Coincheck APIを利用したBitCoinの自動売買ツール作成中のエラーを解決したい

{'error': 'order_type, pair are required', 'success': False}

import configparser
import hmac
import hashlib
import json
from pprint import pprint
import time

import requests



conf = configparser.ConfigParser()
conf.read('config.ini')

ACCESS_KEY = conf['coincheck']['access_key']
SECRET_KEY = conf['coincheck']['secret_key']

# URLの設定
BASE_URL = 'https://coincheck.com'
url = BASE_URL + '/api/exchange/orders'

nonce = str(int(time.time()))

params = {
    "pair": "btc_jpy",
    "order_type": "buy",
    "rate": 3_000_000,
    "amount": 0.005,
}

body = json.dumps(params)

#body = ''
message = nonce + url + body

signature = hmac.new(SECRET_KEY.encode(),
                    message.encode(),
                    hashlib.sha256).hexdigest()

headers = {
    "ACCESS-KEY":ACCESS_KEY,
    "ACCESS-NONCE": nonce,
    "ACCESS-SIGNATURE":signature,
    "Content_Type":'application/json'
}

#  情報を取得する
# params = {
#     'limit':5
# }

# r = requests.get(url, params = params)
# r = requests.get(url, headers = headers)
r = requests.post(url, headers=headers, data=body)
r = r.json()
pprint(r)

{'error': 'order_type, pair are required', 'success': False}