JPS6029452A - High-tension steel having excellent characteristic to stop propagation of brittle fracture - Google Patents

Abstract

PURPOSE:To provide a high-tension steel which has a ferrite structure contg. respectively prescribed ratios of C, Si, Mn and Al as essential components and having high ductility and a dispersion structure having the characteristic to generate brittle fracture inferior to said structure and having the specified max. diameter and the nearest distance between each other and is extremely superior in the titled characteristic to the conventional steel. CONSTITUTION:A titled high-tension steel contains, by weight, 0.01-0.30% C, 0.05-1.0% Si, 0.5-2.5% Mn, 0.01-0.1% Al, or >=1 kind among 0.005-0.2% V, 0.005-0.1% Nb, 0.01-1.0% Cr, 0.1-1.0% Mo, 0.01-0.5% Cu, 0.005-0.05% Ti and 0.1-3.5% Ni in addition. The high-tension steel is formed of a ferrite structure having high ductility and the structure of the 2nd phase which is inferior in the characteristic to generate brittle facture (arresting characteristic) to said structure and is dispersed to the state of 0.01-1mm. max. diameter and 0.05-5mm. the nearest distance between each.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to high tensile strength steel with excellent brittle fracture propagation arresting properties (hereinafter referred to as arresting properties).

(Prior Art) In recent years, the safety requirements for large steel structures have become increasingly strict, and depending on the location where they are used, it is essential to impart high arrestability to steel materials.

This tendency is particularly strong in large tanks used as low-temperature containers and line pipes installed in cold regions.

According to conventional knowledge, the metallurgical guidelines for improving arrest properties include grain refinement and Ni as an alloying element.
It is known that cracks parallel to the plate surface, called rations, can be introduced by adding or controlled rolling. However, although it is clear that grain refinement is an effective method for improving the mechanical properties of steel, especially its toughness, there is a limit to the degree of grain refinement in normal industrial-scale hot rolling heat treatment. There is. Also, N1 as an alloying element
Adding a large amount of Ni not only significantly increases the manufacturing cost of the steel material, but also increases hardenability and has the drawback of increasing the maximum hardness of the weld heat-affected zone.

Depending on the type of steel structure, it may not be preferable from a safety perspective to develop a structure with weak mechanical properties in the thickness direction through controlled rolling with excessive stress, thereby facilitating the occurrence of separation. do not have.

In view of the above-mentioned current situation, the present inventors have studied in detail the metallurgical and mechanical factors that govern the arrest characteristics of steel materials, and have arrived at the present invention.

(Structure and operation of the invention) That is, the average compositional components of the present invention are c: o, o 1 to 0.30% by weight, St: 0.05
~1.0%Mn: 0.5~2.5%, At:
0.01 to 0.1% or to the component V: 0.005 to 0.2% Nb: 0.005 to 0.1
%Cr: 0.01-1. Ofa Mo: 0.1~
1.0%Cu: 0.01~0.5%Ti:
0.005~0.05%Ni:O, i~3.5% Contains one or more types of gold, the remainder being Fe and unavoidable impurities, and has a ductile ferrite structure and brittle fracture from the ferrite structure. The generation characteristics are poor, the maximum diameter is 0.01W+ or more and 1W or less, and the closest distance to each other is 0.05
(2) The gist is a high-strength steel with excellent arresting properties consisting of a structure or a second phase that is dispersed in a state of 5 or more.

The features of the present invention will be explained in detail below.

First, the reasons for limiting each component in the present invention will be explained.

C is an essential element for strengthening steel materials, and 0.01
If it is less than %, it is difficult to secure the full required strength. On the other hand, if it is added in an amount exceeding 0.30%, the toughness, especially the toughness of the welded part, will drop significantly, making it unsuitable for use as a steel material for welded structures.

81 is an effective element because it promotes deoxidation in steel manufacturing and increases the strength of steel materials, but since excessive addition deteriorates weldability, it was decided to limit it to 0.05% or more and 1.01 or less.

Mn is an element effective in improving low-temperature toughness and hardenability, so it is added in an amount of 0.5% or more; however, addition in an amount exceeding 2.5% may increase weld cracking properties; 2. 5
-Limited to:

As is well known, At is a strong deoxidizing agent and is also effective in refining crystal grains, so it is added in an amount of 0.01% or more, but excessive amounts of At can cause inclusions that are harmful to the material. , the upper limit was set at 0.1%.

■As a precipitation hardening element, it is effective in securing the strength of the steel material, and if it is in solid solution, it is thought to be effective in improving the hardenability, so it is added at least 0.005T, but if it is in excess, it will lead to a complete decrease in toughness. Unfortunately, the upper limit was set at 0.2 inches.

Tf, Nb? Although it is an element effective in refining crystal grains, if too much is added, weldability deteriorates, so it is limited to 0.005 to 0.05 and 0.005 to 0.1, respectively.

Cu, Cr, and Mo are all effective as reinforcing elements, but when added in excess, Cu causes hot brittleness and Cr.

Regarding Mo, since it promotes hardening of the welded part, the amount of each component added is 0.01 to 0.5 and 0.01 to 1.0.
H. It was limited to a range of 0.1 to 1.0%.

Since Ni is effective as a toughness improving element, if added, it should be added in an amount of 0.1% or more, but it was limited to 3.5% or less because it is expensive and causes hardening of the welded part.

Next, the reasons for limiting the requirements constituting the basic technology of the present invention will be described.

The present invention is originally based on the results of a detailed study of the metallurgical factors that govern the arrest characteristics of brittle fracture.
The most characteristic feature of this phenomenon is that, prior to brittle cracks propagating at high speed in steel, micro-brittle cracks occur at a certain distance from the same plane in a direction perpendicular to the crack propagation plane, and are diagonal to the main crack. The point is that when the joints are connected, the fracture mechanism is ductile fracture, and a large amount of energy is required. The energy required for this ductile fracture is mainly supplied from the kinetic energy of the propagating brittle crack, and from a different perspective, this ductile fracture has the function of improving the arrest characteristics of the steel material.

It has been experimentally demonstrated that to maximize this function, a highly ductile ferrite structure and an appropriately sized and dispersed precursor microcrack formation region are required.

In accordance with the spirit of the present invention, the dispersed structure or second phase is inferior to the ferrite structure in brittle fracture occurrence characteristics, has a maximum diameter of 0.01 mm or more and 1 mm or less, and has a mutual closest distance of 0.00 mm. We have set a condition for the condition to exist in a state of 05 or more and 5ffiI11 or less.

That is, the maximum diameter of the dispersed structure or second phase is 0.0
If the length is less than 1+ m, it is difficult to expect the effect of diagonal connection of propagating cracks as intended by the present invention, and if the length exceeds 1 m,
There is a risk that the preceding crack will propagate on the same surface, and as a result, there is a risk that the arrest characteristics will deteriorate. Regarding the dispersion state, if the nearest distance is less than 0.05+u+, even if brittle cracks connect obliquely and cause ductile fracture between them,
The effect of kinetic energy absorption of a propagating brittle crack is small, and conversely, if the closest distance is too large, the influence of the stress field of the propagating brittle crack will be too small and no leading microcracks will occur. The upper limit was set to 5th tone.

Here, we would like to explain the difference between the conventionally known methods of improving arrest characteristics, that is, methods of generating separation (or delamination) and introducing excessive second phases (mainly elongated inclusions), and the technical concept of the present invention. Explain briefly.

The basic idea of the above-mentioned conventional technology is to develop a brittle structure parallel to the surface of the steel material by rolling and forging the steel material. This is intended to reduce the stress intensity factor at the tip of a brittle crack. On the other hand, the basic idea of the present invention is to generate a preceding micro-crack at an appropriate position for a propagating brittle crack, connect both cracks with ductile fracture, and absorb the kinetic energy of the propagating crack. However, it is fundamentally different from the conventional technology because it is intended to finally stop the operation. In other words, the above-mentioned conventional technology is a crack arrest technique by relaxing stress conditions at the tip of a propagating crack, and the present invention is a crack arrest technique by improving the toughness value of steel material against propagating cracks.

(Example) Next, the present invention will be explained by examples.

0:0.09%, Si:0.26%, Mn:1.
45%, I't: 0.014chi, AA: 0.037
% was made into a slab by continuous casting, and then rolled to a thickness of 18 mm by thick plate rolling to make a steel plate. These steel plates were heated at 900° C. for 10 minutes and then cooled in air. As is well known, in continuously cast slabs, the degree of central segregation and the state of dispersion change depending on the casting conditions.

Figure 1 shows the microstructures of test steels with different degrees of segregation. Steel A exhibits more pronounced segregation than steel B.

Although both steels A and B are made of the same casting charge material, the stratides used during continuous casting are different, and the slab of steel B was cast with measures taken to particularly suppress the occurrence of center segregation. Temperature gradient type ES was used to evaluate the arrest characteristics of these steel sheets.
SO test and simple DWTT (Drop Weight)
t Tear 'I'est). The latter test piece has a thickness of 14 mm, a length of 180 mm, and a width of 45 vans.
The notch portion at depth 5 is locally embrittled to facilitate the occurrence of brittle fracture. The temperature at which the brittle fracture ratio was 50% was used as the index of the arrest property.

Table 1 shows the brittle fracture properties of Steel A and Steel B.

Table 1 Brittle fracture properties 1) Tδ. = o, 1: In the COD test conducted in accordance with British Standard B55762, the COD value is 0.
1 m f temperature 2) TcDT: 50% brittle fracture transition temperature in simple DWTT 3) Tkca = 300 = Temperature gradient type ES Toughness value Kca measured in So test is 300 kgm 22 Temperature steel compared to steel B It is clear that A is inferior in generation characteristics and superior in arrest characteristics. Moreover, FIG. 2 shows the hardness of the ferrite part of Steel A and the hardness of the hardened structure. The hardness of the ferrite part is 1 on Guickers hardness.
63, whereas in the hardened tissue part it is extremely high at 372.

FIG. 3 shows the relationship between the area ratio of ductile fractures existing isolated within the brittle fracture surface and the test temperature, which was observed and quantified using a simple TM'rT specimen fracture specimen fracture electron microscope. It is clear that steel A, which has relatively better arrest characteristics than @B, rapidly increases as the test temperature rises.

Tables 2 and 3 show examples using continuously cast slag produced in situ. In all of the examples, the steel sheets that were hot-rolled after being heat-treated to completely dissipate center segregation had excellent brittle fracture occurrence properties, but relatively poor arrest properties. be understood.

In addition, in this example, the brittle fracture characteristics of steel materials with and without center segregation are explained, but the dispersed structure or second phase does not need to be limited to only the hardened structure based on center segregation, and in accordance with the spirit of the present invention. It can be selected at will. Further, the steel of the present invention only needs to satisfy the basic requirements of the invention, and the manufacturing method and shape are not particularly specified.

(Effects of the Invention) As explained above, it is clear that the arresting properties of the present invention are extremely superior to conventional steels.

[Brief explanation of the drawing]

Fig. 1 is a micrograph showing the structure of the present invention, Fig. 2 is a micrograph showing the hardness of the ferrite part and hardness of the hardened structure of the invention steel A, and Fig. 3 is the area ratio of ductile fracture in the invention steel. It is a graph showing the relationship between test temperatures.

Claims (1)

[Claims] Average compositional components are C: 0.01 to o, ao* st: o, os to 1.0 by weight.
%Mn: 0.5~2.5% At: 0.01~
0.1' Ir or the component V: 0.005-0.2% Nb: 0.005-0.1
%Cr: 0.01-1.0% MO: 0.1-1
．． 0% Cu: 0.01-0.5% Ti: 0.
A highly ductile ferrite structure containing one or more of 005 to 0.0596Nt: o, 1 to 3.5Nt, the remainder being Fe and unavoidable impurities, and the ferrite structure, which causes brittle fracture. The characteristics are poor, and the maximum diameter is 0.01 m or more and 1 cm or less, and the closest distance to each other is 0.
．． A high-strength steel with excellent brittle fracture propagation arresting properties, consisting of a structure or a second phase that is dispersed in a state of 05 or more and 5 or less.