JPS6029425A - Adjusting method of quality of hot rolled steel material - Google Patents

Abstract

PURPOSE:To enable considerable adjustment of the quality of a hot-rolling steel material contg. specific carbon equiv. with a titled method which rolls and cools said material under limited conditions by setting the grain size of austenite prior to transformation and cooling conditions in such a way that the specific equation for estimating yield strength is satisfied. CONSTITUTION:A hot-rolling steel material contg. 0.10-0.50% carbon equiv. is rolled at the temp. above the Ar3 transformation point and is then immediately cooled to <=700 deg.C at a cooling rate of <=150 deg.C/sec to adjust the quality of said material. The target yield strength is obtd. here by satisfying the equation [YS: yield strength (kg/mm.<2>), Ceq: C equiv. (={(%C+%Mn)/6}X100), CT: coiling temp. or cooling stop temp. ( deg.C), CR: cooling rate ( deg.C/sec), t: plate thickness (mm.), sigma0: the term for precipitation strengthening, etc. A1-A5: constant, n: 0 or 1 m: 1 or 2, B: f(dgamma) or dgamma<1/2> or constant, dgamma: grain size of austenite (mu), C: g(CR) or log CR or (CR)<1/2>].

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing steel products such as thick plates and hot strips by hot rolling.

[Problems with conventional technology]

Generally, the material properties of steel materials are largely controlled by the characteristics of the structure and grain size. For example, yield strength, which is one of the mechanical properties, can be described as in equation (1).

ys: 17fV, +apVp+a,V, -MMVM
+a++d, -2+σ0 °-(1) where σ is yield strength gold, ■ is volume fraction, d is grain size, f is ferrite,
P is perlite, B is bainite, M is martensai)
Show k. Note that a is a constant, and σ0 indicates other strengthening factors such as precipitation strengthening and work strengthening.

Therefore, when dividing the yield strength into nine grades, the key point is how to control the crystal grain size and structure (that is, control the transformation). Regarding control of crystal grain size, heating conditions in a heating furnace, recrystallization conditions by hot rolling, austenite grain size, and conditions for transformation from austenite to ferrite are important.

In other words, the key points in finishing are what the austenite grain size was at the time rolling was completed, and how much ferrite grain size this austenite grain size converted into.

Next, regarding the control of the structure (that is, controlling the type and volume fraction of each structure, and the yield strength of each structure), the austenite grain size and cooling rate before transformation are also important points.

As can be seen from the above, the austenite grain size before transformation and the cooling rate are essential information in order to accurately estimate the yield strength of steel after hot rolling.

However, when looking at the conventional method of adjusting the material quality of steel materials, it was usual to control the temperature at the end of hot rolling and at the temperature at which the coil was wound. This method completely lacks the austenite grain size and cooling rate, which are the important information governing the material properties mentioned above. The reason why such a non-strict method was able to withstand use despite its poor accuracy is because the conventional method kept everything, including the heating temperature, rolling schedule, and cooling method after rolling, almost constant, and the diostenite grain size was kept almost constant. Therefore, particle size and structure control inevitably depend on the components, especially the Cargin equivalent (Ce, = (C+''-Mn)
)) had a high degree of control.

However, the inventors conducted repeated experiments and studies based on the concepts of controlled rolling and controlled cooling, and found that it is possible to form a wide range of materials by combining austenite grain size and cooling rate. It was discovered that the material quality of hot-rolled steel can be estimated with an accuracy that cannot be obtained by other methods, and that steel with the target material can be manufactured with high accuracy if the main factors are known in advance.

[Object of the Invention] The object of the present invention is, based on the above-mentioned knowledge, to provide an innovative hot rolling method that eliminates all the drawbacks of conventional methods and that performs hot rolling by controlling the essential requirements governing the material properties of steel materials. It is in a place where we provide.

[Structure and operation of the invention]

The gist of the present invention is as follows.

(1) Hot-rolled steel material f with carbine equivalent of 0.10 to 0.50 cm Immediately after rolling at a temperature higher than the Ar3 transformation point,
In the method of adjusting the material quality by cooling to 700℃ or less at a cooling rate of 0℃/8e or less, the target yield strength is obtained, and the austenite grain size before transformation satisfies the following yield strength estimation formula. A method for adjusting the material quality of hot rolled steel material, characterized by setting cooling conditions.

YS=AI+A2XBXCTn+A3XCXCT XC
-1-A.

q X Ceq” +As
6) m: 1 or 2 B: fcd) or dr-7 or constant γ d, niostenite grain size (ltrIL) C: g (CR
) or LogCR or [1(2) At:0.001
~0.10% + St: 0.10 to 1.0 inches, and the austenite grain size before transformation is constant A, a-8t-, by cooling the steel material at a cooling rate that satisfies the following formula to completely adjust the material quality. The method described in item 1 characterized in that YS=18.9+128×CeqXCT xtOgCR
+0.0827 x C + 0.25 The method according to item 1, which is characterized by adjusting YS=18.9+128×Ce, XCT xtogCR
+0,0827XCe, 0.25Xt+450X [
JNb] (4) AA: A fourth method characterized by completely adjusting the material quality by cooling the steel material at a cooling rate that satisfies the following formula to At- with a constant austenite grain size before transformation of 0.001 to 0.10 inch. The method described in Section 1.

YS=34.28-0.0166XCT+O,0O03
06XCXC'l"q X togCR+0.00761 XCe," + 0
．． 25X t(5) The method according to item 1, characterized in that the material quality is adjusted by cooling at a cooling rate that satisfies the following formula based on the predicted austenite grain size (dr).

YS=12.76+45.47Xdr-2+47.5
6XCe9X CT””XtogCR+0.305XC
-) -0,25Xt+450[%Nb]q The model will be explained below.

The basic structure is to extract the basic factors that determine material properties, determine the factors that most accurately describe the material by combining those factors, arrange them linearly, and calculate the coefficients determined by multiple regression analysis. This is attached to each explanatory factor. First, the basic factors that determine the material are the composition, cooling rate, 2 winding temperatures (cooling stop temperature), based on past wisdom and new knowledge.

The austenite grain size was determined.

Of these, the components and coiling temperature are factors that were also incorporated into conventional material estimation formulas, and the other two factors, cooling rate and austenite grain size, were determined by the above-mentioned experiments conducted by the inventors. This is a factor that is an essential requirement unique to the present invention and was incorporated from the results.

Next, the explanatory factor is determined by a combination of these factors, which involves sifting the single-phase open sound and single-phase correlation coefficient between all combinations of the four factors mentioned above and the material. . Next, multiple regression analysis was performed using these facet factors, and t
Only those values with 1% significance were retained. As a result, the following formula was obtained for YS.

YS"'AI+A2Xdγ-2XCT0+A3XCXC
T-”XtogCRq +A4XC”+A5Xt1σ.・・・・・・・・・(
2)q where d is the austenite grain size, CT is the winding temperature,
or cooling stop temperature, cn is cooling rate, t is plate thickness, co,
is the carbon equivalent.

Note that n takes a numerical value of O, l, -11, mbal, or 2, and the optimum value is selected depending on the type of steel.

Next, an example of At-8i 4 steel for which these coefficients were determined will be shown.

YS=12.76+45.47xdr-z+47.
56XC,,XCT”X Zo gCR+ 0,305
X Ce q + 0.25 X t + 450
X [%Nb) (3) Here, the unit of each factor is d, is M, CT is °C, and Ceq is ChiX100. CR stands for °C/s, t stands for decoration, and YS stands for cross/body.

In equation (3), each coefficient is significant. The contribution rate of equation 1 is R=0.9
It is 3.

As is clear from the contribution rate of the equation, equation (3) shows the actual YS with extremely high accuracy. In addition, due to the use of multiple regression analysis and the fact that there were limitations on equipment capacity,
The applicable range of Ce9 is 0. The applicable range of CT is 700°C or less, and the applicable range of CR is 150°C/8 or less. However, in implementing the present invention, these limitations should not be assumed. Ceq, CT, in favor of carrying out the purpose of the invention.

What is necessary is just to determine each CR.

Next, in equation (3), when dr is constant, an example where a certain austenite grain size is constant is shown below.

First, At-81- contains steel [) t3. o:o snow 8゛↓0″′YS=18.9+128×Ce,×CTxto
gCR+0.0827xC+0.25xt...
・・・・・・・・・(4,)q ys=189+ l 28 X Ce qXCT X
Zo gCR+ o, o 827XC8, +0.25X
t+450X [CHNb]・・・・・・・・・(5) At
- Steel is (t) 0.001-0.1chi1) O,10
% or less YS=34.28-0.0166XCT+0.0003
06XCe.

X CT XZo gCR+0.00761 XCe
q' +0-25X tgi points, and the following methods are available.

■ Direct measurement method A method of measuring using ultrasound and X-rays on-line.

■ Estimation method from d: The relationship between d and d is determined by the components and cooling rate. For example, a method of estimating d, 754 d, using the effects shown in Figures 1 and 2.

〔Example〕

Next, examples according to the present invention will be shown. An example of implementing the present invention is (
:C)=0.15%, [Mn]=O', 90%,
Ce9-0.30% At-8i- to steel (■~■),
[:C)=0.10%.

[Mn]=1.20chi, Coq=0.30chi, [Nb]
= 0.030T of Nb addition At-8i- to steel (■~■)
, [:C:l=0.1096, (Mn]=1.2
096, C=0.30%, [Nb)=0.02596
Nb moving mouth At-8i-Kq steel (■n, [C] = 0.08%, [Mn] = 0
．． 42%, Ce, = O, l 5% At, - steel (■
Examples of ~■) are shown in Table 1 in comparison with the results of the conventional method.

Table 1 shows normal manufacturing conditions. Naturally, the difference between the estimated value and the actual value is small in both the conventional method and the method according to the present invention. In case (2), the reduction schedule of finish rolling was changed from the normal conditions and the austenite grain size became smaller.

(2) is a case where the cooling rate has changed, and (2) is a case where the heating temperature is low, and the austenite grains have become finer.

As can be seen by comparing the estimated values and actual values in the cases of ■■■ and ■■■■, there is a large deviation in the conventional method, whereas
It can be seen that the method according to the invention is very accurate. In addition, ■ is an example in which Nb = 0.025% and CR = lOO℃/S, and other conditions are exactly the same as ■, but by increasing the cooling rate, Nb was added without decreasing the yield strength. This shows the advantage that it can be reduced and the cost can be reduced.

〔Effect of the invention〕

As is clear from the above explanation, even if the pre-transformation austenite grain size changes due to changes in the heating temperature, hot rolling conditions, and post-rolling cooling conditions, the present invention makes it possible to accurately estimate the material quality. Not only can we manufacture reeds, but also
It is possible to make a wide range of material adjustment by changing the austenite grain size before transformation, and it is possible to make a wide range of ingredient concentration to produce a steel material with a wide variety of uses from a small number of steel types.
Since the alloy can be saved, the manufacturing cost of steel materials can be dramatically reduced, which has great effects.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between d, /dct values and cooling rate for each austenite grain size, and Figure 2 is a diagram showing the relationship between dr/d for each C and MJ.
V indicates the relationship between the value of C1 and the cooling rate. 1st Calculation 2 Figure A Acceleration ('%)

Claims (1)

[Scope of Claims] (1) Hot-rolled steel material k having a carbon equivalent of 0.10 to 0.50 immediately after rolling at a temperature equal to or higher than the Ar5 transformation point.
In the method of adjusting the material quality by cooling to 700°C or less at a cooling rate of 0°C/see or less, the pre-transformed austenite grain size and cooling conditions are used to obtain the target yield strength and satisfy the yield strength estimation formula below. Material adjustment method for hot rolled steel material characterized by setting 0 YS=Al+A3×BxCTn+A3×C0qxCT−
1×C0A4×Ceff+A5×10σO C: Caen equivalent (=((%C)+[:%Mn]/6)
xtoo)q CT: Rolling temperature or cooling stop temperature CR: Cooling rate (°C/5ee) t: Plate thickness (IllII+) σ0: Terms such as precipitation strengthening A! ~A5: constant n: 0 or 1. m: l or 2 B: f (dρ or d, -2 or constant dr niostenite grain size (fi → C: g (CR) or togCR or 2) At: 0
．． 001-0.10%, 83: 0.10-1.0%
The method described in item 1, characterized in that the material is completely adjusted by cooling at a cooling rate that satisfies the following equation for the steel material to At-8i- with a constant austenite grain size before transformation YS=18.9+128XCXCT-1XtogCR+
0.0827q × C 〇, 25Xt q (3) Nb't O,005~(1,10% added At-8i- with a constant austenite grain size before transformation) was cooled at a cooling rate that satisfied the following formula. The method described in item 1, characterized by adjusting the material quality by cooling YS=18.9+128XCXCT-1XtogCR+
0.0827q XC□+0.25Xt+450XC%Nb] (4) A
t: 0.001 to 0.10% The method according to item 1, characterized in that the material quality is adjusted by cooling at a cooling rate of At-, which has a constant austenite grain size before Te transformation, and K1 plus K1. YS=34.28-0.0j66XCT-1-0,00
0306XC89XCT X Zog CR+ 0,0
0761 X Ceq ” + O-25X t(5
) The method according to item 1, characterized in that the material quality is adjusted by cooling at a cooling rate that satisfies the following formula based on the predicted austenite grain size (d, ). YS=12.76+4.5.47Xd-2+4.7.
56XCXCT-'eq XAogCR+0.305XC+0.25Xt+450
X(%Nblq