JPS59177321A - Production of steel bar having excellent toughness - Google Patents

Abstract

PURPOSE:To produce efficiently a steel bar having high toughness by subjecting a steel composed specifically of C, Si, Mn, sol.Al and Fe, etc. to specific rolling in a two phase region of austenite and ferrite after a heat treatment and cooling the rolled steel. CONSTITUTION:A steel which contains 0.02-0.17wt% C, <=0.7% Si, 0.6-2.0% Mn and 0.01-0.07% sol.Al, contains, if necessary, >=1 kind among <=0.01% La, <=0.01% Ce and <=0.01% Ca, or further contg. >=1 kind among <=0.50% Cu, <=0.5% Cr, <=3.0% Ni, <=0.20% Mo, <=0.15% V, <=0.10% Nb, <=0.10% Ti, and <=0.10% Zr, and consists of the balance Fe with inevitable impurities is heated to a temp. range of Ac3 transformation point-Ac3 transformation point +300 deg.C and thereafter the heated steel is subjected to rolling at >=40% cumulative draft in the two phase region of austenite and ferrite and at the draft after the even number pass counted from the final stand of 10-20% per pass. The rolled steel is allowed to cool or is forcibly cooled and a steel bar having excellent cryogenic strength and toughness is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a method for producing a bow 1, particularly a steel bar that maintains high strength and high toughness even at extremely low temperatures of -320°C or lower.

Conventionally, for example, steel bars for reinforced concrete IJ-to have been used for reinforced concrete structures in cold regions and polar regions, reinforced concrete freezers, and for liquid IHI gas such as C+. It is well known that these reinforcing bars are used in tanks, etc., but all of these reinforcing bars are manufactured according to the regulations specified in T-work, S (+3112), and are Because it was designed with the assumption that it would be used at temperatures above or above, it would lack strength and toughness when exposed to extremely low temperatures as described in Section 2 above.

Therefore, in recent years, research and development has been carried out on steel bars that maintain specified high strength and high toughness even when exposed to extremely low temperatures, as well as the above-mentioned low temperatures. Currently, steel bars with low-temperature properties have not been obtained.

From the above-mentioned viewpoint, the inventors of the present invention have developed a method, in particular -1, which sufficiently covers the above-mentioned extremely low temperature usage environment.
As a result of various studies in order to obtain a steel bar that has both high strength and high toughness that can be used even at extremely low temperatures of 2 () degrees Celsius or below, we have developed (11) Valve 4, which is based on the EJ concept. : (,1,02~O,], '7% (hereinafter,
Component ratios are not expressed (% is 11f illusory %).

j,;i: O,'? % or less, Mn: 0
．． 6-2.0%.

5oAAI': O, Ol ~ 0.07%.

Contains Fe and unavoidable impurities, residue.

After heating this steel to a temperature range of A C3 transformation point to [Ac3 transformation point + 3oooC], the cumulative downstream rate in the austenite-ferrite two-phase region is determined by the cross-sectional shrinkage rate. 40% or more, and rolled with a downstream ratio of 10 to 20% in each pass after an even number of passes (2n:n-] to 4 passes counting from the final hole stand, and then allowed to cool or bake. When forced cooling is performed within a range where the temperature does not fall, the initial austenite grains are refined by heating in the pre-rolling process, and further, by the rolling, ferrite grains are refined by transformation from fine-grained austenite, and unrecrystallized austenite is added. - The ferrite grains are made finer by the action of increasing the number of transformation nuclei into ferrite, and an extremely fine [ferrite + pearlite] structure is created. 4) I by rolling an even number of times
A different texture is formed, which improves the formation of separations (layered cracks parallel to the rolling surface that appear in the Charpy impact test) and improves toughness based on the crystal stress distribution. Steel bars of specified dimensions are heated to -120°C.
(b) When producing a steel bar as described above, a predetermined amount of Cu, Cr, or Ni is added to the target steel. , Mo,
Y, Nb,'J)j, .

and Zr, the strength and toughness of the steel are further improved; Based on the findings shown in (a) to (C) above, if one or more of Ca and Ca is contained and the treatment shown in item (a) above is applied, the toughness is further improved. It has come to this.

This invention was made based on the above knowledge, C: 002 to O] 7 sections, Si: 0.7 sections or less.

Mn: Q, 6-2.0%, sol, AQ:
0.01-0.07%.

and, if necessary, 1-+8. : O, O], % or more T'-+ Ce
: 0.01% or less.

Ca: 0.01% or less.

Cu:0.
50% or less, Cr: 05% or less.

N j, ', 3.0 or less, Mo: 0.2
Below section 0.

V: 0.15% or less, Nb: 0.10% or less.

"J'i: O,] Section O and below, Zr: 0.
Less than 10%.

Contains 9 or more types of Ti'e and unavoidable impurities: Remaining.

A steel with a composition consisting of A C3 transformation point ~ [: AC
3 transformation point -1-300℃], the cumulative reduction rate in the austenite-ferrite two-phase region was 100%.
Rolling was performed with a coefficient of ni shrinkage of 4.0 or more and a downstream rate of 910 to 20% for each pass after even passes (2n: n-1 to 4) counting from the final hole stand, and then this was carried out. -12 by cooling or forced cooling
This method is characterized in that it provides a steel bar that maintains extremely high strength and excellent toughness even when exposed to extremely low temperatures of 0° C. or lower.

Next, in the method of the present invention, the reason why the composition of the target steel and the processing conditions are limited as described in "2" will be explained.

Component composition of steel ■ C The C component has the effect of imparting a certain strength to the steel bar, but if its content is less than 0.02 mm, the desired effect cannot be obtained; If the content exceeds 0.02%, the toughness will decrease, so
It was set at ~0.17%.

■ 5i Si component has deoxidizing and strengthening effects on steel, but 0
．． If the content exceeds 7%, the toughness will be significantly lowered, so the upper limit was set at 0.7%.

Mn The Mn component is solid dissolved in the base steel and strengthened as a solid solution.
It has the effect of refining crystal grains and improving strength and toughness, but if the content is less than 0.6%, the desired effect cannot be obtained, and on the other hand, if the content exceeds 2° Since toughness and weldability begin to deteriorate, the degree of doubt was set at 06 to 2 degrees. .

■ The sOl, 8C 5ot, and heC components have excellent grain refining and oxidizing effects, but if their content is less than 0.01%, the desired grain refining cannot be achieved; If the content exceeds that amount, the amount of nonmetallic inclusions will rapidly increase and the toughness of the steel will deteriorate, so the content was set at 0.0 to 007.

■ La, Ce, and Ca These elements have the effect of spheroidizing A-based inclusions and improving the impact value in the ductile region, so they are necessary when further improvement of toughness is desired. However, if more than 0.01 inch is added, the effect will not be saturated, but on the contrary, the amount of inclusions will increase and the impact value will deteriorate. set their upper limit to 0
It was set as 01chi.

Also, Cu, Or, Ni, Ma, V,
The elements Nb, T], and Zr are elements that improve the impact value of steel, or at least increase the strength with less deterioration of the impact value. It is added. The reason why the amount of these elements added is limited is as follows.

■Cu r:υ increases the strength without deteriorating the impact value, but if it is contained in excess of 0.50%, it will cause surface cracking of the steel material. Therefore, the content is set at 0.5% or less.

■N1 Ni is not only effective in improving impact value but also increases strength, but its content is 3.0'%.
Even if it exceeds the above limit, no particularly significant improvement effect is observed, and from an economic standpoint, the upper limit has been set at 30%.

σp Cr cr is also an effective element for improving the strength of the copper phase, but 0.5
Since weldability deteriorates if the content exceeds 0.05%, the upper limit was set at 0.5%.

■ MO, like Cr, is an element that improves the strength of the copper phase, but if its content exceeds 0.20%, weldability deteriorates, so the upper limit was set at 0.20%. .

■ V, Nb, 'I''i, and Zr These elements all increase the strength of copper material by their respective carbonitrides, but ■ is 01.5%.
and Nb, Ti and Zr are respectively O, ], O
If the content exceeds 5%, the strength increasing effect will be saturated and a bainite structure will result, leading to deterioration of toughness. Therefore, the upper limit of It was set at 0.10%.

B. Processing conditions φ) Heating temperature If the heating temperature is lower than the A c3 transformation point, the grains will be uniform and fine, but if heated to a temperature exceeding [AC3 transformation point + 300°C], the grain growth will be significant and the grains will not be refined to the desired size. Since it is not possible to achieve the desired temperature, the heating temperature for refining the grain should be set to the Ac3 transformation point.
[AC3 transformation point + 300°C] was determined.

■ Rolling conditions In order to obtain the desired high strength and toughness by controlling the microstructure and texture of ferrite and pearlite, the cumulative reduction rate in the [ferrite + austenite] region, which is indicated by the area reduction rate, must be 40% or more. In particular, the occurrence of separation is promoted, the crystal orientation distribution develops a texture that makes it difficult to develop cleavage fracture in a cross section perpendicular to the rolling direction, and the difference in the rolling direction of the phase material is In order to reduce the orientation, it is necessary to apply strong reduction an even number of times counting from the final pass of groove rolling.

Since the occurrence of separation realizes a plane stress state,
Combined with the effect obtained by refining the structure, it significantly lowers the fracture surface transition temperature in the Charpy impact test. Is it known in the case of steel sheets that the occurrence of separation is closely related to the texture formed by such low-temperature rolling?33 So far, no reports have been found in the case of steel bars33. The present inventors investigated the relationship between texture and rolling conditions in detail and systematically, and discovered that the development of texture has an important meaning in improving toughness.The details are as follows. It's like this.

That is, due to rolling in the two-phase region of ferrite and austenite, the bar pressure adjustment 'jL is different from that of copper plate (j, OO
l (OO], ) (However, () means parallel to the lower surface, <> means parallel to the rolling direction) Similar texture to copper plate (1], 1) to (211) (O),
], ) of the rotation series around <1-OC1')
This is because, in the case of steel bars, the rolling surface rotates around the rolling direction, and for example, in oval/round rolling, the direction perpendicular to the rolling surface is 4U in each pass.
This is because the rolled steel bar is rotated by 90° around the rolling direction under such circumstances, and the separations that occur in the rolled steel bar are parallel to the (]-001 plane, so a large number of separations occur in a cross pattern. In addition, in such low-temperature rolling, each crystal grain extends significantly in the rolling direction, and cleavage fractures parallel to the rolling surface (100
Cracks along one axis propagate more easily.

Moreover, the mixed (110) orientation group makes it easier to generate separation.

The reason why the number of passes is an even number counting from the final hole mold stand is to reduce the anisotropy caused by the development of the texture described above, and the rolling reduction ratio of the passes is set to 0 to 20%. ,
This is because if the rolling reduction ratio is less than 0 parts, it is difficult to form the desired texture, while if the rolling reduction ratio exceeds 20 parts, it will be accompanied by equipment or operational difficulties.

Note that the steel bar that has undergone the above-mentioned rolling process is allowed to cool or is forcedly cooled, but in this case, forced cooling that causes hardening is not preferable because it deteriorates the toughness of the steel. The preparation structure is preferably a fine [ferrite + pearlite] aggregate.

After the rolling process -) 1. Cooled/Yk, 4-+
Even if J steel is tempered to a temperature below the 1 transformation point, the effects of this invention will not be lost in any way.

Next, the present invention will be explained with reference to examples and comparative examples.

Example] First, steel having the composition shown in Table 1 was melted by a normal melting method, and a billet of 150 mmφ was manufactured from the steel.

Next, each of the obtained billets was subjected to oval/round multi-stage rolling under the heating and rolling conditions shown in Table 2.
A steel bar with a diameter of 30 cages was manufactured.

Note that the * mark in Tables 1 and 2 indicates that the component composition or processing conditions are outside the scope of the present invention.

Next, each obtained steel bar was subjected to a tensile test.

Charpy impact test, observation of primary weave, and measurement of texture were performed.

In the tensile test, nine bar flat '/'f? measure,
In the Charpy impact test, a 1T I 84 test piece was taken parallel to the rolling direction in the same way as the tensile test piece, and the fracture surface transition temperature and V-notch absorbed energy at -120°C were measured, and the fracture surface was measured. The occurrence of separation was observed.

In order to observe the structure, in addition to using an optical microscope to observe the sample after being eaten with 02 Nital, we also used a scanning electron microscope to observe the fine structure in detail, and determined the average crystal grain size of the ferrite. The dimensions of pearlite colonies were determined.

To measure the texture, a thin film was created parallel to one plane perpendicular to the rolling direction, and by using coKam#, the same sample was subjected to Shult, Z reflection method and 1.
) Ecker's transmission method was used in combination to obtain all pole figures.

The measurement results in these tests are also shown in Table 2.

From the results shown in Table 2, it is clear that according to the conditions of the present invention,
j('!The manufactured steel bars have high strength and high i・y
It is clear that the material has extremely high toughness, especially at extremely low temperatures below -]220°C.

On the other hand, steel bars manufactured by a comparative method that does not meet the conditions of the present invention are inferior in these properties. That is,
For those manufactured by Comparative Method 1, the heating temperature before rolling was high, the austenite grains became coarse, and the subsequent rolling did not sufficiently refine the ferrite grains, resulting in poor impact energy absorption at -120°C. The value is low.

The fracture surface transition temperatures of the products manufactured using Comparative Methods 4 and 9 are low, but due to the difference in the rolling reduction ratio between oval and round rolling, the separation parallel to either rolling surface is extremely difficult. It occurs easily and the impact absorption energy value at -120°C is low. The product manufactured by Comparative Method 5 has a rolling reduction rate of less than 10% per pass, so
Low impact absorption energy at °C. And the comparative method lO
and No. 11, the downstream in the two-phase region is not sufficient, so the ferrite grains are not refined sufficiently, the texture is insufficiently developed, and the fracture surface transition temperature is high. .

'' According to this invention, it is possible to efficiently produce steel bars with high strength and excellent toughness even at extremely low temperatures of -120°C or lower.
Industrially useful effects are brought about.

Applicant: Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] (]) C: 0.02 to 017 chi. Si, '0.7φ or less. Mn: 0.6-2.0%. sot, AC: 0.01-O, O'7%. 1.a: 0.01 or less. C4]', 0.01% or less. Ca: 0.01 or less. After heating a steel having a composition (by weight) of 1 (e and unavoidable impurities, the remainder) containing one or more types of vehicles to a temperature range of AC3 transformation point to [ΔC3 transformation point 12,500°C], In the austenite-ferrite two-phase region, the cumulative rolling reduction is 40% or more in cross-sectional shrinkage, and the rolling reduction after an even number of baths (2n: n = 1 to 4) counting from the most shielded winter hole type stand is calculated for each pass. Rolled to a ratio of lO~2(),
A method for manufacturing a steel bar with excellent toughness, characterized in that the steel bar is then allowed to cool or is forced to cool. (2) C: 0.02-10, 17%. Sl: O, 7% or less. Contains Mn: 0.6 to 2.0%. SNL, AQ: 0.01 to 0.07%, and further contains: Cu: 050% or less. Cr: 0.3% or less. Section 30 The following. Ivlo: 0.20% or less. V: 0.15% or less. Nb: 0.10% or less. 'I'i: 0.10% or less. Zr: 0.10% or less. One of the following. Including the above, and as necessary, ■・80
Section 01 and below. (Z C: O, O] Ca: 0.0 After heating the steel (Ac3 transformation point to CAcy, transformation point +3oo℃) to a temperature range of Ac3 transformation point to CAcy, transformation point +3oo℃], the cumulative reduction ratio in the austenite-ferrite two-phase region is 40 coefficients or more in cross-sectional shrinkage ratio, and the final Rolling was carried out with the rolling reduction rate in each pass after an even number of passes (2n: n = 1 to 4) counting from the hole stand to 10 to 20%,
A method for producing 44 steel with excellent toughness, which is then allowed to cool or is forced to cool.