JPS59211528A - Production of non-tempered steel having low yield ratio - Google Patents

Abstract

PURPOSE:To obtain a titled steel having good bendability without adding particularly costly alloy elements and reheating after rolling in a controlled rolling- controlled cooling method of the specifically composed steel by limiting conditions for heating temp., hot rolling draft and cooling. CONSTITUTION:A steel contg., by weight %, 0.03-0.20 C, 0.05-0.60 Si, 0.50-2.5 Mn and 0.005-0.1 Al, contg., if necessary, >=1 kind among <=0.50 Cr, <=1.0 Ni, <=0.50 Mo, <=0.1 V, <=0.05 Ti, <=0.1 Nb, <=0.5 Cu and <=0.01 Ca and consisting of the balance Fe is heated to 900-1,200 deg.C and is subjected to >=30% cumulative rolling down with respect to the finish plate thickness between 900 deg.C and Ar3 in hot rolling. The steel is thereafter air cooled and the cooling is started at >=0.3m<3>/m<2>.min water flow density from between [Ar3 -20 deg.C]-[Ar3 -80 deg.C] surface temp. of the steel plate. The cooling is stopped between 350-600 deg.C temp. of the steel plate. The steel plate having a low yield ratio of <70% and >=50kg/ cm<2> strength while having good low temp. toughness with the fine granular ferrite is thus obtd.

Description

[Detailed Description of the Invention] The present invention relates to a method for producing low yield ratio non-tempered steel. In recent years, in various fields such as shipbuilding, tanks, and industrial machinery,
In order to improve competitiveness, various demands are increasing, such as reducing the number of welding operations, pursuing the ultimate in preparation properties such as bending workability, improving weldability, and reducing steel material costs. 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 less than 70%. Furthermore, in the field of bridges, it is desired to improve weldability and reduce yield ratio in order to improve the safety of structures.

In light of these trends, there is currently a demand for the development of steel materials that are inexpensive, have good weldability and workability, and have a low yield ratio.

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

In order to meet these demands, the present invention has made it possible to manufacture a plow board with a strength of 50 kg or more and a low yield ratio of less than 70%. .03~020chi.

St: 0.05-0.60%, Mn: 0.5
0-2.5%.

Contains AA: 0.005 to 0.01% as a basic component, and optionally Cr: 0.50% or less and Ni: 1.0% or less.

Mo: 0.50% or less, V: 0.1% or less, Ti
: 0.15 or less, Nb: 0.1% or less, Cu:
Copper containing 0.5% or less, one or more of Ca: o, oi% or less, the balance consisting of Fe and unavoidable impurities.
Heating at 00-1200℃, 900℃ in hot rolling
A cumulative reduction of 30% or more is applied to the finished plate thickness between A and A r 5, and then the steel plate is cooled in air until the surface temperature of the steel plate reaches Ar 3-2.
From 0℃ to Ar3-80℃ water density 0.3m3/,
, 2. Cooling starts on the separation stage and the steel plate temperature reaches 350-600℃.
The present invention provides a method for manufacturing low yield ratio non-thermal treated steel characterized by stopping cooling between 1 and 2.

In the conventional controlled rolling-controlled cooling process, in order to improve low-temperature toughness, the grains are made as fine as possible by hot rolling, and
Accelerated cooling from the austenite phase region is employed. However, even with this method, there is a problem that the yield point increases due to the grain refinement and hardening of the ferrite and the conversion of some iR-lite to bainite, resulting in an increase in the yield ratio and a decrease in bending workability.

The title of the present invention is based on the results of various investigations into methods of lowering the yield point using a controlled rolling-controlled cooling process.
Similarly, fine-grained ferrite has a strength of 50%, achieving good temperature toughness and a low yield ratio of less than 70%.
This method has been developed for steel plates weighing more than 1 kg.

That is, after heating at 900 to 1200°C, a cumulative reduction of 30% or more is applied at Ar or higher to achieve fine grain formation, and then air cooling to Ar of 5 or lower to properly precipitate soft pro-eutectoid ferrite, followed by forced cooling. It was found that the structure obtained by appropriately mixing ferrite-to-bainite, which is obtained from soft pro-eutectoid ferrite and the remaining austenite, reduces only the yield point strength without deteriorating tensile strength or low-temperature toughness. This completes the present invention.

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

C is necessary to have a coefficient of 0.03 or more to ensure strength, but if it increases, it will impair the toughness and weldability of the steel, so the content should be 0.03 or more.
The upper limit is 20%.

0.05% or more of St is necessary for deoxidation and is added, but if it is too large, 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.

It is essential to add at least 0.005% of At for deoxidization, but if it is too large, the properties of the steel will be deteriorated due to the presence of too many inclusions, so the upper limit of the content should be 0.1%. do.

In the present invention, in addition to the above-mentioned essential basic components, one or more of the following elements can be selectively included depending on the required characteristics of the steel.

Cr is useful for increasing strength and is added, but too much will impede low-temperature toughness and weldability, so the upper limit of the content is 0.5%.

Ni is useful for improving low-temperature toughness and is added, but since it is an expensive element, the upper limit of the content is 1.0.

MO is useful for increasing strength, but since it is a high-valent element, the upper limit of its content is 0.50%.

(2) is useful for the precipitation effect, but if too much it impedes weldability, so the upper limit of the content is 0.054%.

Ti is useful for refining austenite grains and is added, but if too much it impedes weldability, so the content is 0.15%.
is the upper limit.

Like Ti, Nb is useful for refining austenite grains and is added, but if too much, it impedes weldability, so the upper limit of the content is set at 0.1%.

Cu is useful for increasing strength and improving corrosion resistance and is added, but if too much Cu is added, cracks will occur on the steel surface, so the upper limit of the content is set at 0.50%.

Ca is useful for controlling the form of sulfide-based inclusions and is added, but if too much Ca forms inclusions in the steel and deteriorates the properties of the steel, the upper limit of the content is set at 0.01%.

Next, heating, rolling, and cooling conditions, which are important aspects 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. If the temperature is too high, the austenite grains will become too thick and the properties of the steel will deteriorate.
The upper limit of the heating temperature is 200°C.

The upper limit of the rolling temperature is 900° C. in order to refine the austenite grains, and the lower limit of the rolling temperature is ’fr Ars point in order to roll only the austenite 1′ toe phase region.

The reason why the cumulative rolling reduction rate between 900° C. and Ar 3 is set to 30% or more with respect to the finished plate thickness is to sufficiently achieve grain refinement.

Next, air cooling is performed before accelerated cooling after rolling, and the air cooling is performed immediately after rolling.
It is preferable to air cool to a temperature somewhere between ℃,
This allows a suitable amount of fine-grained soft pro-eutectoid ferrite to be precipitated.

The reason why the upper limit of the accelerated cooling start temperature was set to Ar s -20°C was to lower the yield point, and the lower limit was set to Ar 5-80°C.
The reason why the temperature is set at ℃ is because if the temperature is lower than this, the effect of accelerated cooling will be weak and the tensile strength will decrease, making it difficult to maintain the strength. The reason why the water density was set at 0.3 m3//ln2·preparative is because below this value, there is little increase in strength.

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 strength increases when cooling is stopped at a high temperature range of over 600°C. This is because it will be insufficient.

As explained in detail above, the present invention enables controlled rolling of low yield ratio thick steel plates with good bendability without using special expensive alloying elements or without reheating after rolling. This invention can be manufactured at low cost using a controlled cooling method, has a high strength of 50 kg or more, and has excellent low-temperature toughness and weldability, making it a highly effective invention in industry.

Next, examples of the present invention will be listed together with comparative examples.

Table 1 shows the chemical composition of the test material, and Table 2 shows the heating, rolling, and cooling conditions and the mechanical properties of the obtained copper plate.

Steel A has a compositional system aiming at a strength of 50kg class, steels B and C have a strength of 60kg class, and as shown in Table 2, steel plate AA1. B
1. C1 is an embodiment of the present invention, 50 kg and 6
It has sufficient strength and good low-temperature toughness as a 0 kg class steel,
A low yield ratio of less than 70%, which is the aim of the present invention, has been achieved. On the other hand, copper plate A A 2 has low 1)2+ toughness because the heating temperature is too high. A3 is 900℃～A
The cumulative reduction ratio between r3 is low, and as a result, the low temperature toughness is reduced. In A4, the rolling temperature was outside the upper limit and the low temperature toughness was deteriorated. B2 is an example in which the forced cooling start temperature was too high, resulting in a grain with a high yield ratio.

B3 is an example in which the forced cooling end temperature was too inconsistent, resulting in a material with too much strength and low low-temperature toughness. C2 is an example in which the forced cooling start temperature was too low, resulting in a material with low strength and high yield ratio. C3 is an example of a low water density, and therefore has low strength and a high yield ratio.

Claims (1)

[Claims] C: 0.03-0.20%*Si:
0.05~0.60%, Mn: 0.50~2.5
%, At: 0.005 to 0.196 as a basic component, and optionally Cr: 0.50% or less, Ni: 1
．． 0% or less, Mo: 0.50% or less, V: 0
．． 1% or less, Ti: 0.15% or more, Nb
: 0.1 q6 or less + Cu: 0.5% or less +'Ca:
A steel containing one or more of the following: O, OIS, and the remainder consisting of Fe and unavoidable impurities is heated at 900 to 1200°C and hot rolled between 900°C and Ar sO by 30% or more of the finished plate thickness. After applying cumulative reduction of A method for producing low yield ratio non-thermal treated steel characterized by stopping cooling.