By establishing a high-strength welded structural steel material, which is the first means of forming the HAZ, it has become possible to achieve the objects of the present invention.

In addition, the present inventors investigated the formation mechanism of a structure consisting of crystal grains surrounded by massive pro-eutectoid ferrite in sample steel B, and found that the temperature at which molten Ti and N begin to transform into ferrite is 800°C. Until then, BN is reprecipitated at the austenite grain boundaries, and furthermore, BN is compositely precipitated on the surface of the precipitates, locally reducing hardenability and promoting ferrite transformation.
It was found that this TiN + BN acts as a transformation nucleus of pro-eutectoid ferrite, and massive pro-eutectoid ferrite is generated at the austenite grain boundaries.

Moreover, this structure contains, in addition to the above-mentioned necessary components, Ceq,
It has been found that Ti/N is in a limited range, Ti/N is in a range of 2.0 to 3.2, and N is stably produced in a range of 0.003χ to o, oosz.

Based on the above knowledge, the present inventors have determined that Ti and N dissolved
FS which directly controls the fracture properties by making the pro-eutectoid ferrite into lumps by utilizing the redecipitation of BH and the composite precipitation of BH.
Establishment of a high-strength welded structural steel material consisting of a second means of forming a structure consisting of crystal grains surrounded by massive pro-eutectoid ferrite in the HAZ by reducing the size of P and making it possible to form a HAZ exhibiting a high Kc value. That's what I did.

The present invention is based on the above findings.

<Example> (1) Test steel composition (shown in Table 2)
2) Manufacturing conditions ■ Casting solidification: Continuous casting method ■ Rolling cooling: Plate rolling cooling method using known controlled rolling and controlled cooling (shown in Table 3) (4) Mechanical properties (shown in Table 3) (5)
HAZ Charpy test value (shown in Table 3)6)
HAZ Kc (0°C) (shown in Table 3.

) As is clear from Table 3, Comparative Example No. 13.14 is St.
exceeds 0.4z, and No. 15, 16, and 17 are both Mn/
C is greater than 15 and is 26.3.16.9.17.2,
No.18.19 has Nb exceeding 0.02Z and is 70.030
z, 0.028Z tear'), No. 21 to 26.28 are M
n/C is >15, and No. 27 has Nb>0.02Z, and N.

29.31 has On/C>15 and TsuN>0.003
Z and N.

No. 30 had Nb>0.02χ, and Nos. 13 to 31 had mechanical properties comparable to those of the examples of the present invention, but their Charpy test values were relatively low, and the fracture toughness value Kc was at the lower limit of the required level of 500 kgf. /Nothing reached 5 (0℃).

On the other hand, Nos. 1 to 12 of the present invention examples not only have the required mechanical properties but also have the Charpy test value of 4 and 0.
kgf・- and the fracture toughness value Kc is 500k
A high value of gf/m weight of 1°5 (0°C) or more was obtained.

<Effects of the Invention> The present invention provides FSP that has a direct negative effect on improving fracture toughness.
is controlled by the first means characterized by limited Mn/C and Ceq, and the second means characterized by limited N, Ti/N, B, and Ceq. , we have limited the components and amounts that promote the prevention effect of their formation and ensure the mechanical properties necessary for high-strength welded structural steel materials, thereby reducing the blocky pro-eutectoid formation that has not been seen in the HAZ of conventional welded structural steel materials. It enables the formation of a structure consisting of crystal grains surrounded by ferrite, which eliminates the conventional uneconomical TiO treatment or RE.
We have established a welded structural steel material that satisfies both the HAZ Charpy test value and Deep Notch test value required by users of welded structural steel materials without using M treatment. The safety and economic effects it brings to the field are significant.

[Brief explanation of drawings]

FIG. 1 shows the relationship between Charpy test values and deep notch test values for the steel of the present invention and comparative steel. Figure 2 schematically shows the positional relationships and shapes of the structures of blocky pro-eutectoid ferrite α, IFP, and plate-like ferrite α, FSP, Bu, etc. (1) is that of the steel material of the present invention, (2) ) indicates conventional steel materials.

Claims (2)

[Claims] (1) The heat affected zone or HAZ after welding is in weight%, C: 0
．． 07-0.16%P:≦0.020%Si:0.15
~0.30%S:≦0.020%Mn:1.20~1.
50%Ti: 0.005-0.020%Al: 0.00
Cu: ≦1.0% Nb: ≦0.02% Ni: ≦1.0% Ca: ≦0.1% V: ≦0.1% B :≦0.0015%, and Mn/C≦15 and C%
+Si%/24+Mn%/6+Ni%/40+V%/1
4 = 0.32 to 0.44%, is composed of other unavoidable components and Fe, and has a structure composed of crystal grains surrounded by massive pro-eutectoid ferrite. Superior high-strength welded structural steel. (2) The heat affected zone or HAZ after welding is in weight%, C: 0
．． 07-0.16%P:≦0.020%Si:0.15
~0.30%S:≦0.020%Mn:1.20~1.
50%Ti: 0.005-0.020%Al: 0.00
5~0.10%B:≦0.0015%N:0.003~
0.008%, and if necessary, one of the following: Cu:≦1.0%Nb:≦0.02%Ni:≦1.0%Ca:≦0.1%V:≦0.1% or two or more types, and Ti/N=2.0-3
．． 2 and C%+Si%/24+Mn%/6+Ni%/40
+V%/14 satisfies 0.32 to 0.44%, is composed of other unavoidable components and Fe, and has a structure composed of crystal grains surrounded by massive pro-eutectoid ferrite. A high-strength welded structural steel material with excellent fracture toughness.