JPH0445226A - Production of steel product with low yield ratio - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] (Industrial application field) 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, it is desired to lower the yield ratio in order 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 reducing the grain size by applying a cumulative reduction of 30% or more at a temperature of 30% or more at a temperature of 30% or more, air-cooling to a temperature of 30% or more is performed to properly precipitate soft pro-eutectoid ferrite, followed by forced cooling. 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 partially raise the yield point, it is necessary to increase the area ratio of ferrite and not make the grains too fine, and to increase the tensile strength, it is necessary to increase the second phase carbide (bainite or martensite) that is hardened by rapid cooling. ) was found to be important not to soften 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.

Based on this knowledge, the present invention has made it possible to manufacture a steel plate with a strength of 50 kg f / tsi4 or more and a low yield ratio, and the gist thereof is that C: 0.03 in weight%. ~0.30%, Si: 0.0
5-0.60%, Mn: 0.50-2.5%, AN
: Based on steel containing 0.005 to 0.1%,
In addition, Cu: 2.0% or less, Ni: 4.0% or less,
Cr: 1.0% or less, MO20, 5096 or less, Nb:
0.10% or less, V2 0.10% or less, Ti: 0
．． One or more selected from the strength improving element group consisting of 15% or less, or Ca: 0.01% or less, or one or two selected from the Cu-Ti strength improving element group and Ca. When hot rolling is completed at a temperature exceeding 900°C, or at a temperature between 900°C and Ar, the steel is heated to 900 to 1200°C and the remainder is Fe and unavoidable impurities. 9
Rolling is completed at a cumulative reduction rate of less than 30% relative to the finished plate thickness between 00°C and A r a , and then air cooling is performed to bring the steel plate surface temperature to 1 in the temperature range of Ar -20°C to Ar3-80°C.
Apply light pressure of % to 30%, and immediately reduce the water density to 0.
Cooling starts at 3 units/units, and the steel plate temperature reaches 350~6
This invention relates to a method for producing a low yield ratio steel material characterized by stopping cooling at 00°C. In this case, by regulating the amount of reduction to a low value, it is possible to suppress the unnecessarily fine graining of ferrite and the refinement of carbides in the second phase, and at the same time, by applying a light reduction before water cooling, a yield shelf can be created. The aim is to manufacture low yield ratio steel materials.

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 high, it will impair the toughness and weldability of the steel, so the upper limit of the content is 0.30%.

Sl 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.

A and & are required to be 0.005% or more for deoxidation, but if they are 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, Ti, and Nb are elements that increase strength through precipitation strengthening, but in large amounts they impair toughness.
: 0.1%, Ti: 0.15%, and Nb: 0.10%.

Ni, Cu, Cr, and Mo are elements that increase strength through solid solution strengthening, but in large amounts they impair toughness, so Ni: 4.0%, Cu: 2.0%, Cr: 1.0
%, Mo: 0.50% was the upper limit.

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 was sufficient to heat 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 finer grains of ferrite and finer grains of second phase carbides.

Therefore, the cumulative rolling reduction rate between 900° C. and Ar3 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 Ars 20°C is to lower the yield point, and the lower limit is set to Ars 20°C.
The reason for setting the temperature to 0° C. is that when cooling from a low temperature below 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 at 0.3 rd/d·molar or higher 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 a 20°C and A
By applying a light reduction in the two-phase region at r 3 80° C., active dislocations are introduced into the steel, which can lead to the presence of a yield shelf during tensile testing. If the reduction rate in the two-phase region is less than 1%, a yield shelf does not exist, so the lower limit is set to 1%, and
If it exceeds 30%, 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 for accelerated cooling is set at 350 to 600°C is because low-temperature toughness deteriorates when cooling to a low temperature range of less than 350°C, and if cooling is stopped at a high temperature range of over 600°C, the strength does not increase. Because it will be enough.

(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
, O.

P is 50kg f/d class, steel B, C, D, E
, F, Q.

R, S, T, and U are component systems aiming at a strength of 80 kg f/- class, and as shown in Table 2, the steel plate kA1. A2゜Bl
, CI, DI, El, Fl, Gl, Hl.

I 1. J1. Kl, Ll, Ml, Nl, 01°Pi
, Ql, 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 is the aim of the present invention.
Achieves a low yield ratio of less than % and has a yield shelf.

On the other hand, steel plate No. A3 has low-temperature toughness because the heating temperature is too high. In A4, the cumulative reduction between 900 and A r 3 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.

A6 has a high yield ratio because the light reduction ratio before the start of cooling is 40%. 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) As explained in detail above, the present invention has a high strength of 50 kg f /- or more without using any particularly expensive alloying elements and without performing reheating treatment after rolling. This invention enables low yield ratio thick steel plates having a yield shelf with good bending workability to be produced at low cost using a controlled rolling-controlled cooling method, and is an invention that has great industrial effects.

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, rolling is completed with a cumulative reduction ratio of less than 30% relative to the finished plate thickness, and then air cooling is performed until the steel plate surface temperature is between Ar_3-20℃ and Ar_3.
Apply a light pressure of 1% to 30% in the temperature range of -80℃, immediately start cooling at a water density of 0.3m^3/m^2・min or more, and cool when the steel plate temperature is between 350 and 600℃. A method for manufacturing a low yield ratio steel material characterized by stopping. 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, further comprising one or more of a group of strength improving elements consisting of Ti: 0.15% or less and Ti: 0.15% or less. 3. The method for producing a low yield ratio steel material according to claim 1, which contains Ca: 0.01% or less in weight%. 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, further comprising one or more of the following: .10% or less, Ti: 0.15% or less, and Ca: 0.01% or less.