JPH0445227A - Production of steel product with low yield ratio - Google Patents

Abstract

PURPOSE:To inexpensively produce a steel product excellent in bendability and having high strength and low yield ratio by subjecting a steel having a specific composition consisting of C, Si, Mn, Al, and Fe to heating and to specific hot rolling and then subjecting the resulting hot rolled plate to cooling treatment including specific light draft and to tempering. CONSTITUTION:A steel which has a composition consisting of, by weight, 0.03-0.30% C, 0.05-0.60% Si, 0.50-2.5% Mn, 0.005-0.1% Al, and the balance Fe with inevitable impurities and further containing, if necessary, prescribed amounts of strength improving elements, such as Cu, Ni, Cr, Mo, Nb, V, and Ti, or Ca is heated to 900-1,200 deg.C and hot-rolled. At this time, rolling is finished at >900 deg.C, or, in the case where rolling is finished at a temp. between 900 deg.C and Ar3, cumulative reduction of area is regulated to <30% based on the finishing plate thickness at the temp. between 900 deg.C and Ar3. Subsequently, the hot rolled plate is air-cooled and subjected to light draft of 1-30% at a surface temp. between (Ar3-20 deg.C) and (Ar3-80 deg.C). Then, the cooling for the steel plate is started without delay in >=0.3m<3>/m<2>min water volume density and performed until 250 deg.C steel plate temp. is reached. The steel plate is then subjected to tempering heat treatment.

Description

[Detailed description of the invention] (Industrial use branch) The present invention relates to a method of manufacturing low yield ratio steel.

(Conventional technology) In recent years, in order to improve competitiveness in various fields such as shipbuilding and industrial machinery, various efforts have been made to reduce welding work, pursue the ultimate properties of steel materials such as bending workability, improve weldability, and reduce steel material costs. Demand is increasing. Among these, in order to improve the bending workability of thick steel plates, it is necessary to develop thick steel plates with a low yield ratio of 80% or less.

Furthermore, in the fields of architecture and bridges, a reduction in yield ratio is desired to improve the safety of structures. In addition to simply lowering the yield ratio, introducing a yield shelf significantly increases the amount of energy absorbed by the structural member during an earthquake.

For this reason, the introduction of a yield shelf is also required.

Recently, there has been active development of steel plates with a strength of 50 kg f/- or more using accelerated cooling technology after steel plate rolling, with the aim of improving base material low-temperature toughness and weldability, mainly for shipbuilding, line pipes, etc. The production of low yield ratio steel materials with good workability has not been studied.

In the conventional controlled rolling-controlled cooling process, in order to improve low-temperature toughness, hot rolling is performed to make the grains as fine as possible, and accelerated cooling is performed from the austenite phase region.

However, according to this method, there is a problem that the yield point increases due to grain refinement and hardening of the ferrite and the conversion of some barite to bainite, resulting in an increase in the yield ratio and a decrease in bending workability.

In JP-A No. 59-211528, some of the present inventors studied a method of lowering the yield point using a controlled rolling-controlled cooling process, and found that while obtaining good low-temperature toughness with fine-grained ferrite, We have developed a method for manufacturing a steel plate with a strength of 50 kg f/m4 or more, which also has a low yield point and low yield ratio.

That is, after heating to 900 to 1200°C, Ar
After performing a cumulative reduction of 30% or more at a temperature above a to make the grains finer, air cooling to a temperature below A r s to appropriately precipitate soft pro-eutectoid ferrite, followed by forced cooling, produces soft pro-eutectoid ferrite and the remainder. It has been discovered that by appropriately mixing ferrite-pearlite-bainite obtained from austenite, only the yield point can be lowered without lowering tensile strength and low-temperature toughness.

(Problem to be solved by the invention) However, in response to the subsequent demand for an even lower yield ratio,
As a result of various studies, the cumulative pressure of 30% or more in the temperature range from 900° C. to Ar3 causes ferrite and second phase carbide to become finer and finer than necessary. In order to have a low yield ratio, it is necessary to have a low yield point and high tensile strength. The yield point is the ferrite part, and the tensile strength is the second
Determined by phase carbides (especially high carbon island martensite).

Therefore, if the ferrite becomes finer than necessary, the yield point will be high, and at the same time, the second phase carbide will become finer, and the decomposition of the carbide during tempering will be promoted, resulting in a low tensile strength.

The result is a high yield ratio. Furthermore, by introducing a yield shelf, rather than simply lowering the yield ratio,
The amount of energy absorbed by structural members during earthquakes increases significantly. For this reason, there is a need to introduce a yield shelf, but there is no knowledge of how to achieve this.

(Means for solving the problem) For this reason, in order to continue to lower the yield ratio, we conducted numerous experiments and detailed studies, and found that in order to lower the yield ratio, the microstructure of steel should be changed to ferrite and secondary. Create a two-phase mixed structure of carbide phases. In order to further reduce the yield ratio,
It is important to lower the yield point and increase the tensile strength.

In order to lower the yield point, the area ratio of ferrite should be increased and the grains should not be made too fine, and in order to increase the tensile strength, the second phase carbide (bainite or martensite) that has become hard due to rapid cooling should be tempered. They discovered that it is important not to soften the material more than necessary. Furthermore, regarding the yield shelf, it was found that by applying a light reduction between the two-phase region temperature and water cooling, active dislocations are introduced into the steel, which results in the presence of a yield shelf during the tensile test.

In order to meet such demands, the present invention has made it possible to manufacture a steel plate with a strength of 50 kg f /- or more and a low yield ratio, and the gist thereof is:
C: 0.03-0.30%, Si: 0.05-0.
60%, Mn: O', 50-2.5%, An:
Based on steel containing 0.005 to 0.1%, in addition to this, Cu: 2.0% or less, Nj: 4.0% or less, C
r: 1.0% or less, Mo: 0.50% or less, Nb: 0.
1.0% or less, V: 0.10% or less, T1: 0.1
Contains one or more selected from the strength improving element group consisting of 5% or less, or Ca: 0.01% or less, or one or two selected from the Cu-Tl strength improving element group and Ca. A steel containing Fe and unavoidable impurities is heated to 900 to 1200°C, and hot rolling is completed at a temperature exceeding 900°C, or
Or 900 if rolling is completed between 900°C and Ar3
The cumulative reduction rate is less than 30% relative to the finished plate thickness between °C and A r s, and the steel plate surface temperature is increased by air cooling.
s Apply light pressure of 1% to 30% in the temperature range of 20℃ to Ar380''C, and immediately reduce the water density to 0.3rl.
The present invention relates to a method for manufacturing a low yield ratio steel material, which is characterized in that cooling is started at a temperature of /d·min or more, the steel sheet is cooled until the steel plate temperature becomes 250° C. or less, and then a tempering heat treatment is performed.

(Function) The present invention mainly performs rolling at a temperature exceeding 900°C, and when rolling is performed at a temperature of 900°C or lower, by regulating the reduction amount to a low value, it is possible to reduce the grain size of ferrite more than necessary and The aim is to suppress the refinement of two-phase carbides and, at the same time, to produce a low yield ratio steel material with a yield shelf by applying light reduction before water cooling.

Next, the reason for limiting the ingredients in the present invention will be described.

C is required at 0.03% or more to ensure strength, but
If the content is too large, it will impair the toughness and weldability of the steel, so the content should be limited to 0.30%.

St is added as necessary in an amount of 0.05% or more for deoxidation, but if it increases, weldability will be impaired, so the content should be 0.6% or less.

Mn is useful as an element that increases strength at a low cost, and 0.5% or more is necessary to ensure strength, but if it increases, weldability will be impaired, so the content should be 2.5% or less.

0.005% or more of All is necessary for deoxidation, but if it is too large, inclusions in the steel will increase too much and the properties of the steel will deteriorate, so the upper limit is set at 0.1%.

In the present invention, steel containing the above-mentioned elements as basic components is heated, rolled, and heat treated as specified in the present invention to ensure a low yield ratio. However, depending on the required characteristics of the steel, the following strength-improving element groups, One or more types of inclusion spheroidizing elements can be added.

V, Tj, and Nb are elements that increase strength through precipitation strengthening, but in large amounts they impair toughness, so V:
The upper limits were O: 1%, Tj: 0.15%, and Nb: 0.10%.

Ni, Cu, Cr, and Mo are elements that increase strength through solid solution strengthening, but when their amounts increase, they impair toughness.
The upper limit was 0% and Mo: 0.50%.

Ca is added because it is useful for spheroidizing inclusions, but if too much Ca forms inclusions in the steel and deteriorates the properties of the steel, the upper limit of the content is 0.01%.

Next, heating, rolling, and cooling conditions, which are important requirements of the present invention, will be described.

The lower limit of the heating temperature was set at 900° C., which is the temperature at which sufficient heating can be achieved in the austenite region. On the other hand, if the temperature is too high, the austenite grains will become too large and the properties of the steel will deteriorate, so the upper limit of the heating temperature is set at 1200°C.

Rolling can be divided into rolling at temperatures above 900°C and rolling at temperatures below 900°C, but in applications where low yield ratio steel materials are used, improved toughness through controlled rolling at temperatures above 900°C is sufficient. It is desirable to complete rolling at a temperature exceeding 900°C.

Rather, if the cumulative reduction is increased to 30% or more by controlled rolling at 900° C. or lower, a high yield ratio will be obtained due to the finer grains of ferrite and the finer grains of the second phase carbide.

Therefore, the cumulative rolling reduction rate between 900° C. and A r 3 is set to 30% or less with respect to the finished plate thickness.

Next, air cooling is performed prior to water cooling, and the air cooling is preferably performed immediately after rolling to a temperature between Ar -20°C and Ar3-80°C, so that an appropriate amount of soft pro-eutectoid ferrite can be precipitated. This is what we do.

The reason why the upper limit of the accelerated cooling start temperature is set to A r a −20°C is to lower the yield point, and the lower limit is set to A r 3
The reason for setting the temperature to 80° C. is that when cooling from a temperature lower than this temperature, the effect of accelerated cooling becomes weak and the tensile strength decreases, making it difficult to maintain strength. The reason why the water density is set to 3 nl/d·min or more is because the increase in strength is small below this value.

Prior to this cooling, a light pressure reduction of 1% to 30% is applied. Accelerated cooling start temperature A r 320°C and Ara
By applying a light reduction in the two-phase region at 80° C., active dislocations are introduced into the steel, which can lead to the presence of a yield shelf during tensile testing. If the rolling reduction in the two-phase region is less than 1%, a yield shelf does not exist, so the lower limit is set to 1%, and 30%
If it exceeds the range, the yield ratio increases, which is contrary to the purpose of the present invention, so the upper limit was set at 30%.

The reason why the cooling stop temperature of accelerated cooling is set to 250° C. or lower is that if cooling is stopped in a high temperature range exceeding 250° C. and then tempering heat treatment is performed, the strength will decrease slightly and the low-temperature toughness will deteriorate at the same time. Here, set the water cooling stop temperature to 350
The reason why we adopted a method of water cooling to 250°C or lower and then performing a tempering heat treatment instead of using a method of heating the steel to 600°C is because the latter improves the low-temperature toughness more than the former.

Incidentally, the tempering temperature is set to be below A c +.

(Example) Table 1 shows the chemical composition of the test materials, and Table 2 shows the heating, rolling, and cooling conditions and the mechanical properties of the obtained steel sheets.

Steel A, G, H, I, J, K, L, M, N
, 0゜P is 50) cg f /- grade, steel B, C,
D, E, F, Q.

R, S, T, and U are component systems aiming at strength of 60) cg f /- class, and as shown in Table 2, steel plate &A1. A2゜B
l, CI, DI, El, Fl, Gl, Hl.

11, Jl, Kl, Ll, Ml, Nl, 01°PI, Q
1, R1, Sl, TI, and Ul are examples of the present invention, and each has sufficient strength and good low-temperature toughness as 50.60 kgf/- handling copper, and has a low yield of 70% or less, which is the aim of the present invention. achieve the ratio and have a yielding shelf.

On the other hand, steel plates NO and A3 have low-temperature toughness because the heating temperature is too high. For A4, the cumulative pressure between 900 and A r a is too high, so the ferrite grains become too fine.
It has a high yield ratio due to its high yield point.

A5 does not have a yield shelf because there is no light reduction before the start of cooling.

Since A6 has a high light reduction ratio of 40% before the start of cooling, it has a high yield ratio. B2 is an example in which the forced cooling start temperature was too high, and the yield ratio was high. B3 is an example in which the forced cooling end temperature was too low, and the strength was too high and the low-temperature toughness was low. C2
is an example where the forced cooling start temperature was too low, resulting in low strength and high yield ratio. C3 is an example of a low water density, and therefore has a low strength and a high yield ratio.

(Effects of the Invention) The present invention achieves 5゜kg f /
This invention has great industrial effects, as it enables low-yield-ratio thick steel plates with high strength of m4 or more and a yield shelf with good bending workability to be manufactured at low cost by a controlled rolling-controlled cooling method.

Claims (1)

[Claims] 1. In weight%, C: 0.03 to 0.30%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.5%, Al: 0. 005~0.1% Steel consisting of the balance Fe and unavoidable impurities is heated to 900~0.1%.
Heating to 1200℃ and finishing rolling at a temperature exceeding 900℃ in hot rolling, or 900℃~Ar_3
When rolling is completed between 900℃ and Ar_3, the cumulative reduction rate is less than 30% of the finished plate thickness, and the steel plate surface temperature is continuously cooled in air until the steel plate surface temperature is between Ar_3-20℃ and Ar_3-80℃.
Apply a light pressure of 1% to 30% in the temperature range of A method for manufacturing low yield ratio steel materials, which is characterized by performing tempering heat treatment. 2. In weight%, Cu: 2.0% or less, Ni: 4.0% or less, Cr: 1.0% or less, Mo: 0.50% or less, Nb: 0.10% or less, V: 0 2. The method for producing a low yield ratio steel material according to claim 1, which contains one or more of the strength improving element group consisting of Ti: 0.15% or less and Ti: 0.15% or less. 3. In weight%: C: 0.03-0.30%, Si: 0.05-0.60%, Mn: 0.50-2.5%, Al: 0.005-0.1% The method for producing a low yield ratio steel material according to claim 1, further comprising Ca: 0.01% or less. 4. In weight%, Cu: 2.0% or less, Ni: 4.0% or less, Cr: 1.0% or less, Mo: 0.50% or less, Nb: 0.10% or less, V: 0 2. The method for producing a low yield ratio steel material according to claim 1, which contains one or more of the following: .10% or less, Ti: 0.15% or less, and Ca: 0.01% or less.