JP2008121068A - Steel material for iron shell of converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material for an iron shell of a converter, which has low reheat-cracking sensitivity and low cold-cracking sensitivity at a welding part, is easily manufactured, and is easily repaired by welding. <P>SOLUTION: This steel material has a composition comprising, by mass%, 0.03-0.15% C, 0.01-0.50% Si, 0.30-1.50% Mn, 0.015% or less P, 0.003% or less S, 0.2-0.6% Mo, further one or more of 0.0003-0.005% Ca, 0.0005-0.005% Mg and 0.002-0.02% REM, 0.04% or less sol. Al, 0.006% or less N, 0.003% or less O, one or more of Cu, Ni, Cr, V, Ti, Nb and B as needed, while satisfying the relation of Pcm≤0.22% and P<SB>sr</SB><0, and the balance Fe with unavoidable impurities. Here, Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B and P<SB>SR</SB>=Cr+3.3Mo+10V+Cu+7Nb+5Ti-2, wherein elements in each expression represent their contents (mass%). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel material for a converter iron core, and particularly relates to a material having low reheat cracking susceptibility and low temperature cracking susceptibility of a welded portion and easy to manufacture and repair welding.

  Since the steel for converter cores is repaired by welding when the cores are melted or deformed, SM400 and other low-crack susceptibility materials have been used in recent years. In order to maintain the above, it has been required to have a long life in a high temperature use state, and those having excellent high temperature creep strength such as SB450M to which Mo is added have been used.

  For example, Patent Document 1 relates to a thick steel plate for a converter iron shell and a method for manufacturing the same, and high strength steel for medium-temperature pressure vessels has superior high-temperature strength than SB42 and SM50, but may cause brittle cracks due to aging. However, a thick steel plate for converter cores is described in which high-temperature strength is secured by the combined addition of Mo and V, low-temperature cracking is prevented at a low Pcm, and further embrittlement due to high-temperature aging is prevented by forming a microstructure. Yes.

  Patent Document 2 relates to a method for producing a carbon steel excellent in high-temperature strength used for an iron skin or the like, by continuously hot rolling a Cr-Mo component steel from a temperature higher than Ar3 to directly above Ar1. It describes that it is excellent in high-temperature strength at 450 ° C. and long-term creep characteristics.

Patent Document 3 relates to a low alloy heat resistant steel having excellent high-temperature strength and good weldability. When adding a Cu-Ni-Mo-Cr-V-based component as a basic component and Al, Nb, Ti as selective components, It is described that the addition amount is adjusted so that the parameter formula relating to nitride formation defined by the element and the amount of N in steel falls within a specific range, and excellent high-temperature strength is ensured at 300 to 450 ° C.
Japanese Patent Laid-Open No. 5-271858 Japanese Patent Laid-Open No. 5-320753 Japanese Patent Laid-Open No. 9-184043

  By the way, since the iron core for converters is manufactured by welding and is subjected to SR treatment after manufacturing, it is necessary that SR cracking does not occur in addition to low susceptibility to low temperature cracks. In addition to the high-temperature creep characteristics, it is required to have the above-described excellent weld characteristics.

  Accordingly, an object of the present invention is to provide a steel material for a converter core that is excellent in the above-described various characteristics.

The inventors of the present invention conducted intensive studies in order to solve the above-described problems, and obtained the following guiding principle in component design.
(1) Mo is added in an amount of 0.2% or more in order to increase the creep strength at a high temperature exceeding 400 ° C. and suppress creep deformation. Further, Nb, V, etc. are added as necessary.
(2) Pcm is set to 0.22 or less in order to prevent welding cold cracking during the manufacture of the iron skin.
(3) From the viewpoint of preventing cracking during the production of the iron skin, P, S, N, and O are limited to be low, PSR is limited to be less than 0, and Ca is added. This prevents reheat cracking in the welded part during manufacture and ductile deterioration cracking of the welded part after long-term use during use.

The present invention has been made by further studying the above guiding principle, that is, the present invention
1. In mass%, C: 0.03 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.30 to 1.50%, P ≦ 0.015%, S ≦ 0.003% , Mo: 0.2 to 0.6%, Ca: 0.0003 to 0.005%, Mg: 0.0005 to 0.005%, REM: 0.002 to 0.02% Containing sol. Al ≦ 0.04%, N ≦ 0.006 %, O ≦ 0.003%, Pcm ≦ 0.22%, P SR <0, the converter steel skins steel material and the balance Fe and unavoidable impurities.
However, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
P _SR = Cr + 3.3Mo + 10V + Cu + 7Nb + 5Ti-2
In each formula, the element is the content (% by mass).
In addition to the component composition described in 2.1, Cu: 0.03-0.5%, Ni: 0.03-0.5%, Cr: 0.03-0.7%, V: 0.005-0 0.05%, Ti: 0.005 to 0.05%, Nb: 0.005 to 0.03%, B: 0.0003 to 0.002%, or one or more types A steel material for a converter core comprising Pcm ≦ 0.22%, P _SR <0, the balance Fe and inevitable impurities.
However, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
P _SR = Cr + 3.3Mo + 10V + Cu + 7Nb + 5Ti-2
In each formula, the element is the content (% by mass).

  According to the present invention: Longer equipment life 2.3. Further, when melting damage or deformation occurs, it is possible to easily repair the weld, which is extremely useful in the industry.

Hereinafter, the reasons for limiting the components of the present invention will be described in detail. Note that% is mass%.
C
C is required to be 0.03% or more in order to ensure strength and toughness as a steel material for converter cores. However, 0.15% is made the upper limit in order to increase the toughness and weld cold cracking property with the increase.

Si
Si is effective for securing the strength, but in order to deteriorate the toughness, it is set to 0.01% or more and 0.50% or less.

Mn
Mn is effective in increasing the strength and toughness, but in order to deteriorate the weldability, it is 0.30% or more and 1.50% or less.

P, S
P and S are impurities in the present invention, and P promotes temper brittleness, impairs toughness, and increases reheat cracking sensitivity, so that it is made 0.015% or less. In order to increase the reheat cracking sensitivity, S is made 0.003% or less.

Mo
Since Mo is effective for improving the high temperature strength and creep strength, 0.2% or more is added in order to obtain the effect. On the other hand, if added over 0.6%, the weldability deteriorates and the economic efficiency is also impaired, so the content is made 0.2% or more and 0.6% or less.

One or more of Ca, REM, and Mg are sulfide (sulfide) -forming elements that fix solute S in steel and improve reheat cracking sensitivity. Further, 0.0003% or more is added in order to control the distribution of oxide (oxycide) inclusions and improve the reheat cracking sensitivity. On the other hand, if added over 0.005%, coarse inclusions increase, which deteriorates toughness and easily causes reheat cracking, so the content is made 0.005% or less.

REM
REM is a rare earth element (REM) with atomic numbers from 57 to 71 and is a strong sulfide (sulfide) -forming element like Ca and Mg. ) In order to control the distribution of system inclusions and reduce reheat cracking sensitivity, when added, the content is made 0.002% or more. On the other hand, if over 0.02% is added, coarse inclusions increase, which deteriorates toughness and easily causes reheat cracking, so the content is made 0.02% or less.

Mg
Mg is a strong sulfide (sulfide) -forming element, and is added to reduce the reheat cracking susceptibility by controlling the solid solution S fixation in steel and the distribution of oxide (oxycide) inclusions. Is 0.0005% or more. On the other hand, if added over 0.005%, coarse inclusions increase, which deteriorates toughness and easily causes reheat cracking, so the content is made 0.005% or less.

sol. Al
sol. Al is added as a deoxidizing material, and is also effective for fixing N during normalization and improves toughness, but reduces the reheat cracking susceptibility and high temperature ductility of the welded portion. % Or less.

N
If N is excessively contained, the toughness deteriorates, so 0.006% or less.

O
O reduces the toughness and weldability due to the formation of oxide (oxycide) inclusions, and increases the reheat cracking sensitivity. Moreover, it is 0.003% or less so as not to impair the effect of Ca addition.

Pcm
Pcm (= C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B, element content (mass%)) is set to 0.22% or less in order to prevent cold cracking of the weld.

P _SR
P _SR (= Cr + 3.3Mo + 10V + Cu + 7Nb + 5Ti-2, element content (% by mass)) is set to less than 0 in order to prevent SR cracking of the weld.

  The above is the basic component composition of the present invention, but when further improving the characteristics, one or more of Cu, Ni, Cr, and V are added.

Cu
Cu increases the hardenability and increases the strength as a solid solution strengthening element. When obtaining its effect, 0.03% or more is added. However, when excessively added, the reheat cracking sensitivity is increased, and creep ductility and In order to reduce the hot workability, when added, the content is made 0.03% or more and 0.5% or less.

Ni
Ni is effective in improving hardenability and improving toughness, so when obtaining its effect, 0.03% or more is added. However, if excessively added, reheat cracking susceptibility is increased and economic efficiency is also reduced. When added, the content is 0.03% or more and 0.5% or less.

Cr
Since Cr is effective for increasing the normal temperature strength and the high temperature strength, when obtaining its effect, 0.03% or more is added. However, when excessively added, the weldability is deteriorated. % To 0.7%.

V
V is a carbide-forming element, and improves normal temperature strength, high temperature strength, and creep rupture strength. Therefore, when obtaining its effect, 0.005% or more is added, but if added excessively, reheat cracking susceptibility is increased. When it is, it is made 0.005% or more and 0.05% or less.

Nb
Nb is a stable carbonitride-forming element, which can increase strength, high-temperature strength, and creep rupture strength, but adding excessively impairs toughness and weldability and increases reheat cracking susceptibility. When it does, it is made into 0.005 to 0.03% or less.

B
B enhances hardenability and improves strength, but if added in excess, its effect is saturated and reheat cracking sensitivity is remarkably increased. Therefore, when B is added, the content is made 0.0003 to 0.002%.

Ti
Ti is a strong nitride-forming element and improves toughness by refining. Further, when B is added, it has an effect of fixing N and enhancing the hardenability improvement effect of B. Therefore, when B is added, the content is made 0.005% to 0.05%.

  In addition, in order to increase the reheat cracking susceptibility and creep embrittlement susceptibility, impurity elements such as Sn, As, and Sb are each 0.01% or less, preferably 0.005% or less.

  The production conditions for the steel of the present invention are not particularly specified. The slab heating temperature, hot rolling conditions, and heat treatment conditions are appropriately determined according to the desired size and shape and characteristics, and can be produced by a conventional method.

  Steels having various chemical components shown in Table 1 were subjected to accelerated cooling after hot rolling to produce steel materials having a thickness of 60 mm. About the obtained steel materials, a normal temperature tensile test (based on JIS Z 2201 No. 4 test piece and conforming to JIS Z 2241), Charpy impact test (using JIS Z 2202 2 mmV notch test piece and conforming to JIS Z 2242. Test temperature -10) C, an average value of three test methods) and a creep rupture test (based on JIS Z 2271) were performed.

  In addition, in order to investigate the SR cracking characteristics of the welded part, SR cracking test was conducted using Y cracking test piece of JIS Z 3158, SR (600 ° C × 4hr) was conducted after welding the oblique Y split specimen, and the cross-sectional cracking rate Was measured. The test conditions for the creep rupture test are as follows. M.M. P. = Corresponds to test conditions of 20,000 ° C. and 525 ° C. * 100,000 hours.

  Moreover, in order to investigate low temperature crack sensitivity, the diagonal Y crack test (Test conditions 30 degreeC-80%, JISZ3158 conformity) was implemented using the ultra-low hydrogen type welding material.

  The material strength is a yield point of 215 MPa or more, a tensile strength of 400 to 510 MPa, an elongation of 24% or more is within the scope of the present invention, and the Charpy impact value is within the scope of the present invention with an average value of three being 100 J or more. The low-temperature cracking rate (cross-sectional cracking rate) was set to 0%, which is the result of no cracking, within the scope of the present invention. In the creep rupture test, a rupture strength of 60 MPa or more was set within the range of the present invention.

  Steel No. shown in Table 1 1-31, Steel No. 1, 2, 3, 5, 6, 9, 10, 13, 14, 17, 18, 21 to 27 are steels of the present invention, Steel No. 4, 7, 8, 11, 12, 15, 16, 19, 20, 28, 29 are comparative steels whose component compositions are outside the scope of the present invention. 30 is SM400C, steel no. 31 is a conventional example of SB480M. Table 1 also shows the low temperature cracking sensitivity index (Pcm) and the SR cracking sensitivity index (PSR).

  Table 2 shows the results of a normal temperature tensile test, a Charpy impact test, a creep rupture test, an SR crack test, and an oblique Y crack test. The steel of the present invention has normal temperature strength, creep rupture strength, and Charpy impact value equal to or higher than those of the comparative steel, and no SR cracking and low temperature cracking were observed in the weld. Moreover, it was confirmed that the characteristics are superior to those of conventional steel.

Claims (2)

In mass%, C: 0.03 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.30 to 1.50%, P ≦ 0.015%, S ≦ 0.003% , Mo: 0.2 to 0.6%, Ca: 0.0003 to 0.005%, Mg: 0.0005 to 0.005%, REM: 0.002 to 0.02% Containing sol. Al ≦ 0.04%, N ≦ 0.006 %, O ≦ 0.003%, Pcm ≦ 0.22%, P SR <0, the converter steel skins steel material and the balance Fe and unavoidable impurities.
However, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
P _SR = Cr + 3.3Mo + 10V + Cu + 7Nb + 5Ti-2
In each formula, the element is the content (% by mass). In addition to the component composition of claim 1, Cu: 0.03-0.5%, Ni: 0.03-0.5%, Cr: 0.03-0.7%, V: 0.005-0 0.05%, Ti: 0.005 to 0.05%, Nb: 0.005 to 0.03%, B: 0.0003 to 0.002%, or one or more types A steel material for a converter core comprising Pcm ≦ 0.22%, P _SR <0, the balance Fe and inevitable impurities.
However, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
P _SR = Cr + 3.3Mo + 10V + Cu + 7Nb + 5Ti-2
In each formula, the element is the content (% by mass).